Post by volkerboehme on Aug 10, 2008 12:03:03 GMT -5
I will post again about specific L1649A flight dynamics and handling note issues after my flight dynamics have been released. There are gauge compatibility issues to be addressed each time the gauge complement changes.
In this post I will try to clarify some generic piston aero engine management issues that seem to be widely misunderstood. As the article at Avweb explains 'running square' is an American Urban Myth. The phase in aviation history when superchargers were first introduced to commercial engines happened to coincide with a time when max RPM for each relevant engine was a bit less than max MAP multiplied by 100 and by chance there was a similar relationship of 'all good' operation when that relationship was sustained at lower 'matched' RPM and MAP.
What was true by coincidence when max RPM was around 2500 and max MAP was around 35 before WW2 is not true when max RPM is still only 2800 and max MAP is over 60. A 'rule of equality' that 'worked' for American engines with American superchargers before WW2 was pointlessly repeated and retained. It not only ceased to be relevant, its use could cause damage in more advanced engines, and often caused aircrew to fail to reduce MAP or increase RPM, when it would have been wise to reduce MAP or increase RPM.
There is of course a problem to be avoided. The belief that running square avoids it in all relevant engines is false. Desperate for a 'rule of thumb' some American pilots substituted 'keep it 5 above' or similar 'rules of thumb' that did not work either.
The reason that it is obvious that 'running square' is nothing more than an American Urban Myth is simple. Only American aircraft have MAP gauges. Everybody else controlled boost in the engine using PSI or Ata or Bar (C2). Those values never had the slightest relationship to RPM in any engine. The tiny minority of aeroplanes with MAP gauges, which happen to be familiar to Americans, could be operated 'over square', but the majority without could not, and were not, yet their engine failure and engine damage rate was no worse.
The urban myth took such hold in the United States that it actually passed into airline operating manuals. Those manuals nevertheless carefully promulgated different values to the myth if the engines in question required different values. If an engine had particular problems it was a mandatory requirement to equip it with a torquemeter driving a BMEP gauge so that the situation could be monitored with precision. Unfortunately belief in the universality of the myth became so entrenched in the United States that the manual was sometimes ignored and engine failure ensued.
The article at Avweb correctly debunks the mythology of running (over) square, but then becomes confused. It explains that the engineering issues relate to windmilling and/or braking (backloading) of the engine, but then confuses velocity (KTAS) with profile drag (KIAS). The airscrew is windmilled by the profile drag (KIAS) acting on the screw (and aeroplane), not the velocity (KTAS) of the aeroplane.
The engineering issues are complex, but in FS terms they can be reduced to a simple concept. We must never demand an RPM that the engine cannot produce at the current throttle setting and current IAS. To use the automobile manual gear box analogy, we must always select the correct gear (RPM) for our current throttle setting and energy state.
This links directly to the issue of RPM which must be demanded during the approach.
During approach we must demand and sustain an RPM which both delivers sufficient thrust and which will also allow rapid spool up to the RPM required for a go around. The correct value is related to the IAS range associated with flight after the Final Approach Fix (FAF) and also with the MAP values which may be required to sustain level flight with flap deployed after the FAF, whether or not the instrument runway is the landing runway, whether or not we must circle to land.
Cruise RPM does not provide safe thrust at low KIAS and it delays spool up to go around RPM. Its use inside the FAF is usually inappropriate, but there are always some exceptions to the general rule.
When I write FS handling notes they are extracted from real manuals. If the manual in question is American it may embody American Urban Myths however misplaced they may be. The handling notes help us to recreate the real aircrew operating targets even if they were a strange mixture of safety requirement and operating mythology.
Consider the L1649A
****************************
Approach and Landing:
COWL FLAPS <= 30%
RPM = 2400
MAP => 24 inches *to airfield boundary*
BEFORE INITIAL APPROACH FIX
170 KIAS
BEFORE APPROACH PROCEDURE
FLAP = STAGE 1 (80%)
140 KIAS
Approaching Glideslope
LANDING LIGHTS = DEPLOY
GEAR = DOWN
130 KIAS
subject MAP => 24 inches
Glideslope:
MAP => 24 inches
FLAP = FULL
To cross AIRPORT BOUNDARY 114 KIAS (@ MLW = 123,000lbs)
MAP <= 24 inches
FLARE and LAND
*********************************
We must not shock cool the engines. Cowl flaps are not air brakes. We must apply RPM appropriate to throttle and energy state. We can advance to approach RPM =2400 before the IAF (Initial Approach Fix), but we will NOT use full fine =2900 unless we need to do the equivalent of emergency braking and are prepared to risk some engine damage to apply it. We must be 'in the correct gear'. We are approaching the heavy traffic flows in a built up area after leaving the freeway, not preparing to slow to a stop. We need to 'shift down' from cruise to higher RPM, but not down to first gear and maximum RPM. Approach RPM is roughly equivalent to third gear in a five speed manual shift gear box.
Those of you who have experience of driving wheeled vehicles with manual shift gear boxes will be very familiar with the operating methodology. When descending a hill we do not ride the brakes, we select a gear lower than is appropriate to our speed (causing higher RPM at current fuel flow) to provide engine braking. We control our speed with the gear box using manual gear shifting. We do not slam the vehicle into first gear every time we descend a hill. We impose a gear ratio that delivers adequate engine braking. If we wish to decelerate further we change down further (increase RPM even more).
