Teach me

[jmccaul]

DISCLAIMER : these are just my ramblings on why I don't understand aspects of trim to give someone kind enough to answer and idea of what needs explaining to me. Remember speak s-l-o-w-l-y.


   Ok  when a plane goes faster it noses up, rolls to one side and also yaws (although I haven't particularlily noticed the yaw aspect in this or any other sim and personnally never use rudder trim except on take off).

    I assume the reason the plane noses up as the plane noses up at faster speeds is the increased air flow over the wings causes extra lift (is this correct).
   
   What puzzles me is why the plane rolls and yaws. I assume the roll is due to torque  but surley the horizontal force would be constant at a given rpm, manifold pressure etc. Speed of the aircraft should not effect roll axis, why do I roll more in one direction at 150ias than 400ias despite same engine conditions. Is this another airflow issue?   (and pitch ? This is another thing I am not sure about how constant speed prop, variable pitch props work. Surley if the speeds constant so would the force be despite manifold pressure?)
   
   Also why would a plane yaw at higher speed what force causes the plane to yaw at higher speeds or is yaw just an effect caused by bank and pitch (which needs to be compenstaed for (why was constant left rudder need to applied to keep a 109 in balanced flight at high speed)  )
.
           How actually did trim wheels work, did they just change the position of elevators, ailerons and ruders when the stick centred.

   Is this misunderstanding due to the way sims model trim, in a real aircraft does the joystick at the centre  mean co-ordinated flight and the higher speeds causes the control surfaces to be deflected so force is required to keep it centred and how does this effect yaw.

[Jekyll]

OK, I'm far from an authority on these matters but will give it a try.  Feel free to correct obvious inaccuracies  


Pitch Trim:  the centre of pressure on a wing is not located in the same place as the aircraft's centre of gravity.  The centre of pressure can be taken as the point on which the upward lift force acts on the aircraft, whilst the centre of gravity is that point on which the downward weight force acts.  If the CP is forward of the CG (as on most WW2 aircraft), then increases in speed result in a pitching up of the aircraft.  Therefore, the faster you go, the more down trim you need.

Roll Trim:  Whilst engine torque is the primary reason we need roll trim, it's wrong to say that the actual rolling force is a function of speed.  Rather, imagine flying trimmed out at 150 kias.  Engine torque has been negated by the aileron trim tabs, so you are in stable trimmed flight.   You won't roll because AT THAT SPEED the airflow over the trim tabs counteracts the engine torque.  Now imagine speeding up to 400 kias.  Now there will be an increased airflow over the trim tabs, leading to a tendency to roll, not because of increased engine torque, but simply because the trim tabs now exert more force due to increased airspeed.

Yaw Trim.  Since gyroscopic precession is modelled in AH, yaw trim becomes a factor.  The propeller disk acts as a big gyroscope.  Spin a gyro and put it on your desk, push it forward with your hand and it will actually move at 90 degrees to the direction of force.

So fly an aircraft straight and level, and pitch up.  This is like pushing the gyroscope on the desk.  The reaction will come at 90 degrees to the initial force, which will result in a sideways reaction on the aircraft, leading to a yaw effect.

A secondary consideration is that the airflow past the fuselage is not straight, but rather corkscrewed from the propeller.  So the airflow 'wraps around' the fuselage, eventually striking the left hand side of the vertical stabiliser, creating a yaw moment.  The faster the speed, the greater the force exterted through this corkscrew slipstream.

Yaw also becomes a problem since when deflecting ailerons, the downgoing aileron creates more drag than the upgoing aileron, causing a phenomenon known as adverse yaw.  Roll left, and the aircraft will tend to yaw right because of greater drag on the right wing.  That's why a co-ordinated turn involves inputting rudder in the direction of the rolling moment.


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[indian]

jmmcaul

This might help in real life you fly the plane by using thrust to climb or decend and pitch to increase or decrease speed. The more rpms you have the more torque you will have which will try to roll the plane.

Trim tabs come in two types a fixed tab and a moveable tab (controlled by the pilot). The fixed tab is set by test fly a plane and binding it for best results in flight. The moveable type has fixed tab on it also and is controlled by the pilot in flight, you adjust the trim for level flight everything can effect trim in flight wieght and balance which determines the CG of the plane will change in flight do to fuel useage and ammo useage not to mention parts of plane removed by the enemy. There is no weather modelled here so that dont effect the flight.

I can dig up a more technical explanation if you want just trying to keep it basic.

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