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.
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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.
3 Attachment(s)
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.
2 Attachment(s)
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.