Does CG change airspeed in a descent or glide?

Question

If the CG of an airplane moves forward, would it fly a faster or slower airspeed in a descent? Usually during a descent or landing, you'd use pitch to change airspeed and power to control descent rate. With full nose-up trim, power off, a plane I fly frequently used to fly at a certain speed before some avionics work was done, but now sits at 5 knots faster with the same configuration, same weight (me). This is in a descent to landing. I assumed initially that maybe the CG moved forward a bit and the speed increased due to the plane wanting to be more nose-down with the same configuration and trim. However, I remembered that a forward CG usually decreases performance (and thus speed?) due to a larger tail down force needed to counteract the forward CG, more wing loading and more drag. These two ideas seem to contradict one another! I also don't know if a power off descent vs level flight changes the fact that a more forward CG, increases drag and decreases airspeed.

Thank you for anyone's input.

Answer 1

Moving the CG forward will result in a negative $∆C_m$ (nose-down moment) for the wing, with minor effect on the tailplane $C_m$, which depends on the direction of its loading. So yes, if the original trim setting is preserved, a net negative moment will be produced at the original airspeed, meaning that the aircraft will now be trimmed for a higher airspeed.

It is true that forward CG comes with a small drag penalty (check out this question for more information on that). However, that doesn't affect the trim speed at all.

When a stable aircraft is trimmed for a given AoA (and therefore IAS), it will maintain that by producing an appropriate pitching moment to counter any deviations. When you increase the thrust, the aircraft pitches up to maintain the IAS. Likewise, a reduction in power or increase in drag will cause the aircraft to pitch down to maintain the IAS. Drag has no effect on the trim speed; increased drag will only cause the aircraft to have a greater descent angle with the same original speed.

Answer 2

If the CG of an airplane moves forward, would it fly a faster or slower airspeed in a descent?

Hang gliders and "trikes" (powered hang gliders with seats and wheels) use CG-shift as the sole means of pitch (and roll) control. Anyone who has flown such aircraft will tell you that the answer to your question is "faster".

However, I remembered that a forward CG usually decreases performance (and thus speed?) due to a larger tail down force needed to counteract the forward CG, more wing loading and more drag

If the airplane is re-trimmed to continue to fly at some given airspeed at the original power setting after the forward CG shift, drag will generally be increased due to the increased tail downforce. So the climb rate will be decreased, or the sink rate will be increased, depending on the power setting. Similarly, if the airplane is re-trimmed to continue to fly horizontally at some given power setting after the forward CG shift (or to maintain some given climb rate or sink rate), the airspeed will generally be decreased due to the increased drag associated with the increased tail downforce.

But if you leave the elevator trim fixed as you move the CG forward, the tail isn't going to magically create more down force so as to decrease the airspeed. Rather, you'll end up flying at a lower angle-of-attack, and so you'll see an increase in airspeed.¹ If you are flying on "front side of the drag curve", as is most often the case, this will cause an increase in sink rate (or a decrease in climb rate). If you are flying on the "back side of the drag curve", as is the case in "slow flight", this will cause a decrease in sink rate (or an increase in climb rate).

I also don't know if a power off descent vs level flight changes the fact that a more forward CG, increases drag and decreases airspeed.

There's no fundamental difference in this regard between a power-off descent and level flight. It's just a matter of what are you holding constant as you change the C.G. position-- elevator trim position, or altitude, or airspeed? See above for more.

Footnotes:

1. A careful analysis of this situation shows that the tail downforce has in fact been increased. But the result is not a decrease in airspeed.

Answer 3

From what I know if the CG moves forward it would result in a 'nose heavy' situation. This would cause the aircraft to go into a dive. In other words it would have a negative AOA which you wouldn't want during landing. If the CG moves aft it would cause a 'tail heavy' situation and as a result it would cause a positive AOA such as your nose will pitch up.

In order to compensate these changes the elevator is at your disposal or sometimes pilot would normally lean/push back or forward (depending on the condition) their seat especially if your in a light body aircraft. The decent rate can be manipulated by increasing the lift with the aid of flaps.

It must be noted that any movement of the CG can also affect the center of lift on the wings. For example, if the AOA is increased (pitch up) the COP moves forward which will in fact cause a reduction in lift. Therefore you'd need to add more power to compensate. All in all, what you'd want to do is perform weight and balance procedure to correct the aircrafts CG if you think it was disturbed.

Thanks