Where US fighters failed over Europe

[JDCAVE]

This video entitled: "Where US fighters failed over Europe". It is narrated by Eric Winkle Brown. He explains "tactical" mach numbers, and critical Mach numbers, and why the P-38 (tactical Mach of 0.68…Useless! He said) and the P-47s (tactical Mach of 0.71 again, Useless) failed as fighters as compared to the FW-190 (tactical Mach of 0.75). The P-51 had a tactical Mach of 0.78 and the Spitfire a tactical Mach number of 0.8. Evidently the Spit once attained a Mach number of 0.92, the highest ever recorded, according to Brown.

I found it a fascinating video.


View: https://www.youtube.com/watch?v=VQzSQUQ33OQ

Jim

 

[GregP]

I'm a bit suspicious of M.92 myself. I'm thinking airspeed error.

Based on your post, this is one place I'll have to disagree with Eric Brown. The P-47 shot down over 3,600 enemy airplanes and added a lot more on the ground. That's hardly useless, and the P-47 was vastly superior to the Fw 190 at high altitudes while the Fw 190 would hardly get there and was a bit of a slug above 20,000 feet. At least until the Fw 190D showed up, and there weren't all that many of them. Of some 20,600 Fw 190s built, there were 1,850 D models. So, if you saw an Fw 190, you stood about a 9% chance of it being a D model. In practice, less because the D's didn't show up until later. So ... it depends.

I'd pick the radial Fw 190 as superior at, say, 17,000 feet but I'd take a P-47 at 25,000 and above every single time unless it was an Fw 190D. Then it would be a fight. I'd still pick the P-47 above 35,000 feet.

Eric Brown should KNOW that. I'll have to watch the video ... maybe he does. Never heard any Brit of that generation sing the praises of ANY US airplane, so at least this is consistent.

 

[JDCAVE]

I'm a bit suspicious of M.92 myself. I'm thinking airspeed error.

Based on your post, this is one place I'll have to disagree with Eric Brown. The P-47 shot down over 3,600 enemy airplanes and added a lot more on the ground. That's hardly useless, and the P-47 was vastly superior to the Fw 190 at high altitudes while the Fw 190 would hardly get there and was a bit of a slug above 20,000 feet. At least until the Fw 190D showed up, and there weren't all that many of them. Of some 20,600 Few 190s built, there were 1,850 D models. So, if you saw an Fw 190, you stood about a 9% chance of it being a D model. In practice, less because the D's didn't show up until later. So ... it depends.

I'd pick the radial Fw 190 as superior at, say, 17,000 feet but I'd take a P-47 at 25,000 and above every single time unless it was an Fw 190D. Then it would be a fight. I'd still pick the P-47 above 35,000 feet.

Eric Brown should KNOW that. I'll have to watch the video ... maybe he does. Never heard any Brit of that generation sing the praises of ANY US airplane, so at least this is consistent.

But you have not watched video. Remember, Brown was part of those discussions with Doolittle. Also, Brown flew all of these aircraft at their limits and knew their vices. Have you flown them all? Watch the video. I have no expert opinion, myself.

Jim

 

[SaparotRob]

Does flying all those planes in War Thunder count?

 

[Thumpalumpacus]

But you have not watched video. Remember, Brown was part of those discussions with Doolittle. Also, Brown flew all of these aircraft at their limits and knew their vices. Have you flown them all? Watch the video. I have no expert opinion, myself.

Jim

To be fair, in your own post you called the P-47 "useless", and Greg pointed out that it was pretty useful. That's not about watching the video, that's about his verbiage. The P-47 was obviously "useful", if not in the way that Brown would like.

That and three bucks will get you a cup of coffee at Starbucks.

I'll watch the vid itself tomorrow.

 

[JDCAVE]

To be fair, in your own post you called the P-47 "useless", and Greg pointed out that it was pretty useful. That's not about watching the video, that's about his verbiage. The P-47 was obviously "useful", if not in the way that Brown would like.

That and three bucks will get you a cup of coffee at Starbucks.

I'll watch the vid itself tomorrow.

I didn't call it useless. It was in quotes. Brown did. The P-38 and P-47 were not capable of the speeds of the FW190 in combat. As mentioned above, I have no expert opinion. I'm not a pilot. My point was to get this out there, as I thought Brown has unique expert knowledge on the topic. Greg thought the 0.92 Mach measurement was perhaps measurement error. Possibly, but they put specialized instruments in the aircraft they were testing. This is Farnborough that did the tests on these aircraft.

He explains the terms Tactical Mach, which is the maximum speed in Combat and Critical Mach, which is the maximum speed before critical failure (If I got that correct).

Remember Farnborough did the assessments of all the aircraft discussed in the video. Also, Brown flew ALL of these aircraft.

