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P-38 Lightning and Compressibilty


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Wombatmetal #1 Posted 22 November 2017 - 08:42 AM

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My understanding of this has been wrong until recently. But as I like the P-38 I have been doing research and now have a good grasp of the phenomena. And while this is about the P-38, it applies to all aircraft that approach mach 1. Just that the Lightning was the first aircraft to do so, so it hit the snags first. 

 

The misconception that I have been operating under is that tail redesign and hydraulics were part of the solution to compressibility, that is not the case. The Lighting had two problems with high speed dives. The first one solved was tail flutter, in which when the P-38 entered a high speed dive, the tail would vibrate, and sometimes break apart causing a crash. This problem was also encountered on the P-47, P-51, and Spitfire, and probably other planes as well. This was a simple aerodynamics problem and readily fixed. 

 

The second and tougher problem was compressibility, where the controls would lock in a dive, tail would vibrate and fall off sometimes, and the plane would go into an inverted loop, usually breaking up from the stress. This came from doing a steep high speed dive from about 35,000 feet, and if a pilot was lucky the plane would hold together until he could gain control back at 10,000 feet. 

 

When the Lightning dived from 36,000, it could hit speeds upwards of 0.7 mach. That is the speed of the plan itself. The air going across the top of the wing goes faster, which causes lower pressure resulting in lift. At around 0.7 mach, the air going across the top of the wing hits mach 1. You don't get a big sonic boom, but you get many tiny sonic booms, one of the effects of which is it dumps the air off the wing causing a total loss of lift.

 

The tail now becomes the only part of the plane with lift,  which tends to push the plane into a steeper dive. If the pilot pulls back on the stick to pull out of the dive, the lift of the tail is increased, further increasing the dive angle. One pilot survived without his plane breaking up and completed an inverted loop. 

 

The second effect of the tiny sonic booms is continuous pressure waves, which causes metal fatigue and structural failure in the tail. This is the reason that the engineers thought the tail flutter and compressibility were the same problem for some time, and it still gets reported as such. The symptoms are quite similar. 

 

The planes would pull out at 10,000 feet because of simple physics. At 36,000 feet mach 1 is 540 mph, at 10,000 feet it is 620 mph. Even though the plane is going the same speed or faster, the air over the top wing is no longer hitting mach 1 and there are no more sonic booms, and flight returns to normal. 

 

The fix to it was easy, and it is still employed today. A dive flap is added behind the leading edge of the wings outside the engine nacelles. It is a small piece of metal that comes out at a 15 degree angle to the wing, increasing lift. As long as the wings maintain lift, no problems. 



GhostPrime #2 Posted 22 November 2017 - 05:54 PM

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I have read this as well. I'm not sure if you have seen this video or not, but it's pretty cool and just barely touches on some of the points you mentioned, but is a pretty cool video in its own right. 

 



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Caecias #3 Posted 24 November 2017 - 01:20 AM

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View PostDestroyer_Suzukaze, on 22 November 2017 - 03:42 AM, said:

My understanding of this has been wrong until recently. But as I like the P-38 I have been doing research and now have a good grasp of the phenomena. And while this is about the P-38, it applies to all aircraft that approach mach 1. Just that the Lightning was the first aircraft to do so, so it hit the snags first. 

 

The misconception that I have been operating under is that tail redesign and hydraulics were part of the solution to compressibility, that is not the case. The Lighting had two problems with high speed dives. The first one solved was tail flutter, in which when the P-38 entered a high speed dive, the tail would vibrate, and sometimes break apart causing a crash. This problem was also encountered on the P-47, P-51, and Spitfire, and probably other planes as well. This was a simple aerodynamics problem and readily fixed. 

 

The second and tougher problem was compressibility, where the controls would lock in a dive, tail would vibrate and fall off sometimes, and the plane would go into an inverted loop, usually breaking up from the stress. This came from doing a steep high speed dive from about 35,000 feet, and if a pilot was lucky the plane would hold together until he could gain control back at 10,000 feet. 

 

When the Lightning dived from 36,000, it could hit speeds upwards of 0.7 mach. That is the speed of the plan itself. The air going across the top of the wing goes faster, which causes lower pressure resulting in lift. At around 0.7 mach, the air going across the top of the wing hits mach 1. You don't get a big sonic boom, but you get many tiny sonic booms, one of the effects of which is it dumps the air off the wing causing a total loss of lift.

 

The tail now becomes the only part of the plane with lift,  which tends to push the plane into a steeper dive. If the pilot pulls back on the stick to pull out of the dive, the lift of the tail is increased, further increasing the dive angle. One pilot survived without his plane breaking up and completed an inverted loop. 

 

The second effect of the tiny sonic booms is continuous pressure waves, which causes metal fatigue and structural failure in the tail. This is the reason that the engineers thought the tail flutter and compressibility were the same problem for some time, and it still gets reported as such. The symptoms are quite similar. 

 

The planes would pull out at 10,000 feet because of simple physics. At 36,000 feet mach 1 is 540 mph, at 10,000 feet it is 620 mph. Even though the plane is going the same speed or faster, the air over the top wing is no longer hitting mach 1 and there are no more sonic booms, and flight returns to normal. 

 

The fix to it was easy, and it is still employed today. A dive flap is added behind the leading edge of the wings outside the engine nacelles. It is a small piece of metal that comes out at a 15 degree angle to the wing, increasing lift. As long as the wings maintain lift, no problems. 

 

I could only wish that compress-ability, inertia, and performance metrics / damage such as this were in the game... including the resulting differences from ordnance load-outs.

 

 


Mercsn #4 Posted 26 November 2017 - 06:28 AM

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View PostCaecias, on 23 November 2017 - 07:20 PM, said:

 

I could only wish that compress-ability, inertia, and performance metrics / damage such as this were in the game... including the resulting differences from ordnance load-outs.

 

 

We used to have noticeable differences in performance when different ordnance, or even different gun stetups, were loaded on various aircraft.


 

Regarding the OP, we used to have different speed (and maneuvering) characteristics at "optimal altitude" and sea level.  Now, every plane has a max speed and max speed at sea level, but both numbers are the same.


 

So...if "why is my high altitude engine'd plane going so slow at sea level?!" was too difficult of a thing for new players to find the game "accessible enough" (by developer's standards -developers who are bad at the game, even), I'm preeety sure we're not going to get controls locking up when the screen starts shaking and the speed indicator is in the red and the craft has exceeded the listed "maximium dive speed"...because you know, "comun sence я hart" or something.


Edited by Mercsn, 26 November 2017 - 06:36 AM.

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The below was said to me (Mercsn), from a concerned player:

Edited, on 12 March - 2:01PM , said:

and PS...play more, forum less.  Your opinion might be more credible.

Jyarbro28 #5 Posted 11 February 2021 - 04:37 AM

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Compressibility was solved and the fix incorporated into the J models represented in the game. The F models however, not so much.

 

Now having said that, the F model was the most in demand aircraft in every theatre except Northern Europe, because they were flying at low altitudes and they could maneuver with anything, and out climb everything (shallow high speed climb, not hang on the props climb) they were facing.

 

The 38 was about the only "heavy" fighter to be successful in daylight for most of WW2, the rest were night fighters that were hunting bomber streams. Now, wouldn't it be nice if the 38 F could turn with anything other than other heavies and bombers? Now after Tier 7 there are several heavy fighters that were "successful" designs.



crzyhawk #6 Posted 10 April 2021 - 06:11 PM

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The problem with the P-38 and compressibility was "solved" with the J model but never "fixed" because the problem of the low critical mach number of .68 was unsolvable without a thinner wing.  What that meant was that the Lightning entered compressibility before other fighters like the Mustang and Spitfire.  The Mustang started to really see compressibilty at around .8 mach, and the Spit was ~.89.

 

What that translates to is a Mustang or Spit could /easily/ dive away from a Lightning without worrying about entering compressibility and having to "slow down" to regain control.  Even the 109F and G were in the .78 to .8 range and could remain controllable at higher speeds.


 





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