Question of the day

[szd41a]

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

[Kevin Neave]

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)

[Stefan]

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

[Kevin Neave]

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

>

[Kirk Stant]

Kevin Neave > 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!)

[Jim Britton]

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
>
>
>

[szd41a]

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
> >
> >
> >
>
>
>

[Andy Blackburn]

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!)
>

[Mike Borgelt]

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

[Tim Olson]

In article >,
(Kirk Stant) wrote:

| Kevin Neave > 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

[Buck Wild]

Stefan <"stefan"@mus. INVALID .ch> wrote in message >...
> 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

100m~150m? Even the G103 will climb to 800' or so from a 115kt pass.
I've seen closer to 1000 in standard class dry. About 800' from 100kts
in a Nimbus2 dry.
About 100' in a Pawnee with the prop stopped.
Never did understand reverse Chinese algebra anyway.
-Dan

[Alan Baker]

In article >,
Jim Britton > wrote:

> 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.

Air drag and mass to drag ratio.

>
> 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
> >
> >
> >
>
>
>

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

[Stefan]

Kevin Neave wrote:
>
> And why might you get more height by dumping during
> the climb?

Forget it. There are days on which a brain just doesn't work as advertised.

Stefan

[szd41a]

Final answer :

I solved the equations for my Jantar
mass empty= 360 kg
mass wet= 460 kg

v1 empty= 55.6 m/s
v1 wet= 55.6 m/s (same speed)

v2 empty= 20.8 m/s
v2 wet= 23.9 m/s (speed at Min. sink acording to specs.)

Delta H empty= 135 meters
Delta H wet= 128 meters

So Kevin is right, you theorically go higher without ballast.

Réjean
p.s At my club, nobody belives me. Maybe Newton was wrong!!!

"Stefan" <"stefan"@mus. INVALID .ch> a écrit dans le message de
...
> 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

[dennis]

Wrong kind of science. Others have the right answer - it gets more altitude
for the same reason that a golf ball will hit the ground before a ping pong
ball dropped from the same height. Assuming they are the same size, etc.
except for weight. Just turn it around and say both are going speed x and
suddenly exposed to the air, which will slowdown faster?

Another way to say that - if a reentry module is real light, does it slow
down faster?




In article >, "szd41a"
> wrote:
>Final answer :
>
>I solved the equations for my Jantar
>mass empty= 360 kg
>mass wet= 460 kg
>
>v1 empty= 55.6 m/s
>v1 wet= 55.6 m/s (same speed)
>
>v2 empty= 20.8 m/s
>v2 wet= 23.9 m/s (speed at Min. sink acording to specs.)
>
>Delta H empty= 135 meters
>Delta H wet= 128 meters
>
>So Kevin is right, you theorically go higher without ballast.

[Jere Knuuttila]

Tim Olson wrote:
> 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

Adding energy lost due to drag into the soup:
m = mass
v1 = initial speed
h1 = initial altitude
v2 = final speed
h2 = final altitude

total energy(alt 1) = total energy (alt 2) + energy lost due to drag
1/2*m*v1^2 + m*g*h1 = 1/2*m*v2^2 + m*g*h2 + Ed

dividing both sides by m:
1/2*v1^2 + g*h1 = 1/2*v2^2 + g*h2 + Ed/m

Now compare two gliders, starting with same speed, same altitude, level
flight. Both pull up and level out with same final speed (just to keep
things even). The above holds for both gliders, so:
(1) 1/2*v1^2 + g*h1 = 1/2*v2^2 + g*h21 + Ed/m1
(2) 1/2*v1^2 + g*h1 = 1/2*v2^2 + g*h22 + Ed/m2
substracting (1)-(2):
0=g(h21 - h22) + Ed(1/m1 - 1/m2)
where h21 is final altitude for glider 1 and h22 for glider 2
and masses m1 for glider 1 and m2 for glider 2.
We get:
h22-h21 = Ed/g(1/m1 - 1/m2)

So if m2 > m1, h22 > h21. The heavier glider will get more altitude.
But not very much... Approximating Ed from sink rate (energy lost is
m*g*(sink rate)*time) the altitude difference for masses 360kg and 460kg
is a bit over one meter. But then again, to be exact, Ed is bigger for
the heavier glider (same speed, more mass, need bigger AOA -> more drag).

