Horizontal Stabilizer Shape

[momo95]

Hello all,

Just a quick question about something I'm trying to find out.

Are horizontal stabilizers on most transport aircraft symmetrical aerofoils?

I'm reading about negatively cambered aerofoils on the tailplane, but I'm confused as to how this would work in trying to produce an upward force on the tailplane should the aircraft have its aerodynamic centre situated fwd of the centre of gravity.

Surely with a negative camber, in essence being an upside down wing, means it will only produce a force going down, regardless of its angle of attack. How would this produce an upwards force should the AC be fwd of the CoG?

The only situation I can imagine in which the stabilizer may produce an upward and downward force is if it is symmetrical, in which case a + AoA will give an upward force and a - AoA will give a downforce.

I'm thinking along the line of having a negatively cambered stabilizer at +4 degrees AoA will mean it won't give any downforce, the same way a positively cambered wing stops giving lift at -4 degrees AoA ... but does pushing it to +5 degrees mean it will start to give an upforce?

I appreciate any help

Thanks

[Vessbot]

Cambered airfoils can make lift going both ways. Just less efficiently going the "wrong" way. In that case it will make more drag, and stall sooner (less CLmax).

If they didn't, you couldn't fly a Citabria inverted.

[momo95]

But doesn't the wing basically get 'sucked' into the area of lower pressure which is created by accelerating the air over the cambered side?

How would the flat side be the side of lift when it would still be the side of higher pressure even if inverted ?

[B2N2]

Anything deflected in the airstream will produce extra drag hence trimmable horizontal stabilizers on transport category aircraft.
Center of pressure ( center of lift ) needs to have a certain position relative to CG otherwise stability issues arise.
You've got to see the whole rather then a single component.
Ideally there is no up or down force as this would be the lowest drag scenario. Whether this is achieved for any significant amount of time, I don't know.
With large swept back wing aircraft CG changes with (wing) fuel burn.

[Vessbot]

But doesn't the wing basically get 'sucked' into the area of lower pressure which is created by accelerating the air over the cambered side?

How would the flat side be the side of lift when it would still be the side of higher pressure even if inverted ?

The accelerated, lower pressure isn't on the cambered side. That's a common misconception from overly simplistic teaching. It's on whichever side is turned away from the airflow due to AOA.

You need let go of camber as being some fundamental bedrock of lift production, because it isn't. A sheet of plywood will make lift and serve fine as a wing. Camber just makes it slightly more efficient.

[momo95]

Anything deflected in the airstream will produce extra drag hence trimmable horizontal stabilizers on transport category aircraft.
Center of pressure ( center of lift ) needs to have a certain position relative to CG otherwise stability issues arise.
You've got to see the whole rather then a single component.
Ideally there is no up or down force as this would be the lowest drag scenario. Whether this is achieved for any significant amount of time, I don't know.
With large swept back wing aircraft CG changes with (wing) fuel burn.

OK ... so essentially the ideal situation is to have 0 trim, but this is rarely possible due numerous other factors such as pax seating, cargo in the hold etc?

That offers me a different perspective, because the impression of gotten so far in my studying of stability is that aircraft are intended to have a forward CoG in flight, to give positive stability in case of an upgust etc and that the horizontal stabilizer is designed to counteract this by producing a downforce. I was wondering what happened in the rare instance that it wasn't possible to have a fwd CoG and so my initial question arose ...

[momo95]

The accelerated, lower pressure isn't on the cambered side. That's a common misconception from overly simplistic teaching. It's on whichever side is turned away from the airflow due to AOA.

You need let go of camber as being some fundamental bedrock of lift production, because it isn't. A sheet of plywood will make lift and serve fine as a wing. Camber just makes it slightly more efficient.

I do recall my instructor telling me he could make the laptop on my desk fly so I suppose this is what he was getting it.

I did hold onto the idea though that the upper side produced lower pressure than the lower side (due to bernoulli's theorem), but of course that the pressure still dropped on the lower side, just not as much. It seemed to make perfect sense. So the reality is that the camber just helps the lift production on the upper side of the wing (granted that's where we want our lift produced most time) but the flatter side can still produce lift given the correct AoA?

[Vessbot]

OK ... so essentially the ideal situation is to have 0 trim, but this is rarely possible due numerous other factors such as pax seating, cargo in the hold etc?

