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Bell P-63A Kingcobra

by Bill Reynolds

Plan No.696

Features Construction Articles

Plan Detail:
Plan No. 696
Price: AU$54.00 (2xSheets) 
plus P&H (AU$5.00 within Australia).

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HISTORY

The subject of this construction article is the Bell P- 63A Kingcobra, development of the P-39 Airacobra. The unusual design of the Aircobra was based around a 37mm mounted in the nose and firing through the propeller hub. This arrangement meant the engine had to be mounted behind the cockpit, driving the propeller by an eight foot (2.4m) long drive shaft into a gear case just behind the propeller hub.

Initially designed as an interceptor with a turbo supercharger and a top speed of 400+ MPH initial testing was promising but the Army decided to delete the turbo - supercharger and the designers protested that this would destroy the performance and its intended role. Needless to say that the short sighted Army leadership refused to listen, believing that wars were fought on the ground and aircraft were only good for ground support. As a result the Airacobra did not live up to expectations as an interceptor and was assigned to primarily ground support.

Early experience with the P-39 Airacobra exposed the error of the Army's deletion of the turbo-supercharger and in 1941 an improved design was proposed for what was to become the P-63 Kingcobra, incorporating a hydraulic turbo-charger as well as laminar flow wings, a redesigned tail, four blade propeller and metal covered ailerons and elevators, although the rudder remained fabric covered! The new design Kingcobra was also larger than the previous Airacobra with better access to weapons and ease of maintenance thanks to redesigned access panels.

There were two main production models of the Kingcobra, the P- 63A, the subject of this article and plan, and the P- 63C. P-63's were never used in combat by the USAAF squadrons but large numbers were shipped to Russia and used to great effect against the German ground forces and close air support.

THE MODEL
The plan drawings are accurate and fairly well detailed allowing plenty of scope for experienced model builders to produce a super scale warbird. After completion of this project I found the flight capabilities & characteristics of this model Kingcobra to be exceptional! This is definitely not a project for the novice model builder or flyer. Airborne's Kingcobra is equipped with retractable undercarriage and flaps, including an underside centre section flap, so the builder needs to choose his equipment to suit the model and adjust the plan accordingly. For instance, I used mechanical retracts while some modellers will prefer pneumatic retracts, so some minor plan modifications may be necessary. This is nothing that the experienced builder will have any trouble with. The entire model is skinned with balsa, this can be done with planking or sheeting. When sheeting, wet the balsa on the under side with a 50/50 water and ammonia mix then allowing a time for the mix to penetrate & soften the balsa before bending the sheet over the airframe. This softening can take an hour or two up to overnight depending on the hardness of the balsa sheet. Scoring the wood along the grain to around half depth can speed up the process.

All internal mechanisms need to be sorted out before sheeting or planking. Jigging is required during construction especially the fuselage which has many compound curves. Overall though it is a challenging but very rewarding project for the serious scale modeller. It is always a good idea to thoroughly familiarise yourself with the plan and cut out as many parts as possible to make up a kit of the model beforehand.

TAIL SURFACES
Theses are fully symmetrical components and are skinned with balsa sheet. The tail surfaces as well as the wing must be built up before the fuselage and need to be rigged up in position during construction of the fuselage.
Cut out the spars, leading and trailing edges and all ribs as required, then draw in the centre line of each part which will be your reference line to maintain symmetry.

Build up the fin-rudder and tailplane-elevator over the plan, propping up the outboard sections with spacers and check the pre-marked centreline for symmetry. When the basic structure is set, sheet the top surface with 1.5mm balsa sheet. When dry, remove the tail surfaces off the building board, then sheet the other sides. Now sand to shape. I installed Robart hinges to give the correct scale hinge line.

WING
As previously mentioned the retract units and servos that you use must be checked against the plan to make sure they will suit. Adjust the plan accordingly if necessary. I used standard Eurotrack mechanical retracts for the Kingcobra with a separate servo to operate each unit.

The wing is built up in three sections over the plan, the centre section also having an underside flap.
Note: All the ribs on the plan are marked with tabs which will give the trailing edge the correct position and the 2O wash out on the outer wing panels. Check that you have made all the holes in the ribs for servo extension leads.

Frame up the centre section with ply braces as a unit, including servo mounting plates. Follow on with framing the outer wing panels which will house the retract units and their mounts. When set, join up the three wing sections with the correct dihedral.
You could install the aileron and flap hinges as I did at this point. The ailerons are hinged near the top surface whereas the flaps are hinged from the botton. Install the flap torsion rods into the centre section.

At this stage with the open frame, fit the retract units, wheels, servos, and check their operation. It's a lot easier to make adjustments or alterations before the whole thing is covered!
Plane and sand the leading edge profile to take the balsa sheeting, then sheet the bottom of the wing with 1.5mm balsa sheet. Now you can add the wing tips. When dry pin the wing down, sheeting the top of the centre section first, checking for straightness. Each outer panel should be pinned down with 2O washout set, then when all is correct it can be sheeted with the balsa. This sequence will ensure correct rigging with built in washout.

