The hypothesis:
I have witnessed airframes set up with a 4-cycle engine is capable of a harder corner, with a larger propeller, and further forward CG than an equivalent 2-cycle engine. Windy spoke of this in his development, and marveled at the fact that airframes that were definitely nose heavy (compared to the same airframe that was previously utilizing a ST 60) with a 4-cycle engine appeared to require no real amount of tailweight to achieve the previous amount of hard corner. I have also personally observed other 4-cycle planes that were running excessively large propellers (up to 16”) that appeared to lose nothing in the corner. The usual effect with the 2-cycle being that a larger prop on a constant nose moment gets progressively harder to turn in a sharp corner in our small hemisphere. I think we can all agree to this, being that the effect is created by several factors (precession for example).
Why would a 4 cycle have less resistance to a corner than a 2-cycle? Is it possible that a 4 cycle engine translates power to the propeller in such a way a to reduce a general resistance to a change in orientation (or precession) over a 2-cycle engine?
My theory is as follows:
An engine is not applying torque the blades of the propeller in a constant fashion. Actually, torque is applied in pulses of very short duration. The prop is spinning due to inertia for the majority of the period of one cycle. This is the effect just like a bullet traveling a long distance due to an initial acceleration. The bullet retains some of its energy over the length of its travel due to the engine imparted in the barrel. The bullet is actually losing energy as it goes along.
A spinning propeller is actually resisting a change in orientation due to simple mass (Precession) and is also resisting due effective drag on the blades as they attempt to proceed at constant pitch through a corner. In effect, for an inside corner the top of the propeller disc is traveling backwards and the bottom of the disc is trying to travel forward (I know this doesn’t happen, but I am at the limitation of my vocabulary) while all the time the blade is constant pitch.
In a 2-cycle engine, the blades are affected by imparted force twice as often as a 4 cycle. The frequency of force applied to the blades is twice the frequency of the 4-cycle, even though the average resulting average amplitude is equal. This means that the 4-cycle propeller is “pin wheeling” twice as much as with a 2-cycle engine. If we were to observe the actual speed of the aircraft in microscopic terms, the speed of the aircraft is not constant; it is actually “pulsing” along. With a 2-cycle engine the pulses are twice as fast (one combustion for each 360 degrees of rotation), allowing a shorter distance of travel between each moment of imparted force.
The drag of the prop disc against the corner is elevated at the moment of combustion. At that point, the engines force is working against making a corner.
In summary:
The 4-cycle plane would travel twice as far between each input of torque, allowing less resistance to a change in orientation over the same distance. The average torque imparted to air would be the same in that distance, but would be elavated at a much shorter period.
This would mean precession effects in a hard corner would also be less by the same theory. In our VERY SHORT period of testing one observation with the 4-cycle has been that the usual loss in line tension at the top of the outside squares is somewhat diminished or completely absent.
It’s a theory.