Bf109F vs P-38F

[HoHun]

Hi Mkloby,

>If the aircraft had a constant speed prop, the prop gov would seek to maintain an "ideal" prop rpm speed. If the engine is geared to the propeller at a fixed ratio, as you earlier stated, I don't understand how this is possible.

Hm, if you look at it that way, I could say you're right and that it is not possible to maintain an "ideal" propeller speed.

What the engine computer does is to command a specific speed and manifold pressure for the currently set power lever position. The propeller governor would try and maintain this speed, and if the pilot changed the power lever position, it would try to maintain a different speed as commanded by the engine computer.

However, for lack of an infinitely variable transmission between engine and propeller, the engine computer would have to command a very high propeller-and-engine rpm in order to allow the engine to deliver the desired high power. This speed would be less than ideal for the propeller as it would be hit by a serious loss of aerodynamic efficiency in high-speed flight, especially at altitude, and you'd burn a lot more fuel for just very little more speed, but as this was the only way to keep the engine happy, there was no way around this "abuse" of the propeller in a less-than-ideal operating range.

Regards,

Henning (HoHun)

 

[Juha]

A couple German comments on Bf 109 vs P-38 in North Africa/MTO from Jeffrey L. Ethell's P-38 Lightning p. 21-22 in The Great Book of WWII Airplanes
Johannes Steinhoff, "…Bf 109 was still, perhaps, a little faster. But pilots who had fought them said that the Lightnings were capable of appreciably tighter turns and that they would be on your tail before you knew what was happening…"
Oblt Franz Stiegler, 28 victories, JG 27, "P-38 could turn inside us with easy and they could go from level flight to climb almost instantaneously. We lost quite a few pilots who tried to make an attack and then pull up. The P-38s were on them at once. They closed so quickly that there was little one could do except roll quickly and dive down, for while the P-38 could turn inside us, they rolled very slowly through the first 5 or 10 deg of bank, and then we would already be gone…."

Juha

 

[Soren]

The comment from Steinhoff is odd since he has said the exact opposite before. And even more oddly is that all German accounts I've ever seen mentioning the P-38 considered it to be easy prey.

But Ethell's has also been accused of making stuff up before so, take what is written in his book with a huge grain of salt...

Don't believe tall tales, let real life physics guide you

 

[Soren]

Steinhoff:
"The Lightning. It was fast, low profiled and a fantastic fighter, and a real danger when it was above you. It was only vulnerable if you were behind it, a little below and closing fast, or turning into it, but on the attack it was a tremendous aircraft."

 

[Soren]

LW pilots opinion:

 

[drgondog]

LW pilots opinion:

I know this opinion was widely held by many Luftwaffe pilots. Having said this I wonder how many were interviewed that fought against the post D-Day versions of the J-25s and L's.

The combination of dive brakes, manuever flaps and boosted ailerons really did transform this machine.

By and large only a few LW pilots in LuftReich engaged this ship, as well as remnants of JF77, 27, etc that were still fighting in the Balkans - and those that stumbled on 9th AF ground support missions.

The one intangible that really worked to the LW favor that was not mentioned is that is a very distinctive shape and easy to id from a long way away. At altitude the 109 pilot could often spot a 38 andmake a decision to fight or flee before the 38 was in visible range of a 109. Ditto on spotting a 38 from altitude while it is on the deck.

The intangible made it possible to achieve some initial tactical advantage before attacking.

In my research the P-38's in the 8th AF were by far the least successful against German fighters, the 9th better (but they had the late models) and the 12th/15th still better - perhaps because of better tactics... but the 8th AF ratio was respectable (~2:1 and we can always debate awards to claims) because the 479th had such HUGE success while it was flying the P-38J/L. The 55th, 20th and 364th were above 'even' but they also fought the H and early J's when the Allisons were blowing up, couldn't dive or roll well, and just were outflown.

It remains an interesting question and definitely has a 'break point' between pre J-25s and the rest of the war.

 

[mkloby]

Don't believe tall tales, let real life physics guide you

Soren - doesn't posting accounts of German pilots regarding the P-38 violate this statement you just made a few posts ago?

I tend to think that pilot accounts, performance data, and employment are all very important and to leave one out is ignoring part of the available information.

 

[Soren]

I know this opinion was widely held by many Luftwaffe pilots. Having said this I wonder how many were interviewed that fought against the post D-Day versions of the J-25s and L's.

Yeah I can't answer that question But Steinhoff only got the chance to meet them in the MTO, he had a very long period where he didn't fly, and when he started again it was in the Me-262 against the high flying bombers in 44.

Soren - doesn't posting accounts of German pilots regarding the P-38 violate this statement you just made a few posts ago?

lol, no, it just goes to show exactly why you can't blindly trust pilot accounts or opinions. If you want to know the true abilities of an aircraft then you need to trust in physics and the statements made by modern day pilots of the type.

