Steve Uzochukwu wrote:

> On Wed, 20 Mar 2002 21:17:27 +1100, Peter Garrone
> <pgarrone@acay.com.au> wildly wibbled thus:
>
> >When they certify hang-gliders, do they meet pitch torque stability
> >about centre of pressure or centre of mass of the pilot-glider system?
> >
> >The reason I ask is that I have been following Davis Straub's column,
> >and if the answer is centre of mass, then why hang-gliders tuck is a bit
> >of a mystery, but if its about the centre of pressure, then for me its
> >no mystery at all.
> >
>
> We test for the glider to be pitch stable either with or without the
> pilot weight.
>
> The hardest part of the test to pass is usually the bit where the wing
> is at the zero lift angle, but is still expected to produce a positive
> pitching moment.
>
> We measure the pitching moment about the hang point, which as AFAIK is
> very close to or at the centre of mass of the combination.
>
> The nub of the gist is this:
>
> The vast majority of tumbling/tucking incidents involve uncertified
> gliders, which when subsequently pitch tested are shown to be lacking
> the necessary for a C of A.
>
> The Atos-C [different from base Atos] had no C of A/DHV or whatever at
> the time of the incident. Subsequent pitch testing appears to show a
> slight problem in this area exists.
>
> I've sent Davis a 1996 vintage article on HG testing which be mailed
> to you off line if need be.
>
> --
> Steve U., Yamaha XJ900S, Major Diversion ahead.
> Yamaha Diversion Club Nederland Mailing List: http://www.ydcn.nl
> The UKRM FAQ: www.ukrm.net/faq/ukrmfaq1.html
> ****************************************************************

Thanks for your offer of the article, I will send you my mail address.

I assume that when you say the hangpoint, you mean the hangpoint on the
keel. This point would surely be several feet above the centre of mass of
the combined glider/pilot. Since the pilot would typically be twice the mass
of the glider, the centre of mass would be about 2/3 the distance down to
the pilot, no?

What I am thinking is that with a negative angle of attack, the lift vector
could point downwards and forwards through the hangpoint, because it is
orthogonal to the incident airstream, disregarding drag. This would develop
a significant destabilizing torque about the centre of mass.

I will try to explain with an example from my own experience. Once I flew my
enterprise wings 160B hang-glider, full rope on, airspeed probably about
70-80 KPH, into about 20 knots of descending sea-breeze rotor, behind a
mountain. It rotated forward, taking a second or so, until it was diving
vertically. I was holding on to the base bar, but weightlessly attached to
the glider, like in a balloon drop. Fortunately it righted itself at this
point. So at a zero angle of attack, it stabilised, but at the initial
negative angle of attack, it pitched nose-down, which is unstable behavior,
because that rotation is away from the trim angle of attack.

>

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