AT
Topic Author
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Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 12:11 am

Hello all.
Can someone explain in layman terms what "Trim" is?
How is it controlled on the aircraft and what does it accomplish?
If it is simply to move the nose up or down, wouldn't the elevators take care of that?

I looked on line but there are no good descriptions.
A nice 101 would be most welcome.

Thanks!
 
swiftski
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 12:25 am

Quoting AT (Thread starter):
If it is simply to move the nose up or down, wouldn't the elevators take care of that?

You "trim" an aircraft to reduce the force required on the control column in flight. This makes it much easier to control all stages of flight.

In the case of elevator trim, you set the attitude you wish, then 'trim' is used to maintain that attitude with little or no further control input.
When trimmed accurately, the trim tab will hold the elevator in the same position as you would set it to whilst hand flying.
 
474218
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 12:59 am

The elevators control the aircraft in the pitch mode (nose of the aircraft up and down).

Pitch trim positions the elevators so they are in correct for the current mode of flight.

See: http://www.allstar.fiu.edu/aero/fltmidfly.htm it will provide a good description on how airplanes fly and how they are controlled.
 
timz
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 1:01 am

When you "trim" a 737, you're moving the whole tailplane, not just the elevator?

Same with other airliners?
 
474218
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 1:44 am

Quoting timz (Reply 3):
Same with other airliners?

All airliners trim able horiziontal stabilizers.
 
KELPkid
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 2:16 am

Quoting AT (Thread starter):
Hello all.
Can someone explain in layman terms what "Trim" is?
How is it controlled on the aircraft and what does it accomplish?

Trim=a fine adjustment on a control surface to relieve pressure the pilot (or autopilot) would otherwise have to hold to keep the airplane where they wanted it. When you trim out the pitch correctly for cruise flight in, say, a Cessna 172, you will not have to use any force at all on the yoke after that point, until it's time to descend or climb (in theory, of course-works great on a really calm day with smooth air).

Continuing with our trim example in the good ol' C172, there is a large wheel, horizontal, marked 'TRIM" with little knobbies every so often on it down below the instrument panel, on the console between the pilot's seat and the passenger/flight instructor seat rudder pedals. (probably about an inch between each of the knobbies on the trim wheel). When turned, it actually moves a smaller "servo tab" on the elevator up and down depending on which way you turn the trim wheel (the elevator is the moving control surface at the back of the horizontal stabilizer). This, acting through lift, makes the elevator want to seek the position of least resistance (least amount of lifting force trying to act to move the elevator up or down about it's hinges) and tips the equilibrium of where the elevator wants to rest to your favor (unless you move the trim wheel the wrong way).

The best way to think of things is thus: the elevator puts the plane in the attitude you want it in in the sky, and the trim, when properly adjusted, keeps the plane there with no effort on your part   (if you find yourself having to push or pull on the yoke in cruise, you are not properly trimmed).

Quoting AT (Thread starter):
If it is simply to move the nose up or down, wouldn't the elevators take care of that?

Yes, but you wouldn't want to hold the elevator by hand for the entire duration of a flight. #1, you would get tired. #2, it is also ineffecient (holding the elevator either up or down creates more induced drag on the airframe, causing you to slow down and burn more fuel if you're trying to maintain a speed...). When trimmed properly, there is less induced drag.

Also interesting to note: many other airplanes also have cockpit adjustable trim on other control surfaces. For example, the Cessna 182 usually has rudder trim...   (reduces the amount of rudder you have to hold to counter P-factor).
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
pilotpip
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 2:18 am

In KISS terms.

Adjust pitch with the elevator. Maintain that attitude with trim.
DMI
 
timz
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 2:57 am

One thing that's confusing us non-pilots: we've all heard of "trim tabs", those little flaps on the trailing edge of the elevator, on a C172 anyway. And some? many? all? airliners have them too-- but when you trim an airliner you move the whole tailplane, not the tab. (Right?)

Another point that maybe needs making: in a conventional airliner, the center of gravity is ahead of the center of wing lift, so the tailplane needs to balance that moment arm-- the pilot somehow needs to set the tailplane/elevator combination so the passing air pushes down on it with the right amount of force. As the center of gravity shifts during the flight, the necessary downforce at the tail changes too.
 
474218
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 3:07 am

Quoting timz (Reply 7):
One thing that's confusing us non-pilots: we've all heard of "trim tabs", those little flaps on the trailing edge of the elevator, on a C172 anyway. And some? many? all? airliners have them too-- but when you trim an airliner you move the whole tailplane, not the tab. (Right?)

Wrong, movement of the tab moves the elevator (aileron or rudder) in the opposite direction.

Quoting timz (Reply 7):
Another point that maybe needs making: in a conventional airliner, the center of gravity is ahead of the center of wing lift, so the tailplane needs to balance that moment arm-- the pilot somehow needs to set the tailplane/elevator combination so the passing air pushes down on it with the right amount of force. As the center of gravity shifts during the flight, the necessary downforce at the tail changes too.

All modern airliners have automatic stabilizer trimming. It constantly compensates for changes in CG due to fuel burn and speed.
 
timz
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 3:12 am

Quoting 474218 (Reply 8):
when you trim an airliner you move the whole tailplane, not the tab. (Right?)

Wrong

When you trim an airliner, you don't move the whole tailplane?
 
nightflyer
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 3:26 am

One of the things I remember from my first flight instructor, in a Cessna 172, is that you trim for airspeed. That works in the Cessnas that I used to fly all the way to the MD11 that I fly now. The amount of power or thrust that you have will determine if you climb, descend, or maintain level flight. The airplane will always seek the airspeed that it's trimmed for. That's my flight lesson for the day.  
 
nightflyer
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 3:27 am

Oh, and yes in an airliner you're moving the whole horizontal stab when you move the trim switch.
 
