Air Speed Indicator

I had my first flight yesterday. It was very very exiting after 3 years of building and problems. I seem to have moved onto a new phase of dynamic rather than static problem solving.

During the first flight the aircraft wanted to roll to the left, the controls were very stiff on full right deflection of the flaperons and the engine temps were way too high. I contacted Zenith, Caleb Gerhardt suggested that I had the elevator cables too tight and that I bend the left training flaperon edge to correct the roll. I also had to seal around the radiator to correct the overheating. All suggestions worked even though the EGT are still a bit high.

I now realise that the air speed indicator must be reading slow, I suspected they were on take off it was indicating 40 knots and the aircraft wanted to really go. I had a cheap GPS set to knots which indicated 70 knots whilst the ASI was saying 55.

Can anyone help, what should I do to correct this. The Pitot is as per the plans and I can blow through the pipe OK. I have static vents each side of the cabin just behind the cowl.

Any advice would be appreciated.

regards
Gordon.

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Permalink Reply by Ralph Sanson on October 22, 2009 at 3:42am
I have one static vent placed ahead of the left door (slightly lower than the instrument panel) that reads within 2% of GPS speeds.

If I disconnect this line in flight IAS increases by 15%

My guess is your vents are getting pressure from the airflow around the fuselage, might have to find a more neutral location.

Ralph
Permalink Reply by Gordon Watkin on October 22, 2009 at 4:17pm
Thanks for your suggestions I will try moving the syatic ports
Permalink Reply by John Ellis on October 22, 2009 at 5:37pm
I am not sure if this idea is a good one so if others correct me -- thanks I for the lesson. :-)

As a test would it be a good idea to run the static line inside the epanage rather than poking another hole in the skin. If this corrects the problem then moving the vent is the issue? What I am not sure about is if the internal pressure would be negative or positive the typical outside static vent location. ??? John
Permalink Reply by Gordon Watkin on October 23, 2009 at 1:19am
Thanks I willtry it and see what happens, it cant be any less acuurate than it is know
Permalink Reply by Gordon Watkin on October 24, 2009 at 4:58am
I disonnected the air line to the static ports and flew the plane it seemed to fix the problem. I dont know exactly how much out it is but it seems that the ASI was accurate. Thanks for your help and advice.
Permalink Reply by Andrew Aston on October 28, 2009 at 4:22pm
Hello Gordon,
There was some talk about this topic on the Matronics list some time ago. An internal vent (inside cockpit) will still read low. From memory the position on a 701 that gave the same reading as an accurate GPS was two feet behind the door just under the baggage shelf. I'm not flying yet so this is second hand, please confirm with some flyers. Good luck.
Regards, Andrew.

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