Jim

 

[Geoffrey Sinclair]

Useless is an exaggeration, the 8th found P-38 escort cut bomber losses even if the escorts were not shooting down many interceptors. The Luftwaffe was well aware of the P-38's performance, being more worried about P-38 below them than above given the good acceleration and climb. The 8th Air Force initially chose to convert its P-47 units to P-51 ahead of the P-38 units. Whatever the limits on the P-47D, diving tests showed it could catch Bf109 and Fw190 even though the German types tended to have better initial acceleration. The combat results back this up.

A table supplied during a discussion, source unknown, limiting mach numbers which are slightly lower than the critical mach number, remembering given the time there is a question on how accurate the speed readings were.

Spitfire MK XIV: 0.89
Me-262A: 0.86
Me-163A: 0.845
Me-163B: 0.84
P-51B: 0.84
Gloster Meteor: 0.83
Hawker Tempest: 0.83
P-47N: 0.83
Spitfire Mk IX: 0.79
Bf-109G: 0.78
Yak-3: 0.76
Fw-190: 0.75
F4U: 0.73
P-47C: 0.69
P-38: 0.65

The P-38 without dive flaps was restricted to mach 0.65, the flaps raised the effective dive speed by around 20 mph and made recovery safer. Mach tuck began at about mach 0.74

The P-47 limit was 500 mph IAS at 10,000 feet, or mach 0.82 according to Dean in American's Hundred Thousand. The problem for the P-47 was it could be pushed much faster at high altitude because it could dive so well. So the limit was 400 mph at 25,000 feet. The dive flaps were needed to aid recovery.

The P-51D recommended limit was mach 0.8, though this appears to have been with fabric covered elevators, metal ones were introduced in September 1944. Test dives to mach 0.84 produced structural damage.

Information from Spitfire by Morgan and Shacklady. All the Spitfire are mark XI. Pitot head removed under the wing and moved to the wing tip, a pitot comb was mounted behind the wing trailing edge, reporting airspeeds to a bank of air speed indicators in the fuselage, the instruments being photographed every 1 to 1.5 seconds. The Spitfire was climbed to 40,000 feet, accelerated to its maximum speed and then put into a 45 degree dive under maximum permissible continuous boost. The aircraft was recorded as making mach 0.89 at 29,000 feet.

The P-51A being used was dived from 28,000 feet and registered mach 0.8 at 17,000 feet. The small spoiler flap under the Mustang designed to blank off part of the radiator when in a glide to prevent over cooling was found to permit higher mach numbers in level flight.

Next came Spitfire PL827, with a second pitot head on the other wing, a series of 1/16 holes were drilled into the wings and they found no flow separation at any mach number up to 0.85.

Then came EN409, the 0.92 mach number dive, it had come back from a February dive with popped wing rivets. On 27 April 1944 it was dived to measure the differences between leading edge pitot-static, underwing pitot-static and static vent in a high mach dive. Climbed to 40,000 feet with the altimeter at 1,103, worked up to 170mph IAS and dived. At 27,000 feet there was a loud explosion. Oil on the windscreen blocked any sort of forward view. There was no propeller and "bits of the engine were sticking out". The pilot Squadron Leader A F Martindale decided to try and save the aircraft and photographs, finding the gliding characteristics quite good without a propeller, making it back for a wheels down landing at Farnborough. 600 mph ASA is mentioned as the speed.

Admiralty Fleet Orders, what they knew in mid 1944.

3427.—Compressibility Effects in High Speed Diving
(A.W.D. 1348/44.—29 Jun. 1944.)
The following is a report of a Technical Note issued by the Bureau of Aeronautics, Navy Department, Washington :—
It describes control effects that are associated with compressibility phenomena and suggests certain cautions that may aid in avoiding or reducing the hazard therefrom. This information is to be brought to the attention of all pilots, including those under training at Naval air stations.

2. In recent years the attainable airspeeds of many service types have increased to a point where unusual or unexpected flight characteristics sometimes develop. These effects may take the form of a sudden change in trim, a change in stability about any of the airplane's axes, buffeting or vibration, and the formation of vapour sheets. They may be expected at high speeds, particularly at altitude and under accelerated conditions.

3. These effects are attributed to compressibility. Aerodynamic theory, based on the assumption that air is an incompressible fluid, has been commonly and successfully applied in the determination of magnitudes and distributions of air loads during the era in which possible dive speeds were comparatively low and the actual effects of compressibility were negligible from a practical standpoint. As airspeeds increase, however, certain predictable effects of the compressible nature of air rapidly become of considerable importance. With further increase of air speeds, approaching the speed of sound in air, certain phenomena occur which are difficult of prediction or evaluation. The term "compressibility" has become associated with these latter phenomena which are not completely understood and for which no practicable or thoroughly accurate corrections are yet available.