Please point out any mistakes (well since it's the Usenet, I'm sure you
will :)

Jere
jere at iki.fi

[Kevin Neave]

One trivial point for Jere's post is that the stalling
speed for the unballasted glider will be lower, so
this guy can pull up to a slower speed & therefore
regain more altitude.

Anyone out there want to know why there's a discrepancy
between the maths (which say that there's not gonna
be a measurable difference between the two) and 'popular'
experience which says that the heavy glider wins?

At 17:12 09 September 2003, Jere Knuuttila wrote:
>
>Adding energy lost due to drag into the soup:
>m = mass
>v1 = initial speed
>h1 = initial altitude
>v2 = final speed
>h2 = final altitude
>
>total energy(alt 1) = total energy (alt 2) + energy
>lost due to drag
>1/2*m*v1^2 + m*g*h1 = 1/2*m*v2^2 + m*g*h2 + Ed
>
>dividing both sides by m:
>1/2*v1^2 + g*h1 = 1/2*v2^2 + g*h2 + Ed/m
>
>Now compare two gliders, starting with same speed,
>same altitude, level
>flight. Both pull up and level out with same final
>speed (just to keep
>things even).

> the altitude difference for masses 360kg and 460kg
>
>is a bit over one meter. But then again, to be exact,
>Ed is bigger for
>the heavier glider (same speed, more mass, need bigger
>AOA -> more drag).
>
>Please point out any mistakes (well since it's the
>Usenet, I'm sure you
>will :)
>
>Jere
>jere at iki.fi
>
>

[Jim Vincent]

>Anyone out there want to know why there's a discrepancy
>between the maths (which say that there's not gonna
>be a measurable difference between the two) and 'popular'
>experience which says that the heavy glider wins?

I dunno. According to my math, the heavier one does win. Also, according to
my experience, the heavier one does win.

Jim Vincent
CFIG
N483SZ

[mm]

"Jim Vincent" > wrote in message
...
> >
> >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????
>
>
> It will gain more height with ballast. In level flight, the ballasted
glider
> has more kinetic energy since it weighs more than an unballasted glider.
The
> kinetic energy is defined as 1/2*m*v squared. Taking out the constants,
the
> kinetic energy for the same speed is directly proportional to the mass
(m).
>
> The potential energy is m*g*h, again taking out the constants, it is
directly
> proportional to the mass.
>
> This does not acount for the loss of energy due to drag.
>
> So for example, if a gldier weighs twice as much, it will gain twice the
> height, or at least I think so!
>
>

How can you write the two terms, each directly proportional to mass, and
then conclude that the heavier glider will gain more? No offence, but you
might need some remedial math. The right answer, neglecting drag as you do,
is
(V2^2-V1^2)/2/g=H2-H1. Mass cancels out.

[szd41a]

Both glider have the same speed at start i.e. 100 knts
This point is very important.

"Ivan Kahn" > a écrit dans le message de
news:Kus7b.405642$o%2.181198@sccrnsc02...
>
> "Tim Olson" > wrote in message
> ...
> > 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
>
> What about the fact that the ballasted glider has a higher velocity and
> therefore more kinetic energy? Canceling constants we get: 1/2 * V ^2 = g
*
> height
>
> So then, and just to keep things easy we'll use "32" for g:
>
> With ballast at say 80 knots = 1/2 * 80 ^ 2 = 3,200 / 32 = 100
> no ballast at say 70 = 1/2 * 70 ^ 2 = 2,450 / 32 = 76.5
>
> With ballast it goes higher.
>
> Ivan
>
>
>

[Eric Greenwell]

In article >,
says...

> 100m~150m? Even the G103 will climb to 800' or so from a 115kt pass.
> I've seen closer to 1000 in standard class dry. About 800' from 100kts
> in a Nimbus2 dry.

800 feet! Wow! What elevation are you flying at? I've never seen
climbs like these in my ASW 20 or ASH 26 at 115 kts (more like 400'),
but that's at density altitudes of about 2000'.

And, actually, the Grob should go 20-30% higher at 115 knots than the
Nimbus at only 100 knots, as the altitude gained goes up by about the
square of the airspeed.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

[Andy Blackburn]

Nice add Eric.

If we are going to match up theory and experience we
need to match up based on true airspeed, since energy
is based on TAS.

Adding 5000 feet to elevation increases TAS by 10%
and pullup height by ~25%.

9B

At 02:42 10 September 2003, Eric Greenwell wrote:
>In article ,
says...
>
>> 100m~150m? Even the G103 will climb to 800' or so
>>from a 115kt pass.
>> I've seen closer to 1000 in standard class dry. About
>>800' from 100kts
>> in a Nimbus2 dry.
>
>800 feet! Wow! What elevation are you flying at? I've
>never seen
>climbs like these in my ASW 20 or ASH 26 at 115 kts
>(more like 400'),
>but that's at density altitudes of about 2000'.
>
>And, actually, the Grob should go 20-30% higher at
>115 knots than the
>Nimbus at only 100 knots, as the altitude gained goes
>up by about the
>square of the airspeed.
>--
>!Replace DECIMAL.POINT in my e-mail address with just
>a . to reply
>directly
>
>Eric Greenwell
>Richland, WA (USA)
>

[Buck Wild]

Eric Greenwell > wrote in message >...
> In article >,
> says...
>
> > 100m~150m? Even the G103 will climb to 800' or so from a 115kt pass.
> > I've seen closer to 1000 in standard class dry. About 800' from 100kts
> > in a Nimbus2 dry.
>
> 800 feet! Wow! What elevation are you flying at? I've never seen
> climbs like these in my ASW 20 or ASH 26 at 115 kts (more like 400'),
> but that's at density altitudes of about 2000'.
>
> And, actually, the Grob should go 20-30% higher at 115 knots than the
> Nimbus at only 100 knots, as the altitude gained goes up by about the
> square of the airspeed.

But the Grob has twice the drag of the Nimbi.
My pullups would begin at 5000msl, and level at minimum speed.
Check out the vidio of Carl Heinz doing 2 consecutive loops on final
in an Astire with the gear doors coming open under "G". The first
starts at treetop level & the second starts at about 5 feet. Good
display of available energy.
-Dan

[Kevin Neave]

Ok folks, let's try to put this one to bed.

This post is broken down into 3 sections
1) The Maths
2) An Experiment to prove the maths
3) Popular perception

1) The Maths
This isn't rocket science. Or perhaps it is! I'm sure
there's somebody from NASA out there who wants to take
a few minutes off from designing flying wings.
We have a number of variable here which all contribute
to whether the heavy glider wins.
1.1) Initial velocity - the faster we're going at the
start the greater the advabtage that the heavy glider
has. It's sink rate is lower at higher speed, and the
pull up takes longer.
If we started our pull up at 45kts I'm pretty sure
the light glider wins! If we start at 150kts (or 134
for you Discus drivers) then 'probably' the heavy glider
wins

1.2) Final velocity - If we pull up to the same speed
in both gliders the heavy one wins, no question. However
the light glider has a lower stalling speed than the
heavy one so can gain an advantage there.

1.3) Amount of ballast - This determines how great
the advantage the heavy glider has at the start of
the pull.

However the original post was for a glider at 100kts
with 100kg of ballast & I think the results are too
close to call.

2) An Experiment
This needs someone with a two-seater & a logger set
to 1 sec samples. Start your beat-up, racing finish,
whatever you choose to call it, at something above
the speed that you decide to start the pull-up.
When you get to the designated speed the P2 says 'GO'
& you pull (This why we need a 2 seat, so pilot can
keep their eyes out of the office).
Land, fill with ballast, & repeat. Compare logger traces


3) Popular perception
'Most' people think the ballasted glider wins. I'm
pretty sure it's 'cos they haven't carried out any
calibrated tests as described in (2) above.
What actually happens is...
You arrive back after your cross country, with no ballast,
and about 2.5 miles out you have 1000 ft on the clock.
This definately get you in so you lower the nose to
100kts.
This brings you over the airfield boundary 1.5 mins
later, you do your 'finish' at a few feet which takes
about 10 seconds & then pull up (starting somewhat
less than 100 kts - oops).

