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Bull_dog_
08-05-2004, 05:46 PM
From pg. 43 of flight journal P-38 special edition:

Compressibility, an even worse problem, was first encountered by USAAF Maj. Signa Gilkey in May 1941 while in a very high speed dive. Fortunately, when his controls froze-making the airplane uncontrollable and increasing its dive angle-his dive angle was shallow enough to allow him to ride the aircraft down to denser air and thus reduce the airflow expanding around the wing at speeds below Mach 1.0. This cuased his P-38 to decelerate bleow its critical Mach number of 0.65. Had his dive been 10 degrees steeper, he probably would have crashed without regaining control. This was one of the first encounters with this previously unknown problem that all fighter aircraft manufacturers soon experienced. All fighters prior to the P-38 were required to demonstrate full-power vertical terminal-velocity dives from the service ceiling of the aircraft and to delay their pullout until below 7,500 feet. Because its greater engine power allowed it to start dives at much higher altitudes than previous fighters, the P-38 was the first military fighter to encounter this transonic Mach number phenomenon.

Flight testing the P-38 disclosed that whenever the airflow over the wing exceeded mach 1.0, compressibility effects were encountered. This result was soon predictable when this slippery fighter accelerated in excess of 0.65 Mach in dive angles greater than 45 degrees at altitudes above 15,000 ft. Cockpit-installed Mach meters had yet to be invented. George W. Grey, in his history of NACA, departed from strict engineering terms when he described compressibility effects in the P-38, saying, "The behavior of the P-38 was new to pilots, terrifying, baffling. Several men putting this two-engine fighter through its diving maneuvers experienced a sudden violent buffeting of the tail accompanied by a lunging and thrashing about of the airplane, as though it was trying to free itself of invisible bonds, and then the maddening immobility of the controls, the refusal of the elavators to respond to the wheel control."


Interesting read...several things in there that make me wonder more about aircraft shedding parts in high speed dives. Most importantly, it left me with the feeling that compressibility is not modelled correctly on the Lightning. Elevator authority goes away too soon and it shouldn't be a problem at all under 5000 meters and only above that if certain speeds are hit at certain dive angles.

Imagine diving a plane from its service ceiling to 7500 ft! The comment about terminal dive velocity...if that is true, then maybe that is where the dive model is off in this game...terminal dive velocity should be higher in some aircraft than others....and I don't mean terminal in falling apart...more like terminal in the fact that you can only go so fast and then acceleration stops and deceleration begins as air thickens.

Does this article sound right? If it is, then there is some tweaking to be done on the lightning and many other planes.

Bull_dog_
08-05-2004, 05:46 PM
From pg. 43 of flight journal P-38 special edition:

Compressibility, an even worse problem, was first encountered by USAAF Maj. Signa Gilkey in May 1941 while in a very high speed dive. Fortunately, when his controls froze-making the airplane uncontrollable and increasing its dive angle-his dive angle was shallow enough to allow him to ride the aircraft down to denser air and thus reduce the airflow expanding around the wing at speeds below Mach 1.0. This cuased his P-38 to decelerate bleow its critical Mach number of 0.65. Had his dive been 10 degrees steeper, he probably would have crashed without regaining control. This was one of the first encounters with this previously unknown problem that all fighter aircraft manufacturers soon experienced. All fighters prior to the P-38 were required to demonstrate full-power vertical terminal-velocity dives from the service ceiling of the aircraft and to delay their pullout until below 7,500 feet. Because its greater engine power allowed it to start dives at much higher altitudes than previous fighters, the P-38 was the first military fighter to encounter this transonic Mach number phenomenon.

Flight testing the P-38 disclosed that whenever the airflow over the wing exceeded mach 1.0, compressibility effects were encountered. This result was soon predictable when this slippery fighter accelerated in excess of 0.65 Mach in dive angles greater than 45 degrees at altitudes above 15,000 ft. Cockpit-installed Mach meters had yet to be invented. George W. Grey, in his history of NACA, departed from strict engineering terms when he described compressibility effects in the P-38, saying, "The behavior of the P-38 was new to pilots, terrifying, baffling. Several men putting this two-engine fighter through its diving maneuvers experienced a sudden violent buffeting of the tail accompanied by a lunging and thrashing about of the airplane, as though it was trying to free itself of invisible bonds, and then the maddening immobility of the controls, the refusal of the elavators to respond to the wheel control."


