Got a Bertella(Italian plans) designed SU27 Flanker which has a 9' long fuse. The plans call for horizontal stab and elevator. The full size has all moving stab-actually they're ailevators. I'd prefer scale control surfaces, but wonder as a matter of aerodynamics what actual differences in control there would be at model speeds. I don't know how proven these plans are, but seems balance would have to be nearly perfect to move a 9' fuse around with a 3" elevator. Thoughts, comments? Bill S.

We did a lot of testing in low speed wind tunnels with airplane models smaller than the one you are talking about! Typically for full scale airplanes that use the all flying tail in pitch and roll and they are sized for maneuverability at supersonic speeds where the aero effectiveness is quite low. This usually results in a lot of control authority available at low/model speeds.

For your question ignore the length of the fuse and just look at the elevator chord as a percent of the tail chord. If it is normal (less than 40% and greater than 20% or so) then it would probably be OK for pitch. Might not be too good for roll though. Certainly wouldn't look right :-)

All that being said for a scale model it would seem that the all moving stab is doable and desireable from a scale standpoint. Certainly the numerous F-4, F-100, etc flown in scale work today would indicate that the mechanics of the tail pivot are well worked out.

Thanks for the comments, Ben. If the all moving tail is desirable, again, with respect, of course, to the model, are the ailevators a benefit with respect to the roll function? The questions may appear academic, but the 27 is described as a 'fly by wire' A/C and we're trying to control it with thumbs, albeit, perhaps with a gyro or two.

If you go back to the movie Top Gun and look at some of the close ups of the aircraft you'll see the ailervators in action. On one shot in particular there is a LOT of aileron action in the elevators.

I suspect that as long as you're willing to use the dual rates often to affect the ailevator mix as you move from low to high speeds that the ailevator option may work.

If you're putting a lot of effort into that model it may be worth making a 25 or so sized "hack" test model first.

A lot of that fly by wire stuff refers to the full sized need to move LOTS of surfaces in coordination. That and a fairly rearward center of gravity to enhance the maneuverability.

Hi wsmalley. I'm sure you're aware of this but it caused me some grief when i was experimenting with all moving tails so...

Beware the dreaded flutter! Its important to correctly balance the tail plane so that any potential run away oscillation is damped. On a jet style model its even more important due to the high speeds involved. The tail itself must be nose heavy (i.e. must rotate to the 'up' elevator position when disconnected from any push rods.

From a scale point of view it would be nice to have all moving surfaces, but from a model point of view they are not necessary. Full size jets have them so they can manouver in the transonic region. When they approach mach 1.0, shockwaves form around the aircraft, and on a normal tail plane/elevator arrangement, a shockwave forming at the leading edge of the elevator will render it useless (due to flow seperation i think). Anyway please ignore me if you know all this, or if i'm plane wrong!

- Martin

Hi wsmalley,

I have seen various jets fly with all all moving stabs. One that springs to mind is a scale F15 which flew here in Australia at Shepparton one year. The fuse was around 6 ft long with a 10 - 12 thrust turbine. The only aerodynamic control surfaces it had was the horizontal stab (elevon/ailevon/rolleron) for pitch and roll. It flew perfectly.

I have also flown full scale gliders with all moving horizontal stabs. They allow for a far greater trim range, increased responsiveness but a greater tendency for the unitiated to induce Pilot Induced Oscillations (PIO). This would only be magnified in a scale model, but then again, see comments about the F15.

Thanks guys for the input. This is what makes the hobby interesting! And, no, I didn't know the tail plane needed to be nose heavy! For my part, a lot of the fun is in trying to replicate our particular 'favorite' aircraft into a miniature version. I use autopilots, and try any other gimick, to keep my planes 'reusable'. But while we're on the subject, what about L.E. flaps on a model jet, any thoughts on that one? I also fly some full scale occasionaly and use my simulator just to keep in practice. I've flown a Schweizer sailplane , Cessna 150 (with stabilator), Piper Cherokees and Flightstar U/L- all of which I think are easier to fly than any model I own-maybe 10 or 12. I have a couple of sims of the SU27 and took probably 35+ landing attempts to learn to fly it. But, again, the question I guess is to what degree control surfaces on full scale a/c can be usefully replicated with smaller versions at slower speeds

Bill....The model that comes to mind is the F-16....all the jets I've seen have a "flying Stab".......a friend of mine has 2 of them one being a Byron and the other from Germany....the problems he encountered were getting the linkages "tight" as possible....using bearings in the fuse for the stab mounts...as well as heavy-duty drive system from the servo....keeping the servo as close to the stab drive arm as possible......had some slight flutter problems initially but got those resolved by making drive system backlash free.......Bill.....

Good point! I noticed in a thread in 'Jets' regarding a big SU27 a fellow in Germany was building wherein he had used bearings. Makes me wonder if one could mount the servo sideways and actually attach a stab 'shaft' directly to the servo splined output-running thru a bearing 'mounted in the fuse-no linkage, no slop! Hmmmmm!

The airloads will be too large to put into the servo body. Remember that the servo body has to respond to torque alone. The tail inputs a really really large bending moment into the output shaft. It would break pretty easily. So you need linkages to handle the up and down loads on the tail.

Slats, flaps, etc will all be fine and aerodynamically sound if built correctly.

You asked, "But, again, the question I guess is to what degree control surfaces on full scale a/c can be usefully replicated with smaller versions at slower speeds?"

Exactly - unless you are building a yard flier. For example, in full scale F-15 design work we used a 5 to 6% scale model for anything Mach .5 or above. For low speed work, landing, etc. we used up to a 12 to13% scale. As I set here the exact numbers escape me (getting old) but are certainly representative. The particular sized used were dependent on the size of the wind tunnels that were available to test in, not the fear of not getting representative, good quality data.

The thing is that we were absolutely certain of getting aerodynamic answers that were right on and that were sufficient to convince a stingy (rightfully so) government to give us money.

With that in mind it is very reasonable to go to much lower sizes of models and still get something close to full scale accuracy.

Bob Violetts F-100 and the others mentioned above are good examples of airplane size and flying qualities.

You should feel totally condifent using the scale control surfaces at the sizes you are building. Maybe not if you are building a slow flyer for the backyard.

Good info I personally had never read about. Of course, I've seen articles on L.E. slats and flaps, but they are usually a stand alone. I thought, for example, of coupling a L.E. flap to the flaps with a bell crank arrangement which would engage the L.E. flap at the same the flap. If one had,e.g., 3 position flaps, you would have, I think, 3 position L.E. flaps- don't know if that's good or bad. The sim for the SU appears to also have a 'coupled' flap/l.e. flap configuration, so I'll assume the full size operates the same. I think, Ben, I'm missing the point on the torque/linkage issue. Is the force exerted on the servo arm-therefore, the output shaft-any less if transferred thru a linkage? If the torque produced by the servo is less than the force exerted on the control surface/linkage, then the linkage,servo, or something must give-right? The secondary question then becomes: how does one calculate the force on the control surface- or, in this case, on the ailevator?

Right, the torque at the stabilator (that is what we called them at work rather than ailevator), it is pretty much an industry standard) and the servo is the same. With the pivot at roughly the 25% mac of the tail the torque moment with deflection isn't all that much.

But the bending moment normal (force at right angles to the axis) to the axis of the tube attaching the stabilator to the fuselage or to the servo directly is really high. Remember that there is a tail load up or down necessary to do the pitch maneuvers you want. Just imagine what the down loads on the tail are when deflected full leading edge down. Pretty high. That will occur and yet the moment about the axis can be small if properly located. It would pop the servo arm very quickly.

The normal force is roughly the same as a wing at an angle of attack equal to the tail deflection. This discounts the downwash but it is close. Anyhow, you would multiply the calculated force by 8 or 10 to make sure the fudge factor is enough to be sure that it doesn't break when you whack it against the door frame of the shop (at least I do).

This discussion is really beyond me, but from a practical point of view, I would set up the ailervator mix on a switch so you can turn it off and fly the plane normally in case it doesn't work right. I've known more than one person who experimented with things like this without a way to turn it off and the result was nearly disaster.

By the way, I was thinking about a flying V-tail for my design, Great Gonzo, but I have no idea how to create the pivots. Anyone have a link that I can look at?

Certainly can't disagree with that, I'll ponder that to see if it's possible electronically-within the radio (Fut 8U). I planned to start with a 'Y' connector to run 2 servos on each side, one aileron and one on the 'ailevator'. I guess at a minimum, setting minimum throw with dual rates is possible though my plan at this point is to put a gyro on the aileron function. With the scale wing though the ailerons are pretty small

wsmalley - I just built a computer radio rig for a buddy of mine. You can set up any mix in the Futaba 8 channel to a switch or always on (NULL). I'm guessing most of the Futaba computer radios are like that.

Check this out.

http://www.airfieldmodels.com/computer_radio_rig.htm

Though i have a conventional all moving tailplane (and fin!) on my funfly design you may find this useful...

I simply used two carbon fibre tubes for an axle/bearing arrangement (obviously you need to choose the sizes carefully so that one fits snuggly into the other). The inner tube (6mm) also doubled up as a spar for the tailplane, though you couldn't do this on a V-tail. However you could mount two bearings (8mm on mine) at angles in the fuz, and simply glue on washers so that the axles couldn't pull out (as is done on my fin (only the top half moves) - check out my gallery for pics of the tail arrangement). This may prove tricky, heavy and fairly pointless however! Also be sure to mass balance the tail to kill any flutter, and aerodynamically balance it to save the servo from excessive load. All moving surfaces may seriously damage you're will to live! During experimentation i had several tail failures (including one where the whole thing kinda blew up!) and i reckon i hold some sort of British record for the number of successful tail-less landings - i've never lost one yet!!! I used an all moving tail in order to get silly amounts of pitch authority as the British nationals fun fly events include alot of quick looping. I certainly got it and its VERY twitchy on elevtor so i'm wondering if there is any point (save trim purposes) to you developing this?? Anyway, got to go to lectures.... - Martin

well, in canada we have a ASW/ marine patrol aircraft called the aurora, I think it is very similar to a p-3 orion, but better. Anyway, the ailerons on it aren't directly controlled, I think the elevator and maybe rudder is same. On initial t/o roll, they all flap, completely uncontrolled. As airspeed picks up, they "castor"(for lack of a better term) into position. These control surfaces are controlled solely by trim tabs. If this was done on a model, it would solve the servo/backlash problems. Also, if a full flying surface was properly balanced, i think this servo tab system would work