Erratic Airspped Indicator

[cgruby]

After getting the airplane rebuilt and finally back in the air, I've been trying to improve my land skills. Trying to nail the airspeed at 55 on the approach is next to impossible. The airspeed is all over the place. I usually just wing and discover I'm much too fast.

Should I expect an airspeed indicator in this airplane to be a little more behaved? Or am I'm just going to have to learn how to wing it.

One thought I had in mind was that the static pressure is ported to the cabin, and there is no static port. There's not one on a Cub either, but I've never seen an airspeed indicator swing so wildly as this one does.

Thanks,

[Russell320]

Your pitot tube may be partially blocked.At high speeds you may not notice it, but at low speeds plugged or partially plugged tubes will be more noticeable. Depending on the type, remove the pitot tube and clean it out. Bugs and other stuff love pitot tubes. MAKE SURE THE PITOT TUBE IS OFF THE AIRPLANE. Static pressure from the back of the panel should be is just fine.

[jiott]

I think you should be able to get a more stable airspeed reading than what you describe. Mine doesn't vary more than about + or - 2mph during the approach. There are a lot of opinions on this forum as to whether taking static pressure from inside the cab is good or not. Kitfox recommends a static port location low and just forward of the rear stab on the fusalage (on both sides for best accuracy). Forward slips on final can also mess with airspeed indication accuracy. Outside air vents blowing ram air into the cabin and I believe the stab trim slots on the tail area blow ram air into the cabin can increase cabin air pressure compared to outside. I believe every aircraft may react somewhat differently to all these effects.

[cgruby]

Your pitot tube may be partially blocked...

Thanks a-heap Russ. I guess I should have thought of that, but what tha hey, this 80 year old brain just doesn't tick over as good as it used to.

Thanks again,

[cgruby]

I think you should be able to get a more stable airspeed reading than what you describe...

Thanks Jim, I've got everything else pretty well done, it's time to clean out the old pitot tube.

Cheers,