Aircraft with a 'lifting' tail

[18-Wheeler]

I'm looking at getting back into model aircraft as a hobby (well it's cheaper than the real thing at least) and the kit that I'm going to get, a Hobby Lobby Senior Telemaster, they say the tail generates lift instead of the usual downforce.
I can see how it would work in static conditions, but I can't figure out how it could work dynamically, i.e. with changing airspeed and/or AoA. It should be dynamically unstable.
I can also see how with very narrow CG to CP ranges it would work but again not be dynamically stable.
.... or am I missing something?

[172_driver]

I am not the person to explain, but this article (http://www.av8n.com/how/htm/aoastab.html) is very interesting and explains in easy terms CG range vs. stability. Read 6.1.1. through 6.1.6 at least.

Interesting read in 6.1.6, the author describes how a C172 at forward CG the tailplane produces downforce and at aft CG produces lift. This was apparently done by attaching strings on the tailplane and notice the direction of the vortices. In any case, the aircraft has static and dynamic stability (as one would expect within the CG range ;))

[18-Wheeler]

Okay I worked it out.
I drew a few diagrams of lift/weight and the tail in various configurations and I reckon this is how it works -
The CG has to be fairly close to the CP, but far enough ahead so that with a high AoA the CP doesn't (shouldn't) go behind the CG. The tail would have to have zero or a very low angle of incidence and the wing a pretty good one, say, 4° or so.
That's the only way I can see the system working even close to being dynamically stable.

[Machdiamond]

All airplanes I have come across in detail and analysis have their tail producing positive lift at aft c.g. and are still statically and dynamically stable. It is a common misconception borne of the simplistic explanation of stability found in most textbooks.

The link provided by 172 is, on the other hand, excellent.

[cwatters]

If I remember correctly most books on model aircraft aerodynamics consider the lifting tail to have been a mistaken fad. I think the argument goes that the CoG has to be too far back for good stability.

Some models with so called "lifting tails" do indeed have a tail with wing like lifting section but once trimmed out they end up flying at a negative angle of attack so they still produce a net down force. In other words in some cases it's a nonsense as far as efficiency goes - but efficiency isn't important on a model.

I suspect the good flying charateristics of the Telemaster owe more to it having a relatively long tail rather than a "lifting tail".

[18-Wheeler]

I wouldn't argue with any of that, cwatters.

[Dont Hang Up]

All airplanes I have come across in detail and analysis have their tail producing positive lift at aft c.g. and are still statically and dynamically stable. It is a common misconception borne of the simplistic explanation of stability found in most textbooks.


Another 'simplistic' theory is that if you move pitch control surfaces to the front (canards) then they can provide lift, stability and make the aircraft 'stall-proof'.

I guess there is a flaw in that theory somewhere otherwise all aircraft from models to A380s would be made that way.

[18-Wheeler]

I guess there is a flaw in that theory somewhere otherwise all aircraft from models to A380s would be made that way.

Not really, as it's harder to build a practical aeroplane in that configuration.

[Machdiamond]

Practicality is a red herring.

Canards are not more efficient, it is as simple as that.

For dynamic stability with a positive lift on the tail, consider that as long as the rate at which lift increases with angle of attack on the front wing is higher than it increases on the rear wing, it does not matter which one from the front or the rear is bigger. The rate difference - the lift curve slope difference - is usually and primarily achieved with aspect ratio.

So when you hit an upward gust, the rear wing produces more lift and the aircraft pitches down. Dynamic stability.

[18-Wheeler]

Practicality is a red herring.

I don't think so, as is it more difficult to build a pusher-type that works as well as a conventional aeroplane, due to the requirements for prop ground clearance, etc.



Canards are not more efficient, it is as simple as that.

I doubt that very much and require proof.




For dynamic stability with a positive lift on the tail, consider that as long as the rate at which lift increases with angle of attack on the front wing is higher than it increases on the rear wing, it does not matter which one from the front or the rear is bigger. The rate difference - the lift curve slope difference - is usually and primarily achieved with aspect ratio.

Yes that's what I worked out several posts ago.

