Necessary for elevator to push down?

[IO540]

Traditional teaching seems to be that

1) the wing pushes up,
2) the elevator pushes down (because the CG is ahead of the wing centre of lift),
3) the turning moment (about the centre of rotation in pitch) due to the elevator (both static and dynamic) needs to exceed the TM due to the wing, for any AOA change - this is called "decalage" I believe

and the above give you a human-flyable aircraft.

However, the other day I spoke to a 757 pilot who says they can load it so that 2) is not met, so long as there is enough elevator authority etc.

Model airplane flyers tell me the same - the models fly just fine with a neutral elevator.

It makes sense to me. Any comments?

[TURIN]

The elevator may well be 'neutral' but the stabiliser (tailplane to us limeys) will probably still produce a minimal downward force.

I have never heard of a non fly-by-wire aircraft such as the B757 trimmed to an unstable (IE. C of G aft of C of P) static condition.

Standing by to be put in my place.:O :ok:

[Wizofoz]

Yes, two different things beeing talked about.

Neutral elevator should ALWAYS be present in an aircraft with a full-moving stabiliser (e.g the vast majority of jet transports)

Down force from the stabiliser MUST be present in a conventially configured aircraft for logitudinal stability to be present (this is one advantage of the Canard and three lifting surface configurations).

A more rearward C.G (but still in front of C of P) means less (but not none) downforce is required, which leaves more lift available (this is why C.G figures in the performance calculations of large jet transports).

Aeromodellers (I'm one!!) wouldn't have a clue where the C of P of their models is. They know that more rearward CG = more maneouverability, and probably assume a flat-section tail with a neutral elevator means no down force, but most designs still have Decalage (I THINK you defined it correctly- It's the difference in incidence between the wing and the tail), and would be coming home with a bin-bag full of balsa bits the day they tried to fly a model with the CG behind the c of P!!

[chornedsnorkack]

I do not think that the tailplane necessarily has to push down for the decalage to exist!

Look at it this way: tailplane stabilizes the plane for example because it stalls after the main wing. When the main wing reaches the stalling AoA, the tailplane is at a smaller, unstalled AoA, so nose falls, tail rises and the main wing is turned to a smaller, unstalled AoA.

In a cruise, the plane might have, say, 6 degrees AoA of main wing - the main wing AoA obviously would not be zero because then the main wing would produce no lift at all and the plane would be in free fall.

Now, for the decalage, the tailplane has to have a smaller AoA than the main wing. When the main wing AoA is +6 degrees, the tailplane might have AoA of -6 degrees, and pushing down, or it might be 0 degrees with no lift, or it might be +3 degrees. Obviously if the main wing is at AoA of +6 degrees and tailplane is at 3 degree angle to tailplane, the tailplane is providing some lift, yet it is still stabilizing the plane. It is only if the tailplane has AoA equal or greater than main wing that the stability vanishes.

How is the angle between main wing and taiplane chosen?

[Wizofoz]

I do not think that the tailplane necessarily has to push down for the decalage to exist!


No one was discussing the stall case. In normal straight and level the tail-plane must produce a down force because the C of G is in front of the C of P causing a nose down moment.

The Main Plane does stall before the tail. However, the tail is a different surface with a different plan form, aspect ratio and aerofoil. It won't therefore necessarliy (or even probably) stall at the same AoA as the wing.

Obviously if the main wing is at AoA of +6 degrees and tailplane is at 3 degree angle to tailplane, the tailplane is providing some lift,


This could only happen if the C of G was behind the C of P, in which case the aircraft would be divergently unstable in Pitch,

[CaptainSandL]

The stabiliser does produce a downforce for the reason given by wizofoz. See photo of camber here (http://www.b737.org.uk/flightcontrols.htm) and below. The elevator is normally neutral but is deflected by the yoke to control the aircraft in pitch. If the new pitch attitude needs to be maintained the stab is trimmed (rather than the elevator) to minimise the drag that would be caused by the deflected elevator.

http://www.b737.org.uk/stabprofile.jpg

Your 757 friend was referring to the fact that most airliners are trimmed to the aft limit to reduce the amount of downforce required by the stab (because the weight/CofG is doing the work) thereby giving more net lift & hence more efficient flight.

