Post by volkerboehme on Dec 28, 2008 11:16:06 GMT -5
A recent thread concerning the 'Turbo Constellation' reminded me that the propliner tutorial hardly touches upon Mach number related issues and makes only limited reference to structural failure. Having explained in recent threads why we do not encode engine failures within the aircraft you can download from calclassic.com it may be appropriate to explain why you can nevertheless suffer structural failure when flying the same aircraft in FS9.
If structural failure were not encoded the aircraft you download could attain highly unrealistic velocities. Neither the maximum speed, nor the maximum velocity of complex aircraft is limited by the power available. It is limited by the fragility of their structure. The mooted 'Turbo Constellation' provides a good example of why structural failure must be encoded.
Magazines are written by journalists. They rewrite the truth until they have a good story. Forever after others plagiarise whatever nonsense the original journalist wrote. No version of the Lockheed Constellation could have a maximum speed anywhere near 400 KTAS as reported in magazines over the years. The projected Turbo Constellation eventually turned into the L-188 Electra, which years later, after extensive redesign, with a very much stronger single tail, and a vastly greater understanding of transonic shock, had a practical operating limit of around 350 KTAS.
More than 50 years ago Lockheed issued a data sheet for a mooted turboprop conversion of the Constellation. One line of that data sheet read, Maximum design cruising speed (Vc) = 260 KIAS. Another line in the data sheet explained that the maximum safe altitude for the pressure hull was 25,000 feet. A journalist added two and two together and got the answer 400. It may seem logical that if a Constellation were subjected to a drag of 260 KIAS whilst at FL250 it could attain a velocity not far short of 400 KTAS, but that ignores the all important missing ingredient…….. outside air temperature.
Magazines and the Boys Bumper book of Aircraft may say that an aircraft has a maximum level speed (Vmax) of xyz, but the information is almost meaningless, especially if it does not specify the only altitude at which it could very theoretically be true. In real life, for complex aircraft, there is a significant probability of structural failure before Vmax. Only very simple aircraft have Vmax limited by the power available. Most are instead limited by their structure. The later triple tail Lockheeds, including the YC-121F Turbo Constellation, were guaranteed to survive a drag of 260 KIAS, only in modest turbulence, and only if they stayed in warm air.
When reading articles plagiarised from a source more than 50 years old always remember that the original source, even an accurate, official and authoritative source, untainted by journalism, could not have fully understood transonic shock. The numbers quoted are for warm air only. The speed and velocity achievable in cold air were lower and often much lower. They just didn't understand that fully at the time.
Propensity to generate transonic shock increases as temperature falls, and as you climb temperature tends to fall in a straight line. Within the troposphere, (below about FL360 over the CONUS), the higher you go the more likely structural failure becomes, and the lower the maximum velocity (TAS) that can be survived. That is why access to the stratosphere was such a big deal in the quest to conquer transonic shock which journalists called the sound barrier. Once you manage to climb into the stratosphere air temperature is (nearly) constant regardless of altitude. Commercial aircraft with airscrews cannot access the stratosphere. Lacking turbofans neither could the early jetliners, when carrying a payload.
Those of you who have flown the DZN L-049A will have been observing the following two injunctions very carefully.
Above FL200 NEVER EXCEED 210 KIAS (Mno)
DO NOT EXCEED 236 KIAS (Vno)
The earliest incarnation of the Constellation could safely encounter a drag of 236 KIAS, even in modest turbulence, provided it never encountered air colder than minus 17 Centigrade. Colder than that and drag (IAS) must be reduced progressively and substantially to survive. The injunction within the released handling notes is a simplification of the real data table. It makes not the slightest difference what type of engine is used to create the abusive drag. The tail comes off regardless. More powerful engines just allow pilot flying to rip the tail off more easily. They do not allow the aircraft to go faster.
Lockheed worked hard to make the later versions of the triple tail stronger and in the C-121 it could withstand a drag of 260 KIAS, but since Lockheed did not understand transonic shock they could not make the tail more resistant to that mode of failure. After the L-049, as soon as air temperature fell below minus 7 Centigrade, death by application of transonic shock was more likely than death by application of abusive drag.
There had been no improvement in the safe operating Mach limit so the later triple tails could fly a little faster in warm air, but no faster in cold air.
As I explained in a recent post concerning the comparative operating economics of the CV34 and CV58 the purpose of more powerful engines is to lift bigger payloads, (from the same runway). Any Turbo Constellation could have lifted a bigger payload, but it could not have had a higher Vc. In complex aircraft that depends on the weakness of the structure, not the power of the engines.
