Subject: Number Thirty-Nine in the Series--Bell P-39 Airacobra (1 of 9)

From: [email-masked] (Joseph F Baugher)
Date: 8 Dec 92 17:21:11 GMT

>From Joseph F Baugher <[email-masked]>

The Bell P-39 Airacobra had its origin in June 1936 when the Buffalo, 
New York-based Bell Aircraft Corporation's design team, headed by 
Robert J. Woods and Harland M. Poyer, began the design of a single 
seat fighter.  They conceived the idea of mounting the engine in 
mid-fuselage, driving the propeller via a ten-foot extension shaft.  
Such an arrangement was not exactly new, having been tried earlier by 
the experimental Westland F.7/30 biplane and by the Dutch Koolhoven 
F.K.55 monoplane.  Among the potential advantages offered by such an 
arrangement was the possibility of superior maneuverability, since the 
weight of the plane would be more nearly concentrated at the center of 
gravity.  In addition, it would facilitate the installation of a heavy 
nose armament, since the armament could be mounted near the 
centerline, minimizing the effects of recoil forces.  It would also 
offer good visibility for the pilot, and would permit the installation 
of a tricycle undercarriage.  

Bell's original proposal was to place the pilot behind the engine, 
forcing the cockpit very far to the rear and making the proposed 
aircraft look a lot like the Curtiss XP-37.  A mockup with this 
configuration was built with this configuration and was given the 
company designation Bell Model 3.  However, the problem of visibility 
over the engine eventually forced Bell engineers to move the pilot 
ahead of the engine, and a revised mockup, given the company 
designation Bell Model 4, was used as the basis of a formal submission 
to the USAAC on May 18, 1937.  The Bell submission promised a top 
speed of 400 mph at 20,000 feet and a gross weight of only 5500 
pounds.  

The USAAC was sufficiently intrigued by the proposal that they ordered 
one prototype on October 7, 1937 under the designation XP-39.  The 
serial number was 38-326.  The powerplant of the XP-39 was the 1150 hp 
Allison V-1710-17 l2-cylinder liquid-cooled Vee which was fitted with 
a B-5 supercharger on the portside of the central fuselage.  A 
radiator/oil cooler scoop was fitted on the other side of the 
fuselage.  Provision was made for two 0.50-inch machine guns in the 
forward fuselage and one 25 mm cannon firing through the propeller 
shaft.  In December 1938, it was decided to replace the 25-mm cannon 
by an even heavier 37-mm Oldsmobile cannon.  In the event, no armament 
was actually fitted to the first prototype.  

The fuselage was of all-metal construction and had an oval cross 
section.  It was built up in two main sections: the forward section 
that included the engine mounts and wing center sections which was 
built around two longitudinal strength member and the cockpit deck 
plate and the semi-monocoque rear section built up of bulkheads and 
stringers.  The tail had all-metal fixed surfaces and fabric-covered 
control surfaces.  The wing panels joined at a point 22 inches out 
from the centerline.  The wing structure had three spars, and the 
Frise-type ailerons were fabric covered.  The inboard wing trailing 
edge carried split flaps.  

The cockpit canopy had six transparent panels, and offered exceptional 
all- round visibility.  An unusual feature of the Airacobra was the 
automobile-type door on each side of the cockpit, which allowed easy 
access by the pilot to the cockpit from either side.  The doors even 
had roll-down windows! 

The engine behind the pilot's seat drove the propeller by means of a 
driveshaft mounted under the pilot's seat.  Early Airacobra pilots 
feared what might happen if the driveshaft were to break loose or were 
to start whipping around inside its mount.  However, in practice there 
were no more problems encountered with this driveshaft than with more 
conventional arrangements.  However, there were problems with the 
complex nose-mounted reduction gear, which caused reliability problems 
and resulted in fairly low serviceability rates as compared with other 
fighters.  The exhaust system lead to six ejector stubs on each side 
of the fuselage (changed to 12 stubs on later models).  

The fuel was carried in tanks totaling 60 gallons in capacity in the 
wing outer panels.  There was a reserve tank of 30 gallons in the left 
wing.  

The XP-39 was completed at Bell's Buffalo plant and shipped by truck 
to Wright Field in Ohio.  It was reassembled there and flown for the 
first time on April 6, 1938.  The performance was excellent, the 
prototype reaching a speed of 390 mph at 20,000 feet.  An altitude of 
20,000 feet could be attained in 5 minutes, quite impressive climbing 
performance for the time.  Service ceiling was 32,000 feet.  Weights 
were 3995 pounds empty, 5550 pounds gross, and 6304 pounds maximum 
takeoff.  The USAAC was quite impressed with the performance, perhaps 
ignoring the fact that the XP-39 carried no military equipment or 
armament and was thus much lighter that that which could be 
anticipated for production models.  

