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Nov 14, 2006, 02:35 AM
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Power system suggestions for FF Douglas Skystreak?


I just came across a set of plans for the Comet kit of the Douglas "Skystreak". Ever since I was a kid I've wanted to build one. the original used Jetex for power. With the advent of small light electric ducted fan units it might actually make a practical free flight model.

The original size of the model would have a duct size of about 1 1/4" and 70 sq.in. of wing area. a slightly larger version would have a 1 1/2" duct and 100 sq. in.

For free flight a light weight inexpensive brushed system that would provide a minute or so of moderate climbing would be about right. The model will be of light weight stick and tissue construction. As for batteries, Lipo's would be lighter, but NiMH would be a lot more tolerant as to charging and cut-off voltage and require only a small timer and switch. Ditto for a brushed, as opposed to a brushless motor.

Any suggestions?
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Nov 14, 2006, 09:47 AM
EDF Jet Jam 2019, June 13-16
eatond's Avatar
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As for batteries, Lipo's would be lighter, but NiMH would be a lot more tolerant as to charging and cut-off voltage and require only a small timer and switch.
Stick with the Lipo's. There are several inexpensive Lipo only chargers on the market so charging shouldn't be an issue. Typically, EDF's get flown to a time limit that reflects a discharge level aimed at best cell longevity so the small timer and switch will work just fine. Start with short flights and then keep bumping the timer up until the pack discharges to around 70 or 80% of full capacity. That will occur well before you reach the cell cut off voltage.

Dan Eaton
Nov 15, 2006, 09:56 AM
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stegla's Avatar
1........you could try the Knight&Pridham KP44 on it's recommended 4x 120mAh nicads. This fan-unit is very tolerant of long ducting. You would have to build very light, say, 4 - 5oz AUW. Performance will be nothing special. The model will land when the battery is flat.

2.......the GWS EDF40 on 3s. I understand that this fan-unit is tolerant of long ducting. Motor longevity is not an issue with FF. You will need a timer to stop the motor and to preserve the lipos.
Nov 17, 2006, 02:53 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Good to see someone considering converting this plan to EDF (I assume you downloaded it from my collection?)
I'd considered this conversion myself, I'm sure it is possible but the long duct and quite small nozzle openings will make it quite a challenge. I've got both fans mentioned by Stegla, I think the GWS EDF40 is the one to go for using either the standard brushed motor with two Lipo cells or uprated with a brushless Feigao 12mm motor and three cells. I suspect the thrust offered by 3 cells and the brushless motor would be too great for a small FF model though, however you could always add some form of speed controller.
I think that the EDF40 is also available in a 'H' designation ('H' for high voltage)... you could run this version on 3 cells using the stock brushed motor but thrust would be similar to the standard EDF40 on 2 cells, so there would be little point going this route unless you need to keep the current low.

Details of EDF40: http://www.gwsus.com/english/product...stem/edf40.htm


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