Tails. Pros and Cons

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Winginit

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Thinking about the different types of horizontal tails and wanted opinions on what they do well, and what might be their deficiencies. It seems that an all moving horizontal tail might require less area and therefore exhibit less drag. Also since it's one piece, it should be more rigid and less susceptible to flutter. Also, the best/correct fixed incidence would be a moot point. Any thoughts about this? Is a horizontal stabilizer with an airfoil stronger and more efficient than a flat surface stiffened by wires ?
 

Victor Bravo

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A proper airfoil on the H-stab will obviously be better aerodynamically on a drag and size vs. control authority standpoint.

I'm definitely NOT qualified to offer an expert opinion from a degreed aerodynamicist's point of view, but I will say that three of the more visible proponents of all-flying tails on small light aircraft are Bud Evans (VP-1), John Thorp (Piper Cherokee, T-18), and Michel Colomban (Cri-Cri, Luciole). That's a whole lot of "street cred" in the small/light airplane world.

There are many people on this forum much smarter and more highly educated than I, and they will surely jump right in here and say that there are known technical reasons for and against a flying tail/stabilator on any given aircraft, and that the aircraft's mission/performance/operating environment will determine whether a stabilator is the better option on said aircraft.

My own small/light aircraft sketches and amateur designs (VP-21, Flying Motorcycle) have stabilators because it reduces parts count/build time and makes it much easier to balance against flutter. A stabilator can be built faster than a stab/elevator combination, and reduces the chance of warping during construction. But my engineering resume' is quite a bit less impressive than many other amateur designers on this forum, so take it with a boulder of salt.

With all that said, there must be a good reason that the majority of aircraft have separate stabilizers and elevators. HBA Brain Trust... can anyone explain this simply enough for the purposes of this discussion thread? Dynamic and/or "stick free" stability?
 

autoreply

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With all that said, there must be a good reason that the majority of aircraft have separate stabilizers and elevators. HBA Brain Trust... can anyone explain this simply enough for the purposes of this discussion thread? Dynamic and/or "stick free" stability?
"That's how we always do it".

That's the only reason I could find after 15 years of studying it in detail.

I've studied all the criteria and done plenty of trade-offs. Required area, drag, weight, complexity, ease of removal (after flight...), build complexity in several materials.

Every single criteria is a wash in the end between all-flying and classic elevator.


Just absolutely never ever sweep the .25 chord line. That's very bad for a flying H-tail.
 

Winginit

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"

I've studied all the criteria and done plenty of trade-offs. Required area, drag, weight, complexity, ease of removal (after flight...), build complexity in several materials.

Every single criteria is a wash in the end between all-flying and classic elevator.

Just absolutely never ever sweep the .25 chord line. That's very bad for a flying H-tail.

Please tell me more. I'm surprised that everything becomes a "wash" though. I would think certain things would favor each design.
 

Victor Bravo

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Every single criteria is a wash in the end between all-flying and classic elevator.
From Post #11 in this thread: http://www.homebuiltairplanes.com/forums/showthread.php?t=9307 :

The all moving horizontal is lighter, can generate more lift per drag count ...(VB snip 2-10-2017 VB)... 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.
With no disrespect whatsoever to Autoreply and/or his years of research, the opinion that Orion shares in the above post very much matches my assumptions and/or "gut reaction".

One very well known example of the "fixed stabilizer"principle is the slow speed performance difference between the fixed stabilizer/elevator of the Cessna 172 and the performance of the moveable horizontal stabilizer (still not an all flying tail) of the otherwise identical Cessna 182. The 182 can be trimmed to fly slower and/or at a higher AoA than the 172, because as you raise the nose of the 172 (for slow speed flight) the main horizontal stabilizer is trying to push upward on the rear of the fuselage. This is essentially "fighting" or reducing the "up elevator" force trying to push the tail down. This is a universally understood phenomenon on these two aircraft. It is also responsible for a large part of the increased STOL capability of the Piper Super Cub (moveable stabilizer with elevator) versus the Taylorcraft, Aeronca and its other fixed stabilizer peers.

