SHOULD the P39 have been able to handle the Zero? Was it training or performance?

Shortround6 · Apr 3, 2018

DarrenW said:
Difference in speed was 12.5 mph for this particular aircraft, with and without gun pods. But at least we can see that the speed of the P-39Q WITHOUT the wing guns was 13.5 mph SLOWER at the critical altitude of our one and only P-39N test aircraft . But to be fair the P-39Q was ballasted for the additional weight of the guns and ammo, and it weighed almost 600 pounds more than the P-39N at take-off. What do you gentlemen make of this?

What I get is that the tests were done (or at least written) about 2 months apart, in potentially different weather and with an engine that was going south compared to the first test, not to mention any other wear and tear on the aircraft.

While interesting any data has to be considered in that light.

I do like the note at the bottom where they say the XP-63A prototype showed a 110fpm difference in climb with and without pods and "Such a figure is close to the limites of climb determination accuracy".

Shortround6 · Apr 3, 2018

drgondog said:
If a Stuka initiates a turn too early, he gives the closing fighter a pretty easy (comparably speaking) deflection shot at fuel/pilot/engine. There was a reason that they converted to FW 190 as fast as they could.

I believe a fair number of the JU-87s were operating in a night attack role in the last few years of the war, JU-87Gs and a few others excepted which can skew the results as to losses. The FW 190s taking over the day attack role as you say.

drgondog · Apr 3, 2018

Shortround6 said:
The weight alone has a negligible impact on speed. It is the drag associated with the weight that causes the speed reduction.
For instance if you were add internal ballast with NO exterior changes the only increase in drag would be a very marginal shift in the angle of attack of the wing and the induced drag, since forces on the plane (both drag and lift) go up with the square of the speed adding a few hundred pounds to an aircraft does very little to the speed.
However, pods, protruding gun barrels, gun ports/troughs, cartridge ejection slots can cause a much bigger disruption of airflow and increased drag.

When climbing there is a bit of a double whammy. Since climb speed is relatively low, in fact it is a cross/blend between lowest speed with good controllability and the lowest drag speed ( lowest drag caused by lift and lowest drag caused by speed/shape). I am not explaining that well.
View attachment 488300
Picture worth 1000 words. Climb is done near the minimum drag speed, an increase in weight is going to cause the lift induced drag to rise at low speed in greater proportion than at high speed. This leaves less power to perform the climb with and when climbing you are lifting every pound.

Hope that makes sense?

Hi SR - good chart to discuss L/D and level flight characteristics.

Climbing (and turning) introduce Pressure Drag in a big way - mostly attributable to AOA caused Pressure Drag. In addition Cooling Drag escalated during climb and turn.

Total Drag = Parasite Drag (level flight components of Wing, fuselage, empennage, carb and Radiator ducts, gaps in control surfaces and sheet paneling, friction, exhaust stacks - stuff associated with basic airframe) + Delta Parasite Drag (more level flight components such as cannon/gun ports, radio mast, bomb racks, etc) + Vortex/Pressure Drag due to Angle of Attack = Total Drag (almost done).before Compressibility correction and addition of Induced Drag.

That said a full blown set of forces on the airframe have not yet been calculated. In late 1930s the concept of expressing practical climb, range and turn estimates required that many items be further categorized into attributes that contribute to Power Available and Power Required. This is where the esoteric calcs including equations for Ram Air, Prop design/configuration/efficiencies and Exhaust Thrust all contributed to Power Available as a function of altitude, velocity, throttle/manifold pressure, fuel type and fuel to air ratios.

.In WWII, at least for US airframe designers of piston engine fighters, the calculations for the Cooling drag of both cooling system and carb intake are derived into a delta Power Required.

For increments to Power Required, calcs for Cooling Drag factors for both high speed and low speed are derived from Wind Tunnel tests and also the increased drag due to the Prop Vortex (all fuselage, approx. 25-30% of wing) for the higher velocity than free stream and immersed in the vortex.

