Trimming the Aircraft

Introduction:

  • Trimming is a basic, but critical skill for pilots to understand and practice
  • Thinking back to basics, pitch + power = performnace, and any combination of pitch or power will result in a deflectin of the control surfaces to maintain performance
  • Trimming allows the pilot to relieve the control pressure to stabilize the aircraft
    • Essentially whenever a control surface's neutral position must change, so must the trim
  • It generally does not come easy to most pilots and therefore requires attention and practice

WARNING:
All procedures are GENERALIZED.
Fly the maneuver in accordance with the Pilot Operating Handbook (POH)
and/or current Standard Operating Procedures (SOPs)


Trim Basics:

  • Trim refers to employing adjustable aerodynamic devices on the aircraft to adjust forces so the pilot does not have to manually hold pressure on the controls
  • This is done either by trim tabs (small movable surfaces on the control surface) or by moving the neutral position of the entire control surface all together
    • Trim tabs are likely to be on the aileron, elevator and rudder
  • Trimming is accomplished by deflecting the tab in the direction opposite to that in which the primary control surface must be held
  • The force of the airflow striking the tab causes the main control surface to be deflected to a position that corrects the unbalanced condition of the aircraft
  • Because the trim tabs use airflow to function, trim is a function of speed. Any change in speed results in the need to re-trim the aircraft
  • An aircraft properly trimmed in pitch seeks to return to the original speed before the change due to its stability
  • Trimming is a constant task as soon as you change any power setting, airspeed, altitude, or configuration
  • Proper trimming decreases pilot workload allowing for attention to be diverted elsewhere, especially important for instrument flying
  • In the pattern, if you have trimmed appropriately, you shouldn't have to use back stick at all, which should also prevent you from exceeding approach speed/on-speed
  • Pitch-Power-Trim
  • You are working to trim for an airspeed!
  • If you are changing your airspeed in any way then your trim will be off

Trimming Procedure:

  1. Set and steady your airspeed (can be in a climb, descent, or level)
    • When you change pitch, power, or configuration, wait at least five seconds before you touch the elevator trim
  2. Set the initial trim position by reducing the pressure on the controls (possibly momentarily rleasing the control yoke)
    • If the nose feels heavy, trim nose up
    • If the nose feels light, trim nose down
  3. Reduce pressure on the controls (possibly momentarily releasting the control yoke)
    • If the nose stays level, your trim has been set
    • If the nose wants to go up, you didn't set enough down trim
    • If the nose wants to go down, you didn't set enough up trim
  4. Set the attitude, then trim off the pressure with the appropriate correction
    • Small applications of trim make large changes in the pitch attitude
      • Be patient and make multiple changes to trim, if necessary
  5. Repeat steps 3 - 4 until the aircraft maintains the desired attitude
    • Once the aircraft is in trim, relax on the control yoke as much as practicable
      • When pressure is held on the yoke, unconscious pressures are applied to the elevator and ailerons, which displaces the aircraft from its desired flightpath
      • If the aircraft is in trim, in calm, non-turbulent air, a pilot should be able to release the control yoke and maintain level flight for extended periods of time. This is one of the hardest skills to learn prior to successfully flying in instrument meteorological conditions (IMC)

Trimming by Control Surface:

  • Rudder trim is the most common on general aviation
    • The rudder is trimmed right for power increases and slower airspeeds
    • Trim the rudder left for power reductions and higher airspeeds
  • Elevator trim:
    • Trim the elevator up for power reductions and slower airspeeds
    • Trim the elevator down for power additions and higher airspeeds
  • Aileron:
    • Used as required to level an aircraft
  • Trim Tabs:
    • A servo trim tab moves opposite of the surface it is trimming
    • An anti-servo trim tab moves with the surface it is trimming

Common Errors:

  • Over controlling
  • Trimming when the aircraft's airspeed is changing

Airman Certification Standards:

Conclusion:

  • When transitioning from climb to cruise, keeping in climb power until the aircraft has reached cruise airspeed will make the transition quicker and therefore reduce the amount of time adjusting trim
  • Consider practicing maneuvers on a flight simulator to introduce yourself to maneuvers or knock off rust
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References: