All Flying Tail?

I just received my July/August "Stunt News" (great, as usual) and skimmed through the article on the "All-Flying Tail Stunter". In Figure "7" the top of the flap is shown connected to the top of the stab. Is this correct? I would think that the top of the flap should be connected to the bottom of the stab (just like the standard common flap to elevator connection) to provide a restoring force to drive the stab back to neutral. Am I missing something here?

I'd also like to hear more about the "pros" and "cons" of the AFT (Igor?) Total control system hinge moments, system stability, pitch oscillation damping, mass balance as well as aerodynamic balance etc., etc...

Thanks

Look at the following thread: http://clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=3376&forum=DCForumID1 It covered your question quite well.

It was here already several times. I think the conclusion was every time "do not do it" if I remember.

Beside mechanic troubles of pivot, there is also fact, that compared to deflected elevator on fixed stab which makes undercambered airfoil, all flying tail is flat surface and thus its lift is limited what CAN in some cases limt flying in corners. Also movable leading edge is not very healty solution, may be as a "T" configuration, but certainly not close to wing line.

>It was here already several times. I think the conclusion
>was every time "do not do it" if I remember.
>
>Beside mechanic troubles of pivot, there is also fact, that
>compared to deflected elevator on fixed stab which makes
>undercambered airfoil, all flying tail is flat surface and
>thus its lift is limited what CAN in some cases limt flying
>in corners. Also movable leading edge is not very healty
>solution, may be as a "T" configuration, but certainly not
>close to wing line.
>
>igor

Sorry, Igor. I do not agree. See my previous thread. I experienced none of the problems you allege.

Tom, I never wrote it CANNOT work. I even wrote how to design it well in some old thread few years back on RConline.

My opinion comes from knowledge about two unsuccesfull tries which I saw (and which I never tried ).

It asks for troubles and in my eyes, does not bring any clear advantage. That is all.

>Tom, I never wrote it CANNOT work. I even wrote how to
>design it well in some old thread few years back on
>RConline.
>
>My opinion comes from knowledge about two unsuccesfull tries
>which I saw (and which I never tried ).
>
>It asks for troubles and in my eyes, does not bring any
>clear advantage. That is all.
>
>igor

Sorry, still do not agree with you. Read the thread and follow the lessons learned. Nothing is foolproof, but the chance of failure will be reduced dramatically.

Sorry, it is too long, I read it once, what exactly you mean?

Hmphh. Apparently neither of you have read the Stunt News article
that prompted this thread. It's ok, it just arrived yesterday
at my place. Doesn't have much to do with earlier discussions.

It's very interesting. The author calls it an All-Flying Tail,
but it's not the same as, say, a combat stabilitor.
He has an extra tab, very similar to Howard & Paul's flap,
but driven in the opposite direction. The result
is an articulated surface which ought to generate plenty of lift.

>Hmphh. Apparently neither of you have read the Stunt News
>article
>that prompted this thread. It's ok, it just arrived
>yesterday
>at my place. Doesn't have much to do with earlier
>discussions.
>
>It's very interesting. The author calls it an All-Flying
>Tail,
>but it's not the same as, say, a combat stabilitor.
>He has an extra tab, very similar to Howard & Paul's flap,
>but driven in the opposite direction. The result
>is an articulated surface which ought to generate plenty of
>lift.
>
>Preston

read on... he also talks about using the flaps as a restoring moment rather than the anti servo tab... and implies (at least to me) there is more to it than just pivot point location w.r.t. stab MAC...

There is lots of good information in this and other threads from the past. It's just a little hard to dig out at times. I wish he had presented a little more analytical information. It's like my ol' calculus prof used to say, "...the proof is left to the student..."

>I wish he had presented a little more analytical information. It's like my ol' calculus prof used
>to say, "...the proof is left to the student..."

Glen Allison had his current prototype at the circles on the eve of VSC this year. Although I arrived just after what he termed a successful test, he kindly took a few minutes to explain his system to me. An aeronautical engineer, he had computed the geometrical movements and dynamics of each version of his system, before building. On-field adjustability is limited. I can only surmise that he opted to leave out the math due to space limitations and perhaps a reluctance to incur unpleasant responses from a vocal minority of the mathematically illiterate - probably our loss.

>read on... he also talks about using the flaps as a restoring moment rather than the anti
> servo tab... and implies (at least to me) there is more to it than just pivot point location
>w.r.t. stab MAC...

