lifting stabilizers?

[N74463]

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

[Sparky Paul]

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.

[Brandano]

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.

[peterangus]

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,

[Thomas B]

........................................

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.

[ciurpita]

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.

[HarryC]

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

[N74463]

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. That’s 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?

[vintage1]

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" :D :D)

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..

[peterangus]

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.

[vintage1]

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 ;)

[Iflyj3]

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.

[yoyoML]

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!

[Tom Harper]

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!

[HarryC]

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

[yoyoML]

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!

Nope! You'd like all surfaces to be lifting. I have explained it in this post: http://www.rcgroups.com/forums/showpost.php?p=6933565&postcount=23

And the real problem is not only lift or only drag, but lift/drag ratio. Otherwise you could simply remove all wings and get really low drag... but you wouldn't be flying!

[Brandano]

Not true. Any lifting surface creates drag, including a lifting tail. Ideally you'd have the tail generate no lift at all to have the lowest possible drag, and that's what some sailplanes do by putting their flaps in a reflex position to trim level flight, effectively reducing to the minimum the airfoil pitch-up. The best thing would be to have no tail at all, as someone has already experimented, but that introduces control problems. In media stat virtus, at the end of the day it's all a question of compromise

[HarryC]

Stability does not require net downforce on the tailplane: It merely requires that there is less upforce INCREASE with speed INCREASE,
I didn't say exactly that, and nor is pitch stability about speed stability.
There are other couples at work on an airframe bsedies the lift-weight couple and they may balance the lift-weight couple. For example a high wing plane produces a couple from the drag-thrust lines, which is nose up. It is a small force but larger leverage than the larger force but smaller leverage lift-weight couple which is nose down, and the two may balance under some conditions. A well designed plane would seek to have the two in balance during the cruise so that the tail need produce no load and can fly at its zero lift angle. The plane has conventional nose heavy pitch stability, the tail does not need to be producing a downforce because other forces are providing compensating pitch couples. There may be parts of the flight envelope when the tail needs to produce an upforce to balance any imbalance in the pitching couples. Since the load my be either way and the ideal is no load, the optimum tail section is often symmetrical. I can think of several full size planes with inverted tailplanes due to the downward loads they have to produce, I am not aware of any with upright section tailplanes - not saying there aren't any, just that all the full-size I am aware of are inverted or symmetrical. Whilst full-size are designed with lots of calculations to try and achieve the no-load tailplane as much as possible, purely for drag and hence endurance and range considerations, models are not designed by such detailed calculations, and anyway the data to feed into the equations is not available to model designers. If the Telemaster has an upright section tailplane it is because the designer mistakenly believed it will be contributing to wing lift.
Pitch stability is not about speed stability. It is about re-aligning the aircraft or more pedantically the main wing, into the oncoming airflow. Gusts are not just horizontal but vertical. If a gust comes from below it will increase the main wing AoA but since it alters the tail's AoA in a similar fashion and the tail has leverage, the plane will pitch nose down into the changed direction of airflow and thus maintain its AoA. Stability is often misunderstood to mean providing a stable i.e a smooth ride. It is nothing of the sort, things like pitch and directional stability are concerned with constantly pitching or yawing the plane into the new direction of the airflow and giving an uncomfortable ride! The plane that doesn't pitch and yaw into gusts and carries on unaffected giving a smooth ride is actually the unstable plane!

[Tom Harper]

YoYo,

Interesting analysis.

Problem is that you leave out induced drag. You now have two induced drag terms. A model with the same total wing area in a single forward surface (same AR as one of your tandems) would have far less drag than the tandems at the same lift. A neutral stabilizer has no (or little) induced drag and minimum profile drag.

The AoA*Cl slope of flat bottom airfoils is almost linear. I don't see that different rigging angles offers any stability.

[yoyoML]

To Tom Harper:

The drag that's proportional to AoA^2, which I wrote in the post, is exactly induced drag. It is the parasitic drag that I didn't consider, and assumed to vary little with the small trimming incidence angles.

You can easily imagine a zero-lift stab gives lowest induced drag, but then its share of lift must be generated by the main wing, which then gives more induced drag. It is the total drag that needs considering, not only the stab's drag.

To Brandano:

The same as above. As you lower drag on the tail and maintain total lift, you increase drag on the main wing. Consider the total drag instead of only the stab.