4. Some of these effects may be visualised as resulting from discontinuities in the air flow about an airplane, or "shock waves". Shock waves do not form on slowly moving bodies because the air flow adjusts itself to pass over the object as smoothly as possible with only small changes in the pressure and density of the air. Shock waves do, however, form on high speed bodies because the air cannot adjust itself to pass the body without radical changes in pressure and density and the introduction of what have been termed "compressibility effects". The shock wave phenomenon is comparable to the water wave that forms at the bow of a ship. If the ship is moving rapidly, the water cannot follow the smooth contours of the ship and, instead, piles up as a bow wave.

5. The appearance of shook waves and other compressibility effects in air depend on the ratio of the speed of the object to the speed of sound. This ratio, which is called the Mach Number, is based on true air speed. The speed of sound decreases with altitude, and if the indicated speed remains constant, the true air speed increases with altitude. Hence, the Mach Number increases rapidly with altitude for constant indicated air speed. An indicated speed of 250 knots corresponds to a Mach Number of 0.38 at sea level, but at 35,000 feet this same indicated speed gives a Mach Number of 0.74. This difference is considerable and results in compressibility effects being much more easily encountered at high altitudes than at sea level.

6. As a result of the rearrangement of the airflow when speeds are reached at which shock waves occur, control surfaces may be expected to have varying degrees of effectiveness and the airplane may be expected to change its stability characteristics. These changes are not predictable with accuracy either from analysis or from conventional wind tunnel tests. Flight test is the only means of reliable exploration of these effects and this method may involve undue hazard to the test pilot unless a systematic programme with careful instrumentation is undertaken.

7. Although service types have normal characteristics and no unusual handling qualities may be expected in accomplishing the missions to which they have been assigned, some of them may easily be dived through inadvertence or combat necessity to speeds well above those for which their handling characteristics are thoroughly known. If such a condition should arise, changes in behaviour may occur. Those most likely to occur are listed and described below in order that pilots may recognize them more quickly and thus be able to take more effective steps to counteract or to avoid them :—
(а) Buffeting.—This is one of the most common disturbances associated with high speed flying. It is aggravated by partially open cowl flaps or intercooler doors, damaged or bent fairings, by movements of control surfaces, especially when the leading edges of these control surfaces are sharp, and by other factors that would cause turbulence or irregular air flow. Although buffeting may occur at low speed, it will be more pronounced at high speed. If a severe buffet develops, immediate action to decrease the applied acceleration will probably be more effective in reducing the buffet intensity than a reduction in speed. However, both should be reduced as soon as practicable.
(b) Aileron Snatch.—Ailerons, particularly those with sharp leading edges, may snatch at high speeds. Aileron snatch is a movement, with or without oscillation, of the control stick, caused by air striking or flowing around the leading edge as the aileron is being deflected. In high speed dives the pilot should keep a firm hold on the stick and keep the stick neutral so far as practicable.
(c) High Stick Force.—In high altitude dives with the true speed greater than about three-quarters of the speed of sound, the pilot may find that he cannot hold the airplane in the dive. On the other hand, the opposite effect may occur with the pilot unable to exert sufficient force on the stick to pull the airplane out of the dive. Inability to pull the airplane out of the dive may be due either to a very large diving moment attended by a loss of lift on the wing, which requires a much greater elevator load at high altitudes for a pull-out than the pilot can apply, or it may result from a redistribution of tail loads which increases elevator hinge moments. At lower altitudes, where the true speed becomes less than about three-quarters of the speed of sound, the pilot will probably be able to pull out with normal stick forces.
(d) Loss in Effectiveness of Elevator Trim Tab.—The elevator tab keeps its normal effectiveness up to a true air speed of approximately three quarters of the speed of sound. Beyond this point the elevator tab often becomes ineffective so far as pull-outs are concerned. If this condition is found to exist, the elevator trim tab should always be restored to its initial position, because the tab will regain its original effectiveness and cause a very abrupt pull-out when the airplane reaches lower altitudes where the true speed is less than about three quarters of the speed of sound.
(e) Formation of Vapor Sheets.—Although of no particular significance as regards control effectiveness, the formation of vapor sheets is an interesting phenomenon which evidences high speed. It is a visible indication, in humid weather, of local high velocity of the air over the wing. With increase of speed or "g" the thickness or height of the sheet can be seen to vary. The formation of a thin vapor sheet may or may not be accompanied by other compressibility effects.