To go faster the weak triple tail on the Constellation family had to be abandoned, first in the C130, and then as Lockheed employees, (working outside the skunk works), gradually came to understand transonic shock, in the much stronger L188.
To achieve high velocity (TAS) you must restrain drag (IAS). To restrain drag you must climb high into thin air. An aircraft can only be accelerated significantly by constantly climbing. High air is cold air. Cold air promotes transonic shock. Consequently the real maximum speed of high flying propliners is governed only by their ability to withstand cold. This is measured using Mach (temperature), not IAS (drag), or TAS (velocity).
If a propliner can tolerate a high Mach number it can fly colder. Consequently it can fly higher, and so can continue to accelerate for longer by climbing uphill for longer to a higher TAS (velocity) at constant IAS (drag) in thinner and colder air. The airliner that can climb highest, can accelerate for longest, and can go fastest. In any aircraft acceleration is all about going uphill for as long as possible. Concorde could climb (accelerate) continuously until top of descent, reaching around FL600 and M2.05, but no propliner could reach anything like that altitude and thus could not reach anything like 1137 KTAS.
Under normal operating conditions the L049 was increasingly likely to lose its tail if it exceeded M0.48. By 1956 the single tail C-130 could tolerate M0.56 in normal operation and by 1959 enough reality had leaked out of the skunk works that the L188 could withstand M0.615
The real maximum velocities that could be attained by those aircraft were in proportion to those maximum normal operating Mach numbers. M0.56 was compatible with a genuinely attainable velocity of around 315 KTAS whilst M0.615 was compatible with a genuinely attainable velocity of around 350 KTAS which was pretty much the design goal for the Electra a decade after the Turbo Constellation was first mooted.
In Britain the BR10 Britannia which began route proving with BOAC in 1954 was good for Mach 0.57. The BR30 which entered commercial service with BOAC in 1957 was much stronger and good for Mach 0.6 whilst the VC9 which entered commercial service with BEA in 1960 was a brute of a propliner strong enough to fly at M0.64 in normal use. This took the practical limit for the inferior turboprops developed west of the iron curtain to a fraction over 360 KTAS.
The Soviet Union were years ahead in propliner development and in 1961 introduced the vastly superior TU14 which had a practical limit of around 420 KTAS in normal use. This exactly matched the contemporary turbojet Comet 4C which could also tolerate M0.79 in normal use. All these aircraft could fly a little faster under carefully specified atypical circumstances, but by the mid 1960s only Soviet propliners were truly competitive for long haul operations.
High velocity (TAS) is only possible at low drag (IAS) in thin, cold, air and that requires very high Mach tolerance.
To compete with the FK27, VC7 and VC8 conversion of old Convair airframes to turbine power and CV58 configuration began during the sixties and although Pacific Airmotive did all that they could to strengthen the original tail, it was still prone to fail at only M0.485. Consequently the CV58 achieves maximum safe velocity around FL200. The air above is usually too cold for safe operation at the same velocity. You can easily climb a CV58 above FL200, but in most places you will have to slow down or risk sudden death.
If you only ever fly over California, you will enjoy warmer than average air. It is possible to fly a CV58 without worrying too much about the temperature over California, but don't expect to survive if you take the same liberties over Alaska, (with real weather in use).
The maximum level velocity of a complex aircraft is not a fixed number as books and magazines pretend. It is a complex variable. It depends where you are. It depends on the weather. In warm places it is limited by drag, but in cold places it is limited by temperature. Go any faster than the drag limit (260 KIAS) or the temperature limit (M0.485) and the chances of structural failure in a CV58 rise swiftly. The normal operating limit for any triple tail Lockheed was slightly less at M0.48. No member of the Constellation family could fly faster than a CV58 in the same place in the same weather. The power of the engines mounted is wholly irrelevant.
When you exceed either the normal drag operating limit, or the normal temperature operating limit, FS9 displays an overspeed warning and calculates your chances of survival second by second. Survival depends on whether you are turning, or pitching, both of which apply G to the structure. Most likely however it is the weather that will apply a fraction too much G and will cause structural failure as a result of your loss of control of the aircraft energy state.
Structural failure has almost nothing to do with velocity, (how fast you are going), but since you will perish if you apply abusive drag in pursuit of high velocity, or enter abusively cold air whilst accelerating for too long uphill in pursuit of high velocity, your velocity will be limited realistically anyway.
By the time that the B377 was submitted for certification it had dawned on U.S. regulatory authorities that aircrew needed a means to predict structural failure, whether induced by excess drag, or by excess cold. The B377 was equipped with an ASI that not only provided very accurate drag readings, but that also had a Mach bug to indicate when the airframe might fail due to abusively cold air (Mach 0.52). The Mach bug on the B377 eventually developed into the Barber Pole in the CV58 which serves the same purpose. Both will converge with the ASI needle as soon as structural failure for whatever reason becomes an increasing probability. Always keep a close eye on the barber pole when flying a turbine conversion of a piston propliner in MSFS. Never let the airspeed needle reach the barber pole, (or Mach bug), else FS9 will start to calculate your demise.
Almost all turbine powered aircraft have enough power to achieve structural failure in level flight. Some like the Fairchild built version of the Fokker Friendship known as the F-27A, equipped with very powerful engines, to allow operation from short runways in the California, New Mexico and Nevada deserts, can achieve structural failure in level flight, even in warm air. Despite this their ASI designed in Europe had no barber pole or Mach bug. The next generation of turbine engined cockpits would have Machmeters as well as barber poles or Mach bugs so that pilots had no excuse for ripping the tail off, and for the most part they have managed to avoid that option in real life. Make sure you are just as careful.
Flight simulation is by far the best way to understand how aviation really works. The numbers bandied about in books and magazines mean little in the real world. Some could be true under very peculiar circumstances, but many are just journalistic invention.
The two YC-121Fs Turbo Constellations with a normal structural limit of M0.48 were in reality used to test and evaluate the procedures that would be used by the C-130A when it arrived in large numbers three years later. They were used to work out what syllabus tactical transport pilots would have to go through to convert from simple, low altitude aircraft like the C-119, to the overpowered, turbine and transonic shock prone, high altitude C-130. The YC-121F had all the new energy state management problems that the C-130A would present, but worse, and was an ideal case for test and evaluation en route to introduction of the M0.56 limited C-130 in 1956.
A civilian Turbo Constellation would have had the same Mach limit of M0.48 whatever over powerful turboprop engines were installed. It is just a shame that the same mistake, made by a single journalist more than fifty years ago, is mindlessly plagiarised over and over again in books and magazines claiming daft velocities for the 'Turbo Constellation'.
The only way for aviation enthusiasts to understand what aeroplanes can really do is to fly them in MSFS using realistic flight dynamics that have been carefully integrated with realistic gauge code and matching handling notes. Make sure you have the 'aircraft stress causes damage' realism option enabled else you will inhabit the poorly informed and imaginary world of journalists and magazine publishers where propliners can achieve warp speeds.
If structural failure were not encoded the aircraft you download could attain highly unrealistic velocities. Neither the maximum speed, nor the maximum velocity of complex aircraft is limited by the power available. It is limited by the fragility of their structure. The mooted 'Turbo Constellation' provides a good example of why structural failure must be encoded.
Magazines are written by journalists. They rewrite the truth until they have a good story. Forever after others plagiarise whatever nonsense the original journalist wrote. No version of the Lockheed Constellation could have a maximum speed anywhere near 400 KTAS as reported in magazines over the years. The projected Turbo Constellation eventually turned into the L-188 Electra, which years later, after extensive redesign, with a very much stronger single tail, and a vastly greater understanding of transonic shock, had a practical operating limit of around 350 KTAS.
More than 50 years ago Lockheed issued a data sheet for a mooted turboprop conversion of the Constellation. One line of that data sheet read, Maximum design cruising speed (Vc) = 260 KIAS. Another line in the data sheet explained that the maximum safe altitude for the pressure hull was 25,000 feet. A journalist added two and two together and got the answer 400. It may seem logical that if a Constellation were subjected to a drag of 260 KIAS whilst at FL250 it could attain a velocity not far short of 400 KTAS, but that ignores the all important missing ingredient…….. outside air temperature.
Magazines and the Boys Bumper book of Aircraft may say that an aircraft has a maximum level speed (Vmax) of xyz, but the information is almost meaningless, especially if it does not specify the only altitude at which it could very theoretically be true. In real life, for complex aircraft, there is a significant probability of structural failure before Vmax. Only very simple aircraft have Vmax limited by the power available. Most are instead limited by their structure. The later triple tail Lockheeds, including the YC-121F Turbo Constellation, were guaranteed to survive a drag of 260 KIAS, only in modest turbulence, and only if they stayed in warm air.
When reading articles plagiarised from a source more than 50 years old always remember that the original source, even an accurate, official and authoritative source, untainted by journalism, could not have fully understood transonic shock. The numbers quoted are for warm air only. The speed and velocity achievable in cold air were lower and often much lower. They just didn't understand that fully at the time.
