# Can someone explain trim throttle?

Hi guys, what is the difference between trim throttle and trim airspeed? my F405 WING flight control, flashed arduplane3.9.9 firmware, without airspeed sensor, I set trim throttle in 45%, trim speed at 15 meters, but in the air, the trim throttle sometimes drops to 30% to 40%, trim speed is not accurate in setting values.

Target percentage of throttle to apply for flight in automatic throttle modes and throttle percentage that maintains TRIM_ARSPD_CM. Caution: low battery voltages at the end of flights may require higher throttle to maintain airspeed.
This is taken from the parameter list, and it tells the firmware what to try to maintain throttle at. Of course it will not be fixed at that value, the TECS algorithm will change the throttle based on the flying conditions. If the plane is gaining or loosing altitude, if it is in a turn, all of these conditions will alter the throttle value.

Let us take a practical example to better understand these parameters. I fly the ramler RS without airspeed sensor. I set the FWB _ MIN to 13 m/ s, which is much higher than the stall speed, and keeping this speed flying will not leat to altitude lose. The FWB _ MAX is set to 26 m/ s and TRIM _ throttle was set to 45% ,Trim _ ASPD _ CM to be set to 15 m/ s. The speed of wind is about 8 km/ h. I will set a rectangular course, and there is no special provision for the speed of each waypoint. The fly height is set to 150 M.

It is understood that the throttle should be kept at 45% when the automatic flight mode is flying along the course .

and for the RS, as long as the throttle is kept at 45%, the airspeed is certainly between 13 m/ s and 26 m/ s, and the height is not dropped. However, the actual flight is such that the aircraft is flying more than 45% of the throttle, and the airspeed is also higher than the set speed (15 m/ s), sometimes lower than the set speed. Why are I so concerned about the speed? First, we want two planes to take off the same route, and to avoid a collision, and as close as possible to take pictures, we hope that our speed and height can be kept at a fixed value. Secondly, when I hope to fly automatically, the speed of the airplane can be kept at a fixed speed and a high level when we are not observed for a moment, so that it is not easy to stall. I understand that if trim throttle is set, this value should be fixed.

Yeah, I understand what you want, but it doesn’t work like that. Trim_throttle is like suggested throttle and not a fixed value the autopilot will maintain. As I said, it is managed by the TECS (Total Energy Control System) and it depends a lot on the wind conditions. I noticed when flying in windy weather and with no airspeed sensor the autopilot tends to be on the heavy side with the throttle.
For your application I’d say airspeed sensors is a must, then keeping a set airspeed is a lot better managed by the autopilot. Even in this case, the throttle would not be fixed to a certain value but will fluctuate with the altitude/speed demands.
For example in one of my missions, in 10m/s wind, I noticed sometime throttle going to 0 and plane pitching down to maintain set altitude, but the airspeed is always +/- 1m/s of the set 16m/s mission speed. This is with airspeed sensor attached and calibrated.
Best is to read about how TECS works and than do the suggested tuning, it will fly a lot better.