Post by volkerboehme on Aug 10, 2008 9:10:44 GMT -5
Greetings Al,
I agree with most of what you posted, but you made some errors which led you to a false conclusion. It is very easy to trip up. In this reply I may point readers to earlier posts in other threads and the parts of the PT which deal with the same issues. Some of the illustrations and explanations are explicitly aimed at non aircrew.
<<There are two ways to produce an increase in lift without the use of flaps -- increasing airspeed or .........>>
No. That is conceptually wrong.
In flight, until the wing of an aeroplane stalls;
LIFT = WEIGHT
Regardless of flight path vector.
To grasp why we need to think of a horse and buggy; the kind with two wheels and one axle. The axle supports the weight of the buggy, however much weight varies. Up hill, down dale. The driver does not need to do anything to make this happen. The axle does not power the vehicle, on the flat or up hills. It just supports whatever weight it has to. It is the poor old horse who has to supply the power; to cruise or climb. The axle contributes nothing other than support of weight. It is not an engine. It cannot move or raise a buggy. Lateral motion and vertical motion are achieved by horse power *and nothing else*.
What almost everybody fails to grasp is that aeroplanes are held aloft by POWER not LIFT. Gliders use solar power, *not wing lift*, to stay airborne. It matters. It matters a lot. *Cruise is sustained by power*. Not wing lift. Climb is solely due to *excess* power, not excess lift. The wing is just an axle. It achieves Newtonian equilibrium between lift and weight with no help from the pilot. Up hill and down dale. The pilot can neither prevent it, nor promote it.
In the cruise fuel burns off, weight reduces, KIAS increase, so the aircraft rotates nose down to produce *less lift* at *higher KIAS*.
The PT explains in detail how the crew should intervene to prevent that, in order to promote and then sustain zero pitch cruising, both before and after they have attained certification ceiling. Lift takes care of itself. Aircrew must control cruise pitch via expert cruising level selection below certification ceiling, and then via MAP and/or RPM trickle reduction at certification ceiling.
Flaps have no relevance either. As part 7 of the PT explains we extend flap to pitch the aeroplane nose down, so that we can see where we are going during obstacle clearance and the approach, not to add lift. If we deploy flap the aerofoil pitches nose down until LIFT = WEIGHT, just as it pitches nose down to shed lift if KIAS increase due to weight reduction. Part 7 also explains transient ballooning and sinking. What pilots do is control aircraft pitch independently from flight path vector. The aerofoil (the wing) achieves the equilibrium LIFT = WEIGHT until it stalls. It does that by rotating itself versus the airflow and the rest of the aeroplane just rotates with it. Some aeroplanes allow pilots to stall the wing at low AoA using lift spoilers, but not propliners.
So the key to all this stuff is to grasp that *lift is not even involved*. That is why it barely gets a mention in the PT. What pilots control is pitch independently from the flight path vector (see PT part 7). Of course it's true that we deploy flap to delay wing stall to sustain LIFT = WEIGHT at lower KIAS, but we cannot promote or prevent LIFT = WEIGHT. It's what aerofoils do without any help at all (until we stall them).
The horse does NOT pull the WEIGHT of the buggy. The weight is supported by the axle. The weight of the aeroplane is supported by the wing, up hill and down dale with no input from pilots. But in order to cruise we *require* horse power. To cruise at higher velocity we require more horse power. To climb we require more horse power. To haul more load at the same velocity we require more horsepower. The axle and the wing are not involved.
Suppose we apply normal cruise power in the cruise and trim for the KIAS that delivers level flight at our current altitude and weight in today's weather. If we reduce power the aeroplane dives to sustain the trimmed KIAS. If we increase power it climbs to sustain the trimmed KIAS. Elevator trim demands constant KIAS (profile drag). Elevator trim has no control over either flight path vector or aircraft pitch.
*There is no such thing as pitch trim*
<<Now, a small mental exercise -- >>
<<Well, the airplane has been trimmed to create lift to equal the original weight of the airplane. >>
For the reasons above that is a given; not a trim state.
Elevator trim demands constant KIAS (profile drag). Not constant flight path vector. That is crucial.
Whatever the crew (or the weather) do next the aeroplane pitches to keep that demanded KIAS (profile drag) constant.
Aircrew can suddenly and easily add 10,000 lbs of weight to big aeroplanes. It's not theoretical. They just apply G to turn. Weight is mass multiplied by G.
To add 10,000lbs suddenly to the weight of a 160,000lb aeroplane we just roll on bank and apply a trivial 1.0625 G. That is how pilots control LIFT. They alter WEIGHT. Because lift always equals weight and there is nothing pilots can do about it. Nor can buggy drivers stop the axle supporting some extra weight in the buggy.
If the pitch was zero before we applied 1.0625 G it will become negative in the turn. The aeroplane will dive not sink. It will dive exactly hard enough to deliver the KIAS demanded by the elevator trim status. The elevator trim status demands a KIAS and causes the aeroplane to change flight path vector to deliver the demanded KIAS. It will pitch to achieve the required flight path vector to achieve the target KIAS.
G *is* gravity. Weight *is* gravity. The aeroplane automatically uses the pull of the nearby planet to achieve the trimmed profile drag (KIAS). Whether we increase weight, or reduce power to destabilise the prior equilibrium.
If we add load to a buggy it will slow down. The entire planet prevents it from sustaining its energy state, but an aeroplane will dive to sustain its energy state until it impacts the planet. It will sustain energy equilibrium. It will shed potential energy to sustain kinetic energy. To prevent a spiral dive the crew must apply back pressure to the yoke to override the elevator trim status to demand lower velocity whilst the G applied and the turn endure, or better still the crew must increase power to supply the kinetic energy deficit whilst the (very slight) G load and the (very gentle) turn endure.
Suddenly adding 10,000lbs to the weight of a propliner is trivial. Imposing lower velocity in the turn, with slight back pressure, allows the original power to sustain level flight at the lower velocity. We do not 'hold the aeroplane up' with back pressure. We do not (cannot possibly) add any lift. We just demand lower velocity. Else we must add power instead.
The key error in your post is here;
<<Now.... magically (and instantaneously) slew the airplane to 23000 feet. Push the throttles forward to restore the lost power to where it was before and thusly equal the drag. The final result is that the aircraft maintains the same 200 knots as it was down low. >>
Your assumption that restoring the same power will restore the same profile drag (KIAS) is false. The same power as at 1000 feet won't be nearly enough. The extra power required is exactly the KIAS v KTAS differential of those flight levels (in ISA or in that actual weather).
Newton explained that to propel the same mass at higher velocity requires more power. Whether buggy or aeroplane. There are no special cases. Equality of profile drag (KIAS) does not imply equality of power requirement because profile drag (KIAS) is not velocity (KTAS). We must never confuse profile drag (KIAS) with velocity (KTAS). See PT part 1.
The horse power required depends on vehicle velocity (KTAS) and not on vehicle profile drag (KIAS). Much more power is required to cruise at high altitude because the velocity (KTAS) is higher at the same profile drag (KIAS). We cannot swap KIAS (drag) for KTAS (velocity) in any of Newton's equations.
The thing that actually matters (horsepower required = operating cost) is very different at FL230.
We must not step climb to FL230 (or any other level) until cruise power will sustain zero pitch cruising at FL230 (or any other level). Hence propliners must employ a step climb technique. We need to target maximum velocity (KTAS), else maximum speed (KTS), using only the power our employer allows us to deploy, according to the weather (see PT part 2). We do not target constant KIAS (profile drag) in the cruise.
