physics question about pull ups

On Apr 21, 11:11*am, Darryl Ramm wrote:
On Apr 21, 6:03*am, JJ Sinclair wrote:



On Apr 20, 6:35*pm, jim archer wrote:

On Apr 20, 6:17*pm, "Paul Remde" wrote:

Hi Jim,

It is simple high school physics. *Yes the heavier glider has much more
energy, but it also takes much more energy to lift the heavier glider. *You
would be much more tired after carrying 100 pounds up a flight of stairs
than you would be after lifting 10 pounds up a flight of stairs. *The
physics shows very clearly that the extra speed energy from the higher
weight is exactly cancelled by the extra energy required to raise the
heavier weight.

before pullup * * * * * *after pullup
1/2 mv^2 + mgh *= 1/2mv^2 + mgh

As you can see in the equation above you can divide both sides by m and the
equation doesn't change. So the mass of the glider doesn't matter, but the
speeds have a big effect because the velocity is squared.

Paul Remde

"jim archer" wrote in message

...
On Apr 20, 1:41 pm, Chris Reed wrote:

wrote:
The effect of drag on height recovery isn't too bad, but is enough to
matter.

In a low-performance glider the drag can be extremely significant.. In,
say, a K8 or (I'd guess) an I-26, the height gain is very small in
comparison with 40:1 glass.

A pilot flying at the UK Juniors a few years ago described a racing
finish in a K8, producing no more than a 200 ft climb from a 90kt
pull-up. He said that a K8 in this mode was the ultimate efficient
machine "for converting height into noise".

back to the original question...

Maybe I'm missing something, but I think the approach to the problem
is flawed. * How does mass "cancel out" if they are different masses?
Total energy is not the same in each case. *All things being equal at
the pull up, speed, glider type, etc. a ballasted glider has more mass
and thus more kinetic energy which would result in a higher climbout
compared to a non ballasted glider. *I'm not going to attempt to write
the equation because that would be embarrasing for me. * But what am I
missing? *Even if we start the gliders before the dive at the same
height the result is the same, the heavier glider has more potential
energy and will have a higher climb. *Isn't this simple high school
phyics?

I understand now what you mean, the mass is the same at the bottom and
top for each glider and therefore the climb is the same height if
velocity is the same. *Interesting. *Why does it feel like you climb
so much higher with ballast?- Hide quoted text -

- Show quoted text -

Most of us would be dumping our water ballast as we climbed, does that
make the ship gain more altitude? This is an old argument and I have
always believed the heavier ship gains more altitude.
JJ

Nope dumping the water loses you energy proportional to the mass of
the water, that energy no longer lifts that weight of water higher. In
the simple potential/kinetic energy model there is no effect.

AS stated by others I expect the perceived benefit of extra weight is
due to you more likely to be flying faster if ballasted and therefore
get a higher zoom climb.

Darryl

Just fly the damn thing...

Best explanation I ever heard was the heavier roller coaster car goes
further up the next hill than the lighter one.

Al

wrote:
On Apr 21, 11:11 am, Darryl Ramm wrote:
On Apr 21, 6:03 am, JJ Sinclair wrote:



On Apr 20, 6:35 pm, jim archer wrote:
On Apr 20, 6:17 pm, "Paul Remde" wrote:
Hi Jim,
It is simple high school physics. Yes the heavier glider has much more
energy, but it also takes much more energy to lift the heavier glider. You
would be much more tired after carrying 100 pounds up a flight of stairs
than you would be after lifting 10 pounds up a flight of stairs. The
physics shows very clearly that the extra speed energy from the higher
weight is exactly cancelled by the extra energy required to raise the
heavier weight.
before pullup after pullup
1/2 mv^2 + mgh = 1/2mv^2 + mgh
As you can see in the equation above you can divide both sides by m and the
equation doesn't change. So the mass of the glider doesn't matter, but the
speeds have a big effect because the velocity is squared.
Paul Remde
"jim archer" wrote in message
...
On Apr 20, 1:41 pm, Chris Reed wrote:
wrote:
The effect of drag on height recovery isn't too bad, but is enough to
matter.
In a low-performance glider the drag can be extremely significant. In,
say, a K8 or (I'd guess) an I-26, the height gain is very small in
comparison with 40:1 glass.
A pilot flying at the UK Juniors a few years ago described a racing
finish in a K8, producing no more than a 200 ft climb from a 90kt
pull-up. He said that a K8 in this mode was the ultimate efficient
machine "for converting height into noise".
back to the original question...
Maybe I'm missing something, but I think the approach to the problem
is flawed. How does mass "cancel out" if they are different masses?
Total energy is not the same in each case. All things being equal at
the pull up, speed, glider type, etc. a ballasted glider has more mass
and thus more kinetic energy which would result in a higher climbout
compared to a non ballasted glider. I'm not going to attempt to write
the equation because that would be embarrasing for me. But what am I
missing? Even if we start the gliders before the dive at the same
height the result is the same, the heavier glider has more potential
energy and will have a higher climb. Isn't this simple high school
phyics?
I understand now what you mean, the mass is the same at the bottom and
top for each glider and therefore the climb is the same height if
velocity is the same. Interesting. Why does it feel like you climb
so much higher with ballast?- Hide quoted text -
- Show quoted text -
Most of us would be dumping our water ballast as we climbed, does that
make the ship gain more altitude? This is an old argument and I have
always believed the heavier ship gains more altitude.
JJ
Nope dumping the water loses you energy proportional to the mass of
the water, that energy no longer lifts that weight of water higher. In
the simple potential/kinetic energy model there is no effect.

AS stated by others I expect the perceived benefit of extra weight is
due to you more likely to be flying faster if ballasted and therefore
get a higher zoom climb.

