Aerotow ropes: short or long, breakable or unbreakable?

At 12:39 15 November 2013, GM wrote:
... The angle of attack of the glider during a winch launch is no
different than if the glider was in free flight at the same speed!

Actually, it is significantly different: the wing has to deal with the
tension in the cable (including the cable self-weight) as well as
supporting the weight of the glider. This could easily double the
AoA required in steady flight & smoth conditions towards the end of
a winch launch.

On Friday, November 15, 2013 7:41:22 AM UTC-6, James Metcalfe wrote:
At 12:39 15 November 2013, GM wrote: ... The angle of attack of the glider during a winch launch is no different than if the glider was in free flight at the same speed! Actually, it is significantly different: the wing has to deal with the tension in the cable (including the cable self-weight) as well as supporting the weight of the glider. This could easily double the AoA required in steady flight & smoth conditions towards the end of a winch launch.

Well, James, now you have gone and confused AoA with lift coefficient. You do not necessarily double AOA to double lift coefficient. Lift coefficient generally goes up about. 1 per degree of AoA while you are below separation of flow on the wing surface. At some point on the curve, doubling AoA doubles Cl, but only at that one point. And I would guess that point is no where near where you operate on a winch launch.

Steve

At 16:02 15 November 2013, Steve Leonard wrote:
Well, James, now you have gone and confused AoA with lift
coefficient. You do not necessarily double AOA to double lift
coefficient. Lift coefficient generally goes up about. 1 per degree of
AoA while you are below separation of flow on the wing surface. At
some point on the curve, doubling AoA doubles Cl, but only at that
one point. And I would guess that point is no where near where you
operate on a winch launch.

No confusion in my mind ... when I mentioned doubling I was just
pointing out that the difference was substantial. I could have been
less conservative and said 'tripled' or 'quadrupled'!

If you look at a typical graph of lift coefficient vs AoA you will see
that to double the lift you can expect to much more than double the
AoA. (Although the curve is substantially linear for most of the AoA
range, it doesn't pass through the origin). Not that I said anything
at all about doubling the lift!
J.

On Saturday, November 16, 2013 12:42:24 AM UTC+1, James Metcalfe wrote:
At 16:02 15 November 2013, Steve Leonard wrote:

Well, James, now you have gone and confused AoA with lift

coefficient. You do not necessarily double AOA to double lift

coefficient. Lift coefficient generally goes up about. 1 per degree of

AoA while you are below separation of flow on the wing surface. At

some point on the curve, doubling AoA doubles Cl, but only at that

one point. And I would guess that point is no where near where you

operate on a winch launch.



No confusion in my mind ... when I mentioned doubling I was just

pointing out that the difference was substantial. I could have been

less conservative and said 'tripled' or 'quadrupled'!



If you look at a typical graph of lift coefficient vs AoA you will see

that to double the lift you can expect to much more than double the

AoA. (Although the curve is substantially linear for most of the AoA

range, it doesn't pass through the origin). Not that I said anything

at all about doubling the lift!

J.

James, Steve

not wanting to hi-jack this thread but let me re-state what I meant to say more precisely: the AoA during the winch launch is no where near the the stall AoA. We did fly several winch launches with a side-string attached on the outside of the canopy. Since the line pull forces during the steepest part of the climb are nowhere near the forces shortly before the release and since we are using Spectra, which weighs a few pounds for the 4,500ft we can lay out, the loading on the glider is well below 2g. So the AoA should be that of a glider loaded at 1.5 - 2g.

Uli