Your post conveys the false idea that doing so is 'normal'. It is *wrong* unless our goal is to maximise range (or endurance in a hold). We must NOT target constant KIAS because the *correct* KIAS varies with weight and weather. Minute by minute. Our goal is to maximise either KTAS or KTS, not to 'sustain' KIAS, (unless we must maximise range or endurance in a hold).
The correct altitude for cruising is a key captaincy choice because it controls velocity (KTAS) versus constant MAP and RPM and therefore controls passenger satisfaction, cost and profit. Velocity for any given horse power input maximises at zero pitch. That is the constant. That is the normal operating target.
The operating target changes to constant profile drag (KIAS) at all altitudes only when we need to maximise range. There is then only one KIAS (Vbr) which will maximise range and if ATC permit we will cruise climb (CC) throughout the flight at KIAS = Vbr. See all my prior posts in this forum concerning vintage era constant KIAS range maximising techniques required when flying the B314 and M130 and their Vbr values. However CC clearance was increasingly withheld by ATC once the vintage phase of aviation history gave way to the classic phase.
As I explained at length recently over in the PT thread the crew of an L-1649A would target max velocity (KTAS) from allowed power when flying short trips, but could not target maximum velocity (KTAS) when flying KSFO - EGLL. Their fuel reserves were inadequate. They planned and then flew the POLAR techniques described in my handling notes to reduce, (but not minimise or sustain), profile drag (KIAS) and extend range. The zero pitch technique is the default, but as Part 2 of the PT goes on to explain at other times we must instead maximise speed, or range, or endurance, and each requires a different operating target and technique. The real L-1649A crew would always request CC clearance in the low traffic density Polar regions and 'might' obtain such clearance. They could *not* plan for it. It might be withheld for the reasons also explained in detail in the PT Part 2.
Consequently constant IAS cruise techniques are really not appropriate to the L-1649A at any time. The PT explains when they are appropriate and my handling notes will state a KIAS target if one is relevant for that phase of flight in a given aeroplane.
From other posts;
<<another remark: the inevitable fuel consumption will cause the c.g. to shift minutely within a designed range.>>
An aeroplane is not a seesaw. It is not in contact with the planet in flight. There is no fulcrum in flight. It rotates in pitch as I have explained above and in the PT. CG issues relate only to elevator authority, they have no influence on pitch. On the ground it is a seesaw and the mainwheel contact is the fulcrum. It may need a tail prop else it may pitch hard during loading. The same equations do *not* apply in flight in the absence of a fulcrum.
Big aeroplanes land on their mainwheels else things get broken.
FSAviator.
A continued reply from FSAviator:
When trying to explain the laws of dynamics it is indeed very easy to trip up and there are several errors in my own prior post for which I apologise.
1) Al Green is correct. If we add weight to an aeroplane at zero pitch it will sink (nose up) not dive (nose down).
2) When I said that elevator trim demands a specific IAS that is true, but only at constant weight. The worked example cited weight addition. With constant elevator trim and added weight the aeroplane would sink to ensure that its energy state did not vary and reduce IAS to ensure LIFT = WEIGHT. The aeroplane would do both without pilot input.
3) Jazz5 is correct. CG travel does alter flight pitch (in the absence of pilot corrective action). Loss of elevator authority is only the secondary consequence.
However none of the above alter propliner operating targets. For instance whatever the CG position the cruise target (in the absence of a significant headwind) is zero pitch. It is the (trim) drag and resulting velocity at zero pitch that are variable, not the operating target. We will target and sustain minimal frontal area (zero pitch) cruise at any CG, any weight, indeed at any anything. I will try to explain why those who suggest otherwise are in error.
The real world Aerodynamics 101 or its local equivalent, ( I recommend Mechanics of Flight by Kermode), is required reading for flight dynamics authors, real or virtual, but it is nearly irrelevant to those who want to know how to fly propliners in FS9. I have explained why in many earlier posts and all over again in the Propliner Tutorial (PT).
Commercial propliners were not operated in accordance with the drag curves in Aerodynamics 101; they were operated in accordance with the profit maximisation curve in Economics 101.
The Propliner Tutorial is explicitly not an FD authoring tutorial. It barely mentions lift even though aerodynamics are all about lift. The Propliner Tutorial is about aeroplanes not aerofoils. An aeroplane is not an aerofoil. It's an aerofoil encumbered by a whole bunch of junk. Those precise equations and curves derived from them, in Aerodynamics 101 explain what happens to an aerofoil as things vary, not what happens to an entire aeroplane. Most aeroplanes are mostly junk, not aerofoil. Pilots have to operate the aeroplane in relation to the drag from the junk and the extent to which the junk ruined the precise lift and drag equations relating only to pure aerofoils which are described in Aerodynamics 101.
Propliner operation barely relates to aerodynamics. The economists and the accountants compared engine costs and fuel costs as they varied and gave work instructions to aircrew via manuals which specified how they must mix and match MAP and RPM to control the mix and match of fuel and overhaul costs. Aerodynamic efficiency is a glider pilot issue. They have zero engine cost, zero fuel cost and generate no revenue miles. To the contrary propliner operation revolves around engines not aerofoils; around Thermodynamics 101 and Economics 101, not Aerodynamics 101.
Trucking companies do not tell their drivers to back off and cruise at 40 mph because is more dynamically efficient. The curves in Economics 101 require the truck driver to maximise revenue miles achieved per day, not dynamic efficiency. Consequently profile drag (KIAS) is simply not a cruise target unless range must be maximised. The pilot must apply the mix and match of MAP and RPM appropriate to the current weather as explained in the PT. Neither drag curves nor lift curves from Aerodynamics 101 are involved.
The PT explains at length why (with no significant headwind), since the dynamic inputs are mandated by the accountants, the role of the classic era captain is one of 4D navigation. He must seek the altitude for the current weight, in the current weather, at which the efficiency of those mandated *thermodynamic* inputs maximises. It maximises when the junk presents its minimum cross section (profile) to the airflow. That happens at zero (junk) pitch and (aerofoil) AoA is irrelevant. During propliner cruise, drag from the aerofoil is irrelevant and tiny. It is the awful drag from the junk encumbering the lovely aerodynamic aerofoil that dominates and matters.
That only changes when we need to maximise range. Only then do we have a cruise profile drag (KIAS = Vbr) target and slow to that target. To maximise range we must maximise aerodynamic efficiency not profit. If a truck is going to run out of fuel due to poor planning it may need to slow down to reach the next gas staion, but that is not how truck operators maximise profit. Targeting high mpg and minimum running costs is a truly dreadful way to operate a truck. Profit maximises at a much higher velocity generating more revenue miles per day. To maximise range we must behave as though we were flying a glider. But that is not *normal*. It is an exceptional case in a commercial propliner environment. Maximising range is very unprofitable in propliners, even though it is profitable in jetliners. This is fully explained in the PT parts 1 and 2.
Aeroplanes that must survive very high profile drag (KIAS) may have wings whose angle of incidence is zero. This includes WW2 dive bombers and many jetplanes, but not propliners. The curves in Aerodynamics 101 are not drag v pitch. They are drag v AoA. It is perfectly true that it is always wrong to target zero AoA in the cruise, and that 'best cruising is 'to the left' of zero AOA in an aerofoil drag curve, but unless zero pitch approximates zero AoA there is no reason not to target zero pitch to minimise the frontal area of all the junk attached to the aerofoil and every reason why we should.
