Wing Loading

Is there a practical limit to wing loading?

I'm looking for aero dynamic information, not statements about max gross
weight. Can you load your glider up to the point that, even with strong and
reliable lift, you're at a disadvantage to lighter ships?

I was just thinking of the old days when I read that some contest pilots
tried, or considered, using salt water for ballast because it's heavier.

On Apr 25, 7:38*pm, "Dan Marotta" wrote:
Is there a practical limit to wing loading?

I'm looking for aero dynamic information, not statements about max gross
weight. *Can you load your glider up to the point that, even with strong and
reliable lift, you're at a disadvantage to lighter ships?

I was just thinking of the old days when I read that some contest pilots
tried, or considered, using salt water for ballast because it's heavier.

For every condition there is an optimum wing loading. More is very
often not better. There are weight optimums just like speed optimums
for given thermal strength, thermal width, cloud streeting, and ridge
or wave conditions. Too often I have seen pilots put on too much water
because they falsely believe the old statements that more is better.

You can create models for the correct amount of water if you can
account for all the above factors. Simple models can look at just
thermal strength but the larger circling diameter can make a big
difference on achieved climb rates.

The more you can fly straight the more water is useful. If you are
flying classic thermals without streets then often less is better.

If you need maneuverability to work near ridges less is better also.

I have found using enough ballast to minimize how often I bang my head on the canopy works quite well.

Mike

Thanks, Tim,

I know all these things. This is sort of a mind exercise so let me try a
different approach... I was curious if there's a point analagous to the
drag bucket where the L/D for a given speed would take a sudden dip given
higher wing loading. I'm visualizing the polar curve taking a sudden trip
downward past a certain weight.

Yeah, I know... When the wings break off!

I'm really having trouble putting my thoughts into words...


"Tim Taylor" wrote in message
...
On Apr 25, 7:38 pm, "Dan Marotta" wrote:
Is there a practical limit to wing loading?

I'm looking for aero dynamic information, not statements about max gross
weight. Can you load your glider up to the point that, even with strong
and
reliable lift, you're at a disadvantage to lighter ships?

I was just thinking of the old days when I read that some contest pilots
tried, or considered, using salt water for ballast because it's heavier.

For every condition there is an optimum wing loading. More is very
often not better. There are weight optimums just like speed optimums
for given thermal strength, thermal width, cloud streeting, and ridge
or wave conditions. Too often I have seen pilots put on too much water
because they falsely believe the old statements that more is better.

You can create models for the correct amount of water if you can
account for all the above factors. Simple models can look at just
thermal strength but the larger circling diameter can make a big
difference on achieved climb rates.

The more you can fly straight the more water is useful. If you are
flying classic thermals without streets then often less is better.

If you need maneuverability to work near ridges less is better also.

On Apr 26, 9:39*am, "Dan Marotta" wrote:
Thanks, Tim,

I know all these things. *This is sort of a mind exercise so let me try a
different approach... *I was curious if there's a point analagous to the
drag bucket where the L/D for a given speed would take a sudden dip given
higher wing loading. *I'm visualizing the polar curve taking a sudden trip
downward past a certain weight.

Yeah, I know... *When the wings break off!

I'm really having trouble putting my thoughts into words...

"Tim Taylor" wrote in message

...
On Apr 25, 7:38 pm, "Dan Marotta" wrote:

Is there a practical limit to wing loading?

I'm looking for aero dynamic information, not statements about max gross
weight. Can you load your glider up to the point that, even with strong
and
reliable lift, you're at a disadvantage to lighter ships?

I was just thinking of the old days when I read that some contest pilots
tried, or considered, using salt water for ballast because it's heavier..

For every condition there is an optimum wing loading. More is very
often not better. There are weight optimums just like speed optimums
for given thermal strength, thermal width, cloud streeting, and ridge
or wave conditions. Too often I have seen pilots put on too much water
because they falsely believe the old statements that more is better.

You can create models for the correct amount of water if you can
account for all the above factors. Simple models can look at just
thermal strength but the larger circling diameter can make a big
difference on achieved climb rates.

The more you can fly straight the more water is useful. If you are
flying classic thermals without streets then often less is better.

If you need maneuverability to work near ridges less is better also.

My view: For given conditions, performance is a smooth function of
wing loading. No sharp curves, drag buckets, or discontinuities. If
you add 1 lb/ft^2 you glide a few knots faster, but give up a few
hundred feet in the next thermal.

