Difference in Stall Speeds

[infinity-it]

During the performance testing of a single engine airplane we found a significant difference between the 1 g stall speeds of two identical airplanes. The test items are single engine piston with a high wing and a conventional tail configuration. Both airplanes are equipped with an on board recording system and the with a boom. Both of the recording systems were calibrated. Therefore it is very unlikely that the difference is coming from the instrumentation. The difference is almost 4 kts which is almost 10% of the stall speed.

Does anybody have an idea what the main contributor of this difference could be?

Thank you very much for your help.

[Genghis the Engineer]

A few thoughts:

- Were CGs the same? Aft CG can give substantially lower stalling speeds than forward CG, particularly if the aeroplane is such that at aft CG you are reaching stall before full back stick, but at forward CG you are getting a "mush" at full back stick.

- How good were your low speed calibrations on your airspeed measurement systems? Down to around 1.1Vs calibration tends to be fairly straightforward, below that can be a real headache, and significant nonlinearities in PEC are likely.

- Were they being trimmed identically? I have seen significant stall effects that came down to how the pilot trimmed the aeroplane prior to a stall test, and the mechanisation of the elevator trim tab.

G

[infinity-it]

The CG's were identical. They were within 0.5% MAC.

For the speed calibrations we used a boom. Therefore the should be pretty accurate.

The thought with the trim might good idea to further investigate. It might be that the trim tab was not in the same position. Do you think that this can make such a big difference in stall speed?

Another thing which I came to my mind was that the stall speed were measured at different altitudes. Does anybody have any experience whether this can make a difference?

[BOAC]

Rate of speed decay towards the stall, (if) flapped, any difference retracted between both frames, engine idle RPM or power setting as 3 to look at as well as c of g.

[Genghis the Engineer]

You may have put your finger on it there BOAC; rate of deceleration strongly affects stall speed, and piloting technique is a big player there. Were they the same TP?, Does the FTI (if any) show the same deceleration rates between about 1.3Vs and Vs?

I have seen trim make a big difference to stalling characteristics (particularly if there's asymmetry or internal freeplay in a trim tab design), less so to the actual stalling speed.

G

[BOAC]

...yes, and don't forget the 'idle' power setting - that can have a huge effect. We don't even know how the 1g stall was 'identified' either on both tests.

To answer a question I missed - altitude would not affect.

[Neptunus Rex]

Which is why the RAF had a nominated Unit Test Pilot to do the job. The aircraft on a particular flying station (eg 30+ Jet Provosts at Linton) would be configured in the same way after Major Servicing, then go through the hands of the same pilot. It led to a very standardised fleet as far as predictable handling qualities were concerned.

[infinity-it]

According to the plots we made the rate of speed decay was kept constant from 1.3 Vs to stall. We plotted the speed over the time in one plot and in to other pitch rate and elevator position over time in the other. All stalls on both aircraft were performed by the same pilot. The stall speed was determined as the speed where the elevator reached the up stop and the nose down pitching. Interestingly this happened more or less at the same time for all performed stalls. For each configuration a number of 5 stalls were performed and the resulting stall speed were within 1 kt.

During our investigation the question about the trim condition came up. The regulation states that the airplane has to be trimmed at 1.5 Vs. Does that mean that the airplane has to be trimmed in power idle which would result in a descent or in power for level flight at 1.5 Vs and than the power will be reduced from there before the stall maneuver?

There was indeed a difference of about 2° in the trim tab position between the two airplanes during the tests.

We don't have any information on the idle rpm, but from what I remeber the difference was not more than 100 rpm. Does 100 rpm have an effect?

From what I can take from your replays we should concentrate on the trim tab position, idle rpm and decay rate. Are there any other recommendations?

Thank you very much for your help.

[BOAC]

the elevator reached the up stop and the nose down pitching - a bit puzzled by this - are you saying that you did not actually 'stall'? If the nose dropped with further speed reduction and full up elevator that might suggest the elevator travel/effectiveness may perhaps have caused the difference and it was loss of pitch control you saw rather than an aerodynamic 'stall'. Certainly in 'text book' air test stalls in 737s one often reached full nose up tail authority before classic stall symptoms (buffet, marked nose or wing drop) appeared.

Yes, I reckon the effect of 100rpm's-worth of propwash could affect pitch authority.

[biscuit74]

I'd agree with BOAC's comment that even a small RPM difference can affect pitch authority more than you might expect.

Also, given that, as BOAC says, it seems to be pitch authority loss that is being used as the 'stall' determinant, are the two airframes identically rigged? A small difference in elevator up stop position would have a significant effect - even one or two degrees. As would any slight variation in tail incidence. The older the airframe, the more likely that there are small variations. ??


A thought on trim position, as previously mentioned. On one light aircraft I used to regularly fly, the nose up pitch limit was slightly, but noticeably, improved by appling full nose down trim. Of course, that is unlikely to be relevant in this case because the testing pilot would tend to trim nose up as he slowed towards the stall anyway. My working against the trim force is to ensure adequate pitch authority when landing in a taildragger with a rather light tail load.

[rotorwash4944]

I agree even a small amount of RPM change can have an effect... RPM difference may cause a slight change in flow, i.e. downwash angle between wing & tail.
Saw this on an A-6E flight test... back in the day :sad:...

My other thought was that the airplane might just be built...crooked? are the other control positions instrumented? how do the rudder / aileron positions compare between aircraft?

I'd also agree to try to duplicate with same tail trim settings...

Frank

[theficklefinger]

There is an assumption that two aircraft coming off the line are identical, I can tell you they aren't.

1500 RPM in one aircraft might translate into a higher angle of attack due to reduced prop efficiency, then there are rigging issues, paint, wing attachment accuracy..

There was a time when I was contracted to fly three of the same model jets, interesting to note the cruise speed variances between the different models...such that when taking delivery of a jet I was able to determine during a prebuy that the jet I was buying beat the book by 17 knots at alt..

Not huge, but considering some jets are 15 knots slower then book, that can be a swing of say 5-8 percent...over the course of 10,000 hrs? You do the math and decide if at end of 10,000 hours, you would like to have sat in the cockpit 800 hours longer, x 200 gallons per hour...which also translates into range issues, probably higher stalling speeds and lousier handling characteristics.

Most pilots won't know the difference, but if you sit in the cockpit all day long with nothing do but look up numbers, it kinda sucks to know your in an aircraft that can't even do the book, but it's really cool if you can beat the book, get there a little faster and know that at the end of a 700 hour flying year, you didn't have to sit there for 50 hours longer then you had to, which is a week of your life.

[Genghis the Engineer]

Adding to that, a few things that can vary between identical airframes, that are worth checking:

- Engine power: engines vary between lumps. A cheap way to check this is tie it to a tree with a load cell in the middle, and run the engines at various powers to see what differences you get.

- Propeller pitch.

- Range of available elevator movement: this can vary by as much as a couple of degrees between "identical" airframes, so if you have a pitch authority limited stall, this could be very significant. Very easily checked.


Also, "identical" airframes, even out of the box, are not identical in dimensions, so I'd actually go and re-measure longitudinal dihedral.


There used to be an "interesting" problem with a certain Irish-built military training aeroplane, where about 1 airframe in 10 would, stalled in the landing configuration, roll itself inverted. This was eventually solved by an enterprising SEngO swapping rear fuselages and tailplanes around until the problem went away. The basic problem was tolerances in design adding up to a cumulative significant difference in shape.

G

[infinity-it]

We will continue with the investigation tomorrow. According to all your posts, which I appreciate very much, we will definitely compare the propeller pitch, the idle rpm, the trim tab positions, the elevator travel and the other production tolerances. An we will also repeat the stall tests.

This type of airplane does not have a clear stall. According to the regulation the stall is defined as either an uncommanded pitch down or when the elevator reaches the up stop. Since it doesn't have an uncommanded pitch down the stall in this airplane will when the elevator reaches the up stop.

Thank you very much again for your help and I will keep you posted.

[411A]

Does anybody have an idea what the main contributor of this difference could be?


On some of these types of airplanes (single engine Cessna's for example) the wing angle of incidence is adjustable via a concentric cam.
I would check there, first, to determine if it is in spec...or not.

[Genghis the Engineer]

This type of airplane does not have a clear stall. According to the regulation the stall is defined as either an uncommanded pitch down or when the elevator reaches the up stop. Since it doesn't have an uncommanded pitch down the stall in this airplane will when the elevator reaches the up stop.

I just wanted to emphasise this paragraph - I agree with your analysis of the regulations, but there are many people - even occasionally in flight test - who clearly have not read paragraphs 201-207 properly, and started using daft phrases like "does not stall".

G

[infinity-it]

I fully agree with you. With "This type of airplane does not have a clear stall" I mean that the airplane doesn't pitch down abruptly like we are used from other single engine airplanes. This one just drops the nose slightly and it enters into a kind of a "nodding" (pitch oscillation) motion. Therefore the "The control reaches the stop" will be the determining factor for the stall speed.

There is still the question for the correct maneuver. During our investigation the question about the trim condition came up. The regulation states that the airplane has to be trimmed at 1.5 Vs. Does that mean that the airplane has to be trimmed in power idle which would result in a descent or in power for level flight at 1.5 Vs and than the power will be reduced from there before the stall maneuver?

[Genghis the Engineer]

For determination of the certification stall speed it's normal to trim as required by the regs, power to flight idle, re-trim to that speed at idle power, then decelerate on the primary pitch control at 1kn/s. In virtually any aeroplane (maybe not the odd high performance glider or motorglider) you'll be descending for most of this.

There are of-course many other stall tests you need for handling / cert.compliance purposes, but that's the one for determining Vs (an in particular whether Vso is low enough for compliance with the design code stall speed minimum.

G

[QJB]

I don't have the knowledge or experience to attempt to answer the original question. However, I am intrigued, can someone explain to me how the rate of deceleration affects the stall speed?

J

[dClbydalpha]

QJB, there is an element of hysteresis in the circulation of the airflow. As such, if the aircraft is decelerating at a greater rate, the stall will be "felt" at a lower airspeed.

[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...