jetmech
Topic Author
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Joined: Wed Mar 29, 2006 2:14 am

Servo Tab Question

G'day Techies,

I've got a question about elevator servo tabs which I'm hoping to get your input on. It's a fundamental question of sorts, as I'm not too sure how they actually work in an aerodynamic sense. I was particularly interested in aircraft with floating elevators.

I understand that mechanically, the servo tab moves in the opposite direction that you want the elevator to move, but aerodynamically, I'm not sure how you would get any net force out of this arrangement.

When the elevator tab deflects, it produces a net upward or downward aerodynamic force upon the trailing edge of the elevator. The elevator responds accordingly to this force, by deflecting the other way.

To my thinking at least, I see the elevator just deflecting enough to balance the force produced by the servo tab, such that the tab and elevator system as a whole is in equilibrium. If this is so, where exactly does the net force required to change pitch angle come from?

I can understand that the deflected tab / elevator system may produce a drag force at the elevator hinge line, but I cannot see where any net upward or downward aerodynamic force would come from .

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Regards, JetMech

[Edited 2008-08-13 07:45:02]
JetMech split the back of his pants. He can feel the wind in his hair .

metroliner
Posts: 846
Joined: Sat Jan 20, 2007 4:35 am

RE: Servo Tab Question

 Quoting JetMech (Thread starter):I cannot see where any net upward or downward aerodynamic force would come from .

Hi JetMech,

I believe the net aerodynamic force comes about as a result of this force on the hinge-line, which then changes the AoA of the whole surface, which then results in an aerodynamic force.

For instance, if the servo tab is deflected downwards, then the resultant force means that the elevator will move upwards. This upwards movement of the elevator exerts a force on the aircraft through the hinge line which changes the AoA of the whole horizontal tail so that the aircraft will climb.

Whenever I see servo tabs, I think of the elevator + tab as a whole separately from the whole aircraft - no matter if there is equilibrium between the tab and elevator, if there is still a net force exerted on that hinge line, something will change.

I hope that is more or less clear and that my own reasoning isn't foggy.

All the best,

Toni
Set the controls for the heart of the Sun

ANITIX87
Posts: 2983
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RE: Servo Tab Question

IIRC, the BAE avro has the opposite arrangement, no? I was under the impression that the taps are what the wires control, and when the tab moves down, the elevator moves up, and this is why the elevators sit up instead of down with hydraulic power is off.

Does the MD-80's sytem work similarly or are floating elevators different completely?

TIS
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SlamClick
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RE: Servo Tab Question

Keep in mind that when control column force is applied, the mechanical pressure remains (due to the way it is rigged) to keep the tab deflected, even when the elevator itself begins to move. The forces are only balanced out when they agree with the position commanded by the control column movement.
Happiness is not seeing another trite Ste. Maarten photo all week long.

Lemmy
Posts: 243
Joined: Fri Dec 10, 2004 8:40 am

RE: Servo Tab Question

As I understand it (and I could be wrong), the small servo tab is able to move the much larger elevator because it's on the end of a long lever arm. So the relatively small force the tab exerts has an easy time of moving the large elevator because it's located as far away from the hinge as possible.
I am a patient boy ...

HAWK21M
Posts: 30014
Joined: Fri Jan 05, 2001 10:05 pm

RE: Servo Tab Question

On some types...The balance panels are aerodynamically moving the control surface due differential pressure accross either side of the balance panel initiated by the tab.

regds
MEL
I may not win often, but I damn well never lose!!!

oly720man
Posts: 5813
Joined: Fri May 21, 2004 7:13 am

RE: Servo Tab Question

 Quoting ANITIX87 (Reply 2):IIRC, the BAE avro has the opposite arrangement, no? I was under the impression that the taps are what the wires control, and when the tab moves down, the elevator moves up, and this is why the elevators sit up instead of down with hydraulic power is off.

