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!
