Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: 38ruk on August 13, 2015, 04:06:37 PM
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I was just rereading my favorite P-38 book and remembered asking about this in the past .....
In Warren Bodie's book, "The Lockheed P-38 LIGHTNING " , it reads , compressibilty was never encountered with dives starting below 25,000 ft .
In Chapter 7 "The Specters of War and Compressibility " PG:77 Pharagraph 2 SNIP: "For a number of reasons , it was virtually impossible to encounter compressibilty tuck in any P-38 types if the dive was entered at 25,000 feet or less . Numerous test flights confirm this " SNIP
Ive posted this before and never really got a answer either way about why the 38 here is easily compressed below 25k .
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If I remember right it was at 45 degree angle of attack.
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I was just rereading my favorite P-38 book and remembered asking about this in the past .....
In Warren Bodie's book, "The Lockheed P-38 LIGHTNING " , it reads , compressibilty was never encountered with dives starting below 25,000 ft .
In Chapter 7 "The Specters of War and Compressibility " PG:77 Pharagraph 2 SNIP: "For a number of reasons , it was virtually impossible to encounter compressibilty tuck in any P-38 types if the dive was entered at 25,000 feet or less . Numerous test flights confirm this " SNIP
Ive posted this before and never really got a answer either way about why the 38 here is easily compressed below 25k .
And it's accurately modeled in AH, you can't enter into compressibility under 25,000ft.
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If I remember right it was at 45 degree angle of attack.
I assume you mean dive angle?
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I assume you mean dive angle?
Yes
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Power on, no control imput... probably gonna compress. Power off and cross controlling on the way down, she's like a rock. :rock
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And it's accurately modeled in AH, you can't enter into compressibility under 25,000ft.
I just did in a 0 mph stall at 15000 ft was compressed in a dive by 5k or so.
Edit: I think i understand my problem , compressibility and compressibility tuck are two different things . The nose doesnt try and tuck under from the lower alts where im seeing compression .
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The book never explains the "number of reasons" they wouldn't compress so how do you compare without knowing what they are?
The book says that M .65 is the start of compression and that's when you start to see high speed buffet in AH. Maximum tuck occurs at M .74 and you cannot reach M .74 from 25,000 ft so that may be one of the reasons the book didn't specify.
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I just did in a 0 mph stall at 15000 ft was compressed in a dive by 5k or so.
Edit: I think i understand my problem , compressibility and compressibility tuck are two different things . The nose doesnt try and tuck under from the lower alts where im seeing compression .
What you're experiencing is the result of high speed airflow over the flight surfaces that makes stiffens up the controls and is not compressibility.
Compressibility happens in the P-38 (and P-47) when the airflow over the leading edge of the wing breaks exceeds the speed of sound, and the higher altitude you're at the lower the speed is to hit that mach threshold. Below 25,000ft, the air is too thick for the P-38 to reach compressibility, though you will experience the controls stiffen up because of the high speed airflow over the flight controls.
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I agree with akak on this. ^^
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high speed airflow over the flight surfaces that makes stiffens up the controls
Some times we get too caught up in expecting everyone to know every flight control term and behavior. Like many others I'd like to know the answer to this too.
Other than not diving as aggressively in a P-38 as one would do in commonly known dive monsters (like P-47, FW190, F6 etc.), is there a way to prevent the controls from stiffening or is that an inherent trait of the P-38 when diving?
Thanks for putting the answer to this in English for us P-38 novices. :salute
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I agree with akak on this. ^^
That's a safe bet.
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Some times we get too caught up in expecting everyone to know every flight control term and behavior. Like many others I'd like to know the answer to this too.
Other than not diving as aggressively in a P-38 as one would do in commonly known dive monsters (like P-47, FW190, F6 etc.), is there a way to prevent the controls from stiffening or is that an inherent trait of the P-38 when diving?
Thanks for putting the answer to this in English for us P-38 novices. :salute
Yes, use the p-38l. The ah models in aileron boosters the J in ah doesn't,t have.
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Other than not diving as aggressively in a P-38 as one would do in commonly known dive monsters (like P-47, FW190, F6 etc.), is there a way to prevent the controls from stiffening or is that an inherent trait of the P-38 when diving?
It's not just a P-38 trait. It happens to every WW2 fighter. The speed where it happens varies by the wing design. Some P-47s have the same dive flap as the P-38L to delay the compression control lock.
BTW if you haven't noticed, your Mach speeds are shown on the clipboard E6b if you want to compare to historical max dive numbers.
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Again.......the specificiation for the 25k remark was 45 degree dive angle.