Of course we must be aware that as speed reduces we will encounter a speed at which even the lowest gear (highest rpm) is efficient. At low speed first gear is what we use to accelerate not decelerate. Piston propliners do not have automatic gear selection. The pilot chooses an efficient gear (RPM) to accelerate or cruise and an inefficient gear (RPM) to decelerate. Which gear (RPM) is efficient and which is inefficient varies with velocity (KTAS) even though windmilling varies with profile drag (KIAS). In a piston propliner we simply choose whether to shift down early for extra braking. The lower our velocity already the less effect it will have and eventually at low velocity first gear (high RPM) is efficient and higher gears (lower RPM) are inefficient. In a piston propliner, just like driving a car or truck with manual shift we choose whether we want efficient or inefficient traction and we vary RPM at constant fuel flow accordingly.
When flying the L1649A whether or not we demand 2400 RPM before the IAF, we WILL demand 2400 before the FAF and we WILL sustain => 24 inches because the handling notes are based on TWA = American Urban Mythology. We WILL select 2400 RPM before we reduce to 140 KIAS and we WILL reduce to 140 KIAS before we begin the approach from the FAF. We WISH to reduce to 130 KIAS before the final glideslope but we WONT if we would need to run undersquare to achieve 130 KIAS *because we are simulating American Urban Mythology*.
Regardless we MUST use our personal skill to deploy FULL FLAP at exactly the point on the final glideslope that causes the aeroplane to decelerate to Vref at 50 QFE (above ground). How far up the final glideslope we need to do that depends ONLY on the current headwind and our current weight. We must nail Vref not avoid it. On the final glideslope while contemplating deployment of FULL FLAP we WILL have 24 MAP and 2400 RPM applied so that weight and wind are the only variables we ever need to take into account when timing full flap deployment. If we misjudge FULL FLAP deployment we MUST be too early so that we can easily apply more MAP to correct our mistake. We must never be too late. According to American Urban Mythology we must never make a mistake that requires less than 24 MAP. When simulating TWA operations we operate the R-3350 engines accordingly and we make sure our mistakes have a known and safe remedy because we *will* misjudge the deployment of FULL FLAP and we must misjudge it early.
Everything has a procedure. Every procedure has a reason. Everything has a precise sequence. Every mistake has a pre planned remedy. In practice we plan to make mistakes we know how to remedy and never risk making mistakes for which there is no safe remedy. We plan to extend full flap too soon, never too late.
We must work very hard to reduce the things that are variable within our energy state calculations. Just timing FULL FLAP to nail Vref versus weight and weather reasonably well is as much as we can hope to achieve. Therefore we must develop the skill to make everything else precisely the same every time we make an approach in a given type of aeroplane. We must never fly rushed approaches in random configurations at random KIAS because we will never have the skill to cope with all the variables. This is addressed in more detail in Parts 7 and 8 of the 2008 Propliner Tutorial.
Now lets go back to the American Urban Myth of 'running square'. If the TWA fleet manager published an Operations Manual telling his aircrew they could use 23 inches and 2400 RPM on approach they would soon be banging on his door telling him that this was dangerous because their instructor had told them so 20 years earlier. No such problem existed inside Air France. If the fleet manager calculated that 23 inches would sustain 2400 RPM without damaging the engine down to IAS = Vref the AF pilots would be happy that they had the operational flexibility to apply a lower MAP than TWA pilots were prepared to apply.
There is more than one engineering issue in play, but the things that damage the engine correlate to and are indicated by RPM decay within the engine. No engine will produce maximum RPM at minimum fuel flow. If we demand 2400 RPM from the engine we must use the throttles to demand sufficient manifold pressure to suck sufficient fuel into the engine to deliver the RPM we demanded with the RPM levers.
At low IAS we must have high RPM else thrust will be inadequate for safety. We must avoid RPM collapse because that is thrust collapse and that causes either IAS decay, or rapid sink, or both. We must avoid RPM decay late in the approach at all costs. What those trained outside the United States understood was that different engines have different MAP to RPM relationships which they should exploit to the full whist still supplying sufficient fuel flow to avoid RPM decay.
We know that after engine failure the IAS (profile drag) on the screw will cause it to windmill. It is a windmill.
We must grasp that those windmills are being windmilled all the time, even when power is being applied to them by an engine. When we demand 2400 RPM with the RPM levers the gear box will prevent windmill effects dragging the screw to any greater RPM, but no gear box can prevent an engine spooling down if the fuel flow is inadequate to turn the engine that fast. As IAS (profile drag on the screws and everything else) decays the windmills may start to brake the engine (cause RPM decay) and that may cause damage to the engine. If they were being windmilled to 2400 by the profile drag = 140 KIAS (with power applied) the profile drag on the screw at Vref = 105 KIAS (when we land very light after a trans Polar flight) may not sustain 2400 RPM at the same MAP and fuel flow. The Air France fleet manager had to flight check whether it did or it didn't, but he did not rely on urban mythology.
