CG and Aerodynamics

[dbakeg00]

Any aerodynamics aficionado care to help me remember why aft CG has a higher cruising speed and more difficult recovery from stall as opposed to a forward CG being more stable and crusing at a lower speed?

[specialK]

Not an afficionado but here is what i would say. Higher cruise speed because you are pushing forward on the yoke to compensate for the aft CG which in turn lessens your angle of attack(less drag) higher cruise speed(in short). Stall recovery is more diff. b/c now that you have an aft CG, the center of lift, which is useually behind the CG, maybe be closer to the CG or actually in front of the CG cause the nose to pitch up and you may not have enough forward pressure to break the stall. Hope that helps. I had to get the gears turning to remember how to explain that

[VicariousLiving]

Originally Posted by dbakeg00

Any aerodynamics aficionado care to help me remember why aft CG has a higher cruising speed and more difficult recovery from stall as opposed to a forward CG being more stable and crusing at a lower speed?

CG is forward of center of lift so there is always a nose down rotational force which must be over come by down force at the horizontal stab. As the CG moves aft towards the center of lift the nose down rotational force decreases (think moment-arm) so the need for countering down force at the horizontal stab decreases, too. The down force generated at the horizontal stab is no different than any other downward force (i.e. weight) which must be balanced by lift generated by the wing. The only way to increase lift is to increase angle of attack, but that increases induced drag.

So, stringing it all together, forward CG requires the horizontal stab to generate increased down force (counter the nose down rotational force). The increased horizontal stab down force must be carried by the wing via increased wing AOA, which generates increased induced drag, which slows the aircraft. Move the CG aft and the horizontal stab down force required decreases, which means less weight carried by the wing, which allows flight at reduced AOA, which generates less induced drag, which allows the aircraft to fly faster.

Stall recovery improving with forward CG is based on the same reasoning. Breaking a stall requires lowering the wing AOA. As the CG is moved more forward relative to the center of lift the increasing nose down rotational force (again, think moment-arm) will assist reducing the pitch (and hence the wing AOA).

[CAVOK]

Originally Posted by VicariousLiving

So, stringing it all together, forward CG requires the horizontal stab to generate increased down force (counter the nose down rotational force). The increased horizontal stab down force must be carried by the wing via increased wing AOA, which generates increased induced drag, which slows the aircraft. Move the CG aft and the horizontal stab down force required decreases, which means less weight carried by the wing, which allows flight at reduced AOA, which generates less induced drag, which allows the aircraft to fly faster.

Stall recovery improving with forward CG is based on the same reasoning. Breaking a stall requires lowering the wing AOA. As the CG is moved more forward relative to the center of lift the increasing nose down rotational force (again, think moment-arm) will assist reducing the pitch (and hence the wing AOA).

The best explaination you could ask for. Nice. I was going to say it, but I don't think I could say it any better.

(It takes a lot for me to admit that )

[Dugie8]

Originally Posted by dbakeg00

Any aerodynamics aficionado care to help me remember why aft CG has a higher cruising speed and more difficult recovery from stall as opposed to a forward CG being more stable and crusing at a lower speed?

Draw your typical airplane "picture". Now draw out all of the forces acting on that airplane (lift, drag, thrust, weight). But wait a minute, your missing one, the down force from the horizontal stab. If your airplane weighs 2000 pounds and you have 50 pounds of down force from the stab, the wings are lifting 2050 pounds. As the CG moves forward, more down force equals more lift required, more lift required means either a faster cruise speed, higher angle of attack or a combination of the two. On smaller airplanes really hard to notice the difference, but on a 180 foot long beast, an aft CG is the difference between Mach .8 and Mach .78 with book power settings.

[seagull]

Just a couple of missed points:

1. Trim drag. The drag of the horizontal stabilizer is significant and usually results in more loss of performance than the fairly small increase in effective weight due to "downforce".

2. Stall control issues also hinge on the forward movement of the CP as AoA increases, which can lead to very low static and dynamic stability, or even negative stability in extreme cases.