Now if you're already going super fast as you pass through 25k and push over vertical, then it can easily happen below 25k.
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Again.......the specificiation for the 25k remark was 45 degree dive angle.
Now if you're already going super fast as you pass through 25k and push over vertical, then it can easily happen below 25k.
Super fast requires you start above 25k and trade altitude for speed. Passing through 25k is not starting from 25k. Start at max level speed at 25k and try to make .74 Mach at any dive angle.
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Other than not diving as aggressively in a P-38 as one would do in commonly known dive monsters (like P-47, FW190, F6 etc.), is there a way to prevent the controls from stiffening or is that an inherent trait of the P-38 when diving?
Thanks for putting the answer to this in English for us P-38 novices. :salute
Contrary to public opinion, you can dive the P-38 at high speeds safely. The P-38 dives best at medium to low altitudes, the thicker the air the faster you can dive the Lightning. Below 10,000ft, you can easily and safely reach speeds of 475mph IAS and slightly higher and retain control of the plane, although the controls will be sluggish due to the high speeds you're traveling at.
In a dive if I want to reach the maximum speed possible without suffering from the controls locking up, if in the altitude range of 20,000ft - 10,000ft, my dives tend to be low angle dives so I don't overspeed and avoid the high speed buffet and I will jockey the throttle to keep myself from getting too fast or to slow in the dive. I'll also skid my plane to keep my speed under control, those two rudders make pretty good air brakes and can save your arse in a dive if you get too fast. If you find yourself getting too fast and having a tough time pulling up, use the elevator trim to aid you in recovery.
10,000ft and below, it's balls to the wall in a dive and I'll dive at steep angles and use my roll rate to roll into the bandit as he tries to break to avoid my attack. In high speeds dives like this, I'll again use the throttle and rudders (skid the plane) to keep my speed under control and to avoid overshooting the bandit and giving him the shot, I'll go into a lag turn after my first pass if I miss.
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Ive ran from cons in an L model 500mph+ while maintaining control (on the very fringe limit of complete lock) and pulled out once they broke off of me. I wanna say 530mph+/- but screen was super shakey
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There are a ton of different ways to control the 38's speed in a dive , I use rudder mostly and throttle control . Using elevator trim you can ( at low angles) keep the plane in the buffet for quite awhile when trying to run some one down or egress .
I was just taking some of what I read and interpreted it to mean, then compared it to what i see in AHII .
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Super fast requires you start above 25k and trade altitude for speed. Passing through 25k is not starting from 25k. Start at max level speed at 25k and try to make .74 Mach at any dive angle.
I accept your challenge and will provide film.
Which model of P38 do you want me to use for refutation purposes.
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That's still 80-90mph slower than the 190A
Ive ran from cons in an L model 500mph+ while maintaining control (on the very fringe limit of complete lock) and pulled out once they broke off of me. I wanna say 530mph+/- but screen was super shakey
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I accept your challenge and will provide film.
Which model of P38 do you want me to use for refutation purposes.
Challenge? The challenge was knowing the difference between "starting from" and "passing through". :D
Pick any model and load and let us know what happens when you start above normal cruising speed at 25k.
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I have not been able to break 500 mph ias in a P-38L. I have come real close to 500 mph IAS just as I reached the deck but not break 500.
Not saying it can't be done.
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Challenge? The challenge was knowing the difference between "starting from" and "passing through". :D
Pick any model and load and let us know what happens when you start above normal cruising speed at 25k.
And here is the film.
http://bbs.hitechcreations.com/smf/index.php/topic,204187.msg4988459.html#msg4988459
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Again.......the specificiation for the 25k remark was 45 degree dive angle.
Now if you're already going super fast as you pass through 25k and push over vertical, then it can easily happen below 25k.
:airplane: I'm confused! Both of the 38's we have in the game will compress at any altitude, or at least they have with me! Or, maybe all that shaking and rattling I encounter when diving is just "pot" holes in the road! LOL
If you enter it, just "chop" the throttle and hold down the "K" key and it will come right out of compressing! Just try a little "fishtailing" with the rudder on the way down, it will behave like a little kitten, not to mention helping you avoid ack from he base or town you are attacking!
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:airplane: I'm confused! Both of the 38's we have in the game will compress at any altitude, or at least they have with me! . . .
Well that is true if your going fast enough.
