Torque is the cross product of r and F where F is the force and r is the vector from the rotational axis to the point of action. The magnitude of torque can be found by simply multiply F by d, where d is the perpendicular distance from the axis of roation to to the line of action. In the case of props, d is simply the length from the hub to the center of mass of each blade (find the torque of each blade individually then, add them together)

So, to increase torque, you can increase either F, d, or both. F is a function of mass, so a heavier prop will increase torque. d is function of prop diameter, so a large diameter, low pitch prop will also increase torque.

Also, inertia is defined as mr^2. Where r is the radius (in this case equivalent to d--that is, the distance from the center of mass of one blade to the center of the hub). This shows that increasing m and r will increase inertia, and due to Newton's 3rd law, the rotation will be faster.

Additionally, rotational kinetic energy is 1/2Iw^2. Where I is inertia (above), and w (omega) is angular velocity. Increasing the rotational kinetic energy is done by increasing I or angular velocity (~RPM). For the prop/plane system to be in equalibrium, the plane's rotational kinetic energy must be equal and opposite of the prop. Increasing the kinetic energy of the prop will increase the TR rate.

So, to increase the rate:
Increase mass, angular velocity (RPM), and the diameter.

As I mentioned above, I is a funtion of r squared, and kinetic energy is a funtion of w squared and I. Because m was not squared in any of the equations above, angular velocity and diameter will play a larger roll in TRing than mass of the prop.
As somebody mentioned, increasing mass will increase spool-up/down time and make corrections harder. Due to this, I would try to increase diameter and speed before increasing mass.

The above discussion assumes all other things equal and is just to get the most torque out of the little spinny thing.

Factors hindering the speed of rotation are wind, surface area, and weight.

Weight increases the inertia of the plane and means that more force is required to get it to spin. (Newton's first law--an object will remain with constant velocity until acted upon by another force). Reducing the weight will allow the plane to spin faster (and accelerate faster to that final spin rate)

Surface area increases drag and air resistance, creating more forces to be overcome. Wind is just another source of air resistance. Reducing surface area (wing, fuse, tail, etc.) will make it easier for the plane to spin, and it will be less affected by wind.

I don't think CG will affect spin rate. AoA should be such that the engine thrust line is vertical.

Hope that helps.

-Eli Liechty
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