That offers me a different perspective, because the impression of gotten so far in my studying of stability is that aircraft are intended to have a forward CoG in flight, to give positive stability in case of an upgust etc and that the horizontal stabilizer is designed to counteract this by producing a downforce. I was wondering what happened in the rare instance that it wasn't possible to have a fwd CoG and so my initial question arose ...

There are a several airplane components in the airstream: The wing, the tail, the fuselage, nacelles, etc. Each has its own contribution to stability.

The issue of CG vs. center of lift, is only the wing's contribution. Since it's the total that matters, and the tail is always a huge huge contributor of positive stability, the wing can be slightly unstable and the total still be stable.

How much of each? Well, good thing the engineers figured all that out and gave us, as an end result, the envelope that we have to keep the CG in. Doesn't matter to us why the limits are they are, we just have to stay within them.

As an aside, you'll see that float planes that are converted from land planes, (instead of designed as float planes from a clean sheet) almost always have extra tail surfaces added. Because the majority of the float is ahead of the CG and therefore destabilizing. So we need more stabilizing surfaces to compensate.

[Vessbot]

So the reality is that the camber just helps the lift production on the upper side of the wing (granted that's where we want our lift produced most time) but the flatter side can still produce lift given the correct AoA?

Exactly, that's it.

[momo95]

That's excellent, thanks :ok:

[B2N2]

Keep in kind that when transport category aircraft are in cruise they don't fly 'level'.
When viewed from the outside it's more like "plowing" as they fly 2-4 degrees pitch up in cruise.
You can feel this as you walk through the cabin. It's definitely walking uphill towards the flight deck.
That may put the trimmable horizontal stabilizer slightly negative.
Not sure though as engineers don't talk to pilots :}

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTTxu5OPsLL_0lbjc-0DPH5KTQMJQ-5W1ln_TkgMsiplXwajC9x

[momo95]

you mean in the sense that a positive AOA on a negative cambered aerofoil/stabiliser is negative?

[eckhard]

The B737, A320 and even the Fuji FA-200 all have a noticeable negative camber on the tail-plane.

I don't think that any 'plane (except FBW high-manoeuvre military) can fly with the CG aft of the CP. There should always be a negative force on an aft-mounted tailplane, which contributes to the weight and which is why some folks prefer the canard layout in which the forward-mounted 'tail' contributes to the lift by having an upward force.

Not sure how the Avanti works it all out!

[Vessbot]

Keep in kind that when transport category aircraft are in cruise they don't fly 'level'.
When viewed from the outside it's more like "plowing" as they fly 2-4 degrees pitch up in cruise.
You can feel this as you walk through the cabin. It's definitely walking uphill towards the flight deck.


This is true (sometimes) but also kind of meaningless. In the end, the plane will always fly at whatever attitude puts the wing at the AOA to make the same lift as the amount of weight.

The designers' job is to set the fuselage axis relative to the wing chordline such that the fuselage is straight to the airflow at the design cruise speed/AOA. What number of degrees is displayed on the attitude indicator when it's at that attitude, is arbitrary. The CRJ, for example, cruises at about half a degree to a degree.

The 747 in that picture is probably flying slower than the design speed, for whatever reason... be it that it's going for best range or best endurance, is matching speed with the camera ship, or is flying at 250 below 10K, etc. (Or is actually climbing!)

[galaxy flyer]

Well, looks like it over Mt. Rainer, so probably at photoship speed--250-280 indicated.

[N1EPR]

Airlines save quite a bit of fuel by moving the cg towards the aft limit by loading cargo in the rear bins. This weight in the rear helps to counter the upward lift created by the horizontal stab and reduces the amount of trim needed to counter this lift.

[Dan Winterland]

Fly by wire aircraft can have a more aft c of g due to their artificial stability. Some even move the c of g aft by pumping fuel into a trim tank in the tail (A330). This saves about 1% fuel.

But don't forget that the air over the tailplane is deflected by the wing. If you want to see by how much, have a look at the gutters over the doors which are generally aligned with the airflow in the cruise to avoid drag. You will see from the pic below this can be considerable.

http://l7.alamy.com/zooms/e956e7ad826a446b809915a8ebdeff87/thomas-cook-airlines-airbus-a330-243-airliner-g-omyt-taxiing-for-departure-f4ta77.jpg

Another thing to consider with tailplanes is that they often have greater sweep than the mainplane for stability. if the tailplane is affected by compressibility before the mainplane, this could be disastrous for stability. This isn't the case on all aircraft. For example, the B747 has a higher fineness ratio to achieve this.

