Why do planes spin?

[Hagar]

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

I have often wondered that myself.

[Brett_Henderson]

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

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  :o

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 ?

[chornedsnorkack]

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

[beaky]

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

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.

[beaky]

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

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  :o

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 ?

Good point about the relationship between wing and horizontal tail surfaces... I forgot about that.

I enjoyed flying the Tomahawk, though... very docile, in general. And the visibilty is better than in any typical flight-school or rental aircraft I've ever flown.

[Brett_Henderson]

[quote]So, it is surely better to place the stabilizer so that it may be in wing

[Brett_Henderson]

I enjoyed flying the Tomahawk, though... very docile, in general. And the visibilty is better than in any typical flight-school or rental aircraft I've ever flown.  

Oh man..  Aren't they blast ?  I love flying a Tomahawk.. It just feels more like you're flying a plane (to me). Great cabin space.. GREAT visibilty.. and what FUN they are to land in a X-wind..

Only problem *sigh*.. I weigh 220lbs.. Even If my flying companion is a mere 170.. we aren't going very far...

[chornedsnorkack]

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.

Isn

[Hagar]

Well, once it's fully stalled, a change in geometry is required (something has to move so the nose will come down).

I'm not sure that's necessarily true. In my experience with conventional types (both powered & gliders - full-sized & models) the stall is a series of cycles with the nose dropping then recovering as airspeed increases naturally without moving the controls. The first stall might be very gentle but if recovery action is not taken it will get progressively steeper, eventually becoming quite violent (depending on the design) & possibly dropping a wing converting the stall into a spin.

What happens in a turn made by ailerons alone,

A turn is not possible with ailerons alone. Ailerons control angle of bank on the rolling axis. To initiate & maintain a turn you need to use UP elevator with the bank. We seem to be  discussing conventional prop-driven light aircraft but most jets do not need rudder except for advanced aerobatics & landing/take-off in crosswind conditions. Many R/C model aircraft fly extremely well on aileron & elevator only although most will have a fixed vertical tail surface (fin). Traditionally, beginners models are inherently stable with large dihedral angle on the mainplane & no ailerons. These are known as rudder/elevator models. Most R/C modellers start off on one of these. It's more like flying a boat as control inputs are delayed & you have to give it time to respond.

[chornedsnorkack]

I'm not sure that's necessarily true. In my experience with conventional types (both powered & gliders - full-sized & models) the stall is a series of cycles with the nose dropping then recovering as airspeed increases naturally without moving the controls. The first stall might be very gentle but if recovery action is not taken it will get progressively steeper, eventually becoming quite violent (depending on the design) & possibly dropping a wing converting the stall into a spin.

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?
[quote]
A turn is not possible with ailerons alone. Ailerons control angle of bank on the rolling axis. To initiate & maintain a turn you need to use UP elevator with the bank. We seem to be

[Hagar]

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?

If the mainplane is in a stalled condition with the stick hard back on the stops, Yes. This sequence will get progressively worse until the appropriate recovery action is taken.

[quote]So, let

[beaky]

I'm not sure that's necessarily true. In my experience with conventional types (both powered & gliders - full-sized & models) the stall is a series of cycles with the nose dropping then recovering as airspeed increases naturally without moving the controls. The first stall might be very gentle but if recovery action is not taken it will get progressively steeper, eventually becoming quite violent (depending on the design) & possibly dropping a wing converting the stall into a spin.

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.

[beaky]

Oh man..  Aren't they blast ?  I love flying a Tomahawk.. It just feels more like you're flying a plane (to me). Great cabin space.. GREAT visibilty.. and what FUN they are to land in a X-wind..

Only problem *sigh*.. I weigh 220lbs.. Even If my flying companion is a mere 170.. we aren't going very far...

Too bad they never made a 4-seat version... how would you say the Beech Skipper compares, in terms of utility?

[Hagar]

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...")

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.

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

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.

[beefhole]

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.

It isn't?  Hell, at my school, we flew two cross-countries before we soloed.