MOT_THST_EXPO and need for thrust stand

[jstroup1986 (Joseph G Stroup Jr)]

I’m repeating my tuning process that I followed on my EDU-450 now on my TD-650 - and making a spreadsheet that documents the steps. The steps are:

1. The initial parameters offered in Mission Planner specific to this copter.
2. The Initial Tuning Parameters in the ArduPilot docs.
3. The Alt-A Plug-in suggested parameters.
4. FFT dynamic notch filter settings.
5. Auto-Tune

In the first steps, the value of MOT_THST_HOVER is estimated from a graph. The Tuning Instruction docs say the value for MOT_THST_HOVER is best calculated using a thrust stand.

This prompts a few questions:

1. What is the formula for determining MOT_THST_EXPO from thrust stand data?

2. Would a thrust stand such as this $100 unit from BangGood generate results accurate enough to be worth the trouble?

Racerstar Brushless Motor Thrust Stand V3 For 11mm-59mm Outrunner Motor

3. How sensitive is the copter’s performance to changes in MOT_THST_EXPO?

4. How will the copter perform is MOT_THST_EXPO is set too high or too low?

Thank you!

[AngelAlbeniz (Angel Albeniz Marquinez)]

The MOT_THST_HOVER is calculated by the drone in flight automatically if you have MOT_HOVER_LEARN activated. Even if you change the payload the drone will adjust the MOT_THSH_HVER value to the required one. So, you don’t really need to worry about this.

The MOT_THST_EXPO parameter is different, as you say you have to derive its value from a graph of approximations according to the propeller size. This works very well for most multi-copters, especially small and medium sized ones. But in some cases, with large drones or drones that use “closed” ESCs which may be correcting this exponential, it is good to adjust the value.

Here are two forum posts on this same subject for you to take a look at:

AutoTune in 32inch quad result in strange PIDs, crash in next flight

That thought also crossed my mind, and for my case, in which you have a Motor/ESC combo that is indivisible, and where is almost impossible to take it apart and use it with other different hardware, here can make a little sense. You will obtain a system with a more linear response which is always desirable. But you must tell the client!! Otherwise, as we are doing, we compensate for something that is already compensated. And besides, as you said, compensating for an exponential response in a sy…

LARGE Octoquad instability

I am in the final stages of tuning a heavy lift octoquad and I am running into an issue with the roll axis that appears and disappears seemingly at random. The roll axis will respond very slowly to my inputs, almost like the sensitivity was turned way down. And the copter will be very slow to return to level after being rolled in either direction. This behavior only occurs on the roll axis, and appears and disappears seemingly at random. So one second everything is fine and the next second I hav…

Using "measured" MOT_THST_EXPO: What improvement can one expect?

Hi Olli, This is an extremely important parameter for tuning aircraft. Most ESC’s work on a principle that the commanded input is approximately proportional to the average voltage used to drive the motor. This results in a strongly non linear thrust response for each propeller. I used measurements of approximately 100 propellers to come up with the default MOT_THST_EXPO value of 0.65. The basic principle is the larger the prop the higher the value. So a 30" propeller would be closer to 0.8 for…

As for how to calculate the MOT_THST_EXPO parameter from the thrust stand data. There are several methods but the simplest is to use the following spreadsheet. Just by entering the data it automatically calculates the correct value.

ArduPilot Motor Thrust Fit

Finally, as for the stand you attach the biggest problem, I see is that it does not have a data output. The only interface is a small screen. This is ok to see maximum thrust or thrust at some point of the throttle, but it makes it very difficult to get data from the whole thrust curve. The only way is to do it manually by writing down the thrust at each throttle value.
As I said in a post I attached above, I would recommend that you try to make one yourself if you can. It is the cheapest option and allows you to automate the tests and capture the data from a computer.