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andy120
12-01-2006, 08:18 AM
guys
having trouble working out a power unit for my BT 6.4 scale 69" spitfire.
looking to drive a 21x15 5-blade, cell count open. motocalc cant give me a straight answer, contradicting itself. Pref only Hacker power.

many thanks
andy

redgiki
12-01-2006, 09:17 AM
I suggest taking a look at some of Paul Bradshaw's "E-Power Set-Ups That Work":

http://uterc.org/node/135


Any of the first three (Hangar 9 Funtana 90, tOC-1 30% Yak 54, Great Planes 25% Scale Extra 330) sound similarr enough that you'd probably be right in the ballpark using the following equipment:

Motor-Hacker C50 13XL, ESC- Jeti 77, prop-APC 22X12, Battery-Kokam 3200’s 10 S 20 C. Assuming a flying weight-10.5 pounds, you'll get unlimited vertical.

thunder1
12-05-2006, 04:10 AM
You're going to need a whole bunch of torque to turn a 21x15 5 bladed prop. What kind of top speed are you hoping to achieve?

For that much prop you're going to have to keep the Kv low enough that the amps don't go through the roof. I'd imagine you're probably going to weigh at least 12-14 lbs if not more. But with that much prop you won't have a problem with torque. But getting the pitch speed up is going to take a lot of watts. To keep the amps reasonable (maybe around 70-80) you're going to have to go with at least 9-10 lipo cells capable of the max current draw.

The largest Hacker Outrunner is the A60-18L with a Kv of 145 and it likes a max amp of around 60. With the prop you want to use an a 9s setup it draws 71 amps and generates 25 lbs of thrust at a pitch speed of 60 MPH. With 10s it draws 84 amps and generates about 28 lbs of thrust at a pitch speed of 65MPH.

I'd want a Spitfire to go faster than that. But you're limited by the large prop diameter and blade count.

The C50XL series of motors with a gearbox may be a good way to go as well. But you're still going to need a lot of volts to keep the amps down.

I use this calculator to get a ballpark fiigure of how a certain setup will perform.

http://www.s4a.ch/eflight/motorcalc.htm