Question of the day

A given glider is at level flight, IAS= 100 knots.After a pull-up, will it
achieve more height gain with 100 liters (100 kgs) of ballast than with
empty ballast????
Réjean

The difference in height will be negligible.

The glider's energy, both potential & kinetic is proportional
to Mass so the height gain for a given loss of velocity
will be the same.

However the ballasted glider will have a better sink
rate at 100kts than the unballasted one so during the
few seconds of the pull-up it will 'lose' less height.

On the other side of the equation the un-ballasted
glider will be able to pull up to a lower speed, so
it's change in velocity will be greater so the resulting
height gain may be more.

(My money - if I had any - would be on the un-ballasted
one)

Steve B wrote:

I am curious... what amount of altitude are the different gliders able
to get from a high speed pull up? 15 m 18 m 25 m?

Neglecting drag and other minor inconvencies:

m/2 v1^2 + mgh1 = m/2 v2^2 + mgh2

I think I can leave the rest to you. (Hint: It's something between 100m
and 150m, depending on v1 and v2.)

Stefan

I think you'll find the polar on a 15m glider is quite
a lot flatter than that of a model. So at the entry
to the pull up the model is probably losing height
faster than the full-size ship.

The proportion of ballast you can carry in a model
is probably higher.In my Discus, weighing 320kg dry
(with me in it!) I can carry about 200kg of ballast
i.e about an extra 60%.
The models I used to fly 'when I were a lad' weighed
1-2kg, but could carry about 5kg ballast ('cos I was
less worried about pulling the wings off). This results
in a much greater benefit at high speed than you'll
get in the full-size object :-)

I also suspect the ballasted one is travelling faster
in the first place. How are you measuring the speed
of your models at the point you're starting the pull
up?

At 18:30 08 September 2003, Jim Vincent wrote:
(My money - if I had any - would be on the un-ballasted
one)


From my many years experience flying radio control
slope ships, there is a heck
of a difference in performance based on the amount
of ballast. Without ballast
I can get maybe one vertical roll; with ballast I can
get at least three or
four.
Jim Vincent
CFIG
N483SZ

Kevin Neave k wrote in message ...
The difference in height will be negligible.

Not true. A full load of water makes a HUGE difference in pullup
altitude gained

The glider's energy, both potential & kinetic is proportional
to Mass so the height gain for a given loss of velocity
will be the same.

Again, wrong - check your basic physics. You even say that the energy
is proportional to mass. Therefore, more mass, more energy, more
altitude gained. You appear to be confusing velocity with mass.

However the ballasted glider will have a better sink
rate at 100kts than the unballasted one so during the
few seconds of the pull-up it will 'lose' less height.

True, but the crossover point is quickly reached so this effect is
probably negligable.

On the other side of the equation the un-ballasted
glider will be able to pull up to a lower speed, so
it's change in velocity will be greater so the resulting
height gain may be more.

If you pull up below the ballasted sink rate crossover speed, sure a
heavy glider will gain less. But at those speeds neither glider will
gain much anyway. The real test is what you can gain at redline.

(My money - if I had any - would be on the un-ballasted
one)

Too bad, I love a sure thing!

Kirk Stant
LS6-b (which loves ballasted pullups!)

I just tried this - but had to use a pair of bicycles.

Me (unballasted) and my boss (ballasted) on similar
bikes
at the same speed coasted up a small hill. He was going
significantly faster at the top.

Happens every lunch time so must be true.

At 13:30 08 September 2003, Szd41a wrote:
A given glider is at level flight, IAS= 100 knots.After
a pull-up, will it
achieve more height gain with 100 liters (100 kgs)
of ballast than with
empty ballast????
Réjean

Jim
Next time, challenge your boss on an endless hill, not a small one!!! It is
obvious that at T=0 , if both your boss and you hit me, your boss will hurt
me more than you:-)))). Gee! Are we trying to prove that it easier to move
heavier load up the hill or what ???.
"Jim Britton" a écrit dans le message de
...
I just tried this - but had to use a pair of bicycles.

