Prop Tip Speed Calculations
This information will help you to choose the best MOTOR / PROP / BATTERY combination for your next scratch build.
By calculating and comparing your setup’s tip speed, you can gauge if your powertrain component selection will be suitable. This guide will show you how to calculate the propeller’s theoretical tip speed, and enable you to compare it to other well regarded setups.
Why Theoretical Tip Speed - Not Actual Tip Speed
Propeller tip speed is very important if you want good perfromance from your drone setup. But actual tip speed is a difficult calculation and we actually don’t even need it. What we want to do is compare your proposed setup with other known commercially produced setups. And for this, we only need to do a theoretical tip speed calculation. This is more than adequate to compare setups.
How to Manually Calculate Tip Speed
Use the calculation below to manually calculate the tip speed for your peoposed setup.
NOTE: You can also download the excel file linked at the bottom of this page which contains all the tables and graphs shown on this page.
For Cell Voltage I like to use 4 volts as a nominal voltage per cell.
Throttle Limit can be adjusted to decrease the tip speed of your setup if needed.
We use Miles Per Hour as a good indicator as it relates to the speed of sound.
Tip Speed Calculator
The motor KV averages for each category of drone in the table below are taken from my Motor Size resource. This is a catalog of all commercial FPV drones on the market and also a great resource for seeing how your proposed setup compares with a known quantity.
We are working here with theoretical values. There are actual forces of drag on the propellor as well as motor considerations which affect tip speed in the real world. However the purpose here is to compare one motor, prop and battery setup to another and for that, we don’t need to take into account all the forces acting on a prop in flight, so theoretical tip speed calculations are just fine. If you calculated the actual tip speed the value would be considerably less than shown in the chart below.
In general, we see a trend for freestyle setups to have a higher theoretical prop tip speed approaching the speed of sound while cinematic and long range setups tend to have lower tip speeds for stability (smoothness) and efficiency in flight.
Comparing just freestyle setups we see a good consistency in prop tip speeds. In this data set I have left out whoops as these are not specifically designed for freestyle. Also note that while the 5 Inch Freestyle on 8S has been included in this data the tip speed value is an average of 1 sample size so probably not an accurate indication.
For cinematic drones we tend to see a lower tip speed overall but still a good consistency across different sizes of drones regardless of cell count.
For whoops we can see a good Correlation between 1S setups and a slightly higher average tip speed for 2S setups.
For whatever style of drone you are intending to set up, select your Motor KV, Prop Size and Battery Cell Count to give a tip speed as close as possible to the averages shown in the charts above. Adjust tip speed down if necessary by setting throttle scaling in Betaflight if you can’t adjust it by choosing a lower KV motor or smaller prop. You can download this chart and graph data in the excel file below to make it easier to adjust the values.