8. Certain cautions have been suggested by test pilots and others to avoid compressibility effects. They may not, however, apply in all cases. Each airplane may be expected to have its own peculiarities at high speed at altitude. Pilots who experience any of these effects should make reports of their experiences, including as much thoroughly factual information as possible. The cautions commonly recommended to avoid or to minimize compressibility effects are the following :—
(а) Return 'the elevator trim tab to its initial position if it is found to be ineffective in reducing control forces. As explained in paragraph 7 (d), failure to do so may result in a sharp pull-out when the tab regains its effectiveness.
(b) Attempts have been made to slow down an airplane in a high-speed dive by yawing it. Such attempts have not only failed but have proved dangerous because the dive angle was greatly steepened when the airplane was yawed. Excessive yawing has also resulted in failures in the tail surfaces and in buckled fuselages. Therefore, efforts should be made to prevent a diving airplane from yawing when at high speed and high altitude.
(c) Some pilots of single-engine airplanes have experienced a considerable increase in diving angle when the power was cut in high-speed dives. Since an increase in diving angle greatly increases the difficulty of pull-out from a dive, the throttle should not be cut or the r.p.m. control changed during a high-speed dive except with great care.
(d) Avoid over-controlling by being alert to detect a change or reversal of control force. Unnecessary control surface displacement may provide the disturbance necessary to produce a compressibility effect. For example, at high speeds, rocking the wings may stall first one wing and then the other.
(e) When practicable, dives to high speed should be made as shallow as possible, so that if compressibility effects do present themselves, speed and acceleration will be under better control.
(f) Avoid high-speed accelerated stalls at altitude.

9. As planes become capable of attaining higher and higher speeds, pilots will become more and more familiar with compressibility effects and the peculiarities associated with the high-speed performance of individual airplanes. Good piloting technique requires avoidance of these effects much the same as it requires the avoidance of stalls. The approach of a stall is associated with certain sensations which a pilot normally recognizes and avoids. In the same way, pilots should learn to recognize the approach of compressibility effects by certain of the characteristics described in this technical note, or others which experience will indicate to be present.

 

[Thumpalumpacus]

I didn't call it useless. It was in quotes. Brown did. The P-38 and P-47 were not capable of the speeds of the FW190 in combat. As mentioned above, I have no expert opinion. I'm not a pilot. My point was to get this out there, as I thought Brown has unique expert knowledge on the topic. Greg thought the 0.92 Mach measurement was perhaps measurement error. Possibly, but they put specialized instruments in the aircraft they were testing. This is Farnborough that did the tests on these aircraft.

He explains the terms Tactical Mach, which is the maximum speed in Combat and Critical Mach, which is the maximum speed before critical failure (If I got that correct).

Remember Farnborough did the assessments of all the aircraft discussed in the video. Also, Brown flew ALL of these aircraft.

Jim

No matter who said it, calling them "useless" doesn't accommodate the fact that they were used in great numbers and to great effect. One need not be a great test pilot to see that basic error -- to wit, one flaw doesn't render a plane "useless".

 

[JDCAVE]

No matter who said it, calling them "useless" doesn't accommodate the fact that they were used in great numbers and to great effect. One need not be a great test pilot to see that basic error -- to wit, one flaw doesn't render a plane "useless".

Agreed!

 

[GregP]

But you have not watched video. Remember, Brown was part of those discussions with Doolittle. Also, Brown flew all of these aircraft at their limits and knew their vices. Have you flown them all? Watch the video. I have no expert opinion, myself.

Jim

If you read my post, I said that I'd watch the video. Cheers.

 

[Shortround6]

A what point did the limiting Mach number become a defining measure of the usefulness of a fighter?


From Mr. Sinclair's excellent post.

Hence, the Mach Number increases rapidly with altitude for constant indicated air speed. An indicated speed of 250 knots corresponds to a Mach Number of 0.38 at sea level, but at 35,000 feet this same indicated speed gives a Mach Number of 0.74. This difference is considerable and results in compressibility effects being much more easily encountered at high altitudes than at sea level.

The P-40 was often noted for having a good dive. However it was subject to compressibility problems. This assumes you could get a P-40 high enough to actually run into such problems when not engaged in test flying.

 

[Engineman]

Hi,
An interesting topic, that I think is about Mach number limitations of mid-late WW2 combat aircraft. This is a very complicated subject, that really should not be an open fight about the perceived merits of different types. However, IMO, Mach number limitations should be recognised for what they are, a speed limitation (usually in WW2) at high altitude that is defined as a ratio between the local speed of sound and the actual speed of the aircraft. So, at high altitudes (and particularly in dives with propeller driven WW2 aircraft), the Mach limit represents a common measure of dive speed limitation of aircraft types flown side by side in combat, until (generally) at lower altitude IAS limits become limiting. Of course, for these limiting Mach numbers to become really important, the fight would have to be high speed and at least start at high altitude, where the Mach is limiting. Within the Mach limited envelope, I suspect that Captain Eric Brown is correct in the comparison of different capabilities, however he is probably a little strong in using terms such as "useless", although I am certain that here, he is being precise in the situation WRT Mach number, not the overall merit of the aircraft, even within the high altitude environment.
Furthermore, Captain Brown also wrote about the "Tactical" Mach number ability of aircraft types, whereby they could operate in combat in more-or-less level flight at high Mach number, and here he was quite clear, the Me 262 reigned supreme with his assessment of a "Tactical" Mach number of 0.82M, all piston types requiring near vertical dives to approach that Mach.

