TRIM HELP PLEASE!

[bok269]

I am new to Flight sim, as well as flying in general, and am perplexed by how to use trim?  The lesson in Flight Sim doesn't seem to help me.  Can someone please explain to me how you use it?

[Mobius]

Trim is used to reduce the forces required to move the yoke/stick.  When you fly, the forces around the lateral axis of the airplane are uneven after changes in power, speed, etc..., so the trim tab is used to trim the airplane, or balance out those forces by changing the amount of lift produced by the horizontal stabilizer.  The imbalance of forces is apparent when you let go of the yoke or stick, and the nose moves up or down on it's own, a properly trimmed airplane's nose will stay level.  You trim the airplane by adjusting the deflection of the trim tab in the cockpit using a switch or wheel.  I can't remember what the buttons in the sim are because they're mapped to my joystick, but as an example, if you notice the nose moving up when you release the stick, you would want to trim the nose down, but do so in small increments so you don't trim too much, and end up chasing the proper trim, making your passengers horribly sick.

[beaky]

Use it sparingly; that's Rule #1.
It's very hard to use in FS compared to real life, and unless you have your elevator sensitivity adjusted properly, it's going to be nearly impossible to use.
But here's the idea: the trim wheel or electric trim servo controls a little tab on the trailing edge of the elevator (or sometimes it moves the whole horizontal stabilizer, but not many planes have that).
As you move the wheel, the tab deflects. When you add "nose up" trim, the tab deflects downward, which causes the elevator to "want" to go upwards while in flight (picture it). This of course makes the plane pitch up.The opposite applies to "nose down" trim.

The reason it's there is to "trim off" the force the pilot has to apply in order to maintain a certain pitch angle... to climb or descend, or to maintain a given airspeed at a given power setting. In a real airplane, it's usually possible to trim the plane so you can let go of the yoke or stick, and it will "stay put" until something disturbs it.
Took me a while, but in RL I eventually learned to fly a Cessna in a tight circle hands-off, by setting the bank, then using trim to hold the necessary "back force".

In RL, it's pretty easy, because you can feel how much resistance you're "fighting" with your hand. In the sim, it's much harder, but with a good model and some adjusting, it's very do-able.

You probably remember the lessons sayingto dial in "takeoff trim" before rolling- this helps ease the amount of force you need to rotate, and also guarantees that if your hand slips or whatever, the elevator will still tend to go up, which will keep you climbing.

Keeping in mind what I said about "trimming off force" is the key to understanding how to use it. You have to put the nose where you want it, then apply up or down trim until you can let go and the pitch angle remains more or less stable. This is where the sim makes it harder, even if  you have force-feedback controllers... in RL, you can literally feel when you have it trimmed right.

So try that: hold it where you want it, apply a little trim, let go, put it back where it cshould be, trim some more, etc.

You may find that you think you have it trimmed off, but it still dips or rises a little... some planes need a moment to oscillate before they "settle down", so be patient. Otherwise you wind up "chasing" the nose up and down, and next thing you know, it's so "out of trim" you can barely hold it level.

And use small amounts of trim, always. Trim... wait... check.... trim... wait.

Best way to learn this is in a slower plane, like the Cub... once you get the hang of trimming, try keeping your hand off the yoke or stick and climbing or desending with trim alone.


And again: check your sensitivities... for elevator, and for trim, if you're using a pot on the controller to adjust trim. Experiment- it depends very much on the plane, etc.

[Brett_Henderson]

Remember the old joke, "How long does it take to trim a 172 ?" ...

Trimming for a climb or descent is a little easier than trimming for level flight. Rotty's post covers it all pretty well. I'll just add the "rule" I was taught long ago as it applies to trimming for level flight.

Pitch, Power, Trim ...  In that order.  For example, when leveling off after a climb; Pitch first to stop the climb and let the airspeed come up. You're supposed to wait until you're at target cruise speed (after a while you get a feel for when it's the right time for step two (reference the old joke)). Then reduce to cruise power... and then trim until you no longer have to "hold" the nose down. If it's plane you're familiar with.. you'll get to the point where you blur those steps together.. kinda do them all at once, because you "know" where the plane will settle.