During climb, total drag from the aerofoil, (including induced drag which pilots cannot measure), is potentially huge and we always have either a constant profile drag KIAS = Vy target, else we have a profile drag KIAS minimum which requires termination of climb. These aircraft specific Vy values are always stated in my handling notes. They need to be in all handling notes.
If the PT appears to ignore diagrams in textbooks which explain the behavior of aerofoils tested by scientists in the absence of an encumbering aeroplane, that is entirely intentional.
However some posters in various threads who have mentioned 'zero pitch cruise technique' may not have grasped that the PT calls for zero pitch to be sustained only at our *final* cruising level, which in some propliners may not happen until after 14 hours of step climbing. The PT has always explained that on reaching zero pitch at an intermediate level, the pitch has become too low, and it is time to step climb (again). Until we reach final cruising level all our cruising is above zero pitch, even in a propliner.
On a short haul we will have a low final cruising level, (see PT part 6), and we will attain that level quickly. We may discover that the MAP and RPM mandated by the handling notes are going to cause the aeroplane to pitch nose down (in today's weather). We reduce MAP to prevent that, but we allow zero pitch cruise to develop and sustain. To sustain it we trickle reduce MAP and/or RPM as weight diminishes. We do not prolong the flight to avoid zero pitch. Zero pitch is profit maximising for most propliners from the classic era of aviation history.
The cruise condition has a target (zero pitch), but it also has a two limits, (IAS = Vno and Mach = Mno). If the MAP or Turbine Temperature might cause IAS > Vno or Mach > Mno we must reduce our thermodynamic inputs and we will sustain positive pitch cruising for safety reasons.
Aerodynamics 101 assumes no weather at all. We must be terrified by the weather. We must battle significant headwinds. if we need to cruise nose down to battle the headwind, we always will. Safety overrides profit. Aerodynamics 101 tells us nothing about how to counter significant or severe headwinds, but that is an essential flight simulation skill. Aerofoil theory is almost irrelevant.
In a propliner we have both operating targets and operating limits. Our targets come from the curves of Thermodynamics 101 and Economics 101, it is only our limits that come from Aerodynamics 101. The handling notes should always cite the correct thermodynamic inputs, the operating target, and the operating limits. The concepts 'backing off' and 'trading one thing against another' apply only in so far as we must observe the operating limits from Aerodynamics 101 when the operating targets taken from Thermodynamics 101 and Economics 101 say we could maximise profit by endangering the vehicle. All the other 'backing off' and the 'trading' has already been done by the people who wrote the ops manual, or in FS9, the step by step and phase by phase handling notes.
The 'backing off' the accountants do is thermodynamic not aerodynamic. They specify MAP and RPM or turbine temperature (TIT) for a given headwind vector. As (sim) pilots we are trying to operate the propliner *to its limits* and *taking into account the weather*, using only the mandated thermodynamic inputs which are both the norm and the maximum we are allowed to input for that phase of the flight, in the given weather.
The classic era cruise operating limits are Vno and Mno. Whichever happens first. The cruise target is zero pitch, but only until we encounter a significant headwind.
After we apply the thermodynamic inputs the accountants decided to allow, and issued as our work instructions via an ops manual or handling notes, the profit maximising condition is zero pitch, with no significant headwind, for most propliners *designed* during the classic phase of aviation history. All the exceptions and the reasons for the exceptions are explained in the PT, but I will provide examples when answering the questions below.
Now concerning questions asked so far about propliner operation in this thread.
Decelerating propliners by variation of screw traction is fully explained in Part 8 of the PT. We must 'change down' through the csu gear box using the rpm lever to control (shed) screw traction. Shifting down from 'overdrive' to 'lower gears' will decelerate the propliner, provided the air file is correctly coded. Most propliners do not have speed brakes and are decelerated using manual gear (screw rpm) shifting to force the screws to inefficient pitch in the event that we really wish to squander our hard won energy state. Most of you who live near airfields will have heard aircrew shifting rpm (or screw pitch) suddenly to provide airscrew braking. It's like driving a manual gear shift car with brake failure. Turning power off is NOT what will slow the car down.
Anyway if we are cruising with a profile drag of maybe 190 KIAS up at FL230 and need to be 170 KIAS by the initial approach fix (IAF) after descending for > 30 minutes, its hardly a major problem. We have more than 30 minutes to shed 20 KIAS. See PT Part 2 and propliner TOD planning throughout the PT.
From the IAF (zero delay) propliner approach procedures typically last 8+ minutes for a straight in to the instrument runway or 13+ minutes for another landing runway. Unless the approach is a rushed approach IAS targets should be pretty easy to meet starting from 170 KIAS at the IAF decreasing to maybe 110 KIAS at the boundary fence 8 to 13 minutes later. I think the problem is just rushed approaches and failure to use airscrew braking if actually required. I don't think there is a significant problem in the hard coded FS9 FM. Of course different air files have various errors, but that tells us nothing about whether FS9 is broken. There is also no reason to suppose that any error in third party air files relates to drag. It is far more likely to relate to thrust.
<<Dumb question time, please. Can you explain the rational for designing airliners that adopt a nose-down (and so high drag) profile in normal flight regimes? Specifically I'm thinking of aircraft like the AW.27 Ensign or Airspeed Ambassador. >>
There are no dumb questions in aviation only dumb silences.
I believe you are probably wrong concerning the Ambassador (if you mean in real life), but if not the following also applies to the Ambassador.
The reason that some classic era propliners cruise nose down is fully explained in Part 2 of the PT. Aircraft like the Convair 540 were designed to have medium power piston engines and were re-engined with very powerful turbine engines. Operating the engines in accordance with the curves in Thermodynamics 101 is always far more important than any curves in Aerodynamics 101 and both must always give way to Economics 101. We must operate pimped rides like the CV54 at the mandated turbine temperatures.
These pimped rides were explicitly *redesigned* to cruise with negative pitch because thermodynamic efficiency is far more important than aerodynamic efficiency. We will 'back off' in a pimped ride only if the mandated TIT might cause excedence of Vno or Mno. They were redesigned as nose down cruising hot rods.
For the Ensign which hails from the vintage era the answer lies in Part 7 of the PT where it explains the requirement to control pitch independently from flight path vector during climb out and the approach to see where we are going. It explains that we do that using flap in aeroplanes with effective flaps.
The Ensign was designed at a time (1934) when real FD authors understood Part 7 of the PT, but many did not yet know how to design FD that were compliant with Part 7 of the PT. They did not know how to include sufficiently effective flaps. Unlike many sim pilots, after the pioneer era real airline pilots did not have a death wish, and always insisted on being able to see where the aeroplane was going. The real vintage era FD author, Jimmy Lloyd, had no idea how to deliver that without Fowler flaps, so he designed a series of aircraft for Armstrong Whitworth, (including the more famous Whitley), which cruised very nose down and very inefficiently. Imperial Airways and then BOAC just demanded a bigger tax subsidy to compensate for operating both types. Since LIFT = WEIGHT all of the time when the crew slowed the Ensign or Whitley to approach IAS targets the wing rotated very nose up to produce that lift, but Lloyd had made sure the fuselage pitch was far below the AoA, all of the time, and so the crew could still see where they were going at Vx during obstacle clearance and all the way down to Vref on approach; even though the flaps Lloyd added were nearly useless and provided very little ability to control pitch independently from flight path vector.
