Phil Cole

If an airplane is maintaining level flight, then the CG and the AC must be coincident. Otherwise it would experience a pitching moment, and accelerate around the pitch axis.

Stability could (I didn't do aero, so I don't really know) come from variation in the AC with airspeed or pitch attitude. E.g. if the aircraft pitches up, then the AC moves backward. For this to happen, lift must increase behind the CG relative to the lift ahead of the CG as a consequence of the pitch attitude change.

So, the question is: Is negative lift at the tail a necessary and sufficient condition for the above to occur?

It seems to me that the relative behaviour of the lift at the front and back wings under varying conditions of incidence and/or airspeed is important, not just the direction of the lift at the back.

Craig-RCU

The simulator Cesna is balanced a bit noseheavy (for my taste anyway) and the airspeed in the screen shot is very close to stall speed. With higher airspeeds, the simulated Cessna too requires lots of down elevator trim in order not to pitch up.

Craig-RCU

Many of you may be wondering at this point-- In these times when airliners are crashing into buildings killing thousands of people, what difference does it make whether or not small wings behind big wings produce lift or not. Canards fly well and conventional planes fly well, so just get over it and concern yourself with the more pressing issues of the day.

The real reason I am pressing the issue is just to practice thinking for myself and not to just accept what someone else tells me is true without really understanding why it is true first. The 9-11 highjackers were duped into thinking that mass slaughter is a good thing, and that it is fine to kill people that you think are misguided. Had they really understood the teachings of their own faith (in the name of which they commited mass murder), they could not have killed anyone. Instead, they accepted Mr. Bin Ladin's warped and hateful interpretations (which defy all precepts of living well and happily on this planet) of their holy scripture and performed those indiscribably horrifc acts.

So, exercise your brain muscles with this lifting hstab issue or whatever other issues interest or confuse you. Bite into these issues and think till your brain hurts. Don't be afraid to think for yourself or you could end up doing something akin to the 9-11 slaughters. But then again, maybe thinking is not for everyone. I am a rebel of sorts. I run with scissors and I go swimming less than an hour after eating.

Craig-RCU

I don't know much about the movement of the AC (is this the same as neutral point or center of lift or wing area centroid?). Maybe if you or someone could clarify what this is. I have to think that it is important whether rear wings produce up lift or not just because canards would have much higher landing speeds than they do if the rear wings did not produce positive lift. This is true also for a tandem wing and, by my previous points, not an insignificant source of lift for conventional designs too. I think that I may have missed your point, though. I can't quite determine what side you are coming down on or if you are creating a new side. If you could explain your point a bit more.

mulligan

Craig,

I appreciate your creative, philosophical thoughts, but I do not believe the world is flat (figuratively speaking), nor have I and thousands of aeronautical designers been blindly fooled into believing something because some strong-minded leader/teacher/whatever told us so. The facts I laid out are fairly fundamental in the aeronautical field. No one need believe me because I say so; I encourage people to not believe conjuncture, but rather check out a book or in some other way research the things we're talking about. I obviously am confident about what I know, notwithstanding your insutations that I might not be open-minded enough.

BTW, your paragraph relating wing loading/weight to stall speed, all other things being equal is off the mark as well. The stall characteristics of an airfoil are fixed, no matter what the loading on it. However, there is an indirect relationship (all things NOT being equal). If an aircraft weight is increased, it requires more lift to maintain altitude, therefore, having not changed the wing, increased angle of attack is required for the same speed. Increasing angle of attack gets you closer to critical angle of attack (stall). If you are changing angle of attack, many other variables are changing (e.g., drag, elevator lift, even stability). But, if I understand your conclusion, it is also indirectly correct. If a plane is neutrally stable, it will stall at a lower speed than the same plane at the same weight, but more positively stable IF there is an aft stab. This is because in a positively stable plane, the aft stab is generating downward lift to maintain static equillibrium. To overcome this downward lift in order to keep the plane level, the main wing must generate more lift, thus have a higher angle of attack... closer to critical angle. In a neutrally stable plane, very little/no downward force is required on the tail, thus a lower angle of attack is required to maintain level flight. To be precise, for an aircraft with an aft stab, a more stable aircraft will require a higher angle of attack to maintain level flight. This somewhat illustrates the concept that controllability is inversely proportional to stability.

