Airspeed Calibration Update

istrumit · #1 06-26-2016, 01:40 PM

I am following up on my previous comment that I always seem to have more of a tailwind than forecast, or less of headwind, by about 7 kts.

So, today I did the 3-leg test. I flew 90-360-270, with the autopilot on and dialed in. No power changes...I just let the the AP hold altitude in still air while I rotated the heading bug to the different headings in between stabilized flight.

Never did my TAS read over 165 kts...its was, in fact, steady at 165 kts. Full fuel. Myself and another adult. 5500 feet and 95 def F (on the ground - forget to check the OAT at altitude).

I have a G900X that logs everything for me, so I brought the chip home and typed the numbers into the spreadsheet that everyone (thank you) pointed me to.

The spreadsheet says 174 kts ! 9 kts faster that my TAS reads. Which is about what I thought (I was guessing 7 kts).

Thats good news ! But, now I have to figure out why its not reading correctly.

Static system is the first check.

rv7charlie · #2 06-26-2016, 04:36 PM

Got bumps over the static ports?

Edit; more info:
If able & willing, at your home or other airport, set your altimeter for field elevation while on the ground and fly a high speed pass down the runway at fairly low AGL (10-20 feet, by perception; not by altimeter). Check altimeter reading during the pass. Does it read below field altitude?

istrumit · #3 06-26-2016, 07:32 PM

Quote:

Originally Posted by rv7charlie

Got bumps over the static ports?

Edit; more info:
If able & willing, at your home or other airport, set your altimeter for field elevation while on the ground and fly a high speed pass down the runway at fairly low AGL (10-20 feet, by perception; not by altimeter). Check altimeter reading during the pass. Does it read below field altitude?

No bumps. Just a tiny little hole on each side near on of the aft rivet lines. Another on the bottom of the Bottom of the fuselage.

Carl Froehlich · #4 06-26-2016, 07:39 PM

Quote:

Originally Posted by istrumit

No bumps. Just a tiny little hole on each side near on of the aft rivet lines. Another on the bottom of the Bottom of the fuselage.

Why do you have a static port on the bottom of the fuselage?
Carl

rv7charlie · #5 06-26-2016, 08:30 PM

Try plugging the bottom port, and gluing a reasonable facsimile of Van's recommended static port rivet head over each of the ports on the sides. (Also verify that the side ports are located in the recommended position on the fuselage.)

But before you do, fly the low pass. I'll bet you will find the altitude reading...interesting.

istrumit · #6 06-27-2016, 07:34 AM

Quote:

Originally Posted by rv7charlie

Try plugging the bottom port, and gluing a reasonable facsimile of Van's recommended static port rivet head over each of the ports on the sides. (Also verify that the side ports are located in the recommended position on the fuselage.)

But before you do, fly the low pass. I'll bet you will find the altitude reading...interesting.

I'm not sure I have a static port on the bottom. I was wiping the bottom of the fuselage down yesterday and I noticed a hole that was exactly along the same line as the side ports, but this one was bottom center. All three ports are located just in front of the rivet line prescribed by Vans.

I have noticed also, now that a few folks are suggesting a low speed pass, that, when I review my flights on Google Earth, I frequently show up on Google Earth screen as being below the runway height.

That could be just a Google Earth glitch, but it fits the other evidence.

BTW - Wouldn't a static leak cause a 'high' reading instead of the the 'low' reading that I am experiencing ?

I will update when I have a chance to work on it a bit more.

GalinHdz · #7 06-27-2016, 08:21 AM

Does your TAS difference gets larger the faster you go? If so, place a piece of gorrila tape just in front of the static holes, without covering them, and do another GPS speed test. Your displayed TAS and your calculated TAS should get closer together. Keep adding layers of gorilla tape until you get a difference of less than 2kts between the two. That tells you how big a "shim" you need to instal to compensate for static port position error.

As for the Google earth error, are you using GPS altitude or transponder reported altitude? They are not the same and can point you in a wrong direction.

istrumit · #8 06-27-2016, 08:30 AM

Quote:

Originally Posted by GalinHdz

Does your TAS difference gets larger the faster you go? If so, place a piece of gorrila tape just in front of the static holes, without covering them, and do another GPS speed test. Your displayed TAS and your calculated TAS should get closer together. Keep adding layers of gorilla tape until you get a difference of less than 2kts between the two. That tells you how big a "shim" you need to instal to compensate for static port position error.

As for the Google earth error, are you using GPS altitude or transponder reported altitude? They are not the same and can point you in a wrong direction.

OK...thank you. Yes, the error seems to be more as I go faster...based on only stall speed v expected (2 kts) and this recent box test (9 kts).

istrumit · #9 06-27-2016, 08:31 AM

Quote:

Originally Posted by GalinHdz

Does your TAS difference gets larger the faster you go? If so, place a piece of gorrila tape just in front of the static holes, without covering them, and do another GPS speed test. Your displayed TAS and your calculated TAS should get closer together. Keep adding layers of gorilla tape until you get a difference of less than 2kts between the two. That tells you how big a "shim" you need to instal to compensate for static port position error.

As for the Google earth error, are you using GPS altitude or transponder reported altitude? They are not the same and can point you in a wrong direction.

OK...thank you. Yes, the error seems to be more as I go faster...based on only stall speed v expected (2 kts) and this recent box test (9 kts).

rv7charlie · #10 06-27-2016, 08:41 AM

If your low google map track altitude is based on static system altitude, that would match up with your low indicated airspeed at cruise in pointing to static system error.

Kevin Horton (Canuck; not LA) told me about this when I asked a similar question about 20 years ago. I had flush static ports.

I cut the heads off a couple of large pop rivets & glued them over the flush static ports. Got a free 10 kt increase in cruise speed. :-)

If the hole in the bottom is *not* connected to the static system, it's probably a 'weep hole' to help drain any accumulated moisture/water in the tailcone. Not a bad idea to have one forward of each bulkhead, if it's a taildragger. Don't know which side is preferable for nose draggers.

Charlie