Real FD authors struggle to control the FD too!
The lesson we must learn is the one that the PT attempts to deliver. Each aeroplane is different, and the history of aviation happened in phases. If we seek realism we need aircraft specific handling notes that specify maximum thermodynamic input, the operating targets, and the operating limits, phase by phase as the flight proceeds. We must operate the aircraft according to the date we are simulating. Many aircraft from history are incompatible with modern procedures.
Textbooks which explain how aerofoils in wind tunnels, unencumbered by engines, aircrews or fuselages and tails, behave in isolation, are almost irrelevant, which is why the PT reads nothing like Aerodynamics 101. An FD authoring tutorial would, and the reason no one can download an FD authoring tutorial is that it *is* Aerodynamics 101 and Thermodynamics 101 combined, runs to at least 350 pages with more than 200 diagrams and cites dozens of equations. Anyone can however buy it in any bookshop. My recommendation was at the top of this post.
But the whole point is nobody needs all those curves and equations to fly aeroplanes. Real or simulated. They are only needed by the people who design aeroplanes, (FD authors real or virtual). Pilots need handling notes which specify the thermodynamic input specific to each aeroplane, the operating targets specific to each aeroplane, and the aerodynamic limits specific to each aeroplane. They also need to be specific to the procedures that were in force at the time and that the aeroplane was designed to achieve.
<<Do these FS9 models have flawed FDE's that do not reflect their RL fight characteristics? >>
All FS9 aircraft have flawed FD. It is only the % error that varies and anyway its really not the issue.
The PT now focuses heavily on the relevance of the different eras of aviation history to usage of FS9 and explains that vintage era aircraft are operated to 'design cruise criteria'. Those criteria relate to neither pitch nor drag. They need to be stated in the release handling notes.
**********************************************************
Pilot's Handling Notes for a Savoia Marchetti S.73 with Piaggio X/RC35 engines
The three 625hp Piaggio P.X/RC35 (Gnome Rhone GR9 Mistral) engines have automated mixture controls, and manually applied carb heat controls. Superchargers deliver rated power up to 3500 metres. Each engine can be overboosted briefly to deliver 650hp for go around below 3100 metres.
WARNING - restrain RPM < 2250 at all times
WARNING - Full throttle cruising below 4500 metres is FORBIDDEN
Design cruise altitude is 4000 metres. This variety of S.73 was capable of econ cruising at 5Km, but it seems that the oxygen supply was inadequate for continuous cruising at that level. Flight above 4Km should be restricted to 30 minutes per flight to climb briefly above weather or mountains.
****************************
Max Cruise (4Km and above ONLY):
Ata = (max) 0.866
Plan 720 PPH
Note: Yields 178 KTAS at FL131 (4Km)
****************************
Design Cruise:
Ata = 0.766
Plan 600 PPH
Note: Yields 162 KTAS at FL131 (4Km)
Note: Yields 167 KTAS at FL164 (5Km)
****************************
Econ Cruise:
Ata = 0.7
Plan 520 PPH
Note: Yields 155 KTAS at FL164 (5Km)
****************************
Just like the AW Ensign no variety of S.73 has a cruise pitch target or cruise drag target. The mandated thermodynamic inputs are made, the aeroplane is climbed to design cruise ceiling (not operational ceiling), regardless of the weather, and the crew pay no attention at all to pitch or KIAS. What they have to do in the absence of constant speed airscrews is watch RPM like a hawk and if necessary further throttle the engine < 2250 RPM as weight and weather vary minute by minute.
Because so many real FD authors in the vintage phase of aviation history did not know how to make FD do what they wanted, they could not impose precise targets on aircrew and there was no simple profit maximising strategy. There simply was no profit! The aeroplanes were too badly designed. Airlines of the vintage era needed tax subsidies to survive. Aircrew
operated accordingly. Flight simulation is all about understanding those issues and replicating them during simulation. It is not about L/D curves for pure aerofoils, whether I and others explain them faultlessly or make mistakes when trying to explain their horrible and irrelevant complexity.
Each vintage era propliner was cruised using mandated power inputs and was climbed to 'design cruise altitude'. The user needs to know the values to input and climb to; else vintage era realism cannot emerge from the FD even if the FD are 'realistic'. If there are real values hiding in the FD the user needs to be told what they are so he can apply them and target them. Creating 'realistic' FD and leaving users to apply random inputs and experience random output is pointless. As I post over and over again consumers worry far to much about FD. It is usually other things that are missing or more broken.
In the example above both the values 2250 RPM and 0.766 Ata are bound to be 'hiding' inside the air file. How could they not be? How could it not be 'realistic'? The numbers are all in there somewhere.
That is simply not the problem. The problem in 99.9% of all freeware and payware is that the consumer is not told that to experience realism when flying an S.73 with Piaggio engines (in the absence of a significant headwind) he must climb directly to 4000 M, throttle to 0.766 Ata and then if necessary throttle RPM < 2250 in order to extract the cruise phase realism 'hiding' in the air file.
The problem is not broken air files. It is the absence of handling notes that leave consumers with nothing better to do than make random inputs, seek random targets, and fail to avoid unknown limits. The consumer obviously cannot experience anything 'realistic' because nobody bothers to tell him what inputs real aircrew made, what targets they they pursued (or none), and the limits they were avoiding, so that the consumer can do the same.
For many aircraft the real operating manual will not be available, but the FD author must know what he chose to encode, and the consumer needs the FD author to explain how to extract any realism that was encoded even if the values encoded were only estimates of the real inputs, operating targets and limits.
The consumer does not need an understanding of Aerodynamics 101. It will lead him to believe that a particular equation or curve derived from that equation is some kind of universal solution for all aeroplanes of all types and all dates. Gliders aside operating aeroplanes is all about thermodynamics and economics. Never mind complex propliners, woe betide the real Cessna 182 owner who operates the wing efficiently in accordance with Aerodynamics 101, but has no idea how to mix and match MAP and RPM to maximise time between engine overhauls (and failure). Even real C182 owners must put both thermodynamic efficiency and economic efficiency, far ahead of aerodynamic efficiency, or pay a heavy price.
Whatever the % error in FD they are just radio control model FD until someone tells the user what *inputs* he is supposed to make and what *output* he must target while making that mandated input, for each phase of the flight, else the user cannot experience realism and is just flying a little radio control model of the aeroplane in question.
<<how then do we determine best cruising pitch, (and how do we get it into the airfile)>>
Everything users need to do, weather case by weather case, mission requirement by mission requirement, phase of aviation history by phase of aviation history, is fully explained in the 2008 PT and the aircraft specific handling notes, else the information is not available.
FD authors must study Aerodynamics 101, research the lift slope for the specific aerofoil, research the aircraft specific mean angle of incidence of that aerofoil, and impose their combined product on TBL 404 of the air file (NOT the aircraft.cfg) . That is far beyond the capability of consumers.
Propliner sim pilots should therefore select and fly aircraft with handling notes which explain the thermodynamic inputs they are required to make, the output targets they must use their accumulated skill to achieve, and the limits they must avoid, whilst making each allowed input, phase by phase of each flight. That requires only a full understanding of the PT, not far more complicated aerodynamics textbooks which I fear will only mislead and will trip up even those who are confident they can explain them without simply plagiarising them; including me.