474218
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 4:07 am

Quoting timz (Reply 9):
When you trim an airliner, you don't move the whole tailplane?

I don't know one airliner that does not have a trimable horziontal stabilizer. If you do please tell me.
 
rwessel
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 4:27 am

Quoting 474218 (Reply 12):
I don't know one airliner that does not have a trimable horziontal stabilizer. If you do please tell me.

Concorde? ATR-42? DC-3?

Quoting KELPkid (Reply 5):
it is also ineffecient (holding the elevator either up or down creates more induced drag on the airframe, causing you to slow down and burn more fuel if you're trying to maintain a speed...). When trimmed properly, there is less induced drag.

That's only true if the horizontal tailplane trims. For a tab or force based trim system, the tab or spring just forces the elevator to the position needed to hold the trimmed speed, and the drag is no different than if you input the same elevator deflection via the stick. And even with a trimming horizontal stab there's plenty of drag, it’s just that the resulting flat airfoil is more efficient than one with a big bend in the middle (as happen when the trim system moves just the elevator).
 
tdscanuck
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 5:21 am

Quoting KELPkid (Reply 5):
When you trim out the pitch correctly for cruise flight in, say, a Cessna 172, you will not have to use any force at all on the yoke after that point, until it's time to descend or climb

Even if it's time to descent or climb, you don't need to touch the yoke if you don't want to.

Trim is the condition where the airplane is balanced in pitch (all the torques even out and the airplane doesn't want to change pitch). This condition is related to a particular speed, not to a particular attitude. If you go off speed, and don't touch anything, the airplane will naturally return to the trim speed. So, if you're trimmed for straight and level flight, the pull back the throttles you will slow down, the torques will get out of whack, the airplane will pitch down, and it will pick up speed until it is balanced again (now in a nose-down position).

Tom.
 
DiamondFlyer
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 6:47 am

Quoting 474218 (Reply 8):
Wrong, movement of the tab moves the elevator (aileron or rudder) in the opposite direction.

Wouldn't that depend on if the tab was a servo tab or anti-servo tab?

-DiamondFlyer
From my cold, dead hands
 
flymatt2bermud
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 1:33 pm

Quoting timz (Reply 3):
When you "trim" a 737, you're moving the whole tailplane

Same on the Mooney!

Let me help put this in perspective. With regard to the inputs required to maintain control of an aircraft with a jammed trim. I remember running various trim failure scenarios in the simulator at FlightSafety. While in a nosedown trim (high speed) once you start slowing to approach speed the pilot input required on the control column to maintain control can push 50lbs, that doesn't seem like much until you have to manually fly for 10 minutes or so. Great for the abs in case you have been neglecting your six-pack.

What happens in this situation with a side mounted yoke?
"When once you have tasted flight, you will forever walk the earth with your eyes turned skyward" Leonardo Da Vinci
 
Rj111
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 1:55 pm

The elevator is shift and the trim wheel is caps lock.  
 
AT
Topic Author
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 5:36 pm

Quoting RJ111 (Reply 17):
The elevator is shift and the trim wheel is caps lock.  

Interesting way of putting it!

Thanks guys. This is enormously helpful and explanatory.
 
Fabo
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Fri Feb 26, 2010 6:08 pm

Quoting 474218 (Reply 12):
I don't know one airliner that does not have a trimable horziontal stabilizer. If you do please tell me.

Tu-154. You only set stabilizer for TO and approach phases, and, normally, not via precise setting as for ex. in B737, but rather in 3 sections (FWD/MID/AFT CoG). When you fly, you trim your elevator.
The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
 
pilotpip
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Sat Feb 27, 2010 7:06 am

Quoting flymatt2bermud (Reply 16):
What happens in this situation with a side mounted yoke?

The same thing that happens in any other aircraft.

I believe there is a requirement to maintain control of the aircraft regardless of a runaway pitch trim situation. Still, not fun and I'm not a fan of the "run the trim up as you flare to improve the landing" tecnique because having full nose up during a go around is not a fun proposition. Plus if you're not strong enough to muscle a flare (I know more than a few 5'4", 100lb women than do just fine with it) you shouldn't be flying.
DMI
 
bri2k1
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Sat Feb 27, 2010 5:05 pm

Quoting 474218 (Reply 12):
I don't know one airliner that does not have a trimable horziontal stabilizer. If you do please tell me.

Not positive, but just looking at it, MD-80? DC-9? Probably most, if not all, horizontal stabs mounted on top of the vertical stab?
Position and hold
 
tdscanuck
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Sat Feb 27, 2010 5:24 pm

Quoting bri2k1 (Reply 21):
Not positive, but just looking at it, MD-80? DC-9? Probably most, if not all, horizontal stabs mounted on top of the vertical stab?

The whole DC-9 family and derivatives have a fully moving stabilizer. There's a jackscrew inside the vertical fin that pivots the stabilizer around a hinge.

Tom.
 
tdscanuck
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RE: Can Someone Explain The Use Of Trim Vs. Elevator

Sat Feb 27, 2010 7:40 pm

Quoting tdscanuck (Reply 22):
The whole DC-9 family and derivatives have a fully moving stabilizer. There's a jackscrew inside the vertical fin that pivots the stabilizer around a hinge.

See pg. 13/14 (27/28 of the PDF) here for a good illustration:
http://www.ntsb.gov/publictn/2002/AAR0201.pdf

Tom.

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