As a plane speeds up in the air, the prop has less work to do, and is said to be "unloading", or "unloaded". Less work is required from the engine.

Once the prop hits the theoretical pitch speed, it is about as unloaded as it can get... and the plane cannot move any faster.

A higher pitch prop permits the plane to travel at a faster speed before the prop unloads fully.

However installing a higher pitch prop also has the added effects of slowing the engine down ( loading it more ) at lower speeds, resulting in less thrust produced for the same prop diameter.

Also if the prop pitch is too high, the prop can become "stalled" when the plane is moving slowly or not at all. In effect the prop spins but the plane produces little to no thrust when this happens... it also makes a lot of prop noise.

You see the results of this, as the plane appearing to be quite sluggish getting up to speed, but then picking up a lot of speed in a light dive.

It's all a balancing act where you sacrifice thrust for pitch speed, or the other way around.

On my Funtana I run a very low pitch 13x4W (?) prop. The plane flies SLOOOWLY at high throttle, but it can scoot straight up at 60% throttle without stopping.

With the ST75 I went with a balance between the two, letting the engine/prop unload in the air to get better speed via increased RPM's while maintaining decent verticals. When the plane goes vertical my prop choice let's the engine load down, but not to the point that the engine bogs down a noticiable amount.

With the ST75 my Stik will keep going straight up, but that takes almost full throttle. It hovers around 80% or so.

I'd say the plane clips along around 70 MPH or so when level at full throttle, and the AV behaviour has the flight speeds right.


What prop are you using now?