physics question about pull ups

On Apr 25, 12:57*am, Bruce Hoult wrote:


On Apr 25, 4:27*am, Andy wrote:

The kinetic to potential energy balance yields no difference as has
been pointed out. There are small drag differences that give some
advantage to a heavier glider since it has a higher L/D at any given
speed. *Back of the envelope polar math says the difference in sink
rate at 150 knots with full ballast versus dry is about 100 feet per
mile (for a modern glider). At 100 knots it's about 50 feet per mile.
I'd estimate a typical pullup consumes about a quarter mile. Without
taking the time to integrate the declining sink rate difference over
the entire pullup, I'd guess the overall difference in altitude gain
would be around 20 feet.

I agree.

This ignores any differences in secondary
energy losses associated with pulling G's to make the pullup happen.
My intuition tells me that this would favor the lighter glider
slightly because it takes more energy to change the vector of a
heavier sailplane - how much I don't know except to say that the
harder the pullup the greater the drag losses.

No, for sure not if the heavier glider doesn't pull so hard that it
goes above the angle of attack for max L/D.

Supposing that the speed for best L/D full of ballast (in level
flight) is 75 knots, at 150 knots you'll have to pull (150/75)^2 = 4
Gs before you get to the max L/D angle of attack. *(and the
unballasted guy with a best L/D at 60 knots would have to pull 6.25
Gs)

If both gliders pull the same number of Gs at 150 knots then the
ballasted one will lose a lower percentage of its energy unless they
both pull over 4 Gs.

They are probably equally efficient at around 5 Gs. And the lighter
glider is for sure more efficient at 6.25 Gs -- the ballasted guy is
getting close to stalling by that point.

All in all it's a barely measurable difference. I suspect the reason
people feel like they get a bigger pullup full of water is that they
are generally carrying more speed at the beginning of the pullup when
they are full of water.

Yes, probably, and the smaller loses while cruising along the runway.

I'm not totally sure about this but here's my logic (been a while
since engineering school). If you assume the ballasted and unballasted
gliders fly the same profile then they need to pull the same number of
Gs to execute the pullup. We've already accounted for the steady-
flight L/D effects in the initial calculation so all we need here is
how much energy is lost in pulling the same number of Gs to initiate
the climb. It's the same glider except for the ballast so the form
drag is the same which means we only have to account for the
difference in induced drag. The formula for that is:

D=(kL^2) / (.5pV^2S(pi)AR)

At the start of the pullup all these variables are the same except for
L which equals the weight of the glider times the G's being pulled. If
the heavier glider is 1.5 times as heavy the induced drag is 9 times
as great at 2 Gs. Keep in mind that at redline the induced drag term
overall is small because the speed is high, but still the advantage
should go to the lighter glider for the G-losses part. If you
calculate the L/D in accelerated flight you still end up with a weight
times Gs term in the denominator. I haven't done the math fully
through with real numbers, but that's how the formula looks to me.

Bruce's comment generated one additional thought. The energy balance
calculation we all did assumes the ballasted and unballasted gliders
both start at the same speed (redline) and end up at the same speed.
However, the ballasted glider has a higher stall speed, min sink speed
and best L/D speed - in my case by around 10 knots. If both gliders
pull up to their respective best L/D speeds the unballasted glider
gets about 65 feet higher due to being able to turn that last 10 knots
into altitude. Of course if both gliders went ballistic and did a
hammerhead stall at the top you wouldn't get this difference - but I'm
assuming typically you'd pull up to the same margin above stall speed,
which translates to a slower speed for the lighter glider.

So, by my new calculation the unballasted glider has a slight
advantage. It loses 20 feet to the ballasted glider due to L/D
effects, but gains 65 feet by being able to top out at a lower speed
and gains an unspecified amount (probably small) from G effects on
induced drag at the start of the pullup.

As an aside - the strong G-effect on induced drag is the main reason
why you should try to avoid hard pullups into thermals - you give away
a bunch of altitude.

9B

I was considering two identical gliders - one with water ballast - one
without
both flying at the same speed - and both pulling up to the same speed

The only relevant differences I can see a

- ratio of drag to mass
- slightly different attitude

I believe gliders that take water are optimised for ballast
(so that they have the minimum profile drag for the required angle of
attack at best glide)

Of course, if they are flying the same speed, then they are at different
places on their respective polars to begin with.

Larry



"John Rivers" wrote in message
:

I was considering two identical gliders - one with water ballast - one
without
both flying at the same speed - and both pulling up to the same speed

The only relevant differences I can see a

- ratio of drag to mass
- slightly different attitude

I believe gliders that take water are optimised for ballast
(so that they have the minimum profile drag for the required angle of
attack at best glide)

On Apr 25, 8:21*am, Andy wrote:


As an aside - the strong G-effect on induced drag is the main reason
why you should try to avoid hardpullupsinto thermals - you give away
a bunch of altitude.

9B

Yes, if you both accelerated and are now pulling up in a constant
velocity of transportation field. But by mentioning the thermal, this
is not likely. With discontinuous fluid fields, coupled pullups and
pushovers which are properly timed within a shifting frame of
reference have the potential to gain much more energy than is ever
lost to induced and friction drag- dry or fully loaded. The fully
loaded case has more potential in typical soaring environments because
more time is available to apply the technique and the events can be
further apart.

