cobratraxxas69

My Feedback: (31)

I wanted to find a rule of thumb (grounded in physics and aerodynamics) for determining the diameter and pitch of a prop for maximum speed.


I found some interesting Articles on Thrust, RPM, power, torque and Prop Load Factor, etc.

http://findarticles.com/p/articles/m..._n8978472/pg_1

(Also see the "More Articles of Interest Section" ! )

Prop Load Factor Formula as PLF = D^2 * P

A "thrust" formula, (which is probably more a mass rate formula [ounces of air moved per second] as thrust does not have units of time in it)

Thrust (ounces of air per second) = prop diameter2 x nominal pitch x static rpm x 0.000011127
[divide by 16 to find pounds of air per second]

One interesting caveat I read about noted that high pitch props can stall on the ground (and not produce as much static thrust as a large diameter prop), but once they (the high pitch props) are in the air, the higher airspeeds reduce/eliminate the stall (thus creating more "in-flight thrust" ). Thus maybe we should focus on maximizing in-flight thrust to maximize top speed!?!?


Another formula, Power = Constant * rpm^3 * diameter^4 * pitch

(from http://www.aircraft-world.com/prod_d...p-proptalk.htm )


I came across a website for picking speed boat propellers

http://www.caravelleboats.com/2007/pick.html

It states that:

"The ideal situation is to select a prop with a pitch that allows the motor to reach the maximum RPM suggested by the engine manufacturer without going over. If the prop selected doesn't reach the recommended RPM level, the boat will sacrifice speed and lift."


For maximizing top speed, there are a few possibilities (since it is common knowledge that pitch is more determinant to top speed than diameter, we should focus on pitch and then find a compatiable diameter):

1) Set pitch so that the prop spins at the engine's max torque RPM
2) Set pitch so that the prop spins at the engine's max HP RPM
3) Set pitch so that the prop spins at the engine's max RPM
4) Set pitch such that static thrust is maxed
5) Set pitch such that inflight thrust is maxed

We need a propeller aerodynamicist to weigh in on this one!! Which one do you think maximizes speed?????

ClemenTang

2+5
if hell of an engine no prop big enough then 3

freakingfast

My Feedback: (3)

First you need to know how much drag at a given speed you need to overcome.