You ask a fair question about canards, so I'll expound a little here. Yes, it is possible to have canards on an aircraft and it be stable, especially on smaller aircraft (e.g., models), where the Reynold's number is so much less. The divergent effect of the canards I described will always be present, however two design tricks can help you. One, having the canard angled such that it stalls much sooner than the main wing will assure that the nose may "fall" before an aircraft gets too high in pitch (this only helps in up pitch, such as when landing, but not in negative pitch attitude). Second, as the pitch of a plane increases, so does the lift of the main wing, and thus it's nose-down moment- so if the canard is sized small enough and/or located close enough to the CG AND the AC of the main wing is significantly aft of the CG, the two can be balanced such that the plane overall remains stable. This, however, greatly narrows the allowable CG range beyond what would be required by stability restrictions in a traditional configuration. This is why the vast majority of aircraft do not employ canards- it is not practical. Only the more exotic designs, some of which you mention, employ canards, with other even more non-conventional features. Creative ideas such as forward swept wings and assymetric wings are possible (Burt & Dick Rutan's planes come to mind), but the design/material challanges there are even greater... and hence the cost, too. All this doesn't change the fundamental principal that I explained, but you said I did not prove. I'll try one more time...

If you can visulize that a plane with a CG ahead of the main wing AC (most planes) will naturally nose down, how could a stabilizer (or anything for that matter) located further aft, and hence aft of the CG, keep the nose from falling unless it is pushing down? Everything rotates around the CG, right? Wing goes up, nose goes down... to keep nose up, aft stab must go down.

I apologize for not including my name last time...

Regards,
George

Craig-RCU

Thanks, George. I appreciate your joining in this discusion to try to shut me the heck up about this lifting tail nonsense. I don't think you have been blindly fooled, but your calling lifting tails "nonsense" in your first post leads me to believe that you have a somewhat condescendingly negative attiude towards the concept. If your attitude is typical of most aeronautical engineers, then I can see how the subject could become just plain socially unacceptable among aeronautical engineers. If indeed hstabs cannot lift, then no knowledge is lost. But the hostility does other damage. It establishes hostile behaviour and peer pressure in place of reasoning as acceptable means of getting people to believe what you think is true. It also thwarts the exploration and real understanding of the supposidly obvious. The subject is then just plain avoided by all for fear of disgrace in the eyes of their peers. Planes will fly the same whether you believe hstabs lift or not, so why risk the disgrace. That is how the real knowlege can get burried. Just not an important enough subject for the risk.

For me it is important as a thinking exercise and hopefully a better understanding of the Universe as a whole and I risk nothing. The reason I ever started questioning hstab lift was because it conflicts with my notion that the Universe is symmetrical. You know; for every action there is an equal and opposite reaction, what goes up must come down, what comes around goes around. Both socially and scientificly the laws of the Universe seem to be symmetrical and the idea that a little wing can't produce lift unless it is in front of a big wing is just not symmetrical.

O.K. I see this is your main reasoning behind why you believe an hstab can not produce lift unless the plane is ballanced for negative stability (i.e. the C.G. is behind the AC). If I crack this nut then you have to pay me a dollar. Here it goes. The neutral stability point on a flying wing is around 23-25% MAC. I have tested this myself on a couple of R/C models and it is a widly tested and verified fact for flying wings. Now say you're in your midwing symetrical airfoil conventional planform plane and you have a button you can push that folds up the hstab halves vertically so you loose their horizontal area without changing the C.G. that you took off with. The rule of thumb for stability in conventional planforms is that you can balance them around 30% of the main wing's MAC and have good positive stability. You push the button, up fold the hstab halves, and now your piloting a very tail heavy negatively stable flying wing balanced some 5% behind the neutral point for a flying wing. Since you were cruising straight and level in your symmetrical airfoiled plane the main wing was at a positive incidence when you folded up the hstab. The new tail heavy situation of your plane means that there is a nose up moment since the main wing was at a positive angle of attack when the hstabs were folded. The only way to stop the nose-up (tail down) divergence is to unfold the hstab and the force that the hstab contributes to correct the divergence is up. Where's my dollar!!!

What your example of the plane without an hstab fails to consider is the fact that the AC of the aircraft will move in front of the C.G when hstab area is eliminated. This is the case for all aircraft that are balanced behind 23-25% MAC. And so, a C.G. placed aft of 23-25% MAC of the main wing is were an hstab on a conventional plane produces lift. Have a nice day.

