Every pilot who flies a plane is familiar with prop designations. When you buy a motor from us, you can look at the prop test data and determine whether you need a 6x4 or a 10x3.8 (read as "ten by three point eight") prop. But what do those numbers mean in practice?
Posted by Jodi A. on 7/14/2017
For the MiniBipe
, we recommend a 7035 prop
. (Two of these props are included in the MiniBipe power package.) We could change the designation to 7x3.5, and it would mean the same thing. The first number is the length of the prop in inches.
The second number is the pitch. The pitch of a propeller is defined as the "distance in inches of forward travel per revolution of the propeller." So a 7035 prop is seven inches in diameter, and, under ideal conditions, would move the Minibipe forward 3.5 inches per revolution.
A low pitch propeller like the 7035 provides good thrust at a low speed; in other words, the plane could take off almost from a standstill. These props are great for parkflyers like the MiniBipe. However, the plane will not have a very fast top speed. High pitch props are inefficient at low speeds, but become more efficient at higher speeds and are great for sleek, high speed models.
How fast can my plane go?
We can use this information to determine the maximum airspeed of your plane, but we need a couple more numbers. First, we need to know the RPM of the motor spinning on your plane. For the MiniBipe, we recommend the HURC 250 Slofly. Ideally, you would use a tachometer while running up your motor, while the plane is on the ground or a workbench. Use the following formula:
1. Multiply the RPM by the propeller pitch (e.g., RPM 5699 x 3.5 = 19,946.5)
2. Divide the product of the above calculation by 1056* (e.g., 19946.5/1056 = 18.89)
3. The dividend 18.89 is the speed in miles per hour (mph).
*1056 = 12 (inches per foot) x 5280 (feet per mile)
What if I don't have a tachometer?
If you don't have a tachometer, there is a workaround. In our motor listings, we provide the amount of static thrust produced by various propellers in conjunction with the motor.
On the 3-cell battery (we recommend the 11.1V 350mah 35C EZ Flite Lipo Battery for the MiniBipe), the GWS 7035 prop provides 13 oz. of static thrust. The MiniBipe itself weighs about 8 oz, and with the power system, the weight of the plane to thrust is just about 1:1. This means that the plane can take off with little to no runway.
12928 x 3.5 = 45248
45248/1056 = 42.85
So, under ideal conditions, the MiniBipe could go just under 43 mph.
We don't live in an ideal world
The above formula gives us the mph in ideal conditions. The MiniBipe is never going to reach 43 mph because it is designed for drag. Its aerodynamics work against it, though it's perfect for its purpose.
Wind is another factor, as is air density. If the air is humid, it is denser, and the prop will not be able to work efficiently. Air density causes prop slip. (A prop works like a screw. The prop drags through dense air and is not able to complete its full pitch, the result of which is called prop slip.)
At best, the formula gives you an idea of the performance you can expect from your prop, but keep in mind the environmental factors, as well as the aerodynamics of your plane.