water ballast

[6PK]

Ok, I understand the theory behind the use or need of ballasting, specially into a headwind.
A recent discussion debated the need or rather the "no need" for any ballast for a down wind run.
Would like opinions; pros or con please?
Best
6PK

[Steve Leonard[_2_]]

Ballast and downwind. Is the goal to simply glide as far as you possibly can after the last climb? If so, then no ballast is better than ballast. Unless, of course, you get a boost in L/D max due to higher reynolds number for the ballasted flight.

Generally speaking, ballast lets you have the same glide ratio at a higher speed and thus at a higher sink rate. You won't stay in the air as long, but you would cover the same distance in no wind, with or without ballast (see above possible exception). Since the airspeed for the same L/D is higher with ballast, headwind will have less impact on the distance you can cover. For the tailwind case, you won't be in the air quite as long, so the wind won't "push" you quite as far.

If the thermals are big and strong enough to support carrying water, you are always better off carrying water, whether going upwind or downwind. Until that last glide at the end of the day, where going downwind, you can get a little extra time aloft and wind drift by being dry.

Steve Leonard

[Jim Lewis[_2_]]

On Monday, July 13, 2015 at 7:43:27 PM UTC-7, Steve Leonard wrote:
> Ballast and downwind. Is the goal to simply glide as far as you possibly can after the last climb? If so, then no ballast is better than ballast. Unless, of course, you get a boost in L/D max due to higher reynolds number for the ballasted flight.
>
> Generally speaking, ballast lets you have the same glide ratio at a higher speed and thus at a higher sink rate. You won't stay in the air as long, but you would cover the same distance in no wind, with or without ballast (see above possible exception). Since the airspeed for the same L/D is higher with ballast, headwind will have less impact on the distance you can cover. For the tailwind case, you won't be in the air quite as long, so the wind won't "push" you quite as far.
>
> If the thermals are big and strong enough to support carrying water, you are always better off carrying water, whether going upwind or downwind. Until that last glide at the end of the day, where going downwind, you can get a little extra time aloft and wind drift by being dry.
>
> Steve Leonard

Weight affects Reynolds Number? I didn't think weight was in the formula(s) for Reynolds Number. If Reynolds Number is the ratio of inertial forces to viscous forces where does aircraft weight come in?

[Jim Lewis[_2_]]

On Monday, July 13, 2015 at 8:19:25 PM UTC-7, Jim Lewis wrote:
> On Monday, July 13, 2015 at 7:43:27 PM UTC-7, Steve Leonard wrote:
> > Ballast and downwind. Is the goal to simply glide as far as you possibly can after the last climb? If so, then no ballast is better than ballast.. Unless, of course, you get a boost in L/D max due to higher reynolds number for the ballasted flight.
> >
> > Generally speaking, ballast lets you have the same glide ratio at a higher speed and thus at a higher sink rate. You won't stay in the air as long, but you would cover the same distance in no wind, with or without ballast (see above possible exception). Since the airspeed for the same L/D is higher with ballast, headwind will have less impact on the distance you can cover. For the tailwind case, you won't be in the air quite as long, so the wind won't "push" you quite as far.
> >
> > If the thermals are big and strong enough to support carrying water, you are always better off carrying water, whether going upwind or downwind. Until that last glide at the end of the day, where going downwind, you can get a little extra time aloft and wind drift by being dry.
> >
> > Steve Leonard
>
> Weight affects Reynolds Number? I didn't think weight was in the formula(s) for Reynolds Number. If Reynolds Number is the ratio of inertial forces to viscous forces where does aircraft weight come in?

Jumping to my own possible answer, if the glider is flown at the higher airspeed made both possible and necessary for achieving the same L/D as the glider has when un-ballasted this would raise the Reynolds Number some because Reynolds Number is directly proportional to airspeed.
The higher Reynolds Number would indicate more turbulent airflow. I don't have any idea if the greater turbulence would produce a bump in L/D though. I look forward to the comments from those more knowledgeable than I.

[Steve Leonard[_2_]]

Yes, the higher speed for the same L/D at a higher (ballasted) weight creates higher reynolds number flow and changes the amount of laminar flow on the wing (it increases). Look at the flight tests on the PIK-20B and Nimbus 2 by Dick Johnson. Both showed an apparent increase in L/D max when flown with ballast.

Steve Leonard

[Bob Kuykendall]

On Monday, July 13, 2015 at 8:19:25 PM UTC-7, Jim Lewis wrote:
> If Reynolds Number is the ratio of inertial forces to viscous forces where does aircraft weight come in?

What was the inertia part again?

[Chris Rollings[_2_]]

Some of the confusion is caused by the fact that (at least some years ago,
maybe not now, I don't know) the FAA written (on line) exam for a glider
pilot certificate required one to give the factually wrong answer to a
question about inter-thermal speeds into and down-wind, in order to get the
mark for the question. You had to answer that it was correct to fly faster
between thermals when going into wind than when going down wind. This is
nonsense, but drawing the mistake to the attention of an FAA Inspector only
got his agreement and regret that he could do nothing about it.