Meanwhile the guy with ballast arrives back at the
same point (2.5miles / 1000ft) and again lowers the
nose. Half a mile out this guy still has about 300ft
in hand so puts the nose down even further.
He does his finish at considerably more than 100 kts
& of course pulls up much higher!
(He had best part of 300ft in hand at the airfield
boundary remember)

My guess is that as long as the airspeed is within
a reasonable range (>100kts, well below Vne) people
>are not actually monitoring it that closely during
their 'finishes' so we're not really comparing like
with like

[Martin Gregorie]

On 9 Sep 2003 17:44:24 GMT, Kevin Neave
> wrote:

>One trivial point for Jere's post is that the stalling
>speed for the unballasted glider will be lower, so
>this guy can pull up to a slower speed & therefore
>regain more altitude.
>
>Anyone out there want to know why there's a discrepancy
>between the maths (which say that there's not gonna
>be a measurable difference between the two) and 'popular'
>experience which says that the heavy glider wins?
>

Could this be due to speed difference at cruise or fast cruise speeds?

One point that hasn't been considered by those arguing that the
ballasted glider is faster forget that this is NOT the case at Vne,
which doesn't change with glider weight.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

[Eric Greenwell]

In article >, airthugg1
@yahoo.com says...
> > And, actually, the Grob should go 20-30% higher at 115 knots than the
> > Nimbus at only 100 knots, as the altitude gained goes up by about the
> > square of the airspeed.
>
> But the Grob has twice the drag of the Nimbi.
> My pullups would begin at 5000msl, and level at minimum speed.
> Check out the vidio of Carl Heinz doing 2 consecutive loops on final
> in an Astire with the gear doors coming open under "G". The first
> starts at treetop level & the second starts at about 5 feet. Good
> display of available energy.

At 115 knots, the Grob has (relatively) about 32% more energy than the
Nimbus at 100 knots. So, even though it has more drag, that's a heck
of an advantage, and I'm sure it would allow a higher pull up. I'm
guessing it'd be at least 20%, but it certainly wouldn't be the full
32%.

--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

[Richard Brisbourne]

On Mon, 08 Sep 2003 16:22:06 -0400, Tony Verhulst
> wrote:

>Jim Britton wrote:
>> 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.
>
>Do it (more) scientifically - switch bikes and repeat several times.
>
>Tony V.
>http://home.comcast.net/~verhulst/SOARING/index.htm
>

Nope- try it on bikes with zero friction in the bearings, and put on a
bit of padding so you have the same volume (if still lower mass) than
your boss.

Assuming frictional forces are the same on both bikes, a lower mass
will be subject to a more rapid deceleration (P=mf- remember).

And drag will be proportional (roughly) to the surface area as viewed
from the front.

In the case of the glider, as anyone with an understanding of
elementary physics has pointed out if you just look at a simple
kinetic to potential energy conversion the height gain is independent
of mass (the heavier glider has more kinetic energy at the start of
the pull-up, more potential energy at the end). However, as with the
bike other forces come in. For most of the speed curve the heavier
glider will be subjected to less drag (that's why we put ballast in in
the first place). Intuitively (and correctly) we perceive it requires
more to slow it down.

Reckon the thermal season's nearly over.
--
"Curmudgeonly is the new cool" (Terry Wogan)
(The real name at the left of the e-mail address is richard)

[Kevin Neave]

As the thermal season's nearly over I've got time to
pick nits!

I'm sorry but the heavier glider is subject to more
drag at any given speed than the lighter one.

(Profile drag will be pretty much the same 'cos the
glider is the same shape - Induced drag will be higher
'cos the wings are having to work harder)

> For most of the speed curve the heavier
>glider will be subjected to less drag (that's why we
>put ballast in in
>the first place). Intuitively (and correctly) we perceive
>it requires
>more to slow it down.
>
>Reckon the thermal season's nearly over.