Outside the United States mythology relating to non existent MAP gauges never existed and the fleet manager worked out what manifold pressure would sustain approach RPM at minimum Vref. Nobody had any reason to argue if the manual allowed 23 inches, (or its local equivalent that had nothing to with inches), and 2400 RPM. The same applied to whatever minimum values were mandated by an airline for descent from TOD. Velocity (KTAS) is not an issue. In relation to engine damage the issue is profile drag on the screw (KIAS). The profile drag (KIAS) can be too much or too little for the gear (RPM) we have selected. We can brake piston propliners using the relevant gear box, (the constant speed unit controlled with the RPM levers), but that does not mean we can just slam the engines into first gear (max RPM) at Time of Descent (TOD).
Piston propliner aircrew must monitor RPM continuously. They must never allow it to decay. They must retain control of RPM. They must pay particular attention to RPM during final approach because it is during the profile drag decay to IAS = Vref that RPM collapse might happen if they mishandle MAP and it is exactly then in the flight when RPM decay = thrust decay is most likely to kill them with high sink rates.
Most of the posts in FS forums which say the aeroplane stalled simply confuse stall with sink due to inadequate thrust. In most cases what happened was thermodynamic mishandling not aerodynamic mishandling. The aeroplane was nowhere near the stalling angle. Aeroplanes can descend fast at any pitch. It does not imply stall of the wing. If unintentional it implies inadequate thrust from the screws because the wrong RPM was demanded or having been demanded the pilot failed to provide the fuel flow required to sustain the engine at the RPM demanded. Just demanding the correct RPM for take off, or go around, or approach is not enough. We must supply enough manifold pressure to suck enough fuel into the engine to sustain that RPM as profile drag is reduced to IAS = Vref and windmill effect reduces. Allowing the profile drag to decay below Vref before the airfield boundary is therefore very dangerous (for several reasons). We must nail Vref, not ignore it. Having nailed Vref we simply adjust VSI. See 2008 Propliner Tutorial Part 7.
All of the myths I hope to dispel in this post are parochial myths. What happens is that real pilots who have experience of a narrow range of aircraft and engines post about what 'works' in that narrow range of experience. They may be very careful to explain that they are talking about a single type of aircraft. engine, supercharger, and screw, (or not) but forum (and web page) readers often develop the misconception that the particular case is a generic case with wide application.
No one who owns FS9 could possibly believe the American Urban Myth concerning 'running square' has validity. They have had unlimited opportunity to fly aeroplanes without MAP gauges and to grasp that any procedure that requires a MAP gauge to succeed can only be a myth. Most cockpits simply did not have them. FS9 provides a wonderful opportunity to broaden experience and to study how variable operating procedures really are. This then links directly to the myth that a given MAP has relevance to the ability of an aeroplane to move from rest.
A given MAP is not the same power in different engines. Consequently 20 MAP has no significance at all. Even the same % of max MAP has little significance. Different airscrews deliver different thrust when full fine at the same percent max MAP, or even at the same power. Their gearing, number of blades, full fine pitch and diameter vary. Aeroplanes are not propelled by MAP or even power. They are propelled by thrust. Even the ones that are not jets. It is possible for the static thrust code in any air file to be wrong of course, but the error would not relate to a mythical MAP which is sufficient to move all aeroplanes from rest.
The ability to spot a logical fallacy is not associated with tenure of a pilot's licence. FS9 allows anyone to grasp the huge variation of aircraft, engines and aircrews and understand that each requires specific and different pilot inputs to achieve the same invariant output.
Finally this issue of specific versus generic and failure to grasp the variability of aviation procedures in the real world also explains the 'full fine' on approach misconceptions. Firstly some propliner pilots spent their entire career flying aeroplanes with single speed superchargers and some flew only aeroplanes with two speed superchargers.
When an aeroplane has single speed supercharging it is (usually) safe to push the RPM levers full forward at any altitude (for TOGA). When an aeroplane has two speed supercharging it is always unsafe. High gear blowing must never be combined with full engine RPM. The turbine bearings would shatter and a 'blown turbo' with associated fire often ensued. The maximum safe RPM for HI gear is HI METO, so in the CB16 engined DC6B handling notes AND the CB16 engine CV340 notes we see,
****************************
High Blower METO FL145: (1700hp)
FULL THROTTLE
2500 RPM
****************************
If we have two speed supercharging and the runway in use requires HI gear due to its high altitude we must not use more than HI METO RPM for TOGA. Consequently real Ops Manuals for superchargers with with HI gear tend to say;
GO AROUND - EMPLOY AT LEAST METO
This warns the crew they must not advance the RPM levers beyond HI METO if using HI supercharger gear.
Manuals for engines with no HI gear will tend to call for either full fine screws or more accurately TOGA RPM regardless of altitude. There is no single case. Like everything else a given output requires different inputs in different aeroplanes with different engines, superchargers and airscrews.
Nor is it mandatory to demand either HI METO or TOGA RPM at any time during an approach in every aeroplane. There is no need to demand more than 2400 RPM when flying an approach in an L1649A for instance. After application of full throttle for go around it will spool up just fine from 2400 to either 2650 or 2900 depending on supercharger gear selected and runway altitude. In MSFS our RPM levers do not have a METO gate. If we simulate HI blower we must power up first and only then rev up to HI METO.