Darryl

Just fly the damn thing...

Best explanation I ever heard was the heavier roller coaster car goes
further up the next hill than the lighter one.

Al


Which reminds me....the Boy Scouts' soap box derby needs cars on the TOP
weight limit (and with weight preferably as far back as possible to
maximize potential energy...)

Brian W

Dumping water will give you a little F=ma acceleration upwards. Like a
very anemic rocket.

The heavier roller coaster goes higher by overcoming drag forces
better, ie friction is a smaller % of total force in play.

On a frictionless track, in a vaccum, a feather and a brick accelerate
the same.

On Apr 22, 1:54*pm, Mark Jardini wrote:
Dumping water will give you a little F=ma acceleration upwards. Like a
very anemic rocket.

Seems unlikely. If I pour water out of a jug, the jug is not forced
upwards.

The force in a rocket comes from the fact that the exhaust gas is
being pushed out of the nozzle, not just from the fact it is pouring
out of the nozzle. Thre is no internal force or presure ejecting the
water from a glider and, if there was, the vector would be about 90
deg to the direction required for useful thrust.

Andy

Andy wrote:
On Apr 22, 1:54 pm, Mark Jardini wrote:
Dumping water will give you a little F=ma acceleration upwards. Like a
very anemic rocket.

Seems unlikely. If I pour water out of a jug, the jug is not forced
upwards.

The force in a rocket comes from the fact that the exhaust gas is
being pushed out of the nozzle, not just from the fact it is pouring
out of the nozzle. Thre is no internal force or presure ejecting the
water from a glider and, if there was, the vector would be about 90
deg to the direction required for useful thrust.

Andy

Though I'm with you on the explanation, you open an interesting line of
speculation.
It has stayed with me that a nozzle into an evacuated bottle can develop
a sonic efflux.
It has stayed with me that your average throw-away crinkly plastic soda
bottle can handle 50 psi on up.

It's also true that the bigger the bottle the lower the limit pressure.
But here's the thought: a low pressure pump into the water tank with an
efficient nozzle and you might have somewhat usable temporary thrust.....

Brian W

On Apr 23, 9:56*am, brian whatcott wrote:
Andy wrote:
On Apr 22, 1:54 pm, Mark Jardini wrote:
Dumping water will give you a little F=ma acceleration upwards. Like a
very anemic rocket.

Seems unlikely. *If I pour water out of a jug, the jug is not forced
upwards.

The force in a rocket comes from the fact that the exhaust gas is
being pushed out of the nozzle, not just from the fact it is pouring
out of the nozzle. *Thre is no internal force or presure *ejecting the
water from a glider and, if there was, the *vector would be about 90
deg to the direction required for useful thrust.

Andy

Though I'm with you on the explanation, you open an interesting line of
speculation.
It has stayed with me that a nozzle into an evacuated bottle can develop
a sonic efflux.
It has stayed with me that your average throw-away crinkly plastic soda
bottle can handle 50 psi on up.

It's also true that the bigger the bottle the lower the limit pressure.
But here's the thought: a low pressure pump into the water tank with an
efficient nozzle and you might have somewhat usable temporary thrust.....

Brian W

I think I see where we are going with this. Replace the water with
diet coke and add a mentos injection system. (youtube/diet coke
mentos). A low save system that works every time.

brianDG303 wrote:
On Apr 23, 9:56 am, brian whatcott wrote:
Andy wrote:
On Apr 22, 1:54 pm, Mark Jardini wrote:
Dumping water will give you a little F=ma acceleration upwards. Like a
very anemic rocket.
Seems unlikely. If I pour water out of a jug, the jug is not forced
upwards.
The force in a rocket comes from the fact that the exhaust gas is
being pushed out of the nozzle, not just from the fact it is pouring
out of the nozzle. Thre is no internal force or presure ejecting the
water from a glider and, if there was, the vector would be about 90
deg to the direction required for useful thrust.
Andy
Though I'm with you on the explanation, you open an interesting line of
speculation.
It has stayed with me that a nozzle into an evacuated bottle can develop
a sonic efflux.
It has stayed with me that your average throw-away crinkly plastic soda
bottle can handle 50 psi on up.

It's also true that the bigger the bottle the lower the limit pressure.
But here's the thought: a low pressure pump into the water tank with an
efficient nozzle and you might have somewhat usable temporary thrust.....

Brian W

I think I see where we are going with this. Replace the water with
diet coke and add a mentos injection system. (youtube/diet coke
mentos). A low save system that works every time.

Durn it! You were WAY ahead of me! :-)

Brian W

On Apr 20, 4:24*am, "Paul Remde" wrote:
Hi John,

You are correct. *The physics equations show that you will get the same
height regardless of the weight of the glider.

However, I think it is true that a heavier glider will have a slightly
higher pull-up. *I don't think the difference is very much though. *Both
gliders will have similar frictional losses and losses due to inefficiencies
during the pull-up.

Paul Remde

"John Rivers" wrote in message

...



I was trying to work out the expected height gain from a pull up
Experienced glider pilots say you will get a better pull up with a
heavier glider / water etc.
But I can't see this from my (probably incomplete) equations:

total energy = potential energy + kinetic energy

total energy before pull up = total energy after pull up

m * g * h0 + m * pow(v0, 2) * 0.5 == m * g * h1 + m * pow(v1, 2) * 0.5

with h0 v0 being height and speed before pull up
and h1 v1 being height and speed after pull up

mass cancels out of this equation

I think I need to include momentum in there somehow?

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. 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.

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.

9B

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.

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.

More calculation opportunities for the phyisics groupies.

Which glider gets you to the same altitude faster assuming the same
speed. Ballasted or non ballasted?