Elevator-Downward or upward force?

[mad_jock]

There is mostly downward force on the elevator for conventially designed aircraft.

There may very briefly be upward force due to control input.


But it not something the pilot needs to worry about. Look out the window and use the control inputs you require to make the picture fit to what you want. Be that straight and level or some areobatic manover.

[Lone_Ranger]

Very much within the topic, downforce (or otherwise) on the horizontal tail has rather a lot to do with stability, which will be obvious if you've actually understood any of the replies

[BackPacker]

the answers that are referred to CoG and stability are off topic.

CofG position relative to the main wings CEP, and static/dynamic lateral stability are very much on topic if you want to know why and where the elevator is providing a down or up force. If you don't get the relation between all these items, you'd better hit the PPL theory again.

[BEagle]

The position of the centre of pressure varies with α. The centre of gravity remains fixed at around 30% MAC.

For a typical aerofoil with (CL/CD)MAX of around 21.6 at α = 4°, the centre of pressure will be slightly ahead of the centre of gravity, requiring downforce at the tailplane for balance - e.g. at high IAS. Whereas at low speed and α= 18°, there will be an upward force at the taiplane for balance. To ensure that the aircraft remains stable, longitudinal dihedral is used, so that if the aircraft is disturbed, the change in restorative tailplane force is sufficient to ensure positive longitudinal stability by preventing divergence.

[mad_jock]

These gimps won't understand 0ne word in five of that.

[Silvaire1]

For a typical aerofoil with (CL/CD)MAX of around 21.6 at α = 4°, the centre of pressure will be slightly ahead of the centre of gravity, requiring downforce at the tail plane for balance

Gimps who understand see-saws would be doubly confused.

[mad_jock]

Aye but thats the reason why BEagle was trained to teach RAF pilots to fly both prop planes and also heavy tin.

If you draw a force vector diagram it will become clear.

Its a pity our academc mod plus friends are no longer with us Gengis would have cleared this one up in a couple of posts.

As a pilot I really don' care whats going on at the back be it plus or minus or for that matter where the center of pressure is at various speeds.

The engineers sort that out in the design and flight envelope specifying what I can and can't do.

[BackPacker]

As a pilot I really don' care whats going on at the back be it plus or minus or for that matter where the center of pressure is at various speeds.

True. The only thing that's useful for a pilot to know in this context is that the induced drag of the tailplane reduces when the aircraft is loaded near the aft CofG limit. That may save a few percent fuel. So if you have the option of loading something in the back vs. the front without exceeding CofG limits, try and put it in the back.

[Pilot DAR]

Whereas at low speed and α= 18°, there will be an upward force at the taiplane for balance.

It's this statement which points to the part I do not understand.

If I am flying at low speed, and a relatively low angle of attack, with the plane trimmed (and all other things remaining equal), I must apply a pull force to increase the angle of attack (per FAR Part 23.173, and the stick force must be stable through the speed range FAR Part 23.175.

If the plane went from the tail providing a down force to providing an up force, the required "pull" would no longer be required, and there would be a control force reversal, neither of which are certifiable.

If, in a tricycle plane, in this realm of conditions where the tail is said to provide a lifting force, I go from stopped on the runway (on three wheels), to approaching rotation during takeoff, and the tail suddenly provides a lifting force, I'm going to have a heck of a time getting the plane off the ground - or, the tail tiedown ring was sitting on the ground before I started....

[BEagle]

You are confusing manoeuvre with stability.

When an elevator is deflected by η°, there will be a change in CL at the tailplane of η (dCL / dη) which will therefore pitch the aeroplane. Once the desired attitude has been achieved, the aeroplane's stability characteristics will tend to maintain that attitude.

[Jim59]

Elevator-Downward or upward force?

It all depends. When I'm towing a glider that is in the low-tow position I'm pushing the stick forward and believe that the resultant force from the tailplane & elevator is probably upwards. When the glider is in the high-tow position I'm pulling the stick back and believe that the force is downwards.

[peterh337]

No need for a constant downward force from the elevator. What you need is for the rate of change of the turning moment (with respect to the pitch, and about the pitch axis) due to the elevator to exceed the rate of change of the turning moment (also with respect to pitch) due to the wing. Then you have the required pitch stability. If you don't have that, you need fly by wire control for the pitch axis.