Note that this TOGA RPM or 'full fine' or 'METO only' issue is NOT aeroplane type specific. A given aeroplane type can have engines with single or two speed blowing. It does not apply to all DC3s or all PBYs. It applies to the superchargers in one individual aeroplane's engines.
So much for the take off and go around case. It is being badly muddled with the approach case.
The requirement to demand higher RPM (via finer airscrews) late in the approach (if it actually exists in a particular aeroplane) is NOT for braking. It is because Vref is so low that approach RPM is too low (third gear is too high) and we need more RPM than approach RPM to generate enough thrust to sustain the glideslope all the way to Vref.
Around Vref Max RPM is very efficient. Around Vref pushing to higher RPM than approach RPM will increase thrust.
It is what we do to take off and go around!
It won't provide braking!
We may simply need to shift down from 'third gear = approach RPM' to 'second gear = METO RPM' or even to 'first gear = TOGA RPM' if Vref is low enough in certain aeroplanes with certain varieties of airscrew or engine. The more we slow down the lower the gear (higher the RPM from current fuel flow) we need for efficient traction.
However pushing the RPM levers forward to demand more RPM and thrust is a waste of time if insufficient MAP is applied to sustain that higher demanded RPM and thrust. Many of you may be pushing RPM levers forward to demand more RPM having already lost control of RPM with RPM already decaying below approach RPM! Once RPM (and thrust) is decaying in an aeroplane with constant speed screws the screws are often already full fine, have hit the fine pitch stop, and cannot fine off further to deliver the demanded RPM. This happens when the pilot fails to supply the MAP needed to suck the fuel needed to sustain that demanded and essential safe RPM. This is why there is a minimum MAP associated with approach in some aeroplanes.
Once we are slow enough shifting all the way down to 'first gear' (demanding maximum possible RPM from current fuel flow) will be the 'correct manual gear selection'. It will be generically correct during ground handling in piston engined aircraft. It 'may' also be optimal at an IAS > Vref depending on the screw, the engine, the supercharger, and on Vref.
The rule in MSFS is simple. In flight do not demand an RPM that is not available because you already throttled the engine to prevent generation of that RPM at the current profile drag (KIAS).
There are many reasons to sustain a mandated RPM. Many aero engines suffer harmonic vibration of the crankshaft over quite large RPM ranges. This is true of both the R-2000-7 Twin Wasp in the C-54B and DC-4, as well as the R-985-SB Wasp Junior in the Goose.
The C-54B handling notes explain;
**********************************
The C-54B has four 1100hp Pratt & Whitney R-2000-7 Twin Wasp engines. They are carburetted and modestly supercharged. 1350hp is available for take off from runways below 7,300 feet. The constant speed propellers can be feathered. Automixture control is standard.
WARNING - These engines suffer from harmonic vibration. Avoid use in the rpm ranges 1550 to 1800 and 2310 to 2510.
***********************************
Many FS users fail to comply because they fail to use the handling notes and fail to monitor RPM.
The Goose notes explain,
********************************
CAUTION - NEVER APPLY LESS THAN 2000 RPM IN FLIGHT due to crankshaft harmonic vibration
****************************
Most FS users fail to comply because they fail to monitor RPM for decay during approach.
****************************
Final Approach and Landing:
MAP as required max 30
RPM = 2200 target (minimum 2000)
Downwind:
GEAR = DOWN (RUNWAY)
Base Leg:
REDUCE < 110 MIAS
Final:
FLAP = BOTH STAGES (RUNWAY)
MAP increase to RESTRAIN DESCENT (RUNWAY)
SCREWS = FULL FINE
TOUCHDOWN = 80 MIAS (all weights)
********************************
Our target is 2200 RPM but our engines will not sustain it with low profile drag windmilling the screw (low IAS). Our safe operating limit is 2000 RPM to avoid a broken crankshaft. After we select FULL FLAP in a Goose for a runway landing IAS will decay to Vref (as we intend) and when operating Wasp Juniors we will need a MAP increase and FULL FINE to restrain RPM decay, crankshaft failure, thrust collapse and sink. The supercharger is single speed.
In an aeroplane with that same engine and screws that has much higher Vref the same screws would be windmilled better by the higher IAS (profile drag), the need for full fine might not exist, and in a different engine there might be no need to avoid < 2000 to prevent crankshaft failure.
Each aeroplane, engine, supercharger and screw combination is specific not generic. What may be required in a Goose may not be required in a Starliner. Flight dynamics are highly variable and the procedures needed to operate them realistically are highly variable. That variability is pretty much the point of having more than one aeroplane to fly in MSFS. Concepts cross over from one aeroplane to another, but procedures do not, operating targets do not, and operating limits do not.
We can read about this stuff here or in books, but putting it all into practice in FS9 is what makes it all come alive. That requires understanding of the content of the 2008 Propliner Tutorial and ability to follow the handling notes line by line and step by step. Then we can simulate operation of a particular aircraft rather than just pretending to fly it. The handling notes are the heart of that process. I will say more about the L1649A in particular after release of my FD which will support the procedures discussed, in so far as that is within the capability of MSFS. MSFS can do all of the things discussed in this post to some extent, but it will never deliver these effects with great accuracy. We should nevertheless make use of the techniques in a rational way.
