Optimum CG Range

Eric Greenwell wrote:
Herb wrote:
Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH


Here's another gem piece of advice: With my 3D model airplanes I roll
inverted and check if I need down elevator to stay level. If so, the
cg needs to be moved further back. A well set up model will happily
fly inverted without elevator movement! Haven't tried that in my LS8,
though.
Seriously, Hanks and Eric's methods will both work well. As long as
inside the book range, find the cg that gives you good handling and
enough up elevator to stall the plane in a steep turn. It'll be at
75%-90% aft.

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.

I expect it has to do with efficiency.

If your CG is such that your control inputs are minimised - you reduce
drag. In the case of steep thermalling, it reduces safety - because you
can now stall and theoretically spin.

There can be few things as frustrating as my experience with my (new to
me) Kestrel 19. First flight I wanted to be cautious so set the CG at
35%. Then the day was booming - but with tight strong thermals, and I
was continually running out of elevator. Stick against the back stop and
the thermal is still tighter.

My Cirrus with it's all flying tail never had that problem. Of course
you could depart controlled flight if you got too enthusiastic...

Bruce

Bruce wrote:
Eric Greenwell wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.

I expect it has to do with efficiency.

If your CG is such that your control inputs are minimised - you reduce
drag. In the case of steep thermalling, it reduces safety - because
you can now stall and theoretically spin.

There can be few things as frustrating as my experience with my (new
to me) Kestrel 19. First flight I wanted to be cautious so set the CG
at 35%. Then the day was booming - but with tight strong thermals, and
I was continually running out of elevator. Stick against the back stop
and the thermal is still tighter.

My Cirrus with it's all flying tail never had that problem. Of course
you could depart controlled flight if you got too enthusiastic...

At 35%, you are a long way from stalling in a steep turn. With a more
rearward CG, you would turn tightly enough, but still without enough
elevator to stall in a "tight" turn.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

Eric Greenwell wrote:
Bruce wrote:
Eric Greenwell wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.

I expect it has to do with efficiency.

If your CG is such that your control inputs are minimised - you reduce
drag. In the case of steep thermalling, it reduces safety - because
you can now stall and theoretically spin.

There can be few things as frustrating as my experience with my (new
to me) Kestrel 19. First flight I wanted to be cautious so set the CG
at 35%. Then the day was booming - but with tight strong thermals, and
I was continually running out of elevator. Stick against the back stop
and the thermal is still tighter.

My Cirrus with it's all flying tail never had that problem. Of course
you could depart controlled flight if you got too enthusiastic...

At 35%, you are a long way from stalling in a steep turn. With a more
rearward CG, you would turn tightly enough, but still without enough
elevator to stall in a "tight" turn.

Hi Eric

That is the point I was trying to make.

With the CG so far forward the behaviour is really benign, but I can't
get enough elevator to stall her. Even straight and level the nose just
wallows around at the back stop with ~38kt indicated.

I will be moving the CG back until I can stall it, or the handling
deteriorates, then move it a little forward. There is no virtue in being
able to stall in a tight turn, just efficiency in not holding undue
control deflection.

Bruce