Variometer System Check
Ask G; "My variometer doesn't seem to work correctly. How do I check the system? Maybe the newest pressure transducer variometers with wind gust algorithms have better results?"
Go back to basics for a moment: remember that a pilot should be able to discriminate between going up because he's in lift, and going up because he's pulled the stick back. That's the point of the Total energy system, and if it doesn't work properly then you can't soar properly.
If you have a modern, pressure transducer computer variometer it can measure the speed through the pitot system, measure the height through the static system, and use a super-fast processor to compensate for speed changes, giving a reasonably accurate indication of rising air. Well, it should be easy but...it isn't. The math and physics are simple, but it is really hard to build good pressure transducers and then filter the output so the signal isn't drowned out by noise (turbulence). And - the glider has to have an accurate pitot / static system. This is not as easy as you might think: some types are certified with pretty big errors. To get around these problems:
- Fit a modern variometer - which should have good filtering and fast processing.
- If the airspeed system is poor, fit a second pitot/static probe to the top of the fin and use that to drive the variometer.
If there are no leaks anywhere, and if the pitot/static signal is accurate - then it will all work.
System Leaks
However, most gliders still use a Total Energy (TE) probe to drive the variometer system, because most gliders still use a mechanical variometer as a backup in the panel, which would be no use at all without TE. The TE probe is a clever solution in that only one port - one tube with holes or slots in it - can supply a summation of speed-driven pressure and height-driven pressure to the vario system. I'm not going to get bogged down here in explaining the mechanics of how the air moves around in the system and why, because that requires more space - and besides, I've written about that extensively in "The Soaring Engine" volume four, so if you want to know, get hold of the book. But one thing is very important: there must be no leaks between the total energy probe on the tail and the variometer system in the cockpit. Therefore it is vital to check the system for any leaks at the start of, and occasionally during the season.
Testing
If you're not familiar with the hardware then you could easily damage the variometer in the process of leak-checking, so I'm not going to give you instructions for doing that. You'll have to get the information from your local maintenance shop, or from someone who can show you have to do it without breaking things. This goes double if you're running a Sage vario - they are incredibly easy to damage. But what you can do is to check the system in the air.
You'll need to take a high tow early in the morning when the air is still. Pretend you're Dick Johnson doing flight tests. Accelerate smoothly from 60kt to 100kt: the variometer should go down, down, down, and eventually bottom out at the commensurate sink rate for 100kt. Then raise the nose, and watch the variometer creep slowly upward until it returns to show a knot or two of sink at about 60kt. If the needle moves sharply down when you lower the nose, or sharply up when you pull the nose up - your TE system has a leak between the probe and the vario. It should never read above zero.
If the vario reads "up" when you pull up, there is a leak between the probe and the vario. If the vario reads "up" all the time then you have a leak between the mechanical vario and the flask. Take the glider to the shop and get it fixed before you fly it again because your glider is unserviceable without a good TE system, period.
TE System Alternatives
There is a third option and brand new for the up-to-date pilot who has to have the latest and best gear: the new "HAWK" algorithm from LXNAV. This is a combination of clever filtering, accelerometer data, GPS information, and fast processing that makes it possible for the computer to deduce the airmass velocity at lightning speed. That means it can see the wind (the horizontal component) and the lift (the vertical component) and display both. It isn't exaggerating to call this a paradigm shift. This should remove a lot of the errors that come with conventional TE systems (I haven't even started on those - see volume four) and give you a variometer that truly does tell you if you're going up or down. As a bonus, it will give you an accurate and instantaneous wind. What you do with the information is your own problem...
I hope this is helpful. If you have difficulties with this article - or with your TE system - then get hold of volume four and see if that helps you understand what's going on. Reichmann's "Soaring Cross country:" and Welch and Irving's "New Soaring Pilot" also contains the information you need. If you still have questions then you could "Ask G".
Fly safe,
G
Banner photo by Holger Weitzel, aufwind-luftbilder.de
G Dale is the popular author of The Soaring Engine book series. He follows the endless summer, working for the British Gliding Association, The Gliding Association of New Zealand, and the Gliding Federation of Australia, always teaching cross country flying. He’s also flown and worked at various gliding clubs around the world: at Nympsfield as Chief flying instructor, at Booker again as CFI, at Lasham as DCFI and soaring coach, and at Glide Omarama as head coach, with visits to Minden, Serres, Takikkawa, Narromine, Lake Keepit, and many other clubs as a peripatetic soaring instructor and mountain flying coach.