FSAviator.
In this post I will try to clarify some generic piston aero engine management issues that seem to be widely misunderstood. As the article at Avweb explains 'running square' is an American Urban Myth. The phase in aviation history when superchargers were first introduced to commercial engines happened to coincide with a time when max RPM for each relevant engine was a bit less than max MAP multiplied by 100 and by chance there was a similar relationship of 'all good' operation when that relationship was sustained at lower 'matched' RPM and MAP.
What was true by coincidence when max RPM was around 2500 and max MAP was around 35 before WW2 is not true when max RPM is still only 2800 and max MAP is over 60. A 'rule of equality' that 'worked' for American engines with American superchargers before WW2 was pointlessly repeated and retained. It not only ceased to be relevant, its use could cause damage in more advanced engines, and often caused aircrew to fail to reduce MAP or increase RPM, when it would have been wise to reduce MAP or increase RPM.
There is of course a problem to be avoided. The belief that running square avoids it in all relevant engines is false. Desperate for a 'rule of thumb' some American pilots substituted 'keep it 5 above' or similar 'rules of thumb' that did not work either.
The reason that it is obvious that 'running square' is nothing more than an American Urban Myth is simple. Only American aircraft have MAP gauges. Everybody else controlled boost in the engine using PSI or Ata or Bar (C2). Those values never had the slightest relationship to RPM in any engine. The tiny minority of aeroplanes with MAP gauges, which happen to be familiar to Americans, could be operated 'over square', but the majority without could not, and were not, yet their engine failure and engine damage rate was no worse.
The urban myth took such hold in the United States that it actually passed into airline operating manuals. Those manuals nevertheless carefully promulgated different values to the myth if the engines in question required different values. If an engine had particular problems it was a mandatory requirement to equip it with a torquemeter driving a BMEP gauge so that the situation could be monitored with precision. Unfortunately belief in the universality of the myth became so entrenched in the United States that the manual was sometimes ignored and engine failure ensued.
The article at Avweb correctly debunks the mythology of running (over) square, but then becomes confused. It explains that the engineering issues relate to windmilling and/or braking (backloading) of the engine, but then confuses velocity (KTAS) with profile drag (KIAS). The airscrew is windmilled by the profile drag (KIAS) acting on the screw (and aeroplane), not the velocity (KTAS) of the aeroplane.
The engineering issues are complex, but in FS terms they can be reduced to a simple concept. We must never demand an RPM that the engine cannot produce at the current throttle setting and current IAS. To use the automobile manual gear box analogy, we must always select the correct gear (RPM) for our current throttle setting and energy state.
This links directly to the issue of RPM which must be demanded during the approach.
During approach we must demand and sustain an RPM which both delivers sufficient thrust and which will also allow rapid spool up to the RPM required for a go around. The correct value is related to the IAS range associated with flight after the Final Approach Fix (FAF) and also with the MAP values which may be required to sustain level flight with flap deployed after the FAF, whether or not the instrument runway is the landing runway, whether or not we must circle to land.
Cruise RPM does not provide safe thrust at low KIAS and it delays spool up to go around RPM. Its use inside the FAF is usually inappropriate, but there are always some exceptions to the general rule.
When I write FS handling notes they are extracted from real manuals. If the manual in question is American it may embody American Urban Myths however misplaced they may be. The handling notes help us to recreate the real aircrew operating targets even if they were a strange mixture of safety requirement and operating mythology.
Consider the L1649A
****************************
Approach and Landing:
COWL FLAPS <= 30%
RPM = 2400
MAP => 24 inches *to airfield boundary*
BEFORE INITIAL APPROACH FIX
170 KIAS
BEFORE APPROACH PROCEDURE
FLAP = STAGE 1 (80%)
140 KIAS
Approaching Glideslope
LANDING LIGHTS = DEPLOY
GEAR = DOWN
130 KIAS
subject MAP => 24 inches
Glideslope:
MAP => 24 inches
FLAP = FULL
To cross AIRPORT BOUNDARY 114 KIAS (@ MLW = 123,000lbs)
MAP <= 24 inches
FLARE and LAND
*********************************
We must not shock cool the engines. Cowl flaps are not air brakes. We must apply RPM appropriate to throttle and energy state. We can advance to approach RPM =2400 before the IAF (Initial Approach Fix), but we will NOT use full fine =2900 unless we need to do the equivalent of emergency braking and are prepared to risk some engine damage to apply it. We must be 'in the correct gear'. We are approaching the heavy traffic flows in a built up area after leaving the freeway, not preparing to slow to a stop. We need to 'shift down' from cruise to higher RPM, but not down to first gear and maximum RPM. Approach RPM is roughly equivalent to third gear in a five speed manual shift gear box.
Those of you who have experience of driving wheeled vehicles with manual shift gear boxes will be very familiar with the operating methodology. When descending a hill we do not ride the brakes, we select a gear lower than is appropriate to our speed (causing higher RPM at current fuel flow) to provide engine braking. We control our speed with the gear box using manual gear shifting. We do not slam the vehicle into first gear every time we descend a hill. We impose a gear ratio that delivers adequate engine braking. If we wish to decelerate further we change down further (increase RPM even more).