Propensity to generate transonic shock increases as temperature falls, and as you climb temperature tends to fall in a straight line. Within the troposphere, (below about FL360 over the CONUS), the higher you go the more likely structural failure becomes, and the lower the maximum velocity (TAS) that can be survived. That is why access to the stratosphere was such a big deal in the quest to conquer transonic shock which journalists called the sound barrier. Once you manage to climb into the stratosphere air temperature is (nearly) constant regardless of altitude. Commercial aircraft with airscrews cannot access the stratosphere. Lacking turbofans neither could the early jetliners, when carrying a payload.
Those of you who have flown the DZN L-049A will have been observing the following two injunctions very carefully.
Above FL200 NEVER EXCEED 210 KIAS (Mno)
DO NOT EXCEED 236 KIAS (Vno)
The earliest incarnation of the Constellation could safely encounter a drag of 236 KIAS, even in modest turbulence, provided it never encountered air colder than minus 17 Centigrade. Colder than that and drag (IAS) must be reduced progressively and substantially to survive. The injunction within the released handling notes is a simplification of the real data table. It makes not the slightest difference what type of engine is used to create the abusive drag. The tail comes off regardless. More powerful engines just allow pilot flying to rip the tail off more easily. They do not allow the aircraft to go faster.
Lockheed worked hard to make the later versions of the triple tail stronger and in the C-121 it could withstand a drag of 260 KIAS, but since Lockheed did not understand transonic shock they could not make the tail more resistant to that mode of failure. After the L-049, as soon as air temperature fell below minus 7 Centigrade, death by application of transonic shock was more likely than death by application of abusive drag.
There had been no improvement in the safe operating Mach limit so the later triple tails could fly a little faster in warm air, but no faster in cold air.
As I explained in a recent post concerning the comparative operating economics of the CV34 and CV58 the purpose of more powerful engines is to lift bigger payloads, (from the same runway). Any Turbo Constellation could have lifted a bigger payload, but it could not have had a higher Vc. In complex aircraft that depends on the weakness of the structure, not the power of the engines.
To go faster the weak triple tail on the Constellation family had to be abandoned, first in the C130, and then as Lockheed employees, (working outside the skunk works), gradually came to understand transonic shock, in the much stronger L188.
To achieve high velocity (TAS) you must restrain drag (IAS). To restrain drag you must climb high into thin air. An aircraft can only be accelerated significantly by constantly climbing. High air is cold air. Cold air promotes transonic shock. Consequently the real maximum speed of high flying propliners is governed only by their ability to withstand cold. This is measured using Mach (temperature), not IAS (drag), or TAS (velocity).
If a propliner can tolerate a high Mach number it can fly colder. Consequently it can fly higher, and so can continue to accelerate for longer by climbing uphill for longer to a higher TAS (velocity) at constant IAS (drag) in thinner and colder air. The airliner that can climb highest, can accelerate for longest, and can go fastest. In any aircraft acceleration is all about going uphill for as long as possible. Concorde could climb (accelerate) continuously until top of descent, reaching around FL600 and M2.05, but no propliner could reach anything like that altitude and thus could not reach anything like 1137 KTAS.
Under normal operating conditions the L049 was increasingly likely to lose its tail if it exceeded M0.48. By 1956 the single tail C-130 could tolerate M0.56 in normal operation and by 1959 enough reality had leaked out of the skunk works that the L188 could withstand M0.615
The real maximum velocities that could be attained by those aircraft were in proportion to those maximum normal operating Mach numbers. M0.56 was compatible with a genuinely attainable velocity of around 315 KTAS whilst M0.615 was compatible with a genuinely attainable velocity of around 350 KTAS which was pretty much the design goal for the Electra a decade after the Turbo Constellation was first mooted.
In Britain the BR10 Britannia which began route proving with BOAC in 1954 was good for Mach 0.57. The BR30 which entered commercial service with BOAC in 1957 was much stronger and good for Mach 0.6 whilst the VC9 which entered commercial service with BEA in 1960 was a brute of a propliner strong enough to fly at M0.64 in normal use. This took the practical limit for the inferior turboprops developed west of the iron curtain to a fraction over 360 KTAS.
The Soviet Union were years ahead in propliner development and in 1961 introduced the vastly superior TU14 which had a practical limit of around 420 KTAS in normal use. This exactly matched the contemporary turbojet Comet 4C which could also tolerate M0.79 in normal use. All these aircraft could fly a little faster under carefully specified atypical circumstances, but by the mid 1960s only Soviet propliners were truly competitive for long haul operations.
