minor adjustments to control surfaces on aircraft. used to correct unbalanced loading or to maintain specific flight attitudes without continuous control pressure.

small aircraft generally only have elevator trim; larger, more complex aircraft also have aileron and rudder trim.

TRIM
State, dress. In a sad trim; dirty.--Also spruce or fine: a trim fellow.

The 1811 Dictionary of the Vulgar Tongue.

Trim is a way to make a small but constant and continuous adjustment to a control surface on an aircraft. Control surfaces are the things that make the aircraft pitch, roll, and yaw: the elevators, ailerons and rudder, respectively.

To understand how trim works it's first useful to understand how wings and control surfaces work.

Wings work largely off the principle described by Newton's third law of motion: for every action there is an equal and opposite reaction. They also utilise the coanda effect, which is the tendency of jets of fluid substances (of which air is one) to follow the contours of surfaces they are in contact with. Hold the underneath of the head of a spoon next to a stream of water from a tap to see this for yourself.

A wing is shaped in such a way that air flowing over it is deflected downwards; in doing so, the wing itself is forced upwards. Fine.

Control surfaces change the shape of the surfaces they are attached to, altering the way those surfaces deflect air flowing over them. Take elevators, for instance, which control aircraft pitch. They are mounted on an aircraft's tailplane, aka horizontal stabiliser.

When the elevators move up or down, they change the way that the tailplane deflects air. When they are flat, the tailplane probably has relatively little effect on air flowing over it. When they are moved upwards, they deflect air flowing over them upwards. In doing so the tailplane, which they are attached to, is forced downwards. So the aircraft tail moves downwards. But the aircraft itself doesn't move downwards, it pivots around its center of gravity. So while the tail pivots downwards, the nose pivots upwards and the aircraft, if it's going fast enough, will climb.

Conversely if the elevators move down, they deflect air downwards. The elevators and thus the tail of the aircraft are forced upwards, its nose pitches downwards, and the aircraft descends.

Now we can get to trim. To understand trim, simply think of the control surfaces as wings in themselves. Trim is the control surface of a control surface. Let's use the elevator again as an example:

---------====+++
'-' is the tailplane. '=' is the elevator, and '+' is the trim tab. If the trim tab moves upwards:
               +
              +
---------====+
...(exaggerated, of course) it deflects air upwards. This causes it, and the elevator it is attached to, to be forced downwards:
---------=
          =
           =
            =+++

So, when the trim tab moves upwards, it causes a downward pitch moment. The effect of the trim tab's movement on aircraft pitch is the opposite of the same movement by the elevators.

Trim tabs are typically controlled using a small wheel in the cockpit—one for each set of control surfaces—that are used to dial up a specific setting. That setting then remains until it is changed, unlike the main control surfaces, which in flight tend to return to 'neutral', or flat, positions if the pilot lets go of the control yoke.

Trim is used to apply a constant force to a control surface, without the pilot having to do it using the main control yoke. If an aircraft's weight balance means it tends to pitch up slightly in flight, the pilot can adjust the elevator trim to give a slight downward pitch moment, and level out the aircraft's pitch behaviour without having to apply constant pressure to the control yoke themselves. Trim may be available for the rudder and ailerons depending on the aircraft, but is always present for the elevators.

Trim (?), v. t. [imp. & p. p. Trimmed (?); p. pr. & vb. n. Trimming.] [OE. trimen, trumen, AS. trymian, trymman, to prepare, dispose, make strong, fr. trum firm, strong; of uncertain origin.]

1.

To make trim; to put in due order for any purpose; to make right, neat, or pleasing; to adjust.

The hermit trimmed his little fire. Goldsmith.

2.

To dress; to decorate; to adorn; to invest; to embellish; as, to trim a hat.

<-- to trim a Christmas tree. -->

A rotten building newly trimmed over. Milton.

I was trimmed in Julia's gown. Shak.

3.

To make ready or right by cutting or shortening; to clip or lop; to curtail; as, to trim the hair; to trim a tree.

" And trimmed the cheerful lamp."

Byron.

4. Carp.

To dress, as timber; to make smooth.

5. Naut. (a)

To adjust, as a ship, by arranging the cargo, or disposing the weight of persons or goods, so equally on each side of the center and at each end, that she shall sit well on the water and sail well; as, to trim a ship, or a boat.

(b)

To arrange in due order for sailing; as, to trim the sails.

6.

To rebuke; to reprove; also, to beat.

[Colloq.]

To trim in Carp., to fit, as a piece of timber, into other work. -- To trim up, to dress; to put in order.

I found her trimming up the diadem On her dead mistress. Shak.

 

© Webster 1913.


Trim (?), v. i.

To balance; to fluctuate between parties, so as to appear to favor each.

 

© Webster 1913.


Trim, n.

1.

Dress; gear; ornaments.

Seeing him just pass the window in his woodland trim. Sir W. Scott.

2.

Order; disposition; condition; as, to be in good trim.

" The trim of an encounter."

Chapman.

3.

The state of a ship or her cargo, ballast, masts, etc., by which she is well prepared for sailing.

4. Arch

The lighter woodwork in the interior of a building; especially, that used around openings, generally in the form of a molded architrave, to protect the plastering at those points.

In ballast trim Naut., having only ballast on board. R. H. Dana, Jr. -- Trim of the masts Naut., their position in regard to the ship and to each other, as near or distant, far forward or much aft, erect or raking. -- Trim of sails Naut., that adjustment, with reference to the wind, witch is best adapted to impel the ship forward.

 

© Webster 1913.


Trim, a. [Compar. Trimmer (?); superl. Trimmest.] [See Trim, v. t.]

Fitly adjusted; being in good order., or made ready for service or use; firm; compact; snug; neat; fair; as, the ship is trim, or trim built; everything about the man is trim; a person is trim when his body is well shaped and firm; his dress is trim when it fits closely to his body, and appears tight and snug; a man or a soldier is trim when he stands erect.

With comely carriage of her countenance trim. Spenser.

So deemed I till I viewed their trim array Of boats last night. Trench.

 

© Webster 1913.

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