As in his article, he pointed out that the anti-servo tab enables the degree of stability to be varied through variances in tab area and deflection angle (with pivot location, rod and horn arm length, etc.). Significantly, it does allow the AFT pivot point to be moved out of the normally restricted range (I think he mentioned trying as far back as 50%-chord), while still retaining adequate restorative moment and thus control feedback. But to me this also implies potential weight-saving trade offs in tail arm lengths - with regard to tracking stability, that is (1st. paragraph, p.36). The flap connection surely does open some further possibilities regarding control forces.

I don't remember when this was last discussed, but a stabilator (AFT), needing a smaller deflection than an elevator to achieve the same effective a.o.a. (angle of attack), should be able to turn a stunter or combat plane quicker with less tail area*. It can operate effectively on a longer tail arm, because it can deflect far enough in tight turns to achieve effective a.o.a.'s in places where circular-airflow effects (which grow with distance from center of rotation) increase the normal stabilizer's a.o.a. past the point where elevator deflection ('upward', for inside maneuvers) can further override it. All that's left is to overcome fuselage inertia/drag and produce adequate wing lift for the desired radius of turn, a function of wing area, mass (thus wing loading), and lift coefficient. I think that Glen is correct about reduced control drag in turns. He probably computed it too.

SK

* P.S. In response to Igor's opinion above about limited lift, I wonder whether reversing the modern hinge-line trend and perhaps hinging elevators very near the horizontal-tail leading edge could achieve usefully cambered (almost all-flying) tails with some of the advantages of AFT's - larger control forces though. 'just a thought. Still, Glen's anti-servo tab does give the stabilator (AFT) a flapped camber, just like the normal stab/elevator. I'd better stop musing and get to work...

Serge,

The area and deflection of flying tail is not exchangeable. It means, if your tail cannot make enough lift to pitch to corner, you cannot solve it by more AoA.

Here is example:

We have a model, which needs 30 deg deflection on regular stab/tail configuration (stab:elevator = 1:1) and which needs lift say cl=0.8 to keep proper AoA on wing. It corresponds with AoA of tail –15 deg. It can happen on some older model with rather front CG with rather small tail. But it is real.

Question is, what is happening prior the corner. You deflect the elevator. Its maximal lift is at –5 deg and is say cy=1.2 what makes enough reserve to strongly pitch the wing to at least 5 deg AoA (- on elevator means + on wing) it is angle making enough lift to the corner. So the model starts its circular path which changes incoming air to the tail to those –15 deg where it gets lift cl=0.8 what matches moment from wing and it stops tightening the corner. You see that this standard configuration works WERY well.

Now question is, how to replace it by flat flying tail.

If you use say 9% airfoil, which gets max lift coefficient 0.8 at 12 deg, you will need deflection 15deg (incoming air – known from previous example) PLUS necessary 12deg to make that lift. It makes 27 degrees. So you see that you CAN have equivalent flying tail, but you have no reserve for quick acceleration to the pitch direction. It is because if you deflect the tail to those 27 degrees instantly in straight flight, the real AoA is those 27 degrees what if FAR over stall and you can have lift only cy=0.2 what is FAR under necessary 0.8. More deflection does not help, just because it gives even less lift.

So the only solution is more area. So if we have twice as much area, we need only half of lift coefficient. The lift cl=0.4 is achievable at 5deg AoA so the stab needs only 20 deg AoA to the corner. But 20 deg is still over stall AoA so there is still clear disadvantage to the regular stab-elevator configuration, but useful with back CG position and large tail area.

Question is how to solve it. Longer tail does not help at all. Just because incoming air on long tail has even higher AoA. Short tail will help, but short arm will make problems to overcome flap moment. I would say that relatively short tail on flapless model could be acceptable with flying tail.

There was already thread about hingemoment. I wrote the hingemoment does not grow linearly with angle of deflection (at the same lift production!!!). I think with H.R. cooperation we got 1.5x more hingemoment at 2x higher deflection. The reason is, that you do not change pressure distribution over the chord (too much) if you change the deflection of elevator. You make a necessary lift on whole area of tail, not only on elevator, thus if you have smaller elevator, means smaller area of tail and smaller fraction of whole lift must be supported by pushrod. So if you make more area on stab, less area on elevator, you can have less feedback even you must use more deflection. I think this is better way than flying tail or balancers on elevator. If you do the same on flaps, where almost all feedback comes from, you can very effectively limit control load.

LAST EDITED ON Jul-07-03 AT 10:46 AM (CDT)
 
Igor-

I think that sometimes the -/+ conventions get confusing; at least one such problem crops up in Glen's article - perhaps a misprint. I have reviewed your post and am still pondering the content and ramifications of your paragraph about AFT's:

>If you use say 9% airfoil, which gets max lift coefficient 0.8 at 12 deg, you will need
>deflection 15deg (incoming air – known from previous example) PLUS necessary 12deg to
>make that lift. It makes 27 degrees. So you see that you CAN have equivalent flying tail, but
>you have no reserve for quick acceleration to the pitch direction. It is because if you deflect
>the tail to those 27 degrees instantly in straight flight, the real AoA is those 27 degrees what
>if FAR over stall and you can have lift only cy=0.2 what is FAR under necessary 0.8. More
>deflection does not help, just because it gives even less lift.

