Like many rc hobbyists I enjoy a challenge. Especially a challenge that makes me want to learn more about aerodynamics and building techniques.
This article covers my conversion of a hand-toss foam glider in to an rc airplane. The build was easy, fast, and relatively cheap. I recommend you use an electric motor more suited to an aircraft of this size and weight; like a Park 250 Brushless Outrunner from E-flight. Unfortunately, due to a severe oversight in understanding the concept of torque, the laws of physics prohibited my craft from flying and instead my first flight ended in disaster.
The Foam Glider
When I came across a small 24-inch foam hand-toss glider at a craft store I thought to myself, “I’ve converted a 4-foot version of this in the past so why not this small glider?” And without hesitation, I snapped up the glider and had it on the hobby workbench later that evening.
Needing to keep the costs low so I decided to recycle electronics from my past airplane builds and only use what I had at home. I used a low-temperature glue gun for the build and clear packing tape for any hinge surfaces (e.g., elevator). The electronics I used were:
- (2x) Hi-tech micro servos
- 25amp brushless ESC speed control
- E-flite 450 890Kv brushless motor
- 9×6 Slow electric APC propeller
- 3-cell 1320mAh 11.1v LiPo
Too much power can be a dangerous thing if you don’t know what it’s capable of doing, so be ever mindful when you decide to overpower your aircraft because, torque matters.
The Airplane Build
Weight was possibly going to be an issue for a conversion of this size, however, I’ve flown bricks in the past and was confident I could do the same in this case. Plus, I knew the power my motor would provide should solve any issues getting airborne. The total time it took to do the conversion was a paltry 6 hours over the period of a week. The weight was lighter then I anticipated and I paid particular attention to the center of gravity (CG) to ensure I would have a relatively stable thrust line. While the approach for this particular build & conversion was relatively solid, the oversight regarding torque ended any hopes of seeing this hand toss glider soar into the skies.
RC pilots often talk about how large of motor or engine they have installed in their airplane and this got me wondering how many rc pilots really understand the effects of torque on aircraft. I’ve seen way too many beautiful airplanes crash shortly after take-off or during landings because it seems the pilot doesn’t understand the effects of torque at slow air speeds. It seemed to me that many didn’t understand that too much power can be a dangerous thing if you don’t know what it’s capable of doing – this included me. I just didn’t realize it at the time.
It was time to test out my creation. Since it was a smaller, heavier airplane, I decided to play it safe and set my throttle to 3/4 for my hand launch. My thinking was the worst possible situation would be a wobbly first few seconds of flight mixed with some fast finger work on the transmitter. I powered the airplane up and gave it a solid throw.
The flight ended as quickly as it began. The plane lifted up, immediately rolled left, and slam into the ground. Clearly, that didn’t go as planned. Gathering up the pieces my full-scale pilot spouse 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 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. It was at that moment it dawned on me – torque matters. T he demise of a beloved experiment was because I didn’t understand the concept of torque and its effects on flight. I had overpowered my aircraft so much so that I inadvertently caused my crash. Now my goal was to learn more.
What I Learned From This RC Experiment
I learned that torque and P-Factor matter.
The following information is from the Federal Aviation Administration’s (FAA) manual titled Pilot’s Handbook of Aeronautical Knowledge. Even if you don’t plan on becoming a full-scale pilot, the book contains information that is valuable for every RC hobbyist.
What Is Torque?
To the pilot, “torque” (the left turning tendency of the airplane) is made up of four elements that 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 the engine
- Size of the 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.