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Old Feb 25, 2007, 01:32 PM
Wait...Say again?
Anaheim, California
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lifting stabilizers?

In several Telemaster threads, I've read that they have a lifting stabilizer and that having a lifting stabilizer increases pitch stability. There's something I'm missing, because I would've guessed that using the stabilizer for upward lift would make the airplane unstable in pitch. (I'm talking conventional layout here (big wing in front, little stabilizer in back) not canards.)

Maybe I don't understand the concept. When I read "lifting stabilizer," I'm picturing the CG moved far enough aft that the stabilizer provides an upward force (as opposed to the downward force it provides in most airplanes). Is that right? The Telemaster pictures seem to show a stabilizer with an airfoil shape that has the more curved side on top.

So, in a conventional airplane, when it goes faster than trim airspeed, the stabilizer provides more downward lift, the nose rises, and the airplane tends to return to trim airspeed. Likewise, when it gets slower than trim airspeed, the stabilizer provides less downward force, the nose falls, and the airplane tends to return to trim airspeed. Thus, the plane has positive stability in pitch (at least it has positive static stability).

But here's what I picture if the stabilizer provides an upward force. The plane slows, the stabilizer provides less upward force, the tail falls, the airspeed decreases even more, the tail provides even less lift, and so on right up to a stall. The plane tends to depart from trim speed instead of returning to it. Likewise, if the plane speeds up, the tail provides more upward lift, the nose goes down, the plane speeds up, etc. right through the first half of an outside loop. This does not sound like an airplane that has increased stability.

So, what am I missing? Have I misunderstood the phrase "lifting stabilizer"? And if I haven't, how is a plane with a lifting stabilizer more stable?

Thanks.

Joe
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Old Feb 25, 2007, 01:38 PM
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The lifting horizontal provides a stabilizing force -faster- (at a lower angle of attack disturbance) than a symmetrical airfoil.
It's what free flights depend on to fly at the single best speed for gliding.
The principle of lifting down is still there.
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Old Feb 25, 2007, 04:47 PM
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Think of a lifting stabilizer like of a canard plane with a large canard surface and a really small wing. The, what do you call it, neutral point? point of pressure? comes from a combination of the stabilizer and the wing. As long as the CG is in front of it the plane is stable.
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Old Feb 25, 2007, 06:18 PM
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Quote:
Originally Posted by N74463

So, what am I missing?
Joe

The only thing you are missing is that, for a model like the Telemaster, the "lifting stabiliser" is misguided wishful thinking,
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Old Feb 25, 2007, 07:29 PM
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Quote:
Originally Posted by N74463
........................................

But here's what I picture if the stabilizer provides an upward force. The plane slows, the stabilizer provides less upward force, the tail falls, the airspeed decreases even more, the tail provides even less lift, and so on right up to a stall. The plane tends to depart from trim speed instead of returning to it. Likewise, if the plane speeds up, the tail provides more upward lift, the nose goes down, the plane speeds up, etc. right through the first half of an outside loop. This does not sound like an airplane that has increased stability.


Joe
I have flown a good number of R/C models with flat bottomed airfoiled tails over the years. The well known Chuck Cunningham of RCM fame was a local flyer and designed many models with "lifting" horizontal tails. Besides Telemasters, I have flown 3 or 4 different versions/sizes of Chuck's famous Lazy Ace design, his large Druine Turbulent and others.

Incidence is a much bigger factor than any lift provided by the airfoil.

In reality, most models with the flat bottom airfoiled horizontal tail, the models have enough positive wing incidence that the "lifting" stab effect never overpowers the complete aircraft. Usually, the model climbs from wing incidence effects proportional to airspeed. The Telemaster has some trim change with speed, but it wants to climb, not dive, as it goes faster.

I found that the Chuck Cunningham designs had very pleasant and freindly handling qualities in flight, especially in the pattern and on final landing approach. They felt like they had superor low speed handling qualities, as compared to other designs of similar size and weight. I think is is for te reasons that Sparky Paul mentions.

The pitch up that this poster worries about simply was NEVER evident. Remember, the tail will always "lift" less than the wing as it slows down simply due to the size of the tail area as compared to the wing area. How can it ever "overpower" the wing if it's area is a mere 20% of the wings?