Eng

 

[JDCAVE]

I've carefully read through Geoffrey's reply. It is excellent and worth noting his paragraph on how Farnborough's work was conducted:

"Pitot head removed under the wing and moved to the wing tip, a pitot comb was mounted behind the wing trailing edge, reporting airspeeds to a bank of air speed indicators in the fuselage, the instruments being photographed every 1 to 1.5 seconds. The Spitfire was climbed to 40,000 feet, accelerated to its maximum speed and then put into a 45 degree dive under maximum permissible continuous boost. The aircraft was recorded as making mach 0.89 at 29,000 feet."

From the above, you can see these analyses were carried out quite carefully.

In the video, Brown reported the test with EN409 pretty much as Geoffrey has outlined it. Also, Brown said the P-47 was a capable ground attack aircraft, and the P-38 performed well in the Pacific at heights below 15,000' and the two top US aces during WWII were from that theatre on P-38's.

Important to point out that in combat situations pilots would take their aircraft to their personal operational limits (capabilities) as well as those limits of the aircraft. Dad talked about diving KB865 down searchlights at Magdeburg on the op to Dessau 7/8-March-1945. He mentioned rivets popping. Whether he extemporized this or it was factual, I don't know. I have mentioned on a previous thread of a pilot taking the Lancaster almost to the point of failure during a corkscrew maneuver during and aircraft test. There were skin failures on the wings.1

Edit: 1 I have the "accident" report for this incident, as well as an eye witness account that doesn't "square" with the incident. I don't think the pilot was honest about what actually occurred.

Jim

 

[drgondog]

Useless is an exaggeration, the 8th found P-38 escort cut bomber losses even if the escorts were not shooting down many interceptors. The Luftwaffe was well aware of the P-38's performance, being more worried about P-38 below them than above given the good acceleration and climb. The 8th Air Force initially chose to convert its P-47 units to P-51 ahead of the P-38 units. Whatever the limits on the P-47D, diving tests showed it could catch Bf109 and Fw190 even though the German types tended to have better initial acceleration. The combat results back this up.

A table supplied during a discussion, source unknown, limiting mach numbers which are slightly lower than the critical mach number, remembering given the time there is a question on how accurate the speed readings were.

Spitfire MK XIV: 0.89
Me-262A: 0.86
Me-163A: 0.845
Me-163B: 0.84
P-51B: 0.84
Gloster Meteor: 0.83
Hawker Tempest: 0.83
P-47N: 0.83
Spitfire Mk IX: 0.79
Bf-109G: 0.78
Yak-3: 0.76
Fw-190: 0.75
F4U: 0.73
P-47C: 0.69
P-38: 0.65

The P-38 without dive flaps was restricted to mach 0.65, the flaps raised the effective dive speed by around 20 mph and made recovery safer. Mach tuck began at about mach 0.74

The P-47 limit was 500 mph IAS at 10,000 feet, or mach 0.82 according to Dean in American's Hundred Thousand. The problem for the P-47 was it could be pushed much faster at high altitude because it could dive so well. So the limit was 400 mph at 25,000 feet. The dive flaps were needed to aid recovery.

The P-51D recommended limit was mach 0.8, though this appears to have been with fabric covered elevators, metal ones were introduced in September 1944. Test dives to mach 0.84 produced structural damage.

Information from Spitfire by Morgan and Shacklady. All the Spitfire are mark XI. Pitot head removed under the wing and moved to the wing tip, a pitot comb was mounted behind the wing trailing edge, reporting airspeeds to a bank of air speed indicators in the fuselage, the instruments being photographed every 1 to 1.5 seconds. The Spitfire was climbed to 40,000 feet, accelerated to its maximum speed and then put into a 45 degree dive under maximum permissible continuous boost. The aircraft was recorded as making mach 0.89 at 29,000 feet.

The P-51A being used was dived from 28,000 feet and registered mach 0.8 at 17,000 feet. The small spoiler flap under the Mustang designed to blank off part of the radiator when in a glide to prevent over cooling was found to permit higher mach numbers in level flight.

Next came Spitfire PL827, with a second pitot head on the other wing, a series of 1/16 holes were drilled into the wings and they found no flow separation at any mach number up to 0.85.

Then came EN409, the 0.92 mach number dive, it had come back from a February dive with popped wing rivets. On 27 April 1944 it was dived to measure the differences between leading edge pitot-static, underwing pitot-static and static vent in a high mach dive. Climbed to 40,000 feet with the altimeter at 1,103, worked up to 170mph IAS and dived. At 27,000 feet there was a loud explosion. Oil on the windscreen blocked any sort of forward view. There was no propeller and "bits of the engine were sticking out". The pilot Squadron Leader A F Martindale decided to try and save the aircraft and photographs, finding the gliding characteristics quite good without a propeller, making it back for a wheels down landing at Farnborough. 600 mph ASA is mentioned as the speed.

Admiralty Fleet Orders, what they knew in mid 1944.

