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Thread: Airacobra

  1. #1


    This thread will describe the construction of a Bell P-39 Airacobra for Combat Flight Simulator.

    Over the last several months, I have been heavily involved in discussions about the design issues, performance, and handling issues that resulted from the location of the engine behind the pilot instead of its usual location at the nose to drive a tractor propeller. Some pilots loved the 'Cobra. Some hated it and thought it was the worst production fighter built by the United States.

    I have always thought the Airacobra was a fascinating design and this will be my attempt to build one.
    As with most projects, one has to decide upon a version to build. The Airacobra was a fighter, so the version to build should be one that saw service as a principal front line fighter.

    My own choice to build would be one that most represented the type's characteristics when it served as a fighter with US forces early in the war.
    To me this means it should be the "Flying Cannon".
    It should carry a 37 mm M4 Cannon instead of the very common (for export) 20 mm Hispano.
    It should carry the 4 x .30 Caliber (or .303) MG in the wings.

    As I start this project, I have not yet determined the exact model of the Airacobra I intend to build.
    In the next post, I will explain why the choice is not so easy.

    - Ivan.

  2. #2
    My selection of a subject to model is usually based on what I believe to be the "best performing" of the series that actually served a meaningful role in combat. For the Airacobra, its time as a front line fighter was fairly short. Its first major production version, P-39C, first flew in early 1941 and production ended in July 1944 with the P-39Q-30-BE. After 1943, when more advanced and better performing fighters became available, the P-39 served mostly as Lend-Lease equipment to allies who were desperate for equipment or served as fighter trainers in the United States or as ground attack aircraft.

    The P-39C was the first production model but carried no significant armour.
    Its armament was all mounted in the nose and consisted of
    1 x 37 mm cannon with 15 (!) rounds
    2 x .50 Caliber MG with 200 rounds per gun
    2 x .30 Caliber MG with (based on implied evidence) 600 rounds per gun.
    Engine was the Allison V-1710-35
    170 Gallons of fuel were carried
    Speed of 379 MPH and climb rate of 3000 feet/minute were very good for the time but without armour, it was unsuitable for combat.

    The P-39D was the first combat-ready model and seems to be a popular choice to model.
    It came in three models (five if the export and Lend-Lease models are also counted).
    The first was the P-39D-BE with the following armament:
    1 x 37 mm motor cannon with 30 rounds
    2 x .50 Caliber cowl mounted MG with 200 rounds per gun
    4 x .30 Caliber MG mounted in the wings with 1000 rounds per gun
    Engine was the same V-1710-35 as in the P-39C
    Armour and bullet proof glass were added which resulted in greatly decreased performance.
    Speed was now down to 368 MPH and some reference put the number much lower.

    The export Airacobra Mk.I and P-400 followed.
    Both substituted a 20 mm Hispano cannon with 60 rounds for the 37 mm gun.
    Export models for the British also substituted .303 caliber MG for the US .30 caliber.
    British tests of their new fighters gave only 355 MPH which was pretty far from the almost 400 MPH that they were expecting.

    The P-39D-1-BE retained the same 20 mm cannon of the French / British contract export aircraft and also introduced a rounded fillet at the root of the Fin. Many earlier aircraft were retrofitted with this fillet.

    The P-39D-2-BE also retained the 20 mm Hispano cannon but introduced a new engine: the V-1710-63 with a different reduction gear ratio of 2:1. (Gear ratio was 1.8:1 for the V-1710-35.) The new engine gave a significantly higher (official) Take-Off rating of 1325 HP from the earlier 1150 HP along with a much higher WEP rating at low altitude in some references. Other ratings were identical.

    The P-39D-2-BE would have been my choice of the version to build except that they were all armed with 20 mm cannon.
    There are some references which suggest that there were exceptions, but evidence is very hard to verify.

    The P-39E a series of test aircraft with different Wing and Tail configurations. This designation was not used for a production model.

    The P-39F was created when production of Curtiss Electric propellers was insufficient to meet the demand. The only significant difference between the P-39F and P-39D was that the P-39F used a propeller by Aeroproducts.
    Engine was V-1710-35.

    The P-39G,H, and I designations were not used.

    The P-39J series was yet another engine substitution. The V-1710-59 had automatic boost controls but gave essentially the same output as the V-1710-35.

    The P-39K deliveries began in July 1942. Its engine was the V-1710-63 as used in the P-39D-2-BE.
    The P-39L returned to a Curtiss Electric propeller and added a pair of air scoops near the nose and switch to a redesigned nose wheel.
    All of the V-1710-63's had a very high WEP rating for use at low altitudes (2500 feet) but this is not reflected in the Specific Engine Flight Chart.