The biggest question is how much you can fly straight. There is a
discontinuity in the advantage of ballast when you can fly straight
without thermaling.

There is a maximum desirable wing loading. If the wings let you fly at
50 lbs / ft^2 you wound not want to fill up. Designers make modern
gliders so that they carry enough water to win strong days at world
contests in strong conditions. Most of the time that is far more water
than you need.

Most pilots fly with too much water. They are hoping conditions get
better ahead. But they pay a price until it does, and it often
doesn't.

I've been in several contest flights with me empty, a gaggle full and
3 knot thermals. The water made no difference at all.

Filling up with water and flying too slowly removes much of the
advantage of water. If the gaggle is flying at 65 knots because nobody
wants to be first, then trying to climb at 3 knots with full water,
you will outfly them empty.

Where water really hurts is if you get low and have to core tight
thermals. Even if they are strong, you can waste a huge amount of time
here, then finally dump and magically core the lift. One more reason
that water is not as beneficial for isolated thermals as it is with
streets.

John Cochrane

On Apr 26, 8:58*am, John Cochrane
wrote:
On Apr 26, 9:39*am, "Dan Marotta" wrote:





Thanks, Tim,

I know all these things. *This is sort of a mind exercise so let me try a
different approach... *I was curious if there's a point analagous to the
drag bucket where the L/D for a given speed would take a sudden dip given
higher wing loading. *I'm visualizing the polar curve taking a sudden trip
downward past a certain weight.

Yeah, I know... *When the wings break off!

I'm really having trouble putting my thoughts into words...

"Tim Taylor" wrote in message

....
On Apr 25, 7:38 pm, "Dan Marotta" wrote:

Is there a practical limit to wing loading?

I'm looking for aero dynamic information, not statements about max gross
weight. Can you load your glider up to the point that, even with strong
and
reliable lift, you're at a disadvantage to lighter ships?

I was just thinking of the old days when I read that some contest pilots
tried, or considered, using salt water for ballast because it's heavier.

For every condition there is an optimum wing loading. More is very
often not better. There are weight optimums just like speed optimums
for given thermal strength, thermal width, cloud streeting, and ridge
or wave conditions. Too often I have seen pilots put on too much water
because they falsely believe the old statements that more is better.

You can create models for the correct amount of water if you can
account for all the above factors. Simple models can look at just
thermal strength but the larger circling diameter can make a big
difference on achieved climb rates.

The more you can fly straight the more water is useful. If you are
flying classic thermals without streets then often less is better.

If you need maneuverability to work near ridges less is better also.

My view: For given conditions, performance is a smooth function of
wing loading. No sharp curves, drag buckets, or discontinuities. If
you add 1 lb/ft^2 you glide a few knots faster, but give up a few
hundred feet in the next thermal.

The biggest question is how much you can fly straight. There is a
discontinuity in the advantage of ballast when you can fly straight
without thermaling.

There is a maximum desirable wing loading. If the wings let you fly at
50 lbs / ft^2 you wound not want to fill up. *Designers make modern
gliders so that they carry enough water to win strong days at world
contests in strong conditions. Most of the time that is far more water
than you need.

Most pilots fly with too much water. They are hoping conditions get
better ahead. But they pay a price until it does, and it often
doesn't.

I've been in several contest flights with me empty, a gaggle full and
3 knot thermals. The water made no difference at all.

Filling up with water and flying too slowly removes much of the
advantage of water. If the gaggle is flying at 65 knots because nobody
wants to be first, then trying to climb at 3 knots with full water,
you will outfly them empty.

Where water really hurts is if you get low and have to core tight
thermals. Even if they are strong, you can waste a huge amount of time
here, then finally dump and magically core the lift. One more reason
that water is not as beneficial for isolated thermals as it is with
streets.

John Cochrane

As XX once said.......

"" its hard to find a garden hose at 3,000 agl and put more water
in!!!""

We do have dump valves! Its better to have and dump, than wish you
had more.

Now when to dump and what to dump to, will bring alot of hoop laaaa's
around the fireside chat that goes on and on and on.

# 711.

I am sure there is a point where the polar falls off a cliff, however I
doubt you would ever get enough water in!!

Flying older generation gliders like a standard cirrus with winglets retro
fitted, they climb exceptionally well and carry water very well also. The
day has to be pretty crap (2 knot climbs) for me not to carry water!

In newer generation gliders which already have a higher wing loading, I
think it is much harder to call!

