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  #1  
Old July 14th 15, 03:16 AM posted to rec.aviation.soaring
6PK
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Default water ballast

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
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  #2  
Old July 14th 15, 03:43 AM posted to rec.aviation.soaring
Steve Leonard[_2_]
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Default water ballast

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
  #3  
Old July 14th 15, 04:19 AM posted to rec.aviation.soaring
Jim Lewis[_2_]
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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?
  #4  
Old July 14th 15, 04:31 AM posted to rec.aviation.soaring
Jim Lewis[_2_]
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Default water ballast

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.
  #5  
Old July 14th 15, 05:15 AM posted to rec.aviation.soaring
Steve Leonard[_2_]
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Default water ballast

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

  #6  
Old July 14th 15, 06:37 AM posted to rec.aviation.soaring
Bob Kuykendall
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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?
  #7  
Old July 14th 15, 03:49 PM posted to rec.aviation.soaring
Chris Rollings[_2_]
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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


  #8  
Old July 14th 15, 04:14 PM posted to rec.aviation.soaring
Jim Lewis[_2_]
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Posts: 40
Default water ballast

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.
  #9  
Old July 14th 15, 04:40 PM posted to rec.aviation.soaring
Tango Eight
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Default water ballast

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
  #10  
Old July 14th 15, 04:56 PM posted to rec.aviation.soaring
Steve Leonard[_2_]
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Posts: 1,021
Default water ballast

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
 




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