Elevator-Downward or upward force?

[dimsp81]

Suppose we have an elevator.Lets say of a C172.In the manual I read that there is a negative angle of incidence for the horizontal stabilizer.Probably in order to have a downward force to counteract the upward moment of lift.
I tried to take a close look at the elevator of the cessna that I fly,and noticed that it is a symmetrical wing.Or it looks like.
In the theory,I read that with the downward deflection of the elevator,the downward force will be reduced,and we may have an upward force.
My question:is it possible the downward force exerted by the elevator to be inverted and become an upward force?Even though the deflection is big enough.
Consider an horizontal stabilizer.

[mad_jock]

yes it can be. Its just that in cruise it is the counter to the lift moment so is negative.

Transitory application of control input may get it being positive lift. Wouldn't have thought though it would happen very often in a C172. Aerobatic aircraft more often .

[dimsp81]

So,in a cessna the more we can get is to reduce the downward force cause by the downward deflection of elevator?Or it can be inverted if we deflect it too much?

Thanks for your answer!

[mad_jock]

Depends on what your doing.

Fully forward on the stick and it will produce lift to the limit that the designers deem to be the max the airframe will take or they think you will require. They may stipulate a max air speed which you may use the full range of the controls.

When you do your full and free look at the tail and see the range of movement. If the chord is pointing down it will producing negative lift and if its pointing up its positive.

[Tarq57]

There was an interesting discussion here (http://www.pprune.org/tech-log/485063-tailplane-lift.html) concerning tailplane lift (downforce) and stability.
Should answer most questions about this.

Basically, there is a couple between lift and c of g, with c of g being forward of the c of p. The down-force provided by the tail balances the couple. This provides longitudinal stability. As the airspeed changes, the c of p (for practical purposes) moves only very little* vs the amount of lift - or negative lift - provided by the tailplane, which (like the wing) increases/decreases as a function of the square of the speed (among other things).
What that means is that airspeed is reduced, the amount of negative lift provided by the tail decreases, and the nose will try and pitch down. (C of g doesn't change, so can assumed to be constant.) Vice versa if the nose drops, or is pushed down, and the airspeed increases.

You've heard of phugoid oscillations? The more longitudinal stability the more these will be self- regulating; the aircraft returning to its' trimmed airspeed within only a few oscillations.

When the c of g is at the aft limit, longitudinal stability is reduced, but still present. If c of g is sufficiently aft of the aft limit, a situation arises where there is no longitudinal stability, and the aircraft, left uncorrected will, basically, crash.

I've read a flying report written by someone who flew a Baron severely aft loaded. It was an extreme handful. He calculated that the c of g was way outside the aft limit afterwards. I forget the figure, but it was something around 12 or more inches.

* The C of P moves forward on the main-plane as angle of attack is increased, to the point of the stall, when it moves aft. The value is of the order of inches. This also tends to be pro-longitudinal stability. It's a significant value, but still quite minor in comparison to the changing down-force produced by the tailplane in a correctly loaded aircraft. The tail has a large moment to work through, being way back there, so even if the actual lift/negative lift value is small, it's effective.

[BackPacker]

Couple of points to add.

The downwards lift generated by the tailplane obviously also generates induced drag. Aircraft that are loaded towards the aft limit will be less stable (as stated) but will also have less induced drag and are thus more fuel efficient. Aircraft such as the A380 (I think - not quite sure) automatically transfer fuel aft when the onboard 'puters sense that the aircraft is in a stable cruise and on autopilot, and automatically transfer fuel forward when the aircraft starts its descent to land. All in an effort to reduce induced drag.

Furthermore, it is possible to have the tailplane providing positive lift instead of negative, but in order to maintain the stabilizing effect, this only works if the angle of incidence is lower than the angle of incidence of the main wings. Or something along those lines. It's a very tricky balance and obviously the CofG has to be just right to do so.

[18greens]

I heard a story that I'm still not sure I believe despite the fact I want to.

if you look at a stabiliser on most ac it is rigged at a positive angle of attack. It produces lift not downforce. The story instructors tell about downforce on the tail plane is a convenient way to explain longitudinal stability.

When we draw the lift weight couple it shows the net lift from both surfaces ( main and tail) together not separately. If you draw them separately the main lift is forward of the weight and the lift from the tail stabilises it.

I've never seen a diagram of this and it was so contrary to what I was taught I was in serious denial at the time. However I can't look at aircraft and see negative stabiliser pitch.

This came to me from David Schouler so I'm thinking there's some truth in it. However if I heard it wrong in a fug of classroom overload I apologise for bringing it up.

Anyone else heard this theory?

[Tarq57]

I've heard it, but don't necessarily believe it.
When you look at tailplane incidence, it might indeed appear positive. Add to that the downwash from the mainplane, it might then become negative.

Look at the tail/stabilizer of almost any jet airliner. The side with the most curvature is always the underside.

Look at a Cherokees' all moving tailplane. Note the incidence at full down vs full up. That should tell you a lot about where the lift vs weight moment is.

[srayne]

I was taught that to ensure that the aircraft pitches nose down when stalled there will normally be a downforce at the tailplane to counteract this during the cruise.

[Lightning Mate]

The story instructors tell about downforce on the tail plane is a convenient way
to explain longitudinal stability.

Which is correct, and not "convenient".

Go to AH&N and look at the thread "A26 Upkeep Crash" and see what occurred when the tail came off.