Of course we must be aware that as speed reduces we will encounter a speed at which even the lowest gear (highest rpm) is efficient. At low speed first gear is what we use to accelerate not decelerate. Piston propliners do not have automatic gear selection. The pilot chooses an efficient gear (RPM) to accelerate or cruise and an inefficient gear (RPM) to decelerate. Which gear (RPM) is efficient and which is inefficient varies with velocity (KTAS) even though windmilling varies with profile drag (KIAS). In a piston propliner we simply choose whether to shift down early for extra braking. The lower our velocity already the less effect it will have and eventually at low velocity first gear (high RPM) is efficient and higher gears (lower RPM) are inefficient. In a piston propliner, just like driving a car or truck with manual shift we choose whether we want efficient or inefficient traction and we vary RPM at constant fuel flow accordingly.
When flying the L1649A whether or not we demand 2400 RPM before the IAF, we WILL demand 2400 before the FAF and we WILL sustain => 24 inches because the handling notes are based on TWA = American Urban Mythology. We WILL select 2400 RPM before we reduce to 140 KIAS and we WILL reduce to 140 KIAS before we begin the approach from the FAF. We WISH to reduce to 130 KIAS before the final glideslope but we WONT if we would need to run undersquare to achieve 130 KIAS *because we are simulating American Urban Mythology*.
Regardless we MUST use our personal skill to deploy FULL FLAP at exactly the point on the final glideslope that causes the aeroplane to decelerate to Vref at 50 QFE (above ground). How far up the final glideslope we need to do that depends ONLY on the current headwind and our current weight. We must nail Vref not avoid it. On the final glideslope while contemplating deployment of FULL FLAP we WILL have 24 MAP and 2400 RPM applied so that weight and wind are the only variables we ever need to take into account when timing full flap deployment. If we misjudge FULL FLAP deployment we MUST be too early so that we can easily apply more MAP to correct our mistake. We must never be too late. According to American Urban Mythology we must never make a mistake that requires less than 24 MAP. When simulating TWA operations we operate the R-3350 engines accordingly and we make sure our mistakes have a known and safe remedy because we *will* misjudge the deployment of FULL FLAP and we must misjudge it early.
Everything has a procedure. Every procedure has a reason. Everything has a precise sequence. Every mistake has a pre planned remedy. In practice we plan to make mistakes we know how to remedy and never risk making mistakes for which there is no safe remedy. We plan to extend full flap too soon, never too late.
We must work very hard to reduce the things that are variable within our energy state calculations. Just timing FULL FLAP to nail Vref versus weight and weather reasonably well is as much as we can hope to achieve. Therefore we must develop the skill to make everything else precisely the same every time we make an approach in a given type of aeroplane. We must never fly rushed approaches in random configurations at random KIAS because we will never have the skill to cope with all the variables. This is addressed in more detail in Parts 7 and 8 of the 2008 Propliner Tutorial.
Now lets go back to the American Urban Myth of 'running square'. If the TWA fleet manager published an Operations Manual telling his aircrew they could use 23 inches and 2400 RPM on approach they would soon be banging on his door telling him that this was dangerous because their instructor had told them so 20 years earlier. No such problem existed inside Air France. If the fleet manager calculated that 23 inches would sustain 2400 RPM without damaging the engine down to IAS = Vref the AF pilots would be happy that they had the operational flexibility to apply a lower MAP than TWA pilots were prepared to apply.
There is more than one engineering issue in play, but the things that damage the engine correlate to and are indicated by RPM decay within the engine. No engine will produce maximum RPM at minimum fuel flow. If we demand 2400 RPM from the engine we must use the throttles to demand sufficient manifold pressure to suck sufficient fuel into the engine to deliver the RPM we demanded with the RPM levers.
At low IAS we must have high RPM else thrust will be inadequate for safety. We must avoid RPM collapse because that is thrust collapse and that causes either IAS decay, or rapid sink, or both. We must avoid RPM decay late in the approach at all costs. What those trained outside the United States understood was that different engines have different MAP to RPM relationships which they should exploit to the full whist still supplying sufficient fuel flow to avoid RPM decay.
We know that after engine failure the IAS (profile drag) on the screw will cause it to windmill. It is a windmill.
We must grasp that those windmills are being windmilled all the time, even when power is being applied to them by an engine. When we demand 2400 RPM with the RPM levers the gear box will prevent windmill effects dragging the screw to any greater RPM, but no gear box can prevent an engine spooling down if the fuel flow is inadequate to turn the engine that fast. As IAS (profile drag on the screws and everything else) decays the windmills may start to brake the engine (cause RPM decay) and that may cause damage to the engine. If they were being windmilled to 2400 by the profile drag = 140 KIAS (with power applied) the profile drag on the screw at Vref = 105 KIAS (when we land very light after a trans Polar flight) may not sustain 2400 RPM at the same MAP and fuel flow. The Air France fleet manager had to flight check whether it did or it didn't, but he did not rely on urban mythology.