3427.—Compressibility Effects in High Speed Diving
(A.W.D. 1348/44.—29 Jun. 1944.)
The following is a report of a Technical Note issued by the Bureau of Aeronautics, Navy Department, Washington :—
It describes control effects that are associated with compressibility phenomena and suggests certain cautions that may aid in avoiding or reducing the hazard therefrom. This information is to be brought to the attention of all pilots, including those under training at Naval air stations.

2. In recent years the attainable airspeeds of many service types have increased to a point where unusual or unexpected flight characteristics sometimes develop. These effects may take the form of a sudden change in trim, a change in stability about any of the airplane's axes, buffeting or vibration, and the formation of vapour sheets. They may be expected at high speeds, particularly at altitude and under accelerated conditions.

3. These effects are attributed to compressibility. Aerodynamic theory, based on the assumption that air is an incompressible fluid, has been commonly and successfully applied in the determination of magnitudes and distributions of air loads during the era in which possible dive speeds were comparatively low and the actual effects of compressibility were negligible from a practical standpoint. As airspeeds increase, however, certain predictable effects of the compressible nature of air rapidly become of considerable importance. With further increase of air speeds, approaching the speed of sound in air, certain phenomena occur which are difficult of prediction or evaluation. The term "compressibility" has become associated with these latter phenomena which are not completely understood and for which no practicable or thoroughly accurate corrections are yet available.

4. Some of these effects may be visualised as resulting from discontinuities in the air flow about an airplane, or "shock waves". Shock waves do not form on slowly moving bodies because the air flow adjusts itself to pass over the object as smoothly as possible with only small changes in the pressure and density of the air. Shock waves do, however, form on high speed bodies because the air cannot adjust itself to pass the body without radical changes in pressure and density and the introduction of what have been termed "compressibility effects". The shock wave phenomenon is comparable to the water wave that forms at the bow of a ship. If the ship is moving rapidly, the water cannot follow the smooth contours of the ship and, instead, piles up as a bow wave.

5. The appearance of shook waves and other compressibility effects in air depend on the ratio of the speed of the object to the speed of sound. This ratio, which is called the Mach Number, is based on true air speed. The speed of sound decreases with altitude, and if the indicated speed remains constant, the true air speed increases with altitude. Hence, the Mach Number increases rapidly with altitude for constant indicated air speed. An indicated speed of 250 knots corresponds to a Mach Number of 0.38 at sea level, but at 35,000 feet this same indicated speed gives a Mach Number of 0.74. This difference is considerable and results in compressibility effects being much more easily encountered at high altitudes than at sea level.

6. As a result of the rearrangement of the airflow when speeds are reached at which shock waves occur, control surfaces may be expected to have varying degrees of effectiveness and the airplane may be expected to change its stability characteristics. These changes are not predictable with accuracy either from analysis or from conventional wind tunnel tests. Flight test is the only means of reliable exploration of these effects and this method may involve undue hazard to the test pilot unless a systematic programme with careful instrumentation is undertaken.

7. Although service types have normal characteristics and no unusual handling qualities may be expected in accomplishing the missions to which they have been assigned, some of them may easily be dived through inadvertence or combat necessity to speeds well above those for which their handling characteristics are thoroughly known. If such a condition should arise, changes in behaviour may occur. Those most likely to occur are listed and described below in order that pilots may recognize them more quickly and thus be able to take more effective steps to counteract or to avoid them :—
(а) Buffeting.—This is one of the most common disturbances associated with high speed flying. It is aggravated by partially open cowl flaps or intercooler doors, damaged or bent fairings, by movements of control surfaces, especially when the leading edges of these control surfaces are sharp, and by other factors that would cause turbulence or irregular air flow. Although buffeting may occur at low speed, it will be more pronounced at high speed. If a severe buffet develops, immediate action to decrease the applied acceleration will probably be more effective in reducing the buffet intensity than a reduction in speed. However, both should be reduced as soon as practicable.
(b) Aileron Snatch.—Ailerons, particularly those with sharp leading edges, may snatch at high speeds. Aileron snatch is a movement, with or without oscillation, of the control stick, caused by air striking or flowing around the leading edge as the aileron is being deflected. In high speed dives the pilot should keep a firm hold on the stick and keep the stick neutral so far as practicable.
(c) High Stick Force.—In high altitude dives with the true speed greater than about three-quarters of the speed of sound, the pilot may find that he cannot hold the airplane in the dive. On the other hand, the opposite effect may occur with the pilot unable to exert sufficient force on the stick to pull the airplane out of the dive. Inability to pull the airplane out of the dive may be due either to a very large diving moment attended by a loss of lift on the wing, which requires a much greater elevator load at high altitudes for a pull-out than the pilot can apply, or it may result from a redistribution of tail loads which increases elevator hinge moments. At lower altitudes, where the true speed becomes less than about three-quarters of the speed of sound, the pilot will probably be able to pull out with normal stick forces.
(d) Loss in Effectiveness of Elevator Trim Tab.—The elevator tab keeps its normal effectiveness up to a true air speed of approximately three quarters of the speed of sound. Beyond this point the elevator tab often becomes ineffective so far as pull-outs are concerned. If this condition is found to exist, the elevator trim tab should always be restored to its initial position, because the tab will regain its original effectiveness and cause a very abrupt pull-out when the airplane reaches lower altitudes where the true speed is less than about three quarters of the speed of sound.
(e) Formation of Vapor Sheets.—Although of no particular significance as regards control effectiveness, the formation of vapor sheets is an interesting phenomenon which evidences high speed. It is a visible indication, in humid weather, of local high velocity of the air over the wing. With increase of speed or "g" the thickness or height of the sheet can be seen to vary. The formation of a thin vapor sheet may or may not be accompanied by other compressibility effects.