FSAviator.
I agree with most of what you posted, but you made some errors which led you to a false conclusion. It is very easy to trip up. In this reply I may point readers to earlier posts in other threads and the parts of the PT which deal with the same issues. Some of the illustrations and explanations are explicitly aimed at non aircrew.
<<There are two ways to produce an increase in lift without the use of flaps -- increasing airspeed or .........>>
No. That is conceptually wrong.
In flight, until the wing of an aeroplane stalls;
LIFT = WEIGHT
Regardless of flight path vector.
To grasp why we need to think of a horse and buggy; the kind with two wheels and one axle. The axle supports the weight of the buggy, however much weight varies. Up hill, down dale. The driver does not need to do anything to make this happen. The axle does not power the vehicle, on the flat or up hills. It just supports whatever weight it has to. It is the poor old horse who has to supply the power; to cruise or climb. The axle contributes nothing other than support of weight. It is not an engine. It cannot move or raise a buggy. Lateral motion and vertical motion are achieved by horse power *and nothing else*.
What almost everybody fails to grasp is that aeroplanes are held aloft by POWER not LIFT. Gliders use solar power, *not wing lift*, to stay airborne. It matters. It matters a lot. *Cruise is sustained by power*. Not wing lift. Climb is solely due to *excess* power, not excess lift. The wing is just an axle. It achieves Newtonian equilibrium between lift and weight with no help from the pilot. Up hill and down dale. The pilot can neither prevent it, nor promote it.
In the cruise fuel burns off, weight reduces, KIAS increase, so the aircraft rotates nose down to produce *less lift* at *higher KIAS*.
The PT explains in detail how the crew should intervene to prevent that, in order to promote and then sustain zero pitch cruising, both before and after they have attained certification ceiling. Lift takes care of itself. Aircrew must control cruise pitch via expert cruising level selection below certification ceiling, and then via MAP and/or RPM trickle reduction at certification ceiling.
Flaps have no relevance either. As part 7 of the PT explains we extend flap to pitch the aeroplane nose down, so that we can see where we are going during obstacle clearance and the approach, not to add lift. If we deploy flap the aerofoil pitches nose down until LIFT = WEIGHT, just as it pitches nose down to shed lift if KIAS increase due to weight reduction. Part 7 also explains transient ballooning and sinking. What pilots do is control aircraft pitch independently from flight path vector. The aerofoil (the wing) achieves the equilibrium LIFT = WEIGHT until it stalls. It does that by rotating itself versus the airflow and the rest of the aeroplane just rotates with it. Some aeroplanes allow pilots to stall the wing at low AoA using lift spoilers, but not propliners.
So the key to all this stuff is to grasp that *lift is not even involved*. That is why it barely gets a mention in the PT. What pilots control is pitch independently from the flight path vector (see PT part 7). Of course it's true that we deploy flap to delay wing stall to sustain LIFT = WEIGHT at lower KIAS, but we cannot promote or prevent LIFT = WEIGHT. It's what aerofoils do without any help at all (until we stall them).
The horse does NOT pull the WEIGHT of the buggy. The weight is supported by the axle. The weight of the aeroplane is supported by the wing, up hill and down dale with no input from pilots. But in order to cruise we *require* horse power. To cruise at higher velocity we require more horse power. To climb we require more horse power. To haul more load at the same velocity we require more horsepower. The axle and the wing are not involved.
Suppose we apply normal cruise power in the cruise and trim for the KIAS that delivers level flight at our current altitude and weight in today's weather. If we reduce power the aeroplane dives to sustain the trimmed KIAS. If we increase power it climbs to sustain the trimmed KIAS. Elevator trim demands constant KIAS (profile drag). Elevator trim has no control over either flight path vector or aircraft pitch.
*There is no such thing as pitch trim*
<<Now, a small mental exercise -- >>
<<Well, the airplane has been trimmed to create lift to equal the original weight of the airplane. >>
For the reasons above that is a given; not a trim state.
Elevator trim demands constant KIAS (profile drag). Not constant flight path vector. That is crucial.
Whatever the crew (or the weather) do next the aeroplane pitches to keep that demanded KIAS (profile drag) constant.
Aircrew can suddenly and easily add 10,000 lbs of weight to big aeroplanes. It's not theoretical. They just apply G to turn. Weight is mass multiplied by G.
To add 10,000lbs suddenly to the weight of a 160,000lb aeroplane we just roll on bank and apply a trivial 1.0625 G. That is how pilots control LIFT. They alter WEIGHT. Because lift always equals weight and there is nothing pilots can do about it. Nor can buggy drivers stop the axle supporting some extra weight in the buggy.
If the pitch was zero before we applied 1.0625 G it will become negative in the turn. The aeroplane will dive not sink. It will dive exactly hard enough to deliver the KIAS demanded by the elevator trim status. The elevator trim status demands a KIAS and causes the aeroplane to change flight path vector to deliver the demanded KIAS. It will pitch to achieve the required flight path vector to achieve the target KIAS.
G *is* gravity. Weight *is* gravity. The aeroplane automatically uses the pull of the nearby planet to achieve the trimmed profile drag (KIAS). Whether we increase weight, or reduce power to destabilise the prior equilibrium.
If we add load to a buggy it will slow down. The entire planet prevents it from sustaining its energy state, but an aeroplane will dive to sustain its energy state until it impacts the planet. It will sustain energy equilibrium. It will shed potential energy to sustain kinetic energy. To prevent a spiral dive the crew must apply back pressure to the yoke to override the elevator trim status to demand lower velocity whilst the G applied and the turn endure, or better still the crew must increase power to supply the kinetic energy deficit whilst the (very slight) G load and the (very gentle) turn endure.
Suddenly adding 10,000lbs to the weight of a propliner is trivial. Imposing lower velocity in the turn, with slight back pressure, allows the original power to sustain level flight at the lower velocity. We do not 'hold the aeroplane up' with back pressure. We do not (cannot possibly) add any lift. We just demand lower velocity. Else we must add power instead.
The key error in your post is here;
<<Now.... magically (and instantaneously) slew the airplane to 23000 feet. Push the throttles forward to restore the lost power to where it was before and thusly equal the drag. The final result is that the aircraft maintains the same 200 knots as it was down low. >>
Your assumption that restoring the same power will restore the same profile drag (KIAS) is false. The same power as at 1000 feet won't be nearly enough. The extra power required is exactly the KIAS v KTAS differential of those flight levels (in ISA or in that actual weather).
Newton explained that to propel the same mass at higher velocity requires more power. Whether buggy or aeroplane. There are no special cases. Equality of profile drag (KIAS) does not imply equality of power requirement because profile drag (KIAS) is not velocity (KTAS). We must never confuse profile drag (KIAS) with velocity (KTAS). See PT part 1.
The horse power required depends on vehicle velocity (KTAS) and not on vehicle profile drag (KIAS). Much more power is required to cruise at high altitude because the velocity (KTAS) is higher at the same profile drag (KIAS). We cannot swap KIAS (drag) for KTAS (velocity) in any of Newton's equations.
The thing that actually matters (horsepower required = operating cost) is very different at FL230.
We must not step climb to FL230 (or any other level) until cruise power will sustain zero pitch cruising at FL230 (or any other level). Hence propliners must employ a step climb technique. We need to target maximum velocity (KTAS), else maximum speed (KTS), using only the power our employer allows us to deploy, according to the weather (see PT part 2). We do not target constant KIAS (profile drag) in the cruise.
