Decathlon Accelerated Spin

[djpacro]

Just seeing what happens when the stick is moved forward in a fully developed spin:

 

 

 

[motzartmerv]

Wow... Thanx dave

 

 

[motzartmerv]

Can you explain the aerodynamics behind this phenomena for us Dave? It's very interesting.

 

 

[facthunter]

Be a little wary of pushing the stick well forward. Do something else and you may be spinning inverted quickly. Nev

 

 

[djpacro]

Yes, the Decathlon (and Pitts) will neatly transition to an inverted spin if opposite rudder is applied along with brisk full forward stick. Direction of roll stays the same but direction of yaw is reversed. Quite disorientating.

 

As for moving the stick forward alone, here is part of the short story. Refer to the attached picture.

 

The grey shaded area is "dead air" so the parts of the fin and rudder in there are not effective. Fin and rudder in clear air provides resistance to the spinning motion (roll and yaw). Enough rudder outside that blanketed area is also required to be able to recover from a spin.

 

Consider the tail configuration on the left. Something like the Decathlon and Pitts.

 

Move the stick forward, elevator moves down and increases the "dead-air region" so that more of the rudder is blanketed. Less resistance to spinning so the rate of rotation increases and gyroscopic moment pitches the nose up. Nose down pitch steepens the spin but remains stalled because of the nose up gyroscopic moment. The "ballerina effect" of the reduced radius of rotation also increases the spin rate.

 

With the reduced rudder effectiveness, if opposite rudder is applied in an attempt to stop this accelerated spin: recovery will take longer or it may not recover at all.

 

Now consider the tail design on the right with the horizontal tail moved further aft as for the CT-4, Chipmunk and Zlin 242. The "dead-air region" is moved back behind the rudder. Moving the elevator does not have any effect on the effectiveness of the rudder.

 

The two tail designs can be grouped together approximately wrt control actions for spin recovery (without discussing Beggs- Mueller technique right now).

 

 

 

[motzartmerv]

Excellent , thanx Dave. It's interesting stuff. Is the diagram accurate with the spin axis being so far forward of the cofg?

 

 

[djpacro]

Appearances can be deceiving - imagine that pic superimposed somewhere on this diagram:

 

 

Longer answer here: http://www.flightlab.net/Flightlab.net/Home_files/10_Spins.pdf

 

 

[Bandit12]

I remember doing accelerated spins in the Pitts S2B, and the speed of rotation was beyond what I could comprehend. Almost to the point of making it very difficult to identify direction of rotation, compared to a standard spin. I had only recently got my GFPT and so it was a big shock to see just what the ultimate mismanaged recovery could end up looking like.

 

 

[djpacro]

Exactly, so an unintentional accelerated spin is potentially dangerous.

 

http://www.recreationalflying.com/threads/lecombers-spin.22260/

 

 

[Mick]

Dave, would the effect you describe in post #5 with dead air be as pronounced in an aircraft with the entire elevator ahead of the rudder such as a Cap 232 and most of the Mundry series?

 

 

[djpacro]

I don't have first hand knowledge of the CAPs but the likely extent of the interference can be obtained by drawing the areas similar to that NASA diagram above - draw the lines for different elevator angles to give some indication of the potential reduction in rudder effectiveness. The CAP series has a very large area of the rudder below the elevator therefore in clear air.

 

 

[IBails]

[Geoff13]

I am impressed

 

 

[djpacro]

At least one aerobatic aeroplane in Australia lost its prop due to the common crankshaft failures late '70s/early '80s. I've got the later heavy duty aerobatic crankshaft in mine.

 

 

[Guest Andys@coffs]

Well if your going to lose a prop, that's the way to do it, rather than blade at a time, followed by engine and mount!. Big prop, big spinner at an extreme from CofG perspective wonder if the handling was affected?

 

Andy

 

 

[IBails]

Well if your going to lose a prop, that's the way to do it, rather than blade at a time, followed by engine and mount!. Big prop, big spinner at an extreme from CofG perspective wonder if the handling was affected?Andy

I would think CoG would have moved back a lot. Have seen similar when a Pitts lost a prop and crash landed the pilot had almost full forward stick to sort of fly it to the ground.

 

 

[IBails]

[pmccarthy]

Had me in suspenders for six and a half minutes -great flying.

 

 

[IBails]

Another

 

 

 

[IBails]

Not aeros but Just came accross this and the commentary is very good in defining this accident.

 

 

 

[facthunter]

It is likely you will have some control difficulties with the entire prop and some of the crankshaft gone. Best to keep the speed quite high if you are tail heavy. The only incident I know of the Chipmunk lost ONE blade and the engine came out almost instantly. In that situation the plane is unlikely to be controllable. This one spun but at a much less ROD than normal due to being lighter and it went into Lake Macquarie (near Newcastle). I think the pilot survived.

 

When the crankshaft fails ( due to repeated high loading from gyroscopic precession) it will probably be cracked to some extent first, but the engine will overspeed so fast you are unlikely to be able to do anything about it. This will result in something penetrating the crankcase, like a connecting rod, and you will have engine damage and likely oil all over the place so a fire is on the cards so this is one where perhaps you wouldn't use a ballistic chute, unless you are pretty low. Nev