Putting flaps to reflex is new to me, though. Could you say more about it?

[Brandano]

I have seen it used to improve penetration in high speed stretches. Effectively it subtracts total lift from the wing meaning less pitching moments and less induced drag, approximating the profile more to a simmetric profile wing. It isn't useful to float in a thermal to gain altitude, but it's handy to dash from one thermal to the next at the maximum possible efficiency.
Googling for reflex flaps brought up this: http://www.auf.asn.au/groundschool/umodule4.html as second result, where the technique is explained as a paragraph.
If you consider a glider with a strongly down-pitching airfoil, good for slow flight, as the speed increases you end up needing more and more downforce on the tail (or less and less upforce..). Reflexing th flaps reduces this, improves penetration somewhat (you have lift to spare, since you are going fast) and when optimal has the tail only correct for deviation from the optimal AOA

[Tom Harper]

YoYo,

It is true that the wing must carry the total load. The problem (among others) with the tandem wing is that the two induced drag terms effectively halve the aspect ratio. For a given amount of lift, a tandem wing layout will have significantly more induced drag than a single lifting surface.

[Sparky Paul]

I have seen it (reflexed flaps) used to improve penetration in high speed stretches.
.
The Lockheed U-2 uses reflexed flaps for load alleviation at cruise.

[nfhill]

I'm a bit stumped by my CAD program at the moment, so I thought I'd weigh in here. Hopefully I'm not just muddying things up.

I don't think there is a 'this method always works better' approach to this. It depends a lot on how you want the airplane to fly and how much design effort you want to put in and how much work it will be to fly.

If the h-stab is producing lift opposite of the wing, the wing has to produce more lift to compensate. Thus, designs where the h-stab is producing lift in the same direction as the wing are a little bit more efficient.

However, there is a cost for this: there's always a cost! This 'same way' design increases the 'work' for controlling pitch. Free-flights have no control inputs during flight, so it doesn't matter. Gliders rely on high efficiency and their pilots are willing to pay the price of higher work load.

Another price is non-symetrical h-stabs have non-symetrical pitch response relative to control inputs. Glider pilots are again OK with this because they try not to maneuvor very much: unnecessary maneuvors wast energy. Even with symetrical h-stabs, glider pilots try to operate with minimum lift on the h-stab for efficiency.

For more general flying, pilots prefer giving up a bit of efficiency for easier control. A symetric h-stab changes it's lift the same in both directions with pitich inputs. That's the reason most airplanes, today, use them.

Operating the h-stab at relatively low lift levels also can reduce the pilots control effort. This reduction of effort includes the 'mental' effort involved.

Pitch stability with power changes is a different matter. Most RC models have been carefully designed to not climb/descend with power changes because this is easier for the pilot on the ground to deal with. Changing pitch trim with power changes is the norm when the pilot is in the airplane. In a model, this is usually down with a carefully chosen thrust line that offsets the need to adjust pitch trim.

Of course, I may have this somewhat wrong. :o

Nick

[vintage1]

Not true. Any lifting surface creates drag, including a lifting tail. Ideally you'd have the tail generate no lift at all to have the lowest possible drag, and that's what some sailplanes do by putting their flaps in a reflex position to trim level flight, effectively reducing to the minimum the airfoil pitch-up. The best thing would be to have no tail at all, as someone has already experimented, but that introduces control problems. In media stat virtus, at the end of the day it's all a question of compromise

No. you don't always want lowest drag,. you want best overall lift to drag ratio for lowest sink speed.

Conversely if you want maximum distance (flattest glide) you may opt for a lower lift but lower drag regime..

Sailplanes are either hunting thermals, which needs shallowest glide SLOPE, or in them, which means lowest sink SPEED.

These are not the same trims and you may well find a bit of flap (or reverse flap) helps move from the one regime to the other.

[yoyoML]

To Brandano:

Ok, I see. I think what's going on is the glider is optimized/ballasted for a slower, thermalling flight, but is asked to dash to somewhere else really fast.

Then the real use of reflexing here may be to reduce main wing incidence, so that the fuselage still points into flight path to maintain low drag. If it's trimmed only with the stab, the fuselage will be pointing down too much, at a greatly un-optimized speed.

Simply the optimal incidence decreases with speed, and the flap reflexes to change incidence.

