RAF P-39 Caribou

MIflyer · Sep 2, 2018

On 13 April 1940 the British ordered 675 of the Bell Model 14, the export version of the P-39, which they named the Caribou. They specified two .50 cal nose guns, a 20MM Hispano instead of the 37MM cannon of the P-39, four .303 wing guns, and a maximum speed of of 394 mph at rated altitude, plus or minus 4%.

When the first production Caribou was tested at the Bell plant they found the top speed was only 371 mph, which did not meet the contracted speed requirement. Bell then applied some drag reduction modifications, which also lowered the aircraft weight by 200 lb. The airplane then attained 391 mph at rated altitude and the British determined that the contract requirements had been met.

But the drag reduction mods were not applied to the rest of the production run. Testing in England of a standard production Caribou, renamed Airacobra I in 1941, showed a top speed of only 359 mph.

Source: "Images of War: Fighters Under Construction In Word War Two"

fubar57 · Sep 2, 2018

You are about to drive someone insane

buffnut453 · Sep 2, 2018

Hang on a mo...need to grab some popcorn. This is gonna be fun!

fubar57 · Sep 2, 2018

MIflyer · Sep 2, 2018

I'm thinking it was because of the change of the name from Caribou to Airacobra.

tomo pauk · Sep 3, 2018

And some bacon...

Milosh · Sep 3, 2018

That is Canadian bacon.

P-39 Expert · Sep 6, 2018

fubar57 said:
You are about to drive someone insane

Would that be me?

P-39 Expert · Sep 6, 2018

fubar57 said:
You are about to drive someone insane

Would that be me? It's a short trip.

jmcalli2 · Sep 6, 2018

MIflyer said:
On 13 April 1940 the British ordered 675 of the Bell Model 14, the export version of the P-39, which they named the Caribou. They specified two .50 cal nose guns, a 20MM Hispano instead of the 37MM cannon of the P-39, four .303 wing guns, and a maximum speed of of 394 mph at rated altitude, plus or minus 4%.

When the first production Caribou was tested at the Bell plant they found the top speed was only 371 mph, which did not meet the contracted speed requirement. Bell then applied some drag reduction modifications, which also lowered the aircraft weight by 200 lb. The airplane then attained 391 mph at rated altitude and the British determined that the contract requirements had been met.

But the drag reduction mods were not applied to the rest of the production run. Testing in England of a standard production Caribou, renamed Airacobra I in 1941, showed a top speed of only 359 mph.

Source: "Images of War: Fighters Under Construction In Word War Two"

I remember reading that the British screwed up the ballast on the plane, which is why they had a bad time with it. USAAC aircraft had a reputation of a tight turning circle, better than a P-40.
But I'm old and could be wrong.

P-39 Expert · Sep 9, 2018

The April 1940 order for the British was 170 planes at 6000lbs/plane.
Weight increased almost 1800lbs before rollout in March and testing in April resulting in the 371mph speed.
In June the contract was increased to 675 planes. After the weight increases and testing.

Glider · Sep 9, 2018

jmcalli2 said:
I remember reading that the British screwed up the ballast on the plane, which is why they had a bad time with it. USAAC aircraft had a reputation of a tight turning circle, better than a P-40.
But I'm old and could be wrong.

I suspect we were as good as most when it came to the ballast needed.

Shortround6 · Sep 9, 2018

P-39 Expert said:
The April 1940 order for the British was 170 planes at 6000lbs/plane.
Weight increased almost 1800lbs before rollout in March and testing in April resulting in the 371mph speed.
In June the contract was increased to 675 planes. After the weight increases and testing.

There are quite a number of conflicting accounts about the early part of the Aircobra story.

Not at all helped by charts or statements that make no sense.

I would note that according to one listing of P-400 weights on page 120 of "Cobra" by Birch Mathews. which lists empty, useful load and gross weights for model specifications of Feb 1940, Mar '40, May '40, July '40, Jan 1941, June '41 and July '41 the gross weight of the P-400 had hit 7000lbs as of May 1940 and 7350lbs as of July 1940 with armor and self sealing tanks. These would have been estimates as the first YP-39 didn't fly until Sept of 1940.

