Ball deflected left in spin to the left?

SuperCubRick

Well-Known Member
My instrument instructor stumped me today by asking which way the ball was deflected during a spin to the left.

I quickly said "to the right, as that's where the centrifugal force should be moving it" - but he said, well not really it depends on where it's mounted on the panel.

He said that because it's mounted to the left of the airplanes center line, that's where the ball will fall in a spin to the left - and if it were mounted right of the centerline it would fall to the right in a left spin.

I looked it up online and William Kershner says the same thing as my instructor, in a page in his Flight Instructors Manual.

I'm trying to visualize the forces acting on the plane during the spin and trying to figure out why the placement of the turn coordinator would matter - as they're both in the same plane of rotation, pitch, and bank angle, subjected to identical lateral G loads during the spin - why would there be a discrepancy between the two!?

TGrayson, help me out!
 

tgrayson

New Member
I'm trying to visualize the forces acting on the plane during the spin and trying to figure out why the placement of the turn coordinator would matter - as they're both in the same plane of rotation, pitch, and bank angle, subjected to identical lateral G loads during the spin - why would there be a discrepancy between the two!?
I can verify that what Bill Kershner says is true, since I saw it for myself in his airplane. He had two inclinometers, one on each side of the cabin and the balls did go in opposite directions.

To make it simpler, consider a flat spin. Is it not intuitive that everything will be thrown to the outside of the cabin, with the CG as the center of rotation?
 

SuperCubRick

Well-Known Member
I can verify that what Bill Kershner says is true, since I saw it for myself in his airplane. He had two inclinometers, one on each side of the cabin and the balls did go in opposite directions.

To make it simpler, consider a flat spin. Is it not intuitive that everything will be thrown to the outside of the cabin, with the CG as the center of rotation?
Interesting. I can kind of understand a flat spin with the center of rotation basically through the center/CG of the plane, but in a "standard" spin I can't seem to figure it out. Not a big deal, I guess it behaves as it does but I just like to know why things are the way they are! :D

Even in a flat spin, the two T/C's are out infront of the center of rotation in the same plane of motion/subjected to identical forces, only differences is they're on either side of the rotation center - I can't see how that would make them behave opposite each other.
 

tgrayson

New Member
Interesting. I can understand a flat spin with the center of rotation basically through the center/CG of the plane, but in a "standard" spin I can't seem to figure it out. Not a big deal, I guess it behaves as it does but I just like to know why things are the way they are! :D
The motions of a spin are very complicated, but in a standard spin, the a/c is yawing, rolling, and pitching, and the spin center will not be through the CG. However, the aircraft will be yawing around the CG, but the CG itself will be orbiting some other center.

The reason I suggested visualizing a flat spin is that that this rotational motion doesn't go away as the nose lowers towards a steep spin, but some of it becomes roll and pitch.

Note that the ball isn't a measure of sideslip, per se, but it's a measurement of lateral acceleration. The reason that it normally works to measure sideslip is that a sideslip produces an acceleration of the fuselage along the lateral axis (wingtip to wingtip).
 

tgrayson

New Member
Even in a flat spin, the two T/C's are out infront of the center of rotation in the same plane of motion/subjected to identical forces, only differences is they're on either side of the rotation center - I can't see how that would make them behave opposite each other.
The balls are constrained in their planes of movement, so only a component of the "centrifugal" force along the plane of freedom will move the ball. If the inclinometer were perpendicular to the axis of rotation, the ball wouldn't move at all.

Draw the inclinometers and draw a force line directly from the CG to the ball, and you will see there is a component in the plane of movement of the balls that are opposite.
 

///AMG

Well-Known Member
Regardless of the physics involved, if you press the "I believe" button, then you can just accept that the turn needle is your only accurate indicator of spin rotation direction. :rawk:
 

SuperCubRick

Well-Known Member
Ah, that picture explains it, fairly simple I guess.

I read Rich Stowell's book - Stall & Spin Awareness but I don't ever remember him mentioning this in the book, maybe he did and I just forgot.
Regardless of the physics involved, if you press the "I believe" button, then you can just accept that the turn needle is your only accurate indicator of spin rotation direction. :rawk:
Exactly - but I just gotta know why there's a discrepancy in the ball deflection. I still couldn't explain it to anyone. So it's because the t/c's are on different sides of the centerline pointed towards the center of rotation?
 

tgrayson

New Member
I read Rich Stowell's book - Stall & Spin Awareness but I don't ever remember him mentioning this in the book, maybe he did and I just forgot.
I haven't read Rich's book, but I have seen other spin books warn not to trust the ball. I'd be surprised if he doesn't mention it.
 

