Aerodynamics of a tail stall.

[a&p cfiguy]

Could someone please explain to me what's exactly going on in a tail stall due to icing? I learned about it before, but it was a while ago. I know the recovery procedure is pretty much just the opposite of a normal departure stall, reduce power increase backpressure (right?). How do you know you're in a tail stall? Seems like it'd be pretty hard to convice yourself to do the exact opposite of what you're used to doing. Anyone ever been in a tail stall? Must be pretty scary.

Also, any good websites out there regarding icing training?

 

[sbav8r]

Complete this course, it's full of great information including the aerodynamics of a tail stall.

Nasa Icing

 

[MauleSkinner]

The "approved" recovery procedure for a tail stall never quite made sense to me, and while the NASA icing site is excellent, it kind of reinforced my basic questions rather than answering them, so I sent an email via the link provided on the web site. Here is my email and the reply. Hope it helps!

(the bold section is my emphasis added, as it most directly answers my questions)

David

Question: I am having difficulty with the concept that pulling
back on the yoke in the event of a tail stall decreases tail pitch attitude.

I understand that the tail "lifts" down, and that increasing the
airplane's pitch attitude while maintaining the same flight path decreases
the tail angle of attack. However, it appears to me that we are
increasing the tail's angle of attack (up-elevator) in order to attain
this decrease. If the tail is stalled, how can we use it to change the
pitch attitude of the airplane?

Is it possible that when the tail stall occurs, the resulting pitchover
changes the wing's downwash characteristics and/or converts our flightpath
to a more ballistic path, and the tail's airflow is reattached BEFORE we
initiate the recovery procedure? Or is flap retraction required PRIOR to
initiating the up elevator command?

Thus far, no one has been able to explain this beyond "apply up elevator
because the tail lifts down".

Thank you for your help

David Andersen
ATP/CFI



Hello Mr. Andersen,

Thank you for your inquiry into Tailplane Stall.

Allow me to recap my understanding of your questions:

Q: If the tailplane has stalled, how can up-elevator be effective in
generating a nose-up pitch attitude of the aircraft?
A: Good question - the suction (lower) surface of the tailplane has
completely stalled, and is therefore ineffective. However, the pressure
(upper) surface remains effective throughout the stall.

Q: In application of recovery procedure from a tail stall does the order
between flap retraction and up-elevator matter?
A: Flaps are the bigger driver - they directly impact the tail AOA.
Up-elevator is a secondary effect.
Here is a narrative of the tail stall event we experienced with an ice
shape on the Twin Otter aircraft: The aircraft was configured at 1.3 Vs
with full flaps. The pilot's task was to gradually increase thrust. During
the course of this task, the tail experienced a complete stall. The thrust
was reduced and the action to retract the flaps were taken within 0.2
seconds. [The flaps, however, are slow to move on the Twin Otter.] The
pilot pulled progressively upto 100 lbs but the elevator was sucked deeper
into the separated region off the lower surface. While the pilot was still
struggling to pull the elevator up, the first signs [pitch rate and tail
AOA] that the stall was arrested were noted 2 seconds later when the flaps
had moved 2%. 1.5 seconds later, these signs indicated recovery had begun.
Simultaneously, the pilot's two-handed pull of 172 lbs was starting to move
elevator up toward neutral deflection. FYI, our biggest altitude loss came
after this point.
A video clip of this stall can be found several places, one is at
http://aircrafticing.grc.nasa.gov/courses.html
In an Emergency / Recovery / Tail Stall / p4 / Related Information.

The text at the bottom of the flight footage is this:
TAOA = tailplane angle of attack (-10 deg is OK, -14 is stalled)
KIAS = knots of indicated airspeed
ELEV = elevator deflection (negative is trailing edge down)
CT = thrust coefficient
FYE = force the pilot applies to the elevator (positive is pull)

The data displayed on the screen is updated every 0.5 sec, we recorded at
0.01 sec.


I hope this helps. Please feel free to contact me for more information.
Now that the holidays are over, I'll be spending more time in the office.

Sincerely,
Judy Van Zante


>


~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Judith Foss Van Zante, Ph.D.
Engineer V, QSS Group, Inc.
w/ NASA Glenn Research Center, Icing Branch
[email protected]
216.433-3587 (o), 216.977-7469 (f)

 

[FN FAL]

Could someone please explain to me what's exactly going on in a tail stall due to icing? Seems like it'd be pretty hard to convice yourself to do the exact opposite of what you're used to doing.