Craig-RCU

Guess what, I do not agree that canards are any more or less divergent/stable than conventional designs. It just is not symmetrical to think that a little wing in front of a big wing is less stable than a big wing in front of a little wing. Nor is it symmetrical to think that a little wing can only produce lift if it is in front of a big wing and cannot produce lift if it is behind a big wing. But that is another thread.

mulligan

Craig,

There are so many falicies in your last post, I don't have the time to correct them all. It's obvious to me you are making guesses with limited fundamental knowledge, so if you want to continue with these posts, which you seem to think are provocative but I call mis-information, that is your right. It is my right to share with the other readers my background & knowledge. I've given enough information that I think others can compare to your opinions and make up their own minds after doing a little more research themselves, so I'm done on this thread.

As far as you and I go, we'll agree to disagree- have fun with the hobby.

Jeremy Sebens

Before I give my dissertation, let me tell you that I have a MSME and a BS in Aeronautical Engineering and was an Engineer in the USAF- my focus is stability & control and flight test engineering... I feel silly telling everyone, but maybe someone not so knowledgable will think twice about futher confusing the facts. And the facts (NOT over simplified) are:

OK... I'm not going to list my credentials, but rest assured I'm qualified to speak to these issues...

1) 99% of all planes are positively stable- that is the CG is located in front of the aerodynamic center of the main wing. One notable exception to this is the F-16, which is neutral to negatively stable (or dirvergant)- it would not be controllable by a human if the fly-by-wire system failed (try throwing a dart backwards).

Perhaps it's been a while since Flight 2... A plane is positively stable if the C.G is ahead of the neutral point. This is very different from the AC of the wing, as it is the AC of the entire airplane . Also, remember that it is not the point about which pitching moment is zero, but about which it is constant. This constant can be either negative (nose down, common), positive (nose up, less common) or zero (the holy grail of the neutral fighter or aerobat). Depending on this quantity, trim may be up down or not at all either way. For your standard trainer type, the wing and fuselage have a pretty big nosedown pitching moment at least at low speeds, so the tailforce is probably pointing down. The same applies to many transport category aircraft, which often have static margins (for the uninitiated, the percentage of MAC that the C.G. is ahead of the neutral point in excess of 60-70%. In any case, ideally, we'd like the stab to produce no lift up or down at trim condition, as lift from the stab, which is usually a pretty low AR wing, makes a lot of induced drag (anybody heard of trim drag?) But with a symmetrical airfoil (CM=0), and a fuselage with no nose or tail downsweep, it would be quite likely that the tail would have to produce a subtantial upforce to counter the positive pitching moment of the fuselage.

2) If the aerodynamic center of a main wing is behind the CG, it will produce a nose down moment of the aircraft. The only way to keep it level, then, is to create a nose up moment- for an aft stabilizer, its lift MUST be downward (the idea of less lift than the wing is nonsense); for a forward located canard, its lift MUST be upward. If both are utilized, then the combination of the two must produce a nose up moment. Again, this is straight and level flight (static stability).

All true, except that your forgetting that it is quite possible to have a stable airplane without having the C.G. ahead of the wing's AC. Most airplanes with a reasonable tail volume coefficient in fact do not. Go measure your R/C models and tell me how many of them have the manufacturer's recommended C.G. point ahead of the quarter-chord. Then check where you actually fly it. I don't know many experienced pilots who don't move it back even further for better response and more speed. I'll lay even odds that most gliders or aerobats you have have the C.G. aft of the wing's AC. This often doesn't apply to racers and warbirds, because of their absurdly small tail volume coefficients and even smaller tails from the long moment arm.

In considering the above, it would seem more efficient to use canards than traditional aft stabilizers- this is true, however canards make the aircraft dynamically unstable. For example, if you give nose up command to a stab/elevator (stab goes down relative to the CG), after pitching upward, the stab lift will increase and natually bring the plane back to level (after possibly some Phugoid oscillations, or damping motion). On the other hand, if you give a nose up command to a canard/elevator (canard goes up relative to the CG), the lift on the canard will increase and cause the plane to pitch up even further. This divergent condition will eventually cause the plane to stall... or worse, and it is nearly impossible for a human to controll without fly-by-wire. This is why, even considering the weight implications, it might be advantageous to use both an aft stab AND an eleveator, so as to create an optimum lift condition and maintain longitudinal stability.