What emerges is the expression Thrust = Drag reorganized into Power Available and Power Required. In balance, there is equilibrium at top speed, top rate of climb, best turn rate, best cruise speed, etc. Otherwise, there is acceleration until equilibrium is once again attained

Peter Gunn · Apr 3, 2018

All I can say is that after reading through this thread, I have a better understanding of the Dunning–Kruger effect...

fubar57 · Apr 3, 2018

John Cleese explains this effect on Youtube

pbehn · Apr 3, 2018

Peter Gunn said:
All I can say is that after reading through this thread, I have a better understanding of the Dunning–Kruger effect...

Is that like the Bauschinger effect or more akin to the Jule-Thomson effect?

P-39 Expert · Apr 3, 2018

pbehn said:
My date of 25 Dec 1943 was itself based on how long a new type aircraft which started to be produced in USA in Dec 1942 would actually be expected to be operational in UK. Shipping and training take much longer than you would think.

The first Thunderbolt combat mission is listed at 4/30/43 and was also the first 8thAF bomber mission with at least 100 planes.

P-39 Expert · Apr 3, 2018

pbehn said:
Since most P-39s sent to Russia were the N and Q versions, is it possible these were quite well sorted as far as engine and CoG issues went while the earlier versions just shouldn't have been put into service?

Absolutely. After the P-400s were sent to Russia from the British, they began getting P-39Ns and Qs in ealy 1943. Almost all airplanes get better during production (initial bugs get sorted out etc) and the N was the ultimate version performance wise. The Q was just an N with gondola wing guns which hurt speed and climb. Since the Russians removed the wing guns on both they were basically the same plane.

pbehn · Apr 3, 2018

P-39 Expert said:
Absolutely. After the P-400s were sent to Russia from the British, they began getting P-39Ns and Qs in ealy 1943. Almost all airplanes get better during production (initial bugs get sorted out etc) and the N was the ultimate version performance wise. The Q was just an N with gondola wing guns which hurt speed and climb. Since the Russians removed the wing guns on both they were basically the same plane.

The planes that the British got were not sorted then? especially in the area of tail shape/surface area and CoG

The planes delivered to England for test all had a modified non standard rudder.
Tested on the second British Airacobra (AH571) was a revised rudder of more angular shape and less area. Although the aircraft was delivered to England in this form, this rudder was not adopted as standard. A very small dorsal fin just ahead of the rudder became a standard feature of the RAF Airacobras and was also a distinguishing feature of the American P-39D and subsequent versions

CoG was adjusted again even on the P-39N
The 500 P-39Ns were followed by 900 P-39N-1s (Model 26C). These differed only in some minor internal changes which altered the location of the center of gravity.

Schweik · Apr 3, 2018

XBe02Drvr said:
Didn't VVS pre-war training feature a deficit of all kinds of practice, not just gunnery? Didn't the Japanese in Manchuria in 1938 point out some pilot proficiency deficits in the VVS?
Cheers,
Wes

I would say this was the case for most Allied pilots period. American, British, Australian, New Zealand, French, South African and Soviet. Gunnery training seemed to be rare, in fact it was very common for pilots right out of flight school to have barely enough training on their new (much higher performance) fighters to take off and land (and quite often, clearly not enough because there were a huge number of takeoff and landing accidents in the early years of the war especially in 1940-41)

S

P-39 Expert · Apr 3, 2018

Shortround6 said:
complete and utter bollocks.

