Thrust vs. Prop Pitch

[mattag08]

I've noticed that the Axis aircraft with controllable pitch propellers seem to generate maximum airspeed (which means max thrust) at around 2400 RPM and full throttle. What doesn't sit well with me is when you increase the prop control to max RPM (even past redline on the tach) you actually LOSE airspeed.

This doesn't make sense from a physics standpoint. As someone else wrote succinctly:

The reason you always want maximum allowable RPM for takeoff: All else being equal, each combustion event is going to deliver a certain amount of power, which is generally limited by the amount of air in the cylinder. The more combustion events you can have, the more power you’ll be developing. (More revolutions per minute = more combustion events per minute). In a 4-cylinder engine, you have 2 combustion events per revolution, and in a 6-cylinder you have 3 combustion events per revolution. So, in terms of RPM, the higher RPM will always give you more power (again, all else being equal).

The amount of power we can develop is limited mainly by airflow, which relates to all three engine controls. We want the most air molecules we can possibly get, and the right fuel mixture to develop the most power. Throttle controls airflow by changing the pressure of the air in the intake manifold. Higher manifold pressure = more air = more power. RPM controls airflow by allowing each cylinder’s volume to be filled with air a certain number of times per minute. Higher RPM = more volume = more air = more power. Finally, the mixture control allows us to balance that air with the right amount of fuel to develop the most power. Too lean = Not enough fuel to develop power (air cannot make power on its own, of course). Too rich = the volume of fuel in the cylinder is too high to combust properly and is simply displacing some of that air we need.

So, for takeoff – Always the maximum allowable manifold pressure (full throttle in a normally aspirated airplane), maximum RPM, and best power mixture.

What's the beef here? Full throttle and full prop should always be more power than some other prop setting.