I think the thicker flat bottomed airfoiled tails help prevent the stalls that can happen with flat plate tails, by delaying separate of airflow, to some degree.

A "lifting" stab is a perfectly viable design feature for certain types of sport R/C models....one tool among many in the designers toolbox. Certainly not good for pylon racers, jet models, fast aircraft, most scale aircraft and competition aerobatic aircraft, but appied correctly, they add some benign handling to models like the ones mentioned above.
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Old Feb 25, 2007, 09:27 PM
greg
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there are actually three components to the moment force that balances the pitch of an airicraft: the tail moment, lift moment, and the airfoil moment (Cm). there's an article in RC soaring digest the goes over this
http://www.rcsoaringdigest.com./pdf...CSD-2006-03.pdf

airfoils generate a moment (Cm) because of their shape which is usually negative, meanining it forces the nose down. it only depends on airspeed, not angle of attack. the lift generated by the tail and the wing must balance the airfoil moment, they must produce a moment the forces the nose up, if the Cm is negative.

if the CG is at the aerodynamic center (AC) of the wing, the wing moment is zero, and the tail must produce a downward force to balance the airfoil moment. if the CG is forward of the AC, the lift forces the nose down, and the downward tail force must be greater. if the CG is aft of the CG, the lift forces the nose up, and the tail force can be less or even upward.

at a particular CG position, the lift and tail moments may exactly balance one another, at all airspeeds and angles of attack. in this case, they would not balance the airfoil moment and the aircraft would be unstable. at another CG position, the lift and tail are imbalanced such that their combined moment balances the airfoil moment at all airspeeds. a neutral aircraft. assuming this is near the limit of flyability, it determine how postitive the tail lift can be.

this position depends on size of the tail. at some point the tail becomes more of a wing, and the wing a canard. but for conventional aircraft, the limits on the CG position prevent a stable aircraft from having a lifting tail at all airspeeds.

however, in some free flight models using flat plate wing, the airfoil moment may be zero. in this case, both the wing and tail may produce lift in order to balance total moment of the aircraft.
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Old Feb 26, 2007, 06:28 PM
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Quote:
Originally Posted by peterangus
Joe

The only thing you are missing is that, for a model like the Telemaster, the "lifting stabiliser" is misguided wishful thinking,
Exactly, the "lifting" tailplane is bad design that stems from fundamental ignorance of the mechanics of flight. The tailplane with camber will have to operate at quite a negative angle of attack to create its downward force, thus increasing its drag compared to a symmetrical or inverted section. Models may pretend to be zero rigged, but tailplanes sitting in the downwash of a high mounted wing are actually operating at quite a negative angle of attack.
Certain free flight models have "lifting" tails where competition rules set a maximum size of main wing, but these models only work at a specific speed and go out of control if they depart from a very limited speed range.

H
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Old Feb 26, 2007, 08:03 PM
Wait...Say again?
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Hi Everybody,

Thanks for taking the time to try to help me out. I love lurking around here, knowing that if I really have questions, I can ask and get answers. I also love learning about aerodynamics even though it's far from the field I was trained in.

PeterAngus and HarryC, I have to admit that my skeptical suspicions matched your answers. Sparky_Paul, your answer left me with the question, why is the stabilizer airfoil curved side up, if the stabilizer is producing a downward force? I read the three articles in RC Soaring Digest, but they are more about flying wings and didn't help me much (or maybe they were just over my head).

So I scratched my head and then remembered this link: www.av8n.com/how/. It's John Denker's "See How It Flies." It's sort of Aerodynamics for non-Physics majors. Chapter 6 had just what I needed. (And Thomas_B had the right answer when he said that incidence is the big factor.)

In fact, Denker addressed my question specifically. He said,"Some people are under the misimpression that the tail must fly at a negative angle of attack for the airplane to be stable. Thatís just not true. The real rule is just that the thing in back needs to fly at a lower angle of attack than the thing in front. If the angle is so much lower that it becomes negative, that is just fine, but it is not required. The amount of stability you have depends on the angle of attack of the tail relative to the wing, not relative to zero." (He says "thing in front" because the explanation can fit canards just as well as conventional tails.)

