Describing a spin properly

[TMetzinger]

When an airplane stalls and it's not in coordinated flight, one wing ends up less stalled than the other and thus generates more lift. This makes the airplane roll over into the spin entry and continue to rotate due to the different amount of lift generated by the "fully stalled" wing and the "less than fully stalled" wing.

Correct so far?

My question is... when I'm describing the event where the airplane rolls over, is it better to describe it as the "still barely flying" wing lifting the airplane into the roll, or to describe the "not flying" wing as falling downwards and causing the roll, while the "still barely flying" wing is maintaining it's position.

Put another way, I think the center of lift laterally is out on the still flying wing, and the aircraft is rotating there, not in the center of the cabin, and what's happening is that the stalled wing and cabin are "falling" relative the the flying wing.

 

[brcase]

I think it more correct to say the stalled wing is falling.

My Ah Ha moment came when I realised that the drag goes up exponatially on the stalled portion of the wing. If you stall the tip perhaps with full aileron (incorrect procedure but I have watched pilots inadverntantly do this) this will increase the rotation rate around the stalled wing due to the increased drag.

Brian
CFIIG/ASEL

 

[whifferdill]

My question is... when I'm describing the event where the airplane rolls over, is it better to describe it as the "still barely flying" wing lifting the airplane into the roll, or to describe the "not flying" wing as falling downwards and causing the roll, while the "still barely flying" wing is maintaining it's position.

No, both wings are stalled, it's just that there is a difference in depth of stall between both wings. The down-going wing is more deeply stalled and creating less lift (stalled wings still create lift). In a normal spin entry, you stall the airplane normally (both wings stalled), and induce yaw that stalls one wing more deeply. If the airplane was rotating with one wing flying and one wing stalled, that would be a snap roll rather than a spin.

 

[HPNPilot1200]

My question is... when I'm describing the event where the airplane rolls over, is it better to describe it as the "still barely flying" wing lifting the airplane into the roll, or to describe the "not flying" wing as falling downwards and causing the roll, while the "still barely flying" wing is maintaining it's position.

In a spin, both wings are stalled but one wing is stalled more than the other. The descending wing has an increasing angle of attack beyond the critical angle of attack where relative lift decreases and drag increases.

In a spin, the aircraft rotates around a vertical axis which is shifted out toward the more stalled (lowered) wing since the less stalled (raised) wing is producing lift and shifts the center of lift laterally to that side. In the spin entry, the less stalled (raised) wing aggravates rotation and enters the aircraft into the spin rotation.

I don't believe describing the spin where the "airplane rolls over" is the best way to talk about it (sounds scary to a student I think). I normally discuss stalls first and then emphasize that a spin is merely nothing than an uncoordinated stall where one wing is stalled more than the other. I try not to make it any harder than it is.

The Airplane Flying Handbook (Chapter 4, Page 4-12) is a great reference and I have attached an image that relates to spin rotation around the vertical axis which is somewhere out on the lowered (more stalled) wing.

 

[nosehair]

when I'm describing the event where the airplane rolls over,

Ditto on HPNpilot's comment about 'rolling over'.
My first impression when I read that was the effect it might have on a student.

Most students (and maybe some instructors) fear the dropping of a wing during a stall, and react with aileron. The concept of "rolling over" would encourage that fear, and it is not the 'dropping of the wing that induces a spin.

It is yaw: allowing a departure from a constant straight ahead heading causes the retreating wing to lose lift, and the advancing wing to gain lift. This initial asymmetrical lift starts the retreating wing to descend, increasing the relative AoA and causing the deeper stall.

The point is to focus on yaw control (rudder) rather than roll control with aileron.

 

[TMetzinger]

OK I've seen a difference of opinion here and I still don't feel that anyone has answered the question.

First, I think we agree that there is a difference in lift in the two wings in a spin entry - one wing is generating more lift than the other and that creates the roll tendency that puts the airplane over. I don't care whether you say both wings are stalled or one is more stalled or less stalled than the other, you have a difference in lift produced because of the yaw in the uncoordinated stall.