Your post conveys the false idea that doing so is 'normal'. It is *wrong* unless our goal is to maximise range (or endurance in a hold). We must NOT target constant KIAS because the *correct* KIAS varies with weight and weather. Minute by minute. Our goal is to maximise either KTAS or KTS, not to 'sustain' KIAS, (unless we must maximise range or endurance in a hold).
The correct altitude for cruising is a key captaincy choice because it controls velocity (KTAS) versus constant MAP and RPM and therefore controls passenger satisfaction, cost and profit. Velocity for any given horse power input maximises at zero pitch. That is the constant. That is the normal operating target.
The operating target changes to constant profile drag (KIAS) at all altitudes only when we need to maximise range. There is then only one KIAS (Vbr) which will maximise range and if ATC permit we will cruise climb (CC) throughout the flight at KIAS = Vbr. See all my prior posts in this forum concerning vintage era constant KIAS range maximising techniques required when flying the B314 and M130 and their Vbr values. However CC clearance was increasingly withheld by ATC once the vintage phase of aviation history gave way to the classic phase.
As I explained at length recently over in the PT thread the crew of an L-1649A would target max velocity (KTAS) from allowed power when flying short trips, but could not target maximum velocity (KTAS) when flying KSFO - EGLL. Their fuel reserves were inadequate. They planned and then flew the POLAR techniques described in my handling notes to reduce, (but not minimise or sustain), profile drag (KIAS) and extend range. The zero pitch technique is the default, but as Part 2 of the PT goes on to explain at other times we must instead maximise speed, or range, or endurance, and each requires a different operating target and technique. The real L-1649A crew would always request CC clearance in the low traffic density Polar regions and 'might' obtain such clearance. They could *not* plan for it. It might be withheld for the reasons also explained in detail in the PT Part 2.
Consequently constant IAS cruise techniques are really not appropriate to the L-1649A at any time. The PT explains when they are appropriate and my handling notes will state a KIAS target if one is relevant for that phase of flight in a given aeroplane.
From other posts;
<<another remark: the inevitable fuel consumption will cause the c.g. to shift minutely within a designed range.>>
An aeroplane is not a seesaw. It is not in contact with the planet in flight. There is no fulcrum in flight. It rotates in pitch as I have explained above and in the PT. CG issues relate only to elevator authority, they have no influence on pitch. On the ground it is a seesaw and the mainwheel contact is the fulcrum. It may need a tail prop else it may pitch hard during loading. The same equations do *not* apply in flight in the absence of a fulcrum.
Big aeroplanes land on their mainwheels else things get broken.
FSAviator.
A continued reply from FSAviator:
When trying to explain the laws of dynamics it is indeed very easy to trip up and there are several errors in my own prior post for which I apologise.
1) Al Green is correct. If we add weight to an aeroplane at zero pitch it will sink (nose up) not dive (nose down).
2) When I said that elevator trim demands a specific IAS that is true, but only at constant weight. The worked example cited weight addition. With constant elevator trim and added weight the aeroplane would sink to ensure that its energy state did not vary and reduce IAS to ensure LIFT = WEIGHT. The aeroplane would do both without pilot input.
3) Jazz5 is correct. CG travel does alter flight pitch (in the absence of pilot corrective action). Loss of elevator authority is only the secondary consequence.
However none of the above alter propliner operating targets. For instance whatever the CG position the cruise target (in the absence of a significant headwind) is zero pitch. It is the (trim) drag and resulting velocity at zero pitch that are variable, not the operating target. We will target and sustain minimal frontal area (zero pitch) cruise at any CG, any weight, indeed at any anything. I will try to explain why those who suggest otherwise are in error.
The real world Aerodynamics 101 or its local equivalent, ( I recommend Mechanics of Flight by Kermode), is required reading for flight dynamics authors, real or virtual, but it is nearly irrelevant to those who want to know how to fly propliners in FS9. I have explained why in many earlier posts and all over again in the Propliner Tutorial (PT).
Commercial propliners were not operated in accordance with the drag curves in Aerodynamics 101; they were operated in accordance with the profit maximisation curve in Economics 101.
The Propliner Tutorial is explicitly not an FD authoring tutorial. It barely mentions lift even though aerodynamics are all about lift. The Propliner Tutorial is about aeroplanes not aerofoils. An aeroplane is not an aerofoil. It's an aerofoil encumbered by a whole bunch of junk. Those precise equations and curves derived from them, in Aerodynamics 101 explain what happens to an aerofoil as things vary, not what happens to an entire aeroplane. Most aeroplanes are mostly junk, not aerofoil. Pilots have to operate the aeroplane in relation to the drag from the junk and the extent to which the junk ruined the precise lift and drag equations relating only to pure aerofoils which are described in Aerodynamics 101.
Propliner operation barely relates to aerodynamics. The economists and the accountants compared engine costs and fuel costs as they varied and gave work instructions to aircrew via manuals which specified how they must mix and match MAP and RPM to control the mix and match of fuel and overhaul costs. Aerodynamic efficiency is a glider pilot issue. They have zero engine cost, zero fuel cost and generate no revenue miles. To the contrary propliner operation revolves around engines not aerofoils; around Thermodynamics 101 and Economics 101, not Aerodynamics 101.
Trucking companies do not tell their drivers to back off and cruise at 40 mph because is more dynamically efficient. The curves in Economics 101 require the truck driver to maximise revenue miles achieved per day, not dynamic efficiency. Consequently profile drag (KIAS) is simply not a cruise target unless range must be maximised. The pilot must apply the mix and match of MAP and RPM appropriate to the current weather as explained in the PT. Neither drag curves nor lift curves from Aerodynamics 101 are involved.
The PT explains at length why (with no significant headwind), since the dynamic inputs are mandated by the accountants, the role of the classic era captain is one of 4D navigation. He must seek the altitude for the current weight, in the current weather, at which the efficiency of those mandated *thermodynamic* inputs maximises. It maximises when the junk presents its minimum cross section (profile) to the airflow. That happens at zero (junk) pitch and (aerofoil) AoA is irrelevant. During propliner cruise, drag from the aerofoil is irrelevant and tiny. It is the awful drag from the junk encumbering the lovely aerodynamic aerofoil that dominates and matters.
That only changes when we need to maximise range. Only then do we have a cruise profile drag (KIAS = Vbr) target and slow to that target. To maximise range we must maximise aerodynamic efficiency not profit. If a truck is going to run out of fuel due to poor planning it may need to slow down to reach the next gas staion, but that is not how truck operators maximise profit. Targeting high mpg and minimum running costs is a truly dreadful way to operate a truck. Profit maximises at a much higher velocity generating more revenue miles per day. To maximise range we must behave as though we were flying a glider. But that is not *normal*. It is an exceptional case in a commercial propliner environment. Maximising range is very unprofitable in propliners, even though it is profitable in jetliners. This is fully explained in the PT parts 1 and 2.
Aeroplanes that must survive very high profile drag (KIAS) may have wings whose angle of incidence is zero. This includes WW2 dive bombers and many jetplanes, but not propliners. The curves in Aerodynamics 101 are not drag v pitch. They are drag v AoA. It is perfectly true that it is always wrong to target zero AoA in the cruise, and that 'best cruising is 'to the left' of zero AOA in an aerofoil drag curve, but unless zero pitch approximates zero AoA there is no reason not to target zero pitch to minimise the frontal area of all the junk attached to the aerofoil and every reason why we should.