The only time head or tail-wind affects inter-thermal speed or the need for
ballast is when the glide is to a fixed point on the ground, either a turn
point to be rounded before the next thermal is taken or a final glide to
landing.

At 02:16 14 July 2015, 6PK wrote:
>Ok, I understand the theory behind the use or need of ballasting,
specially
>into a headwind.
>A recent discussion debated the need or rather the "no need" for any
>ballast for a down wind run.
>Would like opinions; pros or con please?
>Best
>6PK
>

[Jim Lewis[_2_]]

Thank you Steve. I'll take a look at Dick Johnson's test report. I love this stuff. I don't understand it but I love it.

[Tango Eight]

On Tuesday, July 14, 2015 at 1:37:39 AM UTC-4, Bob Kuykendall wrote:
> On Monday, July 13, 2015 at 8:19:25 PM UTC-7, Jim Lewis wrote:
> > If Reynolds Number is the ratio of inertial forces to viscous forces where does aircraft weight come in?
>
> What was the inertia part again?

Inertia of the *fluid*.

Re hasn't anything to do with the mass of the glider.

Evan Ludeman / T8

[Steve Leonard[_2_]]

On Tuesday, July 14, 2015 at 10:40:51 AM UTC-5, Tango Eight wrote:
> On Tuesday, July 14, 2015 at 1:37:39 AM UTC-4, Bob Kuykendall wrote:
> > On Monday, July 13, 2015 at 8:19:25 PM UTC-7, Jim Lewis wrote:
> > > If Reynolds Number is the ratio of inertial forces to viscous forces where does aircraft weight come in?
> >
> > What was the inertia part again?
>
> Inertia of the *fluid*.
>
> Re hasn't anything to do with the mass of the glider.
>
> Evan Ludeman / T8

Yep. Rho Vee Elle over Mu. Fluid Density times velocity times chord divided by viscosity. But as the glider gets heavier, it flies faster at the same lift coefficient, so Vee gets bigger.

And, Jim. I am not seeing the "Performance Enhancement Through Airfoil Shape Correction" series in the Johnson Flight Tests section of the SSA Website. Might have to search an issue at a time from about 1977 through 1981. Dick got his Ventus in 1981, so the PIK went away then.

Steve

[Jim Lewis[_2_]]

Thank you Steve. I'll dig into the archive.

[Alexander Georgas[_2_]]

>
> The only time head or tail-wind affects inter-thermal speed or the need for
> ballast is when the glide is to a fixed point on the ground, either a turn
> point to be rounded before the next thermal is taken or a final glide to
> landing.
>

....or when thermals are linked to ground features, such as when flying
above mountains which create energy lines

[Jim White[_3_]]

At 14:49 14 July 2015, Chris Rollings wrote:
>Some of the confusion is caused by the fact that (at least some years ago
>maybe not now, I don't know) the FAA written (on line) exam for a glide
>pilot certificate required one to give the factually wrong answer to
>question about inter-thermal speeds into and down-wind, in order to get
th
>mark for the question. You had to answer that it was correct to fly
faste
>between thermals when going into wind than when going down wind. This i
>nonsense, but drawing the mistake to the attention of an FAA Inspector
onl
>got his agreement and regret that he could do nothing about it.
>
>The only time head or tail-wind affects inter-thermal speed or the need
fo
>ballast is when the glide is to a fixed point on the ground, either a tur
>point to be rounded before the next thermal is taken or a final glide t
>landing.
>
Chris, you should perhaps read Brigliadori's Competing in Gliders. They
make a pretty convincing argument for flying a bit faster into wind between
thermals. Its in the appendices.

Jim

[Brad[_2_]]

so, I gotta ask....wonder what the public is going to think if/when they find out a bunch of guys in their expensive racing sailplanes fill them up with water just so they can fly faster,and then dump it out over the desert.

Brad
GK

[Paul Agnew]

Just tell them it's chemtrails.

[Dan Marotta]

Is that the same public that gets it panties in a bunch at the mere
thought of an SUV?

On 7/14/2015 6:00 PM, Brad wrote:
> so, I gotta ask....wonder what the public is going to think if/when they find out a bunch of guys in their expensive racing sailplanes fill them up with water just so they can fly faster,and then dump it out over the desert.
>
> Brad
> GK

--
Dan Marotta

[Martin Gregorie[_5_]]

On Tue, 14 Jul 2015 17:00:30 -0700, Brad wrote:

> so, I gotta ask....wonder what the public is going to think if/when they
> find out a bunch of guys in their expensive racing sailplanes fill them
> up with water just so they can fly faster,and then dump it out over the
> desert.
>
Much the same as their reaction when they find out that Formula 1 racing
cars have a wooden plank strapped to the underside .

actually its Jabroc, not just a simple lump of dead tree.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |