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Free Lift

Overview

When you fly a tracker with a balloon, you want it to go up

The balloon has to be able to lift:

  • Itself, plus
  • Everything attached to it ("load weight")

This page goes over the basics of how to do this.

Problem -- Everyone measures the load weight wrong

You need to be more detail-oriented than you think.

Solution -- Weigh the load weight carefully

What's a good Free Lift?

Answer

Around 5-7 grams.

Achieving Free Lift

The process is this

  1. Decide a free lift target (eg 6 grams)
  2. Weigh everything you attach to the balloon (your "load weight")
  3. Find something heavier than your load weight by 30+ grams (your "reference weight")
  4. Calculate (reference weight - (load weight + free lift)) = "target scale weight"
  5. Attach your reference weight to the balloon
  6. Get out a scale w/ at least 0.1 gram resolution (don't tare it)
  7. Start inflating the balloon while the reference weight is on the scale
  8. Watch for your target scale weight to appear on the scale

If you're at the target scale weight, you have your desired free lift.

If the scale weight shows lower than target scale weight, you have excess free lift (can burst at altitude if too much).

If the scale weight shows higher than target scale weight, you are under free lift (can fail to fly if too little).

It's not critical to be on-the-dot. Be within +/- 0.5 gram of your target ideally

Weighing Examples

Weigh everything

Sometimes easier to weigh individual "chunks" instead of all together (depending).

Weigh the Tracker

Here is a fully-assembled tracker

Note that the tracker doesn't (yet) have antennas or tape.

Weigh the Antenna

Here is the weight of (say) 20m band antenna of your given wire choice

The 20m band uses 199.2" of wire per "leg" of the dipole antenna (two legs).

The wire I'm using is 36 AWG 4 oz wire.

Using the Wind-O-Matic 5000 I measured out and wrapped together all the wire needed, then weighed the bundle.

For my wire, it's 1.2g total for both dipole legs together.

Weigh the Extras

Example -- do you use a ton of tape?

Do you strain-relief your antennas on the tracker? (you should).

What do you use to attach the antenna to the balloon?

And so on.

Weigh them all independently and make a line item list (in a spreadsheet).

Don't Weigh The Balloon

You don't need to

Calculate

Use a spreadsheet

This is actually the spreadsheet I use. Super simple.

Weight Section -- The line items, summed up.

Helium Section:

  • Calculated target weight
  • Input to check the "actual" free lift I got

Tips

Keep records of your flights

Don't re-use calculator files. Just create new ones.

For example, I use google drive, and a folder for each flight.

I just copy my prior flight files, adjust as necessary. Nice to have a historical record especially as you're iterating on design.