[Jane-DoH]

Machdiamond

All airplanes I have come across in detail and analysis have their tail producing positive lift at aft c.g. and are still statically and dynamically stable. It is a common misconception borne of the simplistic explanation of stability found in most textbooks.

Why has this misconception occurred?

[henry crun]

In his book Model Aircraft Aerodynamics, Martin Simons says a lifting tailplane produces tip vortices and thus drag, and the lift generated by the tail is not enough in proportion to make this penalty worthwhile.

[Jane-DoH]

henry crun

Yeah, but doesn't a regular tail produce it's own vortices too (even if it's negative lift you have a pressure differential at the tips which would spill over into a vortex)?

[Ptkay]

Best example of the effectiveness of the lifting tail.
The fastest prop aircraft:

Piaggio P.180 Avanti - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Piaggio_P.180_Avanti)

[Ptkay]

The Piaggio P.180 Avanti design utilizes both a T-tail and a pair of small, fixed anhedral forward wings that lack control surfaces. The arrangement of the wing surfaces allows all three to provide lift, as opposed to a conventional configuration, where the horizontal stabilizer creates a downward force to counteract the nose-down moment generated by the center of gravity being forward of the center of lift. This is patented as "Three-Lifting-Surface Configuration" (3LSC).

[Machdiamond]

I doubt that very much and require proof.

I wish I could find simple ways to explain that. As already pointed out, the main reason canards proponent say that it is more efficient is that conventional tails must provide negative lift. It is a fact that the statement is not true. And even if it was, we are dealing with small amounts of force. At normal cruise speeds, induced drag is small compared to parasite drag so the additional induced drag on the main wing created by the need to compensate a small negative load on the tail is actually falling into the noise levels (again, at normal cruise speeds). On the other hand the canard has to deal with a wider spectrum of pitching moments (see why below) and must therefore be larger than a tail in the back. More wetted area, more parasite drag, less efficient.

So what else is left for Canards to be more efficient? Nothing that I can see.

On the other hand the list of severe deficiencies is long. I will name just one:

With landing flaps, the wing camber increases dramatically and a substantial pitching moment appears. What is neat with the tail in the back is that with flaps down, the downwash greatly increases as well. That increases the down force on the tail and neatly balances that pitching moment due to the flaps. On well designed airplanes, very small amount of trim is needed when lowering the flaps.

What do you do with the canard then? It does not compensate the new pitching moment since it's so far ahead of the wing. The upwash is comparatively negligible.

The Starship has variable sweep canard for that reason. Variable sweep planforms are not for the faint hearted. Heavy and expensive.

Yes that's what I worked out several posts ago.

I did not read it that way. For dynamic stability, it is the lift curve slope difference that is the key, not the angle of attack delta.

[CliveL]

I would like to offer some comments/explanations on several points in this debate:

You have to distinguish between static trim and stability. It is perfectly possible to design and fly an aeroplane with a tail that gives positive lift and that is stable. When I was a lot younger – in the days before most models were R/C in fact, we used to fly our gliders with the CG almost on the wing trailing edge – and they were stable. Try telling anyone who competed in old fashioned duration contests that efficiency isn’t important on a model!


Exactly the same principle applies full scale, only there they call it relaxed stability and get it by transferring fuel for cruise. How does it work? Basically because if you move the CG aft the nosedown pitching moment of the wing lift about the CG is reduced, so the amount of download you need on the tail to trim the aircraft is reduced. (Remembering that a typical wing section gives a significant nosedown moment even at zero lift).The TOTAL lift has to be equal to the weight, so with a big download on the tail the wing has to provide more lift to balance. This puts the wing lift dependent (induced) drag up and in addition the tail induced drag is high because it also is producing a lot of lift so the overall efficiency is reduced. If you take the aft CG movement far enough you get to a situation where the tail actually produces positive lift at the trim condition. As a matter of interest, Concorde had a lot of CG transfer and actually flew in cruise with a small amount of ‘down’ elevon, which is the tailless equivalent of positive tail load - and yes it was stable.