[Stuck_in_an_ATR]

I am afraid guys, that you're mixing "centre of pressure", with "aerodynamic centre". The CoG may be aft of the former and the plane will still be stable (and the net stab force will be up!). Only when CoG is aft of the latter, the plane is unstable

[Wizofoz]

Stuck,

Looking at the definitions of the two, I'm not sure you're correct. Aerodynamic centre is aerofoil specific. C of P is the sum of all aerodynamic forces on the aircraft.

Can you give me a reference for that?

[Gullyone]

Some contest freeflight model aircraft have the CofG behind the trailing edge of the wing. How does that work?

[Stuck_in_an_ATR]

I don't have reference other than JAA ATPL Aerodynamics textbooks and some (limited) knowledge from my studies. Anyway, the aerodynamic centre is the point on the airfoil, where the turning moment is constant with change of the AOA. Then you add the fuselage, engines, empenage etc., which MOVE the a.c. of the whole aircraft to a different position - but it still exists and is the point of neutral stability. However, I stand by to be corrected by the more knowledgeable :8

[TURIN]

Some contest freeflight model aircraft have the CofG behind the trailing edge of the wing. How does that work?

If it's a swept wing then - quite easily - is the answer. :ok:

[Mad (Flt) Scientist]

"Stuck" is pretty much right on the money. Stability is not a function of what the trim load on the tail is, but a function of the CHANGE in overall aircraft pitching monet as a result of a disturbance in angle of attack. As long as there's a nose-down restoring moment for a nose-up disturbance, the aircraft is stable. Which way the tail is providing the trim load doesn't affect that behaviour.

[chornedsnorkack]

Indeed. Logically, the centre of lift, centre of pressure and aerodynamic centre ought to apply to the whole airframe - "main wing" together with all other surfaces, whether tailplane, canards, tandem wings, fuselage/lifting body or whatever.

[Dick Whittingham]

Nice to see you back, Mad. I was getting worried.

Dick W

[Gullyone]

The model aircraft I was thinking about have straight wings.

[IO540]

Stability is not a function of what the trim load on the tail is, but a function of the CHANGE in overall aircraft pitching monet as a result of a disturbance in angle of attack. As long as there's a nose-down restoring moment for a nose-up disturbance, the aircraft is stable. Which way the tail is providing the trim load doesn't affect that behaviour

That's exactly what I thought - thank you.

In that case, am I correct that one could have the aircraft set up with positive lift from the elevator assembly, for much of the rearward portion of the loading envelope?

[Mad (Flt) Scientist]

Actually, you could set up an aircraft such that the "elevator/tail assembly" always was providing an upwards force. It would look a bit strange, but it can be done. Usually there's other design constraints - tail lift direction isn't an overriding concern.

The terms we use - "wing", "tail", "canard" - all refer to lifting surfaces mounted at different places on the fuselage. If we were to call the one at the front the "wing" and the one at the back the "tail", then any canard-configured aircraft has a "tail" which is much larger than its "wing", and the "tail" always provides up force in normal trimmed flight. Aerodynamically, the names we give to the surfaces is irrelevant; the air doesn't care what we call the parts of the aircraft.

[TURIN]

Gonna be a pedantic barsteward now...

The tail is the empanage surely. IE Fin, tailplane/horizontal stabiliser & the bits that hold them all together.

Back to the thread,

1. A tailplane/horizontal stabiliser produces a downward force.

2. A canard produces an upward force.

Unless the aircraft is designed to be naturally unstable and therefore under computer control through fly-by-wire.

[Mad (Flt) Scientist]

The point I was trying to make by reference to a "canard" layout - small forward lifting surface and larger rear lifting surface - is that such a configuration is stable, despite the cg being well aft of the cp of the forward lifting surface.

Stability is concerned solely with the position of the cg relative to the neutral point of the whole configuration. If the aircraft is so configured that the neutral point is quite a distance aft of the cp of the forward lifting surface, then you can put the cg between the cp and the neutral point, and the rear surface will provide lift, while the aircraft remains stable.