Which Prop!!
 

I am a student at a university and have a project to build a UAV. I have chosen the Sig Kadet Senior as the platform for the equipment, but have gotten a bigger motor than called for to carry the various camera and autopilot equipment. The motor that I have is a Hacker A-60 16M, the recommended prop size was a 20x13 but the maximum clearance I have is around 14" for the prop, I know it is big for it, but.... I guess if I got some bigger wheels then I could get away with a larger prop. Could anyone please help, any suggestions, tips, previous experience stories would be helpful.

Thanks!!

RE: Which Prop!!
 

Hi Mate,

Electric brushless motors have a number of ratings:

KV - how many thousands of RPM per volt.
Volts - maximum voltage
Current - maximum current they can handle.
Watts - maximum current they can handle.

How much power an engine draws relates to how fast the prop spins (input volts) and how big that prop is (a larger prop draws more amps).

If you have a slowish (as in a low rpm/v rating - you can turn a big prop to make it fast) motor, and don't have the clearance to spin a big enough prop you will never draw enough amps to generate enough watts. In short the motor is a mismatch for the plane.

I would also speculate it is too heavy for the plane - at 26oz (~700g) it would be almost a quarter of the aircraft's recommended all up weight (6lbs) - add the battery required to drive it and you are looking at one fat heavy bird.

You need a faster engine (one that spins at a higher kv ) something that can spin say a 12" prop to make good power, and probably one that is a little lighter. I think you will find that even if you engineer around the clearance issue you will still find the engine is not well suited to the model.

Some notes on sizing engines to planes etc is on my blog here - the notes are aimed at parkflyers sized aircraft (so up to 2lbs or so), but the concepts are the same:

http://www.oz********.com/2007/01/el...ic-flight.html
http://www.oz********.com/2007/09/gu...low-heads.html

For a trainer around the 6 pound mark for good flight characteristics around 100watts/pound would be excellent - so you need a power system that delivers say 600 watts. Of say a 4s setup (14.8v) you are going to need around 40A - that's doable, but a little on the high side. A 2p3s setup (to 3s packs in series) would only need 27A putting much less strain on the batteries (and 3s is currently the "popular" size for parkflyers so quite cheap.

Sounds like an interesting project.

Best of luck.

Cheers,
Oz.