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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,
    Iīve 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 Iīll 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 didnīt 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 Iīd 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 21:05.
    "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.

  13. #13
    This is a response from another thread but may help to clarify how the dimensions I posted actually fit together.

    Quote Originally Posted by Aleatorylamp
    P.S. If the Datum line stands at 3 inches ahead of the spinner, and the 30 ft 2 inches is the length of the plane, it would be totally absurd to suppose that the plane length goes from a place in the air infront of the spinner, to the rudder-end at the back.
    Quote Originally Posted by Aleatorylamp
    ...Unless the Datum line is where it is for a specific reason: Would that be because it includes the short barrel of the 37 mm cannon? Holy Cow, this is going round in circles!

    There is a .pdf document about how this plane was built,
    originally published in the May, 1943 issue, Volume 42, number5, of Aviation magazine, published by McGraw Hill. It has a diagram on page 4 where the length includes the short barrel.

    Hello Aleatorylamp,
    Actually for a Datum Line, this is may be absurd, but that is how things are done.
    Note that the VERTICAL Reference is 40 inches ABOVE the Thrust Line.
    There is Nothing there at all, but that was the chosen reference.
    I don't know who picks these references, but they own the design and I do not.

    In the case of the Airacobra, the Longitudinal Datum is not hanging out in space in MOST aeroplanes
    It is part way up the Cannon Barrel.
    Of course if no Cannon was mounted (and plenty of the early aircraft did not actually have a Cannon installed), then nothing was ahead of the Spinner Tip.
    You also Paul Matt's Drawing that show the Overall Length including the Cannon as 30 feet 3.4 inches which is a touch longer than the 30 feet 2 inches normally quoted. I believe this difference is because it includes the full length of the Cannon Barrel. This is consistent with another description I have seen that mentions that the extra length is 1.5 inches.

    I have read the article from Legends in Their Own Time, but I have also read the article that it is based on.
    There are actually some loss in precision because of rounding in both of these articles at least compared to some of the other drawings I have already posted.

    - Ivan.

  14. #14
    Hello Ivan,
    OK! Thatīs very good news! Thanks for your clarifications.

    For a moment I was a bit unsure about what data was more correct about the Datum Line,
    because of how the measurements from that point could vary depending on the source.

    I understand that your deductions from the in-depth research you have been doing, lead to
    the reliable conclusion that the numbers you are managing now are getting more precise.

    Very interesting!
    "Why make it simple if you can also make it complicated?"

  15. #15

    Drawings for Reference

    Hello All,

    I believe the best set of Drawings I have come across to use as a reference in constructing a 3D Model is from the Monografie Lotnicze series of books. As with other non-dimensioned drawings, some work needs to be done to make them suitable for use.
    In this case, I have had to Rotate, Shrink and adjust each drawing for Shear when the error could be identified.
    The result isn't perfect but seems to be quite good.

    Attached are a Side view and a Bottom View of the P-39F.
    I also have a second Side view with the Landing Gear Retracted, but that is just a supplementary drawing.
    Eventually I will probably also need a Top view, but I would like to see how far I can get with just these for now.

    Note that the Extended Main Gear appears to be quite incorrect in the Side view.
    This can be confirmed by measuring the distance between the center of the Contact Points for the Nose Wheel and Main Wheels.
    The Strut appears to be canted too far Aft. An approximate correction has been drawn in.

    Note that whenever possible, the actual known dimensions will be used in place of dimensions scaled from the drawings.
    By this method I am hoping to minimize the inevitable errors.

    Also attached is a very nicely proportioned drawing of the Airacobra's internal arrangement.
    This would have made a good reference drawing except that the drawing is a bit too small and the lines are too thick for precise measurement.

    Note that even though the drawings look pretty good at this stage, the line quality is much degraded from the original drawings because of the Rotations and Size adjustments.
    When annotations, edits and corrections are made the Drawings will look much worse from a visual standpoint but will be a more useful reference for construction.

    - Ivan.
    Attached Thumbnails Attached Thumbnails Reduced_F_Side.jpg   Reduced_F_Bottom.jpg   P-39 SideReference.jpg  

  16. #16

    Creating Templates for Manufacturing

    Last Night and This Morning, I started on the first Template Parts to be used for Reference in constructing the model.
    The Fuselage Template is based mostly on scaled dimensions from the Monografie Lotnicze drawings.
    The Wing Templates are based on Technical Specifications.
    In theory, they should all fit together, but we shall see what actually happens.