[decibel]

You have just destroyed your own conclusion. The light one will damp out
faster and eventually stop sooner.

The weight acts as thrust, not just mass.




In article >, "szd41a"
> wrote:
>Final proof
>Since you don't want to swallow the maths, I thought of this simple test
>that anyone could at home. No need to spend money on tows.
>
>Build two pendulum with equal lenght of string, attach two objects with
>similar shape (same drag), one heavy and the other one less heavy. Lunch
>both pendulum at the same time. Watch them reach the low point at the same
>time, thus at the same speed...Right.....at this point, say at zero
>altitude, both objects have zero potential energy, but the heavy one has
>much more kinetic energy, both travelling at the same speed.
>This is exactly the same system than our two gliders.
>Now just watch wich one will pull up higher??????
>Exactly the same height. Mass have no effect watsoever.
>Galileo demonstrated this hundreds of years ago!!!!!!!
>It is amazing to see people that are supposed to have a minimum of technical
>knowledge refute this. It has been explanined here, they are victim of their
>intuition that tell them that a ping pong ball will hurt yous less than a
>golf ball hitting you at the same speed.
>, which is entirely true
>
>Thank you folks, that was fun
>Réjean Girard happily flying a Jantar in Montréal.
>"szd41a" > a écrit dans le message de
...
>> 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
>>
>>
>
>

[Eric Greenwell]

In article >,
says...
> As the thermal season's nearly over I've got time to
> pick nits!
>
> I'm sorry but the heavier glider is subject to more
> drag at any given speed than the lighter one.
>
> (Profile drag will be pretty much the same 'cos the
> glider is the same shape - Induced drag will be higher
> 'cos the wings are having to work harder)

The heavier glider is flying at a higher L/D than the lighter one, so
it's drag is relatively less (absolute drag might be higher, but so is
it's energy). Because it is flying more efficiently, it will be able
to climb higher.

It's true it won't be able to slow down as much because it's stall
speed is higher, but the difference in energy between a stall speed of
40 knots unballasted and 42 ballasted (20% heavier glider) is very
small (typical values for the kind of gliders we are talking about),
and won't provide a significant height gain. If you doubt this, try it
in flight sometime, and see how much altitude you gain going from 42
knots to 40 knots.

The typical case of a pull up isn't down to stall speed anyway, but
more likely to pattern speed of around 50 knots or so.
--
!Replace DECIMAL.POINT in my e-mail address with just a . to reply
directly

Eric Greenwell
Richland, WA (USA)

[szd41a]

> You have just destroyed your own conclusion. The light one will damp out
> faster and eventually stop sooner.
>
> The weight acts as thrust, not just mass.

Weight acts like thrust when you and your load are travelling towards the
center of the earth. When you try to carry you load away from Mother Earth,
I am nt too sure it is helping you along!! Does that sound right?????Maybe I
put my foot in my mouth here. HELP ME SOMEBODY I AM GOING NUTS

.......but i am having fun. Maybe we are setting a record for the longest and
wittiest sring on rec.avistion ;-))))))

[Jere Knuuttila]

Kevin Neave wrote:

> One trivial point for Jere's post is that the stalling
> speed for the unballasted glider will be lower, so
> this guy can pull up to a slower speed & therefore
> regain more altitude.

That's right. I left that thing out just to make the calculation simpler.

Lots of discussion about different initial and final speeds and their
effects. Having two gliders with different masses, different initial
speeds and different final speeds makes things complicated. Adding drag
to that makes it even worse. We would need the polar curves for
different loads. Taking the dynamic behavior (>1g pull-up and possibly
0g flight path) into account would require the polar curve (or surface)
from 0g to Ng. It would also take some simulating as well I guess.