Yes, the Avro has a tab driven tailplane. It has suffered because of this as well, because of icing that changes the aerodynamic shape of the tab and, consequently, changes the response of the elevator, e.g.

www.caa.co.uk/docs/33/factor200411.pdf
www.aaib.dft.gov.uk/cms_resources/dft_avsafety_pdf_027207.pdf

One problem is that the tailplane can be deiced in flight, but the tab can't so all the wet stuff flows back and re-freezes on the tab.
wheat and dairy can screw up your brain

tdscanuck
Posts: 8572
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RE: Servo Tab Question

 Quoting JetMech (Thread starter):To my thinking at least, I see the elevator just deflecting enough to balance the force produced by the servo tab, such that the tab and elevator system as a whole is in equilibrium. If this is so, where exactly does the net force required to change pitch angle come from?

You need to split the moment and the lift of the surface. A free-floating elevator has to have zero moment about its hinge (because there's nothing to hold it back). Think of the servo tab as a tiny flap that alters the camber line of the elevator. When the servo tab deflects, the camber changes and the moment changes. The elevator will deflect back to the position of zero moment about the hinge (which is different than before because of the tab deflection), and will have a different angle of attack than before. A cambered airfoil has a non-zero moment at zero angle of attack.

Since the angle of attack changed (as well as the camber), the lift produced by the surface changed. If the hinge point is close to the aerodynamic center, the changed lift won't change the moment on the surface so it won't want to rotate any more, but more lift will be transmitted to the airframe through the hinge.

Tom.

Jetlagged
Posts: 2564
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RE: Servo Tab Question

 Quoting JetMech (Thread starter):I understand that mechanically, the servo tab moves in the opposite direction that you want the elevator to move, but aerodynamically, I'm not sure how you would get any net force out of this arrangement.

An aircraft doesn't pitch because of the upward or downward force produced by the elevator. The elevator deflection changes the lift produced by the horizontal tail, which therefore produces a pitching moment.

It doesn't really matter how the elevator gets deflected (whether directly by the pilot, by a hydraulic servo, or an aerodynamic servo). In this case it is held deflected by an aerodynamic moment. The effect on the tail will be the same.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

jetmech
Topic Author
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RE: Servo Tab Question

G'day All,

Thanks for the input!

 Quoting Metroliner (Reply 1): Whenever I see servo tabs, I think of the elevator + tab as a whole separately from the whole aircraft - no matter if there is equilibrium between the tab and elevator, if there is still a net force exerted on that hinge line, something will change.

 Quoting SlamClick (Reply 3): The forces are only balanced out when they agree with the position commanded by the control column movement.

 Quoting Lemmy (Reply 4): So the relatively small force the tab exerts has an easy time of moving the large elevator because it's located as far away from the hinge as possible.

I can see what you are all explaining, but what happens when equilibrium is reached? Is it the transient portion of the deflection that produces the net force   .

To muddy the waters further, there are other types of tab which act in a manner to either assist or resist the pilot in moving the control surface.

 Quoting Tdscanuck (Reply 7): cambered airfoil has a non-zero moment at zero angle of attack. Since the angle of attack changed (as well as the camber), the lift produced by the surface changed.

 Quoting Jetlagged (Reply 8): The elevator deflection changes the lift produced by the horizontal tail, which therefore produces a pitching moment. It doesn't really matter how the elevator gets deflected (whether directly by the pilot, by a hydraulic servo, or an aerodynamic servo). In this case it is held deflected by an aerodynamic moment.

It appears that you are both agreeing that the camber of the tailplane / elevator combination is effected. This in turn, affects the aerodynamic qualities of the tailplane / elevator combination.

If I understand Tds correctly, the camber change produces a net increase or decrease in lift of the tailplane, which then effects the pitch change of the entire aircraft.

If I understand Jetlagged correctly, the camber change produces a net increase or decrease in pitching moment of the tailplane, which then effects the pitch change of the entire aircraft.

I see from both of your explanations, that it was not correct of me to focus on hinge line forces, and the key to it all is the ability to maintain an elevator deflection.

The only bit I don't understand now, is the slight difference in your explanations of the exact type of force responsible for changing the pitch of the entire aircraft. I guess you are saying the same thing in two different ways.

Tds' change in lift, multiplied by the distance from the C of P to the quarter chord of the elevator + tailplane equals Jetlagged's pitching moment.

The issue is much clearer now. Many thanks   .

