"Big Boys Toys?"
I have not had the courage to venture into the world of twin diesels but now
that electrics have advanced to a point where battery capacity and duration is
sufficient, the motors powerful enough and the total weight relatively light a
viable set up is now possible. Switch on, check that the props sound synchronised,
let it go. It could not be easier. Famous last words. The smell of a diesel's exhaust is one of
the sniffing pleasures of aeromodelling but the "keep clean electrickery" is a huge
bonus. All the work that one puts into building a twin is not rotted by the
intrusion of fuel and you could expect it to last for many seasons. 
I have kept the Handley Page electrics as
simple as possible. (See update below) The two motors are straight drive 480s with 7x3 propellers.
The batteries are 7x 500mah SanyoAR cells. These are installed, wrapped in foam,
in the cabin space as seen in the photo. This box is re-enforced with thin ply
to resist the inertia of the batteries exiting through the bulkheads or cabin
sides on landing. The switch is located on the battery box front bulkhead and the actuating
lever exits the bottom of the fuselage so that it is easy to turn on from a hand
launch. The switch is a 20amp toggle switch from a car accessory shop. A blade
fuse (20amp) is used for safety as the amperage pulled by the two motors is quite high.
The wiring is quite hefty. Do not skimp
on this as thin wiring will cause resistance and reduce your power dramatically.
The total weight of the electric package is one pound. This sounds
alarming at first but I have not had to use any ballast in the nose to correct
the C of G and the large wing area of the model carries it very well. Initial
test flights where a disaster and I nearly wrote it off. I
could not get any sort of consistent low powered flights. Back to the drawing
board. I removed the engines and batteries and set them up on a test rig. This
set up was tested repeatedly. This was worthwhile as I discovered that the two
motors, they where purchase years apart, were miss matched by a large margin
even though they were apparently the same. Test before installation. Common
sense really but we all have our up-cocks at times. I purchased two new motors
and they balanced perfectly and produced a slow beat of two to three a second. Sounds
good in the air as it drones overhead. This characteristic beat is essential on
twins as any trimming adjustment cannot consistently make up for the miss-match
of two odd motors. I know, I've tried.
A note on engine thrust lines. The HP is set up with 2 degrees down thrust on
both motors and 2 degrees right thrust on the port engine and 3 degrees right on
the starboard. I was advised by an experience radio twin flyer that the extra
degree on the starboard motor is to counteract the tendency of the motors torque
to pull the model to the left being on the outside of the left turn. (I have
read this last sentence several times and edited it endlessly. I trust it makes
sense)
Update
The HP is now fitted with an electronic timer that works a speed controller.
This enables me to control the duration and power of the motors to a very accurate
degree. I have also fitted a 3 cell, 1200 mah li-po battery . I can now fly a
whole competition on one charge. Easy Life.
Photo
Tony Dowdeswell 
A pair of real twins by Bill Dennis.
Handley Page 0/400 Bomber and an Armstrong Whitworth Argosy. Both powered by
Mills .75s
The
Airspeed Ferry was sarcastically given the alias of Airspeed Hoover by fellow
competitors as it had a very vacuum cleaner like sound and not the rhythmic beat
normally associated with a multi. This, I assume, was due to the motor/ prop
combinations being different. It was fitted with 3 motors. The two outer motors
were 360s with an Olympic gearbox. I tried a straight drive at first but was
unable to obtain enough thrust to fly well. The nacelles were redesigned to take
these units, a very tight fit. I was able to replace 6x4 props with 8x6. The
upper motor was a kyosho racing motor straight drive to a 7x4 prop. The battery
pack was, if memory serves correctly, 7x 600mah AA nicads cells.