The initial XP-39 tests went quite well, and the only problem that was 
encountered being some engine overheating difficulties.  At first, it 
was thought that the overheating problems might be due to bad 
ventilation, and the left-hand supercharger and the right-hand 
radiator intakes and exhausts were both enlarged.  However, this did 
not cure the problem, and it was found later that the problem was 
easily cured by a simple change in the structure of the oil system.  
With this change, the XP-39 was accepted for production with an 
initial order for twelve service-test YP-39s (Bell Model 12) and one 
YP-39A in April 1939.  Serials were 40-027/039.  The YP-39A (40-039) 
was to have been powered by a high-altitude V-1710-31 engine of 1150 
hp.  

In the meantime, the XP-39 underwent a series of full-scale 
wind-tunnel tests in NACA's wind tunnel at Langley Field, Virginia.  
After the tests, the XP-39 was returned to Buffalo for revisions.  The 
rebuilt XP-39 emerged as the XP-39B.  Most of the changes were 
improvements in the streamlining of the airframe.  The cockpit canopy 
was changed to a longer and lower shape.  Changes were made to the 
wheel doors.  The oil cooler and radiator intakes were moved from the 
fuselage right side to the wing roots.  The wing span was decreased 
from 35 feet 10 inches to 34 feet, and length was increased from 28 
feet 8 inches to 29 feet 9 inches.  

The most serious change, however, was the elimination of the 
supercharger.  This change was a result of a shift in philosophy on 
the part of the USAAC.  The USAAC believed that the widths of the 
Atlantic and Pacific Oceans made the USA virtually immune from 
high-altitude attack by enemy bombers.  Therefore, the development of 
high-altitude interceptors was curtailed in favor of strike fighters 
optimized for low-level close support.  The 1150 hp V-1710-17 of the 
XP-39 was replaced by a non-supercharged V-1710-39 engine rated at an 
altitude of 13,300 feet.  The carburetor air intake was mounted in a 
dorsal position just behind the cockpit, where it was to remain 
throughout the Airacobra production run.  

The XP-39B resumed flight trials on November 25, 1939.  Empty weight 
had grown from from 3995 lbs to 4530 lbs, and normal gross weight was 
up to 5834 pounds from 5550 pounds, and the aircraft STILL didn't have 
any armament.  The removal of the supercharger was to have fateful 
consequences for the future of the Airacobra.  Although the Allison 
engine was more reliable and more easily service when the supercharger 
was eliminated, the engine only performed well at low and medium 
altitudes and lost power quite rapidly at altitudes over 15,000 feet.  
Even in spite of the improved streamlining, the XP-39B suffered a 
severe degradation in high-altitude performance.  Maximum speed fell 
from 390 mph at 20,000 feet to 375 mph at 15,000 feet, and it now took 
7.5 minutes to reach 20,000 feet rather than five minutes.  However, 
there was an increase in low-altitude maneuverability because of the 
reduced wing span, and the decrease in low-altitude performance was 
only marginal.  

The USAAC was satisfied with the low-altitude performance of the 
Airacobra, and directed that the twelve YP-39s be completed without 
superchargers.  It was decided to embody the changes made in the 
XP-39B in the service test machines.  The YP-39A was originally to 
have been delivered with the high-altitude V-1710-31 engine, but the 
change in USAAC philosophy caused this plan to be abandoned and this 
aircraft was delivered as a standard YP-39.  

The first YP-39 (40-027) was flown on September 13, 1940 with the 1090 
hp V-1710-37 engine.  It differed from the XP-39B primarily in having 
a larger vertical tail.  Full armament was fitted, comprising the 37 
mm cannon with 15 rounds, a pair of 0.5-inch machine guns with 300 
rounds per gun, and two 0.30-inch machine guns with 500 rounds per 
gun.  All of these guns were mounted in the nose.  Some armor 
protection was provided for the pilot.  Empty and normal loaded 
weights rose to 5042 pounds and 7000 pounds, respectively.  In 
comparison, the XP-39 prototype had a normal loaded weight of only 
5550 pounds.  Consequently, the performance of the YP-39 dropped to a 
maximum speed of 368 mph at 15,000 feet.  An altitude of 20,000 feet 
could be attained in 7.3 minutes.  Service ceiling was 33,300 feet.  