 

[Juha]

Soren
I trust in physics but that doesn't mean that I take too seriously long term (7 days) weather forecasts, even meteorologists who made them using supercomputers admit at least here that one cannot count very much on them. Even much more simple things for ex how certain amount of certain explosive effects known ferroconcrete structure isn't all too easy to predict. So I don't see why should place too much faith on rather elementary calculations on such rather complicated phenomenon as how an a/c behaved in turn.

And if you may enlighten me how a modern day pilot is better to judge behaviour of an a/c than a combat pilot who have had much more stick time on type, who had flown it to limits without restrictions nowadays in force, rightly so, on flying old and rare type of a/c or a test pilot who had found out the envelope limits of the type?

Juha

 

[Soren]

Believe what you wish Juha, your odd comparison of aerodynamics with meteorology speaks volumes for me regarding how you see things.

 

[Juha]

Soren
Meteorology is also a natural science based on physic and weather forecasting is in essence using complicated calculations based on numerous known variables and models based on long experience, much research and physical laws to predict what will happened in near future and a commoner can easily make his opinion on the results.

Aerodynamic is much more simple but for ex you are not professional aerodynamist or computer modeller and you very probably don't have in your use anything near the computer power meteorological institutions have. And even if I don't play with aircombat simulators I would be surprised if all good games give same behaviour for ex Bf 109F-4 or P-38F-15 as it should be if the relatively simple (as compared to those made by a/c manufactures) aerodynamical modelling produces accurate results.

And still waiting the explanation why a modern day pilot is better to judge behaviour of an a/c than a combat pilot who have had much more stick time on type, who had flown it to limits without restrictions nowadays in force, rightly so, on flying old and rare type of a/c or a test pilot who had found out the envelope limits of the type?

Juha

 

[Soren]

Juha,

Again believe what you wish.

As for the modern day pilots, they didn't fly the a/c 60+ years ago during a hateful war, so no memory loss no bias. THAT is why the accounts by modern day pilots are more accurate reliable. And "oddly" enough aerodynamics ALWAYS seems to support what the modern day pilots have to say, something which can't be said about 60+ year old accounts.

And about the meteorologists, sorry no bite, such an awful comparison.

 

[Juha]

Soren
Now it is a bit different to ask today veterans oppinions than to look info from test reports from 40s or even info from what was written in 50s and 60s. For ex E. Brown has his notes to back up his memory, not that I always agree with his oppinions.

And believing that todays pilots had no bias is IMHO a bit naive. Opinions are always subjective, some more and even much more than others. And many were able to remain rather objective and analytical during the war. And as I wrote earlier, IIRC nowadays they don't use full power and had rather strict g-limits when they flew those old planes and not all the modern pilots had fighter pilot training.

Meteorologist comprasion was an answer to your
Quote:"If you want to know the true abilities of an aircraft then you need to trust in physics"

IMHO rather naive sentence. Of course we trust physics but it isn't same as to trust results which were generated by computer models, more complex the phenomenon and less sophisticated the model are, less trustfull the results will be.

Juha

 

[Juha]

And on the question of the poll, now closed. IMHO 109F-4 was the best fighter around when it was in production from May 41 to May 42. So IMHO 109F-4 was better than P-38F even in very late 42. But I must admit that I cannot remember much on P-38.

Juha

 

[drgondog]

And on the question of the poll, now closed. IMHO 109F-4 was the best fighter around when it was in production from May 41 to May 42. So IMHO 109F-4 was better than P-38F even in very late 42. But I must admit that I cannot remember much on P-38.

Juha

Juha - I would agree with you (and Soren) and all the others that voted 109F-4 over P-38 in 1942... and extend that at least through mid 1943 against all versions of the P-38F.

If the introduction of manuevering flaps made enough difference in turn against to 109F (and early G-2) it would have to be in mid 1943 when the 38F-15 (all 221) was introduced, then followed with G.

IMHO that would have been significant only in a sustained turn combat that the 109 chose to not perform a series of opposite rolls and finally reverse and escape. Turning combat is over rated but a nice advantage to have when you are forced into it when you lose all other options.

As good an airplane as the P-38 was it never fully realized its potential until the J-25 finally ironed out the altitude dive compressibility issues with dive brakes and improved roll/turn capability with boosted ailerons and manueveing flap combination. That would be mid 1944 - for first combat introduction

 

[Juha]

Hello Drgondog
have You idea why some US a/c had to went fairly long in alphabetics before they became combat ready? P-38, P-39, then a number which never got into combat service in USAAF, excluding P-40 which in any case was based on P-36, come into mind. Spit, Hurri, 109 and 190 seems to became combat ready earlier in their career.

Juha

 

[drgondog]

Soren
Meteorology is also a natural science based on physic and weather forecasting is in essence using complicated calculations based on numerous known variables and models based on long experience, much research and physical laws to predict what will happened in near future and a commoner can easily make his opinion on the results.

Weather forecasting, IMHO, is the toughest physics problem dealing with Energy/Mass Flow Balances.

The Navier Stokes equation must be solved and there are more variables than equations - requiring huge supecomputers to perform iterative solutions. I intially thought about modelling cloud formation as my Masters Thesis - my Academic Advisor and I had a lot of chuckles on this subject later (thinking I might do a Post Doctoral study with computers roughly capable to your iPod but without the processing and storage capability (CDC 6600/IBM 360/65)

He knew I was delusional and was able to perform an intervention.