Basically, he says that within a normal CG range, the stabilizer may go from producing a downward force when the CG is far forward to an upward force when the CG approaches the aft limit. But it's the difference in angle of attack that ties everything together.

If the tail is flying at a lower angle of attack than the wing, the airplane will be stable, i.e. it will try to return to its trim airspeed. Whether the tailplane is providing upward or downward lift, the angle of attack of the tail changes by a different amount (percentage-wise, not in degrees) than the wing, and the tail lift changes in the correct direction to return the plane to trimmed airspeed. If the CG is forward, the story is pretty much as I described in my OP. What I described happening with a lifting stabilizer is what happens when the CG is aft of its normal limit or when there isn't enough decalage (difference in angle of incidence between wing and tail). Anyway, Denker's explanation of the whole thing is very clear. Take a look if you're curious. You want Chaper 6, Angle of Attack Stability, Trim, and Spiral Dives.

So thanks again, everybody. I'll be able to sleep better now that I understand this.

Joe

P.S. Now I'm curious if the Telemaster's CG range is such that it always flies with the stabilizer lifting upward. If not, you'd think a symmetrical airfoil might be a better choice. But I'm not going to lose any sleep over that. Still, if there are Telemaster owners reading this, I'm curious about the difference in the angle of incidences of the wing and the stabilizer. And has anybody ever mounted their Telemaster stabilizer upside down?
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Old Feb 26, 2007, 08:20 PM
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Quote:
Originally Posted by HarryC
Exactly, the "lifting" tailplane is bad design that stems from fundamental ignorance of the mechanics of flight. The tailplane with camber will have to operate at quite a negative angle of attack to create its downward force, thus increasing its drag compared to a symmetrical or inverted section. Models may pretend to be zero rigged, but tailplanes sitting in the downwash of a high mounted wing are actually operating at quite a negative angle of attack.
Certain free flight models have "lifting" tails where competition rules set a maximum size of main wing, but these models only work at a specific speed and go out of control if they depart from a very limited speed range.

H
No. That is plain wrong.

You can, and do, have true lifting tailplanes.

The secret is to make them big enough and far enough away..

Stability does not require net downforce on the tailplane: It merely requires that there is less upforce INCREASE with speed INCREASE, than on the main wing. I.e that the tail has less angle of incidence than the main wing (or main wing less incidence than the foreplane in a canard, which is the extreme case of a 'lifting tail" )

It is obvious that this condition is satisfied with a main wing at positive incidence, and zero or negative incidence on the tailplane. It is less obvious, but true, that it works with positive on BOTH..think about it..as the speed increases, the main wing needs less incidence to maintain the lift..so in a constant speed dive the nose will be a little more down relative to the airstream..at some point the tail incidence is now almost zero..and once again we have therefore a net nose up force...to pull the plane out of the dive

Machines with long tails and/or large tailplanes can have very rearward CG's to the point where the CG migrates aft of the trailing edge of the front wing, and there is most definitely lift being generated by the rear wing, up to and including a canard, where the CG is somewhere aft of the rear plane leading , which has now become the main wing.


I fly a lot of old timers, some of which have large and long tails..and yes, they are stable, and yes, with a CG around 50% of chord, or more, they are for sure generating tailplane lift.

Stability? they are LAZY ..they float beautifully, but they take longer to respond to elevator input, and longer to pull out of a dive unassisted. On the other hand they tend to show less trim change under power and in the glide.

You take your choice.

Joe: Denke has it dead right as far as I am concerned.
My gut feeling is that the 'neutral tail occurs somewhere about CG=30% of main win chord. You can dredge up figures for the center of lift of an average Clark Y sort of wing and work it out yourself. If CG is aft of center of lift, the tailplane is lifting..
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Old Feb 27, 2007, 04:06 AM
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Let's keep it simple

The original question related to the Telemaster; a very conventional configuration.
--------------------------------------------------------------------------
If we consider all configurations, including canard, tailless, tandem, and high wing camber, then the answer must be lengthy and complicated.
Look at http://www.rcgroups.com/forums/showthread.php?t=415163
posts 7 and 19.
--------------------------------------------------------------------------
However, if we confine the discussion to the Telemaster configuration, then the answer is simple. As follows:

The tail "lift" will always be insignificantly small in relation to the wing. It may be up or down, regardless of tail section camber.
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Old Feb 27, 2007, 07:01 AM
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Its likely in normal flight that it will be neutral to slightly up..as speed increases in a dive, its likely to be a net down..most tailplanes (according to Work in Progress) fail DOWNWARDS..probably attempting to pull out of steep dives
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Old Feb 27, 2007, 07:18 AM
Dan Thompson (MP8K developer)
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I think you fellows are over engineering the situation.

As the aircraft goes faster it tends to climb. The tail also has increased lift as it follows along and therefore it tries to keep the plane from climbing by giving the effect of down elevator. If the horizonal stablizer is planed correctly with the wing, changing the speed of the plane will not cause it to climb or dive.
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Old Feb 27, 2007, 07:45 AM
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I think there's some mistake since the beginning: the "pitch stability" is not about returning to trim speed. It is about an increase in AoA causes a decrease in nose-up pitch torque, which tries to push the nose back down.

"Stability", as in "static stability", almost always describes a returning force related to a displacement (or equivalently, a returning torque related to an angular displacement).

Returning to trim speed is a much more complex process than returning to set AoA. It's something like:

speed change (pitch torque)-> AoA change (lift force)-> flight path change (gravity-drag force)-> speed change (pitch torque)....

where every "->" integrates once in time.

Anyway, Denker is almost completely correct. In fact the front wing (be it conventional main wing or canard) flies not at a higher AoA, but a higher AoA*CL slope. I think it's for this reason you always see canards with a higher aspect ratio and lower sweep than the main wing, on any canard design: higher aspect ratio + lower sweep = higher CL slope. And it helps that a wing with higher CL slope usually stalls at lower AoA, so the canard easily stalls first- wonderful!
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Old Feb 27, 2007, 08:05 AM
Lift is cheap - Drag sucks
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Joe,

The function of the stabilizer is to set the angle of attack of the wing. To do this it must fly at, or near, zero lift. This does not mean that the thick stab section of the Telemaster would have to be rigged at a large negative angle. The downwash of the Telemaster wing does that for you.

Unless you intentionally load the stab by moving the CG aft, it cannot contribute to lift. Any lift from an unloaded stab would have the same effect as down elevator. Loading the stab greatly increases induced drag.

In a conventional layout, with no novel goal, there is no design rational for a lifting stab. That's what the wing is there for. The real problem is decreasing drag not increasing lift:

Lift is cheap, Drag sucks!
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Old Feb 27, 2007, 08:14 AM
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Quote:
Originally Posted by Iflyj3
As the aircraft goes faster it tends to climb. The tail also has increased lift as it follows along and therefore it tries to keep the plane from climbing by giving the effect of down elevator. If the horizonal stablizer is planed correctly with the wing, changing the speed of the plane will not cause it to climb or dive.
No. As it rises due to more lift, the AoA of both main and tail is reduced, or the tail made more negative, thus pitching it up, not down.

"If the horizonal stablizer is planed correctly with the wing, changing the speed of the plane will not cause it to climb or dive." Does not make sense and sounds like you believe that the throttle is a speed control. To increase speed you lower the nose, to control loss or gain of height you adjust the throttle. I can not sensibly change the speed of my plane with throttle and some time back I did measurements in my full size plane to demonstrate to modellers who persevere with their throttle=speed myths. From steady cruise at level height I opened to full power and the speed stabilised 5 knots lower than when at cruise power. Settled back at cruise I closed the throttle to idle and the plane accelerated rapidly, through Vno heading for Vne at which point I pulled back on the elevator to stop the speed rise. More throttle made it go slower, less throttle made it go faster. You can not simply change speed in a plane without altering the elevator trim hence the idea that a properly trimmed tailplane allows you to change speed without a change in height is nonsense because it assumes that the throttle alone is speed controller and that elevator trim plays no part in speed control.

H
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