Is centre of lift variable with airspeed?

[Discharger]

Consider a fairly common design EP sailplane, "V" tail, flat bottom high lift wing, motor mounted up front as usual so thrust line and drag line roughly neutral. The CG and trim seem correct for power off normal speed glide and she flys well but when power is applied she pitches up and requires a fair amount of down elev. to keep her from stalling. My question is does the centre of lift move forward along the chord towards the leading edge with increased airspeed? If this is so then as the CL moves in front of the (power off cruise) CG, would this account for the pitch up? Thanks for any enlightening replies!

[dtknowles]

A couple things to check.

Does it happen very soon after you apply throttle or does the model need to build up speed first?

Does it automatically pull out of a power off dive?

It is either your CG is a little forward of neutral or you need a little down thrust to compensate for prop wash over the center of the wing and tail.

The center of lift is not going to move enough at these low subsonic speeds to make a difference.

Tim

[dtknowles]

Sorry

[Ollie]

"My question is does the centre of lift move forward along the chord towards the leading edge with increased airspeed? "

Not your theory.

To adjust the CG aft and/or the more downthrust angle. Make small adjustmets until you get enough.

[BMatthews]

What you see is a very normal response to adding power. Power adds speed and the model noses up in an attempt to bring the speed back the the trimmed speed as set by the balance point and elevator trim.

If you did this slowly enough it wouldn't over rotate and try to stall. It's only doing that because you're adding the power quickly.

To reduce the effect you can do the dive test and the pull out response to help move the balance point back until the airplane is closer to neutrally pitch stable but so that it still has enough pitch stability for you. Then add downthrust if needed to limit the climb angle. You'll also need to reduce the elevator throw as the pitch control will become more sensitive as the balance moves back.

[Discharger]

Many thanks for the replies. As it seems like a CG issue, I will make some adjustments in this area before the next flying session and also try to put some downthrust in but this could be a bit of a problem. From your replies it seems that the CL point does not move with airspeed, at least within the low subsonic speeds most of us operate in.

[kallend]

In unaccelerated flight, whether climbing, diving, or level, the sum of all the forces must be acting through the CG. If the plane is undergoing angular acceleration, such as during the pitch-up, then the forces are not acting through the CG. A stable plane is one that returns to unaccelerated flight when disturbed, and thus the forces return to acting through the CG as a consequence of wing and tail incidences, CG location, etc.

[globemaster3c17]

Center of lift moves with a change in angle of attack. If I remember correctly from my aerodynamics class, it moves forward with an increase in AoA. Since increasing airspeed requires reducing AoA for level flight, the center of lift moves aft. Like BMatthews said, pitching up with power is a normal characteristic of an airplane, although R/C planes are usually designed to not show much of it. Take a ride in the front seat of an airplane and you will see that there is quite a bit of change in control position with changes in airspeed and power settings. Your plane is basically demonstrating what full-scale pilots refer to as an elevator trim stall.

[vintage1]

Yup. Two seperate issues. Yes, center of lift does change with AofA and speed, but not by enough to cause the problem.

That problem is down to a forward CG being balanced by positive decalage. So as the plane increase speed in e,.g, a dive, the tail gets more downforce and pulls the plane level.

Now add power, and it thinks its in a dive and lifts the nose...into a climb, or in this case too much of a climb.

Three ways to tackle it :

- downthrust, which keps the stbility but pulls the nose down under high power. THE way to trim free flighters.
- mix in a little down with the throttle, which has the same effect more or less
- reduce stability by reducing angle between main and tail surfaces, and move CG back to compensate. That leads to a more neutral plane in the pitch, which is probably better for a salplane, but gets trickier to fly.

In practice you would probably use a little of all three to get the plane trimmed right.