Please explain to me the Long-Eze, then. Lots of those flying around, and the only FBW on them is the electric servo-operated elevator trim. It is quite possible to make a stable (both statically and dynamically) canard. The trick is not the longitudinal stability, but the lat/dir stabitlity. The stall can be ugly, and it is extremely important that the canard stall before the wing, but other than that, the Equations of Motion for a canard are quite straightforward. If you specialized in S+C (I do too, BTW, among other things) you most likely have a copy of Etkin or Nelson or some other S+C text laying around. I suggest you consult it before making such wildly inaccurate blanket statements as this one.

I could go on much longer, but I've addressed this thread- in short, the original post on the F-16 simulator with positive lift stab could be accurate if the sim is modelling a negative stability condition (CG behind the AC).

If anyone has any questions, I will be glad to give some more insight on stability and control.

There, I'm finished- sorry for the vent

Careful... when venting, it's a good idea to be right...

Craig-RCU

Thank you again for participating George. I guess if you are done with this thread, I will have the last word between us. If I may remind the readers of this thread, George never directly explained to me why if canards and tandem wings can have all lifting surfaces what limits conventional designs from doing it. This I feel is important to address. Instead he sidestepped that with:
quote:
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Originally posted by mulligan
If you can visulize that a plane with a CG ahead of the main wing AC (most planes) will naturally nose down, how could a stabilizer (or anything for that matter) located further aft, and hence aft of the CG, keep the nose from falling unless it is pushing down? Everything rotates around the CG, right? Wing goes up, nose goes down... to keep nose up, aft stab must go down.
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I then showed that the hstab of a conventional plane actually lifts up when a conventional plane is balanced aft of the 23-25% MAC of the main wing. With the folding hstab experiment. Reasoning that since a flying wing's AC is at 23-25% if you fold up the hstab of a conventional plane in flight it then becomes a tailheavy flying wing if it was balanced aft of 23-25% at takeoff (if it was ballanced right at 23-25%, it would be neutrally stable with the hstab folded). The new tailheavy condition will cause the plane to diverge nose-up since the AoA and/or lift angle of the main wing is already positive at straight and level cruise. If the hstab is then unfolded it must lift up also to counteract this nose up divergence.

I would call that last arguement the last nail in the coffin to the idea that hstabs can't produce lift. Is this reasoning flawed? I don't see it. Maybe someone could show me the errors in my reasoning if they exist.

Also I don't think it is fair of George to say that I am misguided and not take the time to show me the errors in my thinking.

Craig-RCU

Here is the gist of what we learn fron the folding stab experiment

The CG does NOT have to be ahead of
the aero center of the WING (which would indeed require a
down-pushing tail)

Instead, the cg must be ahead of the aero center of THE WING AND TAIL
SUMMED TOGETHER.

Once you add any amount of hstab area behind the main wing, you will be able to move the C.G. rearward of the main wing's AC without loosing stability. The more area added to the hstab the more you can move the C.G. back. When the area of the hstab equals the area of the main wing you have a tandem wing. Add even more area and you have the main wing smaller than the hstab which is a canard.

Craig-RCU

Thanks Starfire, for your input here.
If you are talking Cessna 172 as your standard trainer type, the wing on a Cessna is at least a couple feet above the C.G. and the induced drag caused by the wing's lift would create a significant nose up moment especially at low speed when the wing is at its highest AoA. For what it is worth, the simulated Cessna (screen shot on first page) showed a lifting tail even with the flaps down at stall speed. Really though, only Austin Meyer X-Planes programmer knows how accurate his simulator is. You would have to exammine the code yourself to determine its accuracy of the simulator. In defense of X-Plane http://www.x-plane.com/ , it has recently been certified by the FAA training toward airline transport certificates.

So you are saying that the off-loading of lift of the main wing to the hstab is not worth the penalty of induced drag that the low aspect ratio hstab would produce in a lifting condition? This is interresting and leads me to wonder why airplanes are designed with low aspect ratio hstabs. If the hstab had a high aspect ratio(same as the main wing), they would not be so draggy when producing lift (at least no more draggy than the main wing). In fact, tandem wings by definition have a fore and aft wing of the same size and aspect ratio. So, what gives with the low aspec ratio hstabs? You'd think that with the low profit margins that airlines have they would be looking for any amount of aerodynamic efficiency the could find. Instead of just carring around the weight an hstab only to provide pitch stability they could be making that hstab really work for its supper (so to speak). Can anyone clarify?