I have been 'out' too long to fully comment on today's state of the art but rougly speaking the models and results published by Gene Lednicer/VSAERO which formed the basis of our pressure distribution comparisons between P-51, Fw 109 and Spits would be a first step and would have to be combined with a very sophisticated Flight Mechanics Model and perhaps Propeller model to interactively work with VSAERO. If VSAERO is incapable of dealing with boundary layer separation and/or asymmetric/spanwise flow then another 'subroutine' would have to be added.

The Flight Mechanics model would have to accurately predict the effects of asymmetric control inputs with initial roll and turn and changes to angle of attack, the effects of these rapid changes would have to be input back to VSAERO, analysis performed on change to pressure distribution and center of lift, back to Flight mechanics model, to calculate the effect of change to Wing Moments on the system as Lift Center moves from Cg - then it gets really interesting when the local angles of attack along the wing cause local chord wise boundary layer separation, and back Flight Mechanics model for for the asymmetrical near stall effects.

Further, a nice model must be developed (if it doesn't exist already) to accurately predict the Thrust/Drag of the Propeller/exhaust system in asymmettrical and symmetrical flight profiles to assist in the Free Body Force balances while slowing from accelerating flight to equilibrium.

The asymmetric flight conditions in a turn must also be combined with the effects of the Propeller system on the aerodynamic wing/body sytem.

It must also be used to determine/speculate on spanwise flow properties across the entire system - which will be different on the high wing from low wing.

Aerodynamic is much more simple but for ex you are not professional aerodynamist or computer modeller and you very probably don't have in your use anything near the computer power meteorological institutions have. And even if I don't play with aircombat simulators I would be surprised if all good games give same behaviour for ex Bf 109F-4 or P-38F-15 as it should be if the relatively simple (as compared to those made by a/c manufactures) aerodynamical modelling produces accurate results.

Juha

Juha - make your own determination regarding how simple modern day aero is but I rank the Full blown/all mauever Flight Mechanics model with VSAERO type 3-D Fluid Mechanics models and Propeller Model and perhaps even complex boundary layer/Chaos theory based subroutines as 'fairly complicated' - and approaching the Earth/Sun model Navier Stokes solutions in complexity.

What we see as simple in the forums is reducing the Aero properties to several coefficients, extracting initial data from sources, and then leaping away into simplified linear equations to a non linear problem.

Having said this, most of the modelling done here is close enough to illustrate approximations to different states in the manuevering, and there are no such 'simple' analogues to predicting weather.. as you noted.

 

[Juha]

Hello Drgondog
I know that meteorology was a rather extreme case, I choose it partly because of that and partly because of, even if my main subject in university was World History, I also studied Geography and so learned something on climate and got some info on weather forecast models of the day, also one of my ex-girlfriends studied Meteorology.

I agree that VSAERO type models are rather complicated but I doubt, and can only doubt, a bad thing in these net discussions is that one cannot really know with whom one was discussing, that Soren used so complicated model in his calculations.
And even rather close approximations can be misleading because deviations can be in opposite directions.

Juha

 

[drgondog]

Hello Drgondog
have You idea why some US a/c had to went fairly long in alphabetics before they became combat ready? P-38, P-39, then a number which never got into combat service in USAAF, excluding P-40 which in any case was based on P-36, come into mind. Spit, Hurri, 109 and 190 seems to became combat ready earlier in their career.

Juha

Good question. In the case of the P-38 (the most extreme example) there were a lot of thorny issues caused by a non traditional approach by Lockheed to achieve very high performance very early in its design gestation.

(Housing turbo superchargers in a boom, intercoolers in leading edge of wing, lack of fuselage filets to improve drag - but finding significant wake turbulence and flutter in the elevator, combined with lack of knowledge regarding compressibility leading to confusion regarding transonic vs flutter effects, combat experience demontrating that an effective fighter in ETO must be able to compete with LW variants in manuever combat rather than serve in interceptor range, etc.

Then combine all of that with the loss of the prototype before WWII started and setting back the flight test back two years because the prototype was hand built (no production contract).

The P-47 and P-51 did not have such great leaps to hurdle and consequently had far fewer significant dash number/model changes. Ditto F6F and F4U.

Is this what you were asking about?

 

[Juha]

Hello Drgondog
Yes. IIRC USAAF considered P-39D as the first combat ready version of the type, and as you wrote P-38 was an extreme case. P-51 was after all first ordered by British and already P-51A was combat ready. IIRC P-47C was the first model used in combat but if we looked its predecessor P-43 one maybe can say, that it also had rather long development history.

USN got its fighters with shorter gestation periods, IIRC and had fewer misses. F5F comes into mind.

Of course one reason might be different procurement policies, and maybe the large number of "duds" simply followed from large number of a/c manufactures and from the fact that late 30s many countries looked for new fighters and US a/c manufactures saw fighter business lucrative and USAAF gave their prototypes P numbers.

Juha