Interesting read...several things in there that make me wonder more about aircraft shedding parts in high speed dives. Most importantly, it left me with the feeling that compressibility is not modelled correctly on the Lightning. Elevator authority goes away too soon and it shouldn't be a problem at all under 5000 meters and only above that if certain speeds are hit at certain dive angles.

Imagine diving a plane from its service ceiling to 7500 ft! The comment about terminal dive velocity...if that is true, then maybe that is where the dive model is off in this game...terminal dive velocity should be higher in some aircraft than others....and I don't mean terminal in falling apart...more like terminal in the fact that you can only go so fast and then acceleration stops and deceleration begins as air thickens.

Does this article sound right? If it is, then there is some tweaking to be done on the lightning and many other planes.

BBB_Hyperion
08-06-2004, 04:17 AM
That the dive model is off is known .)

I reported a almost similar report for a 109 testdive.

It seems like planes accelerate endless and airdrag and prop drag doesnt increase with speed.

High Ground is not only more agreeable and salubrious, but more convenient from a military point of view; low ground is not only damp and unhealthy, but also disadvantageous for fighting.

Sun Tzu : The Art of War

Regards,
Hyperion

Bull_dog_
08-06-2004, 09:36 PM
Elsewhere in another edition I read where a pilot transitioned from Lightnings to Mustangs and he noted how a lightning would zoom climb back up to 10,000 ft. after a dive bombing run where the Mustang would have to take evasive action from AAA much lower because of the lack of zoom...

In addition there is another exerpt about a photo recon P-38 (didn't have dive flaps) cruising at 32,000 ft. and was intercepted by Me-262's...he had to dive and he got stuck in compressibility and bailed (more like sucked out of the cockpit) out at 10,000 ft because he couldn't pull out.



More stuff that makes me think the Lightning is off by a good margin....while I love playing this game and it is the best thing going, these things also highlight how off the dive modelling is. I don't know why planes loose rudders, ailerons and elevators in dives...it just doesn't happen at low altitude where thick air caps the terminal velocity and actually decelarates the aircraft.

Korolov
08-06-2004, 10:07 PM
One thing we all must keep in mind with FB's engine, is that the technology is about 4 years old by now. It's hard to get it to do these things when it was never designed to do them in the first place, and so we have to make do with band-aids to the FMs.

http://www.mechmodels.com/fbstuff/klv_sigp38shark1a.jpg

Cragger
08-07-2004, 12:29 AM
What Korolov said is right.

Also many people need to realize that taking physics that just exists to all our knowledge and requires no universal processing power and converting it into a digital mathematical is just not possible. Your talking about real time calculations here, a single calculation gets caught up and the whole house of cards comes tumbling down.

Even with infinite processing power it is impossible to implemement every detail and faucet of the real world physics into the digital world simply because WE don't know everything there is to know about physics and computers are programmed off our knowledge.

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VW-IceFire
08-07-2004, 07:46 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Cragger:
What Korolov said is right.

Also many people need to realize that taking physics that just exists to all our knowledge and requires no universal processing power and converting it into a digital mathematical is just not possible. Your talking about real time calculations here, a single calculation gets caught up and the whole house of cards comes tumbling down.

Even with infinite processing power it is impossible to implemement every detail and faucet of the real world physics into the digital world simply because WE don't know everything there is to know about physics and computers are programmed off our knowledge.

http://redspar.com/redrogue/cragger_sig.jpg <HR></BLOCKQUOTE>
Well said. Especially the last part...we don't know everything there is to know about how the physics is going to work for an aircraft flying through the air...so even with CPU power not being an issue it'd never be perfect.

I have high hopes that Oleg's new engine for BoB and other theaters has alot of improvements that will mean that by the time they do the P-38 again (which would be in the Med sim no doubt) the details that everyone asks for will be closer to pilot experience than before.

