ORIGINAL: agcatsbest
Now. I had heard in several places statements like "you need a gear setup with a larger prop because if you just keep speeding the prop you reach a point where it's just beating the air and adding nothing.
Some of the these statements were part of the problem with the thread. People seemed to jump on a misunderstanding of how props work -- maybe a danger of asking a question in this forum
.
If you hold a plane on the ground and run your 8x6 prop at 15,000 rpm then the prop blades are stalled and in that case you might say that you are "just beating the air". If you release the plane and let it speed up the portion of the blade that is stalled gets smaller as the velocity picks up. At about 25 mph your prop is not stalled. So whether it is "just beating the air" depends on the velocity also. I still think you would be better off with an 8x4 or 8x5 prop, which will not be stalled on the ground. You will get a lot better pull in the 0 to 25 mph range and I doubt if you care about the extra top speed that the 8x6 prop gives you.
Try the freeware program prop selector. [link]http://www.gylesaero.com/freeware/propcalc.shtml[/link] it gives an indication of when a prop is stalled and how badly it is stalled.
This site has good diagrams showing props and velocity vectors and a little on why they stall
[link]http://www.auf.asn.au/groundschool/propeller.html[/link]
I do note that you had 3 estimates for motor power, 200 W min, 320 W max and a fairly careful calculation stating
The big story is that the 8x8 prop would require something around 620 Watts of input power and the 8x4 would only require 220 Watts.
You went with 277.5 Watts, an 8x6, and seem to be happy. So I guess you did get a some help
. I'm glad it worked out well and I'm sure most people appreciate the update and happy outcome on the project .
As for some of the other comments, come on guys, it's a glider. Under the right conditions it doesn't need the prop at all.
Carl