Question of the day

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.
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Eric Greenwell
Richland, WA (USA)

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)

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

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

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

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)

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)

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.

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

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.
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directly

Eric Greenwell
Richland, WA (USA)