    The P-39M used the V-1710-83 engine with a higher altitude (9500 feet) WEP rating. Deliveries began in November 1942 which was nearing the end of the useful life of the Airacobra as a front line fighter.

    The P-39N used the V-1710-85 engine with an even higher altitude (9700 feet) WEP rating. Later versions of the P-39N reduced the internal fuel capacity from 120 Gallons to 87 Gallons in an effort to reduce gross weight of the aircraft.

    The P-39Q replaced the .30 Caliber MG in the wings with 2 x .50 Caliber in pods under the wings.
    From weight calculations, each gun had 375 rounds.


  3. #3

    Variations on a Theme

    As can be seen from the descriptions of changes,
    The change from P-39C to P-39D was fairly significant.
    The changes from the P-39D to P-39N were very minor equipment changes that were normally not worthy of a change in letter designation so essentially all of those models were just minor variants of the basic P-39D.

    The P-39D would normally be considered the ideal build subject except that none of the 37 mm cannon armed versions were produced with a fin fillet which I consider to be a distinctive recognition feature of this aircraft.
    With this in mind, the model I intend to build would most closely resemble the P-39F or a retrofitted P-39D. With very minor AIR file modifications and nearly invisible Propeller Blade changes, it would easily become the P-39K low altitude fighter or just about any subsequent version.

    - Ivan.

  4. #4
    Hello Ivan,
    Ive been so busy for two days getting the hardware sorted out, that I missed this new thread.
    You have certainly put together a good summary of the different models.

    I agree that choosing which version to build is a difficult decision. I was led to believe you would
    go for the -Q model with the 1200 Hp engine, but that is not so clear now because of the type of
    nose cannon it had.

    I was perhaps thinking of building the P-39D-D2, because of the "nicer" 1325 hp engine, but I would like to build a different one from the one you are planning, so Ill wait a bit until you decide. I could easily go for the -D-D1 with the standard 1150 Hp engine, no problem!

    We shall wait and see!

    Incidentally, did you find any information relative to the top speed of the 1325 Hp powered models like the -D-D2 or the -K? I found one source stating it was 377 mph, but didnt specify if it was 6400 or 10000 ft. Another source said it was 20 mph faster than the -N or -Q versions, but that would make it well over 400 Mph, so Id say it was impossible.

    I found a .pdf document with interesting specs on the "E" series of the V-1710 engine. The -35, -63, -83, -85 and -93 engines are listed.
    Update: Correction: It also includes details on MP settings (page 137). I suppose you have seen this document too.

    Last edited by aleatorylamp; June 21st, 2018 at 20:05.
    "Why make it simple if you can also make it complicated?"

  5. #5
    Hello Aleatorylamp,

    The kinds of information I am going for are not so much about maximum speeds.
    You will find that the maximum speed of the P-39 really didn't change that much from the first production model to the last.

    The improvements of the V-1710-63 engine over the V-1710-35 were such that the engine would tolerate a bit more boost at low altitude but the problem was that neither version had enough supercharger to make a difference at higher altitudes where maximum speed was reached.
    Later versions sacrificed some low level power to move the engine's critical altitude higher where air was less dense and the aeroplane would reach higher speeds.
    The same kind of thing was done with the P-40 series between the P-40K and P-40N. The P-40K was faster at low altitudes but that wasn't where the fights were happening.

    - Ivan.

  6. #6
    Hello Ivan,
    OK! Thanks for your clarifications.
    "Why make it simple if you can also make it complicated?"

  7. #7

    Airacobra's Design Priorities

    In an article in the magazine "Aviation" in July 1941, Larry Bell listed the design objectives and priorities for the Airacobra.
    The priorities were "first, firepower; second, good pilot visibility; and third, good landing and ground characteristics. And these engineering objectives were to be over and beyond ordinary fighter characteristics such as speed, maneuverability, and pilot protection....".
    All of the primary / fundamental characteristics were met but perhaps more attention should have been paid to the "ordinary" characteristics.

    The 37 mm Motor Cannon was seen as an essential part of the design which is why I believe it is necessary to represent this in a CFS project.
    For a single cannon weighing 238 pounds, the only place to mount it would be to fire through the propeller hub. The Allison engine unlike some other V-12 engines was not suited to the installation of a cannon firing between the cylinder banks which led to a remote installation with extension shaft to drive a tractor propeller.