I'm also convinced that the all-flying stabilator is easier to build and lower parts count.
 
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autoreply

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A simple flying tail will have less lift per sqft. (Unflapped airfoil). Also lower drag. A wash in the end.

A flying tail with a tab on the end will have the same drag and max lift as a conventional H-tail if the latter has the right incidence to almost reach stall. Usually it's there (wing down wash)

A flying T-tail with an anti servo tab on a sailplane has exactly the same parts and structural layout as a traditional elevator. So no weight or build difference.
 

Victor Bravo

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OK, school me on aerodynamics:

1) A flapped airfoil whose 60% "main wing" section is at zero angle of attack, and 40% of the wing is deflected at 15 degrees angle of attack... has more lift than an unflapped airfoil whose entire wing (100%) is at a 15 degrees angle of attack?

2) On an airplane in landing configuration (longitudinal axis 15 degrees nose-up), with the fixed part of the horizontal stabilizer also at 15 degrees nose up, the elevator control is deflected upward at 30 degrees.

- This means that the elevator itself is deflected 15 degrees upward from the (true horizontal) flight path and "relative wind". And the forward half of the fixed stabilizer is at 15 degrees THE OTHER WAY to the relative wind.
- This means half of the tail is pushing the nose down, and half of it is pushing the nose up.

2A) Another identical airplane with an all-flying stabilator is flying at the same 15 degree nose-up landing configuration. The entire stabilator is deflected 30 degrees upward (stick full back), which means that this stabilator is also at a 15 degree deflection to the (true horizontal) relative wind.

- However on this aircraft, there is no part of the tail that is pushing the nose down, and all of it is pushing the nose up.

And you are saying that it's a "Wash", neither airplane will be able to fly slower or have better control authority ?
 
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Jerry Lytle

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"- However on this aircraft, there is no part of the tail that is pushing the nose up, and all of it is pushing the nose down."

How do you land a plane if full aft stick is pushing the nose down? It seems to me that all of the tail is pushing the tail down which means the nose is being pushed up.
 

Victor Bravo

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"- However on this aircraft, there is no part of the tail that is pushing the nose up, and all of it is pushing the nose down."

How do you land a plane if full aft stick is pushing the nose down? It seems to me that all of the tail is pushing the tail down which means the nose is being pushed up.
Typo corrected, sorry. In that last example, ALL of the stabilator is pushing the nose upward by pushing the tail down.
 

Aesquire

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Cons on all flying horizontal stabilizers include...

1. All structural loads go through the pivot.

2. More efficient, until the tail stalls. The "flapped" airfoil of a Conv. tail then is better. ( that's an oversimplification.. )

Variable incidence horizontal stabs, like a Cub, aren't the same as an all flying tail. They do offer an advantage in adjustment over tabs, at the cost of weight & usually drag. Because of the "slot" where the H.Stab is next to, but not attached to the fuselage. Lots of work has been done to try and eliminate that air leakage. Bunch of different solutions. All add complexity & weight.

A classic trade off in both cases.

There's issues of stick free stability and control forces too, but those are solvable with careful design. I'd call that part a wash.

I could be wrong. Please correct me.
 

Winginit

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Cons on all flying horizontal stabilizers include...

1. All structural loads go through the pivot.

2. More efficient, until the tail stalls. The "flapped" airfoil of a Conv. tail then is better. ( that's an oversimplification.. )

Variable incidence horizontal stabs, like a Cub, aren't the same as an all flying tail. They do offer an advantage in adjustment over tabs, at the cost of weight & usually drag. Because of the "slot" where the H.Stab is next to, but not attached to the fuselage. Lots of work has been done to try and eliminate that air leakage. Bunch of different solutions. All add complexity & weight.

A classic trade off in both cases.

There's issues of stick free stability and control forces too, but those are solvable with careful design. I'd call that part a wash.

