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6th Apr 2010, 16:16
Hello! The question is: if we set stabilizer in to any down position does it mean that it begins produce the counter clockwise moment. another words- does any know the correlation between the pitch trim wheel and the stabilizer angle of attack?

Field In Sight
6th Apr 2010, 17:17
It works the same as most aircraft. Move wheel forward to nose down position and the Stab moves leading edge more up to produce less downforce.
The zero degree position looks to me almost completely horizontal (parallel to the ground).

Regarding angle of attack, then that is a bit more complicated due to the downwash from the main wing.

What do you actually want to know?

FIS.

6th Apr 2010, 19:03
For the stabilizer position there are dashes down and some up on the fuselage( the stabilizer travel range). I am confused by the up dashes painted on the fuselage. So normally the counter clockwise moment from the wind lift force is balanced by the clockwise moment from the stabilizer lift force. If the stabilizer will be set according to the upper dash does the stabilizer produce the counter clockwise moment? Or what?

FCeng84
6th Apr 2010, 19:21
When referring to stabilizer motion please speak of the direction that the trailing edge of the stabilizer moves (up or down). Trailing edge up generates positive incremental pitching moment and thus airplane nose up response.

Clockwise and counterclockwise are very confusing. When viewed from one side of the plane a clockwise motion will be the same as a counterclockwise motion when viewed from the other side.

Stabilizer position on the flight deck is usually displayed on a scale in increments of degrees. For takeoff there is also a notion of the correct range of stab settings known as the green band. Stabilizer setting for takeoff is most dependent on cg with secondary dependence on weight, takeoff flap setting, and takeoff weight.

rudderrudderrat
6th Apr 2010, 21:06

I think I understand what you are asking. Let's look at the aircraft with the Nose on the Left and the Tail on the Right.

The Centre of Gravity (C of G) is normally forward of the Centre of Lift (C of L) from the wing. Therefore this moment is counter clockwise. To balance the aircraft, the stabilizer must produce a clockwise moment and the leading edge of the elevator will point accordingly. In low speed flight with Flaps Full, it will point a long way down.

In normal high speed flight, let's assume the C of L hasn't moved much, therefore the stabilizer still has to produce the same clockwise moment. Because of the higher air speed, it only needs a smaller angle of attack to the airflow. Therefore the stabilizer will have been trimmed more clockwise to reduce it's angle of attack.

The stabilizer may move into your "upper dashes" position when flying close to VMO with high engine power setting.

Edit. The extra "upper" range on the stabilizer might be for the occasion when all the passengers huddle up in the rear of the aircraft, for some peculiar reason.

Microburst2002
7th Apr 2010, 09:39
I think I know what he is asking.

You need to know the shape of the THS section (symmetrical?) and then, the angle of incidence of the 0º position. These things will determine wether the effect of the tailplane will be ANU or AND for any given mark.

7th Apr 2010, 15:27
Microburst2002, thanks for idea!

7th Apr 2010, 15:57
rudderrudderrat.
I dont think that AOA of the wing will be ever in practice less than 0 deg. Only in that case the wing lift will change direction to the opposit so the stibilizer lift will have been changed accordingly to the opposit in order to provide the a\c stability.
Lets assume that the a\c flys with the Vmo and the wing lift will be close to 0. In that case the wing still produses the counter clockwise lift due to assymetrical pressure distribution on the upper and lower parts of the wing surface.
the only case when I mention the stabilizer in down position (approximately 0.5 down) is during takeoff but after flaps/slats retraction till landing it is only in up position.
The task for me is to find the actual stabiliser position figures as I was kindly advised by the Microburst2002!

rudderrudderrat
14th Apr 2010, 11:39

Out of curiosity, I set maximum nose down on the trim within the green band (with Y hyd pressure) and looked at the elevator position during the outside check. It was within half a division of the maximum "leading edge up" shown on the fuselage.

It's possible to load the aircraft with a sufficiently aft C of G to warrant a stab setting like that. I'm sure the aircraft would have to be speed stable in flight (in case of problem resulting in direct law) and so the elevator must still produce a down force. (i.e. aircraft nose up moment, or clockwise moment in your example)

Microburst2002
14th Apr 2010, 20:24
The tailplane always has a stabilizing effect, since it is well behind the CG.

The amount of up or down tailplane deflection has an effect in the trim angle of attack of the airplane. This deflection has to be such that the THS balances all the moments about the CG due to the wing and fuselage lift plus those due to the wing aerodynamic moment. The final result can either be an ANU or an AND position, depending on the CG, for a given weight and AoA.

rudderrudderrat
15th Apr 2010, 08:59
Hi MB2002,

The final result can either be an ANU or an AND position please can you explain how how it's possible to trim an aircraft to be stick free "speed stable" if the elevator is trimmed AND.

I thought the aft c of g limit (on the aircraft we operate) is to ensure speed stability?

Microburst2002
15th Apr 2010, 11:34
I think you mean longitudinal stability?

An airplanes longitudinal stability depends on the relative position between the CG and the wing and fuselage aerodynamic center, on the one hand, and on the tail plane effect, on the other. GG behind AC: wing-fuselage unstable. CG ahead AC: wing-fuselage stable. The tailplane is always stabilizing, no matter what is its angle of incidence. The sum of both effects determines overall longitudinal stability.

If the tailplane is powerful enough, its stabilizing effect can make stable an airplane with a CG such that it has an unstable wing-fuselage. I think that is the case in most airliners.

The tailplane, however, has another function. To make it possible for the airplane to be in trim (ANU moments=AND moments). Then we have to look at the center of pressures, instead of the AC. If the CG is behind the CP, that is an ANU moment. If the CG is ahead, that is a AND moment. The aerodynamic moment is always ANU, I think.

In flight, I think that the CP can either be ahead or behind the CG, depending on the CG location and the AoA. So the tailplane will either produce an AND or an ANU moment to balance all the moments and make the airplane fly in trim.

It is true that on most occasions, the tailplane produces a downwards force, but sometimes can produce an upwards one (the ideal case for cruise).

rudderrudderrat
15th Apr 2010, 16:54
Hi MB2002,

This Boeing link High-Altitude Handling (http://www.boeing.com/commercial/aeromagazine/aero_02/textonly/fo01txt.html) explains that on an aircraft using flight control computers to enhance the Positive and Relaxed Longitudinal Static Stability, the C of G can be further aft than on a conventional aircraft. But I don't think we load the aircraft so that the elevator force is upwards. I thought it was always downwards - but at a reduced magnitude.

Microburst2002
15th Apr 2010, 18:19
Hi

My idea is that flight computers can push the CG further even more aft than conventional ones, but anyway, I think there can be AoAs for which the tailplane lift is positive. Although in most cases, it is negative.