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-   -   Which aircraft have pos/neg elevator lift? (https://www.pprune.org/tech-log/367457-aircraft-have-pos-neg-elevator-lift.html)

 Dani 26th Mar 2009 10:37

Which aircraft have pos/neg elevator lift?

In context of the Buffalo accident we had a discussion of basic aerodynamics on elevators.

It is widely accepted that "modern" aircraft have a negative lift produced by the elevator.

Question: What does modern mean? Which aircraft are producing negative lift on the elevator?

I stood under the impression that high performance gliders produce also positive lift, so to get a better total lift.
The same rational I applied for airliners with trim tanks in the elevator: To avoid negative lift, they use to equalize the trim by weight and not by negative lift.

True, not? Any list of aircraft that produce what amount of total lift on the elevator on a stabilized horizontal flight in cruise?

Thanks to the aerodynamic wizzards out there :ok:

Dani

 TyroPicard 26th Mar 2009 11:37

All tailplanes produce negative lift.
All foreplanes produce positive lift.

True or false?
TP (not a wizzard)

 Dani 26th Mar 2009 11:41

I remember making my RC models where I had to shape the profile of the elevator of a model glider. The profile was not a negative one. So would that be an exception?

Dani

 Mach Tuck 26th Mar 2009 12:18

I'm no expert the answer must lie in the relationship between the aircraft's centre of gravity and the aerodynamic centre of lift (c:g ahead of the c:l and the tail must have a downforce and vice versa).

More simply though, consider an aircraft's undercarriage configuration.

With a tricycle configuration the aircraft's c:g lies ahead of the maingear (else it'd fall over!) so there must be a downforce on the tailplane to rotate the aircraft into the climb attitude. Coversely, with a tail-dragger the c:g lies behind the maingear (else it'd fall over the other way!) so the tailplane must create lift to raise the tail.

I'm sure there will be exceptions to the rule - watch a B52 take off and you'd suspect the tailplane was producing lift.

As I said, I'm no expert - it could all be BS.

MT

 FE Hoppy 26th Mar 2009 12:48

In order to have positive stability in a convetional layout the tail must pruduce down force and the C of G must be ahead of the neutral point.

I'm not aware of any comercial aircraft which is not positivley stable.

 rubik101 26th Mar 2009 14:15

The tailplane with a faired elevator will produce lift in a downwards direction. If it produced lift vertically upwards, the aircraft would be dynamically unstable. A pitching moment causing the nose to drop would be made worse if the tailplane was producing vertical lift in an upwards direction.
The aircraft must correct the instability without input from the pilot to return to S&L flight.

Ignore all the above if you are a FBW fighter.

 172_driver 26th Mar 2009 14:30

The CG must be ahead of the neutral point, but that doesn't mean it can't produce a positive lift. The important thing is to have a greater angle of attack on the main wings than on the tailplane. That is what provides stability.

This site gives a very good explanation to longitudinal (angle of attack) stability: Angle of Attack Stability, Trim, and Spiral Dives [Ch. 6 of See How It Flies]

According to the author of the article above, you can even get a C172 to produce a positive lift on the tailplane with aft CG. You can read about the experiment in the article.

 Dani 26th Mar 2009 15:39

Thanks so far. I also appreciate the above mentioned site, where it says:

As the center of mass moves farther and farther forward, you will need to dial in more and more nose-up trim to maintain the desired angle of attack. At some point the center of mass will move ahead of the center of lift of the main wing. The tail will then need to provide a negative amount of lift in order for the torques to be in balance, as shown in figure 6.5. There is nothing wrong with this; indeed most aircraft operate with negative tail lift most of the time.
As "most" doesn't mean "all", there must be some aircraft with positive tail lift?

 ChristiaanJ 26th Mar 2009 15:43

Correct me if I'm wrong.

It is not a matter of download on the horizontal tail, but of the difference in angle of attack of wing and horizontal tail.

A straight-wing aircraft needs a horizontal tail with a negative angle of attack (lreading edge down) relative to the wing for dynamic longitudinal stability (leaving canards ou of the discussion).

Under normal circumstances that will indeed result in a download on the tail as well, although the C172 example shows it's not a given.

The story changes with swept wings, and a fortiori deltas. For various reasons these are (or can be made) dynamically stable in pitch, so there is not need for a tailplane for stability.
The average 'modern' airliner has a tail for improved control, trim, etc., less so for stability.
So I would expect under optimum cruise condition to see only small up- or downloads on the tailplane, and considerably less than on an equivalent straight-wing aircraft.

A trim tank can be used to move the CoG and hence minimize trim drag.

CJ

 Intruder 26th Mar 2009 17:14

ALL airliners are designed so there is downforce generated by the horizontal stabilizer in flight.

ALL commercially produced General Aviation aircraft that have horizontal tailplanes are designed so there is downforce generated by the horizontal stabilizer in flight.

Airplanes with canards may not have any downforce produced by a rear horizontal stabilizer (or may not have a rear horizontal stabilizer at all). Those that do not have rear horizontal stabilizers are typically designed so the canard will stall before the wing in most flight regimes (e.g., Vari-Eze).