I could be wrong. Please correct me.
Can you elaborate on #2? Would an All Moving horizontal tail stall more quickly ?
 

Aesquire

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Can you elaborate on #2? Would an All Moving horizontal tail stall more quickly ?
Not sure? If you are talking a one piece H. Stab. with low or zero camber, I'd think a "flapped" conv. tail, which can change it's camber, would have an edge at high AOA on the tail.

Often all moving H. Stabs have "flaps" in the form of servo/anti servo/trim tabs. So the "pure" example isn't the only answer. It's always a trade off.

Look at what gets used, where.

Supersonic planes tend to have all flying stabs, but the reasons don't apply to most GA planes or ultralights.

What do most STOL planes use? High speed touring?

Don't forget there are fads in airplane design. V tails, H Tails, Canards, etc.

And practical "make it simple to build" trends, that are affected by Construction method. Tube & fabric "D" shaped surfaces. Rectangular sheet metal. Swoopy composites.

I don't think it's an accident you see all flying surfaces on home builts at the "trying really hard to be light" and "Simplest high performance" design goal planes.
 

autoreply

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OK, school me on aerodynamics:

1) A flapped airfoil whose 60% "main wing" section is at zero angle of attack, and 40% of the wing is deflected at 15 degrees angle of attack... has more lift than an unflapped airfoil whose entire wing (100%) is at a 15 degrees angle of attack?
It is not at zero aoa, not even close to it:

A simple flying tail will have less lift per sqft. (Unflapped airfoil). Also lower drag. A wash in the end.

A flying tail with a tab on the end will have the same drag and max lift as a conventional H-tail if the latter has the right incidence to almost reach stall. Usually it's there (wing down wash)

A flying T-tail with an anti servo tab on a sailplane has exactly the same parts and structural layout as a traditional elevator. So no weight or build difference.
The down wash from the wing plays a major role in this, and of course also in stability and control calculations. In essence, this is also the answer to your further questions.

Only if you get really close to the ground, this effect gets lots. That's so close (a foot maybe on many airframes) that it isn't relevant in practice.
Cons on all flying horizontal stabilizers include...

1. All structural loads go through the pivot.
So they do on many airframes with a conventional H-stab. Pretty much all T-tail composite airframes for example use a single/double bolt through the main spar. Whether it's a bolt or a pivot isn't that different.
2. More efficient, until the tail stalls. The "flapped" airfoil of a Conv. tail then is better. ( that's an oversimplification.. )
The opposite. A simple flying tail will have lower max lift, and thus require more area than a conventional h-stab. Simple case of Clmax for flapped vs unflapped airfoils.


Oh and the no-sweep requirement has to do with the tips stalling first. Having force reversal in pitch is extremely unpleasant.
 

wsimpso1

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Everything you need to do the basic calcs on this is in TOWS, but no leakage is assumed. A conventional stabilizer and elevator leaks air at the joint between stab and elevator and leaks air between elevator and fuselage. A stabilator leaks air between the stab and fuselage. The difference between big gaps and sealed makes a huge difference...
 

lr27

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Can't you just seal the gap between the stab and the elevator?
 

Aesquire

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Sure.

How?

Is the fuselage curved?

Do you use the Northrup Orion method? A fairing held close by multiple vertical slots & fittings?

VarneyOrion.jpg

Or do you flatten the fuselage in the area the Stab. moves?

I presume you need a "wiper" type flexible seal to block leakage. I leave the detail design to you.

How do they do it on Cubs? Just don't bother?
 

lr27

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Stab and the elevator, not stab and fuse. That's a harder problem, though I think it's quite feasible to solve. Some aircraft have the stab up on a little pylon, so the elevator and stab are continuous. Not just human powered ones. I think there were sailplanes that did this also:
Image78.jpg

Lots of twin boom airplanes evade this problem too. Then there's the Air Truc, although I guess that's a twin boom aircraft too:
AirTruc-EAA.jpg
 
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