I’m back with another weekly Part 107 video test question. This week we are dealing with a question that would fall under the loading and performance section of the Part 107 knowledge test: angle of attack.
Here’s the question, which tracks with the video above:
A stall occurs when smooth airflow over the unmanned airplanes wing is disrupted and the lift degenerates rapidly. This is caused when the wing:
- exceeds the maximum speed
- exceeds the maximum allowable operating weight or
- exceeds its critical angle of attack.
So, let’s start with the first option. It’s not when it exceeds the maximum speed. In fact, this is kind of the opposite of the correct answer. The faster that the airplane would go, the faster that the airflow would be over the wing. So that’s not the correct answer.
Moving to the second option, it doesn’t exceed its maximum allowable operating weight. This one doesn’t really make much sense to me. If the wing is too heavy for the aircraft, why is it part of the aircraft? Maybe it’s related to the overall weight of the aircraft. Either way, this is not the correct answer.
The heavier and airplane is the more lift the airplane will need to create before it can fly. So, that might change the plane’s critical angle of attack but that’s not the correct answer.
Angle of Attack
Finally, let’s talk about the third option, which is obviously the correct answer. It’s when the wing exceeds the critical angle of attack. What is the angle of attack? I’m glad you asked.
Critical Angle of Attack
The angle of attack is the angle at which the wing meets the air that is flowing over it. When you see an airplane actually taking off, it is lifting the nose up into the air. But if that nose continues to rise it’s going to reach a point where the air isn’t able to smoothly flow over the wing or the rotor of a quadcopter. This is kind of intuitive if you think about it. Aside from a fighter plane, or something that has the engine power to flying straight up and down, if you picture a normal passenger airplane taking off, what would happen if the pilot just kept lifting the nose into the air?
Eventually, instead of the air separating and going both over and under the wing (which creates lift), like the picture below, the wing would become a wall and the air would just hit the bottom of the wing. This would not create lift and would cause the plane to drop. And this is when the airplane exceeds its critical angle of attack
Exceeding the critical angle of attack is known as a stall. This has nothing to do with the engine stalling, it just has to do with the wings not creating enough lift to keep the airplane in the air.
I have a whole section of the Ultimate (free) Part 107 Study Guide on forces that effect the flight of drones. If this question and the correct answer don’t make sense to you, head on over to this article and give it a read.