JC
Standard Cirrus 566 (UK)

On 4/26/2012 8:39 AM, Dan Marotta wrote:
Thanks, Tim,

I know all these things. This is sort of a mind exercise so let me try a
different approach... I was curious if there's a point analagous to the drag
bucket where the L/D for a given speed would take a sudden dip given higher
wing loading. I'm visualizing the polar curve taking a sudden trip downward
past a certain weight.

Yeah, I know... When the wings break off!

I'm really having trouble putting my thoughts into words...


"Tim Taylor" wrote in message
...
On Apr 25, 7:38 pm, "Dan Marotta" wrote:
Is there a practical limit to wing loading?

I'm looking for aero dynamic information, not statements about max gross
weight. Can you load your glider up to the point that, even with strong and
reliable lift, you're at a disadvantage to lighter ships?

I was just thinking of the old days when I read that some contest pilots
tried, or considered, using salt water for ballast because it's heavier.

For every condition there is an optimum wing loading. More is very
often not better. There are weight optimums just like speed optimums
for given thermal strength, thermal width, cloud streeting, and ridge
or wave conditions. Too often I have seen pilots put on too much water
because they falsely believe the old statements that more is better.

You can create models for the correct amount of water if you can
account for all the above factors. Simple models can look at just
thermal strength but the larger circling diameter can make a big
difference on achieved climb rates.

The more you can fly straight the more water is useful. If you are
flying classic thermals without streets then often less is better.

If you need maneuverability to work near ridges less is better also.






If I'm accurately understanding what you're pondering, I think you're asking
if there is (are) any physical reason(s) to expect that increasing wing
loading for a given glider will ultimately 'uncover' any presently 'generally
unconsidered' gotchas that will result in 'something like a laminar airfoil's
drag bucket effect' on the glider's polar.

My short answer: "Yes."

Two aerodynamic possibilities: 1) Reynolds number effects (which, arguably,
make laminar flow airfoils possible in the first place...and would eventually
- because of increasing glide speed necessary to support the increasing weight
- result in breakdown of presently-existing laminar flow runs), and 2) mach
effects.

The former might appear as a glider-based (as distinct from a
wing-profile-based) drag increase, while the latter would take us back to
polar-detectable Chuck Yeager days. The time for swept wing sailplanes may be
at hand!

Bob W.

P.S. The operational Me-163B (arguably a non-laminar glider optimised for
assisted climb/speed over thermalling had operational wing loadings varying
from ~20-psf empty to 45-psf full-up; its operational max speeds ranged from
515 mph at sea level to ~600 mph between 10,000 and 40,000 feet. No
thermalling pireps in any of my sources...

At 16:13 26 April 2012, Justin Craig wrote:
I am sure there is a point where the polar falls off a cliff, however I
doubt you would ever get enough water in!!


Must be best to carry as much as possible... even if its only
to dump on any following pilots.
Releasing 10 gallons of (preferably ice cold.) water in the middle of a
5m/s thermal is the easiest way to seed some rain and make it just a 1m/s
thermal !

On Apr 26, 9:13*am, Justin Craig wrote:
I am sure there is a point where the polar falls off a cliff, however I
doubt you would ever get enough water in!!

When I ran the numbers for what later became the Concrete Glider
episode on Mythbusters, that point was where you get transonic flow
and start hemorrhaging energy into shock waves. I worked the numbers
back through L=1/2*rho*v^2*Cl*A and figured that the upper limit was
on the order of 60000 lbs. I thought it was really cool that you could
cast a 15m sailplane in concrete and rebar, ballast it with tungsten
to get the CG right, and it would smash right along at about 40:1 at a
few hundred knots. Of course, launching it would be a real bear, but
that would be somebody else's problem.

But, no, they wanted a concrete glider that could do a roll-off launch
from a hilltop with Jamie or Adam at the controls. That was a non-
starter, because a Part 103 aircraft was not in the cards. They might
have done so if they'd allowed themselves the kind of fiberglass
reinforcing mesh that Rob Wheen used in the University of Sydney's
concrete hang glider, but the Beyond Productions researcher I was
working with said they wanted it all or at least mostly concrete, with
perhaps some pieces of rebar.

Of course, they ended up scaling the episode way back, and made hand-
launch model gliders instead. I think even that could have been a cool
demonstration of how the rho*v^2 works; they could have taken a
styrofoam toy glider, copied it in cement, and then launched it from
an airplane or fast moving car to show that it would achieve the same
glide ratio as its polystyrene cousin, just at a greater speed.

Thanks, Bob K.