Difference in Stall Speeds
 

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.

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

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?

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.

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

...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.

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.

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.

- 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.

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.

My 2 cents :)
 

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

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.

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

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.

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.

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

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?

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

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

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.