It must be an age thing, but having flown r/c models for many years now, I find myself more and more attracted to the relative simplicity of clean electric power, particularly as the majority of ARTF models seem to have been designed with electric power in mind.
Having “assembled” and flown several modern electric-powered, aerobatic ARTF’s I started to consider how to set about substituting electric for glo power in a retro (or classic) F3A model. Don’t get me wrong I have over the last few years scratch built a couple of these types, but as originally intended with a modern 2 stroke glo engine, however the messy exhaust residue, need for a set of starting equipment fuel canister, etc, set me thinking.
Having converted a modern quarter-scale ARTF Extra from 30cc glo to electric, built from scratch Phil Kraft’s “Slik Fli” and Wolfgang Matt’s “Arrow” for electric and converted an old glo powered Frog “Mustfire” airframe to electric, all so far with very satisfying results. Now to tackle a slim nosed aerobat, this throws up the following considerations:
Motor position relative to original firewall and accessibility
Most electric motors are shorter in length than the 61 two-stroke of the original design and with the majority of aerobatic models of the period having a tricycle undercarriage with the nose gear attached to the rear of the firewall, this tends to rule out bringing the firewall forward. Therefore I use standoff mounts such as those made by “Secraft” available from a number of UK sellers.
If, however, the model is a “taildragger” then consideration can be given to bringing the firewall forward, but in this event it will be necessary to redraw it to a revised profile.
In either of the above situations, access will be needed through a suitable cut-out in the cowling between firewall and nose ring for both fitting the motor mount (preferably rear mount ) and to allow cooling air around the motor itself.
Where to place the flight battery
Unless the model has a short nose moment ( such as the Taurus etc.) the probability is that a finished model of this type will end up nose heavy. So try and place the power battery between the original fuel tank position and centre of gravity with the ability to adjust its final position.
Access to this
Some of the earlier models (particularly those of Phil Kraft) had a hatch over the fuel tank, held in place with elastic bands, which would give access. However, if of later design with rear exhaust and tuned pipe, such as the Arrow, the pipe cover could be removable. Otherwise, modifications to the top of the fuselage will be needed, similar to a modern ARTF scale aerobatic model.
Centre of Gravity
As mentioned in point 3 above, there will could a tendency towards nose heaviness, which should be countered by placing the Lipo behind where the original fuel tank would have been. But, you should resist any temptation to extend the leads between the ESC and Lipo, as long leads can give problems. So the ESC should also be moved a similar distance back, to avoid this. With any model of 4S or above I run a separate receiver battery, and the positioning of this can be adjusted to assist with moving the balance point as close to the cg as possible.
Equivalent power of electric to a 61 two stroke
At the period in question 61’s such as those from Merco, Enya, Fox, Super Tigre or O.S. had an output of about 1bhp with silencer attached, some a little more, most running an 11” or 12” diameter propeller in excess of 11, or 12,000 revs.. Around 750 Watts is the equivalent to 1 bhp. BUT sheer power is not the only consideration with electric They run at lower rpm, turning larger diameter propellers which gives better thrust.
I generally use motors of about 580-600 kv, using a 4S lipo for lighter models and 5S for heavier. My Slik Fly flies very well on a 4S, at a weight of 6.75lbs. giving 700 Watts and drawing 45amps on a 14”x7” prop. The Mustfire on 5S with 13”x6” prop flies well, being of large wing area and quite “draggy”. The Arrow is yet to fly ( due to lockdown and eyesight issues…(hopefully now corrected) and is for an 11”x8” prop on 6S power.
Most designs of the period had exposed cylinder heads, with the engine mounted either upright or sidewinder…….(few were mounted inverted). So cooling air can easily be introduced around the substitute electric motor. However, you will need to cut a hole(s) in the firewall to allow that cooling air to reach the ESC, and Lipo, before exiting through a hole in the underside of the fuselage, towards the rear.
This is likely to be less than the original, due to the larger diameter propellers used by electric motors. If a tail dragger, it’s not so critical, but with trike undercarriages, the only solution is to increase the length of the legs, and if retracts are fitted, the “wells” will also need to be adjusted in size.
At that time, landings were marked in competitions, and so the trike u/c model sat on the ground with a negative angle of attack, so that on touch down, the model was less likely to bounce. Therefore simply altering the length of the nose leg/increasing the diameter of the nose wheel can help. A side effect of this is the ability to make take-offs easier.