Comparison of glider classes at Uvalde...

Except heavier gliders don't pull up any further than light ones.
(From the same speed)

Otherwise your total energy vario wouldn't work

PF

At 18:01 22 August 2012, Tom Vallarino wrote:


This gets me thinking that in order to vary the speed of a heavier ship,
which zooms up 300feet on pull up, that this increases the total flight
path over that of a lighter ship which zooms up less and slows down
faster.=20

On Thursday, August 23, 2012 1:39:37 AM UTC-7, Peter F wrote:
Except heavier gliders don't pull up any further than light ones.

If both gliders have the same sink rate at the time the pull up is initiated, and if the transition is lossless, then that would be true. However, neither of those conditions is true.

GY

OK,

So it starts off as a nice day, you're happily dolphining along in your
Discus / Ventus / Nimbus 4 / Quintus (delete as appropriate) full of water
and your TE system is sorted so that pullups don't upset it.

The weather turns to worms so you get low & have to dump water.

You climb away and the weather cycles, so you go back to happily dolphining
along.

Does

1) Your total energy stop working

2) Your vario system that has no idea about the water ballast system
magically works out that something has changed

or

3) Pullups trade Kinetic Energy for Potential Energy and the mass terms
cancel. If you're pulling up from the same speed to the same speed you'll
pull up the same amount. The time taken for the pullup is just a few
seconds so any difference in sink rate at the beginning of the pullup
results in a trivial difference in height gained. (And the light glider can
pull up to a lower speed than the heavy one so will gain benefit there).

TE system doesn't need to know that the ballast has changed 'cos it isn't
affected

PF



At 16:04 23 August 2012, Andy wrote:
On Thursday, August 23, 2012 1:39:37 AM UTC-7, Peter F wrote:
Except heavier gliders don't pull up any further than light ones.

If both gliders have the same sink rate at the time the pull up is
initiated, and if the transition is lossless, then that would be true.
However, neither of those conditions is true.

GY

An open class plane has more drag than a modern 15m. Yes, it also has more lift, but lift is irrelevant when trying to use gravity to speed up. Therefore, a larger or draggier plane will accelerate slower.

An open class ship is also heavier, so it slows down slower than a 15m.

The only way to modulate the speed of an open class like a 15m, is to vary the altitude more (dynamic flight) than that of a 15m. This is often seen as a good thing - but it's not. The more dynamic the flight path, the longer the total flight path becomes.

In general, it is harder to vary speeds in larger and heavier gliders. Consequently, they are probably flown at less optimal speeds, than smaller/lighter gliders, and they total flight path length of the larger ones is probably longer over the same horizontal distance, since their dynamic path is more extreme.

As to pull up height: Weight makes a difference as kinetic energy is a function of mass, the higher the mass, the larger the kinetic energy at a given speed. An insect traveling at 100 knots has much lower kinetic energy than a B747 at 100 knots. In other words, it takes more energy to accelerate a B747 to 100 knots than it does to accelerate an insect to 100 knots.

I have two things to add to this discussion:

First, most of us, when we dump our water, will update the
variometer/computer with that information. This tells the system to change
the speed command for a given MacCready setting.

Second, let's not forget that the kinetic energy of a body in motion
(glider) is equal to one half the mass times the velocity squared. That
means that the heaver glider (water) will be traveling faster (dolphin) so
the conversion from kinetic energy (velocity) to potential energy (altitude)
will be higher.

I hope I said that clearly...


wrote in message
...
An open class plane has more drag than a modern 15m. Yes, it also has more
lift, but lift is irrelevant when trying to use gravity to speed up.
Therefore, a larger or draggier plane will accelerate slower.

An open class ship is also heavier, so it slows down slower than a 15m.

The only way to modulate the speed of an open class like a 15m, is to vary
the altitude more (dynamic flight) than that of a 15m. This is often seen as
a good thing - but it's not. The more dynamic the flight path, the longer
the total flight path becomes.

In general, it is harder to vary speeds in larger and heavier gliders.
Consequently, they are probably flown at less optimal speeds, than
smaller/lighter gliders, and they total flight path length of the larger
ones is probably longer over the same horizontal distance, since their
dynamic path is more extreme.

