AIRPLANENUTS

My Feedback: (2)

Guys, I have a twin electric floatplane that weighs 29.75 oz. empty, and it has two Speed 400 Graupner 7.2v motors which weigh 5 oz., upping the airframe weight to 34.75 oz. The plane has 478 sq. in. of wing area. The prop shafts are 3.75" from the fuselage, giving me a 7.5" dia. max. prop. I don't think these motors are enough, but I would like to hear your ideas. I was thinking of these motors:
http://www.hobby-lobby.com/speed400.htm
The FK5202 long can at the bottom of the page, using a 6X4. Any thoughts?

Thanks, guys!

cwrr5

My Feedback: (32)

Without running the numbers through, just seems like it would be kinda heavy(kinda really heavy for direct drive). The 6v 400 might be better. How about brushless...? (yeah, I know, $$$)

AIRPLANENUTS

My Feedback: (2)

Well, I have two 1:1.7 gearboxes, and I'm going to try 7X4 & 8X6 props and see how much amp draw I get.

Matt Kirsch

My Feedback: (21)

Whatever you do, you'll want the maximum that the motors can handle. That's around 12 Amps at full throttle.

He's right, though. If you want powerful performance, 16oz-18oz per motor, running the motor at full capacity, is about it. You're looking at aroud 25oz per motor when all is said and done.

What a 480 motor will gain you is a moderate increase on power handling capacity, allowing you to run 15 Amps instead of 12. You can also run 10 cells instead of the 8 on a 6V Speed 400.

A 7.2V Speed 400 can also handle 10 NiMH cells, or a 3S LiPoly, but is still limited to 12 Amps.

AIRPLANENUTS

My Feedback: (2)

Thanks, Matt. 15 amps sounds much better. The 480's will be my next choice, for sure. Is the long-can 400 the same as a 480? I notice it will handle 12 volts.

Is this the lipoly you mentioned?
http://www.hobby-lobby.com/kokam.htm
FM15003?

Can that pack discharge at 25-30 amps?

Matt Kirsch

My Feedback: (21)

Long can 400 is generally called a 480, correct.

That LiPoly pack is rated 8C, for a maximum discharge of 12 Amps. You'd want at least three in parallel, using the kokam parallel module also sold by Hobby Lobby. They'll cost more, but still be lighter than a 10-cell 3300mAh NiMH.

Another option is the Kokam 2100 (20C) pack. 2100mAh at 20C is 42 Amps.

AIRPLANENUTS

My Feedback: (2)

Thanks, Matt. I noticed Tower recommends a speed control made specifically for Lipoly's. Will a standard speed control allow the Lipoly's to discharge too low?

AIRPLANENUTS

My Feedback: (2)

UPDATE: I made an 8-cell pack with the 1250 SCR's (?) and wired the motors in parallell. I'm getting 9 volts at the motor, and I'm reading 12 amps using a 1.7:1 gearbox and APC 7X4 electric props. This is my first test with twin motors. Am I drawing 12 amps per motor, or is this for both motors? I have a current/volt meter I made, and I plugged it in between the battery and motors connection.

How do you determine the safe discharge rate of a nicad or nimh if it isn't published? I bought a new 24v cordless drill battery in order to rob these cells, but I don't know the safe discharge rate.