Its an ever increasing level of detail...but its never going to be completely in focus.

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RCAF 412 Falcon Squadron - "Swift to Avenge"

Bull_dog_
08-07-2004, 01:34 PM
When I write something like this, it is because some experience(s) in the game don't jive with my perception of the actual performance of the aircraft...I write these things to get others opinions and I hope that Oleg drops by from time to time and reads something that may trigger him to want to tweak something....

I realize there are limitations to the engine...some of the things I describe are not engine limitations...some are. I wrote specifically about the lightning because as I was flying one of my favorite aircraft I just got tired of not being able B&Z...it was a great B&Z aircraft in real life! I can use energy tactics on a Fw, but not on a 109 because the slow speed qualitites of the aircraft are not well rendered. And of course we all touch on the dive model which I do believe is a limitation of the FM, but which seems drastically off in its manifistation...that is control surfaces breaking off in high speed dives...that is a choice made by programmers.

In the end, I love to fly the Lightning, but there are a few tweaks and I provided some information...it is not bad off, but it I don't think the plane is spot on either and if there are further patches/PF I'd like to see further refinement of the FM...if it were up to me these are the things I'd change:

1) Zoom climb/low speed handling/stall characteristcs: They were improved, but the Lightning should be best in class in these characteristics... the exception being early model light aircraft like Zeke. I'd say 109, La and ki and maybe spit best in class for standing on nose/low speed handling. Once the control surfaces have little enough air flowing over them the torque of the engine takes over spinning the aircraft out....the lightning just didn't have that problem. Engine limitations exist here I know, but from a relative standpoint the Lightning is good, but not great. It should be great.

2) High altitude manuevering. The lightning is not a turner, but for some reason it looses its lift and turning power worse than any other aircraft at high altitude...doesn't seem right to me at all. If a bandit passes me and keeps going, he'll be 3km away before I get my nose pointed at him again. I'm thinking this may be a bug. Not much noticed cause few online fights happen at high altitude and the lightning is useless at above 5000 meteres except going straight and fast. A tweak needed here....nothing major.

3) Elevator authority: probably the single biggest crippler of this aircraft is improper modelling of elevator authority. It is confusion over Compressibility and the whole reason I wrote this. This is a choice not an engine limitation and below 15,000 ft., the plane shouldn't lose elevator authority at any speed...additionally the aircraft had a yoke which allowed a pilot to exert more leverage on the control surfaces...yes they got heavy, but not immovable except in compressibility which didn't happen unless high altitude dives at greater than 45 degree angles were acheived. The aircraft was a legend at B&Z in the pacific at medium and low altitudes...give us this aircraft in FB...please

4) Obviously dive and zoom climb, but this is where we get into the realm of engine capability. Parts didn't fall off in the lightning except in compressibility where the plane was shook apart...same with nearly every US aircraft. I do know that Zeroes shed parts in high speed dives because of their light construction. I don't know if this is Oleg's way of managing the dive envelope due to engine limitations or the characteristics of a few aircraft applied to all of them?

One last note about compressibility...it is modelled in the Lighting, but all aircraft that could start dives from 30 or 35K and had sufficient weight and power suffered from compressibility...Jugs got dive flaps on the D-30 or 40 because of it...Many Hellcat pilots met their end this way etc...always starting from high altitude, in thin air is where the trouble began.

Elevator authority...I'd like to have a B&Z lightning for FB and PF....that is what it did in real life.

p1ngu666
08-07-2004, 02:56 PM
i find it bounces around when i fire cannon alot, and its very sensitive when im lining up shots, im overcorrecting, mind i dont fly it much

maybe its just me

but i do agree with your other points, dunno about high alt tho (not really tested/flown up high)

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&lt;123_GWood_JG123&gt; NO SPAM!

Bull_dog_
08-07-2004, 08:12 PM
Try leaving it trimmed in neutral position (max. climb) and at 100% and just let it climb...you'll find that the climb really falls off and actually slows to a crawl and even stops at about 8000 meters...no other allied plane does this...you have to trim up. The plane really loses lift as it gets to thin air.

