"Chris OCallaghan" wrote in message
om...
I reread this and want to add a few notes for clarity...

(Chris OCallaghan) wrote in message
. com...
Pete,

I got a server error, so this may be a duplicate. Here's one way to
look at the ballast issue.

Think of best L/D as a function of AOA. Now think of speed as function
of AOA and load. Increasing the load increases speed without a change
in AOA. Therefore, increasing load increases the airspeed at which we
will achieve best L/D.

Changing AOA, either higher or lower (slowing down or speeding up),
results in higher drag (more induced drag if we slow down, more
parasite drag if we speed up).

I simplified this too much. Increasing AOA from best L/D will reduce
total drag down to minimum sink speed. These values are getting closer
to one another among newer airfoils. However, the rule of thumb is
that best L/D occurs at the point where induced and profile (parasite)
drag are equal. Minimum sink occurs at the point where profile drag
equal about 1/3 the induced drag.



Now, take identical gliders. One has a gross weight of 100. The other
has a gross weight of 200. The first glider achieves best L/D at a
given AOA. At a weight of 100, best L/D speed is 50. The second glider
achieves best L/D at the same AOA, but because it is twice as heavy,
its best L/D speed is 70 (square root of load factor increase). Glider
two achieves the same L/D at 70 as its lighter twin achieves at 50. In
order for the lighter glider to keep up, it will have to increase its
speed by lowering angle of attack to a less than optimum value,
thereby increasing drag.


ie, it will sink faster at 70.

Coefficent of drag changes with AOA. Additional load allows us to fly
the glider faster at optimum AOA, achieving a lower coefficient of
drag. Since induced drag becomes less important and profile drag more
critical as our speed increases, being able to maintain low C of D at
higher speeds is a significant performance bonus. At lower speeds,
increased induced drag due to loading is a detrement. (The rule of
thumb is that for lift less than 3 knots, don't bother with ballast.
For lift greater than four knots, start adding water. In between 3 and
4 it really doesn't matter, unless there is significant lift
streeting, in which case you speeds will be higher and increased
loading is justified.)

Back to the subject of the thread, I think it is clearer now that if
two gliders, one ballasted, one not, execute a pull up from high
speed, the heavier glider will achieve greater altitude because it has
less total drag throughout most of the maneuver, only becoming less
efficient than the empty one at low speeds, where there is
dramitically less available energy to convert into altitude
(diminishes with the square of the speed).

Counterintuitive, isn't it. But the proof is in the polar. Look at any
polor and note that the sink at at max wing loading at 100 knots is
quite a bit less than the corresponding sink rate at minimum wing
loading.

I think we were asked to ignore Reynolds numbers in this discussion.
So I will, except to say that at the speed we work with, it too is
significant (though second order compared to the effects discussed
above).


Does this help?

I've followed this discussion with interest. I think there is one more
thing that might make a big difference and that is the particular glider's
airfoil. For example, comparing my Lark IS 28 with a G103 at the same wing
loading, the Grob zooms very well and the Lark is terrible. The difference,
I think, is that the Grob has a thick, high lift wing and typical smooth
fiberglass/gelcoat finish. The Grob wing just "grabs" the air better. On
the other hand, the Lark runs much better.

Bill Daniels

Bill Daniels