Propeller/Thrust calculating

[Madwood]

Looking for some information with respect to props...
1. prop diameter verses prop pitch
which gives you speed
which is for torque
what is best for climbout
what is best for speed
which loads the motor more pitch or diameter, draws the most wattage etc
2. electric props verses the others, pros and cons
3. Nylon verses wood verses fiber glass etc

[steve]

As these questions have been discussed in great detail............your best place to look is www.rcgroups.com and look at all the "stickies" and use the search function. Of the questions you have asked, all will yield for some very interesting reading...

The best to start is under the "electric" forum and in the "beginners" section. Although not implying anything, I routinely search that forum for answers to questions as they most likely been asked there...plus I learn a few other things along the way in reading the posts....

Rc groups has a very powerful search engine that allows you to precisely match words and or phrases...........very useful

Steve

[Madwood]

OK here's some links I've found in "RC Groups" as per Steve's suggestion

This links on theory,

http://www.mh-aerotools.de/airfoils/index.htm

A calulator for thust...pretty simple

http://www.lcrcc.net/thrust_calc.htm

Another couple neat calculators... they seem to give all the answers

http://adamone.rchomepage.com/calc_motor.htm
http://adamone.rchomepage.com/calc_thrust.htm

Alot to "chew on" no simple answers here

[steve]

Most Excellent................

By the way, How is the "back" feeling..?

steve

[Madwood]

Backs no bad, but still not right. Off work this week as I had to use some vaction. Been spending time repairing planes (the Kadet and the little Piper Cub). Placed an order last week from Hobby City for batteries and misc stuff that I will need for the Mini Pulse and Spectrum radio I ordered from RC Wings last Thurs...can't wait. That way you can have channel 33 back...at least for the majority of my planes.

[saboo]

I use a simple way to change a prop selection. I found on my SuperFly with an 8x4 GWS I ran about the 90% max amps for the motor speed control combination and running on 3 cells 2200 mah. The 8x6 overloaded with the result being the loss of motor and speed control. However the 8x6 gave a lot more speed, but not as good verticals as the 8x4.

So I knew the 8x4 gave the right AMP range. What is the diameter at 6" pitch that gives about the same amps?

I use a volume calculation. Volume is pi times r squared times h. But pi is a constant in our comparison, so we can ignore it. So the 8x4 gives us 8/2 as radius, squared gives us 16 and times 4 pitch gives us a "volume" number of 64.

What diameter 6" pitch would load the motor approximately the same. So we divide the "volume" number 64 by 6" pitch and get 10.67. This number is the radius squared of our changed 6" pitch prop. The square root of 10.67 is about 3.26 inches. Doubling this gives us a new diameter for a 6" pitch prop or about 6.5 inches. I cut the tips off a GWS 8x6 to give me about 6.625 diameter and rebalanced the prop. I get the great speed from the 6" pitch and I get the amps low enough to protect the motor and speed control.

You may ask why I did not use 6.5" for diameter. The reason is that I do not subtract the hub diameter before doing the calculations. The hub area is not adding to the load of the prop, and the hub area is a greater proportion of smaller props. So I added about 1/8th to compensate.

Note this is not a thrust calculator. It is just way to predict the effect of changing the diameter or pitch versus a known condition of amps with a given prop.

Saboo