What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Flight Testing?

jsharkey

jsharkey

Well Known Member
Suitably inspired by the following post I ran some tests today and input the data into the NTPS spreadsheet.

http://www.vansairforce.com/community/showthread.php?t=50164

TAS as indicated by my temperature and pressure corrected Dynon D100 is consistently reading 5 or 6 kts higher than that calculated from the GPS496 ground speeds.

Anyone know what this means? It implies that the pressure difference between the pitot and static ports is higher than it should be. It's unlikely that the pitot is high so could the static read low if it has a leak?

Jim Sharkey
RV6 - Phase 1

PS - std dev was more like 1 kt instead of the 0.1 kt shown in the post above. Guess I need to fly more accurately :)

PPS - I entered magnetic heading and not GPS ground track - need to do more flying :)
 
Last edited:
Van's Kit

rvbuilder2002 said:
Yes. Or you could have a static pressure error. Can you provide some detail on what you have fro a static system. Port style. location, etc.
Click to expand...

Pop rivets with the mandrel punched out. I checked the pitot system for leaks before flight and it was pretty tight. I didn't check the static system.

Jim Sharkey
 
AC43-13 has leak limits for pitot and static systems, it would be worth testing to confirm both are OK.

How about the Dynon's calibration, a manometer can be used to confirm.

Also, does the dynon have an airspeed correction/calibration table? If so, check the configuration data.

Doug Gray
 
Static leaks are quite common, and the pressure in the fuselage is usually less than ambient pressure, as the air has to accelerate to go around the fuselage. Bernoulli tells us that this accelerated air has a pressure that is lower than ambient pressure, and any canopy skirt gaps allow the pressure in the fuselage to equalize with this low pressure that is flowing around the fuselage. Thus static leaks lead to ASIs that read too high.

To do a complete end-to-end test, put a piece of electrical tape over one static port. Find a way to apply controlled suction to the other static port. I use a piece of surgical tubing - push it hard against the fuselage side to make a seal over the static port, then suck on the other end. Suck hard enough to cause the altimeter to increase more than 1000 ft, then put your tongue over the end of the tube to hold the pressure. The altimeter will jump around a bit until all the pressures equalize, then it shouldn't move more than 100 ft in a minute, starting from 1000 ft above field elevation. Any more than that is an unacceptable leak.
 
Thanks!

Kevin Horton said:
Static leaks are quite common, and the pressure in the fuselage is usually less than ambient pressure, as the air has to accelerate to go around the fuselage. Bernoulli tells us that this accelerated air has a pressure that is lower than ambient pressure, and any canopy skirt gaps allow the pressure in the fuselage to equalize with this low pressure that is flowing around the fuselage. Thus static leaks lead to ASIs that read too high.

To do a complete end-to-end test, put a piece of electrical tape over one static port. Find a way to apply controlled suction to the other static port. I use a piece of surgical tubing - push it hard against the fuselage side to make a seal over the static port, then suck on the other end. Suck hard enough to cause the altimeter to increase more than 1000 ft, then put your tongue over the end of the tube to hold the pressure. The altimeter will jump around a bit until all the pressures equalize, then it shouldn't move more than 100 ft in a minute, starting from 1000 ft above field elevation. Any more than that is an unacceptable leak.
Click to expand...

I think I have a plan to do the static test and repeat the flight tests :)
 
I have had a couple times when my ASI read higher than expected - both times due to static leaks. I bet that when you run Kevin's test, you find a leak. the good news is, that's easy to fix!

Paul
 
Repeated Tests

I repeated the tests on a nice calm day and managed to get a 0.2 kt standard deviation on the results but with the TAS being consistently 6 or 7 kt higher than that calculated from the GPS ground speeds. There had to be a static system leak.

Move forward to this weekend - I replaced all of the fittings on the instruments with ones from SteinAir. A static test to +1000 ft by sucking on a tube puttied to one vent with the other one sealed showed no discernible drop over a minute. An earlier test with the old plastic water fittings from Home Depot could hardly hold +1000 ft while still sucking! (Gus at Van's originally suggested Home Depot but perhaps he didn't mean these :))

Flight tests today were less consistent with a 2 kt standard deviation - my RV is very pitch sensitive, at least that's my excuse - but the difference between TAS and the GPS calculated speed was only 2 kt - so within the accuracy of today's test they are the same.

Clean stall was still around 50 kt IAS with full flap stall a knot or two lower.

I now feel confident about taking it for Transponder certification and can highly recommend the pitot static fittings that Stein sells.

Jim Sharkey
RV-6 Almost out of Phase 1
 
Static port errors

I have had several people do TAS tests with their RVs when doing comparative propeller tests. On each of these tests the IAS was 8-11 mph ,7-9kt high. In each case they were using the rivet static port. Apparently the location of this port is where the pressure is below static pressure. This is easily corrected by putting layers of transparent tape behind the port to raise its pressure. One fellow eneded up with about 0.08" thickness, which he then substituted a small washer cut in half behind and surrounding the port. In your case you might want to start off with a thinny washer cut in half and glued behind the port and re-do your tests.
You might also want to do the two-way, 180 degree opposite ground tracks which will show you your error while you are in the air, and in the time it takes you to do one four-way run you can do two two-way runs and see if your results are consistent. Call 1-800-DONT GO beforehand and get the winds aloft, which are true, not magnetic, correct them for your local magnetic deviation, and use the resulting ground track for the runs. As long as you do not have at least a 20 mph direct crosswind your possible error at 200 mph with everything steady will be below 1 mph. Don't get caught up in all of that standard deviation and other statistical jazz! I've been doing this stuff for years and my results always come out within 1 mph. 'Course if doing all of these different headings and calling up spreadsheet programs and such makes you feel that what you are doing is more scientific, well, have at it. But I used to reduce data from Atlas launches at Vandenberg and determine atmospheric noise models and consistency between our X-band doppler interferometric radar and the X-band position tracking radar and such and use the data to find errors in our guidance radar and I can tell you that the simplest approach is usually the best! Do four-way runs and you can have four errors which are not necessarily zero-mean.
 
rvbuilder2002 said:
Actually the blind rivet static port has proved over and over to be quite accurate. It is the rivet on flush static port that is not.
Click to expand...

I concur with Scott - I have very accurate Static data from stock Van's pop-rivets in the standard location.
 
You must log in or register to reply here.
Back
Top