On Mar 30, 9:29*am, wrote:
On 30 Mar, 03:30, Frank wrote: Bill,

I like the 'neutral-force' idea, and I think I have actually felt that
a little bit on my Ventus. *I'll keep looking at that. *As for
building a drag meter, I looked through the articles you referenced
and beat a quick retreat. *I would be much more likely to do serious
damage to my glider (and/or me) than I would be to do anything
positive! ;-)

Frank

The neutral-force force idea has been around for decades but there is
no particular reason to think that it gives the best glide angle for
the aircraft as a whole for a given speed - not least because it takes
no account of the fuselage drag .

The trailing edge drag probe is useful for research but completely
useless as a real time flap guide during sporting flight - there are
just too many variables and too much noise in the reading.
Furthermore, once again, there is no reason to think that the flap
setting that gives the lowest drag probe reading for that one station
on the wing span is going to be the flap setting that maximises glide
performance for the aircraft as a whole. *If anyone believes otherwise
they are welcome to make me an offer for my old Replogle drag probe
and meter but they will be wasting their money:-)

All the optimum settings change with wingloading (as well as air
density, bugs etc).

So, I would change the question round and ask "what is the best speed
for each flap setting"? *For each flap setting/wing-loading
combination there will be an optimum airspeed when the fuselage is a
its minimum drag angle of attack. *Find information on the minimum
drag angle of attack for the fuselage, set up the glider C of G to
mimimize elevator drag, choose the ballast load depending on
conditions and/or contest tactics, choose the cruise flap setting
depending on the degree of pilot optimism and let the glider run at
whatever speed corresponds to the fuselage angle of attack for lowest
drag - using your handy fuselage angle of attack meter or wool canopy
tufts. *This will still be an approximation to the ideal but at least
it is simple to use.

In this way you end a choice of 2 or 3 three of inter-thermal cruise
speeds - just like Ingo Renner advised all those years ago.

Iain

Lain,

Yes, I believe you have hit the nail on the head. We all drive around
in our flapped gliders, and all we really know is the thermalling and
high-speed flap settings. Interthermal cruise flap settings appear to
be a mystery for everyone, even though we all try to maximize the time
spent in interthermal cruise, and minimize the time spent at
thermalling speeds. And, even if we have good data for a particular
type of glider (V2bx in my case), that really doesn't mean that the
data is completely applicable for a particular glider of that type,
given that they are all essentially hand-made.

It may be that what we wind up with is a recognition that inter-
thermal cruise is pretty low-loss over a broad range of flap settings,
and the difference between 'optimum' and 'good' in the inter-thermal
phase gets lost in the noise between 'good' and 'optimum' thermalling
techniques/performance, as others have pointed out in the past.

Frank (TA)