    The first screenshot shows the Wing Root Template which is located at the split between the Fuselage Center Section which extends 22 inches (1.84 feet) to each side from the Aircraft Center Line.
    The Template is a box which shows the limits of the NACA 0015 Airfoil section.
    The line through the center is the line connecting the Leading and Trailing edges.
    Note that this image also shows the Template rotated at +2 Degrees for the Angle of Incidence.
    My intention is to build the pieces with 0 Degrees Incidence and adjust represent the angle by shear.
    The Leading Edge of the Wing Root Template was located vertically by reference to the Mean Aerodynamic Chord which is shown at 18.99 inches below the Thrust Line in the P-39Q Weight and Balance Chart and was located longitudinally by other aerodynamic specifications.

    The Wing Tip Template was located longitudinally by the matching to a 4 Degree 35 Minute line (Leading Edge Sweep) relative to the Wing Root Template.
    The vertical location was established by matching the Top Surface to a 4 Degree Dihedral line from the Top Surface of the Wing Root Template.
    Note that the Wing Tip Section is NACA 23009 which is a cambered airfoil which is why the upper surface is further from the line connecting Leading and Trailing Edges.
    Note that although Dinedral is specified at 30% for Wing Root and Tip sections.
    My observation is that although the NACA 23009 has a maximum camber at only 15% chord, the upper surface is almost exactly the same height at 30% chord and my belief is that errors from the difference is smaller than the error from representing the section with the AF99 design tool.

    The second screenshot shows Fuselage and Wing Templates together with a point on the underside highlighted because it does not seem to match the contours.
    There are enough lines coming together there and I am working zoomed in to the image and an error in measurement or selection is quite possible.
    As can be seen in the image, the Nose section of the Fuselage Template is quite useable, but the middle and aft sections are not even close to done.

    - Ivan.
    Attached Thumbnails Attached Thumbnails WingRootBoundingBox.jpg   FuselageTemplateV1.jpg  

  17. #17
    Hello Ivan,
    You seem off on a good start for a precision model!
    "Why make it simple if you can also make it complicated?"

  18. #18
    Last Night I added a few more points to the Fuselage Template.
    Note that the points that already existed are still the same, but the appearance is quite different with just a few more vertices between them.
    The shapes need to be adjusted and a vertices may get moved around a bit for better spacing.
    The screenshots show the contrast between the original Template with vertices just at the important dimensions of the Fin / Rudder as compared to the new Template with a few more vertices.

    A new Wing Tip Template was created 27 inches inboard of the first one.
    There are three or perhaps four important Stations on the Outer Wing.
    The Wing Root is located at 22 inches from the Center Line.
    The straight section of the Wing ends at 177 inches from the Center Line.
    The Wing Tip is located 204 inches (17 feet) from the Center Line.
    An additional important Wing Station is located at the joint between the Flap and Aileron.

    The new Template is located at the last non-projected section of the Wing.
    Beyond this Station, is the rounded Wing Tip.

    Note that the Airacobra Design Analysis describes the projected Wing Tip as NACA 23009 with a 50 inch Chord and 4.5 inch Thickness while Paul Matt's drawing describes the section 27 inches inboard of the tip also as NACA 23009.
    These descriptions are not consistent with a constant taper.
    My belief is that the Projected Wing Tip is more likely to be correct and I will be adjusting the Template at 177 inches to match.
    It will actually remain as close as possible to a NACA 23000 series profile though and I estimate that the actual thickness will grow to a bit over 9.5% to match the other sections.

    Note also that although the specifications are often described with very good precision (Wing Root Chord is 98.66 inches), Aircraft Factory 99 is only capable of representing 0.01 foot (0.12 inch) or roughly 1/8 inch increments. When that is combined with the need to make sure vertices line up with each other, the result is quite a bit less precision than one might have expected.

    - Ivan.
    Attached Thumbnails Attached Thumbnails FuselageTemplateV1.jpg   FuselageTemplateV2.jpg  

  19. #19

    Constructing a Wing

    Constructing a Wing is usually a pretty easy task.
    In this case I am taking a bit more care than usual to best represent the Airfoil Shapes.
    The Airacobra was somewhat unusual in that it had a symmetrical airfoil section (NACA 0015) at the Wing Root.
    Because it has no camber, the NACA 0015 section generates no lift at zero degrees Angle of Attack
    The Entire Wing of the Airacobra was set at 2 Degrees Incidence even though the Wing Tip Section was a more conventional NACA 23009 section.

