Scratch Plane

I have been working on a scratch plane that I designed and have a question for you. I'll give you the specs just so you can try to get a picture of it.
Wingspan-41"
Chord-8.25"
Fuse. Length~30"
Motor-Speed 400 w/ 3:1 gearbox
Prop- 9x6 or 10x7
Sky scooter 300Mah
Ok well my questions are, what kind of battery am I going to want, I have a Sky Scooter batter and I tried it out, It seemed to put out a lot of wind but not for very long. It only went for about 2 mins, but I don't know how much power I am going to need and that is another question. Is there any way to know how much air im gunna need to be pushin out? Also I have been trying the skyscooter battery but l don't know if it has enough power. Also it has no duration, doesn't last nearly long enough for a flight. If you have ANY help it would be greatly appreciated. If you need any more info let me know. Thanks all

It sounds like you're just sticking things together in the hopes that it might work.

The best course of action when designing your first electric airplane is to do your homework. Learn about Volts and Amps, as these are the most important values in determining whether your plane will fly or not. A motor calculator program like MotoCalc can help in the early stages of learning. You can get a free 30-day trial at http://www.motocalc.com. Another online calculator at http://brantuas.com/ezcalc/dma.asp will give you a rough idea of how many Amps a particular setup will draw.

I ran the numbers on your intended power system, and they do not look good. The current with a 9x6 prop is 11.67 Amps, and 13.6 Amps with a 10x7!!! Neither that SkyScooter battery, nor that Speed 300 motor will last long at that level of current! If, as I suspect, you are using the Sky Scooter's Speed 400 (FOUR hundred) motor and stock 3:1 gearbox, the current levels are much more manageable. You simply need a battery with a higher mAh rating to get longer flight times.

Your plane has a wing area of 338.25 square inches. That's a pretty good size for a Speed 400 floater. If you can keep the all-up weight under 22oz, that's with receiver, servos, battery, motor, gearbox, etc., it will fly really well, and probably thermal like a sailplane.

For a battery, the only way to fly with a Speed 400 is 8 cells. I recommend an 8-cell pack of Sanyo 600AR or 600AE NiCd cells, or an 8-cell pack of HECELL NiMH cells. Many online vendors can sell you the battery packs, all pre-wired. Check out the vendor section at http://www.ezonemag.com. Remeber that you will also need an appropriate charger. The timer charger that came with your SkyScooter sucks in the first place, and it won't charge an 8-cell pack. I recommend the Hitec CG-340 for light-duty electric flight. It can also double as a charger for your transmitter NiCds.

Hey, thanks for the great reply, I used the motocalc to get the info on the motor,gearbox,and a battery. It told me to use a 8 cell Sanyo 720 NiMh battery. The gearbox and motor are http://www2.towerhobbies.com/cgi-bin...?&I=LXWK47&P=7 .....mistake speed 400 motor! I'll go and edit the first post. Thanks again

720mAh NiMH cells are normally not good enough for Speed 400 applications. Now, I did run the same gearbox and prop, using a Speed 400 motor, and the current is down around 8 Amps. That's on the ragged edge of the 720mAh NiMH's capabilities. It likes 7 Amps maximum much better. If you limit your full-throttle time to short bursts, you can probably get away with that combination, though.

If it were me, I'd consider using the Wattage 5/4AAA 800mAh 8-cell NiMH pack, found at http://www.hobbypeople.net. This pack is only 25% heavier than a 720mAh AAA pack, and still signifigantly lighter than a 2/3A pack of Sanyo 600AR or HECELLs. It can handle up to 10 Amps, and provides 10% more capacity.

That battery looks great. It wieghs about the same as the one motocalc recommended. How long do you think this will let me fly? Also what prop should I use or should I get a different one.
Thanks for the info.

Rcman4life -

I agree with the info that mkirsch is giving you. He obviously knows his stuff.

I live in the Phoenix area and was wondering where you fly your parkflyer. I have designed and built a 280 size parkflyer and so far the only place I've been flying is rather small.

You can contact me thru private message or email if you like. Would be glad to help you also if needed. I am an experienced model designer, builder, and flyer.

feihu