water ballast

On Monday, July 13, 2015 at 8:19:25 PM UTC-7, Jim Lewis wrote:
On Monday, July 13, 2015 at 7:43:27 PM UTC-7, Steve Leonard wrote:
Ballast and downwind. Is the goal to simply glide as far as you possibly can after the last climb? If so, then no ballast is better than ballast.. Unless, of course, you get a boost in L/D max due to higher reynolds number for the ballasted flight.

Generally speaking, ballast lets you have the same glide ratio at a higher speed and thus at a higher sink rate. You won't stay in the air as long, but you would cover the same distance in no wind, with or without ballast (see above possible exception). Since the airspeed for the same L/D is higher with ballast, headwind will have less impact on the distance you can cover. For the tailwind case, you won't be in the air quite as long, so the wind won't "push" you quite as far.

If the thermals are big and strong enough to support carrying water, you are always better off carrying water, whether going upwind or downwind. Until that last glide at the end of the day, where going downwind, you can get a little extra time aloft and wind drift by being dry.

Steve Leonard

Weight affects Reynolds Number? I didn't think weight was in the formula(s) for Reynolds Number. If Reynolds Number is the ratio of inertial forces to viscous forces where does aircraft weight come in?

Jumping to my own possible answer, if the glider is flown at the higher airspeed made both possible and necessary for achieving the same L/D as the glider has when un-ballasted this would raise the Reynolds Number some because Reynolds Number is directly proportional to airspeed.
The higher Reynolds Number would indicate more turbulent airflow. I don't have any idea if the greater turbulence would produce a bump in L/D though. I look forward to the comments from those more knowledgeable than I.

Yes, the higher speed for the same L/D at a higher (ballasted) weight creates higher reynolds number flow and changes the amount of laminar flow on the wing (it increases). Look at the flight tests on the PIK-20B and Nimbus 2 by Dick Johnson. Both showed an apparent increase in L/D max when flown with ballast.

Steve Leonard

Thank you Steve. I'll take a look at Dick Johnson's test report. I love this stuff. I don't understand it but I love it.