The answer to that joke ?   ...   About fifteen hours  

[bok269]

Interesting because in FSX Im trying to trim a 172...In the lesson, it mentions something about the airplane maintaining the speed at which it was trimmed.  How does speed come into all of this?

Thanks for your help.  I really appreciate it!

Also, for a force feedback joystick, how much would that cost?

[Brett_Henderson]

Picture a plane in level flight. The airspeed has to be constant too (that's where your target airspeed comes in). Now... if the airspeed is changing (without any throttle changes) the plane HAS to be climbing or descending. A known power setting (RPM) will yield a known airspeed for level flight. Sooooooo ..  if you've set the throttle for that RPM and trim to maintain that airspeed... you get the idea...

[Brett_Henderson]

Oh.. and if it has to be one or the other.. Put your money toward a yoke and pedals, instead of a force-feedback stick..

[Mobius]

An interesting thing I learned on my BFR was that if you ever get stuck in the clouds in a 172 (assuming you're not instrument rated, because then you really shouldn't be getting stuck in the clouds), trim yourself to what you know will be 85 kt descent, then sit on your hands.  Apparently you will descend slowly enough that you shouldn't crash (quickly at least), and you will be able to get out of the clouds before hitting the ground (if you do though, you should be surprised at how foggy it is and how unlucky you were), and hopefully be able to find someplace to land.

[Brett_Henderson]

Don't mind him.. he thinks he's punchy from over-study (when it's actually because the Packers

[Mobius]

Don't mind him.. he thinks he's punchy from over-study (when it's actually because the Packers  won a game.. the whole state is giddy)

HA! Finally.  Brett's still got it!  (FarveFAVRE that is, Henderson always has it [smiley=thumbsup.gif])

[Brett_Henderson]

Ok.. now you're in trouble... What is the punishment in Cheese-head land for misspelling that name ?

[Mobius]

Eerrmmm, whoops, must be all the studying.  [smiley=cheesy.gif]

[Hagar]

This thread is in danger of becoming bogged down in complex technical explanations which can only end up causing more confusion than it resolves. The question was simple:

how to use trim?

The simple answer is that trim relieves the pressure on the controls so that the aircraft can be flown "hands off" in all parts of the flight envelope. This takes the strain off the pilot & makes the aircraft much easier to control. Although aileron & rudder can be trimmed on some aircraft, when people refer to trim they usually mean elevator (pitch) trim which is the one most likely to require continual adjustment throughout the flight.

The method of adjusting trim varies according to the type of aircraft but can involve simple bias springs, small "trim tabs" on the control surfaces & even completely adjustable main surfaces like all-moving tailplanes (horizontal stablizers). These can be controlled by various means including levers, wheels, rotating handles & electrical switches, knobs & dials. Some aircraft can be flown in normal flight by using the trim controls alone while others have fixed trim which is adjustable only on the ground.

It's not always easy to replicate this in FS especially when relying on key strokes or push buttons. The best type of controller is one with rotary controls that can be assigned to the trim. This makes trimming much easier & intuitive until it becomes second nature. I've "flown" many FS aircraft using the rotary trim wheels alone.

One more thing. I agree that a set of rudder pedals would be more realistic but whether you use a joystick or yoke is a matter of preference. The joystick is regaining popularity in real life & many modern GA types are fitted with them. Most vintage light aircraft & aerobatic types use joysticks. Modern "fly-by-wire" jet fighters & even large jet airliners like the Airbus range feature controllers not unlike a gaming joystick. I can foresee a time when the only aircraft to use a yoke will be trainers & light twins. This is a complete reversal of how it was only a few years ago.

PS. I always thought the most realistic trim controls were those little wheels fitted to the old-fashioned analogue joysticks. By varying the neutral position of the joystick itself these work in exactly the same way as the trim controls on many aircraft.

[FridayChild]

Here's something that will surprise you: my joystick has such a poor response on the elevator axis that during flights I often find myself trimming instead that moving the stick up or down. It's much more accurate... if I move the stick the slightest bit, I tend to get a brutal response. That is so unlike the real thing...  :(

[Hagar]

[quote]Here's something that will surprise you: my joystick has such a poor response on the elevator axis that during flights I often find myself trimming instead that moving the stick up or down. It's much more accurate... if I move the stick the slightest bit, I tend to get a brutal response. That is so unlike the real thing...