HarryC

My Feedback: (1)

For a given area of wing, one single wing is more efficient than 2 half wings. One wing has 2 tips with the attendant loss of lift and extra drag, two wings have 4 tips. We evolved away from biplanes as soon as we could. The most efficient cruise is one wing, no tail. Since we usually carry a tail the most efficient setting is the tail producing no lift so no induced drag only profile drag and that is best done by a symmetrical section at 0 degrees AOA. That may not be 0 degrees incidence if the tail sits in the downwash. If the tail is used to provide lift to keep the aircraft up, then it will create more induced drag than the extra induced drag that one main wing would in creating that same extra lift.

Stall point is affected by aspect ratio, lower aspect ratios have later stalls. Tailplanes would be a disaster if they stalled before the main wing since the tail would drop, putting the main wing into a stall, possibly unrecoverable. By using a lower aspect ratio than the main wing, the main wing will stall first, drop the nose and help put the aircraft back in a safe position.

Harry

Champ-RCU

My Feedback: (132)

Dear Starfire,

re: your post of 05-01-2002 at 3:39PM

In your 4th paragraph you state "I don't know many experienced pilots who don't move it (the C.G.) back even further for better response and MORE SPEED." (CAPITALS ARE MINE)

Please explain how in a conventional aircraft with a lifting horizontal stab. you get more response and more speed.

Mark

Craig-RCU

You know all that sounds good on paper, but in practice, the only plane to ever fly around the world unrefueled used two lifting surfaces(ie. Rutan's Voyager). Are you saying that Rutan is just a glutton for punishment and made things extra tough for himself by designing the Voyager to have two lifting surfaces?

I understand that the low aspect ratio hstab provides an extra level of stability in theory, but when is the last time anyone here flew a tandem wing aircraft and found it so gosh darn unstable and divergent that they just gave up on the whole idea. I am beginning to wonder how much fashion and style play a part in aircraft design and aircraft designers just think tandems look too odd so they don't build them. I think this is a valid point to consider since planes like the the Beech Starship and Avanti don't sell because they look too futuristic not because they don't fly well.

Johng

My Feedback: (4)

Voyager is not a great example to use for this thread, as it was a moderately unstable airplane - which is what happens when you lightly load the canard - for efficiency purposes. The pilot workload stabilizing the plane was unbelieveably high for a flight that lasted for days.

Otherwise, I believe Mulligan's initial error was in this statement:

99% of all planes are positively stable- that is the CG is located in front of the aerodynamic center of the main wing.

I'm assuming he meant the aerodynamic center of the entire aircraft, not the just the wing. Confusing those two causes big problems in this discussion.

Airplanes can indeed be positively stable with the CG behind the AC of the wing. I am flying a Stinger now with the CG set at nearly 33% of the wing chord for 5% stability. For a symmetrical wing, the AC is at 25% chord. Don't try this at home, as mine has enlarged tail surfaces. It behaves just how my calculations predicted, slightly stable but sensitive.

The horizontal tail size and aspect ratio has a lot to do with it. As you make the tail bigger you can move the CG further back. I also have an ooooold flying boat design that is based on free-flight design from way back. The horizontal tail is about 40% of the main wing area. It balances at 60% chord for flight.

Imagine that as you morph the tail larger and larger at some point you get to where you have a tandem wing design, and the CG should be located somewhere slightly forward of the AC FOR THE COMBINED AREA in order for it to be stable. Keep the design morphing and pretty soon the rear wing is bigger than the front and presto, you are talking about a canard. Same principles apply.

Secret #1453 : The aerodynamic center of the aircraft is not just based on relative areas, but also the interactions(downwash, etc) of the various components. These must be accounted for in stability calculations.

Lastly, the simulated aircraft force vectors from x-plane can't be treated as meaningfull without reviewing the calculations that they are based on. They might be right, but they shouldn't be treated as "proof" of anything.

Craig-RCU

.
I think you missed my point for bringing up the Voyager. Starfire said that two lifting surfaces were not as efficient as one. I brought up the Voyager to point out that this two lifting surface plane is the most efficient powered plane built by Man to date. Which really has nothing to do with whether or not pilots like to fly it or not. I guess I changed the subject of the thread, but hey, I started this thread

Jeremy Sebens

As to the response question, moving the C.G. Back dereases the negativity of the aircraft's CMalpha (pardon the lack of subscripts) term. Essentially, the more negative this term is, the more stable the airplane is. A more negative CMalpha also requires more elevator input to hold a given angle of attack, however, so a very stable airplane has a much more limited range of acheivable alphas. Since alpha is what defines acceleratons in the z axis of the aircraft, and since it is these accelerations that give us manuevering capability, a less stable airplane (CG further back) will be more responsive.

