Calibrating Airspeed Indicator

[AlexPeterson]

So, right on time after 1000 hours I decided to formally calibrate my airspeed indicator! Didn't see any reason to rush into it. It is a Van's version, knots only, no TAS calculator built in. Pitot is the heated version with the tip in the same location as Van's plans recommends for the simple tube. While there are various methods for doing this, I chose to fly closed laps, N-W-S-E on the heading, hold altitude, nail indicated airspeed, record ground speed. Of course I also recorded altitude (which was 3500'), OAT and baro. I flew laps at 110 through 170 indicated at each 10 knots. I'm sure the test settings (i.e., altitude) are not optimum for this work, but probably good enough.

With the four ground speeds, I then used Paul Beale's calculator

http://spot.colorado.edu/~beale/TrueAirspeed/TrueAirspeedCalc.html

to calculate TAS for each run. Next I used

http://www.paragonair.com/public/aircraft/calc_TAS.html

in a trial and error fashion to get CAS for each run. (I would enter different CAS until the TAS matched)

The results were well behaved, although something was apparently wrong with the data I took at 170 kias, so I discarded that point. The result is presented here graphically:


The best fit straight line fits quite nicely to the data, and the slope is right around .95. So, this means when I indicate 100 knots, CAS is 95. Indicate 150, CAS is 143. I printed a little table and stuck it next to the airspeed indicator.

Seem reasonable?

 

[rzbill]

Alex,
I am curious what static port shape you have installed.

1) Hollow rivet per our mizerly benefactor?
2) Some version of aftermarket flush?
3) Pitot tube with built in static? Piper unit?

 

[AlexPeterson]

Bill, I have the "per plans" RV6 static ports - a couple pop rivets. If I disconnect those and go with just cabin static, the altimeter jumps maybe 100 feet.

I"m drawing a blank on the pitot tube exact model - I got it from Gretz about 10 years ago, no static.

 

[rzbill]

Thats what I thought

Alex,
Thanks for the confirmation.

If you want to fiddle with the static ports to see if you can make the correction slope closer to 1.0, you might consider reducing the protrusion of the rivet head on the outside surface.

From the posts I have read, the flush ports generally give a static value that is too high, resulting in low IAS. Since yours is slightly low, you might be able to make a mechanical change to get it spot on. Maybe try some tape on the leading and trailing edges of the ports to see if you can fair it into the fuselage and make it act like a shorter port.

 

[AlexPeterson]

Bill, thanks for the thoughts. However, my static system seems to be spot on, since at 140 knots at 10' above the runway, the altimeter reads correctly. I'd prefer that to be correct than the airspeed. My airspeed error seems to be instrument and/or pitot related. Maybe when I hit 2000 hours I'll look into that problem!