ORIGINAL: AB Bob
Thanks for the additional replies!
My instructors have been setting the trim at about 1/2 throttle. This is the setting I have been doing most of my flying. But, I have seen w/o enough power, and a stiff headwind, my trainer will balloon up and stall....(Not good, makes for some anxious moments!
)
Seems to be some good points to doing full, 1/2 or trim as needed perspectives. I'm sure I'll form my opinion based on these inputs, as well as my future experiences.
Bob
AB Bob:
In order to understand the trimming in pitch (horizontal level flight with no nosing up or nosing down), it is good to simplify things.
In order to analyze what happens to the model, let’s disregard some physical facts:
Imagine that it is only the wing what makes your model fly by generating a lifting force.
That force varies in lineal proportion with the angle of attack (AOA) between the wing and the direction of the air hitting the wing.
That force varies in square proportion with the velocity of the wing thru the air.
Now, imagine that the only function of the horizontal tail (stabilizer + elevator) is to force the wing to keep a certain AOA.
The horizontal level flight only happens when the lifting force and the weight of the model have the same value (and opposite directions); it is a perfect balance.
More lift, the model climbs; less lift, the model descends.
That balance of forces that allows a sustained level flight is achieved for a unique combination of AOA and wing velocity thru the air, for a particular model.
In a practical range, less velocity requires a bigger AOA and vice versa.
The velocity of the level flight respect to the wind is controlled by the throttle.
The angle of attack of the wing for the level flight is controlled by the deflection of the elevator respect to the stabilizer, which can be preset with the radio trim tab (normal way to do it) or by constant manual input.
So, as stated here by others, the pilot chooses the velocity for level flight, and then, trims the elevator to achieve the correspondent AOA of the wing.
Now, changes in throttle are powerful, because the air velocity increases lift in a square proportion.
For trainers, which are meant to return to level flight by themselves, the geometry between the wing and the horizontal tail (decalage) makes them more sensitive to any changes in air velocity; some simply cannot fly level for high velocities, even with big adjustment of trim.
When flying upwind, the model moves slower respect to the ground, but the wing encounters the same air velocity and the balance remains; it is only when throttle is added to make it move faster respect to the ground that the model balloons up, due to increased lift.
You may be able to modify the decalage by adjusting the wind incidence, if the problem is too acute; check the following old thread:
http://www.rcuniverse.com/forum/m_75..._1/key_/tm.htm