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Which aircraft have pos/neg elevator lift?

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Which aircraft have pos/neg elevator lift?

Old 27th Mar 2009, 19:18
  #21 (permalink)  
 
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been told that the MD11 does!
tail tank and trough autopilot only!?
not sure
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Old 27th Mar 2009, 19:37
  #22 (permalink)  
 
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Concorde of course used trim tanks to cruise with essentially zero elevon deflection.

Of course, there the issue was complicated by the significant aft shift of the centre of lift during the transition from subsonic to supersonic, and a requirement for essentially zero trim drag during supersonic cruise.

CJ
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Old 27th Mar 2009, 19:56
  #23 (permalink)  
 
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Somebody should tell Burt Rutan that you have to have a tail down force!

I was under the impression that for positive stability, the centre of gravity should be forward of the centre of pressure. How the designer chooses to solve this is up to them.

You can have tail lift at aft c of g and down force at forward c of g.
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Old 27th Mar 2009, 21:29
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Modern fighter aircraft mostly have all positive lift on all surfaces. But then again, they are unstable/unflyable without computers. The all positive lift gives them capability to sustain high G's in turns. Proof of this is the stabilizer of the F-16. From block 25 on I believe they enlarged the stabilizer so it created more lift and the aircraft could carry more load.
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Old 27th Mar 2009, 21:34
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Mad (Flt) Scientist
Quote:
Originally Posted by Dani
Are there some reasonable ways to guess from the trim setting if you are in the positive or negative range? Any types with a marker on the trim wheel or so?
You can make a guess based on estimated the angle of attack at the tailplane.

alpha(tail)=alpha(wing) + tail angle - downwash

Any time you have any significant amount of TE flaps deployed, the downwash will be large, AND you'll have a nose down moment which will usually require a significant negative tail angle. So alpha tail will basically "always" be negative for a flaps config, and so there will be a download.

For the cruise/zero flaps case, you could take a guess at the downwash being about half the wing aoa - and that is a guess. Therefore if the tail angle is greater than 0.5* the wing aoa, you might have a positive alpha-tail, and so upload on the tail.

If you actually KNOW the downwash number, you can use that and not just guess.

If you want a really wild guess, positive tail angle = positive tail load, negative tail angle = negative tail load. That's pretty much assuming that the AoA and downwash are both small in cruise - which isn't quite true, but not a million miles away either.
Don't forget that for anything but a symmetrical Airfoil the zero lift AOA will be negative so even with positive incidence a non symmetrical stab flying in down wash will be creating down force.
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Old 27th Mar 2009, 21:58
  #26 (permalink)  
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OK, so far so interesting...

Let's make it more complicated: Why do we de-ice the top of the elevator, since we know that it's the underside that produces (down)lift? If there is snow and ice on the top of the elevator, the question is easy to answer. If ice is accumulating on both surfaces, then the aerodynamically more delicate part is the underside!

Dani
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Old 28th Mar 2009, 03:58
  #27 (permalink)  
 
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Dani that is way too complicated.
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Old 28th Mar 2009, 12:18
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here's the maths
In what way do you believe that the analysis in that page justifies your assertion that:

In order to have positive stability in a convetional layout the tail must pruduce down force
?

Indeed in their summary, they say:

Despite the drawing above, many tail surfaces are normally loaded downward in cruise.

which merely fuels Dani's speculation that some are not. Denker's analysis in the page c172_driver cites is of course correct, and all that is required for stability with an upforce at the tailplane is that an increase in AoA produces a greater percentage lift increase at the tailplane than at the mainplane. For a symmetric aerofoil in a linear range, that usually means a lower AoA at the tailplane than the mainframe (decalage). For other cases, it's the equivalent expressed in terms of gradient of the lift curve rather than AoA itself.

The lift on the tail varies not just with loading, but perhaps more importantly with speed (and configuration), as MFS observes. At low speeds, the mainplane produces a strong nosedown pitching moment, requiring a high downforce from the tailplane to trim it out. So if there is to be a case at which tailplane lift is positive, that would be at the high-speed low-AoA and of the operating envelope, as well as with the C of G at the aft limit. That makes it hard to achieve, because with a low AoA on the mainplane, there is little scope for decalage. But it is nevertheless theoretically possible, and I would have thought it is something that a designer would strive to achieve to reduce drag in cruise. I cannot offer a specific aircraft/loading/speed example though.
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Old 28th Mar 2009, 17:36
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Having spent hours immersed in books and the Internet, I realise I may have been hasty in my observations!
Perchance I was misled by my aerodynamics instructor many years ago!

Last edited by rubik101; 1st Apr 2009 at 16:47. Reason: Further resarch needed!!!!
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Old 28th Mar 2009, 18:32
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All tailplanes produce negative lift.
All foreplanes produce positive lift.

