Single Prop Engine Turning and Stalling

[Fogg]

Hello,

This is not really a warbird-related question but a technical question - I hope that is OK here. I'm writing a book on the Fokker D.21 fighter - if you don't know it, a sort of mixed construction, fixed landing gear P-36 used by the Dutch, Danish and Finnish forces in WW II - and I have stumbled upon a few thing that I, as a non-pilot, don't quite understand.

The D.21 had a fairly nasty tendency to stall into a spin, both at high speeds, whenever the stick was pulled too hard, and at slow speeds, for example when landing a little slow. Dutch pilots say the unintended spins were always towards the left. Finnish test pilots reported that minimum speed stalls, both for the Bristol Mercury and Pratt & Whitney R-1535-SB4C/G Twin Wasp Junior engined versions of the aircraft, were always 'first to the left and immediately afterwards to the right'. The last part I don't quite grasp - one wing drops and this causes the aircraft to enter into a spin, right?

My basic question is why the stalling would always be on the same (left) side. I have always thought that this would be because of engine torque. However, I'm sure that the Bristol and Pratt engines turned in opposite directions. So has the engine nothing to do with a tendency to stall on the left side?

Thanks for any answers,

Peter

[Cubs]

Since the engines mentioned do spin in opposite directions, I don't have a valid answer. A roll to the left in stall is usually the result of the wing incidence at the tip.
Termed: "wash-in" or "wash-out".
A typical American airplane in cruise flight will try to roll/turn to the left because of propeller effects and engine torque. Because of these factors, the left wing may be built with slightly higher incidence (angle) than the right. Hence, the LEFT wing will stall and lose lift , before the right wing does. Therefore, a roll to the left.
I would think that an aircraft with the Bristol engine would have the opposite built-in design features, hence a stall-- break to the right.
Aerodynamics can be and is a complex subject. Wish I could be of more help.

By the way, any aircraft can stall at any airspeed. Thats just the nature of a wing. The nasty spin afterward is usually particular to a specific aircraft design.

[Fogg]

Thanks Cubs.

Without really understanding it, I was assuming that an otherwise symetrically built aircraft with a left or right turning engine would be inclined to stall on the opposite side simply as a result of propeller and torque effects.

But what you are saying is different. The aircraft may be built slightly assmyetrically to counter these effects, and that assymetry is actually what causes the tendency to stall on on one particular side.

So my assumption that the engine causes that is indeed wrong? If the aircraft is built symmetrically, it will tend to roll/turn towards one side in normal flight but that would have little to do with a tendency to stall one particular side?

Some more input: 'Recovery from a left spin was almost immediate, but more difficult from revolutions towards the right.' This is related to the engine I think? Does it suggest a left or right turning engine? It's about the Mercury version.

By the way, any aircraft can stall at any airspeed. Thats just the nature of a wing. The nasty spin afterward is usually particular to a specific aircraft design.

Yes, but I understand the D.21 stalled easily due to poor lateral stability. The spin in itself wasn't particularly nasty.

[The Inspector]

The F4U had a nasty habit of dropping the left wing when low and slow, it took installing 'trip strips' on the wing to break it of that bad habit.

Among the last improvements ROBERTSON STOL engineers came up with before the company rolled over was to quit adding full span leading edges 'cuff' and instead installing a 'stall strip' on each wing, saved tons of installation time and sanding/bondo/sanding and repainting the wings.

Maybe the idea never occurred to Anthony or his engineers.

[RyanShort1]

By the way, any aircraft can stall at any airspeed. Thats just the nature of a wing. The nasty spin afterward is usually particular to a specific aircraft design.

To be a bit more precise, an aircraft can stall at any pitch attitude - whenever the wing exceeds it's critical angle of attack which is in relation to the relative wind.

Ryan

[The Inspector]

The 'Dark Lady' has an envelope of around 10 kts between too slow and too fast when @ it's operating altitude.

[Fogg]

The F4U had a nasty habit of dropping the left wing when low and slow, it took installing 'trip strips' on the wing to break it of that bad habit.

That's interesting - a more famous aircraft with much the same habit, at least when slow. Now we're getting somewhere - I thought.

