All Moving tail question

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berridos

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Hi Everybody

I understand why a U-Tail has an increase in horizontal effective aspect ratio and can be 5% smaller than a conventional tail. Basically because of the chastity belts the endplates represent.

What I dont understand is why an all moving tail can be sized 10-15% smaller than a conventional tail according to Raymer.
Could anybody shed some light on this?
What about an all moving U-tail?
Could it be 15% smaller? How do these effects affect the calculations/parameter?
 

Toobuilder

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Are we talking about a stabilator, like a Cherokee, or as a trim device, like a Mooney?
 

berridos

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An all moving Ushaped stabilator/finavator , free hanging at the AC with trim tabs that are simultaneusly anti servo tabs and that act asimetrically as inverted slotted flaps when pushing the tail downward and as normal trim tabs when pushing upward .
Need a couple of beers to return to a simple solution.
 

Dana

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Take a look at Mark Stull's ring tail plane (search here for "ring tail").

-Dana

Art and technical perfection are not the same thing in cars, and hard-core car buffs often prefer art. Which is why they own so many tools.
 

HumanPoweredDesigner

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I read that a U-tail has more horizontal stabilizer authority for its size during normal flight, but that during a spin, the rudders can blanket the horizontal stabilizer, making it less effective. The T tail escapes this. Did I read that book right?

I can also understand why an all moving tail could be less draggy for its size. But please correct me where I'm wrong about the following:
A tail with control surfaces that let it change camber should have more authority for its size during cruise conditions, though the extra authority can be diminished during a spin. This contradicts what many people on here say about an all moving tail (like the Daedalus) having greater authority than a similar sized one with control surfaces.
 

PTAirco

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Why do people think an all-moving tail is "more effective" ? A tailplane is a wing. It needs to develop a certain amount of force (lift, though acting both up and down.) Now for a given size, what will generate more lift: a wing with huge 40% chord flaps deflecting 30 degrees or a wing with no flaps (or at most a small anti-servo tab) ?
 
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Kristoffon

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Though that's too broad of a question, definitely the wing with no "flaps" simply because the flaps spoil the perfect airfoil it was once supposed to have therefore increasing drag for a given lift - that is if we assume both cases to be made from the same airfoil and size.

OTOH it's a huge weight penalty since obviously the mechanism for handling a full moving surface is much heavier than a static fuselage with small flap on the side. In the end the weight penalty must not be worth the gained lift efficiency otherwise I suppose they would be common and not rare.

I recall though that in some passenger jets the incidence of the horizontal tail is ground-adjustable by a few degrees.
 

HumanPoweredDesigner

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I don't know how you could fillet an all moving tail to avoid interference drag. Flap junctions have drag too, but they can be smaller.

I read the FAA guidelines for estimating drag, and they said multiply the smooth wing area by 0.01, and the control surface and flap area by 0.015. So I interpret that to mean that the skin behind the hinges gets 50% more drag than without the hinges.

Edit:
Actually I do know how to make a streamlined all moving surface. But it still has to be bigger so it can work during take off at its Cl max. I guess it is fine as long as it is at a reasonably efficient Cl during cruise. But that is the horizontal stabilizer. I'd think the rudder would be less draggy during cruize if it had a flap instead of all moving. But I also think an all moving, larger rudder without a flap would have more dampening during a spin or any other flutter.
 

orion

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The all moving horizontal is lighter, can generate more lift per drag count and does not have a stick-free stability issue associated with it since the whole surface acts to stabilize the airplane. The latter results in a larger allowable CG range or allows for a smaller area (I prefer to take advantage of the latter).

For the operating range of a typical airplane, the flying stab can generate more lift than a deflected flap since it can adjust its own angle of attack whereas a stab is fixed so the pitch change of the fuselage negates the flap deflection.

For GA interference drag is not really an issue since for cruise the lift coefficient is small and by the time the air gets back to the tail the root is pretty much buried in the boundary layer.
 

flat6

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The all moving horizontal is lighter, can generate more lift per drag count and does not have a stick-free stability issue associated with it since the whole surface acts to stabilize the airplane. The latter results in a larger allowable CG range or allows for a smaller area (I prefer to take advantage of the latter).
does this also apply to all moving rudders?
 

orion

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In theory, yes but many designers prefer a fixed vertical since an off position flying vert during spin entry could make the situation worse. How much worse? Don't know but the idea of a flying vert seems to be something that most are staying away from. The flying vert also can't take advantage of the effects of a dorsal.
 

flat6

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In theory, yes but many designers prefer a fixed vertical since an off position flying vert during spin entry could make the situation worse. How much worse? Don't know but the idea of a flying vert seems to be something that most are staying away from. The flying vert also can't take advantage of the effects of a dorsal.
even if the rudder movement range is constraint to an appropriate angle? i like the all moving vertical because its so easy to build.
 

flat6

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even if the rudder movement range is constraint to an appropriate angle? i like the all moving vertical because its so easy to build.
actually all moving rudders can use dorsals. its just a vert stab where the rudder is much larger than usual. like a jodel d11.

you can still use a traditional hinge too. just bury the small vert stab inside the fuselage.
 

flat6

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I don't know how you could fillet an all moving tail to avoid interference drag. Flap junctions have drag too, but they can be smaller.
actually depending on the design, just put a plate with enough clearance from the fuselage skin at the inboard of the surface and put fillets there.
 

Dana

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Possibly the biggest problem with an all moving vertical stab is structural... you'd have a large overhanging load on whatever pivot point you use. This is true, to a lesser extent, of a flying horizontal stabilizer as well. Anchoring the stabilizer rigidly at the front and the back has definite structural advantages, so it can be lighter... which may or may not offset the weight advantage from making the surface smaller, as Bill pointed out is possible.

A number of aircraft have used a tiny vertical fin and a large rudder with significant area (the whole top of the rudder) extending forward of the hinge line, to the leading edge of the fin... a large aerodynamic (and mass, if necessary) balance.

-Dana

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flat6

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Possibly the biggest problem with an all moving vertical stab is structural... you'd have a large overhanging load on whatever pivot point you use. This is true, to a lesser extent, of a flying horizontal stabilizer as well. Anchoring the stabilizer rigidly at the front and the back has definite structural advantages, so it can be lighter... which may or may not offset the weight advantage from making the surface smaller, as Bill pointed out is possible.

A number of aircraft have used a tiny vertical fin and a large rudder with significant area (the whole top of the rudder) extending forward of the hinge line, to the leading edge of the fin... a large aerodynamic (and mass, if necessary) balance.

-Dana

Earthlings: Send more probes. The last one was delicious!
For light aircraft, the weights of the moving tail surfaces are low.
 

orion

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even if the rudder movement range is constraint to an appropriate angle? i like the all moving vertical because its so easy to build.
I thought about that after I wrote the last reply - I did a bit of digging through my files but can't find the original paper where I saw the comment. It however does seem to be that if you constrained the motion of the vertical, possibly in conjunction with a damper, the issue of stall/spin would be no different that with a more conventional configuration. I don't have any first hand experience with this but it does seem reasonable.
 

orion

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actually all moving rudders can use dorsals. its just a vert stab where the rudder is much larger than usual. like a jodel d11.

you can still use a traditional hinge too. just bury the small vert stab inside the fuselage.
Good point - I guess I was picturing the full area of the vertical tail but there's no reason why the geometry couldn't be modified to that which you describe. And in that case the dorsal would still provide the vortex aiding in the control and stability.
 
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