All pilots use it, but the longer you use it and fly with it, the more you forget about the theoretical aspect of it. How does trim work? Most pilots see trim as a way to reduce stick forces on the control wheel. But what happens when we use trim, that’s what I was wondering about this morning and thought it would be good to share this as it is a kind of refresher.
Trim is all about the equilibrium of forces around the aircraft, in specific, the vertical ones. And the momentum around the centre of gravity of your aircraft. So in order to understand the working principle of trim, we have to know what vertical forces are acting on our aircraft, and from where they act.
Almost all aircraft consists of a main wing, and a horizontal stabilizer. Both have forces acting on them because of the aircraft flying through the air. Lift forces acting on the aircraft can be calculated using the formula L= ½ ∙ CL ∙ ⍴ ∙ TAS2 ∙ S. During normal flight, without the use of flaps or slats, our wing area (S) remains the same. At level flight density (⍴) can also considered to be constant. That means that the lift produced by a wing depends on the airspeed (TAS) and the lift coefficient (CL), which has to do with our angle of attack. The resultant force of lift will act from the wing its centre of pressure (CP).
The centre of pressure moves over the chord line of the wing during the different phases of flight, but in most aircraft, always stays aft of the centre of gravity of the aircraft. This is the reason why most aircraft have a negative horizontal stabilizer.
An aircraft remains altitude if the sum of all vertical forces is zero. Vertical forces consists of the gravitational force acting downwards from the centre of gravity; Lift force acting upward from the centre of gravity of the main wing; a upward force component of the thrust delivered by the engine(s) due to the angle of attack of the aircraft in level flight; a lift force downward from the centre of pressure of the horizontal stabilizer and forces up or downward from the elevator and elevator trim.
Besides remaining altitude, we also want the aircraft to maintain attitude, this is accomplished by thinking in of terms of momentum. Momentum is the willingness to start a movement and can be calculated by the force multiplied by the distance to the centre of gravity. If an aircraft has to maintain attitude, the momentum of forces has to be zero as well. Here we can see the importance of trim on our aircraft.
Because of the horizontal stabilizer being in the aft of the aircraft, its force has a way bigger momentum then a same force would have at the main wing. Therefore the trim tab is mounted on the aft side of the aircraft as well. A small area at the aft side has great effectiveness due to the big arm towards the centre of gravity.
Trim normally acts in the opposite direction of the elevator, thereby producing an force in the opposite direction of the elevator. Changing the angle of attack of the trim tab so changes the equilibrium and an aircraft can be trimmed for a specific airspeed.
The bigger get and the faster they fly, the more important it is to minimize drag. Therefore most of these aircraft have an all movable horizontal stabilizer. In these aircraft the pilot changes the angle of attack of the stabilizer and due to the bigger wing area, less change in angle of attack is needed for the same effectiveness. In this way parasite drag is minimized.