My question (which I don't feel has been answered yet), is whether it's better to describe the roll that occurs as:

A wing drop (on the wing generating less lift) where the "pivot point" is out on the wing that's generating more lift. And by Pivot point I'm not talking about the axis the airplane rotates around. I'm talking about the point the airplane ROLLS around at the moment of the entry.
or
A wing lift (on the wing generating more lift) where the pivot point is out on the wing that's generating less lift.

What I remember from the spins I've done is that it felt more like a wing drop than a wing lift. Why is this important, when the end result is the same? I want to accurately show what happens when I'm holding the model aircraft in my hand and demonstrating to a student, and I want to describe the feeling they'll get correctly. It all happens in less than a second anyway.

And of course, the proper response is to use the rudder to counteract the drop, since it's pretty much the only control surface still working AND it will correct the yaw that's causing the difference in lift.

I disagree about not discussing the rollover. The airplane does go on it's back briefly in the normal spin entry, does it not? It may not be appropriate in a discussion of stalls, but if you're going to discuss the actual spin, I think it's better to fully prepare the student. My aerobatic CFI talked about the rollover during our pre-flight briefing before doing the spins.

 

[djpacro]

This is where some information from the competition scene may be useful. Competition spins are rarely more than 1 3/4 turns so there is some emphasis on the transition from normal flight to a spin (incipient spin as competition spins rarely get to the stable fully developed condition). I know that the objective here is far from that of a competition spin so I just want to point out one aspect of the information on this page: http://www.aerobatics.org.uk/judging/judging-spins.htm

Note also the curved trajectory of the down-line. This clearly shows the decay in forward energy of the aeroplane, from stall-speed

..... to the stable, fully developed spn where the axis of rotation is vertical.

If you enter the spin at the stall speed, or just a few knots above it, you can get the wing drop. If you enter significantly above the stall speed - a dynamic entry - the upgoing wing will rise up i.e. a "snap entry" which is where you'll get the airplane going over on its back.

In the Decathlon and Cessna 150 I teach an entry a few knots above the stall speed. In a 152 I generally find that I have to use the dynamic entry.

So, Tim, what type of airplane are you flying?

 

[TMetzinger]

This is where some information from the competition scene may be useful. Competition spins are rarely more than 1 3/4 turns so there is some emphasis on the transition from normal flight to a spin (incipient spin as competition spins rarely get to the stable fully developed condition). I know that the objective here is far from that of a competition spin so I just want to point out one aspect of the information on this page: http://www.aerobatics.org.uk/judging/judging-spins.htm
..... to the stable, fully developed spn where the axis of rotation is vertical.

If you enter the spin at the stall speed, or just a few knots above it, you can get the wing drop. If you enter significantly above the stall speed - a dynamic entry - the upgoing wing will rise up i.e. a "snap entry" which is where you'll get the airplane going over on its back.

In the Decathlon and Cessna 150 I teach an entry a few knots above the stall speed. In a 152 I generally find that I have to use the dynamic entry.

So, Tim, what type of airplane are you flying?

When I did spins they were in a Super Decathalon, IIRC - logbook shows 8KCAB-180. And the spin I'm describing is the one where it's entered at a stall, so I'll describe it as a "wing drop" rather than a "wing lift". The airplane doesn't fees like it "snaps over" onto it's back, but more like it "falls under".

 

[dmccormack]

I found the airplane appears to "go over on its back" in some spins -- not all.

Some -- as you describe -- are simple severe wing drop and then spin. Other times the top wing drops out and you're inverted.

EDIT: Just saw Dave's post -- what he said.

 

[Everskyward]

EDIT: Just saw Dave's post -- what he said.

+1. I think the problem with using the word "roll" to describe the motion is that people connect roll with ailerons and that is not the way to recover. It's been a while since I learned about the different types of spins but it seem like you could either tighten or flatten the spin with aileron but you couldn't recover that way.

 

[gismo]

In any spin, one wing is developing more lift than the other but "centrifugal force" prevents the bank angle from increasing beyond about 45° or so. At the start of the spin, the yaw rate is low which allows a greater bank initially. This is why spins in twins and other "wing loaded" airplanes are different than in "fuselage loaded" singles.

If power (RPM) is high, the gyroscopic forces of the engine and prop decrease the bank when spinning in one direction and increase it when spinning the other way.