Install the servos for ailerons and flaps and check for correct operation. Set up the aileron differential, ratio is approximately 2:1 up and down respectively. Flap movement on the outer panels must be set up so they come down exactly the same amount or else you'll be banked over on final approach. Once set properly however there won't be a problem.

FUSELAGE
The Kingcobra is designed for a .60 two stroke or a .70 to .90 four stroke motor. I used the rotary valve style RCV 90 for my model. The RCV 90 really lends itself beautifully to cowled scale models due to its revolutionary streamline design as well as the fact that due to its 2:1 internal gearing allows it to turn a 3 or 4 blade prop with ease. Great scale looks!
Start the fuselage by making up the engine mount. When gluing the engine firewall and tank box use a good quality epoxy to join up these parts. Drill out holes for mounting bolts, blind nuts and fuel tubing. This box unit will be fitted into the formers during the fame up stage.

Before assembling the fuselage I made up a jig out of 150mm high plywood rectangles, marked with a centre line and all propped up at right angles to the building board and positioned at the former stations. WIth the longeron - crutch pinned on top of the jig you can now glue all the formers into place. (See photo) Leave overnight for the glue to set with everything still in the jig. Add the stringers to the top half only then plank the top fuselage above the crutch, or if you wish you can use balsa sheeting as I did. Allow for the opening in the canopy-cockpit area when skinning the fuselage. When skinning is set, lift the structure off the jig. Add the engine mount-tank box and wing mounting plate and lower stringers. Now check the wing, fin and tailplane for correct incidences and fitment.

Install the nose-wheel retract and check for operation. When happy with this you can proceed to plank or skin the lower fuselage as you did with the top half.

With the engine mounted in place (with all openings sealed with tape) build up the engine cowl from balsa blocks. I actually made up mine out of fibreglass. I also left about a 5mm gap between the spinner and nose ring to form an air intake and omitted the nose wheel doors to help with flow through cooling from the engine. Depending on the engine, you may need to cut out additional holes for the engine cooling. My RCV 90 was fully concealed by the engine cowl as previously mentioned. Now glue the tail surfaces into the fuselage and mount the wing, checking for true alignment and correct incidences. Carve out the balsa wing fillets and fair them into the fuselage. Take your time here as a rushed job will detract from the airtracts appearance.

FINISHING THE MODEL
Sand and smooth the model with sanding sealer in preparation for painting. To save weight I went straight in with primer, however in hindsight it would have been better to cover the balsa with tissue and dope first as later on the timber grain tended showed through the paint. Not what you want on a supposed all metal airframe!

I sprayed on acrylic house paint, dark forest green, in the typical WWII Russian fighter colour scheme complete with the red start insignia. To fuel proof the model I used 1 pack clear Estapol polyurethane. Detail the cockpit, add a pilot and glue on the canopy. An excellent reference source for the Kingcobra is Squadron Signals Publications, Volume 63, ÒP-39 AircobraÓ which features the P-63 variant including many close ups of the cockpit and various walk around photos for those wishing to add extra scale details.

Now it's time to install the radio gear, positioning the batteries to help balance the model to C of G marked on plan. When all is installed and hooked up,re-check all control functions for proper throws.
With my model carrying so much gear I used a 1500mAh battery plus a 600mAh back up system to be on the safe side. There's plenty of servos and heavy servo loading on this beast so an additional battery back up was thought a good safety idea.

FLYING THE MODEL
So how does it fly? This model is aero dynamically very clean. Typical of many warbirds this wing loading is high so you can expect some fast flying action! The RCV 90 weighs in at 2lbs and has awesome power. In fact it swings a four blade 15 1/2Ó x 12.4Ó blade quite admirably which definitely looks right for the Kingcobra.

Throttle up and the engine roars, pulling the model up to a high speed in a most scale like manner. WOW! The Kingcobra really flies ... just like it's on it way to Mars! It moves very quickly through the sky so be alert to keep it in sight. Believe me, I found out the hard way. Only the keen eye sight of a younger club member saved me from possible loss of my Kingcobra on one occasion. Apart from its high speed the model has excellent handling characteristics. It is rock steady in the air, quickly responds to control movements ... nice and crisp. Stalling is a very gentle mush, simply push the nose down to regain full control.

Landing with flaps up is no drama. I've even had a dead stick landing, flaps up with no problems. Flaps though, will reduce the landing speed considerably and work a treat.

Airborne's Kingcobra has given me a great deal of satisfaction, both in building and flying this less common warbird. It has been rewarding to build and looks great.
I'm enjoying this aircraft tremendously!

 


The RCV .90 4 stroke has huge torque allowing it to swing a 4 blade prop & fit neatly inside the cowl.


Canopy is available from Airborne. Nice cockpit detailing adds to the model.


Various stages of the empenage during construction and the finished fin & elevators with hidden hinges. Nice & neat.



At this stage the wing is upside down & the removable 'tabs' allow for building in the correct washout.


Centre flap under the fuselage operates in concert with the two wing flaps & is quite effective.


These next four photos show the fuselage being built on a simple jig. You can elect to strip plank or sheet the fuselage.


The finished fuselage with mechanical retracts and the engine fitted.

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