Trim systems

[legaleagle73]

I'm studying for the ATPL systems exam and I'm pretty confused about trim systems, so I thought I'd ask for some help from those in the know.

The syllabus refers to 2 trim systems - the flying tail and stabiliser trim.

As I understand it, most jet transports have a variable incidence horizontal stabiliser with an elevator. The horizontal stabiliser can be trimmed either manually, or automatically when an autopilot is engaged. I understand this is "stabiliser trim".

I had thought the flying tail was a variable incidence horizontal stabiliser that didn't have an elevator (a stabilator). I then came across a sample exam question from one of the main theory providers asking how the tailplane moves in relation to the elevator on a flying tail, so I guess my understanding is wrong. Would someone please explain to me what a flying tail is.

Many thanks:)

[eocvictim]

A flying tail and a stablator are different. The flying tail moves/trims to keep a neutral elevator position.

[legaleagle73]

Thanks for the reply eocvictim.

Part of the problem seems to be that different people use different terminology. I had a look at a couple of aeronautical engineering texts today. While they are largely beyond me, they clearly showed flying tails as not having an elevator. :ugh:

Since this doesn't seem to be what CASA is referring to, can anyone please explain to me what the difference is between a flying tail (with an elevator) and a variable incidence tailplane? Perhaps an example of an aircraft with each would help.

I know this isn't an exciting topic but any help would be much appreciated.

Cheers

[Bullethead]

G'day legaleagle73,

There are several different types if pitch control systems and therefore pitch trim systems.

An aeroplane such as a C130 has a fixed tailplane and a moveable elevator with a trim tab on the elevator. When trimmed the elevator is held in fixed position relative to the tailplane.

An aeroplane such as a B767 has a moveable tailplane with an elevator. When trimmed the stabiliser changes position until it and the elevator are streamlined, that is there is no elevator deflection relative to the stabiliser.

An aeroplane such as an F111 has a moveable tailplane with no elevator or trim tab. Trim is achieved through the flight control system maintaining a datum tailplane deflection.

There are other pitch control arrangements such as canards but the three methods mentioned above cover the vast majority of aeroplanes.


Regards,
BH.

[legaleagle73]

Thanks for the reply bullethead.

While looking through "Handling the Big Jets" (which is one of CASA's recommended texts for the exam) I think I've stumbled upon what CASA mean. It's noted that all flying tails (which have one surface) may "have the aft portion hinged to increase the surface camber at large angles of displacement". Now whether this aft hinged portion is strictly speaking an elevator or not, I'm not sure but it does explain the question I refer to in my first post.

Thanks again to those who took the time to reply.

[dogcharlietree]

On the DC-9, Movement of the control column moves the control tab (on the rear of the elevator) and aerodynamic forces generated by the tab moves the elevator. As each elevator moves, an additional tab, geared to elevator movement, moves to assist the control tab. The moveable horizontal stabilizer provides longitudinal trim.
As a matter of interest, the two elevators are independent of each other and so quite often on the tarmac you would see one up and the other down. Anyone have a piccy of this?