I finally (blush) see that you are addressing only the moment of initial rotation into a turn. I gather that you are saying that the AFT cannot instantaneously pitch a plane into a sharp turn, because the AFT would exceed its stall angle in the initial instant of pitching - until the radius was approached where circular airflow would subtract from its initial AoA. I do not know what the initial flow would be, but might it remain attached for an instant, before separating? Very small stabilators on (admittedly short-coupled, low rotational inertia) combat planes surely make them turn quickly. If there is danger of stalling the AFT, I think that Glen Allison's tab may help avoid that. Anyway, as the turn is entered, the maximally deflected stab/elevator loses AoA, while the AFT can maintain its maximum AoA with proper pilot input - progressively increasing incidence angle without ever exceeding stall angle. I don't know how well this can be done in a split second.

After the turn is established, though, it would no longer be true that "the real AoA is those 27 degrees". As you have described it, the actual AoA would be the 12 degrees you chose, while only the angle the AFT makes with the fuselage (incidence angle) would still be the 27 degrees.

I think that we are in reasonable agreement on numbers, but just in case, here is where I get mine (please correct me where you feel that I'm wrong):

First, as you mentioned, the free stream air flow is not the same at the tail (or the nose) as it is at the center of the model's rotation in maneuvers. However, my numbers would be a bit different. Let's assume a model doing a circular inside loop of minimum radius - say 12 feet (3.66 m). Assume further that it is a classically dimensioned model with a tail moment arm not exceeding 20" (.51m). This will give conservative figures.

Disregarding downwash from the wing (just for the moment), the horizontal tail will encounter different air molecules at a different angle of attack. If the fuselage axis is tangent to the circular path of the model (it would actually be at a positive angle, which would enhance the effect I describe), then the horizontal tail will follow a path along a greater radius circle and experience a positive angle of attack (air hits it from below). Looked at it from the plane's perspective, each air molecule is traveling in a circular path relative to the plane, with the center of the circle at the center of the circular loop - what Frank Zaic called "circular airflow" in his historic model aviation publications.

A little geometry shows that the horizontal tail's AoA is then equal to the angle between the loop radii to the model's center of rotation (assume c.g.) and chosen position on the horizontal tail. The above assumed dimensions make this equal to arctan(tail arm/loop radius) = ~8 degrees ('~' meaning 'approximately'). To this you would add the wing incidence. For a longer tail arm, this AoA increases. Naturally this affect is always longitudinally stabilizing. The effect on the nose is similar, in that the flow pushes down on the nose, also against the turn. The prop complicates matters in several respects; so I will not attempt to discuss it.

Now, the downwash from the wing must be subtracted from this to find the actual horizontal tail AoA. I haven't computed it, as it would vary with span, weight, air density, etc. If local downwash were around 4 degrees (guess) and the model is pitched about 6 - 8 degrees, that makes the horizontal tail AoA around 10-12 degrees, which would increase with tail length and tighter loops (a couple degrees more for Berringer). This seems to give us close agreement.

Now however, we might diverge (?). Assuming a 1:1 ratio of stab/elevator chords, it would take up to 24 degrees of elevator deflection to neutralize this AoA (I don't know the precise affect of the camber created by the hinge bend). Modern trends (60/40) increase this demand. That leaves a limited, perhaps barely sufficient amount of deflection left to continue pitching the plane to maintain the same radius. Requirements would increase, however, for tighter loops or longer tails. Regardless of how high a stabilizer/elevator's maximum lift coefficient is (due to AoA and hinge provided camber), there comes a point when it cannot be reached with fixed stabilizer, due to AoA limitations imposed by circular air flow. The maximum elevator deflection thus imposes a limit on loop radius and response for a given plane. The AFT, however, can always be deflected far enough to suit the circular air flow direction and fly at its maximum lift coefficient. Adding the "anti-servo" tab just makes its return moment adjustable to the pilot's preference, while adding the missing camber as a bonus. So while the stab/elevator may enjoy an initial advantage, its AoA diminishes with pitch, while the AFT can maintain constant maximum AoA. So I think that there is an advantage here for tight maneuvering, if not for initial quickness. But your point is well taken.

So that's it from my admittedly limited perspective. Do you disagree.