I don't think the 38 was dominant by any stretch up high, but is is really at a disadvantage as the wing lift erodes as does its turn radius...it is odd, but I am confident that is is "abnormal" for a plane designed to be a high altitude interceptor.

I know the fork tailed devil got most of its bad reputation from high altitude escort missions but it was primarily due to poor cockpit heating, poor engine reliability up high, complex controls and long learning curves and of course good old compressibility where axis aircraft could just split S away from a lightning...not from turn radius or climb rate.

The lightning is good down low except that aweful compressibility modelling....try this: airstart at 5000 meters, cut the engine and dive at about 45 degrees...you won't be able to pull out without trim and flaps. That is incorrect.

I am just hoping Oleg continues to fine tune the model and sees some shortcomings as I do. If he doesn't, I'll just fly it just the same, but I won't feel like it is the aircraft WWII airmen flew.

WWMaxGunz
08-07-2004, 09:31 PM
--&gt; Fortunately, when his controls froze-making the airplane uncontrollable and increasing its dive angle-his dive angle was shallow enough to allow him to ride the aircraft down to denser air and thus reduce the airflow expanding around the wing at speeds below Mach 1.0.

Right. It helps that Mach 1 is a higher airspeed as you lose alt, too.

--&gt;This cuased his P-38 to decelerate bleow its critical Mach number of 0.65.

Plus higher drag in denser air. Maybe cranking back on the engine a bit....

--&gt;Had his dive been 10 degrees steeper, he probably would have crashed without regaining control.

And here you have evidence that compressibility can stay with you all the way down.
It can because it's possible to stay over .65 mach all the way down.

--&gt;Flight testing the P-38 disclosed that whenever the airflow over the wing exceeded mach 1.0, compressibility effects were encountered.

That is the condition reached at above .65 mach.

--&gt;This result was soon predictable when this slippery fighter accelerated in excess of 0.65 Mach in dive angles greater than 45 degrees at altitudes above 15,000 ft.

This is only how they were getting P-38's above .65 mach. Compressibility doesn't cut
off just because you're below 15k ft. Just that the P-38 shouldn't be able to get that
fast as a rule of thumb. Wanna bet that a 60 degree dive wouldn't get you there at an
alt where 50 degree dive would not?

***Interesting read...several things in there that make me wonder more about aircraft shedding parts in high speed dives. Most importantly, it left me with the feeling that compressibility is not modelled correctly on the Lightning. Elevator authority goes away too soon and it shouldn't be a problem at all under 5000 meters and only above that if certain speeds are hit at certain dive angles.

Is that if you start the dive under 5km alt?
You got it right about certain speeds but that's it. Exceed the speed and it should
lock, no matter how you do it. When it locks, the nose should drop some too. If you
have a ways down to go then just know that mach rises as you fall, so you may get a
chance to pull out.

Commercial and corporate jets caught in the same condition due to storm cells drop
their gear by advice from the FAA. Modern gear on those planes will take it and still
work. Gotta love hydraulics and blowdown bottles, or electrics in the case of Lears,
at least the ones I knew.

***Imagine diving a plane from its service ceiling to 7500 ft! The comment about terminal dive velocity...if that is true, then maybe that is where the dive model is off in this game...terminal dive velocity should be higher in some aircraft than others....and I don't mean terminal in falling apart...more like terminal in the fact that you can only go so fast and then acceleration stops and deceleration begins as air thickens.

Sure. Before the P-38 came out and maybe for some planes afterwards, service ceiling
was low enough and the plane slow enough they got away with it.

***Does this article sound right?

Does to me.

***If it is, then there is some tweaking to be done on the lightning and many other planes.

You might be right, but take care how you apply that rule of thumb. It's a bit wide.
Did the P-38 debut before the P-40? Not too sure but the P-38 may be the oldest US
plane we have in the set. P-40 is an E model, not the first one of the series is it?
We don't have the Hawk yet, do we? And one look at the I-16... could it get fast
enough to compress or not? Is the Hurricane an old enough design that it should not
be able to get there? What planes should be checked, let alone tweaked?


Neal