8. Certain cautions have been suggested by test pilots and others to avoid compressibility effects. They may not, however, apply in all cases. Each airplane may be expected to have its own peculiarities at high speed at altitude. Pilots who experience any of these effects should make reports of their experiences, including as much thoroughly factual information as possible. The cautions commonly recommended to avoid or to minimize compressibility effects are the following :—
(а) Return 'the elevator trim tab to its initial position if it is found to be ineffective in reducing control forces. As explained in paragraph 7 (d), failure to do so may result in a sharp pull-out when the tab regains its effectiveness.
(b) Attempts have been made to slow down an airplane in a high-speed dive by yawing it. Such attempts have not only failed but have proved dangerous because the dive angle was greatly steepened when the airplane was yawed. Excessive yawing has also resulted in failures in the tail surfaces and in buckled fuselages. Therefore, efforts should be made to prevent a diving airplane from yawing when at high speed and high altitude.
(c) Some pilots of single-engine airplanes have experienced a considerable increase in diving angle when the power was cut in high-speed dives. Since an increase in diving angle greatly increases the difficulty of pull-out from a dive, the throttle should not be cut or the r.p.m. control changed during a high-speed dive except with great care.
(d) Avoid over-controlling by being alert to detect a change or reversal of control force. Unnecessary control surface displacement may provide the disturbance necessary to produce a compressibility effect. For example, at high speeds, rocking the wings may stall first one wing and then the other.
(e) When practicable, dives to high speed should be made as shallow as possible, so that if compressibility effects do present themselves, speed and acceleration will be under better control.
(f) Avoid high-speed accelerated stalls at altitude.

9. As planes become capable of attaining higher and higher speeds, pilots will become more and more familiar with compressibility effects and the peculiarities associated with the high-speed performance of individual airplanes. Good piloting technique requires avoidance of these effects much the same as it requires the avoidance of stalls. The approach of a stall is associated with certain sensations which a pilot normally recognizes and avoids. In the same way, pilots should learn to recognize the approach of compressibility effects by certain of the characteristics described in this technical note, or others which experience will indicate to be present.

Lot of good information in the post. One oddity was citing P-51A as having small spoiler in front of radiator? Source for that one would be interesting as only NA-73 Mustang I had that small spoiler.

 

[drgondog]

This video entitled: "Where US fighters failed over Europe". It is narrated by Eric Winkle Brown. He explains "tactical" mach numbers, and critical Mach numbers, and why the P-38 (tactical Mach of 0.68…Useless! He said) and the P-47s (tactical Mach of 0.71 again, Useless) failed as fighters as compared to the FW-190 (tactical Mach of 0.75). The P-51 had a tactical Mach of 0.78 and the Spitfire a tactical Mach number of 0.8. Evidently the Spit once attained a Mach number of 0.92, the highest ever recorded, according to Brown.

I found it a fascinating video.


View: https://www.youtube.com/watch?v=VQzSQUQ33OQ

Jim

I watched and commented on this video.

A couple of comments:
The first true dive testing (of record) was Cass Hough VIII Air Technical Services in Jan-Feb 1943 with P-47C-2. He was a.) shocked at the close call recoverng from first terminal dive test, and b.) proceeded to document the 'do's and don'ts" - specifically back pressure moderate on stick, no yaw, no trim tab would yield safe pullout at lower altitudes.

The dive flaps, based on those early reports from ATS in early 1943 were designed by Republic and inserted in P-47D-30 block.

Jimmy Doolittle never 'came to Brown' to request assistance during 1943. Doolittle was in North Africa until January 1944. In any case Doolittle in February 1944 had already made up his mind to trade his VIII FC P-47D and P-38J for P-51B & D. Based on Range primarily. The Relay system of P-47 for Penetration and Withdrawal Escort worked but more complicated planning required than an inventory (P-51B/D) that could perform Penetration, Target, Long Range Sweep and Withdrawal.

A lot of smoking holes caused by chased (and caught) Bf 109 and FW 190s by P-47C/Ds in dives. Maybe Brown was confused?