For most gliders, the optimized multiplier is so substantial that you
run out of positive g maneuvering envelope (based on JAR standards)
with a mere 2-3 knots of lift.

Best Regards,

Gary Osoba

On Jun 5, 2:30*pm, Gary Osoba wrote:
On Apr 25, 8:21*am, Andy wrote:

As an aside - the strong G-effect on induced drag is the main reason
why you should try to avoid hardpullupsinto thermals - you give away
a bunch of altitude.

9B

Yes, if you both accelerated and are now pulling up in a constant
velocity of transportation field. But by mentioning the thermal, this
is not likely. With discontinuous fluid fields, coupled pullups and
pushovers which are properly timed within a shifting frame of
reference have the potential to gain much more energy than is ever
lost to induced and friction drag- dry or fully loaded. The fully
loaded case has more potential in typical soaring environments because
more time is available to apply the technique and the events can be
further apart.

For most gliders, the optimized multiplier is so substantial that you
run out of positive g maneuvering envelope (based on JAR standards)
with a mere 2-3 knots of lift.

Best Regards,

Gary Osoba

If you mean dynamic soaring then the airmass velocity gradient needs
to be horizontal, not vertical as is the case with thermals - plus the
magnitude of the gradient in a thermal is way too low to be useful,
even if it were in the correct orientation.

If you aren't referring to dynamic soaring then all I can say is
"huh"?

9B

If you aren't referring to dynamic soaring then all I can say is
"huh"?

No, Gary means it. In theory, we can gain a lot by strong pull ups and
pushovers in thermal entries and exits. In fact, in theory, you can
stay up when there is only sink. You push to strong negative g's in
the sink, then strong positive gs when you are out of the sink. Huh?
Think of a basketball; your hand is sink and the ground is still air.
When you push hard negative g's in the sink, the glider exits the sink
with more airspeed than it entered, just like the basketball as it
hits your hand. The opposite happens when you pull hard for the first
second or two after entering lift.

To work, you have to pull hard while the glider is still descending
relative to the surrounding air in the thermal, and ascending relative
to surrounding air in the still air or sink. You only get a second or
two. In my experiments I haven't gotten this to work, though it may
account for some of the aggressive zooming we see in Texas
conditions.

Really, to make it work well, I think we need to surrender pitch
control to a computer that handles pitch based on very fast update
vario and g meter. The optimal pitch control is not a hard problem to
solve. It does take a faster feedback than human -- or at least this
human -- can seem to manage.

Don't laugh. Handing over pitch control to a computer might give the
same performance boost as several meters of span. It would definitely
be worth it, though the occupant might need an iron stomach.

John Cochrane
BB

On Jun 5, 2:41*pm, Nine Bravo Ground wrote:
On Jun 5, 2:30*pm, Gary Osoba wrote:





On Apr 25, 8:21*am, Andy wrote:

As an aside - the strong G-effect on induced drag is the main reason
why you should try to avoid hardpullupsinto thermals - you give away
a bunch of altitude.

9B

Yes, if you both accelerated and are now pulling up in a constant
velocity of transportation field. But by mentioning the thermal, this
is not likely. With discontinuous fluid fields, coupled pullups and
pushovers which are properly timed within a shifting frame of
reference have the potential to gain much more energy than is ever
lost to induced and friction drag- dry or fully loaded. The fully
loaded case has more potential in typical soaring environments because
more time is available to apply the technique and the events can be
further apart.

For most gliders, the optimized multiplier is so substantial that you
run out of positive g maneuvering envelope (based on JAR standards)
with a mere 2-3 knots of lift.

Best Regards,

Gary Osoba

If you mean dynamic soaring then the airmass velocity gradient needs
to be horizontal, not vertical as is the case with thermals - plus the
magnitude of the gradient in a thermal is way too low to be useful,
even if it were in the correct orientation.

If you aren't referring to dynamic soaring then all I can say is
"huh"?

9B

9B:

The physics apply in all directions, but the potential is greatest
with positive vertical velocity gradient since that vector directly
opposes gravity- and that's our job if we're going to stay up. The
reason the horizontal gradients are more readily recognized is that
they are often sustainable in a cycle, witness the Albatross. However,
I'm not wanting to argue about it. I know the physics and the math and
have been using them effectively for about 15 years now.

Best Regards,
Gary Osoba

On Jun 5, 3:00*pm, John Cochrane
wrote:

Don't laugh. Handing over pitch control to a computer might give the
same performance boost as several meters of span. It would definitely
be worth it, though the occupant might need an iron stomach.

John Cochrane
BB

Hi John:

Precisely what Taras Keceniuck, Paul MacCready and I were doing in a
DARPA funded study when Paul passed away.

Best Regards,
Gary Osoba

Hi John:

Precisely what Taras Keceniuck, Paul MacCready and I were doing in a
DARPA funded study when Paul passed away.

Best Regards,
Gary Osoba

Is the study finished and any publication done? I want the pitch
controller for the worlds!
John Cochrane

On Jun 5, 3:33*pm, John Cochrane
wrote:
Hi John:

Precisely what Taras Keceniuck, Paul MacCready and I were doing in a
DARPA funded study when Paul passed away.

Best Regards,
Gary Osoba

Is the study finished and any publication done? I want the pitch
controller for the worlds!
John Cochrane

Let's go private, John.

-Gary