Why can it be stable? Because what matters now is the CHANGE in lift and pitching moment when hit by a gust. Suppose one hits an ‘up’ gust; the wing AoA is increased and the lift increases. If the CP is ahead of the CG this will produce a noseup pitching moment which will increase the AoA still further, i.e. it is unstable. But the tail will also see this AoA change and its lift will see an upwards increment. It doesn’t matter whether the tail starts with an upload or a download; the change in AoA will either make it less download or more upload. Either way the airplane gets a nosedown pitch increment which depends on the tail area/wing area ratio and the tail arm. Since most tails have an arm about 3/3.5 times wing chord this will be a good solid nosedown pitch big enough to overcome the noseup pitch from the wing; the aircraft pitches down back towards the original (trimmed) AoA; i.e. it is stable.


Canards more efficient? Canards more difficult to produce a practical design? “Three surface” aircraft?
To answer the last two points first:
You don’t see many (any) commercial aircraft with canards, so there must be some important disadvantages. One problem would be that the necessary location for the canard is just where the passengers normally get on and off, and the canard would get in the way of moving jetways. Maybe the gains in efficiency aren’t as big as they are cracked up to be? I do know that Airbus had a very serious look at a three surface design, but concluded it wasn’t any advantage. On other designs (small aircraft) where this is not a problem you sometimes see canards.


Canards actually REDUCE stability. The arguments rehearsed above for tailplane contribution to stability go into reverse. Because the ‘tail’lift is ahead of the CG it produces a noseup moment in a gust. To make the aircraft stable the CG must be well ahead of the wing so that the wing lift now produces the necessary nosedown recovery pitch. OK, this means that to get TRIM, the aircraft must fly with upload on the canard and that must be good, mustn’t it? Yes so far as it goes, but in producing that lift the canard generates its own downwash which acts on the wing to reduce wing lift. Again to balance the total lift against weight the wing must be made to fly at a higher AoA which reduces or eliminates any drag gain.


So why have canards at all? Why do so many fighters have them? Because they give more rapid response to pitch commands and because the lift they develop is an instantaneous generator of ‘g’ whereas an aircraft with a rear tail suffers from an initial negative lift and must generate a positive pitch angle to get the AoA and lift necessary to pull the ‘g’.
Do canards give a ‘stallproof’ airplane? I once had a long discussion with a USAF colonel on this one, where he claimed that one of the Rutan designs was in fact stallproof. So it was, but only because the canard stalled before the main wing and limited the control power to less than was needed to get to the wing stall condition!


Hope this is not too confusing!

[SpanWise]

I'm looking at getting back into model aircraft as a hobby (well it's cheaper than the real thing at least) and the kit that I'm going to get, a Hobby Lobby Senior Telemaster, they say the tail generates lift instead of the usual downforce.
I can see how it would work in static conditions, but I can't figure out how it could work dynamically, i.e. with changing airspeed and/or AoA. It should be dynamically unstable.
I can also see how with very narrow CG to CP ranges it would work but again not be dynamically stable.
.... or am I missing something? Hi Machdiamond,

The horizontal stabiliser producing +ve lift does not automatically mean the aircraft is unstable.

In the case where the horizontal stabiliser produces +ve lift, the CG is behind CP. The CG can move rear behind the CP up to a certain point where neutral stability happens (aircraft nose stays where it is disturbed to), moving it any further back makes the aircraft unstable (it continues to pitch in the direction of disturbance). Positive stability region is still there while CG is aft between the point where CG and CP coincide and where neautral stability happens. Therefore, it is possible to have a stable aircraft with a CG rear of CP up to a certain point, in other words, a "lifting tail" can also be a stable aircraft.

[photofly]

The rate difference - the lift curve slope difference - is usually and primarily achieved with aspect ratio.That can't work. The only way it can work is for the wing at the back to have a higher angle of attack than the wing at the front".

Example: if the pitch of the aircraft increases by 2 degrees, the angle of attack of both wings changes by two degrees. If the wing at the front was previously at 3 degrees it is now at 5, and its lift has increased to 5/3 of its previous value.The wing at the back was at 1 degree and is now at 3 degrees, and so its lift has increased by a factor of 3. Net result, a nose-down force. Aspect ratios don't come into it.

(See John Denker's treatment at Welcome to Av8n.com (http://www.av8n.com) as previously posted.)

