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As a non-pilot writing a book on a 1930s fighter aircraft type, I hope you can answer a question that bothers me about its flying characteristics.
The Fokker D.21, a conventional low-wing single-engined fighter, was not too stable laterally, and tended to stall into a spin in certain situations, both at low and high speeds. Its pilots say it was always the left wing that dropped, and I would like to know, maybe even understand, why.
As a non-pilot I expected the turning direction of the propeller to cause this, but pilots and engineers I have talked to so far didn't seem to think this was a very likely cause, especially in situations were the engine was at idle.
If you agree with this, could you suggest some other reasons why this aircraft would have stalled towards the left? Or do you think the engin/prop would have caused this?
[Edit] Originally I didn't mention the aircraft type, but Brian in Reply 1 is probably a wiser man than I am, thanks Brian.
The Wikipedia article is fairly crappy, please let me know if you need some specific information, I might be able to give it.
I'll obviously tell you later which aircraft type I'm talking about. I'm sorry, I don't want to play guessing games, but discussions in other places about this tended to go horribly off-topic.I hope you understand. Please ask me if you need more information.
Some aircraft have idiosyncratic handling qualities, and without knowing what particular aircraft you are referring to it's difficult to proffer advice. There are people here with a wealth of experience, and it's possible that some have experience with the type, despite it's age. Without that info you will find the discussion going incredibly off topic, as people debate issues which may have absolutely no relevance to the particular type of aircraft you have an interest in.
I note that the Fokker is a radial-engined fighter so perhaps the bias was caused by the direction of rotation. On later jets with metal wings, the overall methodology of producing the wing itself can induce a twist that causes a particular wing to drop - don't know if your aircraft has metal-covered wings but that also could be a factor.
The D.21 wing was built of wood and clad with plywood. But a twist caused by the methodology of producing the wing of a jet fighter sounds like a very small twist, so I guess we are possibly talking about something very small.
Fokker's production methods were not the most exacting in the world. For example, the upper wings of Fokker biplanes were not exchangeable due to the holes for the struts not being in the exact same places, even in the mid 1930s. Fuselages were welded steel tube frames that might very well turn out slightly asymmetrical I suppose.
However, the bias was not in individual airframes but in the aircraf type as a whole, including airframes built in Finland. Some Finnish aircraft also had an engine turning in the opposite direction, and supposedly all had the same bias towards the left.
Looking at the three-view on Wiki, it seems there is a pitot probe mounted on the LH wing; that might have been enough to introduce an element of consistent stalling on that wing.
While the prop/engine at idling may have been a small factor, it certainly is an asymmetric effect, and might be expected to introduce some bias in one direction.
The three-view appears to show tabs on both ailerons. If one were a trim tab, and the other a gear tab, perhaps, then you could end up with a routine asymmetric effect. Hard to say more without knowing more about the roll controls in detail.
The three view also seems to show an asymmetric feature on the cowling, at about the 7 o'clock position viewed from the front - some kind of scoop or intake? Might it have had an effect near the stall...?
I'm reminded of an old joke about the South African pilot during WW2 who recounted in the bar being bounced by "a pair of Fokkers"; a British pilot who didn't think that there were any Fokker aircraft in the area inquired "you say these were Fokkers", "Nah", replied the South African "These fokkers were Messerchmidts".
More seriously, I'd have a look at the design of those trim tabs. I once had a project - a 2 seat light aeroplane with a very significant wing drop at the stall, which after a lot of mucking about we determined was a function of trim speed, which of-course is a function of trim tab angle. The aeroplane had trim tabs - mechanically linked on both sides of the elevator. However, it turned out that there was significant freeplay between the two sides, and because the trim linkage was onto one side only, with the trim set to a lower speed, and under air loads, we got signicant asymmetry between the two trim tabs, which translated into 45 degrees plus wing drop at the stall, always in the same direction. We solved it with the introduction of a couple of extra low-friction washers that eliminated about 90% of the freeplay.
A few years ago I was part of a test team gaining UK certification for a high performance microlight aircraft. We had great problems initially with very severe wing drop at the stall. Even making absolutely certain that we had no slip on at the stall, the aircraft would instantly drop a wing, not always the same one.
We tried vortex generators towards the wing tips to no avail, and changed the angle of dangle of the ailerons, again no change. In the end, we put stall strips on the inboard couple of feet of the wings' inboard leading edges. This made the stall acceptable, but to gain certification we had to include an aural and visual stall warning system.
During this process, we considered the fact that the aircraft was a two-seater with side by side seating. There seemed to be no difference whether a light tp or the standard heavy one flew the tests, nor which seat was occupied, and we began to log lateral CG as well as longitudinal on each of the test points. We could find nothing in the data to suggest the lateral CG had any effect. We also approached one of the UK's legendary test pilots, and asked him for his thoughts. He replied,
"I cannot remember any great difficulties. The critical factors would have tended to be:-
1. Wing loading and span-loading towards the tip. 2. Wing tip chord and type of section. 3. Even an offset seat, when flown solo, did not cause significant trouble, because the wing loading was reduced - and with it the Reynolds number. Of course much could depend upon the Rn in theory, whether it is translated from turbulent to laminar, and the thickness ratio of the wing section near the tips....."
It's an interesting subject. Wish I could find our reports!
Wyvern, I believe I know what programme you mean and saw similar things in the same aeroplane in the few hours I got to fly the prototype.
However, my recollection was of generally poor stalling characteristics - a tendency towards severe wing drop and some tendency towards an incipient spin. I don't recall it showed a particular tendency towards consistently dropping the same wing?
That is fairly unusual. The project I mentioned showed that, but I can't recall any other aeroplane which showed such a tendency towards always dropping the same wing when otherwise happily in balance?
Great care was taken to have the ball in the middle, as in your involvement
I do wonder sometimes the extent to which this is possible in the transient conditions between, say, 1.1Vs and the stalled condition - can we really achieve consistent zero sideslip? - particularly in a piston single. And if we can't, as Test Pilots, of-course that's likely to be true of any other pilot in the same aeroplane later.
Regarding the OP's Fokker, I wonder if there's a difference there between the flight test reports, and what was later found in squadron service? And whether there's any evidence to tell.
One piston engined trainer I instructed on in the RAF usually (but not always) dropped the left wing at the stall. The stock answer when asked was that it was due to the rotation of the slipstream from the prop. The residual slipstream, even at idle would reduce the angle of attack on the right wing and increase that of the left. As the design had washout to ensure the wing stalled evenly over the span, or even root first, this angle of attack change, though slight, would be enough to make the left wing break first.
This type's engine was British and woulf have rotated the same direction as the Fokker's Mercury.
It's fairly obvious to me now that I'm not going to establish the exact cause - I'll just leave my readers a footnote saying it could have been any small asymmetrical feature on the aircraft, intended or otherwise. Does that sound OK?
The information comes from Dutch service pilots and Finnish military test pilots. The latter were pretty thorough and their information that both the Mercuy and Twin Wasp versions stalled the left wing first should be reliable.
However, my source says 'flight reports tell that both stalled first to the left and immediately afterwards to the right.' This is not exactly my layman's view of what happens when an aircraft stalls into a spin, but maybe that is just me, or a translation problem?