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Mick
Chadwick discusses the planning behind his successful
Lockheed
P-38 Lightning design.
Planning:
Having
built four PSS models previously from plans and being keen to
try something new, I decided to take the plunge and design my
own from scratch. I had built and flown a P-38 Lightning
control line model in 1962 (showing my age now!) and always
liked the design, it's also a bit different and there are lots
of colour schemes to choose from.
I
found an article in RCM&E with a three view, decided on a
span of 60 inches and scaled up the drawing to suit. The
wing taper looked as if it would be susceptible to tip
stalling, so I decided to reduce this by increasing the tip
chord, this would be the only deviation from scale. The
root chord worked out to 12 inches, and the tip to 4 inches,
which I increased to 5 inches. This together with
3 degrees of washout at each tip
would
hopefully eliminate any tip stalling tendencies.
The
wing area worked out to 3.6 sq. feet so I was aiming for an
all-up weight of around 58oz to give a wing loading of 16oz/sq.ft.
The
aerofoil section would be good old E205.
Construction:
The
model was built to be four piece, this being the two booms, a
one-piece wing with fuselage pod attached and a plug in
tailplane centre section.
The
booms are basically a full length 1/4" balsa crutch, with
1 / 16" ply doublers at the wing mounting area and
1/32" ply doublers at the tailplane mounting point. There
is just one former, this being at the wing leading edge
position. The balsa crutch is clad with white foam from
the former to the front of the fin, carved and sanded to
shape, then sheeted with 1/16" balsa. The area in
front of the former, i.e. the engine cowling, is carved from
pink foam with glasscloth and epoxy covering. The former
has a hole to accept the wing dowel and a captive nut is
cemented to the side of the crutch for the wing bolt.
The port boom carries a servo for the elevator.
The
wing is of standard white foam and veneer construction, with
1/2"x2" balsa trailing edge and glasscloth bandage
to strengthen the boom mounting points. A servo is
fitted in each wing panel, hidden within the booms, with
torque rods operating the ailerons.
The
fuselage is a rectangular box of 1/8" balsa, clad with
white foam on the top and sides and pink foam on the bottom
for strength. This is then carved and sanded to shape,
planked with 1/16" balsa. with a balsa nose block fitted
it is then cemented to the wing. The canopy is a
13" WW2 Mustang type and cost £4 from Pegasus Models,
they also make them in 9", 11 " and 15" sizes.
The
receiver and battery are housed under a hatch in the fuselage
pod, the aileron servos being connected via a "Y"
lead and the elevator servo by an extension lead.
| The
P-38 broken down into its four component parts. |
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Finishing:
I
used the usual rule of "keep the rear end light, build
the front end strong" and covered the tailplane &
fins with Litespan. The booms were covered
with
tissue and dope, the wings with Solartex and the fuselage pod
with glasscloth & epoxy.
I
spent some time searching for materials with which to make the
radiator fairings, until one evening while
lying
in the bath, I noticed a Tesco Foam Bath bottle of my wife's
which had the perfect curves (the bottle - not the
wife!). I cut the shoulders off the bottle and hey
presto (or is that hey Tesco -Ed.), two radiator
fairings. A second bottle provided the other set and the
tops were used to make part of the turbochargers - real Blue
Peter type modelling!!
Painting
was a combination of car aerosols and Humbrol enamels.
Flying:
The
model was flown for the first time at the 1996 Spring Hole of
Horcum event - hardly the ideal time for test flying but at
least the wind
was
good. The first flight proved to be reasonably without
incident (trousers still unstained!) although the model was
quite sensitive to elevator and an attempt at a loop resulted
in a flick roll. Reducing the elevator movement to
1/4" each way and adding
2ozs. of nose weight has resulted in a model which is
responsive but predictable and a real pleasure to fly.
The final position of the C.G. is 28% of the mean chord.
Conclusion:
The
all up weight is now 4lbs. giving a wing loading of around
18oz/sq.ft. and the model has flown easily in winds of
10mph. I have not, as yet, fully explored the flight
envelope but I think it would fly in much lighter wind
conditions and has not shown any nasty tip stalling
tendencies.
The
only aspect I would alter in the design would be the underfins,
which would benefit from strengthening with ply doublers or
inserts, as they tend to suffer occasionally in a fast
landing.
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