The faster issue is a little more complicated. There exists a case for most aircraft where the aircraft will trim with no lift being produced by the tail (this is only true for one airspeed, mind you.) For a lot of extremely stable aircraft, this may be acheived by moving the CG a bit more aft. This question all depends on how the airplane's trim plot looks to begin with, though. I'll be glad to speak more to this issue if this doesn't slake your curiosity, but I'll have to draw some pictures to attach.

As a case in point, it is not uncommon for glider pilots (both R/C and full scale) to balance their aircraft further back to increase speed and best L/D. It's a long experimental process, as it is quite possible to go too far and start increasing drag again, or worse, go unstable, so it requires a fine touch.

Jeremy Sebens

On the Voyager (which is BTW, the one of the most awesome aircraft to date, and a homebuilt to boot), it is indeed a special case. My implication was not that one wing is better than two, but that on conventional aircraft , it would be nice to have the tail produce no lift. If both wings (tails, canards, whatever) have roughly the same aspect ratio, than the most efficient case would probably be (and I'll have to run some numbers on this) with both wings having the same wingloading. This would acheive a minimum of total vorticity and therefore induced drag.

In the case of the Voyager, I think that it might have been unstable dynamically (With that huge L/D, it'd probably take a SAS to stabilize the phugoid), but I don't think it was statically. In fact, I think I can give evidence that it was stable. On one of the test flights, the airplane entered a rainstorm and began descending. Nothing seemed to bring the nose up. It turned out that the rain had spoiled the super-high-lift canard airfoil and it could no longer produce enough lift to hold the nose up. This indicates that the canard operates in a lifting capacity for normal flight, which would seem to me to indicated positive "pitch stiffness" or static stability. Not sure though - it might depend on the ration of loadings of the two wings. I'll think on it some more before rambling at any greater length on the topic...

Hey, wait a minute... wasn't it IFR capable? You can't certify an unstable airplane for IFR, can you? Ah, well... I suppose I can ask Dick Rutan about it this year at Oshkosh...

MiL

phew, i just read this whole thread... i think i need some tylenol and a nap now!

Craig-RCU

MiL, LOL

Why exactly is the Voyager a special case that we can't compare it's efficiency with other aircraft? Is it the airfoils? The C.G.? I don't see what you mean. We can measure the lift/drag ratio or glide ratio (flying efficiency) of anything that movess through the air (birds, bricks, lifting/nonlifting hstab aircraft) And I stand by saying that the two lifting surface Voyager is Man's most efficient i.c. powered flying machine thus, two lifting surfaces can't be all that inefficient for any aircraft (conventional or canard). Unless anyone has data to prove otherwise, I've got to believe that unloading the main wing with a lifting hstab is more efficient than not for any existing conventional plane.

O.k. it was Harry C. that said two lifting surfaces were less efficient. But you said quote:
--------------------------------------------------------------------------------
Originally posted by Starfire
In any case, ideally, we'd like the stab to produce no lift up or down at trim condition, as lift from the stab, which is usually a pretty low AR wing, makes a lot of induced drag (anybody heard of trim drag?)
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This seems to me to be saying --It is not efficient for the hstab to produce lift.

You also say that you could get more speed with a lifting hstab (I agree, because you offload the main wing which reduces its AoA for any given airspeed by useing the hstab for both lift and stability). So what is "nice" about having the tail produce no lift? In order to get more speed from the same HP drag must be reduced. More speed, less drag---that is what I would call nice. Also, more efficient.

Champ-RCU

My Feedback: (132)

Starfire, That's what I thought. Sh*t flows down hill and the Arc was designed and built by Amatures while the Titanic was designed and built by experts. On the aforementioned continumm (Poor Spellers of The World --Untie) if you have a very aft C of G you are unstable if you have just alittle aft C of G you are stable if you are balanced on the aircrafts center of lift you are stable gut less manuverable (?), if you have aforward C of G you are more stable but not as fast? My kids seesaw says I gotta have a BIG glass of milk to swallow that.
Or as my poor old dead Father in Law used to say "to err is human but to really screw up you need a computer." (I knew that would come in handy some day!)
Isn't this a wonderful Country! Later Dude.

