Proper way to trim?

[clamman]

I seem to have a lot of trouble gatting aircraft to trim properly. Seems I go to much one way than too much the other way. Is the trim wheel supposed to be real sensitive or do I need to adjust it some how? Also if I notice I am starting to drop altitude should I be using trim to bring it back or should I pull back on the yoke than trim afterwards? Thanks for any advice>

[xxmikexx]

clamman,

The correct way to trim is to do it until you don't have to apply any control forces to fly straight and level. After that you would control rate of climb/descent trend with power. However ...

The required amount of trim required will change with airspeed. Another complicating factor is that if the autopilot is on it will control angle of attack by operating the elevator trim, effectively negating any manual trim elevator inputs you may have made.

For the benefit of those of you who fly props aircraft, the first thing you should do is to use rudder trim to remove any yaw tendency. (Check the ball in the turn/slip indicator.) Then you should trim the elevators. Finally, if there is any residual rolling moment you should counter it with aileron trim though you're not likely to encounter the need for this -- typically only if there were a fuel imbalance in wing tanks.

I hope this helps,

[tigisfat]

IT's a matter of choice whether you use trim or elevator pressure to correct smal errors during cruise.

Personally, once the aircraft is trimmed out, I use small amounts of elevator to counteract pithc moments in cruise. The reason is, you're likely to have to set the trim back to where it was in a short minute. Thermals and descending air greatly affect small aircraft with high wing-loading under 10,000 feet. While it may seem that the aircraft has gone out of trim; it hasn't. You merely need to get to the other side of the phenomena causing the small climb or descent.

Think of every trim setting having a specific speed and drag configuration that it's good for. If you're perfectly trimmed out for 90KIAS for an approach and you add power, you'll get an initial 2-3 knot increase in airspeed as the nose pitches up. The nose will go further than it needs to because of the extra knots. A true instrument pilot will know how far it needs to go up and 'guide' the aircraft's nose to the right place while letting your trim setting do most of the work. The added power but unchanged with trim will eventually have you climbing at 90KIAS. The same is true about descents.

In an aircraft that is both statically and dynamically stable longitudinally will always try to return to a flight attitude that makes your speed and trim agree. It may porpoise (very slowly, and only up and down 30-40 feet) a few times, but that's only if you fail to help it find the right attitude without overshooting. What causes porpoising and overshooting is extra and too little speed. If you're trimmed out for 90KIAS and you enter a thermal, you'll get high and slow. Because of the reduced lift and trim setting, the nose will drop. Going below your cruising altitude now, the aircraft picks up speed that is too fast for the trim setting and the nose rises. You now climb slightly past your desired cruising altitude. This will go on and on a few times, high and low, until the aircraft settles pretty close to where you initially set up your trim and at the right speed.

Set MSFS up with no winds and give it a shot. These concepts are most true in small, stable propeller-driven aircraft when in normal weather conditions. Once you see how it all works, you'll probably develop your own technique.

When in airliners, I rarely touch the elevators except for the flare. At those speeds and departure (from level flight) rates, I find trim is all I need to gently maintain altitude.

[xxmikexx]

tigisfat,

I agree with you completely about making small corrections with elevator. That's why I used the word "trends", which is where power changes come in.

[tigisfat]

Where it gets REAL fun is this:

aircraft with high wing loading and excess power have different rules. I fly a seminole pretty often, and I set power once for the whole approach. I know it well enough that 18" manifold pressure will yield me blueline at 88KIAS with a certain trim setting and 25 degrees flaps will add the correct descent at around 500FPM. All changes in airspeed and descent are made by pitch only. I had a conversation with a DC-10 pilot about such a technique, and he admitted that it was his secret to the perfect approach as well.

[bstolle]

clamman,

You have to take into account that when using the elevator (or stabilizer) trim there is a considerable lag.
You have to trim a bit, wait a few seconds and then trim further.
If you try to crank in the correct amount at once you are likely to trim too much, hence the impression that the trim is oversensitive.

E.g. in IRL I NEVER use the stab trim during the approach for small corrections but this implies that the plane has been perfectly trimmed initially.

When using elevator/stab trim I most of the time even let go of the yoke/stick for a second or so after trimming, to make sure that the plane is perfectly in trim.

tigisfat

Confirmed, as long as the speed is within plus or minus 5kts of my target speed I don't touch the thrustlevers or stabilizer trim (in the 767)

[clamman]

Thanks for the suggestions guys. I think my biggest mistake is making too big of an adjustments instead of small ones and waiting. Now if I could just land without crashing I'd be all set.

[lnuss]

Thanks for the suggestions guys. I think my biggest mistake is making too big of an adjustments instead of small ones and waiting. Now if I could just land without crashing I'd be all set.

A couple of common mistakes in landing are in flying the approach too fast and in trying to "land" the aircraft. Approach speeds in airliners will typically be in the 120 to 150 knot range, depending on the aircraft and its weight, and for light single engine aircraft approach speed is typically in the 70 to 90 knot range, again depending on the aircraft.

You don't actually land an aircraft. Instead, you want to descend to a foot or so off the ground and maintain that altitude by gradually increasing back pressure until (with power way reduced or at idle, depending on the aircraft) the aircraft settles to the ground of its own accord. And, if you are using autothrottle you must disengage it separately from the autopilot. It takes practice to perfect the landing, of course...

[xxmikexx]

lnuss,

I will add to your remarks that a spot landing typically must be a "wheels" landing, in which the aircraft is driven onto the runway at a planned spot at a low rate of descent.

I will also observe that wheels landings are easier to perform than classic full stall landings. So maybe clamman could start out with wheels landings and work his way up to doing it right.

[bstolle]

I disagree. In a tailwheel aircraft my spotlandings are always 3 pointers.
Only in conventional planes with a nosewheel I never do threepointers
E.g. a few weeks ago I flew a tailwheel motorglider which MUST be landed in a threepoint attitude as the single mainwheel suspension too rigid for a wheel landing.
Furthermore in a threepoint attitude you are slower, also not bad for a spotlanding

If you do a full stall landing the plane naturally stays on the ground, but you can screw up a wheel landing quite easily.