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stick-free, small tails & stabilators - help!

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Ted

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Jan 12, 2004
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Hello folks

Could someone please help me to understand stick-free stability a bit better? (I'm thinking mainly about pitch stability and horizontal tail sizing but I realise it applies to other control surfaces.)

Have I got this right:

Stick free stability is only different to stick-fixed stability in reversible systems. Thus in non-reversible systems stick free = stick fixed = no difference = one can have a smaller overall tail size than with a reversible control system (since stab and elevator count together).

In a reversible system, stick free is a hands-off scenario where the elevator will float (weathervane) with the relative wind. The elevator will therefore play no part in stability, stick free, and therefore when sizing the tail one discounts the elevator and sizes just the fixed stabiliser (or else one ends up with a tailplane that is too small and has consequent reduction of stability, which, particularly in turbulence, then requires constant control input to make up the difference).

If the fixed stabiliser is too small but overall sizing correct then you have correct stick fixed stability but reduced stick free stability. You can't use trim tabs or trim devices such as springs to make up the difference. For better stick free stability you just have to increase the size of the fixed stabiliser or make the control system non-reversible.


If the above is correct then I'm having trouble understanding how an all-flying tail or stabilator provides any stick free stability at all in a reversible system. Have I got this right:

The stabilator is hinged at 20-25% mac. It therefore provides little to no control feedback and is also free to weathervane. Stick force is usually created using an anti-servo tab. This tab is attached to a movable point and thereby also acts as a trim.

But since any tab is going to be subject to the same relative wind as the stabilator, what stops the stabilator weathervaning? In other words, what gives the stabilator its stick-free stability?

I must be missing something very obvious because if an elevator can float and therefore has to be removed from the stability equations for tail sizing then surely a stabilator must behave in the same way and would provide virtually no stick free stability for the aircraft (since the whole surface is floating, not just an elevator). Yet that must be wrong since there are many light aircraft using reversible control systems with stabilators and which fly along quite happily with excellent hands-off stability.

What am I missing? Thanks!
 

orion

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Flying stabs are generally attached at or very near the aerodynamic center of the surface. At that point the surface will not weathervane since the lift vector is though the ac and thus through the hinge point (zero camber surface only - normal angles of attack) so in essence only the stick fixed conditions apply. This is sort of an oversimplification but the picture should be close.
 

Ted

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Jan 12, 2004
Messages
9
Hi Orion

Ah ha! I was thinking stabilator while having the wrong mental picture - of something like a Nieuport vertical surface which, of course, now that I think again, is hinged like an elevator (rather than a stabilator), is free to weathervane and hence why it gives virtually nil stick free stability. Well that seems clearer now, thanks!

So that should make a stabilator an interesting proposition since it can be sized smaller than a stab/elevator setup and therefore be more efficient for the same static stability. However, I gather from your comments in a thread called "Pitch control theories" that there is a drawback. You say:
"the flying stab does not contribute to the [dynamic] stability characteristics as much as a fixed stab. As a matter of fact, several airplanes that incorporate a flying stab are actually dynamically a bit unstable."

Would that be just because one has been tempted to size the stabilator smaller (and thereby reduce dynamic stability proportional to the square of changes in chord) or is there something inherent in a stabilator that makes it less effective at dynamic damping than a fixed stab?

I would have thought the obvious thing to do would be to reduce the stabilator size but lengthen tail arm a touch, thereby gaining efficiency while maintaining static & dynamic stability - win win.

Must have missed something again! If you can spare a moment to explain I'd be most grateful - thanks!
 

Hot Wings

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So that should make a stabilator an interesting proposition since it can be sized smaller than a stab/elevator setup and therefore be more efficient for the same static stability.
Stick free static stability isn't the only parameter to consider for sizing. Pitch control at low speeds may dictate a much larger stabilator than a conventional tail and elevator. It may help to think of a stabilator as a little flying wing, with all of the associated limitations, trailing along in the down wash of the main wing.

The simplicity of construction may also be offset by the need to incorporate servo tabs if the control forces turn out to be high.
 

orion

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The dynamic stability is a function of tail length squared - this is why many designers like longer tail arms.

An all flying stab may have some drawbacks, most of which have to do with control authority and thus one of the sizing criteria may be a function of your forward allowable CG limit. The flying stab is a low Rn, low thickness surface, flying in a downwash field so one of the things you must consider is the limit of deflection since too much might cause the tail to stall. This therefore may require the flying stab to be as large as a conventional tail. But the benefit is still there is the Cd/Cl of the flying stab is much better than for a stab/elevator configuration.

And yes, if the tail is too small, while it may suffice for static stability, it may not do as well for dynamic however I don't have any firm examples for you on that account. I have been told that Cherokees have a slight dynamic instability however, since the phugoid has a relatively long period, it was deemed acceptable.
 

GESchwarz

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Ted, Check out the NASA video of a stabilator fluttering in the post titled "Rudder Flutter".

Stabilators have always kinda spooked me so to get the authority I want I'm going with a trim adjustable stabilizer (using a jackscrew), and a good sized elevator.
 

wsimpso1

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Stabilators theoretically can be reduced in size relative to conventional tails, but by the time you make the clearance to the fuselage/vertical tail (to keep friction down enough for good flight characteristics), and make them behave in ground effect at max forward CG, well, they generally end up being as big as if they had been a stabilizer and elevators.

When you make the stabilizer trimmable with a jack screw, you can glove it pretty tightly. Using the assumption that the elevator contributes nothing to stick free stability is conservative. It usually still adds something, and then you know that you will have more stability if the trim or downweights and the like tend to center it a little.

Billski
 

Ted

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Jan 12, 2004
Messages
9
Hi guys

Thanks for the input - much appreciated. I'm thinking about a stabilator as a reasonably simple way to improve stability in an existing design. So far I'm liking the fact that it's an 'easy' way to gain an extra foot or so of tail arm (by having the stabilator ac at the rear bulkhead rather than the rear of the fixed stab at that point, as at present) without adding significant extra weight.

Can I just check with you all that I did get the first part right above - that there are no easy fixes for a fixed stab that's too small?

Either you make it bigger or further back or you stop elevator float by
a) adding aerodynamic balance horns to the existing elevator and also adding springs or bungees for the necessary control feel or
b) redesigning the control system to be non reversible.

There's nothing else blindingly obvious that I'm missing I hope?

Thanks again.


Oh yes, there was one other thing - has anyone heard of "canted tabs" and more to the point, does anyone understand them enough to know if they might also be used on an elevator or rudder to increase stability?
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