Jeremy Sebens

OK... the reason the Voyager is a special case (I should have been more specific) is that the wing and the canard have roughly the same aspect ration - makeing them have roughly the same k factor (1/(pi*e*AR); e=Oswald efficiency factor ) which is the measure of how much induced drag a wing produces. Therefore, they're have roughly the same efficiency, so we have no preference as to where the lift comes from. In point of fact, most canards share this equal-AR configuration - take a look at the long skinny stab on a Long-Eze sometime. This is mostly the case so that the canard will stall before the main wing (It's very bad if the wing stalls first - in general we want to keep the pointy end INTO the wind).

The reason a conventional aircraft should have NO lift (or at least very little) from the htail is that the htail has a much lower AR than the main wing in most cases. Because of this, lift from the htail costs more than lift from the wing in terms of drag. This goes for lift that's pointed down as well. Ideally, we'd like that lift (and the relatively large induced drag term which goes with Lift squared) to go away for best efficiency. But this only applies to standard tails with a lower AR than the wing.

I'll get to the stability versus responsiveness issue later... must go take a Structures exam.

Hal deBolt

Hi Craig,
Two things first.
1. J have designed,built and flown a mess of models of all types
over my many years, this lifting tail saga is a highlight of my experiences.
2.It is hard to relate what full scale does (uses) to models from an
observation made years ago. It sometimes seems they have a
penchant for doing things the hard way. For example a pilot not
only uses the main controls but he also is constantly trimming the
craft for the various modes of the flight. Model experience would
suggest proper design would simplify all that.
Lifting tails: They go back to early free flight where they became
mandatory to controlling the speed differential between power and glide while also contributing some desired lift.
Myself: When R/C was in the embryo stage models were more or
less free flight style. My first r/C was one of that type, Experience
soon showed a great need for ,lets say. level flight. Details would
be extensive. So a desire was born for "level flight" over a wide
speed range. Actually this was easily accomplished and is so desireable all my designs have featured it.
The fact that a horizontal tail is nothing more than a wing and thus can and does produce lift is the clue. That lift can be put to work,
As speed increases so does lift with ANY surface.
The htail is used to stabilize the wing.
One need is to control the airfoils natural down nose tendicy.
Neutral point is the center of lift of ALL surfaces.
Stability requires the center of gravity to be forward of the neutral point.
With a liting tail the neutral point is BEHIND the wing's C/L.
Considering item No. 3, if tail lift is present this control can be achieved by having the tail force (leverage, etc) somewhat less than the wing's force.
A pattern model design is a good example of this design philosphy. An excellent pattern design will fly from take off thru all
maneuvers to landing without a trim change.
How is it accomplished: Tail lift is adjusted to match wing lift, that
is a proportion is struck between them. When properly achieved with speed changes wing lift changes and with the proportion so
does that tail lift, thus the line of flight remains constant.
The proper proportion of tail lift is achieved using several factors.
Area, airfoil and incidence. What and how much is chosen from the
efficency factor, mostly considering what creates least drag.
I could go on and relate to various models but expect you will
have gotten the drift. Others sure have offered much.

Hal deBolt

drela

One thing that needs to be corrected here is the confusion about the aerodynamic center (AC). This is NOT the center of lift, sometimes called the center of pressure (CP). Many posters here assumed that these are the same.

If the CG is ahead of the wing's CP, then the tail is downloaded to balance the nose-down moment.
If the CG is behind the wing's CP, then the tail is uploaded to balance the nose-up moment.

For stability, the CG must simply be ahead of the AC, which is a different matter. This is possible with either an uploaded or a downloaded tail, depending on the wing and tail areas, tail moment arm, and the wing airfoil mean mean aerodynamic chord and pitching moment.


Here are some related facts about tail upload/download:

If the wing has the usual negative pitching moment, then a very small tail must indeed be downloaded for positive stability.

But as the tail size is increased, more and more upward load can be added to the tail while maintaining a positive pitch stability. At some particular tail size, the net tail load can be upward.

But let's keep going...

The tail size is increased until it is much larger than the front wing, in which case its total upward load can even exceed that of the front wing. All while remaining stable. This extreme case is nothing more than the usual canard setup. But the load/area (i.e. the CL) of the front wing must always be greater for stability as another poster has pointed out.

- Mark