True or false?
False

Cunard type aircraft ala the Beechcraft Starship, and Burt Rutans Long-EZ
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Old 28th Mar 2009, 19:01
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All conventional aircraft have tailplanes that produce downwards lift in normal flight.
Deflection of the elevators will change this dynamic for a short time, while the input is maintained.
If the tailplane produced upwards lift in normal, straight and level flight, any deviation would cause loss of control of the aircraft. A nose down pitch would be made worse by the lift of a tailplane producing upward lift. It would cause the aircraft to pitch even further forward. As this is not the case, then the assumption that the tail produces positve lift is false.
Get a wind tunnel and an Airfix model of a Jet Provost and see for yourself.
Anyone who thinks differently is reading the wrong book.
The design of aircraft and tailplanes/foreplanes is over 100 years old and hasn't changed since then.
Once again, FBW fighter may well be very different in that they are designed to be inherently unstable, but for conventional airliners, this is not the case.
Maths do not enter into it. Physics does.
It's a good thing pilots don't need to have a clue what makes their aircraft fly. If they did, pilots wouldn't last very long...

I think the practical reason why tailplanes usually produce negative lift, is the need for sufficient decalage in all parts of the envelope. To achieve sufficient decalage for the most rearward CofG, you end up having negative lift for a more forward CofG.

If possible, you'd probably prefer zero lift from the tailplane, when not maneuvering. Positive lift would mean you are generating induced drag, and the wing probably has a better lift to drag ratio than the tailplane. I.e., you want the lift to be generated by the wing, not by the tailplane.
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Old 28th Mar 2009, 19:21
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There are aircraft out there - either canard/foreplane configurations, or the 'double wing' types - where both airfoils produce lift in some normal flight regime.

Since the mathematics and physics behind the idea of pitch stability cares not one jot whether I call them foreplanes, mainplanes, tailplanes, wings or chickens, these aircraft alone are sufficient to show that download on the aft-most airfoil is NOT required for stability.

All that is required for stability is that the cg be forward of the neutral point (and manoeuvre point) for the configuration. Since the neutral point is derived by considering all the airfoil surfaces, it follows that it may well be aft of the aerodynamic centre of the foremost airfoil, and also may well be aft of the cp of that airfoil. In such circumstances there might have to be upload on the after surface to trim in pitch.

The key is that stability is concerned with the CHANGE in forces and moments in response to a disturbance - the absolute values matter for trim and control, not for stability. Which is why the branch of aerospace engineering concerned with these matters is called "stability and control" - both need to be considered, and they are not the same thing.
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Old 28th Mar 2009, 20:34
  #33 (permalink)  
 
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Red face

It is possible to have positive lift on the tailplane.This condition is met if the CG is aft of the centre of lift of the wing (roundabout quarter chord point) but not too far behind.This is because the combined wing plus tailplane centre of lift must remain ahead of the CG in order to ensure positive static stability.
I can assure you of this fact from experience.Many many years ago,as a flight test observer I was given the task of flight testing a so called tail load restrictor in a Lancaster.It simply froze the power controlled elevator motion when a certain upward tail load was reached (designed for bombers recovering too smartly from evasive manoevres).I tested over a range of CG's and at the aftmost the inevitable happened--we pulled sufficient g to lock the elevator in steady flight, but couldn't push the stick forward, because that would transiently
increase the upload further.Fortunately I had the foresight/luck to install a cut-out switch (I can see it until this day)
Keith
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Old 28th Mar 2009, 20:36
  #34 (permalink)  
 
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Could some one please post a commercial aircraft of standard configuration either past or present that in any trimmed regime produces either a) no aerodynamic force from its tail plane or b) lift from it's tail plane?

I understand the theory and also know that in practice the need for stability AND control mean that unless the tail volume is huge ie a twin plane layout then you cannot create an effective lifting tail. The problem with twin layout is that it's not as good as a single lifting surface. Too heavy and too much drag. BWB tailless is the ideal layout and it will come soon. Horten led the way in this layout.

Decalage is the difference between the rigging angle of the upper and lower main plane of a biplane.

Longitudinal dihedral is a better term for the difference in angle of incidence between main and tail planes.
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Old 29th Mar 2009, 01:17
  #35 (permalink)  
 
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Originally Posted by captjns
Cunard type aircraft ......


Cunard= Cruise liner company offering floating holidays.

Canard=French for 'duck' applied to aircraft with main lifting surfaces aft of any stabiliser.
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Old 29th Mar 2009, 03:53
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Cunard type aircraft ala the Beechcraft Starship, and Burt Rutans Long-EZ
They may have canards, but they aren't ocean liners...

