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If "all flying rudders" for LSA aircraft need 33%-40% less area than a typical vertical tail (stabilizer and rudder) ( subject to final stability analysis), what about "all flying elevators" (stabilators) with:

  1. anti-servo trim tab?
  2. no anti-servo trim tab?

I understand anti-servo tabs add stability to "all flying rudders/elevators" by auto-centering.

According to a design guide from Zenith: Flying on Your Own Wings, by Chris Heintz, pg 229:

The vertical tail area should be 12 to 15% of the wing area (15% if the rear fuselage has a small rounded section, and 12% if the rear fuselage has large flat sides). These percentages may be reduced to 7% and 10% for an "all flying rudder".

A wing is a wing, no matter what attitude, whether it be horizontal or vertical. So I should be able to reduce my horizontal tail area by roughly 33-40% when using only a "all flying elevator" (stabilator) (subject to final stability analysis). Is this correct?

My personal view is that a Zenith can fly a lot slower with an all flying rudder, 39 mph or slower, and that is why Heintz recommends that based on the type of aircraft he manufactures: basically LSA types, @ 1320 lbs MTOW, 39-4 5mph stall speed. Somewhere I read that a Zenith 701 won a STOL competition in Australia with a 25' takeoff, and that a 3 second takeoff is common and by the time you reach full throttle, you're aloft albeit still in ground effect.

I understand the Zenith CH200 had a classic vertical tail, but that most or all current designs use an all flying rudder.

Summary: So I assume that Heinz's above recommendation is for sizing a rudder at a 39mph stall speed which would need to be bigger for slow speed handling than for a plane that stalls at 60mph.

Revised question: at 39 mph would an "all flying" elevator be more efficient, thus letting it be 33% to 40% smaller?

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Zenith gets away with the little All Flying Rudder without an anti-servo tab because the fuselage itself is used to provide sufficient fixed fin area for acceptable passive weathervaning. If you look at their designs they all have slab sided fuselages that are quite deep all the way back. So there is a kind of "fin" with the rudder; it's just built into the fuselage. I got to fly the Zenith 750 demonstrator once in Mexico Missouri and I recall that its yaw stability was perfectly acceptable even though the rudder just wanted to trail if if left it to do its own thing.

Horizontal tails have more work to do and horizontal surface area of the fuselage aft won't cut it without a fixed stabilizer surface; unless you use a stabilator/anti-servo tab. If you wanted to use an all-flying surface without an anti-servo tab, you have to have a means to "fix" the surface to provide stick free static stability. If you're in an F-86, the all flying tail is hydraulically powered and can be fixed easily with a bungee that controls where the hydraulics wants to center the surface when you're not applying force. Some gliders use all flying tails with no tabs of any kind, and they depend on bungee springs in the pitch control circuit to provide a tendency of the surface to want to hold a certain angle if you aren't hanging on to it.

You could design your ultralight with an all flying tail with no anti-servo tab by using a fairly stiff spring bungee as your "stick free" stabilization device, and for trim. You may find the anti-servo tab method provides better handling characteristics however and if it was me I would go that way.

Note that airplanes sometimes use bungee springs in the rudder control circuit to provide a passive centering force that has the effect of increasing the fin area to improve passive weathervaning. Cessna 180/185s do this to improve yaw stability so that when fitted with floats they don't need extra ventral fins. Problem is, it makes the rudder unpleasantly heavy.

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  • $\begingroup$ Yes, but Chris Heintz is saying, in the quote above, that even round fuselages can reduce rudder area by 33% (15-10/15) if using an all flying rudder. A flat slided fuselage even more at about 40%. $\endgroup$ – Fred Apr 23 at 21:58
  • $\begingroup$ The downside of slap sided fuselage is difficulty in cross winds. The Curtiss C-46 with its double bubble fuselage was like a flying billboard with way too much side area and insufficient rudder area and was very limited in x winds. Anyway, the point is that it doesn't work so well in the pitch mode because the interactions are far more complex. $\endgroup$ – John K Apr 23 at 22:11
  • $\begingroup$ @ John K: re difficult in cross winds: Yes, but I think the Zenith 701 has a max x wind of 25mph, which is almost double that of a Cessna 172 at 15mph(?), so it's almost twice as good even though it has slab sided fuselage. $\endgroup$ – Fred Apr 23 at 23:44
  • $\begingroup$ @ John K: Re my revised question above, would a stabilator be better at low speed handling below 39mph same as an all flying rudder, allowing it be smaller than a classical stab and elevator? $\endgroup$ – Fred Apr 23 at 23:45
  • $\begingroup$ Yes you will get more tail power per unit of surface area but you have to figure out the gearing for the anti-servo tab, which will determine your stick free stability and stick forces per G. The Cessna Cardinal's stabilator is quite large and is able to drag the tail skid on the ground as low as 30 mph (I used to own one), which is why the Cardinal has a really heavy duty tail skid. $\endgroup$ – John K Apr 24 at 0:09
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It seems the question contains a false premise.

Why should an all-moving vertical tail need less area than the traditional arrangement, in terms of providing stability?

For the case where the pilot's feet are firm on the rudder pedals, there should be no difference, and for case where rudder is free to float, it would seem that the all-moving tail should provide less stability.

I can see why an all-moving vertical fin gives more control power to command a sideslip than a rudder of equal size attached to a fixed fin, but that's a different subject.

The above comments are for no anti-servo tab. I'm not aware that anti-servo tabs have ever been used on all-moving vertical tails of light aircraft with simple mechanical control linkages.

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  • $\begingroup$ The determining size of the all flying rudder may not be due to stability in this kind of aircraft with it's typical lever arm length. Zenith could be sizing their rudder based on sideslip requirements, crosswind requirements ( 25mph vs Cessna 172 @ 15mph(?), or slow speed requirements ( 35-39mph at low weight, single pilot, I'm told). $\endgroup$ – Fred Apr 23 at 22:06
  • $\begingroup$ @JohnK and Fred-- I've seen a page on a Zenith website explaining the superior crosswind capability due to the powerful effect of all-flying rudder. Sure the slab side increases required bank angle but you aren't going to run out rudder in a wing-down correction. The quoted material in question seems to imply though that the sizing being discussed is for stability considerations, so I still don't quite understand it. After all it's talking about vertical fin area, and fixed vertical fin area doesn't help you deal w/ crosswinds at all, it only gives you stability. So, I'm still puzzled. $\endgroup$ – quiet flyer Apr 23 at 22:26
  • $\begingroup$ @Fred -- see comment above. $\endgroup$ – quiet flyer Apr 24 at 1:48

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