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Models I

List of models

Piper J3 Cub

The famous American two seat light sport plane sport scale model. Original was produced in 1938-47, more than 14.000 planes.
Built and flown by Alois Raffaj. Great Planes kit electric conversion.

Piper and his father
Span 2 285 mm
Wing area 72,5 dm2
Empty weight (estimation) 4 000 g
Flying weight 6 000 g / 24 x 1900
Wing loading 83 g/dm2

Motor MEGA S7
Gear belt gear 2,4:1, self designed
Prop Bambula 16 x 8

Building remarks
Model is of classic design, some parts are overdimesioned and the used balsa is often very heavy. Plane is covered with the "Cub" yellow Solartex imitating the original covering perfectly, but is a little bit heavier. Because of quite complicated wing mounting it is not possible insert the battery under the wing. That's why a battery box is made of 2mm ply and placed between the fire wall and gear bulkhead. The hatch is on the bottom. After the crash the firewall and main bulkhead were remade from sandwich ply-balsa-ply to reach less weight and original thickness. The gear mounting plate was lighten by two holes (this saved 30 g).
The motor controller is JETI JES60. The receiver is supplied from the 4 Twicell 1600 mAh pack. Model is controlled with the FC-18 transmitter.

Battery box The woden struts were replaced by carbon ones Motoru cover with ventilation holes
Gear mounting plate with lighten holes Homemade belt gearbox

Model has enough power for basic aerobatics. Flying characteristics are realistic, "Piper like". Model has unbeliveable low minimal speed. When slowing down its reaction is also slowing down. Flying time with Sanyo 1900mAh NiCd is between 5,5 and 7,5 minutes, with Sanyo 3000mAh NiMh between 8 and 10 minutes. When the Great Planes floats were used the flying weight reached 7,2 kg and weight loading 99 g/dm2. The electric Piper has enough power for raise of water. Because of greater drag more throttle is necessary.

Piper uncovered, .. and with floats Flying on water is great!

What to improve next time?
1. Many heavy parts could be replaced and up to 500g could be saved.
2. Motor is running on its limit and good cooling is necessary. Some bigger motor (MEGA L) or bigger LRK (MEGA Magic RC 41) could be tested
3. The aerotowing hinge is planned to be built in (probably the 18 x  8 prop will run better in this case). I think aerotowing the gliders of 3-3,5 kg is possible.

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Monocoupe 90A

The American golden years sport plane sport scale model.
Built and flown by .
Inspired by this picture of model published on E-Zone. Model was bulit from TrueScale(USA) plan, drawn by Frank Mizer.

Monocoupe 90A - 1650mm, 10 cells
Span 1650 mm
Wing area 34 dm2
Empty weight 1 715 g
Flying weight 2 380 g / 10 x CP1700
Wing loading 70 g/dm2

Pohonná jednotka
Motor AXI 2820/10
Prop APC E 11 x 5,5

Building remarks
Model is of classic design, except the main spar made from carbon tube. It is covered with Solartex what is fantastic material for working with but is of higher weight. Execpt the basic functions also flaps and high emissions LED lights are controlled. The lights are controlled and supplied via the module produced by the Czech company BEL. This module plus 4 standard servos are suplied from the TMM40 controller with BEC. The lights can be switched on via separate switch and also with full flaps. Model is controlled by the FC-16 transmitter.

light design Removable motor mount unit ... ... is fixed to the firewall with four M3 bolts.
Plastic motor mount is produced by Kovo Najman Using of total different drive (geared f.e.) is then possible by buildineg new exchangeable unit only. Ready for assembly
  Rudder light detail. Later it was equipped with saver made from 0,8 mm piano wire.  

Despite the model is little bit heavier than expected the current drive seems to be of enough power. The flying time is up to 6 minutes with careful throttle control, what means the average current is about 15A. Using higher capacity cells will bring longer flying time.

Full flaps pass Monocoupe on final with flaps an lights
Full throttle ... ... and next pass from opposite side
These nice pictures were made by Mr. Milanu Cvenar from Ostrava. Thanks. Despite he is not flying electric (yet ...) I'm appreciative for his advice and help.

What to improve next time?
1. The landing gear is very hard. It could be better to have softer gear or design with adjustable strength.
2. Wheel pants and motor cowl - these parts were made from balsa. Moulded GFK parts will be lighter and more durable.
3. Flap hinges - they are made same way as ailerons hinged on the wing top side. Next time I will make flaps with rotating axis below the wing.

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Knight Twister

American racer sport scale model
Designed and buldt by P.Doubrava, current owner .

Original plane was designed and built in 1928 - 1932 by Wernon Payne and was named Knight Twister. With 33kW engine it was successful pylon racer.
During 1971 the modified Knight Twister with Lycoming O-290-D2 engine of 100kW started it's another successful pylion race career - new world record of 309.53 km/h in its category in 1984.

Knight Twister - 1300mm, 10-12 cells
Span 1 300 mm
Area 53 dm2
Flying weight
2 500 g / 12 x 1700SCR
Wing loading 47 g/dm2

Motor AC MEGA Magic RC 41
Prop Aeronaut folding 15 x 8
Flying time 5-7 minut

Building remarks
The base is a foam last covered with 1.5mm balsa. Wing with symetrical airfoil has a D-form. Upper wing is fixed to pylon of wood and balsa, the interwing struts are from 6mm light balsa. The tailfeathers are made of 4 mm balsa sheet, Landing gear is made of 3 mm piano wire, with aditional reinforcement. Wheels are light. Tailwheel is controled from rudder. Ailerons are controled by one servo, all servos are of standart size.

Nice evening ...

Original cell pack Sanyo 1700 was about 4 years old with some cells renewed. Calm flying without lot of aerobatics the flying time is about 6 minutes. With more aerobatics the flying time is about 4-5 minutes.
Now the GP 3300 Nimh pushed cells are used - the flying time is about 8-10 min. The throttle matters, the plane is flying also below half throttle very realistically.Model is able to start on half throttle, with full throttle it is overpowered. Full throttle is used for vertical maneuvres only. The minimal speed is very low. Also behavior on minimal speed is very nice.

New Mega Magic (370g) front view, ... side and rear view ...
... and built in His Master's plane ...

What to improve next time?
1. Make bigger ailerons or ailerons on both wings - rolls could be then faster.
2. Landing gear is too weak. The piano wire shall be of larger diameter or fully redesigned.

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