How to PROPERLY trim a medium jet aircraft

[error_401]

Hi flight testers,

I'm having a little issue on day to day operations on medium jets as to how correctly and properly trim a medium jet. I say medium jet because that is my actual work-horse but in general I'm referring to a swept wing jet aircraft with powered controls.

A long time ago an ex test-pilot on light aircraft (Piper) told me this:

You start with the rudder trim in order not to sideslip the aircraft. Once it flies straight and tends to roll you may apply a bit of aileron trim.

This also worked out nicely on the ATR turboprop and in cruise flight a gain or loss of 1 - 3 knots could be observed between a good trim and a poor trim. (Not talking about loading here).

Anybody a good insight into how to properly trim a swept wing jet and why?

Thanks for your insights.

[Gerz]

How I used to do it

For the Boeings I've flown:

First we would set the rudder trim to "neutral" at first trim point

1. At 250KIAS
2. Ensure symmetrical power and fuel balanced
3. Hold wings level and use rudder trim to hold a constant heading
4. Don't touch the rudder trim again!
5. Record aileron trim to keep level wings

Repeat trim point at desired speeds and configurations

[LOMCEVAK]

There is no difference in the philosophy of trimming a swept wing or a straight wing aircraft; always trim in yaw then in roll. However, all other factors being equal, a swept wing has greater lateral stability (dihedral effect) than a straight wing. Therefore, any sideslip that is present due to directional mistrimming will generate a greater rolling moment with a swept wing than with a straight wing, and this will appear as a greater out of trim condition in roll.

One aspect to consider is the definition of 'in trim' in yaw. Typically, the slipball (or other lateral acceleration symbol) is trimmed to neutral because that is the primary indication. However, note that in wings level flight this may not be zero sideslip, especially at high power settings in a single prop aircraft or in a twin prop when both propellers rotate in the same direction. This is due to a lateral aerodynamic force due to an asymmetry in the airflow over each side of the fuselage and fin which, for equilibrium, must be countered by an opposing aerodynamic force that is generated by sideslip. Similarly, if you are in wings level flight and OEI with the 'slip ball' centred you most certainly will not be at zero sideslip and the resolution of the forces here follows the same logic.

There are some aircraft and some circumstances when you may wish to maintain zero yaw rate/constant heading but to do so the slip ball will not be centred. Typically, a decel to the stall in a prop aircraft or the case of an aircraft such as the Buccaneer which had the speedbrakes extending into the jet efflux such that unless they were perfectly symmetric a yawing moment would be generated but with little effective lateral force. The Bucc was flown in the landing pattern and on the approach with varying amounts of speedbrake and you just trimmed to constant heading and ignored the slip ball.

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Thanks for your insight.

LOMCEVAK - great summary.

Looks like I've been correct in how I trim the aircraft. On the EMBRAER E190 it works exactly this way but I wanted to make sure I'm not on a wrong path.

Cheers