Airspeed accuracy .....w

[Michel_MTR]

Hi,

I installed my D10A between the airspeed indicator (classic certified) and the altimeter (electronic) witch are already installed.
I checked that altitude and airspeed indicated by the EFIS and the others intruments, are strictly the same values.

No problem with the instrument accuracy ...

But, I know (thanks to the GPS !) that I have an airspeed error (+ 6% at 140 kts) and I know that the problem is the static intake position. I know too that the right place is not easy to fix.

Can we imagine a calibration process witch alow to not drill the fuselage side for each attempt in searching the right place ?

Cheers,
Michel.

 

[dynonsupport]

Note that if your problem is with your static, you should also be seeing issues with altitude as well.

It is not possible to use the EFIS to "calibrate out" incorrect airspeed or altitude readings that arise from bad pitot/static data. You need to have good pitot and static locations for any airspeed/altimeter, including our EFIS, to measure properly.

If you are building from a kit its manufacturer may have thoughts on where a good location for a static port is.

 

[khorton]

But, I know (thanks to the GPS !) that I have an airspeed error (+ 6% at 140 kts) and I know that the problem is the static intake position. I know too that  the right place is not easy to fix.

Can we imagine a calibration process witch alow to not drill the fuselage side for each attempt in searching the right place ?

Cheers,
Michel.

How did you use GPS data to measure your airspeed error?  There are accurate ways to do this, and there are inaccurate ways to do this.  Some builders use methods that are not mathematically correct.  Let's make sure you are using a good method before you go too far.

I have some information on my web site that might be relevant.  See:

Doug Gray's mathematically correct method to determine TAS using GPS data:
http://www.kilohotel.com/rv8/rvlinks/doug_gray/TAS_FNL4.pdf
http://www.kilohotel.com/rv8/rvlinks/doug_gray/TASCALC.XLS

More info on converting GPS data to TAS:
http://www.kilohotel.com/rv8/phplinks/index.php?&PID=49

Information on converting TAS to CAS:
http://www.kilohotel.com/rv8/rvlinks/airspeed.zip

Specific information on how to determine static source errors:
http://www.kilohotel.com/rv8/rvlinks/ssec.html

More information on determining static system errors:
http://www.kilohotel.com/rv8/phplinks/index.php?&PID=48

Kevin Horton
Engineering Test Pilot

 

[Michel_MTR]

Hi,

I agree with you Kevin, and it is precisely the Doug Gray's method that I used !!!!

If a bad position of statics make an 0.5 hpa error, that produce a 14 ft altitude error and 10 mph airspeed error ..
One is not perceptible, the other is !

Cheers,
Michel.

 

[khorton]

Bon soir Michel,

Sorry for the slow response, but it seems that the e-mail notification of responses isn't working.  I will try to pay closer attention to this thread.

How many other aircraft of this type are flying?  It looks like one of the MCR family.  Have other builders done credible static system accuracy testing?  If other builders have a similar problem, then perhaps the static source is the cause.  But if other builders don't have a problem, then maybe there is something unique to your aircraft.

We should rule out other possible errors before focusing on trying to modify the static system.

Be sure there are no leaks in the pitot or static system.  Do another leak test to confirm.

Are your results repeatable?  When you converted from TAS to CAS,  did you use pressure altitude (i.e. altimeter set to 1013.25 mb)?  Did you use a spreadsheet to do the calculation of CAS?  If so, which one?

In order to calculate CAS from TAS, you need an accurate OAT.  Do you have an on board OAT?  If so, where is it located, and have you been able to compare its reported OAT with any other sources?

Where is your pitot tube located?

Your static source is located on the rear fuselage.  Did you make any changes from the plans?  I.e. different location, different type of static port, different contour ahead or behind the static port?

Is your airspeed reading too high, or too low?  How does the error vary with airspeed?

If we rule everything else out, then perhaps we need to look at the static port.  But it is very difficult to predict what other location might be better.  It may be more practical to modify the pressure at the current location with a small dam ahead of, or behind, the current static port.  A dam behind the port will increase the pressure sensed at the port, and lower the IAS.  A dam ahead of the port will have the opposite effect.

Sorry to ask 20 questions, but I want to be 100% sure you really have a static source problem before you start doing static port modifications.