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]).

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.

AIAA YP Member of the month – April ’17

http://www.aiaa.org/Secondary.aspx?id=29094

Night rating complete

Finally! Night rating complete. Best part, the Toronto night tour at 2,000′ MSL (~1,750′ AGL) passing by the CN Tower just above the lighted sky-scrapers. Just magic…

Technical manuals write-up and text editing tools…

For the last weeks I have been heavily involved in the write-up of technical academic manuals for flight test. Writing of academic manuals (textbooks) is not a simple task, as beside the technical content that has to be included, the most important part is the way it is presented so it can be followed and understood by the students. Beside those primary points, a significant characteristic of an academic manual is the consistency of the technical symbols, equations and figures as it drastically facilitates readability and clearness of the content. I was fortunate enough that during my PhD studies I did extensive work on revision and editing of academic manuals under the supervision of my professor, who was insisting for the slightest technical detail going down to symbolic differences between “ν” and “$v$” – and he was right!

For those who do not know, or for those who know and still insist not to change their habits, the best and only option for technical editing excellence is $\LaTeX$, which I also use in the equations of this blog. The consistency and automation in text formatting and presentation of complicated math equations is incomparable to the mainstream editing tool, which is definitely easier and faster on a short term basis but when you are looking for technical detail it can drive you crazy and even when you finish our documents does not look that good – as with most things in life, the easier option is not the best option. So, for all those out there that want to deliver technical writing excellence, get to use $\LaTeX$, and as soon as you dive in its world, you will comprehend the weakness and inadequacy of all other tools. Peace!

Delivering supplemental training in Baltimore, ML

A short trip to the area Baltimore ML took place to deliver supplemental training on Unmanned Air System testing to a US customer. Pictures or further information cannot be disclosed. I was just amazed for one more time by the magnitude of the aerospace industry in the US, the employment opportunities it offers and the appreciation the Americans have for aviation. Really cool…

Weapons testing patch

Due to my technical and flight simulation contribution in a recent Weapons Testing course at ITPS, I was honored to be included in the course’s patch indicated by the Greek flag. A great gathering of expertise around the world delivered a valuable course for the customer.