[BEagle]

It all depends. When I'm towing a glider that is in the low-tow position I'm pushing the stick forward and believe that the resultant force from the tailplane & elevator is probably upwards. When the glider is in the high-tow position I'm pulling the stick back and believe that the force is downwards.

The tow rope tension imparts a pitch moment towards the glider position. Thus if the glider is below the tug, the moment will be nose up and forward control column pressure will be necessary for the tug pilot - and vice versa if the glider is above the tug.

As the magnitude of the tension is rarely constant, it will often be necessary to cope for out-of-trim situations with residual control column forces.

[Silvaire1]

As a pilot I really don't care whats going on at the back be it plus or minus or for that matter where the center of pressure is at various speeds. The engineers sort that out in the design and flight envelope specifying what I can and can't do.

Longitudinal static stability does not have to be 'rocket science' when viewed in simple terms. The wing's center of lift is behind the center of gravity in cruise. A tail down force balances the resulting nose down moment. When the aircraft slows, the nose is raised which (if you look at a typical wing section) makes the top of the wing effectively more cambered, producing more lift at a given speed. Its also fairly obvious (when you look) that the wing's center of lift will simultaneously move forward, which is closer to the center of gravity. This would reduce the nose down tendency of the aircraft at reduced speed, an unstable and undesirable characteristic, but happily since the horizontal tail has a negative angle of attack in level flight, and is fixed in relation the the wing by the fuselage, its down force goes away as the wing's angle of attack increases. The designer arranges for the pitch stable effect at the tail to be greater than the destabilizing effect of the wing's center of pressure shift and as a result, you have to increase the tail down force with some up elevator/stick deflection in order to fly slower.

As long as the tail's nose down moment increases faster with angle of attack than the reduction of the wing's nose down moment, the plane has static pitch stability. At high angles of attack the wing's center of pressure moves forward of the CG, but by now the tail has transitioned to producing lift by virtue of the aircraft's increased nose up attitude (relative to the flow of air around it), so static stability is maintained.

A canard aircraft is different in that both wings produce lift at all times and the CG is located between them. Static stability is maintained by making the rear wing gain lift with angle of attack faster than the front wing (higher dCl/dAlpha in the obtuse vernacular ) That's why when you look at the section of a canard's front wing, it is so uniformly round on top - so it is relatively insensitive to angle of attack. To maintain lower speed, you increase the front wing's lift by pulling the stick back, deflecting what amounts to flaps on the forward wing. The good news about canard aircraft is that both wings always push upward, the bad news is that the front wing section might not be quite as efficient due to its role in generating static pitch stability.

[Silvaire1]

If the plane went from the tail providing a down force to providing an up force, the required "pull" would no longer be required, and there would be a control force reversal, neither of which are certifiable.

If the CP is right on the CG at a certain angle of attack, but the fixed horizontal tail is set to produce a nose down moment (lift) at that same angle of attack, in level flight the elevator must be deflected upward by the pilot to balance and overcome the fixed horizontal tail's force. That means he is pulling back on the stick.

Then if the angle of attack increases from there, the fixed tail lift increases, over-balancing the developing nose-up CP shift on the wing (by design). So then you have to add still more back elevator input... which is by definition static pitch stability.

I can see this becoming complicated in design, assuring static stability over the whole CG range. I think you have to make sure that at the angle of attack where the CP has moved forward to the CG, the fixed tail has already transitioned to lift.

God help us if we have to go through the mental exercise for a stabilator

[mad_jock]

I am more than happy as an Mechnical engineer in a life before being a Pilot understanding the principles of stable static flight.

As a pilot its a fixed feature of the aircraft by design for most if not all civilian aircraft.

[bubbers44]

The elevator only pitches you up or down. The horizontal stabilizer is always set to have a negative lift or down force so it keep the aircraft stable with speed changes. Airbus and others use aft CG to minimize this down force because it costs the same fuel as weight. We transfered fuel to the rear tank in the Lear Jet in the 70's and could reduce power to hold the same speed.

[BEagle]

The horizontal stabilizer is always set to have a negative lift or down force so it keep the aircraft stable with speed changes.

Not always.


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