On 29 Sep 2003 05:10:39 -0700, (Chris
OCallaghan) wrote:

Flew on Sunday in non-stable conditions. David Pixton flew his Ventus
2b at approx. 800 lbs, and I flew my Ventus 2bx at 1150 lbs. Because
conditions were turbulent, we were unable to do any smooth air
testing, but during the course of 10 pull ups (entry 100 knots, exit
60 knots) we observed the following:

Wing abreast with about 100 to 150 feet of separation, pull ups were
either even or slightly better (10 to 20 feet) for the heavier glider.
Because we were following a cloud street, we were unsure whether the
apparent parity was actual or lift related. Dave was selecting the
path, so it is possible that he was better centered in the lift
street.

I repositioned directly behind Dave (less than 100 feet). With each
pull I was forced to maneuver to avoid him, and would generally gain
between 10 to 30 feet. On one pull however, I gained more than 50
additional feet. In each case, I was forced to deploy spoilers to get
back into position for the next pull. It appeared that the stronger
the core we pulled in, the more advantage I got. On a few occasions,
Dave pulled to less than 60 knots, and while this cut down on my
advantage, I regained it immediately (and then some) as we returned to
cruising speed.

It is interesting to note that during these tailored pull ups and
during subsequent street running with freeform pulls, Dave never
gained on me. I found myself making S turns to keep from overtaking
him. When at last I took the lead, the overall advantage to the
heavier glider was apparent and substantial.

We'll try this again in still air, but initial results indicate that
there is a marked difference in climb during each pull up. Even at a
conservative 15 feet per pull, a hundered such pulls during a task
would account for 1500 extra feet, and this doesn't include the
advantages water offers during the cruising phase of flight.

So to answer the question, "is there a difference?" the answer is
yes... in theory and in practice. Dave commented after the flight, "I
need to get back into the habit of putting water in the wings."

If I can muster Dave for some early moring flying, we'll get some more
quantitative results.

OC

Here's what I posted earlier in this topic:

Let's define the problem a little better - a pull up from 100KIAS to
50 KIAS, level flight in both cases.
Pull to a flight trajectory of 30 degrees up relative to the horizon.
This gives a vertical velocity of 50 knots immediately after the
pullup. That 50 knots requires an extra 1 g for about about 2.5
seconds(some simplification and approximation here)or a suitable other
combination of G load and time). At the high speed the extra induced
drag is quite small for a short time so can be neglected to a first
approximation. The pullup will take only a few seconds,10 so that
difference in height gain is the difference in ballasted and
unballasted sink rates for a few seconds. At the low end the sink rate
difference is very small and at the high end the ballasted glider has
lower sink rate. This difference might be as high as 200 feet/min but
we are only talking for a small fraction of a minute so we get maybe
30 feet difference in favour of the heavy glider, maybe only 10 to 15
feet.

Please note in the kinetic/potential energy equation the mass cancels
out so to a really rough first approximation neglecting the effect of
ballast on the polar the height gain is the same.


It's nice when the experimental results match the theory.

Note that all you are seeing in your flight test is the difference
between the polars of two nearly identical gliders with different
ballast loads at high speeds. (faster than best L/D)
The pull up/pushover manoevering is largely irrelevant.

Mike Borgelt