During climb, total drag from the aerofoil, (including induced drag which pilots cannot measure), is potentially huge and we always have either a constant profile drag KIAS = Vy target, else we have a profile drag KIAS minimum which requires termination of climb. These aircraft specific Vy values are always stated in my handling notes. They need to be in all handling notes.
If the PT appears to ignore diagrams in textbooks which explain the behavior of aerofoils tested by scientists in the absence of an encumbering aeroplane, that is entirely intentional.
However some posters in various threads who have mentioned 'zero pitch cruise technique' may not have grasped that the PT calls for zero pitch to be sustained only at our *final* cruising level, which in some propliners may not happen until after 14 hours of step climbing. The PT has always explained that on reaching zero pitch at an intermediate level, the pitch has become too low, and it is time to step climb (again). Until we reach final cruising level all our cruising is above zero pitch, even in a propliner.
On a short haul we will have a low final cruising level, (see PT part 6), and we will attain that level quickly. We may discover that the MAP and RPM mandated by the handling notes are going to cause the aeroplane to pitch nose down (in today's weather). We reduce MAP to prevent that, but we allow zero pitch cruise to develop and sustain. To sustain it we trickle reduce MAP and/or RPM as weight diminishes. We do not prolong the flight to avoid zero pitch. Zero pitch is profit maximising for most propliners from the classic era of aviation history.
The cruise condition has a target (zero pitch), but it also has a two limits, (IAS = Vno and Mach = Mno). If the MAP or Turbine Temperature might cause IAS > Vno or Mach > Mno we must reduce our thermodynamic inputs and we will sustain positive pitch cruising for safety reasons.
Aerodynamics 101 assumes no weather at all. We must be terrified by the weather. We must battle significant headwinds. if we need to cruise nose down to battle the headwind, we always will. Safety overrides profit. Aerodynamics 101 tells us nothing about how to counter significant or severe headwinds, but that is an essential flight simulation skill. Aerofoil theory is almost irrelevant.
In a propliner we have both operating targets and operating limits. Our targets come from the curves of Thermodynamics 101 and Economics 101, it is only our limits that come from Aerodynamics 101. The handling notes should always cite the correct thermodynamic inputs, the operating target, and the operating limits. The concepts 'backing off' and 'trading one thing against another' apply only in so far as we must observe the operating limits from Aerodynamics 101 when the operating targets taken from Thermodynamics 101 and Economics 101 say we could maximise profit by endangering the vehicle. All the other 'backing off' and the 'trading' has already been done by the people who wrote the ops manual, or in FS9, the step by step and phase by phase handling notes.
The 'backing off' the accountants do is thermodynamic not aerodynamic. They specify MAP and RPM or turbine temperature (TIT) for a given headwind vector. As (sim) pilots we are trying to operate the propliner *to its limits* and *taking into account the weather*, using only the mandated thermodynamic inputs which are both the norm and the maximum we are allowed to input for that phase of the flight, in the given weather.
The classic era cruise operating limits are Vno and Mno. Whichever happens first. The cruise target is zero pitch, but only until we encounter a significant headwind.
After we apply the thermodynamic inputs the accountants decided to allow, and issued as our work instructions via an ops manual or handling notes, the profit maximising condition is zero pitch, with no significant headwind, for most propliners *designed* during the classic phase of aviation history. All the exceptions and the reasons for the exceptions are explained in the PT, but I will provide examples when answering the questions below.
Now concerning questions asked so far about propliner operation in this thread.
Decelerating propliners by variation of screw traction is fully explained in Part 8 of the PT. We must 'change down' through the csu gear box using the rpm lever to control (shed) screw traction. Shifting down from 'overdrive' to 'lower gears' will decelerate the propliner, provided the air file is correctly coded. Most propliners do not have speed brakes and are decelerated using manual gear (screw rpm) shifting to force the screws to inefficient pitch in the event that we really wish to squander our hard won energy state. Most of you who live near airfields will have heard aircrew shifting rpm (or screw pitch) suddenly to provide airscrew braking. It's like driving a manual gear shift car with brake failure. Turning power off is NOT what will slow the car down.
Anyway if we are cruising with a profile drag of maybe 190 KIAS up at FL230 and need to be 170 KIAS by the initial approach fix (IAF) after descending for > 30 minutes, its hardly a major problem. We have more than 30 minutes to shed 20 KIAS. See PT Part 2 and propliner TOD planning throughout the PT.
From the IAF (zero delay) propliner approach procedures typically last 8+ minutes for a straight in to the instrument runway or 13+ minutes for another landing runway. Unless the approach is a rushed approach IAS targets should be pretty easy to meet starting from 170 KIAS at the IAF decreasing to maybe 110 KIAS at the boundary fence 8 to 13 minutes later. I think the problem is just rushed approaches and failure to use airscrew braking if actually required. I don't think there is a significant problem in the hard coded FS9 FM. Of course different air files have various errors, but that tells us nothing about whether FS9 is broken. There is also no reason to suppose that any error in third party air files relates to drag. It is far more likely to relate to thrust.
<<Dumb question time, please. Can you explain the rational for designing airliners that adopt a nose-down (and so high drag) profile in normal flight regimes? Specifically I'm thinking of aircraft like the AW.27 Ensign or Airspeed Ambassador. >>
There are no dumb questions in aviation only dumb silences.
I believe you are probably wrong concerning the Ambassador (if you mean in real life), but if not the following also applies to the Ambassador.
The reason that some classic era propliners cruise nose down is fully explained in Part 2 of the PT. Aircraft like the Convair 540 were designed to have medium power piston engines and were re-engined with very powerful turbine engines. Operating the engines in accordance with the curves in Thermodynamics 101 is always far more important than any curves in Aerodynamics 101 and both must always give way to Economics 101. We must operate pimped rides like the CV54 at the mandated turbine temperatures.
These pimped rides were explicitly *redesigned* to cruise with negative pitch because thermodynamic efficiency is far more important than aerodynamic efficiency. We will 'back off' in a pimped ride only if the mandated TIT might cause excedence of Vno or Mno. They were redesigned as nose down cruising hot rods.
For the Ensign which hails from the vintage era the answer lies in Part 7 of the PT where it explains the requirement to control pitch independently from flight path vector during climb out and the approach to see where we are going. It explains that we do that using flap in aeroplanes with effective flaps.
The Ensign was designed at a time (1934) when real FD authors understood Part 7 of the PT, but many did not yet know how to design FD that were compliant with Part 7 of the PT. They did not know how to include sufficiently effective flaps. Unlike many sim pilots, after the pioneer era real airline pilots did not have a death wish, and always insisted on being able to see where the aeroplane was going. The real vintage era FD author, Jimmy Lloyd, had no idea how to deliver that without Fowler flaps, so he designed a series of aircraft for Armstrong Whitworth, (including the more famous Whitley), which cruised very nose down and very inefficiently. Imperial Airways and then BOAC just demanded a bigger tax subsidy to compensate for operating both types. Since LIFT = WEIGHT all of the time when the crew slowed the Ensign or Whitley to approach IAS targets the wing rotated very nose up to produce that lift, but Lloyd had made sure the fuselage pitch was far below the AoA, all of the time, and so the crew could still see where they were going at Vx during obstacle clearance and all the way down to Vref on approach; even though the flaps Lloyd added were nearly useless and provided very little ability to control pitch independently from flight path vector.