Just look at the number of books, articles and websites that claim the XP-39 flew at 390-400mph and climbed to 20,000ft in 5 minutes before the NACA ruined it by taking out the turbo charger. The P-39 may have had more complete trash written about it than practically any other fighter if we count the pages of wink and bandwidth. The XP-39 never flew at full power due to a feared drive shaft problem before the turbo was taken out. A new, heavier drive shaft was installed in all later models. What is interesting is that nobody can point to WHEN this supposed flight of 390mph and climb to 20,000f took place, no date is ever given unlike many other planes where early flights and achievements are given. Also no pilots name is mentioned. Some books/accounts even go so far as to claim it was done on the first flight. Which, given the chronic overheating problems the XP-39 had is hardly creditiable let alone the sometimes mentioned flight duration of 20 minutes for the first flight.
Test pilot took-off retracted landing gear, immediately went into a full power climb to 20,000ft, leveled off, accelerated to 390mph, slowed down, descended to airfield, lowered landing gear and landed, all in 20 minutes and in an airplane that had never flown before??????
However this little "fact" did not stop Bell from advertising/ marketing the P-39 as a 400mph fighter. The "fact" that the XP-39 was over 500lbs overweight (about 10%) when delivered to Wright field was exactly pointed out in advertising brochures either. This is for the unarmed prototype with no guns. Bell claimed the performance figures were for a 5500lb gross aircraft. When weighed at Wright field it went 6104lbs. Which makes nonsense out of a lot of the later development weight figures.
The XP-39B (original XP-39 rebuilt) doesn't fly until Nov 25th 1939. French (desperate) are already trying to buy it. In fact by April of 1940 the French have given Bell a 2 million dollar cash advance.
Now please note that the first YP-39 (2nd P-39 airframe) to fly does so on Sept 13th 1940 and the first P-400 airframe flies in April of 1941 almost a year after being ordered with contracts signed. This is rather late in the game for the British to start specifying cast iron coal fired cabin heaters or whatever they did to run the weight up to get out of the contract.
Somethings on the prototypes just did not work very well. Like the first plane had ejection slots in the nose for the machine gun cartridges but these tended to be ingested by the radiator intakes in the wing so the spent cartridges were collected in bins in the weapons bay.
Bell in 1939 and the first half of 1940 ws promoting the P-39 as a 400mph fighter and that is what the French and British thought they were buying. However the empty weight of the fighter grew by 1026lbs (although this may be debatable, this was based off the 5,849lb gross of the XP-39B which may have been under stated)
We next have a bit of shall we say "trickery" in which the first P-400 was tested in late April 1941 by both a Bell company pilot and Wing Commander Adams and speeds of 391mph at 14,350 ft are recorded (after corrections). However this aircraft differed from planes on the production line by.
1.A considerably modified fin/rudder and horizontal stabilizer/elevator assemblies. The moving parts (fabric covered) were made smaller and the fixed parts (metal covered ) made larger but the finished assemblies were a bit smaller over all than standard.
2. Different fillets were used (or eliminated) on the fixed tail surfaces.
3. Plastic wood was used around all the edges of all cockpit frame work and sanded smooth.
4. gun access doors were covered over with 0.064 sheet aluminum to prevent partial opening during flight
5. Stronger landing gear linkage installed to prevent landing gear deflecting up to two in in flight
6. Longer outlet shutters of restricted area installed on oil cooler and radiator ducts to improve local airflow.
7. Standard 6 port exhausts replaced by 12 port exhaust angled down 15 degrees as tuft testing showed local airflow to be 15 degrees below the thrust line.
8. .50 cal gun ports cleaned up
9. removal of antenna mast
10. one piece engine cover and exhaust stack fairing.
11. unspecified other modifications.
12. 20 coats of Dupont grey primer sanded between coats.
13. Standard British camouflage applied but lightly sanded to remove seams from camouflage templates.

The contract speed was 394mph at rated altitude and thus the test flights were within 1 % . The contract allowed a 4% tolerance.
However while some of the modifications could be incorporated in production aircraft some could not (like filling the the cockpit structure and the 20 coats of sanded primer) and the production aircraft fell well below this test aircraft.
A Bell company test report dated Aug 18th 1941 is supposed to show that aircraft AH579 was 20mph slower than AH 571 (the modified aircraft) had a cruise speed 24mph slower, a lower critical altitude by 250 ft and took 2 minutes and 16 seconds longer to get to 26,230 ft.

This is from "Cobra" Bell Aircraft Corporation 1934-1946 by Brich Mathews.

