Airspeed and altimeter errors

[Cozylover]

Hi Canardiens,

 

Have a COZY(3) I recently bought. Really enjoy flying it!

 

The Airspeed indicator seems to be quite optimistic. I crosschecked with GPS Groundspeed on minimum 3 legs and then entered the data in one of the online programs ( http://www.reacomp.com/true_airspeed/index.html ) to convert IAS into TAS.

 

After comparing the IAS converted into TAS (mathematically done, taking into account Temp, density alt...), with the figure I found with the TAS calculator online, it looks like my ASI is optimistic by more than 10kts. Only checked so far for the average cruise speed, not for higher and lower speeds.

 

Also, a while ago I was formation flying with another Canard (Gyroflug SC-01). This guy just had his Altimeter calibrated. Flying at exactly the same altitude, I was reading +200ft on my altimeter than he was on his. I suspect mine over reading the altitude.

 

Bringing the airspeed and the altimeter problems together, I would suspect a "Static pressure" problem, since ASI and altimeter are in error it can hardly be a Pitot problem.

 

The Pitot is in the nose position as per plans as are the static ports.

 

Did anybody of you guys had a similar problem in Cozy or Long-EZ?

What should I check first....?

Any suggestions.....?

 

 

All good advice welcome,

 

 

Regards,

 

Björn MOERMAN

 

flybjorn@emirates.net.ae

 

[Aaron]

Not a flyer yet, but I would suggest checking the static port plumbing with a manometer to make sure there aren't any leaks. A leak into the cabin would mean IAS reads fast and altitude reads high.

 

 

If you want to calibrate your pitot:

 

http://www.crixbinfield.freeserve.co.uk/Misc/ASI%20Check.doc

 

This is from cozybuilders.org mailing list:

http://www.cozybuilders.org/mail_list/topics99/instruments.txt

 

Date: Sun, 17 Oct 1999 19:47:55 -0400

From: bil kleb <kleb@mciworld.com>

Subject: Re: COZY: Airspeed calibration problems

 

Reid Siebert wrote:

>

> Prior to flying your new bird for the first time the pitot and static

> systems should be certified by a calibrated test box.

 

the way i read the regs (91.413, 43 Apnds E&F) that these boxes are made

for, they don't test the pitot system: just the static. basically they make

sure that your altimeter doesn't leak, is calibrated, and that your encoder

/transponder pair is squawking the right code given the some simulated altitude.

 

if you'd like to check your pitot, make a water manometer (a u-shaped tube

partially full of water with a ruler strapped alongside) and have at it;

 

dh = 0.5*rhoa*(v*n2m*5280/3600)^2/g/rhow*12

 

where (for typical conditions),

 

dh = difference in water column height (inches)

rhoa = density of air (0.002378 slugs/ft^3)

v = speed (mph or kts according to n2m, see next item)

n2m = conversion factor (1.152 for speed in knots or 1.0 for speed in kts)

g = 32.19 ft/s^2 (gravitational acceleration)

rhow = density of water (1.937 slugs/ft^3)

 

or the inverse relation:

 

v = 3600/5280/n2m*sqrt(2*rhow*g*h/12/rhoa)

 

so you can generate a table like:

 

knots inches

40. 1.04

45. 1.32

50. 1.63

55. 1.97

60. 2.35

65. 2.76

70. 3.20

75. 3.67

80. 4.17

85. 4.71

90. 5.28

95. 5.89

100. 6.52

105. 7.19

110. 7.89

115. 8.62

120. 9.39

125. 10.19

130. 11.02

135. 11.88

140. 12.78

145. 13.71

150. 14.67

155. 15.67

160. 16.69

165. 17.75

170. 18.85

175. 19.97

180. 21.13

185. 22.32

190. 23.54

195. 24.80

200. 26.08

 

--

bil <http://www.geocities.com/~kleb/>

[John Slade]

Hi Björn,

I'm not flying my Cozy IV yet, but I've read of a few flyers who had static problems. For the ASI and Alt to read high, you'd need a lower than true static. I don't recall anyone who had this specific problem, but it might be something to do with airflow over the vent.

 

A common solution to static problems has been to T static vents from both sides. Of course it's also possible that both instruments need calibration. If it were me I'd start with calibration.

Regards,

John Slade

[dust]

When i was in marietta ga, the velocity builder was grinding away small amounts of material on the outside of the plane, adjusting his static port?? Didn't go into details with him, too much to cover in too short of time, but i did work, pushed the plane for him to calibrate his compase, he he he, no wrk, no see

[Cozylover]

Hi Canardiens,

 

Thanks for all your replies. Some pretty good ideas among them.

My Cozy is about 3000NM away from the place where we're living...Live and work in Dubai (U.A.E.) and the aircraft is still based in my home country, Belgium, Europe.

So, unfortunately I can't just go and check things out.

 

End of April I'm going to fly and work on the Cozy for about 3 weeks. I need to do the annual inspection beginning of May and this might be a good moment to check things out.

 

I'll keep you advised on the progress.

Any further hints or tips are still appreciated.

 

CU all soon,

 

BJORN MOERMAN