DfwRevolution
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Trim Question

Mon Jul 05, 2004 7:46 am

This should be a rather straight forward question- what is trim?

I keep thinking of something like the system on boats with outboard engines that lets you pivot the engine up or down. Does the trim do something simmilar to the control surfaces of an aircraft?

Regards,
DFW
 
dl757md
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Joined: Mon May 24, 2004 9:32 am

RE: Trim Question

Mon Jul 05, 2004 7:55 am

Yes, trim does move the control surfaces to affect AC attitude and stability.

Sorry for not having time to elaborate myself but here's a link to a site with a good description of 737 control and trim systems.

http://www.b737.org.uk/flightcontrols.htm

Dl757md
757 Most beautiful airliner in the sky!
 
rendezvous
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Joined: Sun May 20, 2001 9:14 pm

RE: Trim Question

Mon Jul 05, 2004 8:21 am

Bascially trim is designed to make flying the plane easier for the pilots. At nearly all stages in flight the elevator is required to be held in a position that's not neutral, ie, you need to apply force on the controls to keep the plane where you want it. This gets pretty tiring, and trust me I know because my instructor just simulated a trim failure the other day.

Trim is designed to reduce the workload. There are a variety of different systems, but basically it keeps the elevator displaced from the neutral position without a constant imput from the pilot.

Trim is also available (on some aircraft) for rudder displacement and also ailerons to control roll. On a twin engine if you have an engine failure you don't want to be having to keep your foot on the rudder pedal for the 3 hours to the nearest airport, you'd get pretty tired of it!
 
QantasA332
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RE: Trim Question

Mon Jul 05, 2004 10:46 am

Rendezvous pretty much summed it up already. Basically, to keep an aircraft balanced, the pilot would have to apply a constant force on the elevator to accommodate imbalances (very rarely is a plane perfectly balanced in flight) - a very tiring obligation - if not for trim. As Rendezvous said, trim is simply a system that corrects for any imbalances, without the need for constant control input. There are trims for the rudder and ailerons as well, but elevator trim is the one most commonly referred to.

There are two main trim methods. One, which is used on most larger aircraft, is simply an all-moving tailplane (like a stabilator, but with a screw system to lock it in place). Rather than moving just the elevators, the whole horizontal stabilizer moves to produce either a downward or upward moment. The grey/silver ellipses you see on either side of most airliner tailcones are the areas 'cleared' for the tail's movement.

The second trim method, found mostly on smaller aircraft, is the trim tab. Trim tabs are basically little elevators on the elevators, which are deflected (usually electrically, I think) in the appropriate direction to create and up or down moment on the elevators. Depending on the amount of deflection of the tabs, the elevators can be held in the correct position to maintain a trimmed condition, without any pilot exertion.

Cheers,
QantasA332
 
411A
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RE: Trim Question

Mon Jul 05, 2004 1:16 pm

The Lockheed TriStar is unique amoung jet transport aircraft, in that the entire horizontal tailplane is moved when 'elevator' forces are applied, for pitch change.
In addition, the stabilizer trim, like on other jet transports, is trimmed, to relieve control forces.
The TriStar does have an elevator, however its movement is linked directly to the stabilizer (ie: no separate movement possible by the pilot) to provide enhanced authority.
This feature, combined with DLC (direct lift control, ie: spoilers for pitch assist, after landing flaps are selected), provides a very stable and constant body angle for automatic landings.

 
oly720man
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RE: Trim Question

Mon Jul 05, 2004 11:11 pm

My parents were once on a BA L1011 MAN-JFK and the pilot complained about too many people going down to the back of the plane and how it was affecting the cruise trim.

Andy
wheat and dairy can screw up your brain
 
FinnWings
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RE: Trim Question

Tue Jul 06, 2004 2:14 am

Once happened in the real life on Saab 340 flight:

Quite a big man was seated on the last row of the Saab and just prior the gate departure F/A kindly requested if the man would like to be seated in the cockpit at jumpseat to get the aircraft balanced... You can imagine, that the passenger was more than happy with that.

This is other way to get the aircraft better balanced.  Smile/happy/getting dizzy

Regards,
FinnWings
 
rendezvous
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RE: Trim Question

Tue Jul 06, 2004 8:33 am

The better balanced the plane is without trim the less fuel you're going to use. If you have the elevators/trim applying a lifting force (either upwards or downwards) for the flight you get more drag. Drag is a biproduct of lift. The difference in fuel burn may not be that much, but for a fleet of say 100 planes, over a period of a year, it may add up to a nice sum of money on fuel that is unnecessary. I guess that's why some planes also use fuel to trim them, ie moving it to a further forward tank rather than deflecting a control surface.
 
QantasA332
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RE: Trim Question

Tue Jul 06, 2004 8:45 am

If you have the elevators/trim applying a lifting force (either upwards or downwards) for the flight you get more drag. Drag is a biproduct of lift.

Drag isn't only a byproduct of lift. Sure deflected elevators and/or trim increase induced drag ("lift-induced drag") by creating some lift force, but that's not all. Elevator/trim deflection also increases parasite drag, as well as interference drag to some degree.

Cheers,
QantasA332
 
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Starlionblue
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RE: Trim Question

Tue Jul 06, 2004 11:09 am

If it's not quite clear yet to a friend DfwRevolution, the aircraft has to be retrimmed ALL THE TIME, so autotrim is a very important part of flight systems. Even the weight of one passenger walking back to the crapper will, combined with the long moment arm (80kg tired pax 1m from center of gravity compared to 30m from center of gravity is a big difference in force), make the trim change significantly. Old 737s are good to check out in the cockpit, because those trim wheels never sit still.

As a matter of curiosity, the Concorde had to be retrimmed so much between subsonic and supersonic regimes that it had a whole system of automatic pumping between fore and aft fuel tanks. If this trimming was not done properly, the plane would have been unflyable.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
rendezvous
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RE: Trim Question

Tue Jul 06, 2004 5:54 pm

Yeah you're right QantasA332, I forgot to mention about those other things! I did mean to say "induced drag is a bi-product of lift", not just "drag".
 
FredT
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RE: Trim Question

Sat Jul 10, 2004 5:38 am

"The better balanced the plane is without trim the less fuel you're going to use. If you have the elevators/trim applying a lifting force (either upwards or downwards) for the flight you get more drag."

I'd like to fill in with a bit more information, if you don't mind.  Smile

For stability, you want the horisontal stab to generate some negative (downward) lift. The amount of negative lift is reduced as you move the CoG aft, reducing drag. However, move the CoG too far aft and you render the aircraft unstable. That is why there is an aft limit on the CoG position.

Fly-by-wire aircraft can cope with this and thus, in theory, they can fly with no lift generated by the stab and thus less drag.

Regards,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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