Airspeed indicator (ASI)


Had a long chat with Derek in the UK the other evening and we both found that we have something in common. No, not that we both fly Tanarg 912S’s with Bionix wings, but that we both have inaccurate ASI’s.

Now I tend to not bother much with mine and use the GPS speed more, but Derek believes that his was more accurate before he fitted the Bionix wing and the taller windscreen. That would make me think that the only thing that has changed is the air, and maybe pressure, behind the dash.

All ASI’s measure two things, pitot and static pressure. I know this from the 25 years that I served in the Royal Air Force (thank you queen). The pitot is measured with the little pipe that sticks forward somewhere on the front of the trike pod, but the static is measured at the back of the instrument, by a stub where a static pipe should fit.

The best place to measure the static would be next to the pitot probe. Derek is making another pipe to fit next to the pitot probe that will have a hole in the side rather than the end to measure static pressure (just like big aircraft have). I will do the same when I have found a suitable bit of pipe. When we both have them fitted we will do some highly technical experiments to see if the ASI is more accurate. These experiments will probably consist of flying backwards and forwards to see if the ASI agrees with the GPS.

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3 Responses to Airspeed indicator (ASI)

  1. Mike Gillam says:

    Here’s what I do:
    Calculate your TAS at a “given height” and RPM setting (I use hand throttle for this) – air temperature will affect (Density Altitude and therefore) TAS a lot in hot Summer weather. Then set your GPS to give “average speed” and fly in a wide 360-degree circle at your “given height” – I set a radius of about 2 miles. Then examine the difference between your GPS’s average-speed-over-the-ground and your TAS.
    It’s best to pick a calm day so as to avoid errors due to wind gusts.

    • rouffignac says:

      Very similar to the way I do it Mike. We are no so much trying to test the accuracy of the ASI, but to improve the accuracy. The fact that the static porrt is behind the dashboard and probably in a low pressure area with the bigger winscreen cannot be much good for the accuracy.

  2. rouffignac says:

    Have just spoken to Derek this evening. He has fitted the twin pitot/static pipe setup and has had a short flight. Conditions were a bit rough, but he is of the opinion that the ASI is now much more accurate. More to follow.

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