The only job of a propeller is to take the torque generated by the motors and translate it into movement through thrust. Thrust is a great way to understand how much torque our motors can generate, but this is not the case when trying to correlate thrust to speed.
A few weeks ago, I did a speed run and found out my 5045 propellers were actually the fastest props I had out of the nearly dozen of different prop types I have. I figured it was time to find out why and it was an eye opener.
The Basics: What Do Prop Numbers Mean?
The first two numbers tell us the diameter of the prop in inches – just put a decimal in between the two numbers. Therefore a 5045 prop has a diameter of 5.0 inches. The last two numbers tell us the pitch of the prop – same thing here, just add a decimal in between the numbers. A 5045 prop has a pitch of 4.5 inches.
Thrust ≠ Speed
One of the largest factors in determining your quads top speed are the propellers. You’ve seen all the thrust results of various propellers and you are determined to get that 6040 prop that is throwing down 1600g of thrust. That has to be the fastest one, Right? Well, thrust can help fight the effect of wind resistance (the main drag force acting on quads) which does translate to some speed, but RPM is an even bigger factor as it turns out.
I’m letting the cat out of the bag early here: a 5045 prop that puts out 1200g of thrust is going to give your quad a faster top speed than that 6040 1600g monster. How? It has to do with the pitch and RPM. Theoretical RPM can be figured out by taking your motors kv and multiplying it by your battery voltage. For a 2300kv motor with a 4s (14.8v), that would be 34,040 RPM. Now we need to determine the theoretical speed.
Since a 6040 prop has a pitch of 4 inches, that means for every rotation the prop makes, the quad will (theoretically) move 4 inches.
So now we need to see how many inches it will move in an hour. Since we know the prop will rotate 34,040 times a minute, we multiply that by 60 to get 2,042,400 revolutions per hour. Then we multiply by 4 inches to get the number of inches it travels in an hour, then divide by 12 to get the number of feet traveled, and lastly divide by 5280 to get the miles per hour traveled. That works out to be nearly 129 mph. If we go through the same calculations with a 4.5 inch pitch, that works out to 145 mph! However, we will never see this speed due to battery voltage drop and air drag (both on the props and on the quad frame).
More Amps and Less Drag
As far as voltage drop goes, we are limited by battery technology. Just be sure to use a battery that can handle as much amp draw as possible. Using the B1 project as an example, the voltage dropped to 13.6 volts at a top speed of 113 mph. If we do the math using the actual top speed voltage of 13.6, 2300kv, and the prop pitch (4.5 inch), we get a theoretical speed of 133.3 mph. However, the B1 was still 20 mph shy of this meaning it was robbed of that much speed due to air drag. Reducing air drag is the easiest way to increase speed as we will eventually see.
What Does Thrust Do For Us?
You are still probably thinking “What about the thrust difference between the props? Doesn’t that help at all?” A little bit yes but mostly no.
In the next part of this series, I will explain how and why a quad with 6040 props and 6400g of total thrust is slower than 5045 props and 4800g of thrust.