Effect of C of G on elevator responsiveness?

Question

I keep forgetting the actual descriptions of the EFFECT of Centre of Gravity on elevator responsiveness and feel.

Rearward C of G makes the elevator more/less responsive?

Rearward C of G makes the controls "feel" heavier/lighter?

This is for an ATPL exam...so thinking heavy jet level.

THANKS!

Answer 1

aviator [pilot] is mostly correct in what he says but his answer is far from complete, especially as far as the heavy jets you are concerned about. and he is dead wrong when he says the yoke is 'much  harder' to move with an aft cg vs a forward cg. it actually moves with very litlle effort as cg moves aft and requires much more force as it moves toward the nose. i'll talk about that with fighters later here.

good  question though, and important to know for an atp. either, too far forward cg or two far aft will render the elevator ineffective but for different reasons. here's the deal. the elevator controls pitch about the lateral axis right? call it wingtip to wingtip for simplicity sake but we know that's not really so. stay with me here. the lateral axis is located fore and aft on the airplane where the center of pressure, think lift, is. but the airplane can be loaded so that the cg is in front of or  behind the center of lift/lateral axis.

now if the cg is too far forward, the elevator simply may not be able to create enough down force to overcome the weight of the load forward of the wing. think of a lever, the shorter the lever that you use to lift a weight, teeter totter?, the less weight you can lift. so there's the problem with a forward cg. and to make matters worse, that center of lift/lateral axis we talked about , moves even further aft in a swept wing jet as speed increases, thereby shortening that lever even more. eventually a phenomenon we call 'mach tuck' will occure and only bad things come next. mach tuck may not be recoverable in some aircraft. in a lear 20 series for example if the uncontroled dive to an altitude where the air is thicker and TAS decreases does not return elevator effectiveness, all you can do at that point is lower the gear and s***w the doors that  will fly off the aircraft. flight spoilers are a bad idea because although they do add drag, they also have the effect of moving the center of pressure even further back so keep your hands off those suckers.

by the way, a forward cg becomes less and less efficient as it moves forward because the increased down force required at the tail to keep the nose up and away from an overspeed is really just induced drag. maybe you'll recall that in light aircraft, what becomes the real problem in an uncontrolled high speed dive is the horizontal stab failing downward.

a cg is more efficient as it moves aft for that reason to a point of course. concord used to pump fuel to for and aft tanks to control the cg so trim forces did not have to be applied to the aircraft thereby reducing induced drag. but a too far aft cg in a transport jet is bad for the same reason it's bad in a cessna. a cg aft of the lateral axis makes an aircraft aerodynamically unstable. unlike a forward cg where if you were to not touch the trim during cruise, and simply pulled the elevator back, let the nose pitch up and let it go, the aircraft pitch oscillations would gradually reduce in amplitude until the aircraft once again stabilized back to normal cruise.

with a cg load aft of the wing however, a pitchup will not recover on its own. in fact, the pitch oscillations will worsen with each nose up/down cycle and the aircraft will soon become a reall bucket of worms. fighters are being designed with neutral to aft cg's now so as to be very maneuverable requiring a very light touch, read easier not harder to pull back the stick aviator [pilot], but it requires computers to keep things in hand.

Answer 2

http://www.avweb.com/news/leadingedge/le...

There ya go.

Answer 3

Well, an aft CG will cause limited forward elevator effectiveness in a stalled condition...

Answer 4

John B hit it right on.  Excellent answer.

Answer 5

Good question! Now think of the airplane as a wrench, the longer the handle the more easy it is to open or pull a bolt. The airplane has the same physics where the Load, Fulcrum, and Arm is playing a vital role.

As the CG moves forward the arm increases, therefore arm times force equals greater momentum. The controls will be more responsive and easier to move. With each elevator movement the airplane is much more stable and sensitive to the pilots yoke movements. This is not such an advantage as it makes the airplane cruise speed slower, stall speed higher, raising the nose for a landing harder.

When it moves Aft/rearward, the opposite happens, it is much harder to move the yoke for a demand of aircraft  performance. If stalled you very much will be in a Flat spin that is 98% unrecoverable. However, stall speed decreases, Cruise speed increases, less stable, and raising the nose on landings is quiet easy.

The controls wont be a lot noticeable where every the CG is but if you really focus on this matter it can be felt.