Fly Electric!

Optica

A very exciting but challenging model! I built the canopy and shroud in 1996 after seeing the full size at Farnborough. The plane inspired me and I and thought it would be an ideal project for an Electric. Work pressures, and the realisation that this probably should not be my FIRST electric (!), dampened my enthusiasm a bit. However, after hearing Nigel Nixon describe his experiences developing a 1/3rd scale model for a film, I knew I had to complete the aircraft which I did in April 2000. I'm now pleased because it is an interesting subject and it flies well. Thanks to Tony Shortell and Phil Metcalf for some excellent in-flight photos...

Underside view in flight
32 kb
Landings are great
12 kb
Graceful flights
15 kb
Close-up of the canopy
24 kb
First public outing
27 kb
Full size at 1996 Farnborough Airshow
17 kb
The original aircraft has a ducted fan motor but my model is really just a 'pusher'. The prop in the model is about 10mm from the trailing edge of the shroud, and it is most effective there. There is no internal ducting although this would probably help a little. The output (static thrust) is about 15% less than what I get in comparative tests on a conventional fusalage. Another test I performed was to move the motor forward to the widest area of the shroud but I lost 40% of my thrust (even with a larger diameter prop). A heavier pitch also did not generate more static thrust in any position. The inlet area of the shroud is 16% larger than the outlet which helps makes it more efficient in flight. I'm very happy with the power and take-offs are quick and flight times are pretty good. Here are the mouldings and some construction photos.

new Croc motor
3-view drawings of the full size
Canopy and shroud moulds
Structure #1
Structure #2
Structure #3
I have been told by someone involved in it's orginal design and manufacture (and also by a later pilot) that the sound is very realistic. That's a change for electrics! As with the full size it is extremely stable in flight and does not readily stall if you have the CG between 25 and 30% of wing chord.

Wing span: 108"
Wing area: 1041 sq"
Wing loading: 25 oz/sq'
Weight: 11.5 lb
Motor: Astro 40 Cobalt #640
Changing to a 25mm Crocodile motor
Gearbox: Astro standard 1.63 : 1
Changing to direct drive
Prop: Graupner 11x7 3-blade
Prop speed: 7700 RPM
Batteries: 20 x RC2400
Changing to 6 Lipos
Motor current (full power): 33A
Probably changing to 40A
Motor power: 700w
Power loading: 60 w/lb (input)
Static thrust: 1.9kg / 4.2lb (take-off; fresh cells)
Flight times: 10 minutes (excluding thermalling!) conservative flying in good weather (8 mins with 2000's); 6 or 7 minutes with heavier throttle usage on 2400's such as in brisk winds.
Receiver: Futaba 6ch single conversion
Servos: 2 x FMA360 (Rudder/Elevator) 2 x Futaba S148 (Ailerons) 1 x Futaba S6 (Nose wheel)
Speed control: Jeti JES80 (JES40 would be adequate)
Rx battery: 700mah 4 cell nicad
Model design: Scratch built
Airfoil: Eppler 210 (slight undercamber). The full size actually uses a NASA GA(W)-1 airfoil but I only found this out later from the airfoils site you can find on my Links/Further info page.
Wing construction: Conventional; 6mm spruce spars; 3/32 balsa ribs; cap strips; 1/16" leading edge balsa sheeting; built-up ailerons. Two pairs of spars pass through the shroud to support booms, u/c and wings. Only the front spars are used in the outer wing panels. The Rx is housed in one of the centre wing panels to minimise electrical wiring lengths.
Canopy/Shroud: Plugs made from balsa (shroud) and insulation foam (canopy). Finished product moulded out of fibreglass, carbon and epoxy resin. Home-made windows pulled with 0.25mm vacuum-forming plastic over separate moulds (using a domestic 2-bar heater to soften the plastic).
Booms: Built-up from 1/8" balsa formers and 3/32 balsa sheeting (cut to 1/8" strips). Glassed with 1.5oz cloth after sanding. Each epoxy'd in place between two ply wing ribs.
Undercarriage: 4mm piano wire (a bit 'wobbly' but adequate); 100mm main wheels; steerable 70mm nose wheel on coiled 5/32" leg.
Covering: Solarfilm on flying surfaces; canopy and shroud sprayed.
Extra features: Detailed cockpit; full lenght Pilot.
Windows held by retaining screws which only need about 1 turn to loosen to allow plastic to be released.
Wings panels attach with 1/4" ply joiners, retained with two strips of Velcro.
The plane thermals in reasonable lift.
What would I do differently?: The incidences and perhaps motor thrust are probably slightly out as the plane requires a bit of up-elevator trim for level flight but then climbs when power is reduced.
To minimise electrical wiring, I used long plastic pushrods (on rudder and elevator) which change length slightly in different temperature ranges.
I increased the wing area proporationately from 100" wingspan to 108" to reduce wing loading. This was not necessary.
Home-made wheels have not been successful on this aircraft as they seem to take a beating (especially the nose wheel).

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