I have been rolling the following idea in my head after seeing a couple of pylon racer designs.

I was thinking of using a convetional T-tail design with a smaller than normal h-stab. at the top of the fin. Then placing an all-moving elevator mid way (more or less) up the fin. The sum of the two areas would be that of a normal h-stab. and elevator.

Would there be any advantages/disadvantages in doing this?

-Q.

Here is a picture of what I'm proposing:

Opps. Can't seem to upload it. Seems there's a problem with RCU. Try looking at my gallery by following this link http://www.rcuniverse.com/gallery/sh...php?photo=9357

You might try flying something before you "improve" it.

Are you suggesting that I should fly a pylon racer before I "improve" apon it's design? My intention is not to improve apon a pylon racer design, but to investigate all things aerodynamic so that I may better understand how things work and interact.

I would like to hear from people who might have actually built and/or flown such a configuration and what they found as a result of flying it. I also want to hear from people who might have some aerodynamic background and can highlight any problems and/or possible plus points on the configuration.

-Q.

Q,
Here's what I learned when trying to improve on "one design" club pylon racers.
Frequently the intersection of the fuse and stab (stab and stab in your case) creates more drag than the surface it self. With the double "T" stab you would be roughly doubling the drag vs. a single T-tail. In addition you would see drag from the pressures between the two stabs and a loss of efficiency from the lower aspect ratios.
The benefit of a T tail (besides keeping the horizontal out of the weeds) is that it only has 2 plane intersections vs. 4 on a conventional tail. The greater the angle (>90 deg.) between the surfaces the less drag those intersections induce. This is also why 110-120 deg. V tails are so popular.

Chris

a088008,

My guess would be:

1) somewhat more drag than a standard horizontal stabilizer
2) somewhat less effectiveness for a given total area

banktoturn

Think about this setup in a turn, requiring some amount of up elevator. Now, the airplane is at an angle of attack, commanded by the moving tailplane. Meanwhile, the non moving tailplane is at an angle of attack, lifting against the moving tailplane to try to restore the plane to straight & level. So, it is creating induced drag, and reducing the effectiveness of the moving tailplane.

Big whoa.

Drag has various forms, one of which is induced (or vortex) drag. Vorticies predominantly form at the tips of flying surfaces such as wings and tails so having two horizontal tail surfaces ( and therefore 4 tips) would effectively double the induced drag produced by the tail i guess.

But would this be noticeable? I doubt it!

I can't see much in the way of an advantage from this setup, and all flying surfaces are prone to flutter if not done properly but having said that it would look a bit different wouldn't it?!

- Martin

I was hoping someone would confirm the stability aspect, where the fixed stab would fight the movable stab.

I was also wondering about the elevator effectiveness. I know a all-moving stab is more effective. I was trying to keep the stab surface clean and flying while the all-moving stab would be allowed to stall. What do you think of this characteristic. Would it work this way?

-Q.

It certainly would look different. I'm a BIG fan of different.

I did not realize this until you mentioned flutter, but there is the redundancy aspect. If the elevator (all-moving stab) were to flutter and fall off, or just plain break off, the plane would still be controllable since the fixed stab would still be there. (This is just an observation as I'm not one to include mechanical or structural failure in my designs)

-Q.

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Controllable?
By what?
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There's nothing "wrong" with "different", but much of what you propose has been looked at and rejected as too complex, unworkable, and different just to be different without considering any actual utility.
For example:
At the top of the heap in the field of competition those sitting there are possessed of extremely find and quick minds. They look for ALL advantages over their competition, and use it.
Wierd stuff doesn't appear at the top of the heap because mostly it just plain doesn't work.. in competition.
If a specialized situation arises where a unique shape can be exploited, it usually is discovered quickly and used.
Discovering what won't or can't work is valuable though, in that it saves time for those interested in progress.

Along with the problem of the control forces of the movable stab being fought by the fixed stab there's the problem of the sizes of the stabs. In our model sizes there's already a bit of a problem with the tails being so much smaller than the wing. To compensate for this it's commonly realized that the stabilizer aspect ratio has to be considerably less than the wing aspect ratio when gliders are being designed. Now you're suggesting making the tail surfaces even smaller and splitting them up. The resulting Reynolds numbers for these surfaces will be super low resulting in even worse "scale" effects than we have with one surface.

I like different too but in this case I just think it's a bad idea to use multiple tail surfaces in this manner.

Full moving tails are more efficient? Yes and no. It depends on how much travel you want. For gliders where the tail is more of a trimming surface full flying is the way to go as it eliminates the hinge line (but be sure to seal the stab to fin joint with something like felt or the leakage drag will eliminate any advantages). But for a 3D aerobatic model the fixed stab with movable elevator forms a variable camber airfoil that is less prone to stalling until higher deflection angles are involved and, if done right, are capable of higher lift coeficients thanks to that camber.

Regarding the full flying stab-
I've been looking into this for greater authority in 3D manuvers. The mechanics are pretty simple:
1. Put the pivot point ahead of the MAC so that ther is always a positive load on you servo. To avoid "snatch" as the stab rotates.
2. Use a carbon or aluminum tube as your spar, nylon, delrin or another tube as the bushing surface.
3. Control the surface via a pull-pull wheel or elongated horn mounted inboard of the surface between it and the fuse.
In use a full flying stab is very tricky, you either use it when the stab size makes it less effective (Fokker Dr-1 rudder) or you use very small throws. My attempts at a full flying stab with +/- 45 deg. of throw resulted in a plane that was prone to pilot induced occilation. Some slope soarers and the Diabolitin have aerobatic flying stabs but I thing that the long tail moment allows them to be more controllable.
Basically, unless you are copying an exhisting design you'll just have to try it out.
The fixed stab would not be controllable if you lost the flying stab. You would still not have pitch control. With flaperons you might be able to coax the plane down.

Chris

In my years as a prof. aero. eng. we had the opportunity to try a lot of things. In the 100 years since the Wright brothers a lot of things have been thought of and tried. Honestly there is not all that much that is new. Most modern aero work is just refining designs as different structural materials, such as those that allow things like the extreme aspect ratios of the gliders, become available.

I have heard it said that aero types working on transports (targets) would sell their moms for a one drag count reduction. Aero types working on aerobatic airplanes (fighters) would do the same to get a few degrees of angle of attack before buffet onset occurrs. There have been a lot of man hours of work to try to get any edge in airplane design.

Saying all that I have to believe that the configurations that we see presently in use are in all honestly pretty well optimized. What you don't see in a flying design are the hundreds of pertubations that were tried in the wind tunnels. For instance your horizontals. If someone had even a hint of performance (drag or maneuvering) benefit it would have been bolted on during the wind tunnel testing.

This is not to say that there is not anything new left to be discovered but it is almost the case. We prof. aero. eng. types like to think that we are free enough thinkers to have worked out optimum configurations for most stuff.

My boss in Commercial Engineering Flight Test (Sam Wyrick) said that very thing, but offered his grandmother...
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Along these lines, there's just so many ways to get from the leading edge to the trailing edge and have a useable shape. I think the various on-line libraries contain a summary of ALL possible worthwhile shapes. It's up to the designer to find the one he needs,: it's been done already!