As to pull up height: Weight makes a difference as kinetic energy is a
function of mass, the higher the mass, the larger the kinetic energy at a
given speed. An insect traveling at 100 knots has much lower kinetic energy
than a B747 at 100 knots. In other words, it takes more energy to accelerate
a B747 to 100 knots than it does to accelerate an insect to 100 knots.

Yes Dan, but even from the same start speed, a heavy glider will climb higher than a lighter one of identical design, when slowing down to a slower target speed - or it will take longer and travel a greater horizontal distance to do so - or both. In other words it will have a more dynamic flight path, unless the pilot chooses to modulate less and keep the speed more constant, regardless of the changing air masses. In the latter case, the plane will fly less optimally than one able to modulate better.

Perhaps these disadvantages are more significant than thought.

In response to Steve's observation that the open class gliders were only marginally faster than 18m and even 15m, I think it is good to have a discussion as to the reason for this.

I guess I wasn't clear enough. You're correct that given the same start
speed, the heavier glider will climb higher: 1/2mv**2.


wrote in message
...
Yes Dan, but even from the same start speed, a heavy glider will climb
higher than a lighter one of identical design, when slowing down to a slower
target speed - or it will take longer and travel a greater horizontal
distance to do so - or both. In other words it will have a more dynamic
flight path, unless the pilot chooses to modulate less and keep the speed
more constant, regardless of the changing air masses. In the latter case,
the plane will fly less optimally than one able to modulate better.

Perhaps these disadvantages are more significant than thought.

In response to Steve's observation that the open class gliders were only
marginally faster than 18m and even 15m, I think it is good to have a
discussion as to the reason for this.

When the basic equations of physics are questioned they should be tested by real experimental data.

Having recently worked on the compensation of the vario in my Ventus 2, I just happen to have such data. Several pull ups from 180 km/t to 95 km/t were recorded by the igc-logger (and on film).

Theory first (in SI units):

Potential energy: m * g * h
(m: mass, g: acceleration due to gravity - about 9.8 m/s^2, h: altitude)

Kinetic energy: ½ * m * v^2
(v: speed)

As a result, the theoretical lossless altitude gain by a pull up is:

dh_theory = ½ * (v_start^2-v_final^2) / g

This equation does not depend on the mass of the glider !

Experimental data:

24 pull ups from three different days in relatively calm air.
Average start speed: v_start = 49.8 m/s ± 0.4 m/s
Average final speed: v_final = 26.3 m/s ± 0.7 m/s
Average altitude gain: dh = 90 m ± 3 m

Using the equation above and the average start and final speeds, I find the theoretical altitude gain to be: dh_theory = 91 m ± 4 m

Actually, I was a little surprised to see such a close agreement.

No variation between days or direction of flight is seen (i.e. correct wind correction). The duration of the pull ups is 10 seconds. The quoted uncertainties are the statistical standard error of the average. Further analysis shows that the uncertainties on dH and dH_theory are highly correlated. I could think of several potential error sources but have not investigated their influence.

The mass of the Ventus 2? Well, it doesn’t matter…

Jan

PS! The mass-independent conversion from speed to altitude was actually given as an example in my school physics book when I was 14 years old. At that time I questioned the physics book due to the general (incorrect) understanding of this topic among glider pilots.

While I agree that the height gain on a pull up is not in principle
dependent on glider mass - being simply an exchange between two forms of
energy less some drag, maybe there is a real difference for the heavier
glider..

I suggest that the heavier glider will probably normally have a greater
initial speed - and thus the G for the pull up can be maintained for a
longer time, albeit small. I think it is the G which is the key thing
here, since it multiplies the effect of the lift. Hard to believe but
apparently true for positive and less practically for negative G...
Reference:- F.G.Irving, the Paths of Soaring Flight, pp86-88, a good
bedtime read.

At 10:07 25 August 2012, Jan wrote:
When the basic equations of physics are questioned they should be tested
by=
real experimental data.

Having recently worked on the compensation of the vario in my Ventus 2, I
j=
ust happen to have such data. Several pull ups from 180 km/t to 95 km/t
wer=
e recorded by the igc-logger (and on film).