Outside the United States mythology relating to non existent MAP gauges never existed and the fleet manager worked out what manifold pressure would sustain approach RPM at minimum Vref. Nobody had any reason to argue if the manual allowed 23 inches, (or its local equivalent that had nothing to with inches), and 2400 RPM. The same applied to whatever minimum values were mandated by an airline for descent from TOD. Velocity (KTAS) is not an issue. In relation to engine damage the issue is profile drag on the screw (KIAS). The profile drag (KIAS) can be too much or too little for the gear (RPM) we have selected. We can brake piston propliners using the relevant gear box, (the constant speed unit controlled with the RPM levers), but that does not mean we can just slam the engines into first gear (max RPM) at Time of Descent (TOD).
Piston propliner aircrew must monitor RPM continuously. They must never allow it to decay. They must retain control of RPM. They must pay particular attention to RPM during final approach because it is during the profile drag decay to IAS = Vref that RPM collapse might happen if they mishandle MAP and it is exactly then in the flight when RPM decay = thrust decay is most likely to kill them with high sink rates.
Most of the posts in FS forums which say the aeroplane stalled simply confuse stall with sink due to inadequate thrust. In most cases what happened was thermodynamic mishandling not aerodynamic mishandling. The aeroplane was nowhere near the stalling angle. Aeroplanes can descend fast at any pitch. It does not imply stall of the wing. If unintentional it implies inadequate thrust from the screws because the wrong RPM was demanded or having been demanded the pilot failed to provide the fuel flow required to sustain the engine at the RPM demanded. Just demanding the correct RPM for take off, or go around, or approach is not enough. We must supply enough manifold pressure to suck enough fuel into the engine to sustain that RPM as profile drag is reduced to IAS = Vref and windmill effect reduces. Allowing the profile drag to decay below Vref before the airfield boundary is therefore very dangerous (for several reasons). We must nail Vref, not ignore it. Having nailed Vref we simply adjust VSI. See 2008 Propliner Tutorial Part 7.
All of the myths I hope to dispel in this post are parochial myths. What happens is that real pilots who have experience of a narrow range of aircraft and engines post about what 'works' in that narrow range of experience. They may be very careful to explain that they are talking about a single type of aircraft. engine, supercharger, and screw, (or not) but forum (and web page) readers often develop the misconception that the particular case is a generic case with wide application.
No one who owns FS9 could possibly believe the American Urban Myth concerning 'running square' has validity. They have had unlimited opportunity to fly aeroplanes without MAP gauges and to grasp that any procedure that requires a MAP gauge to succeed can only be a myth. Most cockpits simply did not have them. FS9 provides a wonderful opportunity to broaden experience and to study how variable operating procedures really are. This then links directly to the myth that a given MAP has relevance to the ability of an aeroplane to move from rest.
A given MAP is not the same power in different engines. Consequently 20 MAP has no significance at all. Even the same % of max MAP has little significance. Different airscrews deliver different thrust when full fine at the same percent max MAP, or even at the same power. Their gearing, number of blades, full fine pitch and diameter vary. Aeroplanes are not propelled by MAP or even power. They are propelled by thrust. Even the ones that are not jets. It is possible for the static thrust code in any air file to be wrong of course, but the error would not relate to a mythical MAP which is sufficient to move all aeroplanes from rest.
The ability to spot a logical fallacy is not associated with tenure of a pilot's licence. FS9 allows anyone to grasp the huge variation of aircraft, engines and aircrews and understand that each requires specific and different pilot inputs to achieve the same invariant output.
Finally this issue of specific versus generic and failure to grasp the variability of aviation procedures in the real world also explains the 'full fine' on approach misconceptions. Firstly some propliner pilots spent their entire career flying aeroplanes with single speed superchargers and some flew only aeroplanes with two speed superchargers.
When an aeroplane has single speed supercharging it is (usually) safe to push the RPM levers full forward at any altitude (for TOGA). When an aeroplane has two speed supercharging it is always unsafe. High gear blowing must never be combined with full engine RPM. The turbine bearings would shatter and a 'blown turbo' with associated fire often ensued. The maximum safe RPM for HI gear is HI METO, so in the CB16 engined DC6B handling notes AND the CB16 engine CV340 notes we see,
****************************
High Blower METO FL145: (1700hp)
FULL THROTTLE
2500 RPM
****************************
If we have two speed supercharging and the runway in use requires HI gear due to its high altitude we must not use more than HI METO RPM for TOGA. Consequently real Ops Manuals for superchargers with with HI gear tend to say;
GO AROUND - EMPLOY AT LEAST METO
This warns the crew they must not advance the RPM levers beyond HI METO if using HI supercharger gear.
Manuals for engines with no HI gear will tend to call for either full fine screws or more accurately TOGA RPM regardless of altitude. There is no single case. Like everything else a given output requires different inputs in different aeroplanes with different engines, superchargers and airscrews.
Nor is it mandatory to demand either HI METO or TOGA RPM at any time during an approach in every aeroplane. There is no need to demand more than 2400 RPM when flying an approach in an L1649A for instance. After application of full throttle for go around it will spool up just fine from 2400 to either 2650 or 2900 depending on supercharger gear selected and runway altitude. In MSFS our RPM levers do not have a METO gate. If we simulate HI blower we must power up first and only then rev up to HI METO.
