Centre of gravity, flaps and elevators

[plantain]

So, my understanding is that as a general rule in flying, the further aft the centre of gravity, the less drag.
This is because as the centre of gravity moves further ahead of the centre of lift, the higher the torque that must be counteracted by the horizontal stabilizer generating downforce. If you can move the centre of gravity to the same place as the centre of lift, your horizontal stabilizer would not be producing a downforce - less drag (more unstable, more dangerous etc etc let's keep this on topic)

Something I've been struggling to understand is how the optimum centre of gravity moves when you add flaps and elevators to the mix.

Q1) I understand that negative flaps reduce the amount of torque generated by the difference between centre of lift and centre of gravity, but I'm not quite sure how - I assume the centre of lift has to move forwards because I don't see how the centre of gravity would change. Surprisingly there seems to be very little information about this on the internet.

Q2) My next puzzle comes from flying my unflapped glider aft, usually 90% aft. This means that when I am at cruising speed, I need substantial forward stick/elevator deflection to maintain that speed.

This doesn't feel efficient - so my question is, is it more or less efficient than if I had the centre of gravity forward such that neutral elevator produced my intended cruise speed?
Does further aft always mean less drag when you consider the elevator is required to maintain your cruise speed?

[Bruce Hoult[_2_]]

On 2014-08-24 10:55:45 +0000, plantain said:

> So, my understanding is that as a general rule in flying, the further
> aft the centre of gravity, the less drag.

Well, no. Once the CofG is far enough back that the elevator has to
produce lift then you are inneficient again, because the elevator has a
worse aspect ratio than the wing.

The safe rearward CofG limit is well beyond the most efficient rearward CofG.


> Q1) I understand that negative flaps reduce the amount of torque
> generated by the difference between centre of lift and centre of
> gravity, but I'm not quite sure how - I assume the centre of lift has
> to move forwards because I don't see how the centre of gravity would
> change. Surprisingly there seems to be very little information about
> this on the internet.

If the flaps aren't producing lift then something else further forward
has to instead.


> Q2) My next puzzle comes from flying my unflapped glider aft, usually
> 90% aft. This means that when I am at cruising speed, I need
> substantial forward stick/elevator deflection to maintain that speed.
>
> This doesn't feel efficient - so my question is, is it more or less
> efficient than if I had the centre of gravity forward such that neutral
> elevator produced my intended cruise speed?

That definitely sounds inefficiently far rearwards.

[J. Nieuwenhuize]

First of all, the stab doesn't HAVE to lift down persé. As long as the derivative of the lift slope is negative, it's stable and that can well be with an up-lifting tail.

Also there's the downwash from the wing, meaning that the amount of up/down lift of the stab is a function of not only stick position, but down wash angle as well. Fly slower (or pull G's), steeper down wash, more down-lift on the stab, without changing stick position.

Q1. Look at the pressure field. Negative flaps yield higher pressure on the upper surface of the flap, yielding a nose-up moment. You also get lower pressure on the lower surface of the flap, adding nose-up moment.

You can only get zero stab lift at one angle of attack. Realize that the extra lift of the tail is induced drag, proportional to the tail lift coefficient, so assuming 1G, it decreases with the square of speed. Over a whole flight it's optimal to keep induced tail drag low during slow flight and accept a bit of a lifting tail during cruise. Where the optimum is varies per glider, though for flapped ships the variation in tail lift is tiny.

It seems that 50-80% of the CG range is optimal in most modern sailplanes.

[Eric Greenwell[_4_]]

plantain wrote, On 8/24/2014 3:55 AM:
> So, my understanding is that as a general rule in flying, the further
> aft the centre of gravity, the less drag. This is because as the
> centre of gravity moves further ahead of the centre of lift, the
> higher the torque that must be counteracted by the horizontal
> stabilizer generating downforce. If you can move the centre of
> gravity to the same place as the centre of lift, your horizontal
> stabilizer would not be producing a downforce - less drag (more
> unstable, more dangerous etc etc let's keep this on topic)
>
> Something I've been struggling to understand is how the optimum
> centre of gravity moves when you add flaps and elevators to the mix.
>
> Q1) I understand that negative flaps reduce the amount of torque
> generated by the difference between centre of lift and centre of
> gravity, but I'm not quite sure how - I assume the centre of lift has
> to move forwards because I don't see how the centre of gravity would
> change. Surprisingly there seems to be very little information about
> this on the internet.
>
> Q2) My next puzzle comes from flying my unflapped glider aft, usually
> 90% aft. This means that when I am at cruising speed, I need
> substantial forward stick/elevator deflection to maintain that
> speed.
>
> This doesn't feel efficient - so my question is, is it more or less
> efficient than if I had the centre of gravity forward such that
> neutral elevator produced my intended cruise speed? Does further aft
> always mean less drag when you consider the elevator is required to
> maintain your cruise speed?

My ASH 26 E (a flapped glider) manual says the higher the wing loading,
the more forward the CG should be, and that the CG automatically moves
forward the correct amount when ballast is used (the ballast tanks are
ahead of the CG).

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

[Steve Leonard[_2_]]

On Monday, August 25, 2014 10:52:52 PM UTC-5, Eric Greenwell wrote:
> My ASH 26 E (a flapped glider) manual says the higher the wing loading, the more forward the CG should be, and that the CG automatically moves forward the correct amount when ballast is used (the ballast tanks are ahead of the CG). -- Eric Greenwell

Or, maybe that was just their way of saying "We aren't giving you a tail tank because we are going to tell you that you don't need one."

:-)

Steve Leonard