Regards, JetMech

[Edited 2008-08-14 07:22:58]
JetMech split the back of his pants. He can feel the wind in his hair .

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: Servo Tab Question

 Quoting JetMech (Reply 9):The only bit I don't understand now, is the slight difference in your explanations of the exact type of force responsible for changing the pitch of the entire aircraft. I guess you are saying the same thing in two different ways.

No real difference. Pitching moment in this case is tail lift times tail moment arm. Tail lift produces the pitch change by means of the pitching moment it generates.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

jetmech
Topic Author
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RE: Servo Tab Question

 Quoting Jetlagged (Reply 10):Pitching moment in this case is tail lift times tail moment arm. Tail lift produces the pitch change by means of the pitching moment it generates.

I see. I thought you meant the pitching moment of the elevator / tailplane combination   !

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair .

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: Servo Tab Question

 Quoting JetMech (Reply 11):I see. I thought you meant the pitching moment of the elevator / tailplane combination

I did, when I said tail that included the elevator. Tail lift naturally includes the effect of elevator deflection. I can't see what's confusing about this.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

411A
Posts: 1788
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RE: Servo Tab Question

Ahhh yes, balance panels, located in balance panel bays, just forward of the primary control surface.
Works good, lasts a long time.
First used on the B707 and CV880.,,,all primary control surfaces, including the rudder.
With both aforementioned types however, hydraulic boost for the rudder was also used.

Another type that used tabs was the CL44...the column connected to control tabs only, not directly to the primary control surfaces....as I recall.

Caravelle...a similar arrangement, except that the primary control surfaces could also be directly moved by the column, if need be.
A nice arrangement.
The Caravelle was very advanced for its time...including autoland, using the Lear/SUD system, exclusive to the type.

[Edited 2008-08-15 19:14:46]

jetmech
Topic Author
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RE: Servo Tab Question

In reply 10, you described the pitching moment of the tailplane as;

 Quoting Jetlagged (Reply 10):Pitching moment in this case is tail lift times tail moment arm. Tail lift produces the pitch change by means of the pitching moment it generates.

This would be in line with Tds' explanation.

However, along with coefficients of lift and drag, airfoils also have a property known as the quarter chord pitching moment (C of M). This is defined as the moment about the quarter chord of the airfoil, and is due to the fact that the centre of pressure moves with respect to the chord of an airfoil as the angle of attack changes.

http://en.wikipedia.org/wiki/Pitching_moment

I was under the impression that it was the C of M you were talking about in this reply;

 Quoting Jetlagged (Reply 8):The elevator deflection changes the lift produced by the horizontal tail, which therefore produces a pitching moment.

In other words, I was under the impression you were saying that the tailplane / elevator applies a pitching moment / couple to the airplane as a rigid body.

If you apply a couple to a rigid body, it is the same magnitude at any point on that body. The result of this would be a pitch change of the aircraft as a whole.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair .

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: Servo Tab Question

Would it make more sense if I had said:

"which therefore produces a change in pitching moment"

Tail lift is of course only a contribution to the total pitching moment on the aircraft.
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jetmech
Topic Author
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RE: Servo Tab Question

LOL  . I think I really read too much into some of the explanations given in this thread! Nonetheless, the issue is much clearer to me than before. Thanks again for the input    !

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair .

bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: Servo Tab Question

 Quoting Jetlagged (Reply 8):An aircraft doesn't pitch because of the upward or downward force produced by the elevator. The elevator deflection changes the lift produced by the horizontal tail, which therefore produces a pitching moment.

Isn't lift an upward or downward force?

Jetlagged
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RE: Servo Tab Question

 Quoting Bio15 (Reply 17):Isn't lift an upward or downward force?

Yes, what I meant is not the force on the elevator alone. The pitching moment comes from the change in lift of the whole tailplane as a result of elevator deflection.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

DC8FriendShip
Posts: 235
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RE: Servo Tab Question

 Quoting JetMech (Thread starter):I can understand that the deflected tab / elevator system may produce a drag force at the elevator hinge line, but I cannot see where any net upward or downward aerodynamic force would come from

Keep in mind that along with the tab, as the elevator deflects the horn also sticks into the wind, providing an extra assist to the tab.
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