Ratty's Ramblings - Trim and Trimming

By Ian Radcliffe

There are many aspects of flight that are not necessarily obvious, and one of these is the role of trim. Not everyone understands trim, and a lot of people who think they do don't. It doesn't help that in our simulations it's often poorly implemented. I've also found that amongst sim pilots there are two common misconceptions that I should disabuse you of right now. The first: trim is not a flight control. The second: it's not an "elevator booster". Let's take a look at the purpose of trim, and then at how to use it.

Most modern aircraft are dynamically stable; that is, once established in a steady flight state they will tend to stay in that state, or return there if displaced. In cruise flight, the pendulum stability of high-wing aircraft and the dihedral of (most) low-wing aircraft help maintain wings-level flight, and help restore it if the aircraft banks. Apply rudder briefly and the nose will swing from side to side until the vertical stabilizer brings the airplane back to straight-ahead flight. Raise the nose a little and release the controls and the nose will drop and rise a few times before settling back into level flight.

Early aeroplanes with no throttle had one cruise speed; the power options were only All or Nothing. If, at cruise speed, your plane had a tendency to roll or climb or turn it could be rerigged on the ground to fly straight so you didn't have to hold control forces all the time. But add a passenger behind you and suddenly your plane is tail heavy, and heavier, and your elevator forces have changed.

The biggest variations in control forces take place in pitch. Changes in speed, weight, power, and airfoil configuration all contribute to how hard you have to pull or push. Increase power in level flight and the plane will start to climb; if you want to stay level and go faster you'll have to hold forward pressure on the stick. Decrease power, and to maintain level flight you'll have to hold constant back pressure. And that's what trim is for: to stop your arm getting tired.

The first trim device apparently was developed in 1910; all I've been able to find out is that it was "ground adjustable", but I'd bet it was created to accommodate changes in pitch. There are many ways that trim is implemented, most being aerodynamic and involving adjustment to the elevator and/or the entire horizontal stabilizer. The simplest system is a bungee or spring device connected to the stick.

Aileron and rudder trim appear mostly on larger aircraft, because planes are most stable in roll and yaw. About the only way to make one wing heavier than the other is to mismanage fuel in the wing tanks. Propeller-driven airplanes do have yawing tendencies that vary with different power settings and speeds, but they're usually rigged for cruise, so a P-51 Mustang REALLY wants to swing left on takeoff, but bores straight ahead in level flight.

In practice, I find I can fly for quite a long time holding control pressure, like holding a fixed accelerator position when driving, so my use of trim may be different than others', and I encourage you to look around and see what others have to say. For what it's worth, here's how I use trim:

Before flight I set the elevator trim to the takeoff position. After takeoff, I establish a climb at the desired airspeed; to maintain that speed I need to maintain a particular stick pressure, but if I do it too long my arm will get tired. This is where I adjust the trim until I don't need to push or pull any more. In a real airplane you can feel the pressure going out of the controls. I have nose down/nose up buttons on my stick, and as I run the trim I can almost feel the pressure going out of the controls.

When I get to my cruise altitude and level off, I lower the nose first, then adjust the throttle as necessary, then wait (because it can take a while to settle on a speed) and then retrim.

After that, as I burn fuel my airplane will get lighter and want to climb. If I drop ordnance or fuel tanks my airplane will want to climb. If I increase power my airplane will want to climb. If I reduce power, my airplane will want to descend. If I add weight (as in air-to-air refuelling, for example, frankly the only example I can come up with) my airplane will want to descend. To maintain level flight, or establish a particular climb or descent rate, I will now have to apply force to the stick again. I wait for the speed to settle and then retrim.

In the landing phase there can be numerous changes in the stick pressure you use. Slowing, power adjustment, lowering gear and flaps, all change the trim of the plane. I generally trim when established on downwind at a steady speed with the gear extended. Once cleared to land, reducing power puts me into a descent without the need to touch anything else. Speed adjustments will require pressure on the stick, but since I still have the flaps to deploy I don't touch the trim again until I'm established on a steady final approach with everything hanging and no major power changes in the offing.

And then I won't touch the trim again; unless I have to go around, in which case I will apply power, clean up the aircraft, establish a climb...and trim.

FS20...

...or, yes, I know, Emesseffess.

I bought it, installed it, flew the 152 over my house, and uninstalled it.

It's very pretty, and I guess has a lot of possibility but, for me, without realistic aircraft and without the ability to tinker like FSX and P3D it really is just pretty pictures. I go all the way back to FS2: for all its primitive graphics it was an amazing simulation of the Cherokee, and since then I've always made flight realism my priority over how it looks.

Maybe one day...

More Hang-ars/ers

Ratty's Ramble

Although the Misfit Squadron have flown all over the world, there are still regions we haven't explored. I've been looking at eastern and south-eastern Europe recently, and I've found several areas that make for spectacularly pretty VFR flights. This is one of them, comprising a piece of northern Greece and a bit of southern Albania. (Note: I flew this in FSX with Orbx Europe Landclass scenery. Your viewing experience may vary.)

It's a 170-mile trip, tailored for Cherokee and T-6 speeds, but suitable for faster aircraft, though some of the canyon turns get a little hairy over 200 knots. Flying at two thousand feet provides a great view of the scenery and is a comfortable flight altitude, or there are canyons and valleys aplenty for the more adventurous. There are one or two "surprises" that you need to be at 4,500 feet to clear.