kd6veb wrote:
Hi Gang
There is a caveat to what Eric is saying. With a cranking mid summer
day around Minden it is true if you are going distance most pilots
would balast their gliders to max gross and expect to fly to FL 180.
However in wave flying where you might be close to FL 180 or above
with ATC permission for a good portion of your flight and with
temperatures perhaps around -25 degrees F you would not use water
ballast.

This is a legitimate concern, but not the "performance reasons" Bill was
asking about.

I have never known anyone to use water ballast for a wave
flight. Now having said that it may be that having the wings full of
water might reduce flutter at high speeds which could be advantageous.

That's an interesting idea. I'm not aware of a discussion of on how
ballast changes the flutter characteristics, but it seems like the
differences might be substantial.

PS I have researched flutter without finding any really definitive
papers on the subject. It is widely said that if flutter occurs at say
200mph at sea level it will occur at the same speed at any altitude.

The handbook values for "many" gliders built in at least the last 20
years or so usually have the Vne as a constant IAS up to about 10,000',
then a (mostly) constant TAS limit after that. My ASH 26 E is like that.
It's more complicated than just a TAS limit, but a TAS limit is
conservative. "Fundamentals of Sailplane Design" notes that some people
think a limit half way between TAS and IAS would be more appropriate.

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

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org