P-39Q Compared to the Earlier Models

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M

MIflyer

1st Lieutenant
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May 30, 2011
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An article by Ed FurIer, Jr. that ran across in the Fall 1995 issue of the USAF Museum Friends Journal has some interesting data on the P-39Q. It seems that the externally mounted .50 cal guns on the wings caused stick shudder at high speeds due to the to the effects of turbulence on the ailerons. This also made the P-39Q more sensitive to proper rigging of the ailerons. Also, "the Q models shook pretty badly in a dive at about 300 mph while the earlier models could get to 400+ with nary a twitch. Late models also came apart once in a while."

An option was to remove the two wing mounted .50 cal guns and replace them with bomb racks capable of carrying a 500 lb bomb. On the P-63 the ability to carry either a bomb or a drop tank was added, without removing the .50 cal guns.

Small wonder that the Soviets chose to remove the wing mounted .50 cal guns on some Q models. I'd guess that by the time the Q model came out the P-39 was being used mainly for ground attack by the US, so the problems associated with the guns were not as much of a factor.
 

Hello MIflyer,

Is there a place I can get a hold of this article? I have actually been working on getting performance and handling information on the Airacobra for a while and I have some suspicions that some of the "normally accepted" data isn't really correct.

That the Soviets removed the .50 caliber gun pods from the P-39Q is no real indication of any great faults in that installation. They also typically removed the .30 caliber wing guns from earlier Airacobras they received as well.
The long nose P-40 "Tomahawk" models that they used also typically had their .30 caliber wing guns removed.
They were great believers in all centerline armament and IIRC, of operational Soviet designs, only some versions of the MiG fighters carried wing guns.

- Ivan.
 
Okay, here is the article, along with a chart they published with it showing the performance difference between the 3 blade and 4 blade versions.

You know, I recall reading that after the war Bell signed a contract with the USAAF to deliver two of the four bladed props. They were unable to find one, but one of their people noticed that P-39Q's with the four bladed prop were available surplus for something like $500 each, far less than what they were being paid for the props. So they bought two of them, took the props off and delivered them to the USAAF.
 
Great articles, thanks for posting.

Where to start on the .50 caliber wing guns? Who in the world came up with this idea? Compared to the existing .30 caliber wing guns, they weighed 150 pounds more (330# vs 180#) which reduced climb by about 200 feet/minute and the external mounting reduced top speed by 14mph. The increase in firepower was negligible, about the equivalent of part of one .50 caliber MG. Quite an improvement, eh? The AAF should have eliminated all wing guns from all the P-39 models since the 37mm cannon and two .50 caliber fuselage guns were plenty for aerial combat or ground attack.
If these .50 caliber wing pods were such a good idea, why didn't the P-51D get them when armament was increased from 4 guns to six? Sure that's crazy, about like putting them on the P-39Q.
 
I guess that comparing the 25,000 ft time to climb numbers, it looks rather strange.

All other things being equal (and they probably weren't, or at least not exactly) a 4 blade P-39 would be heavier than a 3 blade P-39.

Interesting that all the other airplanes tended to move from narrow cord props to wide cord props - and it appears that the Germans got there first - but the P-39 did not.

I'll admit that I am quite surprised at the 394 mph top speed at 20,000 ft.

I think it was Erich Hartmann that said that the P-39 performed like the 109 at low altitude.

I wonder why the Soviets never replaced the 37MM gun with something better for air combat. In general their aircraft guns were unusually good.

One RAF pilot that met the Soviet forces in northern Norway near the end of the war said they brought with them P-39's still in the shipping crate.
 
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Shortround6 said:
Can you spot the probable misprint in the chart?
Click to expand...

Hello Shortfound6,

Looks to me like the climb rates and time to altitudes don't have much relationship to each other, unless of course the 4 blade was taking a break in between checkpoints to let the 3 blade catch up.

P-39 Expert said:
If these .50 caliber wing pods were such a good idea, why didn't the P-51D get them when armament was increased from 4 guns to six? Sure that's crazy, about like putting them on the P-39Q.
Click to expand...

Hello P-39 Expert,

The P-51D had enough volume to fit the guns inside but as I understand, had to have a small change to its airfoil to be able to mount the guns vertically. I don't think there is the volume inside the Airacobra's wing to do the same.

Slightly off topic, but have you ever tried to calculate the CoG of the Airacobra in empty-equipped condition without any of its disposable loads? (But with a pilot of course.)

- Ivan.
 
What I am questioning is the 4 bladed plane being 0.2 minutes (12 seconds out of about 450) quicker to 20,000ft, a full minute behind at 25,000ft but only 0.2 minutes (12 seconds again) slower to 30,000ft.?
with both planes rated at nearly identical climb rates at all three altitudes.

time to 25,000ft of anywhere from 10.2 to 10.6 would be a lot better fit to the other data and the most simple explanation is a typo of 11.4 instead of 10.4.
 
I think they have the data for the 4 blade and 3 blade reversed.

Otherwise the 4 bladed showed almost no improvement at all, only 50-60 fpm ROC increase at 3 different altitude, and a actual decrease in top speed at several, and no increase in top speed at any altitude.
 


Such a result is possible. If you over prop a plane (put on too big a propeller) you will decrease the performance of the plane as the larger propeller has more drag than a smaller propeller. If the smaller propeller can efficiently transfer all the power available then fitting a larger propeller may be counter productive.
If you car's engine can barely chirp the tires, fitting large. heavy drag racing slicks will not improve performance.
 

