Prop Design


What do the prop design speed parameters mean?

The design speed of a prop determines the twist in the prop blades, and in the case of a fixed pitch prop, the overall pitch. At its design speed, each part of the prop blade is at the ideal angle to meet the oncoming airflow, given its rotational speed.

The design speed parameters are given as the prop rpm and the airspeed through the prop. Together with the prop diameter, these determine the airflow angle at each point on the prop blade.

How do I set the design speed parameters?

For example if I know that the cruise speed is 97 knots and the prop RPM should be around 1860 rpm should I enter those values? Or would it be better to use the max aircraft speed and MAX prop RPM speed??? Or does it matter as long as it's accurate???
 
Ideally, you get the data from the aircraft specs. If that's not possible, you'll have to pick something. The prop design speed is a tradeoff; the prop will be most efficient operating at its design speed. So, for example, you can set the design speed to optimize top speed, economy cruise, or climbout. It's one of the factors you can tweak to make your model match the published performance data for the plane.

Say you've picked a design speed for optimum cruise. You test the plane and discover that it cruises too fast (or uses too little fuel) but its climb rate is below par. In this case, you could retune the prop for climbout instead.

To set the prop design speed for a certain flight configuration, enter the desired prop rpm and the sum of true airspeed plus half the propwash. (That's the actual airspeed through the prop.) You'll have to determine propwash by testing the plane. Start by setting the design speed to the plane's desired speed plus 10-20 knots. Then fly the plane in X-Plane and use the Settings -> Data Output menu to display the propwash velocity on screen. Then you can go back to PlaneMaker and use the actual propwash value to correct your design speed.

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