Difference in Stall Speeds

[Genghis the Engineer]

Plus pitch rate inertia - the higher deceleration rate equals a higher nose-up pitch rate, and the aircraft will tend to go further into the stalled regime. For this reason, high pitch rate stalls in non-laminar flow wings tend to be more exciting (laminar flow wings can be the reverse, since getting deeply into the stalled regime quickly usually ensures that both wings stall together and avoids much risk of an asymmetric stall, and thus possible incipient spin).

G

[QJB]

Thanks Genghis,

So what I was taught in aerobatics training about there being a "stall stick position" was not quite correct. One stick position does not always correspond to one angle of attack for a given configuration, C of G etc? Is angle attack also a function of rate of pitch as well?

J

[Genghis the Engineer]

I've heard that as well, but am fairly (but not totally) convinced it's cobblers.

One of these days when I've got a few hours to spare, I'll work through the flight mechanics equations and try and prove, or disprove, that.

G

[Pilot DAR]

This one just drops the nose slightly and it enters into a kind of a "nodding" (pitch oscillation) motion.

I had a Cessna 206, with a Robertson STOL kit do this repeatedly during flight testing. Throuble was that in the test configuration, it did not ever reach the aft pitch control stop (though I did reach the forward stop several times - but that's a different tale!). Therefore, with the nodding going on, and a 10 knot speed variation, I could not pin down one particular stall speed.

I had the opportunity to refly the aircraft with a regulatory authority test pilot, and he made the same observation. He explained that for the purpose of certification, the speed at which the nodding began (being the higher speed in the range of nodding) would be the stall speed, as that was the first speed at which an uncontrollable pitching down began. Flying more slowly, though sometimes possible, was not required, to demonstrate design compliance.

A 2 degree trim tab difference would indicate to me that there are other differences between the two aircraft too (all other things beign equal I presume). I have this with two different Piper Navajos, I have been flight testing recently. Quite different trim tab posititons, for otherwise similar configurations. I'm not far enough into the flight testing yet to compare stall speeds between them though.

On another stall test program years ago, I found that the factory new aircraft I was flying, though a correct stall speed per the flight manual, had wild spin tendancies during a very carefully entered stall. Other aircraft produced in the same group did not. The reason went undiscovered for some time, and was later found to be manufacturing variation, which resulted in the leading edge skin installation (and hence the airfoil) varying from aircraft to aircraft (and in the case of this aircraft - wing to wing). Once corrected, the aircraft are delightful to stall.

My experience is that small factors, including so many mentioned here, can affect stall speeds. You might find that by triming one of the test aircraft in pitch, so as to be well out of trim, you might affect the tailplane effectiveness, and thus stall speed. Though not useful for showing design compliance, if you get the two aircraft to stall the same way, with different trim (or other control positions) it might offer you some clues as to where to look next.

Good luck...

[Tarq57]

Not mentioned in this thread, and seems so bleeding obvious as to be firmly in the ''dummy" category, but do both these identical aircraft weigh the same?

[flysebi]

We have an Apollo Fox ultralight plane on wich in the apropiate future we want to put vgs(vortex generators). I am curios to see the new stall speed. I will write about it here, after we mount and test the vgs

[infinity-it]

After four days of investigation we made the following discoveries:

1. The two aircraft had a difference of 1.5 degrees in elevator deflection. After we increased the elevator deflection in one aircraft by 1.5 degrees the stall speed was 3 kts lower!

2. The trim tab position seemed not to have a noticeable effect on the stall speed. I did the stall speed testing once with the trim tab at the full up position and once at the full down position. We could not find a noticeable difference in the stall speeds.

3. We found a difference of about 100 rpm in the idle rpm of the two aircraft. After increasing the idle rpm of one aircraft the stall speed was 1 kts lower!

4. I did the stall speed flights in different altitudes once in 8000 ft and once in 3000 ft. As expected there was no difference in the stall speeds.

5. We also found out, that the two aircraft have different pitot tubes, but we did not investigate the effect yet. Today we will change the pitot tube of one aircraft and do the test again. I will keep you posted about the outcome.

I have on more question. Does anybody remember how the deceleration rate in the different stalls can be mathematically corrected to 1kt/s?

Thank you very much for your help again.

[Genghis the Engineer]

You can't mathematically correct for different deceleration rates - you just have to test at the right rate I'm afraid.

Well done, sounds like you're about there.

G

[BOAC]

infinity - I cannot fathom your 'maths', but the two differences add up to 4kts which is where we started - do I deduce they are now identical or have we moved further apart?

Re 'decel speed' - I have always assumed it did not matter much as long as the intent was there and you were close. The aim is, as said, to limit any 'dynamic' pitch rate.

[Genghis the Engineer]

Hi BOAC, I have seen a number of light aeroplanes where the deceleration rate is significant in both stall speed and stall characteristics. Probably the most marked I've ever seen was the French Aviasud Mistral, but there are significant effects in most sub-2000kg aeroplanes if you look at them.

That said, I think that if the rate is kept in the order of 0.5 - 1.5 kn/s, the effects should be acceptably small.


G

[BOAC]

0.5 - 1.5 kn/s

- yes, that would do for 'close' in my book - in fact I might actually look for better since that is a 300% bracket there. It should not be difficult.

[infinity-it]

It looks like the mystery of the different stall speeds of two similar aircraft is solved. The difference was mainly caused be the pitot error. It turned out that the two airplanes had different pitot tubes. Though they looked almost the same from outside they were not identical. The dVpc in the speed range around the stall of one pitot tube dinverged much more than from the other. That was the reason for the differences in the stall speeds. After we changed the pitot tube the speed were almost identical - within +/- 1 kt.

Thanks again for all your advice.

[Pilot DAR]

Thanks Infinity,

Are you able to offer more detail on the differences in the pilot tubes? A few questions I'd be asking were I to be with the two planes:

Is the bore diameter the same in each? Is there an entry point chamfer in one and not the other? Is there an angle of incidence difference? Is there a difference in the direction changes for the air within the pilot tube?

The pitot tube should not be a path for the flow of air, but rather the entry point at which air pressure is measured without flow. Thus, I would not expect that the air path or bore diameter should make a difference.

I'll be interested in your observations...