Serge

P.S. I still think that Glen Allison's flap discussion (p.36 of his article) implies that the flap drives the AFT directly in response to upsets and thus must also do so in control deflections. I think there must be a connecting control rod operating the flap and AFT differentially.

>>>I think that sometimes the -/+ conventions get confusing<<<
Definitely, but I think we know what we speak about

>>>you are saying that the AFT cannot instantaneously pitch a plane into a sharp turn, because the AFT would exceed its stall angle<<<
Yes, ALSO, but not only. I say there can be problem of stability. I mean situation if something is opening the turn, and tail comes to stall. In this case the tail does not make MORE lift necessary to damp disturbation and keep proper pitching. It LOSES the lift because of too high AoA. It is not happening on stab/elevator just because it does not come to such situation thanx its cambered airfoil with lot of reserve of useful AoA and its operating point in linear section of polar.

>>>combat planes surely make them turn quickly<<<
Yes, exactly, combat models have short tail and no flaps. Short tail lovers the difference of AoA on tail between “in loop” and “in line” and missing flaps and aft CG not needs too much AoA at all. So as I wrote in #17: short tails and no flaps are better suited for AFT than long tail on flapped model.

>>>If there is danger of stalling the AFT, I think that Glen Allison's tab may help avoid that. <<<
Yes, but I say fixed stab is even better. The only real advantage of Glen Allison's AFT is limited feedback (from its construction) – it is difficult to have with fixed stab, may be beside balancing tabs at tips, or higeline “inside” the elevator.

>>>Anyway, as the turn is entered, the maximally deflected stab/elevator loses AoA, while the AFT can maintain its maximum AoA<<<
Here I must disagree. If you have flat AFT making lift cl=0.3 and you have chambered airfoil made by stab and elevator making equivalent lift cl=0.3 and it enters the circular path (means lowering its AoA) and if they are in linear numbers (means do not stall, separate or whatever qualitatively different) than they BOTH loses cl=0.11 (theoretically) per 1 deg AoA.

>>>After the turn is established, though, it would no longer be true that "the real AoA is those 27 degrees".<<<
It can be I did not explain it well. Does not matter what tail type/size/angle or whatever you have, one thing is true. You must fulfill two things for wanted corner diameter:
1/ AoA of your wing must match cl exactly necessary to balance the centrifugal force out of the radius.
2/ Lift (negative ) of your elevator must balance the moment opening the radius which is coming from CG and centrifugal force plus pitching moment of flapped wing. Thus you must have SOME PROPER AoA on your tail.

Those 27 AoA is “in turn” and comes from my example, where you have some AoA on wing, plus AoA coming from difference between tail and wing, plus AoA necessary to balance the negative pitching moment of CG and flaps.

You cannot compare 20deg-deflected AFT and 40 deg deflected elevator on 1:1 stab/elevator. It is so just because that if relative incoming airflow at 20 deg does lift cl=0 on AFT, but on stab/elevator you will get large lift cl=1!!! The stab/elevator comes to cl=0 somewhere at 30deg!!!

>>>First, as you mentioned, the free stream air flow is not the same at the tail (or the nose) as it is at the center of the model's rotation in maneuvers. However, my numbers would be a bit different. Let's assume a model doing a circular inside loop of minimum radius - say 12 feet (3.66 m). Assume further that it is a classically dimensioned model with a tail moment arm not exceeding 20" (.51m). This will give conservative figures. <<<
Well I wanted to have some extreme numbers to make it more visible.
But OK to your example…
>>>~8 degrees ('~' meaning 'approximately').<<<
Ok, it is another example, but do you agree you are speaking about LARGER radius, not tighter where you want get with AFT?
>>>If local downwash were around 4 degrees (guess)<<<
It is hard to count with such downwash on such tight radius and long tail, but OK …
>>> and the model is pitched about 6 - 8 degrees, that makes the horizontal tail AoA around 10-12 degrees, which would increase with tail length and tighter loops (a couple degrees more for Berringer).<<<
OK, so you say the AIRFLOW at the tail is at 10-12deg, but you need some lift which depends on CG position and flaps pitching moment. It can be somewhere at cl=0,3 to 0,5. It means you need another 3-4 degrees. It makes final angle 13-16deg. What typically reaches stall on such thin airfoils.
>>>Now however, we might diverge (?). Assuming a 1:1 ratio of stab/elevator chords, it would take up to 24 degrees of elevator deflection to neutralize this AoA<<<
Here I must really disagree. You must go different way. You must find AoA on such chambered airfoil making that lift, and you must add that relative incoming air angle. I modeled 9% airfoil with 50% flap. I tried to find deflection giving those necessary 0,3 to 0,5 lift at –10 to –12 deg AoA. That is what is happening on tail. I am attaching lift polar at deflection 20 deg. You can see that it is very close to what we need. Maybe even little less. So it is 15 versus 20 degrees in my eyes.