[FlightDetent]

momo95: I don't think that any 'plane (... ...) can fly with the CG aft of the CP. Do you insist that CP is forward of CG?

Anyways, the nose-down moment of the arm between CP and CG behind is not the sole reason we need downwards force on the tail.

On top of that is the aerodynamic moment, again nose down, of the wing itself. It will be most prominent with full flaps extended.

To counteract that in an effective manner some tails are designed with upside down camber. This is more typical for non-moving horizontal stabilizers, especially in combination with high-wing design without a T-tail.

http://www.kolmanl.info/images/l410_vop_1.jpg

[wiedehopf]

@dan

as far as i know civilian transports as well as military ones are naturally stable even if they have FBW.

only military fighters and not all of them use artificial stability.
i don't know a reference i'm sorry.

[Concours77]

Hi Dan,

The gutters are sloped aft to keep them from filling and getting the slf damp whilst parked and doors open. Also. If level, they confuse the rain runoff.

[FlightDetent]

CC77, true. The secondary design objective would be to slope them in such an angle that they contribute to as little drag as possible. Dan's suggestion remains a viable one.

[spannersatcx]

but the fwd doors the gutters slope the otherway!

[Dan Winterland]

The gutters are sloped aft to keep them from filling and getting the slf damp whilst parked and doors open. Also. If level, they confuse the rain runoff.

That's a given. But their angle of dangle is not arbitrary - it's calculated. Best seen on the A321 where the mid doors are at a lesser angle that the fore and aft doors.

And this is good gen from a mate who was on the Airbus design team.

[Concours77]

Thanks Dan. Good design suggests its purpose to those who can appreciate it.
Conforming the slope to cruise airflow is one example. In the course of an airframe's life, saving drag goes directly to the bottom line...

[DaveReidUK]

as far as i know civilian transports as well as military ones are naturally stable even if they have FBW.

only military fighters and not all of them use artificial stability.
i don't know a reference i'm sorry.

Good intro to the subject here: The 3 Types Of Static And Dynamic Aircraft Stability (http://www.boldmethod.com/learn-to-fly/aerodynamics/3-types-of-static-and-dynamic-stability-in-aircraft/)

[Volume]

On aircraft with powered control surfaces it doesn´t matter, but on aircraft with manually controlled elevator, negative camber of the horizontal stabilizer results in a "stick forward" zero elevator force, the faster you go, the higher the stick forward tendency, the aircraft is free-stick-instable. So basically you need to have either an S-sloped airfoil, or one with at least a positively combered elevator to make the aircraft stable. Symmetrical airfoils are neutral with that respect, all stick-free-stability comes from the fixed-stick-stability, which causs an elevator deflection over speed curve with enough gradient, to also produce enough stick force over speed gradient.
Positive camber plus trim tab may work as well, but is not very efficient. Additional springs in the elevator system may provide the desired force gradient as well.
Several gliders have symmetrical horizontal stabilizer airfoils up to the hinge but positively cambered elevators aft of that, often created by a the naturally concave surface of the laminar flow airfoil combined with an (easy to build) flat upper elevator surface.
The Wilga (https://static.rcgroups.net/forums/attachments/1/4/4/9/6/a2890781-153-wilga%20elevator.jpg?d=1258946115) even has a fixed (negative) slat on the elevator horn.

[TURIN]

There was a long running thread on this subject about ten years ago. Can't find it at the moment.

On a naturally stable a/c, which is every large civilian type I can think of, the horizontal stabilizer has a cambered lower surface. Usually, completely flat on top. That suggests to me that in all flying conditions the stab is designed to create a downward force to balance a c of g forward of the c of p.
It is also notable that on a trimmable stabilizer the range of movement is far greater leading edge down than leading edge up.

https://s15.postimg.org/ngkcqrhev/HS_2.png (https://postimg.org/image/ngkcqrhev/)

Which also suggests that a downward force is almost always the requirement.

Storing fuel aft, either tip tanks in a swept wing or stabiliser (horizontal or vertical) is just a very efficient way of reducing the required stabiliser incidence in cruise and therefore reducing drag.

Storing fuel in the tips also aides wing bending relief but thats another topic.