Longitudinal trimming

[David Lewis]

For small rubber power free flight models, it's usually best to locate the CG as far back as stability requirements will allow. Then trim the stab and thrustline to maximize the L/D during climb, and max lift coefficient during the glide. This will maximize flight endurance.

If we look at the climb diagram, we see that when the wing operates at max L/D, the rate of climb (ROC) is maximized. Even though the climb angle is lower than at max lift coefficient, the airplane flies faster, and therefore after a given amount of time, reaches a higher altitude.

[David Lewis]

In gliding flight, the situation is reversed. We'd like to trim the airplane to maximize the lift coefficient. Although the glide ratio is lower, the airplane flies more slowly and therefore stays aloft longer.

glide.GIF

To select the airfoil that maximizes glide duration, use the endurance parameter:
Endurance parameter = (lift coefficient^3/2)/drag coefficient.

[davidchoate]

Does'nt moving the CG back make it tail heavier? I thought tail heavy was bad? And I get the Climbing thing. As a Car Mechanic I try to relate to what I know, and It's like driving up a 40 deg incline compared to a 25 deg grade. The steep hill needs lower gearing and therefore lower speed to go up without stalling . But a lesser grade takes less torque at the wheels allowing a faster speed.

[David Lewis]

On small free flight scale models, tail heavy is indeed bad because you will need to add noseweight. Otherwise the airplane will be unstable in pitch. Therefore my advice is to set the CG as far back as possible without making it too unstable. That means, when the airplane goes into a dive, it pulls out gradually.
If it pulls out quickly, CG is too far forward.
If it tucks under (dive gets steeper) CG is too far back.

The distinction between the best rate of climb and best angle of climb is important in full scale pilot training:
Best RATE of climb = hit the control tower at mid height
Best ANGLE of climb = break off the roof antenna