>>>Modern trends (60/40) increase this demand.<<<
Yes, exactly, somewhere to those well accepted 30 degrees.

>>>That leaves a limited, perhaps barely sufficient amount of deflection left to continue pitching the plane to maintain the same radius.<<<
Do you still think? Even if you mean model without flaps and CG at 25% MAC (means no lift of tail necessary). The AFT tail needs those your 10 to 12 degrees, but if I go to simulator, I found that the same tail 1:1 stab/elevator needs only 15 degrees deflection. So now it is 11 versus 15.

>>>Requirements would increase, however, for tighter loops or longer tails. Regardless of how high a stabilizer/elevator's maximum lift coefficient is (due to AoA and hinge provided camber), there comes a point when it cannot be reached with fixed stabilizer, due to AoA limitations imposed by circular air flow.<<<
I am not sure what you mean is the really limited diameter. I think the real limitation is wings lift. You cannot go to any AoA on wing, because it will stall. This limitation can prevent such stall. You are right about tail only if you go to such extreme numbers, but you must take care to design of whole model.

BTW, if you go to real extreme and you try 90 deg elevator deflection, you will be surprised, it still makes lot of lift which ends at somewhere –70 deg AoA (equivalent to similar AFT configuration) what I think is enough also for WERY TIGHT turn. And it is also not out of sence because of expected drag. Did you already saw spoilers on F1 racers? How unbelievable amount of deflection the wing has? The drag grows to large numbers only at low lift AoA – it means at those –70 degrees. Or if you SEPARATE the elevator from stab. But sealed flap at 90 degrees deflection but at 0 AoA is still acceptable extreme (hmm … yes, it is not what is happening in our example, but I think 70 degrees at AFT is also not typical )

>>>So I think that there is an advantage here for tight maneuvering, if not for initial quickness.<<<
Well – it can be, question is, if useful. What I say by whole that example and calculation, is, that stab/elevator moves the operating point to linear numbers. If you take a look to that polar, you see that those –10 degrees of upcoming air, moves the operating point of slope to its middle, where the slope is linear, where its one end (maximal lift) is at 0 AoA where you start the corner and the second is at 0 lift where it can end unloaded – means giving all what one designer can dream about, while the flat AFT and especially not properly designed is at its peak. Glenn’s tab solves this problem to some extent, if nothing else for flapless model. But still not what I would like to have, or what well designed stab/elevator gives.

Igor,

I have read your following discussion of all flying tails and believe you are making the problem too difficult. I will quote my earlier response in the thread I listed in the first response to your original question. Here is my experience with all flying tails: "I made two removable horizontal tails for the NF-104, a normal elevator hinged at 50% of the chord and a stabilator of the same size and shape hinged at 18% of the Mean Aerodynamic Chord (MAC) on surface that is 24 inches long, Average Chord 5.25 inches and total area of 126 sq in. The tail area is 22% of the wing area of 583 sq in.
My experience with a stabilator on my take apart NF-104 Starfighter, both wing and horizontal tail, yielded the following:

The first six flights of the Starfighter were with the normal elevator to get the ship trimmed for tip weight, leadout rake as well as elevator to flap ratio. I had to change the original 1 to 1 flap to elevator ratio to approximately 2 to 3 with the elevator having more movement. Once trimmed the Starfighter showed itself to just as competitive as my other ships. Outsides were just as crisp as insides with equal movement.

The seventh flight was with the 18% hinged stabilator and it flew stable and straight from takeoff. But, the control pressures were extremely heavy. It took a very heavy hand to get the ship to maneuver as the stabilator restoration loads were quite large. Outside maneuvers were challenging as the ship would only make very large radius turns. I took the ship home after 3 flights with the 18% stabilator. I spent the next several days and rehinged the stab at 26% MAC and then on the following weekend went out and tested it again.

The NF-104 flew well from takeoff to level flight again with no discernable hunting or abnormal flight characteristics. I tried inside loops and was pleased at the low control loads and quick response of the plane to control commands. I then did inside squares, triangles, outside loops and retrimmed the flight controls for the stabilator. I reasoned that a stabilator of the same dimensions of a conventional elevator tail would only take half of the movement of the elevator. My thesis was confirmed. I then tried inside and outside loops, inverted flight, horizontal eights and square eights and then vertical eights, hour glass, overhead eight and finished up with the cloverleaf. I found some pitch sensitivity in landing after the engine quit. I fixed that later by adding a Prather 1 ounce spinner weight on the engine propeller shaft.