Hello Tyrotom,

There is also the possibility that the 4 blade prop is heavier enough to cancel out any gains in low speed thrust in the climb. I also remember being quite surprised that the earlier Thunderbolts with the Toothpick propellers were just a touch faster than ones with Paddle Blade propellers even though the Paddle Blade accelerated faster and climbed better. It wasn't a matter of difference in drag between Razorback and Bubble Tops because they were not different versions being compared.
It could also be a matter of blade profile. Of the Paddle Blades, the Hamilton Standard was slightly faster than the Curtiss Electric.

- Ivan.
 
No one prop is ideal under all conditions. At low altitude a smaller prop can be used in the thicker air but such a prop is far from ideal at high altitudes in thin air.
On the other hand the large prop at low altitude will slow the plane down. The extra drag of the larger airfoils in the thick air take more power to turn.

a lot of people want to clip wings on hypothetical modifications for less drag and yet want to add extra blades or larger blades to props on existing aircraft.
Please remember that some props were approaching (and few exceeding) supersonic speeds at the prop blade tips and since drag goes up with the square of the speed (and power required goes up with the cube) a large diameter multi-blade prop could suck up an awful lot of power if used at the wrong altitude.
 
Wing airfoil was never changed from the B model to the D model Mustang and the myth keeps letting repeated.
 
Hey Ivan1GFP,

re P-51D wing airfoil.

warbird51 is correct, the basic airfoil of the wing did not change. My understanding is the only part of the wing that changed in form was the innermost 3 ft or so (all inboard of the guns) which received a new a new leading edge fillet to the fuselage in order to decrease drag and improve handling in high speed dives. I have never seen any real data on how much this change improved the performance of the P-51D vs the old leading edge fillet, but supposedly it did. (I have also read that the new leading edge fillet was actually in order to accommodate landing gear modifications, but I have never seen anything official. Maybe someone else has some info on this?)

The changes to fit the 6x .50 cal were minor and internal to the wing, with the major change being a rotation of the guns to a more vertical alignment and modification of the feed mechanism/arrangement for greater reliability.


re the 4-blade v 3-blade issue.

During various propeller tests early in the war the UK found that props with wider bases on the blades significantly increased climb rates and acceleration at lower speeds. The Rotol Jablo wooden bladed props used on the Hurricane/SeaHurricane and Spitfire/Seafire variants is a prime example of the type I am referring to, particularly for the 4-blade Seafire models where acceleration for take-off and climb rate were considered more important than speed. They also found that the increased 'solidity' of the wide based blades started to increase drag at speeds of over 350 mph, becoming significant at 400-450 mph, the increase in drag being enough to noticeably reduce the maximum level speed, the acceleration in a dive, and the maximum dive speed.

I have never seen any specific data on the difference in performance for the P-51D cuffed and un-cuffed, but I have read this effect was the reason why the cuffs were later removed from the 4-blade Hamilton-Standard props on the P-51D.

While the P-47 is not the P-51D, I would suspect this range of effects is the reason for difference in performance of the 3-bladed toothpick vs 4-bladed paddle P-47s you mentioned.

And perhaps this is also part of the reason for the reversion from 4-blade to 3-blade on the P-39Q?
 
I understand that the cuffs were removed from most blades postwar because water would get in between the cuff and blade and cause corrosion. The F-51's referbed for Korea had Aeroproducts props installed.

By the way it seems that the USAAF tested the XP-51 (Mustang Mk 1) and recommended that cuffs be added to the prop because adding cuffs had been so successful with the P-39. But other than making a prototype they did not produce that design. And I've never seen a P-39 with cuffs on the prop.
 
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Couldn't agree more about Soviet weapons. I always thought the Volkov-Yartsev VYa-23 would have been a great replacement for the M4 in the P-39. I pondered this for a while, my conclusion for what it is worth - is that in mid 42 the Soviets had the plane configured for their style of fighting and it was very effective. Pokryshkin thought it was the best fighter in the Soviet arsenal til mid 44. The advantages to keeping the M4 were 1) Price is right (free) 2) Ammo price is right (free).
 
Sure, the Russian 23mm cannon would have been a great weapon in the P-39. So would the 20mm M2 that went into the P-38. The P-39D-1 and similar P-400 carried the 20mm M1 but with only 60 rounds or abut 5-6 seconds firing time which was way too short.

Nothing really wrong with the 37mm M4 installed in the P-39 after the exit vents were installed in the nose of the P-39L/M/N/Q allowing more cockpit heating air to flow through the armament bay to keep the cannon from freezing at altitude. Very reliable after that. The Russians considered it more reliable than the 20mm M1.

One strike from a 37mm shell and most all single and twin engine planes were going down. Rate of fire was a little slow but certainly adequate for shooting down bombers or ground targets.

The widely noted looping trajectory was not really a problem. Trajectory drop was only 21" more than the accompanying 50cal machine guns at 400 yards which was just about the outside limit for air to air combat anyway. There were separate triggers on the stick for the cannon and machine guns, so the cannon could be fired by itself if need be for accuracy past 400 yards.

Altogether a pretty solid installation after the initial bugs had been wrung out.

But personally, I would have preferred a rapid fire 20mm or 23mm cannon with at least 100 rounds to go along with the synchronized twin .50s. Delete the peashooter .30 caliber wing guns and their accompanying weight for better climb.
 
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