Re: Optimum Prop Diameter

From: Tapio Linkosalo <tapio.linkosalo_at_helsinki.fi>
Date: Thu, 6 Sep 2007 06:52:16 +0300 (EEST)

On Wed, 5 Sep 2007, calgoddard wrote:

> Assume that the rules for a particular class or contest specify a
> maximum prop diameter. In general, is it preferable to use the
> maximum prop diameter allowable, assuming the plane is still built to
> the minimum weight allowed under the rules?

The prop theory says yes. There is a good chapter in "Model aircraft
aerodynamics" by Martin Simons about this. Already in the late 19th
century Froyde postulated (while studying boat props, no airplanes at that
time!) that prop efficiency can be estimated as:

eff = 1 / (1 + dV / V), where

V is flight velocity and dV is the velocity increase of the prop to the
slipstream. In other words, the prop is the more efficient the less it
accelerates the sliptream, and this difference is the more crusial the
slower is the flight speed. For this reason, fast planes like pylon racers
get god results with small props, but slow models like indoor need to have
big props.

The need for big prop comes from the fact that the momentum that the prop
gives the the slipstream, and thus (based on Newtons second) the
slipstream gives to the model is equal to mass * velocity of the air in
the sliptream. To get the _required_ momentum and have a small (increase
on sliptream) velocity, you need to have a larger mass of air accelerated.
And, as the flight speed is slow, and thus the "thickness" of the air mass
cylinder that the prop flies through in a given time period is small,
therefore the only way to increase the air mass is to increase the
diameter of the air mass cylinder = diameter of the prop.



-Tapio-
Received on Wed Sep 05 2007 - 20:53:42 CEST