I then flew the Starfighter with the stabilator for over 10 flights and found it to be manageable with no quirks. The only anomaly I have observed is some stabilator flutter during inverted flight. I plan on balancing the stab dynamically to see if this will reduce or eliminate the flutter. But, in the interim I am using the elevator exclusively because it is crisper in maneuvers and hold a point better coming out of the turn.

This has been quite and experience and I am glad I did it. It has been fun and I have for the first time designed my own CL Precision Stunt ship which is quite competitive. It presents itself very well in flight and stands out as a very different aircraft than the rest. It is take apart, has anhedral in the wing, has a very high T-tail and yet stays up with the rest."

The bottom line is an all flying tail at our low speeds of CLPA perform very similar to a conventional horizontal tail and elevator. Total area should be the same, CG the same as well as aerodynamic center. Long tail moments are desirable too. Short tail moments will have the same problems that they have on stunt ships.

a stabilator has one clear advantage over a stab/elevator- it is more forgiving of the CG position. A conventional tail can run out of control power if the plane is noseheavy. this is not so much of a problem with the current large tails in vogue. A good comparison would have to take into account the fact that the stabilator would be smaller and lighter than a stab/elevator which would make up for some of the added mass of the heavier pivot hardware.

The design certainly has to be optimized for the stabilator. Fine tuning the stabilator hinge point, the balance point of the plane, and the amount of flap needed would be a major undertaking. Much easier to pull out some numbers from a current good flying plane and put a different silouhette on them.

>a stabilator has one clear advantage over a stab/elevator-
>it is more forgiving of the CG position. A conventional
>tail can run out of control power if the plane is noseheavy.
> this is not so much of a problem with the current large
>tails in vogue. A good comparison would have to take into
>account the fact that the stabilator would be smaller and
>lighter than a stab/elevator which would make up for some of
>the added mass of the heavier pivot hardware.
>
>The design certainly has to be optimized for the stabilator.
> Fine tuning the stabilator hinge point, the balance point
>of the plane, and the amount of flap needed would be a major
>undertaking. Much easier to pull out some numbers from a
>current good flying plane and put a different silouhette on
>them.

yeah, true. another advantage might be a lower hinge moment...

LAST EDITED ON Jul-03-03 AT 12:38 PM (CDT)
 
>a stabilator has one clear advantage over a stab/elevator-
>it is more forgiving of the CG position. A conventional
>tail can run out of control power if the plane is noseheavy.
> this is not so much of a problem with the current large
>tails in vogue.

This is what I have found certainly utilizing counterbalances. I added 1 oz to the noze of the Mr. Hyde, and frankly, it did everything better and still did not load up the handle. Keep in mind I already have a 17 oz motor and a 2 oz spinner.

In fact, it seems with the counterbalances of any real size, a forward Cg is mandatory. This is certainly true of the Berringer plane, and I was hoping the same would be true on my design. So far all is well. The tracking and line tension is certainly more like a forward CG plane and the turn more like an aft CG plane. My only problem is underestimating how forward the CG could be.

Don't get me wrong, I still have a lot of work to do but the approach is realizing some of its potential.

I'm not going to do a tell-all, but anyone interested in the Stunt News article being discussed really must drop by the Rush Stunt Pits at the Nationals this year.

While there one and all are prompted--no, strongly urged--to ask Howard Rush about Mr. Howard Tang. No further hints will be dropped.

Ever'body in this thread...

Hi!

Glenn is Cholla Choppers' Pres & I'm his VP. Down here it takes a few extra days for SN to show up. I've had much opportunity to hear Glenn's explanations, and it IS a servo tab arrangement to 1) unload control forces, 2) allow further aft CG for crisper turns, and 3) to assure positive neutralizing returns.

Well developed. Haven't seen it fly (70 miles from my mile-high mountainside down to blazing Tucson's ~1,000' forest fire fiesta charms me little in high Summer...)

As I recall Glenn's thing, the servo tab is mounted to "deploy" counter to ordinary stabilator control inputs, sort of like Al Rabe's wiggly rudder. Different purpose, here, of course.

Now, I have to wait to see what the fuss is all about... Again....!

I am surprised, S/N was in mailbox, so I saw the article already .

