Guidelines or formulas for how much water to dump in narrow or weakthermal racing?

Hi, all.

When racing a typical 1-2 hour flatland race in, say, a Diana2 or Ventus3-15 (15 meter ships with water ballast and flaps), are there any guidelines or formulas for how much water to dump, particularly when thermals are weak or narrow?

Here are the parameters I'm thinking would apply (but I'm no expert yet- thus the question):

1. Average thermal strength
2. Average thermal width
3. Average distance between good thermals
4. Height band for the day

I was hoping to find something on this subject in a glider racing book or the Diana2 flight manual, but no luck so far finding either googling.

Ben Ethridge

On Friday, May 15, 2020 at 7:18:47 PM UTC+2, Charles Ethridge wrote:
Hi, all.

When racing a typical 1-2 hour flatland race in, say, a Diana2 or Ventus3-15 (15 meter ships with water ballast and flaps), are there any guidelines or formulas for how much water to dump, particularly when thermals are weak or narrow?

Here are the parameters I'm thinking would apply (but I'm no expert yet- thus the question):

1. Average thermal strength
2. Average thermal width
3. Average distance between good thermals
4. Height band for the day

I was hoping to find something on this subject in a glider racing book or the Diana2 flight manual, but no luck so far finding either googling.

Ben Ethridge

The designer of the Diana 2, Krzysztof Kubrynski, covers this in his paper "HIGH PERFORMANCE SAILPLANE DESIGN STRATEGY USING INVERSE DESIGN AND OPTIMIZATION TECHNIQUES", and the results can be summarized with:
3kts, tank it
2kts, empty it

Mostly ASW ships....but if the water load is keeping you from making climbs....dump water until you climb.
Consideration is....are you in a soft area with better later lift?
Yes.....do enough dump now to climb now but save wing loading for later.
No....maybe dump it all and do a save, slow speed on course is better points than a landout.

Well Charles....very little if any information is published on the science of ballastology. The pros are even hesitant to lecture on the subject as it falls into the realm of Boson Particle Theory. I actually thought you were asking about etiquette while dumping water in a thermal, reading your title.
I feel the general opinion is to learn and tune your skills by being out on course. Intuition plays into it followed by Monday quarterbacking after the flight if you’re not the winner. Your best bet for information is to research who the big dogs always seem to be and call them up, get their feedback.
Try here and the thread will drift over to hook releases or lithium batteries.
Cheers,
R

Try here and the thread will drift over to hook releases or lithium batteries.
Cheers,
R

Lithium batteries? What's wrong with Lithium batteries?

....and so it begins :-)

Charles, rumors I have heard is there was a Canadian who wrote a computer program years ago to address the factors you mention. He may have been from the BC area.

For the V2 I have found that I think in terms of dry (about 7.4 lbs/ft2), 8, 9, 10, and full (11). Rough approximation in flat lands without much streeting is less than 2 knots, 2, 3, 4 and 5 or more match the above wingloadings.

If you have ridges or streets you can carry more water in weak conditions because you are cruising more of the time.

Most pilots I know start heavy and dump some if they can’t climb. I often load to the expected climb and conditions so I know my gliders weight and cg. CG Is the hard part to control when dumping. I think many racing pilots fly too heavy in weaker conditions.

There are good answers in this thread already, To add a new concept, the amount of water depends also on the glider. The newer gliders, particularly 18 m hold water well in weaker conditions. The older gliders, not as well..

The 2-3 kt rule above is a really good starting point, but it really depends on the nature of the day.

The most important consideration is "can you climb" if you can't you are too heavy and that makes you circling speed higher and your radius wider so you climb more slowly because of the combination of the two factors (speed = drag, radius = weaker lift).

This is where the nature of the day comes into play. Are the thermals narrow? Are the thermals bubbly or turbulent? Are they smooth and wide? These conditions also affect the amount of water that should be carried.

I remember many years ago when racing in an ASW-20, I was heavy with water and bombing along between thermals thinking I was smoking. My buddy in his 1-35 (dry) was keeping up. While I was loosing him on the run he gained it all back in the climb. I learned a good lesson that day. Heavy and fast is not always right. The time spent climbing is just as important as the time spent running. Both have to be minimized.

On Friday, May 15, 2020 at 9:26:29 PM UTC-4, Charles Ethridge wrote:
...and so it begins :-)

The important bit, of course, is to learn how to make the glider go, without water.

T8

The biggest challenge (for me at least) is deciding on the amount of ballast to carry when launching early, when the lift hasn't yet developed to the strength I hope to see later in the day when it reaches its maximum power. Many times I have been forced to "curb my enthusiasm" by dumping part or all of my water to remain aloft, only to curse that decision when the lift peaks midafternoon and I am flying lighter than I want.

Another challenge is trying to figure out what CG range works best for the type of conditions I will encounter. Many manufacturers and pilots suggest about 80% aft in the recommended CG range, but that seems to work best with a climb/cruise ratio of 50%. If I am in strong, streeting lift conditions, I sometimes have about a 20% climb vs. 80% cruise ratio, meaning the drag associated with elevator deflection is much higher during cruise. Drag goes up exponentially as speed increases, and since I spend more time running than circling, the drag penalty is much higher. I hope to be able to quantify these data this year to be able to determine an "optimum" CG point for weak, vs. strong conditions.

And then I will just forget what I learned and do the best I can to make some distance and avoid as much yardwork as possible.