Sailplane wing with water ballast
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water ballast how much water ballast to use

How much water ballast should you use on any given day? This is not an all-or-nothing type of tool. Some pilots will takeoff full every day 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 wing ballast with no tail ballast and an extreme forward CG. A forward CG can lead to difficulty climbing in thermals.

So we can take some tips from Leo and Ricky Brigliadori's book Competing in Gliders:

Start by answering five questions. I have included the following tables that will help guide you to a recommended wing loading.

  1. What is the average lift strength for the day?
  2. What is the thermal size, consistency, and workability (to get an idea, you could look at the buoyancy sheer ratio)?
  3. What is the workable height band?
  4. What type of Soaring flight do you expect? Thermal, wave, slope, convergence, streeting
  5. Are there 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   Thermals Wave Ridge (=cloud streets)
  Value 60 100 100
         
Wind Knots <20 >20  
  Value 0 100  

What is the wing-loading range on your glider? For example,e 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 following table is the theoretical maximum value for each parameter used to calculate the result.

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 have everything to determine what wing loading we should be flying at. The example day is: 4knt thermals to 6000ft with evenly spaced thermals, no streeting, a B/S ratio of 10+, and a long final leg into a 20knt 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 wing loading. 87.5% x 18 = 15.75 suggested wing loading 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 wing loading. 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 rarely got the wing loading to be heavy enough.

The newer gliders carry their water better than the older ones. They have a wider range than 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.

Banner photo by Mika Ganszauge

garret willat  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.