Crockrocket94

Well-Known Member
Yeah true about the ball, but if you have a Turn Coordinator you can step on the "high wing" provided it is not an inverted spin right? It has been a while since I have done spins and aerobatic flight.

Guess I should freshen up if I make statements :p
 

SuperCubRick

Well-Known Member
I haven't read Rich's book, but I have seen other spin books warn not to trust the ball. I'd be surprised if he doesn't mention it.
Yeah he did mention not to trust the ball and refer to the needle as an indicator of spin direction. I guess I didn't think about what he said as meaning the ball would be on either side of the t/c on different sides of the panel.

So if you hold up a weight on the end of a string, it'll be deflected back (towards the tail) and to the left in the left seat and back and to the right in the right seat? Doesn't seem like it makes sense - I'd think they'd both be deflected straight back towards the tail and toward the lower wing, or does that not have anything to do with how the balls behave? The acceleration force wanting to pull the plane away from the center of rotation is constant all over the airplane right?
 

Brian Z

Well-Known Member
The way I see it is there are two different axis of rotation. One is around the vertical axis through the CG and the other is the axis the whole plane rotates around. You can picture one axis by pointing your finger at the top of your desk. Now move your finger in a circle. That is the rotation the aircraft is making around an imaginary point. When you do that you notice your fingernail always faces the same direction. Does an airplane face one direction, say east, while in a spin. No, it points towards the inside of the spin. To do that it has to rotate around another axis. When you add the clearer visualization of what the ball does in a flat spin I hope it helps seeing what is going on.
 

///AMG

Well-Known Member
Yeah true about the ball, but if you have a Turn Coordinator you can step on the "high wing" provided it is not an inverted spin right? It has been a while since I have done spins and aerobatic flight.

Guess I should freshen up if I make statements :p
Actually the turn needle should indicate the proper direction in either upright or inverted spin. It is purely a turn rate indicator, and it doesn't care what side of the horizon you are looking at. So always step on the high wing.......though some inverted spin recovery procedures aside from rudder input differ from their upright counterparts (for example in the T-34C we use stick forward of neutral for upright, stick centered neutral for inverted)
 

Copperhed51

Well-Known Member
So does this mean that if you sit in a seat in the middle of the airplane that your balls will stick to your legs?:nana2: Sorry, I had to. Interesting conversation though. I'd never even considered the balls could go two different directions.
 

tgrayson

New Member
So if you hold up a weight on the end of a string, it'll be deflected back (towards the tail) and to the left in the left seat and back and to the right in the right seat?
Remember that the ball is constrained...it can only move left (or right). It would want to fly forward AND to the left if it were set free. If you hang a ball on a string, it's not constrained in that way and will point directly away from the axis of rotation. In a flat spin, if you held it in your hand, it'd probably point to the left and somewhat backwards, depending on where the CG was. In the more complex motion of a steep spin, who knows?

Why don't you try it and report back?
 

tgrayson

New Member
Actually the turn needle should indicate the proper direction in either upright or inverted spin.
My understanding is that a TC will only indicate correctly in an upright spin, whereas a turn and bank indicator will be correct in either an upright or inverted. A TC also measures rate of roll, whereas a T&B does not.
 

tgrayson

New Member
Dang, you studied under Bill Kershner?
He lived fairly near me in Sewanee, TN, and I flew over there a few times to fly with him. I did a little aerobatics training in his Aerobat and then returned later for my CFI spin endorsement. I would have loved to have spent more time with him.

I'm surprised you even know his name....I haven't run into *anyone* locally who has ever heard of him. :rolleyes:
 
R

Roger, Roger

Guest
I'm surprised you even know his name....I haven't run into *anyone* locally who has ever heard of him. :rolleyes:
Yeah, one of the texts for my CFI groundschool was his "Flight Instructor Manual". Good book, and I've heard lots of good things about Kershner around the interwebs.
 

///AMG

Well-Known Member
My understanding is that a TC will only indicate correctly in an upright spin, whereas a turn and bank indicator will be correct in either an upright or inverted. A TC also measures rate of roll, whereas a T&B does not.
Yes, you are indeed correct....sorry I fly with a T&B indicator so I wasn't really considering the little airplane style TC. So to clarify/reiterate/corroborate/high-five/:panic: T&B will always show the correct direction of spin
 
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