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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

Thanks for your input.

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


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