curtrc

I am plan-building a 24% Pitts that has a very scale tail. I want to be able to 3D this model and while the horizontal stabilzer may do in a pinch, there does not seem to be nearly enough rudder authority, according to a friend who has built from this plan.

So I would like to redesign it. I've been reading everything I can find from full-scale design equations to rules-of-thumb and articles written by modellers and there are a lot of factors that go into tail design, I know, so Iam trying to narrow down my questions to topics that actual designers (as opposed to plan-hacking engineers with delusions of granduer, like myself) can shed light on.

First off, is there any magic formula for the movable as opposed to static portion of the elevator?or is larger always better? I ask this because it seems to me that the extreme is stabilators (fully flying elevator) would make the most sense, if they could be engineered into the tail of a model aircraft, which doesn't seem like it would be THAT much of a challenge.. run a keyed mini wing-tube through the two elevator halves, place a well-supported rotating assembly in the tail and viola. But no one does.. there must be a reason, what is it? If I were to design/machine a suitable center asembly, should I?

To that end, Is this design:

Limited in its A:A ' ratio only by what the wood+hinges will support? or is there some other reason for the fully-articulated outer edge..? Is there some suitable ratio for A:B or is it just "as much as the structure can support"?

It seems to me that the more moveable area the better, since it would mean less throw for a given moment, and the smaller the tail (and thus p-factor affected area and drag-producing structure)could be.

Any insight would be appreciated.

pimmnz

Well, I'm up for it, though this is more a design/construction question, rather than aerodynamics. Of course you can redesign the tailplane/elevator, if you have a need. All moving tailplanes are very common in modelling, not so much in powered models, but many gliders use the idea, keep around 25% of the total area in front of the hinge, and make sure that the hinge is strong enough for all your flight loads. What they will be I cannot guess. The tail you have drawn is fine, the dangly bit at the elevator tip is an aerodynamic balance, to assist the pilot in moving the control surface, by reducing the load needed, and should be no more than 10% of the elevator area. it might even help your elevator servo by doing the same thing. You can enlarge the whole thing by about the same amount over the existing, use the bracing wires to really brace the fixed portion, and hinge accordingly. Make sure that the tailplane and elevator spars actually work for a living, a good 'C' section, locally thickened to accept the hinges using a hard balsa web and spruce caps will be fine. Try to keep the weight of the rest of the structure down so the new tail weighs the same as the original. You can add a little balance weight on the elevator projection to help balance the surface about the hinge line, this will reduce the tendency to flutter. Diagonal ribs in each part will help the surface siffness. The rest is up to you...
Evan, WB #12.

rmh

Your real problem with a 1/4 scale Pitts?
weight
spend your time reducing weight from the model
the basic force setups is fine but in small sizes weight easily gets out of hand and no amount of changing the tail group will fix things
the aerodaynamic balances are fine in about the ratio you drew them.

BMatthews

I have to agree that "lighter is righter" as a starting point. LIghter extremites are easier to accelerate and also tend to stop moving more easily as well so you get less overshoot.

As for the A:A' thing you can ever do that. The aerodynamic balance horns are not there to gain leverage for the controls. They are there to counteract some of the air loads on the servo or pilot.

Within reason the further forward the hinge line is so that the movable portion is bigger compared to the fixed portion the more the control response will be from a given surface displacement angle. But at somewhere around 30:70 to 40:60 ratio between fixed to movable you hit the point of reducing returns.

If you were to go with an all moving stabiliator then you want to consider using a rather thick and stall resistant airfoil section. While a flat plate at a 15 to 20 degrees angle of attack still generates lift an airfoil which isn't stalled will generate MORE lift at that same angle.

A fixed stab and elevator combo can be flat and still work well at high angles of attack. This is because the two parts form a variable camber airfoil.

Put all this together and you end up with some decisions to make about how you wish to go.

curtrc

Good info thank you very much! I have been racking my brains on how to create a practical all-flying elevator, and for 1/4-scale 3D loads it just doesn't seem practical..

As I see it the key is the center joint. It has to be able to rotate very slop-free, and captured in 2 other axis, while also being securely affixed to the airfoil. I have been considering a 3/8" hollow aluminum tube with a slug of balsa and slots cut into it to provide a surface for the tail spars to be epoxied to, then put a teflon sleeve in the center for rotation. Problem is I would have to machine the teflon to be mechanically fastened to the tail structure (unless someone can sugges a glue that sticks to teflon and that means more mass exactly where I don't want it, at the extreme aft of the airframe.

So I've temporarily abandoned that plan and am currently planning on a simple flat tail with perhaps some small rib buildouts to form a NACA0010 or NACA0008 -ish airfoil section. Then again maybe not. I will try a few different designs and see if any crack under strain, and of course if I can keep the weight acceptable.

I'm sure I will end up with a conventional barn-door flat tail but I want to be able to at least say I tried

BMatthews

Frankly I'd say that a Pitts of any sort would look a bit odd with a full flying stabilator anyway.

If you did want to persue it you'd want a very solid carbon tube as a carry through spar. It would have to be in a size which fits into some sort of thin line ball bearing as well since as you suggest you want to achieve a low rotational resistance. And in this case a thin line style lightweight ball bearing on each side of the fuselage would do the trick. So yeah, some engineering required to do it right.

superlouis555

Ateflonconnection I see would be good at reducing friction, butit may not be importantas it would be better to go up one size in servo strength, and concentrate entirely on amore structural connection. For an entirely moving stabilizer I would run a brass tube across the vertical stabilizer and fasten it at each sideof the vertical stabilizer with plates like plywoodwhich glues nicely, or metal,like aluminum which could be fastened. The wider the vetical stabilizer is, the more robust the connection will be, so try to make the structural reinforcements at the greatest width aswill be allowed by the vertical stabilizer. If using a carbon tubeglue a brasstube atthe center so it canrotate without wearing the carbon.

rctech2k7

My Feedback: (1)

If you look on aircraft like F-15, 16 & 18 they have a full stab system. In your aircraft this would be simple in comparison other than accessibility because of the exhaust tube present in between that horizontal halves. However there's an structure fitted to support the load on this system as it works like an ailerons as well as elevator. BTW, Do you mean mechanical equation that determines load and stresses for the strength of materials? Or for tail volume ratio, sizing and controlling stability?