The main reason for reflexing mustn't be the trimming torque. Simple reasoning: using the tail to trim is way more effective than using reflexed main wing, or else you'd see trim tabs located on flaps, not elevators. Oh, and you should see airliners with reflexed flaps during cruise. I guess not.

To Tom Harper:

The tandem wing is just a quick example. I didn't compare its efficiency with a conventional config.

Anyway, the choice here is a) half the (effective) aspect ratio but keep alpha, or b) keep aspect ratio but double alpha. Please just do the calculation either according to my post or according to http://www.auf.asn.au/groundschool/umodule4.html#span_ar, taking CL proportional to alpha. You will see that b) gives more total induced drag.

A simpler view is: since lift ~ alpha, but drag ~ alpha^2, if you ask for 120% lift, you get 144% drag at the same time.

[Tom Harper]

yoyo,

Then, by your calculations, a triplane is a low drag configuration?

[biber]

Reflexing the wing is primariiy an issue of reducing the drag coefficient of the airfoil at low lift coefficents.
All other effects are rather secondary for that matter.

biber

[yoyoML]

yoyo,

Then, by your calculations, a triplane is a low drag configuration?

A triplane has lower induced drag when all 4 wings provide balanced lift, but it has higher induced drag when only one wing provides lift.

I never compared config to config, only the distributions of wingloading in a certain config.

[vintage1]

A triplane has lower induced drag when all 4 wings provide balanced lift, but it has higher induced drag when only one wing provides lift.

I never compared config to config, only the distributions of wingloading in a certain config.

Sadly triplanes tend to have short stubby wings, which gives greater induced drag, and also the wings interfere with each other..which reduces lift.

[nfhill]

The aerodynamic community has known for over 60 years the the most efficient shape for lift is a single wing. Anything other than the single wing reduces lift efficiency. If multiple wings gave increased efficiency, we would have airplanes like that flying all over the place!

Multiple wings interfer with each other all over the place and the extra wing tips, by themselves, kill the design even if the interference issues could be minimized.

The single lifting wing with trailing stabilizing surfaces is still the most common used because it still provides the best overall solution for most applications. There is no shortage of qualified designers with big budgets that would be producing multi-wing aircraft if it was a better solution.

The airline industry is one example of traditional design for traditional reasons: the ignorant public will only fly on airplanes that look like airplanes 'should' look. If people would be willing to fly on efficient designs (especially without those dammed windows), airliners would be flying wings with, perhaps, small canards.

Nick

[vintage1]

Its not that simple..at low speeds NOTHING beats a multi wing for manoeuverability..at low speeds nothing beats a thin under cambered wing for slow speed lift either.

Sadly the structural constraints on thin under cambered wings mean lots of bracing wires..and drag..

Aircraft design is a succession of compromises and developments.

Canards have their own problems..the chief of which in the olden days was the chances of having a couple of hundredweight of engine up your...well you know ... in a crash. Since balance dictated that a front engined canard was almost impossible to balance..

[yoyoML]

I may not have explained very well...

If we are to focus on a certain config, for example a certain triplane with its 4 fixed wing sizes/shapes and locations, then it is better to let all 4 wings contribute lift than to have 1 contribute and the other 3 idle.

This adjustment is done by shifting the CG and tuning incidences for all 4 wings, not by adding or removing wings.

So no, what I said doesn't lead to multiple wing designs; it leads to all surfaces lifting on each design. Please read my calculations more carefully before drawing conclusions.

[nfhill]

Vintage1, I doubt that you and I disagree significantly about the aerodynamics here. Low aspect ratio wings are the common way to reduce roll inertia. But, I believe, all modern competition aerobatic planes are monoplanes. Minimizing roll inertia is an opposing requirement to lift efficiency.

I aggree that camber is the primary factor producing lift and wing thickness is mostly about enclosing the necessary structure for strength. The slower you want to fly, the more camber you want. I'm sure that that's the reason modern sailplanes use variable camber wings.

Canards are too tricky for an amature like myself to argue very much about. I find it interesting that the latest 'no holds barred fly the farthest on the least fuel' airplane is a monoplane, from Scaled Composits no less. I think its stabilizing surfaces are also behind the CG.

You're absolutely right that any airplane design requires compromise between opposing requirements.

Nick

[Tom Harper]

yoyo,

I see the point you are making.