Canards are not more efficient, it is as simple as that.All else being equal, canards are more efficient. Why? Stability requires the wing-at-the-front to fly at a more positive angle of attack than the wing-at-the-back. If the wing-at-the-front is Canard then both wings are always flying with a +ve angle of attack and generating lift. This is more efficient than a tailplane which (most of the time, depending on c-of-g position) flies with a negative angle of attack,, generates downforce and requires more lift (and hence more drag) from the main wing.

EDIT: I accept Machdiamond's points about why, in practice, canards tend not to be more efficient; I take away from what he says that the practical difficulties involved in implementing what seems to be a "good idea" mitigate most if not all of the gain.

Another 'simplistic' theory is that if you move pitch control surfaces to the front (canards) then they can provide lift, stability and make the aircraft 'stall-proof'. If you have canards you can't get close to max alpha of the main-wing because the canard will stall first (bad). That means slow flight is out of the question, and landing speeds are high.

The Avanti appears to have the the best of both worlds (like the King Katmai C182 conversion - look it up if you've not heard of it) at the expense of extra complexity.

For dynamic stability, it is the lift curve slope difference that is the key, not the angle of attack delta.
Can you give more detail? It seems clear that it is in fact the angle of attack delta that is the key, as described above.

[fantom]

I was informed by a fellow Co. pilot (he was a test pilot Eurofighter, by the way) that there were no tail-lifting aircraft except those with a config similar to the Typhoon.

I can't argue.

[Machdiamond]

Can you give more detail? It seems clear that it is in fact the angle of attack delta that is the key, as described above

Your example does not work at all. Let's assume that instead of starting from 3 degrees you start at 1 degree. You pitch up two degrees so now you have 3 degrees, do you really believe that suddenly the wing produces 3/1 or three times as much lift than it did before? And what happens if you start from zero degrees and you go to two, that's infinite acceleration to the stars now. Keep in mind that when dealing with dynamics, you are dealing with derivatives, not with static values.

[Machdiamond]

Rereading my prior posts, I have to confess that my attempts at explaining longitudinal dynamic stability are not good to be of any use. I should probably stick to what I know :\

Nonetheless, I stand by the rest. Tail lift, canard etc.

[photofly]

Your example does not work at all.I think it does.

Maybe I should get one source of confusion out of the way, by saying the "zero" angle of attack is the angle of the airfoil that generates no lift. In that it differs from any other angle of attack definition by a constant angle it's just a convention as to the zero you measure from. Let's assume that instead of starting from 3 degrees you start at 1 degree. You pitch up two degrees so now you have 3 degrees, do you really believe that suddenly the wing produces 3/1 or three times as much lift than it did before?Yes, it does. For "sensible" values of alpha, lift is directly proportional to alpha. And what happens if you start from zero degrees and you go to two, that's infinite acceleration to the stars now. Zero angle of attack -> zero lift. Two degrees angle of attack, yes, the lift has increased "infinitely". You'd never be flying with angle of attack of the main wing though, you'd be accelerating towards the earth, because zero angle of attack -> zero lift. Keep in mind that when dealing with dynamics, you are dealing with derivatives, not with static values.The derivative of lift with respect to alpha is almost dead-on a constant value, reflecting their linear relationship, up to near the stall.

If you'd rather use a different definition of the angle of attack, say based on the chord, then define another quantity as the "angle of attack minus the zero-lift angle of attack". The analysis works just as well.

I have to say, it's not my treatment - it's John Denker's. I'm curious to see if you can fault it though.

[Machdiamond]

I can't, I was mistaken.

[18-Wheeler]

Interesting stuff guys, thanks.
Too much to reply to, so I'll just read most of it.

[18-Wheeler]

... and while we have some aerodynamic gurus here, would any of you like to have a go at this one please?
http://www.pprune.org/tech-log/451827-pusher-prop-vs-tractor-prop.html

[glhcarl]

What most of you are calling the tail is actually the "horizontal stabilizer". It could be on the aft end of the aircraft of the front (canards). Since its purpose is to stabilize the aircraft in the pitch axis sometimes it creates lift and sometimes it creates down force. Simple as that.