In spite of the degradation in performance, the USAAC was generally 
pleased with the Airacobra, and an initial order for 80 production 
examples (Bell Model 13) was issued on August 10, 1939.  Serials were 
40-2971/3050.  For some reason, the USAAC allocated the new 
designation of P-45 to these machines, even though they were almost 
identical to the YP-39 service test aircraft.  However, in the 
political climate of 1940, it was virtually impossible for the USAAC 
to acquire any new aircraft.  But it could order more examples of an 
already-existing model.  Consequently, the designation of the 
Airacobra was changed to P-39C prior to the delivery of the first 
aircraft.  

The first P-39C (Ser No 40-2971) flew in January 1941.  In the event, 
only twenty Airacobras were actually completed to C-standards--serial 
numbers 40-2971/2990.  The rest, serial numbers 40-2991/3050, were 
completed as P-39Ds.  

The P-39C was powered by a 1150 hp Allison V-1710-35 engine.  Weights 
were 5070 pounds empty, 7075 pounds gross (combat weight), and 7300 
pounds maximum takeoff.  Maximum speed was 379 mph at 13,000 feet.  An 
altitude of 12,000 feet could be reached in 3.9 minutes.  Service 
ceiling was 33,200 feet.  Armament was one 37-mm cannon, two 0.50-inch 
and two 0.30-inch machine guns, all in the nose.  

Combat reports coming back from Europe led the USAAC to conclude that 
the P-39C was inadequately armed and armored for its intended 
ground-support role.  On September 13, 1940, 344 P-39Ds were ordered.  
The serials were 41-6722/7115.  These differed from the P-39C in 
having four wing-mounted 0.30-inch machine guns with 1000 rounds per 
gun, two fuselage-mounted 0.50-inch machine guns with 200 rounds per 
gun, plus the 37-mm cannon with 30 rounds.  Bulletproof windshield 
panels were added, and some armor protection for the pilot was 
provided.  Self-sealing fuel tanks were introduced, which reduced 
internal fuel capacity from 141.5 Imp.  gal.  to 100 Imp.  gal.  This 
internal fuel could be supplemented by a 72.4 Imp.  gal drop tank.  In 
addition, a very small dorsal fin was added just ahead of the rudder.  
The bulletproof windshield and armor protection added 245 pounds to 
the weight of the aircraft, causing the climb and altitude performance 
to suffer.  

The engine of the P-39D was the same as that of the P-39C, a 1150 hp 
Allison V-1710-35.  Maximum speed of the P-39D was 309 mph at sea 
level, 335 mph at 5000 feet, 355 mph at 10, 000 feet, 368 mph at 
12,000 feet, and 360 mph at 15,000 feet.  An altitude of 5000 feet 
could be reached in 1.9 minutes.  It took 5.7 minutes to reach an 
altitude of 15,000 feet and 9.1 minutes to reach 20,000 feet.  Service 
ceiling was 32,100 feet.  Maximum range (clean) was 600 miles at 
10,000 feet at 231 mph.  Range with one 145.7 Imp gal drop tank was 
1100 miles at 196 mph.  Weights were 5462 pounds empty, 7500 pounds 
gross, and 8200 pounds maximum takeoff.  Dimensions were wingspan 34 
feet 0 inches, length 30 feet 2 inches, height 11 feet 10 inches, and 
wing area 213 square feet.  

On the day after the P-39D order, the initial contract for the P-39C 
was modified, and the 21st and subsequent machines on the P-39C order 
(serials 40-2991/3050) were ordered completed as P-39Ds.  

The first P-39D Airacobras entered service with the USAAC in February 
1941.  The P-39D (along with the P-400) was the first to see combat in 
US service.  There were a great many weaknesses in the Airacobra, 
apart from the general problem of poor high-altitude performance.  
Among these were the lack of gun heaters which caused the guns to 
freeze up and jam at altitudes over 25,000 feet, the lack of hydraulic 
chargers which made it difficult to charge the guns in the air, and 
the forward gear box just behind the propeller which had a tendency to 
throw oil.  

Sources:

  War Planes of the Second World War, Fighters, Volume Four, William Green,
  Doubleday, 1964.

  The American Fighter, Enzo Anguluci and Peter Bowers, Orion Books, 1987.

  United States Military Aircraft since 1909, Gordon Swanborough and Peter
  M. Bowers, Smithsonian Institution Press, 1989.

  P-39 Airacobra in Action, Ernie McDowell, Squadron/Signal Publications, 
  1980

  The Calamitous 'Cobra, Air Enthusiast, August 1971.

Joe Baugher       AT&T Bell Laboratories    2000 North Naperville Road 
		  Naperville, Illinois 60566-7033.   (708) 713-4548