Optimize Water Ballast in 5 Questions
The topic is how much water ballast are we going to use? This is not an all or nothing type of tool. Some pilots will takeoff full everyday and dump down in timed increments (20 seconds for example). However if you have a stock water system the tail ballast will dump at a faster ratio, meaning it will drain the tail before the wings are empty, moving the CG forward as you dump. So you might still be at 3/4 ballast with no tail ballast and a very forward CG. Forward CG is not where you want to be if you are having difficulty climbing.
So we can take some tips from Leo and Ricky Brigliadori's book Competing in gliders:
You have to look at answering the next 5 questions. The following tables will help guide you to a recommended wing loading.
- The average lift strength for the day?
- What is the thermal size, consistency, and work-ability (to get an idea you could look at the buoyancy sheer ratio)?
- What is the workable height band?
- What type of Soaring flight do you expect? Thermal, wave, slope, convergence, streeting
- Significant headwinds?
Look at the table to determine the associated values:
| Parameters | Units | Possible Value for your choices | ||
| Average thermal strength | Knots | 2 | 4 | 6 |
| Value | 10 | 85 | 100 | |
| Thermal size and irregularity (B/S is a good reference) | Diameter | Narrow | Normal | Large |
| Value | 20 | 85 | 100 | |
| Available working band | Feet AGL | <1600 | 1600-3300 | >3300 |
| Value | 20 | 80 | 100 | |
| Type of Soaring | Themals | Wave | Ridge (=cloud streets) | |
| Value | 60 | 100 | 100 | |
| Wind | Knots | <20 | >20 | |
| Value | 0 | 100 | ||
What is the wingloading range on your glider? For example we will use a glider with a range of 34 kg/m² to 52 kg/m². That means we can change it by 18 incremental points.
The next table is the theoretical max. Value for each of the parameters.
| Average thermal strength | 30% |
| Thermal size and irregularity (B/S is a good reference) | 10% |
| Available working band | 30% |
| Type of Soaring | 20% |
| Wind | 10% |
| total | 100% |
We now have everything to determine what wingloading we should be flying at. The example day is: 4 knt thermals to 6000 ft with evenly spaced thermals, no streeting, a B/S ratio of 10+, and a long final leg into a 20 knt headwind.
| Parameters | Choice | Value | Result |
| Average thermal strength | 4 | 85 | 25.5 |
| Thermal size and irregularity (B/S is a good reference) | large | 100 | 10 |
| Available working band | >3300 | 100 | 30 |
| Type of Soaring | thermals | 60 | 12 |
| Wind | >20 | 100 | 10 |
| total | 87.5% |
Based on the above tables I would ballast to get 87.5% of my wingloading. 87.5% x 18 = 15.75 suggested wingloading increments. The plan would be to take off at 49.75 kg/m². Now I can have the CG at the optimal location for that wingloading. The tables above are a great way to start figuring out your glider and understanding how it handles. You might want to change your values or parameters depending on your experience. In my Nimbus 3 I could rarely get the wingloading heavy enough.
The newer gliders carry their water better then the older ones. They have a wider range then previous generations. This was noticeable between the Discus and the Discus 2. I would routinely carry the water longer in the Discus 2ax (part of that might have been I had more experience and didn't get stuck nearly as often).
The key to remember is that water ballast is not an all or nothing tool we can use. This should be a good guide to help make your water ballast more efficient.
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Garret Willat holds a flight instructor rating with over 8000 hours in sailplanes. His parents have owned Sky Sailing Inc. since 1979. He started instructing the day after his 18th birthday. Since then, Garret has represented the US Junior team in 2003 and 2005. He graduated from Embry-Riddle with a bachelor's degree in Professional Aeronautics. Garret represented the US Open Class team in 2008 and 2010 and the Club Class team in 2014. Garret has won 3 US National Championships.
