Ok, for math geeks, here’s a nit-pick point. 

 

When doing the poor man’s dyno test the propeller slip stream will cause the wing to create some lift (small amount).  The amount of lift is of course dependent on slip stream velocity and angle of attach.  All is ok so far, as the actual weight is not really a consideration, only the rotational force (i.e. the difference in the right and left scale indications).

 

However, the slip stream has a rotational component to it.  Thus, the actual angle of attach on each wing is a bit different.  Thus, the lift created by each wing is bit different.  Thus, there is an additional rotation (torque) force that is not due to “the engine using the prop to fight against the air” (i.e. the engine HP).  Since the left wing will have a higher angle of attach it will create more lift and tend to reduce the force (torque) as measured on the scales.

 

Also the rotational component of the slip stream impacts the tail, which also causes a (small) induced aerodynamic rotational (torque) force.  This also is in the direction of lifting the left wing, which reduces the force (torque) as measured on the scales.

 

While these are effects presumed to be insignificant in relative magnitude. They are essentially part of the asymmetric force affects taught during twin (with an engine out) training - with a little editorial liberty.

 

Rick