If you trace Brown's operational and testing assignments, it appears that he was back at Farnborough in late 1943.

His comments about P-47 being 'useless' was just sad. Kinda defines his view of the class of 'not useless' fitting into point interception type performance. Would he be shocked if the Spitfire was deemed useless for high altitude bomber escort, medium and long range sweeps, or CAS vs P-47, by US war fighters?

The combination Placard dive speeds and Do not exceed dive speeds of P-47D-2 through end of models were .75M and .80M respectively. That said they were stated in IAS vs altitude as no Mach meters were installed.

I exchanged letters with Brown in mid 1970s. A Gentleman. Curiously enough, however, he was of the opinion that the FW 190 outdived the P-51. I sent him several flight tests, including the P-51D test in which it was tested to M=0.84. That was a surprise for him - and to his credit he admitted it.

Respect but I suspect memory was not up to par. I totally get that a my age.

 

[Geoffrey Sinclair]

Went back through Morgan and Shacklady, they only say P-51, I must have added the A in error when I first used the data. Tried to find the P-51 serial number but the A&AEE had a number of Allison Mustang on strength. A further note the Mustang's guns and radio were removed for the trials. Morgan and Shacklady report on the P-47 indicates it had dive flaps fitted.

The Spitfire and P-51 trials were being done in winter, speed of sound at 30 degrees C is about 5% faster than at 0 degrees.

 

[JDCAVE]

Thanks for your reply DRGONDOG. Very informative!

Jim

 

[drgondog]

Went back through Morgan and Shacklady, they only say P-51, I must have added the A in error when I first used the data. Tried to find the P-51 serial number but the A&AEE had a number of Allison Mustang on strength. A further note the Mustang's guns and radio were removed for the trials. Morgan and Shacklady report on the P-47 indicates it had dive flaps fitted.

The Spitfire and P-51 trials were being done in winter, speed of sound at 30 degrees C is about 5% faster than at 0 degrees.

Hi Geoffrey - I am even more curious now. A&AEE likely to have a variety of Mustangs, but why refer to them as "P-51"?

The NA-73/Mustng I/XP-51 had that small flap in front of intake scoop, but AFAIK it (flap) did not even make it into NA-83 Mustang I. Then referring to P-47 with dive flaps moves the discussion into the P-47D-30, earliest arrival in ETO in July-August 1944?

If by some chance the timelines align then RAF had Mustang I, Mustang IA, Mustang II, III and IVa and IVb when P-47D-30 in UK. I wish I had the book.

 

[Engineman]

Hi Geoffrey - I am even more curious now. A&AEE likely to have a variety of Mustangs, but why refer to them as "P-51"?

The NA-73/Mustng I/XP-51 had that small flap in front of intake scoop, but AFAIK it (flap) did not even make it into NA-83 Mustang I. Then referring to P-47 with dive flaps moves the discussion into the P-47D-30, earliest arrival in ETO in July-August 1944?

If by some chance the timelines align then RAF had Mustang I, Mustang IA, Mustang II, III and IVa and IVb when P-47D-30 in UK. I wish I had the book.

Hi,
Reference to the Merlin Mustang developments by RR at Hucknall written by David Birch in his RRHT book ROLLS-ROYCE and the MUSTANG includes specific info about the forward radiator scoop being fitted to AM121 which was an NA-83 type. Detailed info is on p.61 of my copy.
It is surprising to me, how so many very specific trials and tests are written-up with limited or missing details of the tested aircraft.
Cheers

Eng

 

[Geoffrey Sinclair]

"The Secret Years" by Tim Mason, ISBN 0 951899 9 5, which covers Flight Testing at Boscombe 1939-1945

Looked up a copy online but it was a low quality scan making it hard to be sure of the serial numbers via the index, there were a number of Mustangs on strength over time, the index also lists 7 Thunderbolts, 3 with USAAF serials and only 1 in 1943, the rest seem to arrive July 1944 or later. The Morgan and Shacklady are Spitfire, the other types mentioned in passing, really need the A&AEE reports. Or the output of the late David Birch.

Most people use P-51 and Mustang as the generic interchangeable label for the aircraft yet P-51 is one of the few WWII USAAF designations for a production version without a trailing letter and there was no USAAF designation for the NA-73 Mustang I.

The first Farmingdale P-47D-30 was accepted on 31 August 1944, then production from mid September, Republic seems to have had a habit of building an example or two of a new version/block slightly ahead of the actual change over. Evansville began D-30 production late October.

According to the end June 1944 RAF census there were 323 Mustang I, 58 mark IA and 46 mark II and 405 mark III in Britain plus 72 III in the Mediterranean and 48 III en route. Operational units were using a mixture of all marks. No mark IV but 3 Mustang X, 1 Mustang 36 and 2 Mustang (American Type) were in Britain. First Mustang IV imports reported in September. Losses to end June 293 I, 34 IA, 4 II, 126 III, 1 X and 4 American Type.