Note that this TOGA RPM or 'full fine' or 'METO only' issue is NOT aeroplane type specific. A given aeroplane type can have engines with single or two speed blowing. It does not apply to all DC3s or all PBYs. It applies to the superchargers in one individual aeroplane's engines.
So much for the take off and go around case. It is being badly muddled with the approach case.
The requirement to demand higher RPM (via finer airscrews) late in the approach (if it actually exists in a particular aeroplane) is NOT for braking. It is because Vref is so low that approach RPM is too low (third gear is too high) and we need more RPM than approach RPM to generate enough thrust to sustain the glideslope all the way to Vref.
Around Vref Max RPM is very efficient. Around Vref pushing to higher RPM than approach RPM will increase thrust.
It is what we do to take off and go around!
It won't provide braking!
We may simply need to shift down from 'third gear = approach RPM' to 'second gear = METO RPM' or even to 'first gear = TOGA RPM' if Vref is low enough in certain aeroplanes with certain varieties of airscrew or engine. The more we slow down the lower the gear (higher the RPM from current fuel flow) we need for efficient traction.
However pushing the RPM levers forward to demand more RPM and thrust is a waste of time if insufficient MAP is applied to sustain that higher demanded RPM and thrust. Many of you may be pushing RPM levers forward to demand more RPM having already lost control of RPM with RPM already decaying below approach RPM! Once RPM (and thrust) is decaying in an aeroplane with constant speed screws the screws are often already full fine, have hit the fine pitch stop, and cannot fine off further to deliver the demanded RPM. This happens when the pilot fails to supply the MAP needed to suck the fuel needed to sustain that demanded and essential safe RPM. This is why there is a minimum MAP associated with approach in some aeroplanes.
Once we are slow enough shifting all the way down to 'first gear' (demanding maximum possible RPM from current fuel flow) will be the 'correct manual gear selection'. It will be generically correct during ground handling in piston engined aircraft. It 'may' also be optimal at an IAS > Vref depending on the screw, the engine, the supercharger, and on Vref.
The rule in MSFS is simple. In flight do not demand an RPM that is not available because you already throttled the engine to prevent generation of that RPM at the current profile drag (KIAS).
There are many reasons to sustain a mandated RPM. Many aero engines suffer harmonic vibration of the crankshaft over quite large RPM ranges. This is true of both the R-2000-7 Twin Wasp in the C-54B and DC-4, as well as the R-985-SB Wasp Junior in the Goose.
The C-54B handling notes explain;
**********************************
The C-54B has four 1100hp Pratt & Whitney R-2000-7 Twin Wasp engines. They are carburetted and modestly supercharged. 1350hp is available for take off from runways below 7,300 feet. The constant speed propellers can be feathered. Automixture control is standard.
WARNING - These engines suffer from harmonic vibration. Avoid use in the rpm ranges 1550 to 1800 and 2310 to 2510.
***********************************
Many FS users fail to comply because they fail to use the handling notes and fail to monitor RPM.
The Goose notes explain,
********************************
CAUTION - NEVER APPLY LESS THAN 2000 RPM IN FLIGHT due to crankshaft harmonic vibration
****************************
Most FS users fail to comply because they fail to monitor RPM for decay during approach.
****************************
Final Approach and Landing:
MAP as required max 30
RPM = 2200 target (minimum 2000)
Downwind:
GEAR = DOWN (RUNWAY)
Base Leg:
REDUCE < 110 MIAS
Final:
FLAP = BOTH STAGES (RUNWAY)
MAP increase to RESTRAIN DESCENT (RUNWAY)
SCREWS = FULL FINE
TOUCHDOWN = 80 MIAS (all weights)
********************************
Our target is 2200 RPM but our engines will not sustain it with low profile drag windmilling the screw (low IAS). Our safe operating limit is 2000 RPM to avoid a broken crankshaft. After we select FULL FLAP in a Goose for a runway landing IAS will decay to Vref (as we intend) and when operating Wasp Juniors we will need a MAP increase and FULL FINE to restrain RPM decay, crankshaft failure, thrust collapse and sink. The supercharger is single speed.
In an aeroplane with that same engine and screws that has much higher Vref the same screws would be windmilled better by the higher IAS (profile drag), the need for full fine might not exist, and in a different engine there might be no need to avoid < 2000 to prevent crankshaft failure.
Each aeroplane, engine, supercharger and screw combination is specific not generic. What may be required in a Goose may not be required in a Starliner. Flight dynamics are highly variable and the procedures needed to operate them realistically are highly variable. That variability is pretty much the point of having more than one aeroplane to fly in MSFS. Concepts cross over from one aeroplane to another, but procedures do not, operating targets do not, and operating limits do not.
We can read about this stuff here or in books, but putting it all into practice in FS9 is what makes it all come alive. That requires understanding of the content of the 2008 Propliner Tutorial and ability to follow the handling notes line by line and step by step. Then we can simulate operation of a particular aircraft rather than just pretending to fly it. The handling notes are the heart of that process. I will say more about the L1649A in particular after release of my FD which will support the procedures discussed, in so far as that is within the capability of MSFS. MSFS can do all of the things discussed in this post to some extent, but it will never deliver these effects with great accuracy. We should nevertheless make use of the techniques in a rational way.
FSAviator.