It is possible to have positive lift on the tailplane.
The original question was
Which aircraft are producing negative lift on the elevator?
The simple answer is NO commercial airliners and NO production general aviation aircraft with conventional (wing-and-tail, non-canard) lifting surfaces, and NO military transport aircraft; only a relatively FEW military fighter aircraft!
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Old 29th Mar 2009, 08:34
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Intruder is correct.

Heres something to try at home:

Go out and tie some lead to the tail of your cessna. Just enough so that the cg is behind the centre of lift. Now wind in a bunch of forward trim and takeoff.
Surprise! It flys - and maybe even lifts off a knot or two early. Now if you are still alive wind the trim back as you accelerate in order to keep the nose down. Thats right - back. Get yourself up to a nice safe altitude if you can for the next bit. Trim the aircraft for level flight. Now reduce power slightly and do not make any pitch inputs. Observe the nose rising as speed reduces. And as the nose rises speed reduces more, causing the nose to rise until...stall! But don't worry; plenty of height to recover. Don't cheat by making any pitch inputs yet! Now as the nose drops observe the speed increase. And observe the tendency for the nose to keep pitching down as speed increases. Try and recover. You'll need to be very hands on.

And that is why all commercially produced, conventional aircraft with ordinary cambered wings always have an allowable cg range that is in front of the centre of lift. Hence always a downforce on the elevator when in stable flight (ie not maneuvering) throughout the allowable flight envelope.

On a canard the reverse is true. Always an upforce when in stable flight . This gives them the advantage of no trim drag. Why aren't they more popular? I don't know.

PS - Don't really try this!
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Old 29th Mar 2009, 10:52
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Next time you see a B737, have a look see which surface of the tailplane the vortex generators are nailed to.

btw, Cunard did operate an airline for a time. They bought out Eagle Airways from Harry Bamberg because he was stealing a lot of their transatlantic passengers.
Eagle bypassed the status quo/monoploy of the UK/USA airlines by registering the aircraft in and flying via Bermuda, much to the Government and BOACs horror.
Cunard Eagle Airways was quietly liquidated a few short years later, all the while having made a profit.
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Old 29th Mar 2009, 12:05
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Seems we're running in circles; still new replies appearing as to why tail downforce is required for stability, despite multiple explanations and references as to why that is not at all necessary.

I guess we can at least agree that most aircraft have a tail downforce in most parts of their flight envelope. How many that actually have a tail upforce in some remote corner of their flight envelope is still an open question (but if the C172 does, as demonstrated, then I'd imagine it can't be all that uncommon!).

But just to pick up on this line then:

On a canard the reverse is true. Always an upforce when in stable flight . This gives them the advantage of no trim drag.
There will be trim drag. When lift is produced, whether it is upwards or downwards, induced drag will always be generated.

But isn't the drag from the canard offset by the reduced lift requirement from the wing? No it's not! Because the wing is far more effective in generating lift than the canard is, you get an overall reduction in lift to drag ratio as more lift is generated by the canard rather than the wing.

It is true that a canard always has to produce an upforce in stable flight. The positive decalage requirement (the thing in the front needs a higher angle of attack than the thing in the back), together with the obvious fact that the wing always has to have a positive angle of attack [*], means the canard has to fly at a rather high angle of attack, always. The possibility for very low downforce, or even a slight upforce in some remote corner of the flight envelope, that exists for conventional aircraft, is not possible for a canard. Therefore, a canard always produces significant lift and significant trim drag.

To try to get rid of the canard trim drag while still maintaining positive decalage throughout the envelope, people are using canard flaps, variable incidence canards, variable canard sweep, or even retractable canards. Such devices are also needed to keep the stall speeds down and get reasonable takeoff and landing performance (and meet the 61 kt stall speed requirement for singles, if certification is desired). It adds complexity, and it adds weight.

So I guess that is why we see so few successful canard designs.

Now, can anyone sort out which surface produces what forces on the Piaggio Avanti? Canard, wing and tailplane...


[*]: ... with zero angle of attack defined as the angle of attack giving zero lift, as can be done without loss of generality.
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Old 29th Mar 2009, 15:04
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Rubik, where are the vortex generator on a 737? I only know these on the tail fuselage between the vertical and the horizontal stabilizer. I couldn't find one picture that showed me some on the elevator.

According our theory, they should be on the lower side?

bjornhall, good you mentioned the Piaggio Avanti. The most succesful (the only?) commercial canard design wouldn't have a for- and a tailplane, if the lift of both would cancel themselves out?

Are there really no genuine aerodynamic engineers on actual commercial projects out there on PPRuNe?

Thanks again for this very interesting discussion
Dani
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