Me (unballasted) and my boss (ballasted) on similar
bikes
at the same speed coasted up a small hill. He was going
significantly faster at the top.

Happens every lunch time so must be true.

At 13:30 08 September 2003, Szd41a wrote:
A given glider is at level flight, IAS= 100 knots.After
a pull-up, will it
achieve more height gain with 100 liters (100 kgs)
of ballast than with
empty ballast????
Réjean

I think Kevin's right. (at least to the first order).
In a pullup you trade kinetic energy for potential
energy, so (neglecting friction effects) the physics
are (mV^2)/2=mgh -- kinetic equals potential energy.

Or (solving for h): h=(V^2)/2g (the mass cancels out).

You can try to add the drag parts back in, but the
time is so short, I don't think it will not add up
to much.

I think maybe ther reason people associate ballast
with taller zoomies is because the cruise speeds with
ballast are higher. For my ship a McCready 10 pullup
yields 700 feet with full ballast, 530 feet dry, but
the speed is 15 knots higher.



At 21:00 08 September 2003, Kirk Stant wrote:
Kevin Neave wrote in message news:...
The difference in height will be negligible.

Not true. A full load of water makes a HUGE difference
in pullup
altitude gained

The glider's energy, both potential & kinetic is proportional
to Mass so the height gain for a given loss of velocity
will be the same.

Again, wrong - check your basic physics. You even
say that the energy
is proportional to mass. Therefore, more mass, more
energy, more
altitude gained. You appear to be confusing velocity
with mass.

However the ballasted glider will have a better sink
rate at 100kts than the unballasted one so during
the
few seconds of the pull-up it will 'lose' less height.

True, but the crossover point is quickly reached so
this effect is
probably negligable.

On the other side of the equation the un-ballasted
glider will be able to pull up to a lower speed, so
it's change in velocity will be greater so the resulting
height gain may be more.

If you pull up below the ballasted sink rate crossover
speed, sure a
heavy glider will gain less. But at those speeds neither
glider will
gain much anyway. The real test is what you can gain
at redline.

(My money - if I had any - would be on the un-ballasted
one)

Too bad, I love a sure thing!

Kirk Stant
LS6-b (which loves ballasted pullups!)

On 8 Sep 2003 13:08:22 -0700, (Kirk Stant)
wrote:

On the other side of the equation the un-ballasted
glider will be able to pull up to a lower speed, so
it's change in velocity will be greater so the resulting
height gain may be more.

If you pull up below the ballasted sink rate crossover speed, sure a
heavy glider will gain less. But at those speeds neither glider will
gain much anyway. The real test is what you can gain at redline.

(My money - if I had any - would be on the un-ballasted
one)

Too bad, I love a sure thing!

Kirk Stant
LS6-b (which loves ballasted pullups!)


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.

This is used in the design of total energy probes which DO NOT require
changing for different ballast amounts.

With a little mathematical jiggery pokery it can be shown that the
kinetic/potential energy equation is equivalent to the equation for
the pressure produced by the TE probe.

Mike Borgelt

Borgelt Instruments

In article ,
(Kirk Stant) wrote:

| Kevin Neave k wrote in
| message ...
| The difference in height will be negligible.
|
| Not true. A full load of water makes a HUGE difference in pullup
| altitude gained
|
| The glider's energy, both potential & kinetic is proportional
| to Mass so the height gain for a given loss of velocity
| will be the same.
|
| Again, wrong - check your basic physics. You even say that the energy
| is proportional to mass. Therefore, more mass, more energy, more
| altitude gained. You appear to be confusing velocity with mass.

Total energy = kinetic energy + potential energy

kinetic energy = 1/2 mass * velocity squared

potential energy = mass * gravitational constant * height

total energy (altitude 1) = total energy (altitude 2) [conservation of
energy]

Since mass is a constant factor on both sides of the equation, it
cancels out. Therefore there should theoretically be negligible
difference in the pullup altitude gained between the ballasted and
unballasted cases.

-- Tim Olson