Aerodynamics of aero towing

On 3/7/2011 2:47 PM, Mike the Strike wrote:
On Mar 7, 2:14 pm, Eric wrote:
On 3/7/2011 7:24 AM, Mike the Strike wrote:



We have discussed this many times before. The nose-up attitude of the
glider results in extra forces that have to be overcome by the wings
and tail surfaces. In particular, modern standard-class ships with a
low angle of incidence of the wing have a pronounced nose-up attitude
that results in loss of control at speeds well above free-flight stall
speed.

I'm having trouble picturing the influence of a low angle of incidence
of the wing (I assume that's compared to the fuselage) on loss of
control. Why would the wing be affected by the fuselage attitude being a
few degrees higher, relative to the air, than on an older glider?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)

The angle of incidence of the wing (relative to the fuselage) is a lot
lower on modern standard class ships. The result is that they have a
pronounced nose-up attitude when flying slowly - you can see this both
on tow and when thermalling. On aero-tow, a nose-mounted tow hook
will result in a downward force on the nose that has to be compensated
by an upward force from the tailplane and extra lift from the wings.
The point at which you lose control under these dynamics is at a lot
higher speed than the free-flight stall speed. The effect doesn't
seem to be as strong with older gliders or flapped ships, but
definitely is a lot less in low tow.

The incidence difference is less that 5 degrees, isn't it? That would
mean the nose hook was only about 6" higher than otherwise. I just can't
see how moving the rope up 6" is going to change the forces
significantly. There is a bigger difference between a CG hook and nose
hook vertical separation, and nobody comments on how they lose control
sooner with the nose hook. I sure didn't notice any difference on my
previous glider, an ASW 20 B, where I used neutral flap to tow.

Maybe the difference between older and newer gliders is wing loading?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)