I'm not going that far. But leaving the drag out of the question we can
get some simple results. Starting with the good ol' conservation of energy:
1/2*m*v1^2 + m*g*h1 = 1/2*m*v2^2 + m*g*h2
This time, dividing by m cancels it out completely, as we've seen
multiple times. So:
1/2*v1^2 + g*h1 = 1/2*v2^2 + g*h2
For the two gliders that is:
(1) 1/2*v01^2 + g*h0 = 1/2*vf1^2 + g*hf1
(2) 1/2*v02^2 + g*h0 = 1/2*vf2^2 + g*hf2
v is speed, where subscript 0 means initial, f final and number is the
glider.
substracting (1)-(2):
1/2(v01^2 - v02^2) = 1/2(vf1^2 - vf2^2) + g(hf1 - hf2)
that is
hf1 - hf2 = 1/2g * (v01^2 - v02^2 - vf1^2 + vf2^2)
But it's kind of hard to see what's happening there. So say the initial
speed of glider 2 is a times the speed of glider 1:
v02 = a*v01
and similarly for final speeds:
vf2 = b*vf1
So we get:
hf1 - hf2 = 1/2g * ((1 - a^2)*v01^2 + (b^2 - 1)*vf1^2)
Then, say the speed change for glider 1 from initial to final is c:
vf1 = c*v01
So:
hf1 - hf2 = 1/2g * ((1 - a^2) + (b^2 - 1)*c^2)*v01^2
that is:
hf1 - hf2 = v01^2/2g * (c^2*b^2 - a^2 - c^2 + 1)
Reality check:
If c=1 (glider 1 has constant speed) increasing a (glider 2 faster in
the beginning) makes the altitude difference negative (glider 1 is
lower). Increasing b (glider 2 faster in the end) makes the difference
positive (glider 1 is higher).
Seems OK.
When c < 1 (so at least glider 1 pulls up to a slower speed) a^2 has a
bigger factor (1) than b^2 (<1).

What that means:
20% more speed initially has more effect than 20% in the end.
So if glider 1, starting with speed 150 km/h, pulling up to 70 km/h and
glider 2, starting with 180 km/h (150 km/h +20%), pulling up to 84 km/h
(70 km/h +20%) are competing, number 2 gets higher.

Some people have been comparing two gliders flying at Vne. In that case
the lighter wins since it's able to pull up to a slower speed. Plus, I'm
not sure about what different glider manufacturers say about flying at
Vne with full ballast. Might be OK, since the weight and the lift are
both in the wings...

Jere
jere at iki.fi

[Jere Knuuttila]

Eric Greenwell wrote:
> The heavier glider is flying at a higher L/D than the lighter one, so

I don't think that can be right.
The heavier glider will achieve the same max L/D ratio than the lighter
one, but at a higher speed.

Jere
jere at iki.fi

[Eric Greenwell]

In article >, says...
> Eric Greenwell wrote:
> > The heavier glider is flying at a higher L/D than the lighter one, so
>
> I don't think that can be right.
> The heavier glider will achieve the same max L/D ratio than the lighter
> one, but at a higher speed.

It's true they will have the same max L/D ratio; however, at any speed
above the ballasted glider's max L/D, the ballasted glider will have
an L/D better than the unballasted glider (same speed, lower sink
rate). That L/D will be lower than the max L/D, of course, since it is
flying faster the max L/D speed.

Take a look at a polar plot for a glider that shows the both the
unballasted and ballasted polars, compare them for the same speed, and
you will see what I mean.
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directly

Eric Greenwell
Richland, WA (USA)

[Jere Knuuttila]

Eric Greenwell wrote:
> It's true they will have the same max L/D ratio; however, at any speed
> above the ballasted glider's max L/D, the ballasted glider will have
> an L/D better than the unballasted glider (same speed, lower sink

Oh yes, of course. On the other hand, at a speed slower than max L/D
speed for the lighter, the sink rate will be smaller for the lighter
glider. And the gliders will have the same sink rate at a speed
somewhere between their max L/D speeds.

So, during a manouver where two gliders pull up from a speed greater
than max L/D speed for the heavier one to a speed slower than max L/D
speed for the lighter, the altitude difference is a function of the
speed profile during the manouver. It's quite hard to compare the
altitude gains without heavy calculations, but I wouldn't say the effect
would clearly benefit either glider. What it _does_ say, in my opinion,
is that the pilot should probably only pull up to the max L/D speed or
minimum sink speed (depending on the situation) for _his_ glider.

Jere

jere at iki.fi