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For STP at SL, the data for M=1 is as follows
25K = 692.7mph TAS, at 20K = 706.9mph TAS, at 15K=720.8mph. MCr = >65M or 450mph at 25K, 459mph at 20K
at 25,000 feet during published flight tests with full internal fuel in the P38L the max level speed at WEP is 411mph TAS (,59M), at 20K it is 402mph (.57M). This is the flight regime when CDo starts going up exponentially (for a Mustang M=.65 for comparable Drag divergence)
So starting a dive at say max level speed in a chase at WEP at 25,000 feet, that P-38 only has to accelerate to .65M in 5000 feet to get to Mcr or 459mph TAS. Sound feasible in a 45 degree dive?
It does to me. I think Bodie is wrong.
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To those of you on the first page who are quoting Bodie as stating the P-38 "did not compress" below 25,000ft, you are reading too lightly what Bodie actually said. DrgnDg is correct in stating that the P-38 would compress below 25k.
Specifically to AckAck(and others making this claim): The air below 25,000ft is not too thick to allow for compression. Rather, the change in air density causes the onset of compression EARLIER in terms of indicated airspeed at higher altitudes. It is 100% possible to compress at ANY altitude given enough speed. If you exceed your mach limit, it doesnt matter what altitude your at. It is just less likely that you would compress the lower you go because A: less space to accelerate B: Speed of sound changing due to altitude.
I think some of you might be misunderstanding what Bodie was saying in his P-38 book. It was not that the P-38 never compressed under 25k, but that under 25k, it was not the crippling operational hazard that it was higher up. IE: in a most engagements below 25k, it was difficult enough to compress that it was encounters less often and was not as big of a deal. Look closer in Bodies book and you will see that he posted charts in there that show that the P-38 WILL compress at lower heights, just at much higher IAS.
CASE IN POINT:
(https://upload.wikimedia.org/wikipedia/en/f/fe/Compressibility010.png)
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From the manual
DIVE RECOVERY FLAPS
The airplane without these flaps becomes very nose heavy and starts to buffet above diagram dive speeds (Dia.2.). The dive recovery flaps which are installed under the wings between the booms and tile ailerons restore the lift to this portion of the wing and thus cause the uncontrollable nose heaviness to occur at a higher speed. The flaps also add some drag to the airplane which in conjunction with the higher allowable dive speed permits safer dives at a much steeper diving angle. The dive recovery flaps should be extended before starting the dive or immediately after the dive is started before a buffeting speed has been reached. If the airplane is buffeting before the dive recovery flaps are extended the buffeting will momentarily increase and then diminish. With these flaps extended, the nose heaviness is definitely reduced but the diving speed should never be allowed to exceed the placard by more than 15 or 20 mph. With the dive recovery flaps extended before entering the dive, angles of dive up to 45 degrees may be safely accomplished. Without dive recovery flaps extended the maximum angle for extending dives is 15 degrees. Diving characteristics are better with power off than power on.
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The 'nose heaviness' is caused by the change to the CMac when the onset shock wave migrates from ac (~T/C max) toward the middle of the wing - which moves the center of Pressure aft... which in turn causes the Nose Down Pitching moment.
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I doubt Bodie was referring to max level speeds and full power dives. Isn't there is a significant control response difference between Mach .65 and Mach .74? What were the unnamed reasons Bodie referred to? Could they have included 'power off' and 'mind your max speed' ? Are there combat reports that contradict Bodie?
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I doubt Bodie was referring to max level speeds and full power dives. Isn't there is a significant control response difference between Mach .65 and Mach .74? What were the unnamed reasons Bodie referred to? Could they have included 'power off' and 'mind your max speed' ? Are there combat reports that contradict Bodie?
Possibly - but a P-38 in full chase WEP in level attitude at 25k rolling to follow a 109 split essing is going to accelerate past .62 very rapidly if he does it 'clean'.. which is more reality, than quiet contemplation. So, the comment that it 'can't go into Compressibilty below 25K" just doesn't make much sense (to me)..
As to significant control response between .65 and .74 M? I dunno. I do know that the P-38 wing was in onset drag divergence at .62-.63M with a full blown shock wave developing shortly thereafter. The shock wave would form at about 25% chord and the nose tuck would begin shortly thereafter as the shock wave migrates toward trailing edge of the wing.
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The dive acceleration is obvious enough that it's hard to believe Bodie was unaware of it. Unless it's an editing error it seems likely Bodie had a reason for the comment even if it's not obvious from the book.
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From the manual
:airplane: Hard to beat the old flight Manuel for accurate information! Do you reckon that anyone who flew these great birds, really knew any of this technical stuff these guys are throwing out? Never flew one of these Randy, in RL, but always wanted to, it had to be a blast! Did you ever read the "bail" out procedure for one these things?