Comparison of glider classes at Uvalde...

The evidence is really undeniable. What has been driving the 18m and open class, is the perception that the bigger spanned classes are far superior than what they in fact are.

The 15m other sailplane builders still have no answer to the Diana-2. Maybe the Duckhawk is an answer - we'll see. Diana-2 has sold poorly IMHO, because of perception.

Maybe we should replace wingspan with wing aspect ratio, when looking at the layout. The Diana-2 and now the Duckhawk, for me, seem to indicate that this is the more important metric. In these two designs, the low weight is used advantageously in an aerodynamic sense to produce a larger aspect ration wing with a smaller wing areas, than would otherwise be practical.

Wing profiles today, show little differences from one to the other.

In the analysis, it should be noted, that the EB29 can be flown in a 25.3M configuration, reaching about 58kg/m wing loading.

On paper the larger ships should have been faster, but they're not. So we are not taking everything into consideration.

Speculating here, does it take longer to accelerate a long spanned ship, due to the higher profile drag, through sink or upon thermal exit? If so, they would be flying slower than the smaller ships through sink.

One point that I think has been missed is that I believe the open class was
tasked with longer tasks than the other classes during the WGC. That would
make their final glide a smaller percentage of their entire flight - which
would make their overall speed a bit slower that it would be if they flew
the same length tasks that the 15m and 18m ships flew.

Also, I image the difference in speeds would be greater at a soaring site
with weaker lift than found in Uvalde. The 21m and 23m ships and their high
wing loadings seem tailor made to Uvalde's very strong soaring conditions.

Paul Remde

In the recent edition of AeroKurier, there is an article describing the increasing flight path length associated with a stronger variation of the speed in an ASW-27.

From memory, in the example they gave, increasing the speed from 160KM/h to 180KM/H just through the modeled sink area, increases the flight path by up to 4%. But it is still faster over the horizontal distance, as 180 is the optimal speed for that example.

I assume the same is true in lift (increased flight path due to pull up).

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.

I have a hunch the higher profile drag requires a stronger push over, over that of a shorter spanned plane, increasing the flight path due to this or causing a slower speed gain.

It would be interesting to compare the effects of 1) longer flight paths,or 2) less speed changes, or 3) slower speed changes - of heavy, long winged ships to a baseline ASW-27.

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.