    The Airacobra was designed as a small and light-weight interceptor with a Turbo-supercharged engine. Design speed was over 400 MPH at 20,000 feet.
    In actual tests, 390 MPH was achieved but in a lightly loaded prototype with no military equipment and this was after aerodynamic improvements with NACA assistance. It was pretty obvious that THIS aeroplane was never going to meet performance requirements with any military equipment. In addition, the Turbocharger was not very well developed and the Intercooler was not adequate to allow sustained full power operation.
    The requirements were then changed to 400 MPH at 13,500 feet which seemed achievable with the technology that was available.


  8. #8
    Attachments for the last message.

    These projections turned out to be overly optimistic for the production aircraft.

    - Ivan.
    Attached Thumbnails Attached Thumbnails NACA_Original_XP-39.jpg   NACA_XP-39_Small.jpg  

  9. #9

    Design Features of the Airacobra

    The main reason I like the Airacobra is because of its streamlined appearance.
    "It looks like it is going 400 MPH even when it is parked."

    Much of this appearance was due to the sleek nose which was possible because of the mid-engine location.
    Other obvious aspects of its design were the heavy cannon firing through the Propeller Shaft and a Tricycle Landing Gear. The ground handling and visibility were good enough that pilots would talk about rolling down the side windows and taking an Airacobra on an extended trip on the local highways.

    There were a few more features and consequences of the design of the Airacobra that were not so obvious.

    The Airacobra used a single Coolant Radiator flanked by two Oil Coolers that were entirely enclosed within the Fuselage between the wing roots. There were two intakes at each Wing Root. The inboard one on each side fed the Coolant (Prestone) Radiator. The outboard Intake on each side fed the Oil Cooler for that side.
    This was a very streamlined setup with no protruding intakes but also was not very efficient at cooling.
    The Airacobra would tend to overheat on the ground.

    One not very obvious feature of the Airacobra is that it used a NACA 0015 Airfoil at the Wing Root and a NACA 23009 Airfoil at the Wing Tip.
    The angle of incidence was a constant 2 degrees along the Wing.
    The choice of a symmetric 0015 airfoil is rather odd for the circumstances because the airfoil depends entirely on a positive Angle of Attack for Lift. It is a more typical choice for Stabilizers and for wings on aerobatic aircraft: Aircraft that might do a lot of inverted flying.
    Sustained inverted flying was prohibited in the Airacobra as it was in most fighters of the time.

    The Lateral control of the Airacobra was characterized by a rather poor Roll Rate that peaked at 75 degrees / second and by somewhat poor lateral control at low speeds.

    At its typically Normal Loaded Weight (more on this later), the Center of Gravity was at just over 30% Mean Aerodynamic Chord which is pretty far back in comparison with typical aircraft. This was probably a contributing factor to a very sensitive Elevator. Elevator authority was very good and the Nose Wheel could be raised at speeds as low as 50 MPH or perhaps even lower (NACA Test). It was also determined that too little force and control movement were required to operate the elevator.

    All models of the Airacobra from the P-39D though the P-39Q had a "Normal Loaded Weight" of around 7500 pounds.

    There were several factors that contributed to this.

    First factor was that there was not really very much difference between the P-39D and P-39N and not that much difference except for Wing Armament between the P-39N and P-39Q.

    A Second factor was that the rather light "Normal" Loaded Weight was achieved by using only PARTIAL Fuel, Oil, and Ammunition capacity. Although 120 Gallons of Fuel is not much, it was reduced to 104 Gallons in Normal Loaded configuration. More than that would be considered "Overload" condition. Similarly, only 300 rounds would be carried for each .30 Caliber Wing MG when full capacity was 1000 rounds.

    A Third factor was that in later series Airacobras (beginning with the P-39N), the internal Fuel Capacity was actually reduced to 87 Gallons and armour was reduced in some models. Kits were available to restore Fuel Capacity back to 120 Gallons.

    Maximum Take-Off Weight which ranged from 8100 to 8800 pounds was a better though still misleading comparison because not every model was able to carry the same external stores. Some of these limits also appear to be administrative in nature rather than practical.

    This brings us back to the subject of Center of Gravity.
    The typical "Normal" loaded weight puts the CoG at about 30%-31% MAC. This Normal loaded condition is an interesting statistic but somewhat meaningless in my opinion.
    The CoG limits range from about 24% to 34% MAC depending on aircraft load. At the forward CoG limit, the Airacobra behaved well but at the aft limit, the aeroplane was much less predictable.

    - Ivan.

