A quick, layman explanation would be appreciated. I searched google but couldn’t really find anything that explained the basics.
I was under the impression that trim was something that reset control surfaces on an aircraft to a new default - ie if you pulled back on the stick 20%, then setting the elevator trim there would result in the “center” stick being equivelant to the 20% back position of the previous setting. That way you could hold “automatic” climbs, etc.
But, given some experimentation with trim settings in flight simulators, it appears to be absolutely nothing like that.
So can someone explain to me the basics of what the trim controls on aircraft are, and how they’re used?
Trim doesn’t usually affect the entire control surface. If you look carefully at the rudder or elevators of a typical small plane, you will see a small part of that control surface that is hinged to move by itself. That is the “trim tab”.
In a nutshell, the trim tab is a tiny control surface in its own right. It has a small amount of the same authority that the whole control surface has. The trim settings adjust the trim settings to provide just enough control input to maintain whatever flight attitude you wish (often straight and level flight).
After re-reading my first post, I realize that I may have been misleading. The trim tabs do affect the entire control surface but not directly. The trim tab is basically a control surface for the larger control surface. It works to add or subtract preasure to the primary controls.
Not directly rjciii apart from aircraft with hydraulic flight controls and no trim tabs. Using trim tabs to hold a control position takes conserably less force than forcing the flight controls to hold that position.
Right you are, Padeye. Iwas just giving an explanation from “inside the cockpit” that was very similar to the dumbed-down version I got as a student pilot.
You may have already got your answer, but here is how I use trim (Just got down from 1.7 hours in the clouds myself).
When I take off in a 172, the trim is set neural. The small tab on the elevator is inline with the rest of the control surface. As I am rolling down the runway, I pull back on the yoke and hold it there, trying to maintain a certain climb airspeed. Now, it takes a bit of effort to hold that yoke back, so I start to gently roll the trim wheel back, which moves the trim tab downward. The trim tab then starts deflecting air downwards, pushing it and the rest of the elevator assembly up. As I roll it back, the force to hold the yoke back gradually decreases until it will simply stay there on it’s own, climbing at the speed I want.
When I get to my cruise altitude, I push the yoke forward to level off. However, it won’t stay there because I have the aircraft trimmed for climb. I roll the trim wheel forward, which helps to pitch the nose down so I don’t have to fly the whole way pushing forward on the yoke.
You can also use it to compensate for balance issues. I had a rear seat passenger this week whose weight was making the airplane want to fly nose high. So I trimmed it nose down some, and added some more power back in to keep my altitude.
Actually, it depends on the aircraft in question. There’s no trim tab - the trim control on the Piper Colt I fly manipulates the entire elevator. I believe the Piper Warrior has the same setup.
One thing I’d like to add is to point out that when you “trim” the elevator, the airplane will tend to hold that airspeed. If you subsequently move the stick, then let go, the airplane will oscillate for a bit, then return to that airspeed. If you’re trimmed for level flight at a given airspeed, take your hands off the stick, then reduce power, the airplane will begin to descend at that same airspeed. Adding power will get you climbing, again, at the same airspeed.
The reason your simulator probably doesn’t do anything with trim is that you don’t have any force feedback in the stick. The main purpose of trimming is to reduce the stick force to zero.
One of my flyin’ stories has to do with an electric trim system. The Piper Archer I was flying had an electric thumb switch you could use to adjust the trim. After you get used to it, you fiddle with it almost without thinking. Well, once the switch simply stuck, cranking in nose down trim without my noticing. That is, I didn’t notice the stuck switch, I sure did notice that the airplane wanted to go down in a real bad way. I had practically all I could do with both arms pulling back just to keep things level. The spinning trim wheel caught my eye just about the time I figured I was about to die.
Something similar happend to one of the planes ( C-130) I used to work on, only the trim tabs got stuck in the up position. Which is rather fortunate, because the plane was flying at about 300 ft when it happened. The crew had to cut the hydraulics and manually push the yoke back down, with the trim tabs trying to pull up. Now I know what those foot rests on the instrument panel are for. Since then I’ve had to move the yoke without hydraulics a few times, and it is quite hard to move. I got the honor of having to re-wire all the associated wiring in the cockpit.
A Robinson R-22 helicopter has a two-position trim. Pulling the knob out puts a bungee on the cyclic control to reduce the stick forces. They call it “right trim” because that’s all it does. We used to joke that the “prelanding checklist” in an R-22 is: “Turn off the trim.”
Incidentally the fuel mixture control has a plastic tube about an inch in diameter that fits over it. It’s not held there in any way; it just sits on the panel and around the mixture control. (That part of the panel is horizontal.) This is so that you don’t accidentally pull the mixture to idle cut-off when you were meaning to pull on the trim. I discovered the utility of that little device once when I gave a sharp pull on the “trim” and would up with a piece of plastic in my hand. (This was after I got my license, which means that even someone who is familiar with the aircraft can make mistakes!)
The Schweizer 300CB has an honest-to-gods real-live “helicopter cyclic” instead of the T arrangement of the R-22. It has an electric trim which you can get used to in about 0.2 seconds. So much better than Robinson’s kludgy thing.
[sub]The sad thing is that the only “trim” I get is the control in the cockpit! And even that has been a while…[/sub]
In addition to the elevator trim, you can also have rudder and other trim. The idea is to take some of the physical effort out of flying.
To the non-flyer, I usually compare it to cruise control on the car - it helps you maintain a steady speed with less effort.
And the simple explanation is that “aircraft trim” slightly modifies the shape and/or positioning of the airfoil it’s attached to in order to make subtle adjustments to the amount of lift generated by that control surface, which allows more precise control of the aircraft.
That explanation covers trim tabs - both ground and in-flight adjustable - as well as wonders like stabilators and flying tails and forth.
Aside from trim tabs, aircraft can also be trimmed by using springs and bungees in the control system. Rotate the trim wheel, and it tightens tension on a spring or bungee, which causes the control surfaces to move until the airplane is in trimmed flight again.