The problem is induced drag. The stabilizer has a lower aspect ratio than the wing. When it is loaded it contributes to the total induced drag. Since it has a low AR it contributes in greater proportion, relative to area, than the wing.

Your AoA*Cl issue is interesting, Do you have an aircraft configuration in mind that would allow you to test it.

[yoyoML]

yoyo,

I see the point you are making.

The problem is induced drag. The stabilizer has a lower aspect ratio than the wing. When it is loaded it contributes to the total induced drag. Since it has a low AR it contributes in greater proportion, relative to area, than the wing.

Your AoA*Cl issue is interesting, Do you have an aircraft configuration in mind that would allow you to test it.

Then let's calculate it.

This is from Brandano's link:

Induced drag = (k × CL² / A) × Q × S,

where
"k equals 1/Pe where P [pi] equals 3.14 and e is the span effectiveness factor which might vary between 0.8 and 1.0."

Q = ½*rho*V², the dynamic pressure;

A is aspect ratio;

S is wing area.

Let's take e=1 for now, so k = 1/pi.

Then the total induced drag = [(CL1²/A)*S+(CL2²/mA)*nS]*Q/pi,

where mA is stab aspect ratio and m<1; nS is stab area and n<1. CL1 and CL2 are CL of main wing and stab respectively.

The constraint is Lift=Weight, or

L = Q(CL1*S+CL2*nS) = W =constant.

=> CL1 = [ W/Q-CL2*nS ] / S

Put this into drag:

Drag = {[ W/Q-CL2*nS ]²/(A*S) + (CL2²/mA)*nS} * Q/pi

To find minimum, differentiate w.r.t CL2:

d Drag / d CL2 = { -2[ W/Q-CL2*nS ]*nS/(A*S) + 2*CL2 * nS /mA} * Q/pi = 0

=> CL2 = [ m / (nm+1) ] * W/QS

Aha! The optimum CL2 is not zero! It is positive due to everything on the right being positive. Yes, even when the stab has lower aspect ratio ( m<1 ).


Let's downgrade to the tandem case just to verify:

n=1, m=1
=> CL2 = 1/2 * W/QS
=> CL1 = [ W/Q-(1/2 * W/QS)*S ] / S = 1/2 * W/QS = CL2 !!!!

About the alpha * CL slope thing, I was being a bit stupid... Since alpha * CL slope itself is just CL, duh :p

[Tom Harper]

yoyo,

Interesting analysis. Give me a little time to look at it.

[Tom Harper]

YoYo,

You have a little numbers game going here. If you make the stab and wing the same planform and same section and ignore downwash and Reynolds number then everything becomes proportional to Cl^2. The derivative of X^2 is X and the rate of change in Y per unit change in X is = 2X+1.

So, since the stab is operating at near zero AoA it will have a smaller rate of change of Induced Drag per unit of AoA increase than the wing. I yield that point.

Then you must add profile drag:

Assume the stab is 30% of the wing area. For 'classical' airfoils, like the Clark Y, the 0 Cl and minimum drag Cd do not coincide. For the first degree or two that you raise the LE the profile Cd decreases. For a Clark Y the minimum profile Cd is at Cl=.1 (+.5 degree from zero lift). The wing would be rigged at a Cl of .9 (+6.5 degrees from zero lift) to get Max L/D during level cruise. To sustain level flight would require that the stab carry .30*W*(.1/.9)=.033W. That will place the CG behind the neutral point. Not a stable condition.

However, If you use a symmetrical section for the stab the above is not true and the profile drag of the stab will increase at a faster rate than the profile drag of the wing.

I have to agree that there are rigging angles that will lower the drag of the stab. I don't feel that these angles yield practical solutions. I prefer to rig the stab at zero lift which gives me predictable control of the wing.

But, hey, I'm old and grouchy and set in my ways! Give it a try. You may be on to something.

(If you like number fondling then rig one surface near max Cl and watch what happens when the derivative goes to zero or negative)

[ciurpita]

am i missing something?

isn't induced drag is non-linear? drag is always positive regardless of whether the lift is positive or negative. therefore you can't minimize the total induced drag by having negative lift (Cl2) that negates the induced drag from Cl1.

[BMatthews]

am i missing something?

isn't induced drag is non-linear? drag is always positive regardless of whether the lift is positive or negative. therefore you can't minimize the total induced drag by having negative lift (Cl2) that negates the induced drag from Cl1.