You do the opposites of tail stalls? No you don't...land on the mains and hold the nose wheel off until the tail stalls. What happens? The nose drops.

If the nose drops while you are flying, what would the natural reaction be? Pull back on the yoke? Or would you "unstall" the tail by pushing the forward on the yoke when the nose drops?

 

[FN FAL]

While you're thinking of an answer...consider this, you're confusing wing stalls with tail stalls. By pulling back on the yoke during a tail stall, you are increasing camber on the horizontal stablizer. Think about it for a while.

What do airliners do to prevent stalls at slow speed? Lower the nose and speed up? No, they create camber by putting down the flaps and extending slats.

Better yet...you're in a 182 with flaps retracted and you pull power to idle while holding altitude. What happens? At the bottom of the green arc, you stall. Extend flaps before you get to flaps clean stall speed and you bought some time. Why? It's not because you lowered the nose to maintain airspeed, it's because you increased camber.

 

[mar]

Just avoid it.

How do you know you're in a tail stall? Seems like it'd be pretty hard to convice yourself to do the exact opposite of what you're used to doing. Anyone ever been in a tail stall? Must be pretty scary.

Just say "No."

My silly little answer would be this: If your tail is gonna stall because of ice you're pretty much in a world of hurt already. If your plane can't handle the ice (and here I'm thinking of the Caravan) you should avoid it at all costs.

I flew the Caravan a little and basically I'd climb through the ice or descend through the ice but I'd never hang out in ice no matter what the intensity was.

FN FAL--Hang on for a sec. I think you're actually confusing the original issue of tail icing by introducing wing flaps and touchdown aerodynamics into the subject.

Horiz stabs *do* ice up and they do stall but I don't think you can start making comparisons between clean wings at touchdown and iced up stabs in flight.

[Editted for rampant spelling and formatting problems. More caffeine required.]

 

[FN FAL]

FN FAL--Hang on for a sec. I think you're actually confusing the original issue of tail icing by introducing wing flaps and touchdown aerodynamics into the subject.

Nah...the guy said it's opposite to what you're used to doing.

First, it's not opposite to what you're used to doing. You're not USED to having a yoke vibrate in your hands...that's one difference between a wing stall and tail stall.

Second, you increase camber on the tail to generate more lift in a tail stall. In a wing stall, you decrease angle of attack of the wing. Two separate things, not opposite things. The only thing that's opposite is the control input to the yoke.

I simply tried to correlate why you increase camber of the tail by using analogies to illustrate the difference between the recoveries.

To make things even more confusing, when you recognize the onset of tail stall, you are going to quit holding altitude to maintain control. You are going to keep your airspeed up and descend...slowing down by holding altitude will guaranee you'll need even more back pressure, which will eventually lead to the fully developed tail stall.

We do this scenario in the Level D Caravan sim in MEM...you can't miss the onset of tail stall, because the yoke will be shaking. It is not opposite of a wing stall, it is different.

...Seems like it'd be pretty hard to convice yourself to do the exact opposite of what you're used to doing. Anyone ever been in a tail stall? Must be pretty scary.

Also, any good websites out there regarding icing training?

 

[mar]

Alright, I think I get it now.

Thanks for clearing that up. I really need a strong coffee.

 

[FN FAL]

Thanks for clearing that up. I really need a strong coffee.

Yea, a lot gets lost through the medium of typing.

If you think about it, you're going to gradually add backpressure naturally as your horizontal stabilizer loses efficiency, in order to maintain altitude. Between onset of tail plane icing and the fully developed tail stall, there are going to be clues...it's not like you're driving down the street at 55 mph and deer jumps out in front of you and you have to make an instant decision to swerve, drive over the deer or jump on the binders.

From what I read (unverified) about the Caravan, lift generated by the tail can be up or down in direction depending on CG loading. Cessna engineers say that pilots don't need to know which it is doing or when, they just need to know that pushing forward on the yoke points the nose down and pulling back on the yoke makes the nose go up.

So how does that fit into recovering from tail stall recovery? Hmmmm?

 

[mar]

Egads.

Like I said, it's my humble opinion (and self-preservation instinct) that if you're picking up ice in a Caravan you need a Plan B.

I think the Caravan is a wonderful airplane, it's comfortable, handles nicely and made my former employer a lot of money...

...but something is pretty freaky when an airplane like that has spoilers and vortex generators...

Yikes.