PDA

View Full Version : Trim set at take-off

mbenkhort
22nd May 2011, 23:17
Hi all;
Would like to know why pilots must set a trim value for take-off and how is this trim value calculated?

freightforlife
22nd May 2011, 23:30
Trim is set prior to takeoff to counteract the pitching moment of a rear or forward centre of gravity. For example if an aircraft has been loaded at the aft CG limit the pilot will set a certain amount of nose down trim so that excessive control force is not required on take off.

EW73
23rd May 2011, 02:26
Every commercial jet has a range within which the CofG must be located after all the effects of basic weight, pax, cargo and fuel have been considered.
When that CofG has been calculated by the use of either a computerized program, or manually with a proforma that has a series of steps of filling in the squares and adding all the weights, the end result of which will give you a Zero Fuel Weight (the weight of everything except the fuel), and adding the total fuel will give you the gross weight, and minus the taxiing fuel will give you takeoff weight.

So, together with many other factors (temp, OAT, runway length, etc), the pilots figure out their takeoff performance numbers (like V1, Vr and V2) using this takeoff weight calculated figure.
In addition, because the calculated CofG can be within a range of percent MAC (mean aerodynamic chord), the pilots simply use the adjustable pitch trim to account for where the actual CofG is located.

For a forward CofG, the pitch trim is adjusted slightly nose up, and vice versa.
This allows for predictable pitch control at the point of liftoff, it's up to the pilot to retrim as usual from then on!

That's the idea anyway! ;)

Capn Bloggs
23rd May 2011, 07:20
In my machine, the ZFW, Fuel on board, Flap Angle for takeoff, the three V Speeds, the ZFW C of G and the Takeoff C of G (both from the loadsheet) is inserted into the FMS. The FMS then calculates and displays the Stab Trim setting, which is then set by the crew with the pitch trim buttons.

bingofuel
23rd May 2011, 07:46
To add my tuppence worth, if I remember correctly, the trim setting should also mean that in the event of an engine failure(on a multi) or normal take off in a single, the correct setting should mean the aircraft is trimmed for a initial climb speed of V2 or Vy accordingly , so the aircraft naturally adopts roughly the correct climb attitude.

john_tullamarine
23rd May 2011, 09:35
The loaded CG, via a graph generally in the AFM, dictates a nominal trim setting for a given flap configuration.

redED
23rd May 2011, 12:15
Our trim setting manually calculated from:

Flap setting
CofG
TOWeight

mbenkhort
23rd May 2011, 12:37
Hi again,
Thanks for all your replies, but as a matter of trim set to avoid excessive force on yoke by pilots to counter pitching moment (this is what I have heard often) , this does not seem to be logic for me because ( the case of commercial aircrafts), the effort that moves the moving surface (elevator) is furnished by hydraulic servos (or electric motors), not directly by the pilot muscles, and thus these hydraulic servos and motors are designed to move the elevator at all conditions of flight (included take off at all allowed cg's).
Just think of FBW (fly by wire) aircrafts where the pilot's arm just feels an artificial feedback force proportional to the real force applied to the elevator.
Then, the purpose of trim set at take-off is still not clear for me.

Al Murdoch
23rd May 2011, 13:26
What you say is partly true, however the servos act a bit like power steering in a car. A force on the control column is still required (in the case of non-airbus types) to maintain the correct attitude on takeoff. With an incorrect trim, the stabiliser will be producing an incorrect amount of downforce and therefore the pilot would need to apply an immediate large force on takeoff to achieve the correct attitude. By setting a takeoff stabiliser trim, we are setting a sort of null position for the liftoff speed and therefore making rotation easier and safer. An incorrect stabiliser trim can rotate the aircraft prematurely on takeoff or make liftoff at the correct speed slow or difficult.
Imagine if your car steering wheel was permanently set to hold the wheels 10 degrees off to one side. You could still steer as you have power steering right? But it would be a major pain in the neck and a bit unsafe as you would have to constantly apply a force to drive straight on? Same idea with trim.
Trust me, takeoff trim is important!

MarkerInbound
23rd May 2011, 15:38
While you are correct that in most transport jets the flight controls are powered (the Direct Cable 9 is one exception) there is some feedback to the pilots from the system. Pilots over time learn the "feel" of the aircraft and know how much force they must apply to raise the nose. Triming the plane keeps this feel constant so they do not have to think, "We are nose heavy today so I will have to pull more" or "We have an aft CG so I will not have to pull as much as normal to raise the nose."

mbenkhort
23rd May 2011, 18:52
Hi and thanks.
If I have understood well your answer, as a matter of trim at take-off, it is more a question of the necessary aerodynamic force to be developed by the horizontal tail(as a whole, HS incidence(trim)+elevator) at the specified rotation speed (so that rotation is possible at this speed) than a question of mechanical force needed to deflect fully the elevator surface.
For example, if the specified rotation speed is let's say 140 knots and trim is set 0 (incorrectly), even at full elevator up defelection, rotation does not occur ( fwd cg case) and the aircraft will have to consume more runway and rotation occurs at a higher speed. Inversely, for an aft cg case, an incorrect trim at take off leads to a nose up before the specified rotation speed even when full down elevator is applied.
On another hand, I think runway elevation and outside temperature count for trim value calculation at take off since q (the dynamic pressure) depends on these items.

galaxy flyer
23rd May 2011, 21:15
The only entering parameters are CG and Flap Setting--for a given weight, rotation and V2 speeds will always be at the same Indicated Airspeed. True airspeed will vary with pressure altitude and OAT.

Otherwise, you have got it.

GF