However, a quick search on the net reveals that some people think the F4U dropped the right wing.

I'm hardly doubting you, but I guess the basic question is: why does one particular wing drop first.

Among the last improvements ROBERTSON STOL engineers came up with before the company rolled over was to quit adding full span leading edges 'cuff' and instead installing a 'stall strip' on each wing, saved tons of installation time and sanding/bondo/sanding and repainting the wings.

Maybe the idea never occurred to Anthony or his engineers

Anthony was enjoying life in New York and St Moritz in the 1930s. The Finns ultimately reduced the stalling tendency by fitting 6-hole slots in both wing tips.

[JDK]

It's an interesting question - unfortunately there's not so much on the Fokkers in print, so sources are fewer than many other designs - I can't help, in short! A long shot would be to talk to the Aviodrome or MLM Museum in Holland. The Aviodrome (IIRC) has a lot of Fokker papers and material.

Anthony was enjoying life in New York and St Moritz in the 1930s. The Finns ultimately reduced the stalling tendency by fitting 6-hole slots in both wing tips.

The Lockheed 14 and military derivatives got a similar 'fix'. (As did other types of the period.) The issue was as much a higher-performance aircraft than biplane pilots were used to as much as 'vicious' stalling characteristics. However it was probably a factor in a very damaging accident to Australia's war effort.

http://en.wikipedia.org/wiki/1940_Canberra_air_disaster

Bear in mind that slats were patented mechanism of the Handley Page Company, with royalties payable (they had been invented by Lachmann in Germany, earlier) so the stall remedy options was also driven by a commercial choice.

All peripheral, but of interest, I hope.

[John Dupre]

I have been waiting for a book in English on the D.21 for years. One publisher listed one years ago but then cancelled it. I think up through the 1930s the tendency among designers was to design symmetrically, and let the flight characteristics fall where they may. Pilots were expected to fly. Some military organizations, notably the USN, realized that making the aircraft easier to fly made it easier to fight with and particularly to land on a carrier. This lead directly to the extended development of aileron design and stall strips on the F4U.

I had never noticed the slots in the Finnish built aircraft but checking some of my books I see them now. Fixed slots cut behind the leading edge and Handley Page slats that rise out of the leading edge are two different things though both used to make a wing more controllable near stall. I don't know who might have patented slots.

Have your read "Lentajan Nakokulma II"? It means "Pilot's Viewpoint II" written in Finnish and has a brief English summary that confirms the flight characteristics of the Fokker and most all other Finnish fighters. It would be a marvellous book to see translated to English. Finnish test pilots even measured "stick force gradient" something I only heard about from magazine pilot reports in the 1980s.

[JDK]

I had never noticed the slots in the Finnish built aircraft but checking some of my books I see them now. Fixed slots cut behind the leading edge and Handley Page slats that rise out of the leading edge are two different things though both used to make a wing more controllable near stall. I don't know who might have patented slots.

Given the Wright's litigation over wing warping and ailerons, and the attempts to avoid that by mid-strut ailerons on Curtiss biplanes (IIRC) it would be interesting to know if slots were outside the Handley Page patent scope - or not.

Fogg - do you have a publisher?

Regards,

[Fogg]

I have been waiting for a book in English on the D.21 for years. One publisher listed one years ago but then cancelled it.

do you have a publisher?

My book will only have a brief summary in English I'm afraid. I'd be interested in publishing a full English version of course.

There's talk about an Osprey Aces series book on the D.21 (pilots).

An earlier version of my book appeared in French years ago, strangely enough. I couldn't find a Dutch publisher back then. Now I have one. Not too much time left before we're closing for print.

I had never noticed the slots in the Finnish built aircraft but checking some of my books I see them now.

The final production aircraft got them and they were retrofitted on other Finnish aircraft during major overhauls.

Have your read "Lentajan Nakokulma II"?

No, I'll look into that. Thanks.

I think up through the 1930s the tendency among designers was to design symmetrically, and let the flight characteristics fall where they may. Pilots were expected to fly. Some military organizations, notably the USN, realized that making the aircraft easier to fly made it easier to fight with and particularly to land on a carrier. This lead directly to the extended development of aileron design and stall strips on the F4U.

