ela1967

Hi, almost done building a scratch built extra 300 micro. About a 16 1/4 wingspan. Looking like it will be in 3-3.5 oz auw range(depending on motor). How much power should I try to put in it? 40 watts going to be enough? Thanks, Eric.

ela1967

This is balsa btw.

049flyer

My Feedback: (18)

You want about 100 watts per pound for sport aerobatics and 125 or more for 3d. For a 4 oz ready to fly plane, 40 watts should put you within the 3d realm depending on your choice of props.

ela1967

Thanks. Thats what I was hoping for. Not really a 3d pilot, but I wanted to make sure the plane was not underpowered. Thanks for your help!

skaliwag

My Feedback: (1)

Having no see what you are working on but I think this would be more than enough for a power plant.

http://www.headsuphobby.com/Power-Up...otor-E-525.htm

Mr Cox

A Cox .010 or .020 should work well.

skaliwag

My Feedback: (1)

I agree, a TD .010 would be very happy on the front but it appears Mr Eric has chosen the Spark Side.

ela1967

Lol, yeah I chose electric for this one. I just built a gp extra 300s .60 (finished and maidened about 2 weeks ago). Put an os 91 fx up front...love this plane! I had alot of scrap balsa and ply left, so I scaled it way down and started cutting parts. When its all done I'll put up some pics. Rest of the electronics should be here in a couple weeks.

skaliwag

My Feedback: (1)

Matter of interest??.. What is the Hardware list for this tot.

ela1967

I have a 5 gram ntm propdrive motor-40 watts, 8 amp esc, 1.7 gram actual servos, orange rx r415 dsm2 receiver, 180 mah 3-cell nano tech. All surfaces I used really small ca hinges.

ela1967

For tail wheel and landing gear I picked up some micro stuff from local hobby shop. Plane with servos installed weighs in at just about 2 oz. Should be a bout 3.5-4.0 when all together.

049flyer

My Feedback: (18)

Another rule of thumb is that you need about 1000ma of battery per pound of aircraft weight. Since your plane will be about 1/4 lb you would need about 250ma of power for decent flight times.

Rhzip

What is the best way to determine size of electric motor for bi-plane, wing span 72 inches, wt about 4 lbs.,

049flyer

My Feedback: (18)

Everything starts with the ready to fly weight. A 4 lb plane x 100 watts per pound = 400 watts required. Then you decide what voltage battery you will use, probably 3 or 4 cell I would guess. Take the desired watts and divide by the battery voltage to get amps required. So 400 divided by 11.1 volts (3 cell) = 36 amps. So you need a motor that can handle at least 36 amps (3 cell) and an ESC that can handle it as well, be sure to add a bit of a margin say 10%. Finally consider the anticipated speed of the aircraft, fast planes like higher KV motors and higher voltage batteries than slow planes because more volts and more KV mean more RPM. Lower volts and KV mean lower RPM but usually more torque for bigger props which are nicer on slower planes.

Therefore I would guess you should consider a motor of around 800 KV or so for 4 cell batteries and about 1000 kv for 3 cell batteries. You will NEED a wattmeter to find the prop that draws the amps and watts you calculated in step one. Then go fly and tweak the prop selection to get the performance you want. Check watts and amps with each prop change to be sure you don't exceed the limits of your motor, esc or battery.

Your questions might be more suitable for the electric guys on the proper forum. However we like to be helpful here on the 1/2a glow forum.

The electric guys are probably more knowledgable than this 049flyer!