Why do planes spin?

There is a obvious reason why planes can glide. Translatory flight is stable... with horizontal stabilizer, the airframe will correct changes of angle of attack.

There is an obvious reason why a helicopter rotor might autorotate. At no translational airspeed, every blade is gliding forward, with leading edge in front and trailing edge behind, just as a wing of a fixed-wing plane. Though I cannot see how the AoA would be stabilized... and it is said that helicopters cannot autorotate directly down, but have to gain translational speed if unpowered.

But why do fixed-wing planes spin? Note that the wings have defined, dissimilar leading and trailing edges. In a spin, one wing is moving leading edge forward, the other is moving trailing edge forward. Why is this a stable state? And why cannot a plane in a spin spiral out and return to a forward glide at a reasonable translational speed and low yaw rate?

I depends on the plane. And depends on what type of spin; which can even depend on how the plane is loaded.

There isn't just one answer to that question.

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You may jest, but sometimes it can be due to the girth of any crew member...

In order for a plane to spin, it must be fully stalled (both wings); but one has to be more stalled than the other, giving a strong turning tendency towards the more stalled wing.

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Right, should have clarified that.  If one stalls before the other it usually leads to it being MORE stalled than the other.  Thanks

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Anyone who's trained in a 172 knows to stomp on the right rudder in a stall.

So, how do you design a plane to ensure it returns to flight from a stalled state?

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Yes, that makes sense - washout means that the wing root stalls first and the plane stalls straight ahead while keeping roll stability, instead of spinning.
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Which is probably where washout works.

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For example, T-tails are notorious for deep stalls, even straight ahead. What advantages do they have?

T-tails, and spins.. Now THERE'S something to talk about 

You can even talk to the same "expert" on different days of the week and get different answers on those subjects.

We had a Tomahawk and a Skipper in the club at the same time and I've flown (and spun) both. Just as a quick reference between the two: They're both very easy to spin, but the Skipper recovers quicker/easier. The Tomahawk had a nasty habbit of  jumping right into a spin IN THE OPPOSITE direction 

Two things come up when you're talking about T-tails and I'm not sure if they're advantages or not (in the big, aerodynamic give/take). One, they're never in the prop-wash. Two, they're never in the wing's wake. The very thing that makes wings happier in ground-effect, can make a horizontal stabilizer  unhappy. A T-tail's elevator can also feel "un-effective" at low speed (especially approach)...  which can lead to over-controling, if you're not used to it. Take a look at the rear tie-down hook, next time you see a Tomahawk on the ramp 

The T-tail itself isn't why the "Traumahawk" is a notorious spinner, as much as the constant-chord, "less than rigid" wings are. I've never flown, or talked to some who has, over even heard, that a T-tailed Piper Arrow is a spinner. Have any of you ?

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Well, once it's fully stalled, a change in geometry is required (something has to move so the nose will come down).
The smarter and easier thing is to build it so it won't stall in the first place.
The simplest way to do this is to limit elevator travel (so you can't pitch up enough to exceed the necessary A of A for a given airspeed), but of course this inhibits performance somewhat, so it's not very popular. Canards help, too, even more so than washout, because the canard is set at a higher A of A than the main wing, so it stalls first, causing the nose to drop which brings the wing back into the correct angle range before it can stall.
But canards are still unpopular because they look funny to most people, even though a canard bipe with no tail is technically  a superior design to what we usually see... aside from the stalling thing, a "properly" designed airplane shouldn't require a rudder at all, and there's no reason why elevons can't be used, which greatly reduces complexity and weight. and of course a delta planform produces less drag than the familiar wing-with-empennage design.
Airplanes like the LongEze are not good short-field performers, but they are wonderfully stable and easy to turn in a nice coordinated fashion. But I digress...

Both concepts work very well most of the time, but an Ercoupe (limited elevator travel) and a LongEze(canard) can both be coerced into sort of a "deep mush" if you're careless enough, and pilots have managed to crash both types that way, although they will both be moving forward pretty slowly when that happens, so it's usually not as bad as stalling a more conventional aircraft at a low altitude.