  10. #10

    Airacobra - General Arrangement

    All production models of the Airacobra shared the same overall dimensions and basic Aerodynamic Characteristics:

    Wing Span: 34 feet 0 inches
    Length: 30 feet 2 inches measured from near the Tip of the Spinner to the End of the Rudder.
    Thus the length of the Cannon barrel extending past the Spinner was not included.
    The protrusion of the Cannon barrel varied depending on the actual weapon that was installed and would move under recoil.
    Wing Area: 213.2 feet^2
    Wing Root Chord: 98.66 inches (22 inches from Aircraft Center Line) -- NACA 0015 Section
    Wing Tip Chord: 50.0 inches (155 inches outboard of Wing Root Station) -- NACA 23009 Section
    Dihedral: 4 Degrees Positive (measured at 30% Chord on Upper Surface of Wing)

    Wheel Base: 119.78 inches
    Wheel Track: 136 inches

    This half page of statistics may seem nearly meaningless at this point, but the data will be used to confirm the measurements of the visual model to be built for this project.

    For this aircraft, the Horizontal Reference line was "Station 0" located about 3 inches forward of the Tip of the Spinner.
    The Fuselage Reference Line (FRL) was the Vertical Reference which was located parallel to and 40 inches above the Engine Thrust Line.

    Attached are some Drawings that will be used for positioning of various aircraft components.

    - Ivan.
    Attached Thumbnails Attached Thumbnails P-39 Station.jpg   P-39Q_Loading.jpg   P-39D_3V.jpg  

  11. #11
    Attached are an additional Diagram from a manual written / translated by the Russians for the Lend-Lease P-39D-1 fighters that they received. The manual itself is more descriptive than any US / English manuals I have been able to find, but the translation for me is rather tedious and time consuming.

    Also attached is a page from the "Pilot's Flight Operating Instructions for Army Model P-39Q-1 Airplane".
    This Weight and Balance Chart was used to calculate the Aircraft Center of Gravity for the Visual Model and for the AIR file.
    (More on the calculations later.)

    Although it isn't terribly relevant to a model for Combat Flight Simulator, a breakdown of armour locations and weights is included for the more common variants of the Airacobra. Note that this does not include field modifications or some of the Photo Recon modifications.

    Attached Thumbnails Attached Thumbnails Aerokobra_Cxema.jpg   P-39Q-1_Weight&Balance.jpg   P-39_Armour.jpg  

  12. #12

    Weights & Balance

    Calculating the Center of Gravity of the Aeroplane without Fuel and Ammunition is important, especially for a Combat Flight Simulator model.
    While the newer simulators can specify an offset from the origin (Coordinates 0,0,0) of the model to the CoG as displayed and for the Flight Model, this feature does not exist for CFS.

    There are a couple tricks to get around this, but it is much better to get this location as close to correct as possible before building the model.

    To do this, I used the Weight and Balance chart from the Pilot's Manual for the P-39Q-1 (Second Image in the prior post).
    The process was to deduct the weights and moments of the Fuel and Ammunition from the Loaded Weight to calculate the new weight and moment.
    From the new weight and moment, the new Moment Arm can be calculate which would then be the Center of Gravity of the Aeroplane with (CFS) disposable loads expended.

    P-39Q Normal Loaded:
    Weight ---------- 7570 pounds
    Moment Arm -- 134.2209 inches
    Percent MAC -- 0.2810

    P-39Q - Ammunition & Fuel Expended:
    Weight ---------- 6678 pounds
    Moment Arm -- 135.8829 inches
    Percent MAC -- 0.3016

    P-39D - Ammunition & Fuel Expended
    Weight ---------- 6536 pounds
    Moment Arm -- 135.8829 <-- From the P-39Q calculated value.
    Percent MAC -- 0.3016 (obviously the same)

    P-39D - Overload (No External Stores)
    Weight ----- 7705 pounds (I have a little bad math here because I am ignoring Engine Oil differences.)
    Moment Arm -- 134.1377 inches
    Percent MAC -- 0.27998

    The calculated value for the CoG of a P-39Q-1 is 135.8829 inches from the Datum line but the later models of the P-39 had some adjustments in a attempt to move the CoG further forward so the value I will use for the Longitudinal CoG for the model will be
    Fuselage Station 136.2 or 11.35 feet from the Datum line.
    The Vertical CoG will be 0.75 feet below the Engine Thrust Line.
    While these are estimates, they should be close enough so that errors in the 3D model are not visible if any adjustments need to be made.

    Note that the actual CoG migrates noticeably with raising or lowering the Landing Gear and that change is actually larger than the expected margin of error. (We will be examining the side effects of this CoG change later in the discussion.)

    - Ivan.

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