Electric Mini Ellipse

by Mike Shellim,  during foot and mouth crisis, 2001

Electrifying a Classic

I'm a great fan of the original glider version of the Mini Ellipse, so when I decided to have a dabble at electric, a Mini Ellipse conversion seemed the obvious choice. Before I go furhter, I should say I am not an e-expert - the purpose of the exercise was simply  to get something going during our Foot and Mouth crisis in 2001...

Completed model after test flights

Components

Fuselage

Apart from the power train this was the only component I needed to buy. 

The electric fuse is similar to the glider version in construction and layup (see photo left). As expected it is very light indeed. The front bulkhead is marked for various bolt configurations, ready for drilling.

 

Wings

The wings from the glider version were used.

Power Train

On my trip to Dortmund, my friendly e-expert Malcolm Corbin helped me select the following:

Two battery packs were used:

  1. 8-cell Sanyo 600 AE pack (end-soldered, 3x3x2).
  2. 8-cell Sanyo 700 AR's (braid, 4x4).

Assembly

Power system

I soldered the motor power cables from the ESC directly to the motor. The order of the connections doesn't matter as reverse rotation can be programmed in the ESC if necessary. Clever these ESC thingies.

Input to the ESC is the Rx Throttle output which also provides power to the Rx using a BEC.

Heavy duty silicon cables were used from the battery, terminating in 4mm connectors. The connectors also serve as the power switch. 

Motor

The gearbox was bolted to a false bulkhead using tiny countersunk 2mm bolts I had spare. The rear of the motor is supported using 1/4" balsa.

Radio

Two Hitec 85 MG's are used for the ailerons. I decided on separate rudder and elevator functions, so two Hitec 55's were installed in the fuselage as (there wasn't enough room for two 85's). Although the servos are tiny, they are quite fast for their size and I've had no problem with wear so far.

The batteries are secured using velcro. The 600 pack fits easily through the canopy. The 700 AR pack just fits but it's very tight! (Photos above and below show 600AE pack).

Setting Up

R/C

The model was set up with aileron, rudder, elevator and throttle controls on my mc4000. Butterfly and V-Tail mixers were used. 

Two timers measure total flight time, and cumulative motor run. The latter is triggered when the throttle stick goes beyond 1/4 throttle. Most of my flying is either with the motor either at full power or off, so the timer provides a fairly accurate guide to full-power duration.

Cooling Considerations

Since the Hacker's magnets can destroy themselves above 110º C, I was worried about cooling. To play safe I slip the canopy back about 1/2" to provide a gap for airflow - not very pretty! However I understand that Farnell sell temperature sensitive dots which change colour permanently to indicate maximum temperature. These could be very useful in deciding  whether it's safe to run without provision for cooling - something to try.

Weights

Here's a breakdown:

Component Weight (gm)
Fuselage (inc radio, motor) 290
Wing (inc. servos) 302
Battery pack 600 158
TOTAL 750
Battery pack 700 250

Flight Tests

Performance

Rate of climb is pretty good. We're not talking ballistic vertical climbs, but it's still impressive. How that tiny little motor does it I don't know, must be on steroids!

Duration

The 600 AE cells give around 2.75 minutes under power. The 700 AR's give 3.5 minutes. 

Given a choice I would use the 600's. They are lighter and the flight duration is sufficient. On a reasonably thermic day I get around 20 minutes per flight, using a combination of short (~15 second) full-power bursts followed by thermal seeking.

Handling

Handling characteristics were a little disappointing after the glider version. I wasn't prepared for quite such a deterioration in glide performance. The model must be flown fast and wide in turns, otherwise the sink rate increases rapidly. This means you the air must be more buoyant than I'm used to, before you can do any proper thermalling.

Summary

In its present state, my electric Mini Ellipse falls between two stools - not particularly aerobatic, and not as efficient as I'd hoped as a pure soarer. If anything it tends to the former. 

However I'm sure it has a lot of potential. It still needs a spinner (can't find one suitable so far!), and it deserves different cell and prop combos. Before people write in complaining that I've not done the machine justice, I acknowledge that there is room for improvement - it's just that lack of time and suitable equipment (e.g. WattMeter) mean that I cannot devote concentrated effort to the project for the time being.