It's a little unfair to say T-tails are "notorious for deep stalls", because they're not any easier to stall than other types: they just have a tendency to be harder to recover, particularly  if they happen to be loaded on the "rear CG" side of the W&B envelope.
I'm not 100% sure why T-tails are used (probably several reasons, including just to look different, like swept vertical stabs), but having flown a Piper Tomahawk, I know that one small advantage is that when climbing, adding power gets a lot of performance out of the wing (by simply increasing airspeed) without causing the airplane to pitch up very much, because the elevator is clear of the airflow off the prop. This can actually prevent stalls and enhance forward visibilty when, say, you're climbing off the runway. It's not a dramatic difference, but I believe that's one of the reasons it's used, particularly on the Tomahawk.

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That makes good sense to me.

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Isn´t it decalage in all cases? Canards set at higher AoA than main wing (so canards stall and drop first, and return main wing to lower AoA), or a conventional tailplane at lower AoA than main wing (so main wing stalls and drops but the tailplane, unstalled, lifts the tail and returns main wing to lower AoA), or a single reflex airfoil with lower AoA in the rear, needing no separate horizontal stabilizer at all, as on a Concorde?
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Why? What is actually the technically superior design: a conventional main wing and tailplane, or main wing and canards, or main wing with both tailplane and canards like Piaggio Avanti, or a single reflex airfoil with no separate horizontal stabilizer like Concorde?
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What happens in a turn made by ailerons alone, with no use of rudder? Why use a rudder and why have a rudder? Etrich Taube has no rudder at all, and turns solely by warping the wings...
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Really? The arguments about wingtip vortex sound convincing...
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Agreed. Rear CG tends to cut into the decalage and make it easier for the plane to pitch up and stall...

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Ah. So the conventional planes are dynamically unstable in pitch? A pitch-up would be followed by return to pitch-down and then, instead of damping-down, successively weaker pitch-ups, successively more violent pitch-ups and pitch-downs follow?
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So, let´s consider the simple aileron use to roll the plane into a modest bank, then stopping roll at that bank.

Yes, the plane will start losing altitude unless the plane increases pitch (by elevators) or true airspeed (by thrust). Increasing pitch and leaving the thrust unchanged would in any case cause loss of true airspeed.

But if a plane simply rolls into a bank, no changes in pitch and thrust, and therefore loses some altitude (until the true airspeed grows in the dive), does it initiate a turn or does it not?

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Right you are... although given enough altitude, some aircraft, will go through these oscillations several times while losing energy, and if they have a healthy dihedral and the air is calm, dropping a wing is unlikely. The Cub is a pretty good example of that, and many trainer-types like the 150 and 172 will also "schwoop" a few times rather sedately when left to their own devices after a mild stall  (I've done it). I've been advised more than once by instructors that leaving it alone for at least one stall/recovery cycle is often the best thing to do, particularly in a situation where you've blundered into IMC and somehow stalled... might want to use rudder slightly to keep the ball centered, but there seems to be a precedent for people screwing things up by using the elevator too much and/or banking inadvertantly in that scenario ("witnesses saw the plane exit the bottom of a cloud in a dive, then pull up back into the cloud steeply"or "plane exited cloud in a spin...")

More unstable higher-performance ships can definitely get nasty, though, if left alone like that... they will pick up more airspeed than you'd like as the nose drops each time.

  But they all will still  come down sooner or later without any input (we can blame Sir Isaac for that, but of course, without him, there'd be no flying in the first place ), and of course, even at a very low entry speed, a stall is a stall and you sure don't want to stall close to the ground.

I guess I only stated half of the story: initially the pilot should drop the nose a bit to recover by regaining "flying speed" (only so as not to lose more altitude while you're waiting for the plane to do so itself), then of course you have to return to level flight with elevator input and/or elevator trim.


In normal flight, best policy is: don't let it stall.

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OK, I see what you mean. I was describing a deliberate stall where you throttle back (powered types) & gradually pull the stick back until it's on the stop. All the time you keep it like this it will never recover. Of course, it's perfectly possible to stall accidentally without reducing power or using extreme control movements.