Here some notes:

1/ Glen wrote the pivot for flying tail (without servo tab) must be little front of 25% MAC. It is because symmetric airfoil has very low pitching moment coefficient, which is by definition, calculated or measured to that 25% of MAC – it is called aerodynamic center (AC) also by definition. So if pivot is close but front of AC, the “hingemoment” or better “pivot moment” is very low but negative just because AC is almost at the center of pressure (thanx missing moment) and pivot is front of it. Low is good because of low load of handle and negative because of stability. (Until now no comment – just little more letters)

2/ He adds a servo tab. Its mechanical functionality is clear. I did not find explicit reason for that trick, but I think the main idea was to add some chamber to the flying tail to solve one of disadvantages – flat airfoil with limited lift. So now we have chambered airfoil, which already HAVE some moment. So the center of pressure is on different place, but also the pushrod makes some moment to tail. This clearly moves that optimal pivot point to another place. Calculation of its position is NOT trivial and it will probably need lot of testing. It is because of two very important thinks for its calculation. The first is fact not mentioned in whole article and it is that angle of incoming air to the wing is DIFFERENT than incoming to the tail. And the second is, that also air around the tail is not straightly moving. The air stream is curved. It all makes all analytical calculations very difficult.

BUT … he speaks about flapless model. I am asking myself if we really need to limit feedback on flapless models. I also did some tests to limit the feedback (on large flapped models). Successful, but I missed feedback in round figures. I think good stunter needs proper feedback.

3/ May be my English language ability, or limited brainpower today morning prevents me to understand well the section about flapped version. Did I understand well that the flying tail is self-orienting to the incoming air? I think yes, in this case I am asking what is it for? If it takes direction to the incoming air, it does not make any lift, and in this case it does not act like a stabilizer. Isn’t it joke? Didn’t I miss something?

Igor-

>2/ He adds a servo tab. Its mechanical functionality is clear. I did not find explicit reason for
>that trick, but I think the main idea was to add some chamber to the flying tail to solve one
>of disadvantages – flat airfoil with limited lift.

In my discussion with him, he indicated that the function of the "anti-servo" tab is to give a restoring (centering) moment to the all flying tail (AFT), which translates into a greater "tracking" (longitudinal) stability". That is, it functions as a stabilizer does on a conventional tail. Tab size and deflection changes correspond to changes in stabilizer area or tail moment arm length on a conventional stab/elevator tail. He says this on page 36 in the paragraph just under the caption to Fig. 5.

>BUT … he speaks about flapless model. I am asking myself if we really need to limit feedback
>on flapless models.

I think his device is intended to adjust feedback to any value desired. In general, by adding stability, it increases feedback, while at the same time increasing the aerodynamic effectiveness of the AFT through increased camber. This seems ideal.

>Did I understand well that the flying tail is self-orienting to the incoming air?

I think this might have been better introduced/worded in the article, but he says "without control input...". I think he was just emphasizing that if it were unintentionally deflected, it would be self righting. He was just emphasizing its stable nature.

>I am also asking myself if I understand it well as I already wrote, but I think that line just
>means that the pivot position is fixed to the fuselage together with the wing. It is not any
>pushrod as I understand. (or better said as I do not understand )

I think this means that the AFT and flap are connected, but not literally as indicated by the dashed line. I think they would be connected by a rod between arms on opposite sides of the flap and AFT. For instance from above the flap to below the AFT. At least that's what the paragraph indicates by asserting that flap and AFT rotate oppositely - e.g. flap moving down (clockwise, cw) while the AFT is moving counter clockwise (CCW).

I am still pondering your post #17 in response to my #12. For the first time, I have been unable to understand. I may be unable to respond for a time, as this is our Independence Day holiday, and I do not know how much time I will have. I will try to respond when I can make it understandable. For now, however, I think I will end up disagreeing. I'll think more about it

Serge

>>>I am still pondering your post #17 in response to my #12. For the first time, I have been unable to understand.<<<

Shall I make a picture?

>>>In Figure "7" the top of the flap is shown connected to the top of the stab. Is this correct?<<<

I am also asking myself if I understand it well as I already wrote, but I think that line just means that the pivot position is fixed to the fuselage together with the wing. It is not any pushrod as I understand. (or better said as I do not understand )

>>>"pros" and "cons" of the AFT (Igor?) Total control system hinge moments, system stability, pitch oscillation damping, mass balance as well as aerodynamic balance etc., etc...<<<

+ses:
+can be used to limit feedback (but someone can need it to load the handle)
+can suppress separation (or bubble) on high pressure side of stab at LE appearing because of too large AoA on stab (but someone wants it for easier turn) (thick blunt stab solves it also)
+can make high effective AoA and thus can pitch model quicker (do we need it?)

