All-moving tail surfaces on pivoting, tubular spars

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BJC

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Years ago I flew a prototype ultra lite wit an all flying elevator.
The designer missed the design a bit.
The control stick forces were so light as to be non existent.
The pivot point was too close to the center and no travel stops
Absolutely could not fly hands off.
SCARY
Dennis
Did it have (the usual) anti-servo tab?


BJC
 

cluttonfred

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Even for a handmade homebuilt aircraft, making three of the same part is easier and faster than making three unique parts. That's especially true if those parts are in turn made up of many unique smaller parts. Look at the tail surfaces of the Boulton Paul Phoenix at the beginning of this thread. I believe that they are made with wood spars, wood ribs, and a bent metal tube to form the perimeter. They are pretty curves, but since they are identical you could lay out the curve with blocks on the bench and bend three sets of identical tubes to form the perimeter curve, built three identical spars, make three of every rib, and you're done. Standardization like makes a project seem and be more achievable for an amateur builder, at least, that's my thinking.

What benefits are there to having three identical tail surfaces on a plane that will be made one at a time by hand? Genuinely curious here.
 

cheapracer

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I believe it is standard practice for the stabilator to be mass balanced, and have an anti-servo tab so the axis of rotation can be moved farther back.
Did it have (the usual) anti-servo tab?

The Morgans have quite a heavy mass-balancer, but no servo tabs.

Stab balance 2.jpg

stab balance.jpg


Even for a handmade homebuilt aircraft, making three of the same part is easier and faster than making three unique parts.
Yes, the ability to use one pattern, or use the first made part as a pattern (as I did a few days ago for my HS Skins), is a little faster, but I haven't found a big difference to necessarily go out of your way to do it that way, and I am referring to hand made, not CNC.

.

By the way, with the right knowledge of lever ratios and how they can increase and decrease pressure, servo tabs aren't really a requirement IMO.

.
 
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cheapracer

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What benefits are there to having three identical tail surfaces on a plane that will be made one at a time by hand? Genuinely curious here.
I was referring to aluminium above, but I guess for a composite manufacturer, you lower the mold count. You also have to keep less spares on hand for if someone damages a tail surface.
 

BJC

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By the way, with the right knowledge of lever ratios and how they can increase and decrease pressure, servo tabs aren't really a requirement IMO.
I asked about anti-servo tabs, not servo tabs. Anti-servo tabs that also function as trim tabs are useful.


BJC
 

cheapracer

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I asked about anti-servo tabs, not servo tabs. Anti-servo tabs that also function as trim tabs are useful.
My deepest apology for erring with my possessive "s", here it is again with the grammar duly corrected:

... but no servo tab's
 

Tiger Tim

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Even for a handmade homebuilt aircraft, making three of the same part is easier and faster than making three unique parts... Standardization like makes a project seem and be more achievable for an amateur builder, at least, that's my thinking.
I wonder though if you’re just moving the complexity to some other place. Forcing a compromise where you may not need to. My line of thinking is that perhaps you can have a lot of duplication in internal parts but put together in slightly different ways. For example you could change the rib spacing for the vertical vs horizontal tail to get the ideal aspect ratios and/or areas for each. Likewise I imagine each will have slightly different needs for spars so you design to be appropriate for both. You still have one jig or forming block or whatever but they make parts that can be used more appropriately.

Of course, if your plan is mass(-ish) production of major assemblies at some central location then yes, I can see how having one mould would save shop space and stock inventory.
 

Dan Thomas

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Years ago I flew a prototype ultra lite wit an all flying elevator.
The designer missed the design a bit.
The control stick forces were so light as to be non existent.
The pivot point was too close to the center and no travel stops
Absolutely could not fly hands off.
SCARY
Dennis
There's the problem. The location of the pivot point is crucial; too far forward and conrol forces get too high. Too far aft and you get the problem you describe, and it can be fatal. Remember that the center of lift moves forward as AoA increases so that you could get a condition where the surface "snatches" and goes full-deflection on its own.

With large control deflections you can stall the surface. The first versions of the Cessna Cardinal had an issue with the stabilator stalling in the flare, dropping the nose hard and sometimes breaking the nosegear. Cessna had to put a slot in the stab near the leading edge to prevent that.

Anti-servo tabs have several purposes. They increase control forces as deflection increases; that allows minor adjustments with small control forces but prevents overstressing the airframe with too-easy forces by providing the necessary feedback. With a mechanism to adjust the tab's actuator rod anchor point, they become trim tabs.

All I know is that I'd have trouble coming up with something safe the first time around. I've seen enough of these things to know that it's easier just to build a conventional tail. The hinge points on a stabilator are so close together that they are rather heavily loaded by aerodynamic forces that try to twist the stab. Think of the spiralling slipstream off the prop: it will try to lift the left end of the stab and push down on the right end. In turbulence it will wiggle a lot. These forces are not small. In a spin in a Cessna 150, if you look at the conventional stab through the back window, you can see it flexing mightly scarily. A designer has to make sure his engineering is going to stand up to such stuff.
 