Real FD authors struggle to control the FD too!
The lesson we must learn is the one that the PT attempts to deliver. Each aeroplane is different, and the history of aviation happened in phases. If we seek realism we need aircraft specific handling notes that specify maximum thermodynamic input, the operating targets, and the operating limits, phase by phase as the flight proceeds. We must operate the aircraft according to the date we are simulating. Many aircraft from history are incompatible with modern procedures.
Textbooks which explain how aerofoils in wind tunnels, unencumbered by engines, aircrews or fuselages and tails, behave in isolation, are almost irrelevant, which is why the PT reads nothing like Aerodynamics 101. An FD authoring tutorial would, and the reason no one can download an FD authoring tutorial is that it *is* Aerodynamics 101 and Thermodynamics 101 combined, runs to at least 350 pages with more than 200 diagrams and cites dozens of equations. Anyone can however buy it in any bookshop. My recommendation was at the top of this post.
But the whole point is nobody needs all those curves and equations to fly aeroplanes. Real or simulated. They are only needed by the people who design aeroplanes, (FD authors real or virtual). Pilots need handling notes which specify the thermodynamic input specific to each aeroplane, the operating targets specific to each aeroplane, and the aerodynamic limits specific to each aeroplane. They also need to be specific to the procedures that were in force at the time and that the aeroplane was designed to achieve.
<<Do these FS9 models have flawed FDE's that do not reflect their RL fight characteristics? >>
All FS9 aircraft have flawed FD. It is only the % error that varies and anyway its really not the issue.
The PT now focuses heavily on the relevance of the different eras of aviation history to usage of FS9 and explains that vintage era aircraft are operated to 'design cruise criteria'. Those criteria relate to neither pitch nor drag. They need to be stated in the release handling notes.
**********************************************************
Pilot's Handling Notes for a Savoia Marchetti S.73 with Piaggio X/RC35 engines
The three 625hp Piaggio P.X/RC35 (Gnome Rhone GR9 Mistral) engines have automated mixture controls, and manually applied carb heat controls. Superchargers deliver rated power up to 3500 metres. Each engine can be overboosted briefly to deliver 650hp for go around below 3100 metres.
WARNING - restrain RPM < 2250 at all times
WARNING - Full throttle cruising below 4500 metres is FORBIDDEN
Design cruise altitude is 4000 metres. This variety of S.73 was capable of econ cruising at 5Km, but it seems that the oxygen supply was inadequate for continuous cruising at that level. Flight above 4Km should be restricted to 30 minutes per flight to climb briefly above weather or mountains.
****************************
Max Cruise (4Km and above ONLY):
Ata = (max) 0.866
Plan 720 PPH
Note: Yields 178 KTAS at FL131 (4Km)
****************************
Design Cruise:
Ata = 0.766
Plan 600 PPH
Note: Yields 162 KTAS at FL131 (4Km)
Note: Yields 167 KTAS at FL164 (5Km)
****************************
Econ Cruise:
Ata = 0.7
Plan 520 PPH
Note: Yields 155 KTAS at FL164 (5Km)
****************************
Just like the AW Ensign no variety of S.73 has a cruise pitch target or cruise drag target. The mandated thermodynamic inputs are made, the aeroplane is climbed to design cruise ceiling (not operational ceiling), regardless of the weather, and the crew pay no attention at all to pitch or KIAS. What they have to do in the absence of constant speed airscrews is watch RPM like a hawk and if necessary further throttle the engine < 2250 RPM as weight and weather vary minute by minute.
Because so many real FD authors in the vintage phase of aviation history did not know how to make FD do what they wanted, they could not impose precise targets on aircrew and there was no simple profit maximising strategy. There simply was no profit! The aeroplanes were too badly designed. Airlines of the vintage era needed tax subsidies to survive. Aircrew
operated accordingly. Flight simulation is all about understanding those issues and replicating them during simulation. It is not about L/D curves for pure aerofoils, whether I and others explain them faultlessly or make mistakes when trying to explain their horrible and irrelevant complexity.
Each vintage era propliner was cruised using mandated power inputs and was climbed to 'design cruise altitude'. The user needs to know the values to input and climb to; else vintage era realism cannot emerge from the FD even if the FD are 'realistic'. If there are real values hiding in the FD the user needs to be told what they are so he can apply them and target them. Creating 'realistic' FD and leaving users to apply random inputs and experience random output is pointless. As I post over and over again consumers worry far to much about FD. It is usually other things that are missing or more broken.
In the example above both the values 2250 RPM and 0.766 Ata are bound to be 'hiding' inside the air file. How could they not be? How could it not be 'realistic'? The numbers are all in there somewhere.
That is simply not the problem. The problem in 99.9% of all freeware and payware is that the consumer is not told that to experience realism when flying an S.73 with Piaggio engines (in the absence of a significant headwind) he must climb directly to 4000 M, throttle to 0.766 Ata and then if necessary throttle RPM < 2250 in order to extract the cruise phase realism 'hiding' in the air file.
The problem is not broken air files. It is the absence of handling notes that leave consumers with nothing better to do than make random inputs, seek random targets, and fail to avoid unknown limits. The consumer obviously cannot experience anything 'realistic' because nobody bothers to tell him what inputs real aircrew made, what targets they they pursued (or none), and the limits they were avoiding, so that the consumer can do the same.
For many aircraft the real operating manual will not be available, but the FD author must know what he chose to encode, and the consumer needs the FD author to explain how to extract any realism that was encoded even if the values encoded were only estimates of the real inputs, operating targets and limits.
The consumer does not need an understanding of Aerodynamics 101. It will lead him to believe that a particular equation or curve derived from that equation is some kind of universal solution for all aeroplanes of all types and all dates. Gliders aside operating aeroplanes is all about thermodynamics and economics. Never mind complex propliners, woe betide the real Cessna 182 owner who operates the wing efficiently in accordance with Aerodynamics 101, but has no idea how to mix and match MAP and RPM to maximise time between engine overhauls (and failure). Even real C182 owners must put both thermodynamic efficiency and economic efficiency, far ahead of aerodynamic efficiency, or pay a heavy price.
Whatever the % error in FD they are just radio control model FD until someone tells the user what *inputs* he is supposed to make and what *output* he must target while making that mandated input, for each phase of the flight, else the user cannot experience realism and is just flying a little radio control model of the aeroplane in question.
<<how then do we determine best cruising pitch, (and how do we get it into the airfile)>>
Everything users need to do, weather case by weather case, mission requirement by mission requirement, phase of aviation history by phase of aviation history, is fully explained in the 2008 PT and the aircraft specific handling notes, else the information is not available.
FD authors must study Aerodynamics 101, research the lift slope for the specific aerofoil, research the aircraft specific mean angle of incidence of that aerofoil, and impose their combined product on TBL 404 of the air file (NOT the aircraft.cfg) . That is far beyond the capability of consumers.
Propliner sim pilots should therefore select and fly aircraft with handling notes which explain the thermodynamic inputs they are required to make, the output targets they must use their accumulated skill to achieve, and the limits they must avoid, whilst making each allowed input, phase by phase of each flight. That requires only a full understanding of the PT, not far more complicated aerodynamics textbooks which I fear will only mislead and will trip up even those who are confident they can explain them without simply plagiarising them; including me.
FSAviator.