How accurate this list may be is subject to question as the 1000lb jump in gross weight between March of 1940 (2 machine guns added to each wing?) and May of 1940 (wing gun caliber increased????) seems to need a little more explanation.

P-39 Expert · Sep 9, 2018

Shortround6 said:
There are quite a number of conflicting accounts about the early part of the Aircobra story.

Not at all helped by charts or statements that make no sense.

I would note that according to one listing of P-400 weights on page 120 of "Cobra" by Birch Mathews. which lists empty, useful load and gross weights for model specifications of Feb 1940, Mar '40, May '40, July '40, Jan 1941, June '41 and July '41 the gross weight of the P-400 had hit 7000lbs as of May 1940 and 7350lbs as of July 1940 with armor and self sealing tanks. These would have been estimates as the first YP-39 didn't fly until Sept of 1940.

How accurate this list may be is subject to question as the 1000lb jump in gross weight between March of 1940 (2 machine guns added to each wing?) and May of 1940 (wing gun caliber increased????) seems to need a little more explanation.

If you have an explaination we're all ears.

Shortround6 · Sep 9, 2018

Don't have one.
No accounts speak of increasing the caliber of the wing guns, unless somebody thinks that changing from French 7.5mm to British .303 was going to make any real difference in weight/drag.

I would also note that the when the British ordered the batch of 675 aircraft in 1940 (accounts differ as to month) the first YP-39 had yet to be delivered.
The XP-39B (rebuilt XP-39) never had guns and had completed 28 hours of flight testing when it was written off after a heavy landing Aug 6rh 1940. . Even here accounts differ.
Some say destroyed, others say damaged and written off (months later, Nov/Dec) as uneconomical to repair (with 13 YP-39 already ordered and being built /delivered) . So no, neither the British or Bell had any flight experience whatsoever to base performance numbers off of for an armed, fully equipped plane when either order was placed.

Zipper730 · Sep 10, 2018

Milosh said:
That is Canadian bacon.

Nothing wrong with Canadian bacon

Zipper730 · Sep 10, 2018

MIflyer said:
On 13 April 1940 the British ordered 675 of the Bell Model 14, the export version of the P-39, which they named the Caribou. They specified two .50 cal nose guns, a 20MM Hispano instead of the 37MM cannon of the P-39, four .303 wing guns, and a maximum speed of of 394 mph at rated altitude, plus or minus 4%.

378.2 - 409.8 mph

When the first production Caribou was tested at the Bell plant they found the top speed was only 371 mph, which did not meet the contracted speed requirement. Bell then applied some drag reduction modifications, which also lowered the aircraft weight by 200 lb. The airplane then attained 391 mph at rated altitude and the British determined that the contract requirements had been met.

By the way, what drag modifications were used?

But the drag reduction mods were not applied to the rest of the production run.

Why?

Shortround6 · Sep 10, 2018

It's in another thread, but part of the drag modifications included using plastic wood filler around the canopy/windscreen and using 20 coats of paint, hand sanded between each coat to fill in imperfections. In other words, hundreds of man hours that would not stand up to service use.

MIflyer · Sep 10, 2018

Well, that's a good explanation but I wonder how they did those drag reduction mods and also lowered the weight by 200 lb. I wonder if they replaced metal in spots with the wood filler, although that likely would have increased the weight. Maybe if they used balsa wood fairings that would have helped both weight and drag. I suppose that the drag reduction mods could have been one thing and also deletion of some equipment could have reduced the weight.

Shortround6 · Sep 10, 2018

The aircraft in question, serial # AH 571 also had modified tail surfaces, with a larger vertical fin but smaller rudder and trim tab, slightly larger horizontal stabilizer but smaller elevator and trim tab.
There were also longer shutters/doors to the radiator/oil cooler ducts to increase the velocity of the exiting airflow. the standard 6 exhaust stacks were replaced by 12 stacks angled downwards at 15 degrees as tuft testing had shown that the airflow in this area was angled down 15 degrees from the thrust line.
there were fairings over the .50 cal gun ports, the antenna mast was removed, a single piece (instead of multiple panel) engine cowling and exhaust stack fairing was fabricated.
The modifications done to the gun access doors and to the linkage of the landing gear doors might be excused as correcting an initial teething defect that could be applied to all subsequent production aircraft?
I too, wonder was left out to get the weight down.

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