Stall speed testing for certification: To idle trim or to PLF trim?

It has been more than once that I have been involved in discussions about stall speed testing for Part-23 certification purposes and, as far as I have experienced, opinions seem to split over a certain part of the testing technique. (To add to this, reference suggest that there have been controversies over stall speed testing even within the FAA organization [1]).

bearhawk1

More specifically, “§23.201 Wings level stall” indicates the conditions under which the stall should be demonstrated and 23.201(f)(6) states “Trim: At 1.5 VS1 or the minimum trim speed, whichever is higher.” (same applied for VS0 testing). What is not very clear is whether the regulation require this trim configuration with power idle (off) which would result in a constant descent rate, or trim with power for level flight (PFL) and then bring the power back to idle. Depending which condition is used, the trim setting will be different, with the idle condition resulting in a larger trim tab deflection downwards (reaching even maximum limits), which will result in a larger upwards elevator deflection in order to generate the pitching moment required for equilibrium. So does this small (or not) difference in the trim tab position effect stall speed and is it important?

Running back to basic aerodynamics, for a fixed configuration, stall speed depends on weight, density, and maximum lift coefficient (stall angle of attack). Where does the trim tab get into play? Well, the point is that in certification testing the definition of the stall speed is not always resulting from an aerodynamic stall, but stall speed can be defined by “§23.201(a)(3) The control reaching the stop.” which means that the airplane is actually elevator power limited. So the question now changes to “Does the trim tab position affect the elevator power/effectiveness?”. As in many flight test related questions, the answer is “It depends”. There have been airplanes that have not shown any change in stall speed between max down and max up trim tab position. There have been other (undocumented) cases where a nose down trim tab position provides an increased elevator effectiveness and results in a smaller stall speed than a nose up trim tab position, which would actually mean that we should trim in an idle condition to get the worst case (higher stall speed) scenario – which would be the case in a trimmed approach of an engine-out landing.

Enough analysis for certification purposes, risking to go too far to capture a small difference. The bottom line of this post is that it is my opinion, as well as others as far as I know, for stall speed certification testing at power off conditions, the airplane should be trimmed at 1.5VS at power idle. This would mean that for a stall speed determination at a target altitude, the first descent pass should have trim only purposes and by the time the desired trim tab position is established, power should be added to climb above the altitude band (without changing the trim position) and during the second descent pass on the desired stabilized condition a stall speed test point should be executed at the target altitude.

The next question would be whether trim tab position affects stall characteristics, and while someone might rush to say that those are governed by an aerodynamic stall with the trim tab having minor effect, some undocumented results indicated significant difference [2].

References

[1] Kimberlin R.D., Flight Testing of Fixed-Wing Aircraft, AIAA Educational Series, 2003.

[2] http://www.pprune.org/archive/index.php/t-445920.html, Accessed April 23, 2017.

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