Theory first (in SI units):

Potential energy: m * g * h =20
(m: mass, g: acceleration due to gravity - about 9.8 m/s^2, h: altitude)

Kinetic energy: =BD * m * v^2 =20
(v: speed)

As a result, the theoretical lossless altitude gain by a pull up is:

dh_theory =3D =BD * (v_start^2-v_final^2) / g

This equation does not depend on the mass of the glider !

Experimental data:

24 pull ups from three different days in relatively calm air.=20
Average start speed: v_start =3D 49.8 m/s =B1 0.4 m/s
Average final speed: v_final =3D 26.3 m/s =B1 0.7 m/s
Average altitude gain: dh =3D 90 m =B1 3 m

Using the equation above and the average start and final speeds, I find
the=
theoretical altitude gain to be: dh_theory =3D 91 m =B1 4 m

Actually, I was a little surprised to see such a close agreement.

No variation between days or direction of flight is seen (i.e. correct
wind=
correction). The duration of the pull ups is 10 seconds. The quoted
uncert=
ainties are the statistical standard error of the average. Further
analysis=
shows that the uncertainties on dH and dH_theory are highly correlated.
I
=
could think of several potential error sources but have not investigated
th=
eir influence.

The mass of the Ventus 2? Well, it doesn=92t matter=85

Jan

PS! The mass-independent conversion from speed to altitude was actually
giv=
en as an example in my school physics book when I was 14 years old. At
that=
time I questioned the physics book due to the general (incorrect)
understa=
nding of this topic among glider pilots.

Your analyses are great! I'd been thinking about dH and it's clear that the
mass falls out of the equations, however, the heavier glider will most
likely be flying faster which is, I think, the error most of us, myself
included, make. Given the same entry and exit speeds, the altitude gain
should be the same.

Thanks for the clarification.


"Steve Thompson" wrote in message
.com...
While I agree that the height gain on a pull up is not in principle
dependent on glider mass - being simply an exchange between two forms of
energy less some drag, maybe there is a real difference for the heavier
glider..

I suggest that the heavier glider will probably normally have a greater
initial speed - and thus the G for the pull up can be maintained for a
longer time, albeit small. I think it is the G which is the key thing
here, since it multiplies the effect of the lift. Hard to believe but
apparently true for positive and less practically for negative G...
Reference:- F.G.Irving, the Paths of Soaring Flight, pp86-88, a good
bedtime read.

At 10:07 25 August 2012, Jan wrote:
When the basic equations of physics are questioned they should be tested
by=
real experimental data.

Having recently worked on the compensation of the vario in my Ventus 2, I
j=
ust happen to have such data. Several pull ups from 180 km/t to 95 km/t
wer=
e recorded by the igc-logger (and on film).

Theory first (in SI units):

Potential energy: m * g * h =20
(m: mass, g: acceleration due to gravity - about 9.8 m/s^2, h: altitude)

Kinetic energy: =BD * m * v^2 =20
(v: speed)

As a result, the theoretical lossless altitude gain by a pull up is:

dh_theory =3D =BD * (v_start^2-v_final^2) / g

This equation does not depend on the mass of the glider !

Experimental data:

24 pull ups from three different days in relatively calm air.=20
Average start speed: v_start =3D 49.8 m/s =B1 0.4 m/s
Average final speed: v_final =3D 26.3 m/s =B1 0.7 m/s
Average altitude gain: dh =3D 90 m =B1 3 m

Using the equation above and the average start and final speeds, I find
the=
theoretical altitude gain to be: dh_theory =3D 91 m =B1 4 m

Actually, I was a little surprised to see such a close agreement.

No variation between days or direction of flight is seen (i.e. correct
wind=
correction). The duration of the pull ups is 10 seconds. The quoted
uncert=
ainties are the statistical standard error of the average. Further
analysis=
shows that the uncertainties on dH and dH_theory are highly correlated.
I
=
could think of several potential error sources but have not investigated
th=
eir influence.

The mass of the Ventus 2? Well, it doesn=92t matter=85

Jan

PS! The mass-independent conversion from speed to altitude was actually
giv=
en as an example in my school physics book when I was 14 years old. At
that=
time I questioned the physics book due to the general (incorrect)
understa=
nding of this topic among glider pilots.