Some Fly-by-wire military airplanes may fly without downforce on the horizontal stabilizer in some normal flight regimes (e.g., F-16).

 Mark1234 27th Mar 2009 00:02

Dani,

I think you may be missing a piece of the puzzle - It's not type specific, it depends on how the aircraft is loaded.

In a conventional wing & tail config, you're only likely to see tail download towards the rear extreme of the allowable COG - depending on the aircrafts aerodynamic stability the most aft allowable COG might not be far enough back to produce tail download.

I'm reasonably sure that at most forward COG, EVERY a/c will have the tail producing lift downwards - i.e. upload on the tail.

I also think the sailplane thing is a red herring - flown them a fair bit. I'm quite sure (from memory) that the duo discus (for example) has a 'inverted' section on the tailfeathers.

 FE Hoppy 27th Mar 2009 00:18

172 driver.
Beware the internet. It's not all correct.

I say again "I'm not aware of any commercial aircraft which is not positivley stable."

There are no commercial jets in service today that have lifting tails.

 Machdiamond 27th Mar 2009 02:52

Static stability does not require a downward force on the horizontal tail if you care to read past the second chapter of any self respecting aerodynamics testbook.

In the first chapter, static stability is introduced with a nice little schematic of a CG ahead of the center of lift, and a downward force on the horizontal to balance the aircraft.

If you believe things are that simplistic, you are mistaken.

Most airplanes are perfectly stable with a small positive lift on the horizontal tail when the CG is near the aft limit. Think airfoil camber and pitching moment (nose down) for example, to give you a lead...

 Mark1234 27th Mar 2009 03:57

Yes machdiamond, correct - I don't think anyone's said otherwise

All aircraft are designed to operate within a *range* of COG. They'll be most stable with a forward COG, least with a rearward. Depending on design, certification etc (which attempts to leave a good margin for average pilot skills), the most rearward allowed COG position may or may not result in tail download (mostly not I suspect).

Also, the COG range available between start of tail download and too unstable is a LOT smaller than that between start of tail upload and running out of elevator authority.

Hence aircraft will operate with tail upload (downward lift) *most* of the time. ( I try to avoid absolutes, people have a habbit of coming up with the exception...)

 FE Hoppy 27th Mar 2009 07:20

here's the maths

 L337 27th Mar 2009 07:31

On a related note, we (BA) load the B747-400 deliberately with an aft C of G at hot and high airfields if we are struggling for weight and need that last extra hundred odd kilos.

Stab tank fuel ftw.

 Capn Bloggs 27th Mar 2009 08:17

load the B747-400 deliberately with an aft C of G at hot and high airfields if we are struggling for weight and need that last extra hundred odd kilos.
That would be to lessen the required downforce on the tailpane. Actually, the most efficient way to fly is with a rearward C of G. "Throw it in the back, will ya?" :ok:

 Dani 27th Mar 2009 15:00

Thanks a lot for this discussion.

Also for you, L337, who confirms my thoughts about trim tanks. You would sacrifice stability for more lift. Still we don't know if the total lift of the elevator would be positive or negative. It very well depends on the distribution of the loads and finally the stab trim setting.

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?

Dani

 Mad (Flt) Scientist 27th Mar 2009 17:45

Originally Posted by Dani (Post 4819000)
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.

 Mach Tuck 27th Mar 2009 18:13

I didn't realise it could be so complicated.

As stated by many above, all modern conventionally configured civil transport aircraft operate with a downforce (negative lift) on the tailplane. The only thing that varies is the amount of downforce and that is influenced by the aircraft's centre of gravity (the further aft the c:g the smaller the required downforce).

For pilots stability shouldn't be an issue unless you're in the business of operating your aircraft way outside the published c:g limits.

MT

 mupepe 27th Mar 2009 19:18

been told that the MD11 does!
tail tank and trough autopilot only!?
not sure

 ChristiaanJ 27th Mar 2009 19:37

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

 Miserlou 27th Mar 2009 19:56

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.

 Piper19 27th Mar 2009 21:29

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.

 FE Hoppy 27th Mar 2009 21:34

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.

 Dani 27th Mar 2009 21:58

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

 Machdiamond 28th Mar 2009 03:58

Dani that is way too complicated.

 bookworm 28th Mar 2009 12:18

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.

 rubik101 28th Mar 2009 17:36

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!

 captjns 28th Mar 2009 18:32

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

 bjornhall 28th Mar 2009 19:01

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... :E

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.

 Mad (Flt) Scientist 28th Mar 2009 19:21

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.

 keith smith 28th Mar 2009 20:34

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

 FE Hoppy 28th Mar 2009 20:36

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.

 The Beer Hunter 29th Mar 2009 01:17

Originally Posted by captjns
Cunard type aircraft ......

:rolleyes:

Cunard= Cruise liner company offering floating holidays.

Canard=French for 'duck' applied to aircraft with main lifting surfaces aft of any stabiliser.

 Intruder 29th Mar 2009 03:53

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!

 NO LAND 3 29th Mar 2009 08:34

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!

 rubik101 29th Mar 2009 10:52

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.

 bjornhall 29th Mar 2009 12:05

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? :8 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.

 Dani 29th Mar 2009 15:04

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 :ok:
Dani

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