High velocity (TAS) is only possible at low drag (IAS) in thin, cold, air and that requires very high Mach tolerance.
To compete with the FK27, VC7 and VC8 conversion of old Convair airframes to turbine power and CV58 configuration began during the sixties and although Pacific Airmotive did all that they could to strengthen the original tail, it was still prone to fail at only M0.485. Consequently the CV58 achieves maximum safe velocity around FL200. The air above is usually too cold for safe operation at the same velocity. You can easily climb a CV58 above FL200, but in most places you will have to slow down or risk sudden death.
If you only ever fly over California, you will enjoy warmer than average air. It is possible to fly a CV58 without worrying too much about the temperature over California, but don't expect to survive if you take the same liberties over Alaska, (with real weather in use).
The maximum level velocity of a complex aircraft is not a fixed number as books and magazines pretend. It is a complex variable. It depends where you are. It depends on the weather. In warm places it is limited by drag, but in cold places it is limited by temperature. Go any faster than the drag limit (260 KIAS) or the temperature limit (M0.485) and the chances of structural failure in a CV58 rise swiftly. The normal operating limit for any triple tail Lockheed was slightly less at M0.48. No member of the Constellation family could fly faster than a CV58 in the same place in the same weather. The power of the engines mounted is wholly irrelevant.
When you exceed either the normal drag operating limit, or the normal temperature operating limit, FS9 displays an overspeed warning and calculates your chances of survival second by second. Survival depends on whether you are turning, or pitching, both of which apply G to the structure. Most likely however it is the weather that will apply a fraction too much G and will cause structural failure as a result of your loss of control of the aircraft energy state.
Structural failure has almost nothing to do with velocity, (how fast you are going), but since you will perish if you apply abusive drag in pursuit of high velocity, or enter abusively cold air whilst accelerating for too long uphill in pursuit of high velocity, your velocity will be limited realistically anyway.
By the time that the B377 was submitted for certification it had dawned on U.S. regulatory authorities that aircrew needed a means to predict structural failure, whether induced by excess drag, or by excess cold. The B377 was equipped with an ASI that not only provided very accurate drag readings, but that also had a Mach bug to indicate when the airframe might fail due to abusively cold air (Mach 0.52). The Mach bug on the B377 eventually developed into the Barber Pole in the CV58 which serves the same purpose. Both will converge with the ASI needle as soon as structural failure for whatever reason becomes an increasing probability. Always keep a close eye on the barber pole when flying a turbine conversion of a piston propliner in MSFS. Never let the airspeed needle reach the barber pole, (or Mach bug), else FS9 will start to calculate your demise.
Almost all turbine powered aircraft have enough power to achieve structural failure in level flight. Some like the Fairchild built version of the Fokker Friendship known as the F-27A, equipped with very powerful engines, to allow operation from short runways in the California, New Mexico and Nevada deserts, can achieve structural failure in level flight, even in warm air. Despite this their ASI designed in Europe had no barber pole or Mach bug. The next generation of turbine engined cockpits would have Machmeters as well as barber poles or Mach bugs so that pilots had no excuse for ripping the tail off, and for the most part they have managed to avoid that option in real life. Make sure you are just as careful.
Flight simulation is by far the best way to understand how aviation really works. The numbers bandied about in books and magazines mean little in the real world. Some could be true under very peculiar circumstances, but many are just journalistic invention.
The two YC-121Fs Turbo Constellations with a normal structural limit of M0.48 were in reality used to test and evaluate the procedures that would be used by the C-130A when it arrived in large numbers three years later. They were used to work out what syllabus tactical transport pilots would have to go through to convert from simple, low altitude aircraft like the C-119, to the overpowered, turbine and transonic shock prone, high altitude C-130. The YC-121F had all the new energy state management problems that the C-130A would present, but worse, and was an ideal case for test and evaluation en route to introduction of the M0.56 limited C-130 in 1956.
A civilian Turbo Constellation would have had the same Mach limit of M0.48 whatever over powerful turboprop engines were installed. It is just a shame that the same mistake, made by a single journalist more than fifty years ago, is mindlessly plagiarised over and over again in books and magazines claiming daft velocities for the 'Turbo Constellation'.
The only way for aviation enthusiasts to understand what aeroplanes can really do is to fly them in MSFS using realistic flight dynamics that have been carefully integrated with realistic gauge code and matching handling notes. Make sure you have the 'aircraft stress causes damage' realism option enabled else you will inhabit the poorly informed and imaginary world of journalists and magazine publishers where propliners can achieve warp speeds.