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Indeed. That's why I always thought stalling & spinning should be part of the PPL syllabus - before a student is allowed solo - as it was in my day.

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It was for me as well, we had to do it before starting to learn to land, I even think it was one of my first couple of lessons.  Not spinning though, my instructor tried a couple of times to put a Cessna 172 into a spin, but they don't spin very easily, so he actually couldn't do it.

During my initial training (many moons ago), spin recovery was "required", by my instructor. My first spin was in a 140hp Archer.

A couple years ago, when I started flying again, I requested spin recovery practice. Spun both a Tomahawk and a Skipper. And of course for the CFI certificate, you must complete spin recovery training.  No spinning in C172s for me (that I'll admit to). For one.. it's tough to get them to spin..And two, with me @ 220lbs, with an instructor.. the CG is too far forward for utility loading (and too heavy for utility loading with someone in back).

Nowadays, it's like Hagar says.. Spin awareness.. As in: add a little rudder during a stall and feel the plane yaw and roll.

I've mixed emotions about mandating spin recovery. People wiser than I have made those decisions. But I think a pilot should take upon himself to seek spin training.

I actually just now realised I haven't had any spin training at all-spin 'awareness' or full spins or whatever (after making it sound as if I had-d'oh ).  Will have to go up with an instructor some time.

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Let´s consider, say, an airplane banked by 6 or so degrees with no compensating changes to pitch or thrust.

It is accelerating sidewards at 0,1 g. It also is accelerating downwards at 0,005 g. The downward acceleration will cause a slow increase of AoA which over time would increase lift, and which might cause the nose to pitch down through stability provided by tailplane. However, the sideward acceleration, being 20 times larger, would add up much faster.

Does a high-performance plane enter into a pure sideslip in that situation, or does it yaw into a turn?

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If a plane is banked, then the lift still operates at a right angle to the wing - but no longer vertically upwards. Thus, as the lift is tilted from the vertically up direction towards the lower wing, the plane would accelerate towards the lower wing.
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Surely it would increase?

If the pitch, the angle between wing and horizon, stays unchanged, but the plane accelerates down, the relative airflow is no longer coming horizontally - it is coming from an increasing angle below the horizon. Thus the AoA between airflow and wing should increase.

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You're splitting hairs here, and over-simplifying things. If the plane is "falling".. and that rate-of-decent, as compared to forward airspeed, is enough to change the AoA.. Then the turn would start becoming more of a control surface thing (with the banked wings accelerating downward being the control surface).. so THAT acceleration would counter it all (relative airflow) and again make the AoA be a decreasing thing.

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Blame to Sir Isaac, all planes come down sooner or later if and when they stop being powered - excluding the gliders that manage staying powered by airflow.

However, what about a powered plane that is stalled by control movements or updraughts? Can it undergo a series of increasingly less violent stalls and recoveries, and eventually recover for good, then proceed to climb back as the plane is still powered?

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Take at least an hour of aerobatics dual sometime in an approved airplane... lots of fun, and it's a good confidence-builder, as well as being educational.
The rental rates on C152 "Aerobats" are usually about the same as a stock 152; that'd be a good first-time choice.

Reminds me I haven't actually done full spins yet (my one aerobatic lesson didn't include vertical-line maneuvers because the vis. was poor that day, bad horizon); I should remedy that.

Did about 1/2 turn once in a Skyhwak during my PP training after practcally begging my CFI; kinda disappointing but I did learn that in a Skyhwak at least, you really have to screw up to get it to spin inadvertently.

Or get into a nasty shear or wake-upset situation... which is why I think fully-developed spins should still be demonstrated, instead of just "R and R".
  Less training accidents nowadays since they stopped doing that, but oddly enough, licensed pilots are still spinning in from the pattern, or after loss of control in IMC... maybe for some, there's just no hope.

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Whoops- yes, I think you told us about that. A Pitts sounds like fun... i think for my next aerobatic flight I want to try an Aerobat. something about getting upside-down in a 152 really appeals to me...