[custardpsc]

Two things worth mentioning, one is that the tail is indeed a net downforce (In a straight and level situation), Backpackers point is dead right, an aft c of g means the tail needs todo less work, less induced drag giving better fuel consumption. The other thing to say, more generally, is to not think about elevator deflection, but relative wind, as that is the true cause of lift, or otherwise, and what is actually happening. The static angle of incidence is only of note if the relative wind is from dead ahead.

The stability point doesnt really relate to downforce, it would be true if it were an upforce too, the stability explanation is more about moments produced by the tail to oppose the disturbance, and where the planes c of g is relative to the tail. An aft c og g means the tail is less effective stability wise as the distance betewwen te tailforce and pivot (cofg) is reduced, so the restorative moment is smaller

[Tigger_Too]

http://i486.photobucket.com/albums/rr224/jythill/CofG.jpg (http://s486.photobucket.com/user/jythill/media/CofG.jpg.html)

Longitudinal stability. With the Centre of Pressure behind the Centre of gravity, the force from the tailplane must act downwards to counteract the pitch down moment. If the aircraft then pitches down, the (negative) angle of attack of the tailplane increases, so the downward force increases causing the nose to pitch back up. Converse is true for an initial pitch up. Thus the aircraft is longitudinally stable. If it deviates from a given pitch attitude, the tendency will be for it to return to that attitude.

Put the CoP ahead of the CoG and the force from the tailplane now has to act upwards. Sounds good - you now need a smaller wing. But if the aircraft pitches down, angle of attack of the tailplane increases, upward force on tailplane increases, aircraft pitches further down. So aircraft is longitudinally [U]unstable[U].

Unstable configuration can be made to work, but you need a very clever computer between the stick and the elevator!

[mad_jock]

That's all in stable unaccelerated flight.

When you come to manoeuvre the aircraft it may come to positive lift depending how aggressive you are on the stick.

[phiggsbroadband]

Hi, if you are an aero designer (or easier still an aero-modeller.) you can put your front wing on the airplane in any position you want.

You can get the Centre of Pressure to coincide with the CofG, and the tailplane would need no angle of attack at all.
You could put the CofP in front of the CofG, and you would then need a lifting tailplane.

However for best Stability it has been proved, since the days of Wilbur and Orvile, that CofP behind CofG is best. This results in the tailplane having about 2-3 degrees less angle of attack that the Mainplane.
It also means that the Tailplane does not normally stall before the Mainplane, which could result in something like a tail-slide stall.

[Cobalt]

And if you do not believe the explanations above, here is simple proof.

Get a pilot and pax in your (nose-wheeled aircraft).

Chop off the nosewheel --> nose goes down and hits the ground. Why else would we need a nosewheel?

Put a fat bloke on the tailplane --> voila, straight and level attitude.

Do not forget to put the nosewheel back on afterwards... :-)

[Tarq57]

(Make sure the engine has been stopped before trying this.)

[Pilot DAR]

This topic came up on a different forum recently, with some really persuasive writing and math supporting the possibility of the tail providing lift [nose down direction] in some C of G configurations. I'm not persuaded yet.

To support the theme that Cobalt and others have presented, if there were a configuration where in stable flight the tail lifted up, rather than down, as convention has it, how would you rotate for takeoff in a tricycle aircraft? The tail would already be on the ground! A Cessna Caravan, as other types has a "pogo stick" for loading when parked, but not 'cause you need it for taxi takeoff and landing!

I remain convinced that in sable flight, in a conventional aircraft, the tail always provides a downforce to some degree. I have flown Cessnas a bit behind the aft C of G limit, and they still required a nose up control input to rotate on takeoff, because they did not rotate themselves - happily!

[BackPacker]

I would imagine that if you have an aircraft with a permanent positive-lift tailplane, the main wheels (assuming a nosewheel configuration, and not a tailwheel aircraft) would be rather further backwards and not roughly below the center of pressure (CEP) of the main wings. After all, a positive-lift tailplane means the CofG is behind the CEP of the main wings, and if you put the main wheels roughly at the CEP of the main wings, the aircraft will fall over backwards.

But I don't think airplanes exist that have their CofG permanently behind the CEP of the main wings. For starters, that would mean they would have an extremely narrow CofG range, so they would not be exactly practical.

To the best of my knowledge, the A380 is loaded on the ground, takes off and performs the initial climb just like any other aircraft, with a tailplane providing negative lift. So the CofG is in front of the CEP of the main wings, and in front of the main wheels. Only once it's in stable cruise, or maybe at the last stages of the cruise climb, is the fuel transferred aft and will the tailplane reach some measure of positive incidence/lift - with the CofG slightly aft of the CEP of the main wings. My gut feeling is actually that the fuel transfer is done until a precise positive incidence of the tailplane is reached. After all, if the positive incidence of the tailplane would exceed a certain value (which is more or less the incidence of the main wings), the inherent dynamic stability would be lost.

Well before landing (I think once the cruise altitude is left), the fuel will be transferred forward until a normal situation, with negative lift of the tailplane, is reached again.

(And if that final fuel transfer fails, or is forgotten, I would imagine the landing and rollout would be rather spectacular - imagine an A380 sitting on its tail...)

[phiggsbroadband]

Hi, please don't fly with too aft a CofG.

This Dutch Roll occurred after very heavy cargo shifted on take-off.

Bagram Airfield Crash 29 APR 2013 - YouTube

Very sad for all on board and their families.

[dimsp81]

Thanks for your answers,but the answers that are referred to CoG and stability are off topic.I only asked for the downward force of the tailplane....

[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. :ok:

[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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