I strongly suspect Fokker would have followed the traditional approach. So the D.21 was probably symmetric except for the engine.

[JDK]

Fogg - check your PMs!

[Ken]

As was pointed out above, some airplanes tend to drop one wing first because of their construction or because they acquired a "bend" sometime in rough service. Others aren't inclined to drop either wing, so they are said to stall "straight ahead".

As Ryan was saying, aerodynamic stall happens by reaching the critical angle of attack (AOA); this can be done in any attitude and any speed. The key is AOA; the angle between the wing and the relative wind. Realize it's not a "wind" per se blowing on the airplane, it's simply the air rushing past as a result of the plane moving forward. In other words, if you stick your hand out of a car window at 60 mph on a calm day, your hand feels 60 mph of "wind", right? Note that the "wind" is coming from straight ahead.

If the concept of relative wind doesn't quite make sense, imagine an airplane in level flight - the relative wind is essentially "in your face" because that is the vector of the airplane, just like the "wind" your hand felt out of the car window. Now picture that airplane in a pure vertical dive. The relative wind striking the front of the wing is still coming from "straight ahead" ~relative~ to the airplane but to an outside observer, that "wind" would be coming straight up from the earth, right? If our pilot tries to pull too abruptly to return to level flight and exceeds the critical angle of attack, he will stall despite being pointed straight down and indicating a high airspeed. (BTW, this condition is called an "accelerated stall".)

If you're wondering how an airplane ever gets back to level flight from a vertical dive without exceeding this angle, think of it in slow motion, frame-by-frame. As the nose tracks up, the airplane's vector changes. As the vector shifts, so does the relative wind - it's dynamic. If you've ever been out on the bow of a sailboat (Never let go, Jack!) and felt the wind on your face and then felt the relative wind shift to your cheek while the boat was in a turn, that's the concept.

(Here is an advanced part of the concept: if the boat was going straight and a gust caused you to briefly feel wind on your cheek, that's an example of how gusts/turbulence can momentarily change relative wind even though the vector of the boat is the same - one of the reasons pilots carry a few extra knots "for Mom and the kids" in those conditions; they are reducing AOA to increase their margin from the critical angle.)

You've probably also seen relative wind in video where a high performance airplane, like an F-18, is pulling out of the bottom of a loop with an extremely nose-high attitude, but the airplane's vector is still downward ... the airplane is not going where it is pointed. The angle between its vector and its body is a good illustration of high AOA.

Spins require two ingredients: stall and yaw. Take a docile airplane like a Citabria, pull the power to idle and allow the airplane to slow while trying to maintain altitude (not attitude) by easing the stick continuously back. The nose rises and when critical AOA is reached, the wings stall. Lift doesn't disappear, but it is reduced below its ability to support the weight of the aircraft. Because both wings are seeing nearly the same AOA, the nose (aka the heavy end) drops and the wings stay fairly level. With the nose lowered, the airplane's vector becomes more "downhill", AOA is reduced back below the critical angle and the airplane resumes flying.

Take the same situation and add yaw - this can be intentional yaw caused by rudder input or not-so-intentional yaw caused by the build of the airplane, gyroscopic force of the prop, or sloppy piloting. Bottom line is some amount of fishtailing, if you will ... this makes the relative wind "felt" by each wing different - each will have it's own AOA. As a result, one may stall first, and in the stall, if yaw is still present, one will be "more stalled" than the other. As I mentioned, lift doesn't disappear during a stall, it's reduced. If one wing is at a lower AOA, it is "less stalled" and will bring more residual lift to the table than its mate. In layman's terms, that's what causes the rotation, or spin; one wing is providing more lift due to its individual AOA.

The standard spin recovery can be summed up like this: IDLE, NEUTRAL, AFT, RUDDER, STICK, RECOVER.

Check the throttle in idle. Neutralize all controls (rudder and stick). Bring the stick full aft. Apply full rudder opposite spin direction to get the rotation to stop. Use forward stick to reduce AOA and recover from the stall. Quickly recover from the dive you now find yourself in. It took stall and yaw to get into the spin - removing both gets you back to flying.