Now are the British to be blamed for specifying too much "stuff" in order to get out of the contract or did Bell promise way more than it could deliver?

pbehn · Apr 3, 2018

P-39 Expert said:
The first Thunderbolt combat mission is listed at 4/30/43 and was also the first 8thAF bomber mission with at least 100 planes.

Exactly my point, when did they first roll out of the factory in USA?

P-39 Expert · Apr 3, 2018

Glider said:
I do get a little annoyed when you keep saying the British tried to get out of the contract. Have you got any evidence to support that, if so put it up or stop saying it.

Well, they DID get out of the contract with Bell and Lockheed. Whether it was Pearl Harbor causing a need for planes by the AAF or just refusal to accept, the British did not buy those planes. Lockheed just refitted the undelivered British order with turbos and handed propellers and they became P-38Fs.

P-39 Expert · Apr 3, 2018

pbehn said:
The planes that the British got were not sorted then? especially in the area of tail shape/surface area and CoG

The planes delivered to England for test all had a modified non standard rudder.
Tested on the second British Airacobra (AH571) was a revised rudder of more angular shape and less area. Although the aircraft was delivered to England in this form, this rudder was not adopted as standard. A very small dorsal fin just ahead of the rudder became a standard feature of the RAF Airacobras and was also a distinguishing feature of the American P-39D and subsequent versions

CoG was adjusted again even on the P-39N
The 500 P-39Ns were followed by 900 P-39N-1s (Model 26C). These differed only in some minor internal changes which altered the location of the center of gravity.

Only one plane had the modified rudder etc. (SN AH-571) for the performance test.

P-39 Expert · Apr 3, 2018

pbehn said:
Exactly my point, when did they first roll out of the factory in USA?

The prototype rolled out in Dec '41.

pbehn · Apr 3, 2018

P-39 Expert said:
Well, they DID get out of the contract with Bell and Lockheed. Whether it was Pearl Harbor causing a need for planes by the AAF or just refusal to accept, the British did not buy those planes. Lockheed just refitted the undelivered British order with turbos and handed propellers and they became P-38Fs.

Can you stop banging on about this, at the time the P39s were on the way to UK, the UK had loaded 40 Hurricane MkIIs and 550 mechanics/pilots to defend Murmansk. In total 3,000 Hurricanes and 1,400 Spitfires were sent. British and Canadian tanks shipped to Russia totalled over 5,000 and were first used in November 1941 on the Volga. The contract for 675 fighters is absolutely utterly insignificant in terms of cost. The significance really was they were available and crated up and of far more use to the USA and the Russians than they were to the British.

pbehn · Apr 3, 2018

P-39 Expert said:
Only one plane had the modified rudder etc. (SN AH-571) for the performance test.

So the aircraft on the performance test was unique?

Shortround6 · Apr 3, 2018

pbehn said:
So the aircraft on the performance test was unique?

One might say that

See post 961 on page 49 for a LIST of differences between the test aircraft and a noraml production aircraft.

Also consider that fabric covered control surfaces "ballooned" at high speed and created drag so the smaller you could make them the less drag you had.

Peter Gunn · Apr 3, 2018

P-39 Expert said:
*SNIP*

Lockheed just refitted the undelivered British order with turbos and handed propellers and they became P-38Fs.

Or you know... they didn't and they actually were taken by the U.S.A.A.F. as the P-322-I and P322-II as advanced trainers ( 143 according to Wiki ). The rest of the French/RAF order were completed as P-38F's ( 420+/- ). Semantics perhaps but still...

pbehn · Apr 3, 2018

Shortround6 said:
One might say that

See post 961 on page 49 for a LIST of differences between the test aircraft and a noraml production aircraft.

Also consider that fabric covered control surfaces "ballooned" at high speed and created drag so the smaller you could make them the less drag you had.

I saw it (good post), now imagine you are the fresh faced bunch of pilots presented with your new steeds and one has a different tail to the others? Imagine you are the clients contract manager and you notice that this cut down version was the one tested?

SHOULD the P39 have been able to handle the Zero? Was it training or performance?

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