Like many aviation hobbyists I enjoy a challenge. Particularly I enjoy a challenge that makes me want to learn more about aerodynamics and building techniques. So when I came across a small foam glider at a craft store I thought, “I’ve converted two of these before, why not this small 24″ glider?” So without hesitation I snapped up the glider and had it on the workbench later that evening.
Wanting to keep the costs low, I decided to use what I had on hand which included a park sized E-flite 450 brushless motor and some small three cell 900mAh lithium polymer batteries. I knew weight was going to be an issue for a build of this size, however, I was confident the thrust my motor would provide should solve any issues getting airborne.
A week later it was time to test out my creation. Since it was a smaller, heavier airplane, I decide to play it safe and set my throttle to 3/4 for my hand launch. My thinking was a worst possible situation would be a wobbly first few seconds of flight mixed with some fast finger work on the transmitter sticks. I powered it up and gave my experiment a solid toss skyward.
It ended as quickly as it began as I watched the plane shoot up and immediately roll left and slam into the ground. Hmm, that didn’t go as planned. As I gathered up the broken pieces my wife (who is also a pilot) made a rather poignant statement, “Do you think the engine was too powerful? It looked like it took off pretty straight and level but then rolled left like it had too much torque.”
I pondered for a few seconds, then realized she was on to something. In my obsession to have enough power to get this creation skyward I naively forgot about a thing called torque. At that moment it dawned on me; torque matters. By not understanding the concept of torque and its effects during flight, it became the demise of a beloved experiment. I had overpowered my aircraft so much so that its response to the laws of physics had inadvertently caused my crash. Shortly thereafter my goal was to understand this force and its effect on my aircraft.
The following information are excerpts from the Federal Aviation Administration’s (FAA) manual titled Pilot’s Handbook of Aeronautical Knowledge (its free to download). Even if you don’t have plans to become a full scale pilot, these books contain flight physics information that is valuable for every model hobbyist. Because no matter the size or shape, the physics of flight remain the same. And by educating yourself down here, you’ll only get better up there.
Pilot’s Handbook of Aeronautical Knowledge
Understanding Torque and P-Factor
To the pilot, “torque” (the left turning tendency of the airplane) is made up of four elements which cause or produce a twisting or rotating motion around at least one of the airplane’s three axes. These four elements are:
- Torque reaction from engine and propeller
- Corkscrewing effect of the slipstream
- Gyroscopic action of the propeller
- Asymmetric loading of the propeller (P-factor).
Torque reaction involves Newton’s Third Law of Physics—for every action, there is an equal and opposite reaction. As applied to the aircraft, this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the aircraft in the opposite direction.
When the aircraft is airborne, this force is acting around the longitudinal axis, tending to make the aircraft roll. To compensate for roll tendency, some of the older aircraft are rigged in a manner to create more lift on the wing that is being forced downward. The more modern aircraft are designed with the engine offset to counteract this effect of torque.
When the aircraft’s wheels are on the ground during the takeoff roll, an additional turning moment around the vertical axis is induced by torque reaction. As the left side of the aircraft is being forced down by torque reaction, more weight is being placed on the left main landing gear. This results in more ground friction, or drag, on the left tire than on the right, causing a further turning moment to the left. The magnitude of this moment is dependent on many variables. Some of these variables are:
- Size and horsepower of engine
- Size of propeller and the rpm
- Size of the aircraft
- Condition of the ground surface.
This yawing moment on the takeoff roll is corrected by the pilot’s proper use of the rudder or rudder trim.
P-factor is a term for Asymmetric Loading. When an aircraft is flying with a high angle of attack the “bite” of the downward moving blade is greater than the “bite” of the upward moving blade. This moves the center of thrust to the right causing a yawing moment toward the left.