The Glasair III uses stall strips also. They are aprox 10" strips with a triangular cross-section, one on each wing located at the inner third of the span (ideally preserving good flow out at the ailerons). The presence of the shape affects air flow at that part of the wing and helps induce a stall earlier than it would occur otherwise. During flight test, the strips are taped in place and the plane is stalled. If one wing tends to stall first and drop, the strips are moved laterally until the wings stall simultaneously. Once the right locations are found, the strips are bonded in place.

So, to say that an airplane always drops one wing first or stalls straight ahead depends on a number of things. The largest variable in stall behavior is what the pilot is doing, (creating or removing yaw) at the moment of stall and whether those inputs can have a greater effect than the other factors at play (torque, wing shape, etc). It all boils down to the AOA seen by each wing. And yes, the wing itself may have twist (or wash) that causes different parts of the wing to see different AOAs along its span in a stall, but we'll leave that for another day.

Ken

[Fogg]

Hi Ken,

Thanks for taking the time to go back to the basics of stalling and spinning - it is useful and much appreciated. It also makes perfectly clear to me, though, that I shouldn't attempt to understand everything about the Fokker D.21's stalling and spinning characteristics, let alone to explain it to others.

I would appreciate it if you could answer the following question - does it strike you as odd that this aircraft type (not an individual airframe) would tend do drop the left wing first, while having an engine that turns anti-clockwise, and would still tend to drop the left wing first with an engine that turns clockwise? Or are you saying: no, I can see a couple of possible reasons for that.

If the latter is the case, I am fine with not knowing the exact reasons. I just don't want to get my book shot down with 'Hey, it's utterly impossible that the version with engine x would tend to drop the left wing first' or 'it's utterly impossible that both the left-turning and right-turning engine versions would tend do drop the left wing first'.

The standard spin recovery can be summed up like this: IDLE, NEUTRAL, AFT, RUDDER, STICK, RECOVER.

Check the throttle in idle. Neutralize all controls (rudder and stick). Bring the stick full aft. Apply full rudder opposite spin direction to get the rotation to stop. Use forward stick to reduce AOA and recover from the stall. Quickly recover from the dive you now find yourself in. It took stall and yaw to get into the spin - removing both gets you back to flying.

One more question about this. The D.21's spinning wasn't judged to be difficult to recover from. However, a newbie test pilot panicked and allegedly increased the spinning rate by applying ailerons against the direction of the rotation. Another pilot said the fast spin was easy enough to recover from by applying aileron in the rotation of the spinning. So, not just the rudder?

Thanks,

Peter

Last edited by Fogg on Wed Sep 21, 2011 4:30 am, edited 1 time in total.

[RyanShort1]

One more question about this. The D.21's spinning wasn't judged to be difficult to recover from. However, a newbie test pilot panicked and allegedly increased the spinning rate by applying ailerons against the direction of the rotation. Another pilot said the fast spin was easy enough to recover from by applying aileron in the rotation of the spinning. So, not just the rudder?

This one's fairly easy. Basically, it has to do with the concept of adverse yaw. If you are beginning a spin, let's say to the left, dropping the left wing, and you forget your training, and try to correct the wing dropping by using aileron, then you are actually feeding in pro-spin forces. In this instance if the left wing is more stalled, and begins to drop, an improperly trained pilot would instinctively go hard right on the stick. The left aileron then goes DOWN increasing the drag on the left wing, similarly to a flap, and further causing the aircraft to rotate in the direction of the spin, while the right aileron comes up, doing very little the outboard wing. Actually turning the stick or yoke into the spin is an anti-spin input in some aircraft, although you probably want more training if you are trying this in an aircraft... Spins, when properly understood, can be fun, but a rookie is more likely than not going to make one worse.
If you want to understand more about basic aerodynamics, a good start for anyone is the FAA's Pilot's Handbook of Aeronautical Knowledge, available for free download here: http://www.faa.gov/library/manuals/avia ... _handbook/

Ryan

Last edited by RyanShort1 on Tue Sep 20, 2011 9:55 pm, edited 1 time in total.