-ses
-no fixed well installed part of tail (important for those who can not make flying tail without free play) – did you read article by D.F. how important is to have well installed and well shaped stab to have good tracking and repeatability?
-structural integrity – it does not mean that it must break apart, the problem can be “soft” installation; the forces on both sides of tail are not equivalent and tilting or tweaking can limit ability of sharp and repeatable turns
-movable LE – the LE shape and position is very important, if it moves (as the pivot is far from the LE) it can makes troubles

I fly with Glenn on a regular basis. He is an extremely intelligent man. He does more experimenting in a week then I do in a year. As for the plane, it is light 33oz, very straight. Watching it fly I see no advantages with it. Bart Klapinski has had a hand in trimming it, so I'd say it's flying at it's peek. Those that have flown it say "it fly's OK"

One does have to admire Glen for his engineering abilities. He has a heart of gold and you would be hard pressed to meet a finer person

I personally think a conventional flapped, stab & elevator design is a much better way to go.

I too have read the article and was intrigued. Larry brings up a point that the AFT flies well but sees no advantage. I see one. Simply stated. More efficiency, less drag. You do not have to overcome the fixed horizontal stab lift or negative lift in a turn and the trim tab reasserts itself into the wind reattaching any wind that may be turbulating. Couple the AFT together with a flap combination and you should have a turning machine with less drag, hence better performance in the turns and/or less power required to turn the plane, less speed reduction. That's it.

I have to admire this type of experimentation. Without it the sport would soon waste away. Keep it up.

Bob Smiley
A Lancair 360 builder/pilot trying to be a modeler

Today I am a pilot departing Bremerton, WA for Houston, TX. about 9 flying hours with 180 hp for 1700 nautical miles.

After following this thread for the last week or so, I finally got my Stunt News yesterday to see if what I had gleaned from this thread is what the article was about. My experience with stabilators is fairly extensive, since 1959, so Ican see a few errors in Glen's thinking. The views he shows relative airflow are erroneous as axplained when he is talkiing about the self aligning characteristics of the AFT. The only time you would get a 0 degree AoA is flying straight up or straight. In any other flight condition, i.e., the wing developing lift, the elevator has to produce negative lift to hold the nose up, so there is no self aligning there as a cotrol input is required. He states that stunt ships are not nose heavy. To my point of reference as noted in several Flite Streak threads, the CG shoiuld be very close the teh center of pressure, but still must be ahead of it (on a Streak, that means the CG should be on the spar). Stunt guys want the CG far ahead of the Center of pressure for more stability, making them more nose heavy than I would prefer. But it is a necessity to keep the smooth flowing maneuvers that the stunt judges like. At any rate, the more nose heavy the model is, the more negative lift required to hold the nose up.
Everything done on a modern stunt ship is used to slow the response down; large airplanes, forward CG, flaps, long moments, large bellcranks, high wing loadings, etc, and now Glen wants to throw something in there to speed up the response and lose the one thing that stops the turn where you want it to stop, the stabilizer.
As far as the stabilator pivot goes, one must be careful,as hinted at in one of the responses, as the pivot moves rearward there becomes a great tendency fo the stabilator to flutter. The antiservo tab will help stop the flutter, but the real reason for them on a Cherokee is to feed a load into the elevator control so the pilot has a sense of control input. Balanced stabilators offer little of no control feedback and can actually give a feeling of control reversal. As the leading edge loads up, in some situations, the air loads can push the stabilator in the direction it is moving rather than trying to neutralizing. It will tend to make a model wind up more in maneuvers rather than straighten out. Some of my balance elevator combat models exhibited this characteristic. I never that much about it, but I had other people comment on it. The anti-servo tab will reverse this trend by forcing the trailing edge of the elevator in hte opposite direction.
On the other hand, in the case of my Melamij combat models, I used to use a 1-1/2" bellcrank, cut down from a 3" and still use the outer hole of the pushrod arm, along with a large EZ-Just, so my total control movement from full up to full down was less than a half inch at the handle! I had to keep the control loads light so the elevator would move! My first Xterminator ahd a bit too much balance area forward of the pivot in the stabilator and did a wonderful fluttering job in level flight. It was OK in manuevers, which it did very well, and was a successful design after I removed the balance area. Later ones had some balance area added again, but not as much as the original and flew well.

Bare

All:
I used a different thread title in a post that's a bit further down in the general index, the gist of which follows. While I agree with most of the comments in this thread, no one seems to have picked up that Glenn DID use a stabilator pivot significantly AFT of its ac. Fred Bachl's Prof Cardinal, p70, same SN, has the stabilator pivot much further aft, but uses a fork and pin to drive it, not a usual horn and pushrod. Greater leverage to resist the tendency to drive to the stop from its own "lift"?

There may be a few more illuminating thoughts in that other post, titled All Flying tails -- an explanation (or somehting similar...)

And, Larry F's remarks about Glenn are spot-on! Good to hav a guy like him around...