W

wadem

Member
Hi guys, have an issue with my airspeed on my 4 that's puzzling me. For the record I have a Dynon 10A and also a standby analog airspeed gauge. I was doing some work behind the panel in December and noticed that my main static line was unplugged from the instruments. I assumed that I had dislodged it somehow and just plugged it back in not thinking twice about it. The next time out flying (month later) I quickly noticed that my plane was around 13 mph slower (indicated). I am a true airspeed fanatic and so I'm always doing GPS triangulation runs to confirm my TRUE airspeed. My 4 has always been a solid 170 Mph @ 2400 2450 rpm. Sensenich fixed pitch and an E2D 150hp Lycoming.
So that said I quickly did another GPS 3 legged run and confirmed that my bird was still doing its standard 170MPH true airspeed.
When you do the math on a wiz wheel or computer using what my new indicated is my true speed is down the equivelant, or approx 13 mph, but again that's not the case as the GPS math says its where it should be (170). Also when you know what your real true airspeed is at, you can do the math backwards and determine what your INDICATED should be for a given altitude,temp, press alt.
Again this confirmed that I should be indicating approx 13 mph more then I am.
Also, as stated above, both the analog and dynon are within 2mph of each other, so it's not the gauge.
So I went through the entire pitot static system checked for leaks, blocks kinks etc......everything a ok,blew lines out etc. After reading for hours on the Internet about static and pitot issues, as well as alternate static sources, I decided to create my own alt source and simply unplugged the line that I found disconnected originally. That solved the problem! Everything back to normal.
All the math checks ok, indicated,true,etc...all good.
So why would my indicated speed be higher when using the cockpit static as opposed to the regular fuselage Vans setup? As I remember from my commercial days, cabin pressure is somewhat lower than outside while in flight.
The way I see it, with everything working the way it was designed, ie,plugged into the design static source, my indicated is incorrect, but when using the cabin source it's bang on..........what problem or flaw would cause this in the designed intended system? I have owned the plane less then a year and now believe the previous owner had the static line disconnected for the above reason
Sorry to ramble on, wanted to give all details.

Cheers

Wade
 
wade do this first, at the runway put the altimeter in "0" make a fly by at full speed over the runway the lowest posibly and see the alt indication, if you see -cero indication then your static system is pressurized and you have tu put half washer in front of the static sensors and try again untill the alt goes in
 
OK, not quite sure I understand this. If the alt is set to a given value, zero in your suggestion, when you do a low and over at let's say 10 feet, if the static line is pressurized would you see a higher number then zero or lower? Or am I way off base here?

Wade
 
if your static sensors get pressure the alt will be lower, so if you pass at 10 feet could show 9 or below, in mi case i have 100 ft lower indication at a full speed pass.

the cabn difference pressure will be higer always because the air getting in by the air ventilation system, so your alt source will show lower alt
 
I was always told that the cabin pressure is slightly lower not higher?
if the pressure was higher in the cabin and I'm using the cabin static as the source, would the airspeed not Under read? seems to make sense to me that with less static pressure, the resistance to the pitot side of the airspeed aneroid would be less therefore resulting in higher airspeed, or in my case the correct airspeed.

Wade
 
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wadem said:
I was always told that the cabin pressure is slightly lower not higher?
if the pressure was higher in the cabin and I'm using the cabin static as the source, would the airspeed not over read? seems to make sense to me that with less static pressure, the resistance to the pitot side of the airspeed aneroid would be less therefore resulting in higher airspeed, or in my case the correct airspeed.

Wade
Click to expand...

cabin pressure can be higher or lower. You can change it by opening fresh air vents or turning on the heater.
Regardless of what it is, you need to use an accurate external source.
Take a look at your static ports and tell us what they look like (better yet, post a photo)
 
I'm pretty sure the static source is built as you say, flush to the skin. Will try to get out to the field to check today.

Wade
 
Wade,

Did you using the standard Van's method with a blind rivet or did you use a third party port?

Like Scott and several others mentioned, it would appear the static port is too flush to the skin. This was problematic in many of the early designs of third party static ports.

Without a photo, it's hard to recommend a solution. Some folks have glued a blind rivet head on to a flush static port. It sounds like a kludge, but I've seen it work.

While not a RV-4, the theory is the same. Tim Olson has a good write up on his gtrial and tribulations. http://www.myrv10.com/tips/flight_testing/staticports/index.html

bob
 
Just did a hurried read through this, & your answers seem to exactly reflect my experience with the 1st -4 that I owned. With flush static ports in Van's 'stock' location, airspeed indicated ~10 kts slow, & at full throttle, altitude indicated ~50 feet low. The high speed pass recommended is the easy way to detect the problem (and it's how I noticed mine). Kevin Horton turned me on to the solution, which was to simply cut the heads off a couple of pop rivets & glue them on the a/c skins over the static port holes.

The 'set altitude to zero' trick is just to make it easy to observe the relatively negative reading you'll get at full speed. You can do the test just as easily if you set field alt while stationary on the ground, then do the high speed low altitude pass & note your altitude indication during the pass. If you're anywhere near the ground, your altitude will indicate below field altitude.

Charlie
Thanks, Kevin; I still owe you a meal & it's been 15 years!
 
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