Aerodynamics of carrying water

"Gene Whitt" wrote in message
nk.net...
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt



All good answers.

I would add that an examination of two polars for a particular glider, one
ballasted and one not, will show a crossover airspeed above which the sink
rate with ballast is less than without - very counterintuitive.

Most modern gliders flying in conditions with greater than 2Kt thermals will
spend 80% of their X/C time flying at airspeeds above the crossover point
where the sink rate is reduced by using ballast.

Flying fast with reduced sink is dramatic.

Bill Daniels

On 11 Oct 2005 03:52:06 -0700, "Andy" wrote:


Edit to 25 words.

"A glider is a gravity-powered machine. The heavier it is, the more
power it has."

16 words, beat that!

-Pat

The polar documents the phenomenon, it does not explain it. Any
attempt to explain the use of ballast by reference to the polar should
leave any intelligent person asking - but why does the polar do that.
I believe the effect can only be explained by considering the system
energy exchange. Once that is understood the effect on the polar
family is intuitive.

But I need the same amount of power to go the same speed don't I? Why
does more power reduce my sink rate?

I think you'll need more than 16 words.


Andy

The polar documents the phenomenon, it does not explain it. Any
attempt to explain the use of ballast by reference to the polar should
leave any intelligent person asking - but why does the polar do that.
I believe the effect can only be explained by considering the system
energy exchange. Once that is understood the effect on the polar
family is intuitive.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~

but loses all the effects due to Reynolds number which is an aerodynamic
phenomena and the original question was about the aerodynamics, not the
dynamics.


Rgds,

Derrick Steed

Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt


E=M(CxC)

Paul

Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase=20
performance in gliders to argumentative airplane student.
=20
I need a simple explanation in 25 words or less.

Gene Whitt

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~

buy glider, put water in, launch, do 100 knots, note sink rate, dump
water, land, fly, do 100 knots note sink rate, job done


Rgds,

Derrick Steed

Thermalling stores energy in the glider, but thermals stop at
cloud base. Water carried needs to be lifted too, thus more
energy can be harnessed.

jk


"Gene Whitt" wrote in message
nk.net...
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to
increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt

Thanks everyone. Now for a 'yes' or 'no' answer to my follow-up
question.

Does the addition of weight have the same effect on airplanes with power off
and prop stopped as on gliders but at a less efficient level.?

Gene
Aside: I once dove a C-150 to 120 mph before stopped prop
would start the engine,

A glider is a gravity-powered machine.

A sled is a gravity-powered machine. A glider is solar
powered.........
==============
Leon McAtee