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cheapracer

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The hinge points on a stabilator are so close together that they are rather heavily loaded by aerodynamic forces that try to twist the stab.
This.

Even though the Morgan doesn't seem to have this issue, it was still enough to put me off a stabilator when I was considering replicating their setup.

Funnily enough, now I have a very wide tail now that would suit a stable stabilator, but the conventional tail is easy enough to build, but more importantly, more acceptable for consumers.
 

cluttonfred

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As I've said before, there are benefits to simplicity and ease of building, and there are also benefits to perceived ease of building, though ideally you want both. My first small boat build was a Bolger Brick, which looks like mattress box with a sail, but it encouraged me to actually start building. Later boats had more curves, but it was the Brick that got me started.

That's the enduring appeal of the Volksplane concept, that is seems more doable than other homebuilt designs, so it encourages folks to actually jump in and try. Bud Evans was an accomplished aeronautical engineer and as I have thought about ways to tweak the VP designs I have often discovered that there was a lot of thought that went into things that seem very simple.

If there are some reliable ways to design safe, reliable all-moving tail surfaces without anti-servo tabs, I'd love to learn more.
 
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lr27

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snip
Remember that the center of lift moves forward as AoA increases so that you could get a condition where the surface "snatches" and goes full-deflection on its own.

With large control deflections you can stall the surface. The first versions of the Cessna Cardinal had an issue with the stabilator stalling in the flare, dropping the nose hard and sometimes breaking the nosegear. Cessna had to put a slot in the stab near the leading edge to prevent that.
snip
Someone once wrote that the center of lift was invented by the manufacturers of oxcarts to prevent the manufacture of airplanes. Consider a Clark Y airfoil at 0 lift. It's still trying to pitch down. The center of lift would have to be at Alpha Centauri. There would probably be relativistic effects when you tried to change the angle of attack and center of lift had to move by light years in a fraction of a second. ;-)

In any case, if the tail surfaces are symmetrical, the pitching moment will be very small. Post stall is another issue. According to charts in Theory of Wing Sections, the pitching moment will then be negative, i.e. nose down if the lift is up*. I don't think that's "snatching". Quite the opposite.

*The tests were at Reynolds numbers of 3 million and above. I haven't seen, or don't remember, tests of post stall pitching moment at lower Reynolds numbers.

Moral of story:
Don't use cambered foils or hinged control surfaces if you aren't a relativistic physicist. ;-)

P.S. Any tail surface can stall, depending on the design and the situation.
 

Riggerrob

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Yes, Zenith 700 has a cambered horizontal tail, but that is driven by its specialized flight enevelope. Zenith is best in the low and slow corner of the STOL envelope and all other aspects of performance suffer. Zenith 700 has a higher parts count (e.g. slate) than a “cruising” airplane. Since those slats create extra drag, it also cruises slower than a general purpose airplane.
 

Riggerrob

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The easiest way to avoid tail surfaces stalling is to make them big (light wing-loadings) and mount them on long tail moment arms. Long tail moment arms also aid stability while dampening pilot-induced oscillations.​

A classic example is the Pilates PC-3 military trainer. Aerodynamic engineers wanted a long aft fuselage for stability, but structural engineers wanted to shorten the aft fuselage by 2 feet (60 cm) to reduce weight. Structural engineers won that debate.
Soon Swiss Air Force instructor pilots complained about poor spinning characteristics. After an exhaustive series of spin tests, Pilatus bolted on a variety of dorsal fins, ventral fins, etc. All subsequent Pilatus airplanes have all those complex add-on fins. PC-9 even has leading edge extensions (LERX or horizontal fins) on its horizontal stabilizer!

The simpler solution is a longer aft fuselage.
Even if a small workshop requires bolting on the aft fuselage .... you end up with a double-strong seat-back bulkhead ... where you need a roll-bar anyways.
 
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Riggerrob

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Looking at the counter-weight inside the Morgan Aeroworks .... has anyone bolted control cables to the counter-weight arm? ... to make it also serve as a bell-crank?
 

BJC

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Looking at the counter-weight inside the Morgan Aeroworks .... has anyone bolted control cables to the counter-weight arm? ... to make it also serve as a bell-crank?
The Glasair II has weights at the tips of the elevator as well as inside the vertical stabilizer. It uses springs for trim, and a push-pull tube for elevator actuation. The trim spring cables attach to the “J” shaped counterweight arm as does the push-pull tube.


BJC
 
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cheapracer

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Looking at the counter-weight inside the Morgan Aeroworks .... has anyone bolted control cables to the counter-weight arm? ... to make it also serve as a bell-crank?
It's actually a better idea as you're eliminating flex/control delay, and helping as an inertia damper.
 

cheapracer

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Yes, Zenith 700 has a cambered horizontal tail, but that is driven by its specialized flight enevelope.
One Member posted here about all his extensive mods to a Zenith 7** and felt strongly that his changing to an un-cambered symmetrical HS foil was one of the best mods of them all (if I remember correctly).
 
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