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

On Saturday, May 16, 2020 at 10:59:37 AM UTC-7, Charles Ethridge wrote:
Thinking about Tim Taylor's comment: Yes, it feels like this is something that software would be good at solving. (I'm a 35 year software engineer, BTW...just not an aeronautical engineer :-) )

A few years ago I wrote a script that computes the climb rate of a glider based on the factors you mention plus air density (thermals widen significantly at altitude) and the glider's performance polar. The calculations for climb rate given thermal strength & width are actually much simpler than you might expect, the vast majority is just trigonometry.
Modeling thermals is a little more tricky and the lift distribution varies drastically depending on which model you believe.
The model I have is by no means perfect for several reasons, a big one being that glider polars are measured in straight flight and as such, they are not necessarily representative of performance during circling flight (this effect is amplified for gliders with significant di/polyhedral and/or winglets). At some point it might be worthwhile to actually measure circling performance to avoid misappropriating polar data for this. Plus, this model does not take into account individual gliders' handling characteristics at different wing loadings, which is a non-negligible issue in real life.
To be clear, I don't suggest blindly following the numbers provided below.

If you aren't a flight dynamics expert but have some fundamental understanding of aerodynamics, I highly suggest reading the following source if you wish to learn more.
https://booksite.elsevier.com/978012...SAILPLANES.pdf

The results for a few gliders we
Discus 2a:
For lift stronger than 5kn, max gross.
For lift [3kn, 5kn], half ballast.
For lift weaker than 3kn, no ballast.

Standard Cirrus:
For lift stronger than 3kn, max gross.
For lift [2kn, 3kn], half ballast.
For lift weaker than 2kn, no ballast.

ASW-22BL (750kg):
For lift stronger than 4kn, max gross.
For lift [2kn, 4kn], half ballast.
For lift weaker than 2kn, no ballast.

This is only useful for average climbrate throughout your entire flight. If you're hauling for most of your flight with strong conditions and fail to switch gears with changing conditions, get low and have to take a weak climb to get back up, you might as well throw this information out the window.

The non-binary decision-making involved in resolving situations like this one or (avoiding it altogether) is what separates the great pilots from the decent ones, and the results of such decisions are reflected in big differences in average speed.

I am open to discussion regarding sharing my code (written in MATLAB) but in its current state it may not be easily comprehensible to an average user.

Thomas

On Saturday, May 16, 2020 at 2:53:29 PM UTC-4, Thomas Greenhill wrote:
If you aren't a flight dynamics expert but have some fundamental understanding of aerodynamics, I highly suggest reading the following source if you wish to learn more.
https://booksite.elsevier.com/978012...SAILPLANES.pdf

The results for a few gliders we
Discus 2a:
For lift stronger than 5kn, max gross.
For lift [3kn, 5kn], half ballast.
For lift weaker than 3kn, no ballast.

Thomas

Thanks, Thomas. I read the article, and being a former CFI-I/MEI, I get the concepts. Being a current software engineer (vs a math engineer), with a bit of help breaking down the formula(s) into non-MATLAB software syntax, I can put formulas into LARGE bodies of software at proper places, without fully understanding the formulas. I'm working in the Kotlin language for Android phones presently.

What you show for the Diana2 fits for me, since that's about how much water I've had to lose to keep up with the leader in a flatland 3-5kt narrow thermal day race in Condor (running against his Condor "ghost" over and over).

He told me he thinks there's a "sweet spot" for water ballast given all the parameters, but I don't think he knows the formula(s) either. Just does it intuitively after years and years of experience (which I don't have).

Bottom line: Yes, I'd love to see your code. Maybe we could make an Android and iPhone app and give it out to the Condor racing community for evaluation? Nobody's going to die using it in Condor, if we "put a decimal point in the wrong place or something" (software engineer in the Office Space movie :-) )

Ben

My view: The main issue is not so much climbing as centering. As others have noticed, around 2-3 knots if you're flying with a gaggle that's doing all the work of finding thermals, water vs. no water is about a wash. It feels bad to watch them glide past you, but you do gain it back again in the climbs.

The real issue comes when you're out there alone, low, and you have to center a punchy thermal. It can literally be impossible to climb. You flail around in the nasty bits and pieces watching the fleet go by overhead. Eventually you give in and dump the water and now magically you climb at 5 knots.

Flying with water you're going faster, so thermal circles are larger, and you're less maneuverable.

So really, yes, the main consideration is how punchy the thermals will be and how often you're going to have to find and center one on your own.

Many of my if Idas are memories of great days blown by getting low and not dropping the water fast enough

John Cochrane BB