You didn't miss anything. You're right.

The drag on any surface will reduce to a minimum at the zero lift angle and then start rising as the lift reverses.

[JetPlaneFlyer]

The drag on any surface will reduce to a minimum at the zero lift angle and then start rising as the lift reverses.
Is this really true?... For a cambered airfoil surely the minimum drag occurs when the section is producing lift?... which is why a 'lifting' (i.e. cambered) section stabiliser will be more efficient than a symetrical one, at least in some trim configurations.

[yoyoML]

YoYo,
Then you must add profile drag:

Assume the stab is 30% of the wing area. For 'classical' airfoils, like the Clark Y, the 0 Cl and minimum drag Cd do not coincide. For the first degree or two that you raise the LE the profile Cd decreases. For a Clark Y the minimum profile Cd is at Cl=.1 (+.5 degree from zero lift). The wing would be rigged at a Cl of .9 (+6.5 degrees from zero lift) to get Max L/D during level cruise. To sustain level flight would require that the stab carry .30*W*(.1/.9)=.033W. That will place the CG behind the neutral point. Not a stable condition.


I think the above just shows you wouldn't want Clark Y for that particular stab.

For any given CL requirement, it is possible to choose the airfoil to miniize profile drag, right? In this case you'd like something more symmetric than Clark Y, of course.

The stability problem is still there, though. A "conventional" plane may need the CG slightly behind NP both to neutralize wing pitch down torque and to balance the wingloading/miniize induced drag. The static stability is gone, so the plane must be stabilized by computers.

It becomes a compromise between human flyability and reducing drag.

[ciurpita]

for each airfoil, there is some Cl where the Cd is minimized. that Cl can be achieved by the elevator trim, and the aircraft will fly at the airspeed dictated by that Cl.

the trailing edge of the wing (flaps and ailerons) can also be adjusted to shift the Cl curve up or down. presumably this can be done with minimal affect on the Cd curve. so trailing edge adjustments can be used to select the Cl at the minimum Cd point, so that the airspeed or trim setting are also adjustable at that minimum Cd point. of course there are limits to these adjustments.

[BMatthews]

Is this really true?... For a cambered airfoil surely the minimum drag occurs when the section is producing lift?... which is why a 'lifting' (i.e. cambered) section stabiliser will be more efficient than a symetrical one, at least in some trim configurations.

Actually you're right. My bad for encouraging him... :D

If you look at a coefficient of lift vs coefficient of drag graph there's a point where each airfoil has a minimum value. For higher cambered airfoils this usually occurs at a higher than zero value for the coefficient of lift. Only for symetrical or very low camber values does this occur at a CL of zero.

Not sure why I posted that when the answer was only a couple of grey cells away if I'd chosen to use them.

The funny thing about all this is that many, many model airplanes fly with a positive lift coefficient on the stab. And I'm not talking about the real odd ones either. Once your CG moves back to near the 30% MAC point or even further back your stabilizer will be positively loaded. At that point if you're looking for the ultimate in low drag for ONE SPECIFIC FLIGHT SPEED I suppose it would be a good idea to determine the tail loading and thus the required coefficient of lift for the tail and use an airfoil with it's lowest drag coefficient occuring at that lift coefficient.

But our models fly in a very dynamic manner and any such calculation for such a miniscual advantage would be pretty much a waste of time.... unless it's for a free flight model that does indeed fly at this magical point for most of it's time aloft.

[vintage1]

Don't confuse two things..

Minimum drag, and minimum drag to lift ratio.

For speed, you want minimum tailplane drag..plenty of lift on the main wing, and that will be a thin section at a low angle of attack..so for out and out speed you will have that thin main wing plus a tailplane as near zero as it can go.

For best overall lift to drag, you probably want a low speed wing - heavily undercambered..and a large tailplane that contributes lift as well.

That will give you the lowest sink rate.

But as the speed varies the tailplane loading may well swing from positive to negative through neutral anyway. Essentially it makes little sense to talk about a lifting tailplane except on the case of a one-speed model.

[biber]

But as the speed varies the tailplane loading may well swing from positive to negative through neutral anyway.
Essentially it makes little sense to talk about a lifting tailplane except on the case of a one-speed model.One of the most important statements in this thread.

biber