 

[Areeda]

+1. I think the problem with using the word "roll" to describe the motion is that people connect roll with ailerons and that is not the way to recover. It's been a while since I learned about the different types of spins but it seem like you could either tighten or flatten the spin with aileron but you couldn't recover that way.

I believe the problem with using ailerons is that their effect depends on how far the stall has progressed along the wing.

If the stall is only at the root they will work normally to correct bank angle which is more of a symptom than a cause. So I agree with Mari that doesn't produce a recovery.

However if the stall has progressed far enough to cover the aileron they work backwards. Since we're passed the critical angle, down aileron, increasing the angle of attack at that point, produces less lift causing that wing to fall further.

At least that's the reason I learned we keep the ailerons neutral.

Joe

 

[djpacro]

There's an excellent document on the subject here - some bedtime reading! http://www.flightlab.net/Flightlab.net/Download_Course_Notes_files/10_Spins.pdf

 

[dmccormack]

There's an excellent document on the subject here - some bedtime reading! http://www.flightlab.net/Flightlab.net/Download_Course_Notes_files/10_Spins.pdf

Great resource -- thanks!!

 

[TMetzinger]

It was a great resource - I was out in 10 minutes last night. Then this morning I read it again and stayed awake and it was still a great resource.

 

[xaminmo]

I always heard it as the more stalled wing dropped more, and this makes more sense to me.

The less stalled wing doesn't "come up", it just falls less. It only appears to come up because it's relative to the low wing, and therefore relative to the cockpit.

The increased drag from the more stalled wing causing the pivot is also a good point to add in there.

 

[Rob Schaffer]

My words....

"Holy Crap!!!" as the wing spun over and we entered the spin.....

Then the CFI recovered,...

and We did it again, this time My turn,....

"Wahoo!!!, Let's do that Again!"

Six spins total that day,... and lots of

Thats my non-technical description

 

[elmetal]

My words....

"Holy Crap!!!" as the wing spun over and we entered the spin.....

Then the CFI recovered,...

and We did it again, this time My turn,....

"Wahoo!!!, Let's do that Again!"

Six spins total that day,... and lots of

Thats my non-technical description

Agreed. I did this in a Zlin 242 prior to some aerobatics and my god. Spins were the greatest thing ever!

Then we started doing rolls, and hammerheads and the like.

My life was changed

 

[Dr. O]

My highly, technical, description of the spin condition in an airplane is as follows...




WHEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE


denny-o

 

[spiderweb]

Agreed. I did this in a Zlin 242 prior to some aerobatics and my god. Spins were the greatest thing ever!

Then we started doing rolls, and hammerheads and the like.

My life was changed

ANOTHER ZLIN PILOT! YES!

I've done aerobatics in the Zlin. I love it!

Before that, I'd done spins in a C150 and spin entry in C172. They were NOTHING!

 

[elmetal]

ANOTHER ZLIN PILOT! YES!

I've done aerobatics in the Zlin. I love it!

Before that, I'd done spins in a C150 and spin entry in C172. They were NOTHING!

Now, I've never flown a pitts

but the Zlin is EASILY the most fun airplane I've ever flown.

I can just take the stick and boom, 90 degree bank

I like how "unstable" it was. If I put it in 45 degree climb, I don't really need to trim, it just stays there... the control pressures are so much less strong than a cessna. everything took so little power to do. I guess it's safer to have a more stable aircraft, but it's much more fun to fly the Zlin

 

[spiderweb]

Now, I've never flown a pitts

but the Zlin is EASILY the most fun airplane I've ever flown.

I can just take the stick and boom, 90 degree bank

I like how "unstable" it was. If I put it in 45 degree climb, I don't really need to trim, it just stays there... the control pressures are so much less strong than a cessna. everything took so little power to do. I guess it's safer to have a more stable aircraft, but it's much more fun to fly the Zlin

Oh yeah. And it you can wrack it around the sky however you like, and it's totally happy. Even an aileron roll is a pleasure!

 

[elmetal]

Oh yeah. And it you can wrack it around the sky however you like, and it's totally happy. Even an aileron roll is a pleasure!

Man I wish I were flying

 

[spiderweb]

Me too!