It’s 7 AM on Saturday, and you woke up extra early today to finish off your quad from the awesome parts you just ordered from GetFPV. For the past week, you have been binging on Youtube to learn how to FPV, and have stayed up late many days, trying to get that perfect solder joint on your flight controller.
Everything is ready, except that you forgot which motor goes where and what direction they’re supposed to spin. Not to mention, the propellers, which are confusingly labeled “L” and “R” in microscopic letters, just add to the confusion. To top it off, after securing your propellers on the quad, you find that after arming it and taking off, it flips wildly and crashes, splattering broken plastic propeller bits all over the place!
Never fear, a simple understanding of how motor and propellers work together to get the quad flying will set you straight on the path to success with your aircraft the FIRST TIME it flies.
This article was submitted through the GetFPV Community Program by Lawrence Ro.
Disclaimer: This article was written solely by a member of the FPV Community. Views and advice in this article are that of the author and does not reflect the opinion or views of GetFPV.
First off, we need a general understanding of how things fly in the first place. A guy named Daniel Bernoulli figured out a long time ago a unique relationship between speed and pressure. Basically put in terms of aerodynamics, the faster the air flowing over a wing, the lower the pressure on that part of the wing becomes. This is called Bernoulli’s principle. As shown in the diagram below, we can see this principle applied to an airplane wing:
Notice how there is a lower pressure where the air is flowing faster over the wing (on top), and how there is higher pressure where the air is flowing slower over the wing. This is because that the air traveling over the upper “hump thingy” on the wing has to travel the same distance that the air on the lower flat side has to travel in the same amount of time.
This makes the air above the wing travel faster, and the air below the wing, slower. The higher pressure underneath the wing pushes the wing up, and whatever is attached to it, whether its a fuselage to an RC plane, or the 400+ people in a Boeing 747. Again, this is a very general idea of how things fly.
From Planes to Multirotors
Now, does this apply to multirotors and propellers? Absolutely! If we take a look at a miniquad prop here, we can directly see where Bernoulli’s principle is taking place. Instead of a wing flying forward, the propeller has blades that spin in a circular motion. In both cases, lift is generated by the high pressure pushing the wings and blades up.
This is why props with a higher number of blades (ex. 4 vs 2) can generate a higher amount of thrust at the cost of efficiency due to there being more “wings” connected to the propeller.
However, the propeller markings can be somewhat confusing. The L and R generally mean CW and CCW. Props with the L label mean that they go on a CW rotation motor. Props with the R label mean that they go on a CCW rotating motor. If there is no marking, then generally assume that the propeller is a CCW rotation, as the screw on the motor would screw on normally (lefty-loosey, righty-tighty).
Motor / Propeller Direction
But why do our motors need to spin in different directions in the first place? Well, if our quadcopter motors were to spin only in the CW direction, then the quad would spin out of control in the yaw axis due to the angular momentum of all four motors. If we rotate an even number of propellers in the opposte direction adjacent to each other, as seen in a quadcopter, the angular momentum of the motors cancels each other out, resulting in a stable yaw axis.
This comes to another point: CCW motors have normal threads, while CW motors have reversed threads as seen in the image below. These days, many people just use CCW motors and screw their prop nuts extra tight to avoid confusion.
In some aspects, this can be beneficial because it makes manufacturing and buying a lot easier. However, you would only get the self-tightening effect of the prop nuts by two of the motors, not all of them. This is because as the motors spin in their set direction, their rotation keeps the prop nut secured on, so the extra tightening of the nuts is not needed.
In fact, some propellers have threads built into them. Many aeriel photography rigs have self-tightening props. These props don’t need a prop nut because the nut is practically the blade itself. As the motors spin up, they self-tighten the motors, thus keeping the props nicely secured. When new props are needed, a quick flick of the wrist frees the prop from the motor and makes swapping blades quite convienient.
As you can see in the image below, the self tightening propellers effectively utilize the use of differing prop rotation. Also, it is more aesthetically pleasing, as manufacturers are able to add stylistic caps to the top of their props.
Using this new power of knowledge, you never need to be scared of not knowing which way your propeller spin. In fact, you don’t even need to check the very tiny markings on your blades. Here’s the foolproof rule I use: The prop spins towards the higher part of the blade, otherwise known as the leading edge. It’s really that simple! Now, you just need to make sure that your motors are spinning in the correct direction. Never again do you need to worry about your quad flipping out after arming due to the wrong propeller type and direction! Happy FPV!