    The first image shows what a NACA 0015 Airfoil should look like.
    The second image shows my attempt at representing this section using only 5 segments each for the Top and Bottom of the Wing.
    This is certainly crude, but it is pretty typical for Aircraft Factory 99 designs with limited resources.
    The third image shows the airfoil section rotated 2 Degrees and with the points adjusted to match the Top and Bottom longitudinally.
    There is also a slight adjustment at the Leading Edge because I know there will be a set of Intakes mounted there.

    The points dividing the segments are roughly at 10%, 30%, 50% and 70% Chord.

    Next comes the Wing Tip Section.

    - Ivan.
    Attached Thumbnails Attached Thumbnails NACA_0015.jpg   NACA_0015.jpg   NACA_0015_2_Degrees.jpg  

  20. #20
    Attached are a couple screenshots of the first attempt at building an AF99 Assembly using the newly created Wing section.
    Note that these shots are mostly for "Instant Gratification".
    It is nice to be able to see SOMETHING in the simulator even if it doesn't look much like an aeroplane yet.
    These images also confirm that the general shape of the Fuselage Template is reasonable and that the Parts forming the main Wing section are close enough to planar to work without bleeds.

    - Ivan.
    Attached Thumbnails Attached Thumbnails Assembly1A.jpg   Assembly1B.jpg  

  21. #21
    Wing Tips are complete now.
    Attached Thumbnails Attached Thumbnails WingTips1.jpg   WingTips2.jpg  

  22. #22
    I was pretty sure that eventually I would need a Top View of the Airacobra to determine the Fuselage Width in various places.
    Attached is a reduced size version of the drawing I am using.
    In this drawing, I noticed that there is a slight discrepancy: The Wing appears to be slightly further back than it should be according to calculations.
    Other than that, there were the typical minor errors such as a slightly different scaling factors for Lateral and Longitudinal dimensions and a very slight shear error. Oddly enough, there was not a rotational error.

    The maximum width of the Airacobra's Fuselage is 34.75 inches.
    This works out to 1.4479 feet on each side of the Center Line. I will be using 1.45 feet.
    The scaled dimension from this drawing works out to be 1.46 feet which is pretty close.
    The next step is to take measurements to build a three dimensional Fuselage to replace the Profile Template that is in place now.

    - Ivan.
    Attached Thumbnails Attached Thumbnails Reduced_F_Top.jpg  

  23. #23
    Good morning, Ivan,
    Thanks for the fuselage width data, which came in very handy indeed. I know that Iīd already had it in the new drawings you sent, but I hadnīt checked it yet. Better late than never!
    "Why make it simple if you can also make it complicated?"

  24. #24
    Hello Aleatorylamp,

    There are a lot more issues that still remain with that AFX that you are working with.
    Basically you really can't get a dimensionally correct (by AF99 standards) model using that AFX without replacing EVERY original polygon.
    I did not see the point of doing that much work to end up with a model that I would have to credit to another author anyway even though nothing remained of the original, which is why I put certain limitations on the changes I would make.
    That model was good enough to use for visualization while working on a Flight Model though.

    I don't really know that my own Project will be all that accurate but I do know that the errors in it will be mine and not those inherited from the choices made by the original author. I have already had to make a few decisions that rely on Eyeball measurements rather than scaling from a drawing or using accurate specifications, so there are certain to be some errors.

    As for references, nearly nothing is new. The documents have been around for decades and most of them date from well before the Microcomputer Era.
    The Design Analysis article quoting Larry Bell is from a magazine published in 1943.
    The P-39Q-1 Manual was in use during the war.
    NACA Report L-602 dates from the same period. (It describes a P-39D-1)
    The Russian Manual was written by them when they received their first P-39D-1 aircraft probably around 1941.
    The Paul Matt Drawings were published in the 1960s.

    It is really a matter of finding the references and trying to put together a consistent set of data despite the contradictions.

    - Ivan.

  25. #25

    Spinner & Propeller

    Attached is a screenshot of the new Spinner and Propeller.
    Note that the colour choices are somewhat odd but they are a consequence of a bug in Aircraft Factory 99.
    The Spinner is split into a Forward Structure and a Rear Structure.
    The Forward Structure has a "Forward Bulkhead" (Wall) specified.
    While both Structures are specified as Yellow, Aircraft Factory 99 handles Structures with Bulkheads differently.

    - Ivan.
    Attached Thumbnails Attached Thumbnails Spinner&Propeller.jpg  

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