IAS problem

cdr.rschultz · #1 06-11-2011, 07:55 AM

Was just wondering if anyone has ever had a IAS problem with a Piper Pitot Blade (static port is actually in the blade itself)? The blade is mounted at the pitot tube mount site on an RV8. In flight the IAS is exactly 20 KIAS slower than the GPS indications (no wind of course). The Aircraft is stalling at 32 KIAS when that number should be 52 KIAS. The GPS numbers are constant with ATC ground read outs. Anyone have a thought? Cheers, Bob

jjconstant · #2 06-11-2011, 08:17 AM

Many of us have learned this lesson the hard way!!!! I was using a different brand pitot/static tube but the issue has to do with the static source being located under the wing, in a (go figure) high pressure/pulsing pressure area. You may also find that if you have a modern auto pilot it will bob up and down in turns. After fighting similar issues for months I finally just installed the vans standard static in the standard position.

Jeremy Constant
Rv7a 110 hrs

Kevin Horton · #3 06-11-2011, 10:29 AM

Quote:

Originally Posted by cdr.rschultz

Was just wondering if anyone has ever had a IAS problem with a Piper Pitot Blade (static port is actually in the blade itself)? The blade is mounted at the pitot tube mount site on an RV8. In flight the IAS is exactly 20 KIAS slower than the GPS indications (no wind of course). The Aircraft is stalling at 32 KIAS when that number should be 52 KIAS. The GPS numbers are constant with ATC ground read outs. Anyone have a thought? Cheers, Bob

Are you sure you don't have any leaks in the pitot system? Also, there is always a chance that you have a bad ASI. You can have it checked by an instrument shop, or do it yourself using a homebuilt water manometer. I created a spreadsheet to convert between water manometer height and ASI reading.

There are many different part number Piper pitot-static blades, each with a different angle on the bottom face of the blade. The angle on the bottom face of the blade affects the sensed static pressure, so each different part number will give a different effect on static system accuracy. You might get lucky and a different part number blade would be acceptably accurate. Or, you can take the path of least resistance and simply use Van's simple, cheap pop rivet static ports in his recommended location.

cdr.rschultz · #4 06-11-2011, 12:11 PM

Thanks for your response Mr Horton. The insight will help to run down this gremlin of sorts. Was curious if you have encountered any aileron flutter with full deflection aileron rolls? Cheers, Bob

Jaknjoan · #5 06-11-2011, 12:27 PM

I used a Piper blade on my first RV-4, and found it terribly inaccurate, even after using a manometer to test speed with a water column and for leaks.

Possibly something comes into play from where they are mounted on RVs. A separate bent stainless steel pitot tube and separate static port, using the standard rivet-head port took care of all of my problems.

JT

johnfurey · #6 06-11-2011, 12:47 PM

Like many others have documented. My RV7 recently made it's first flight and I noticed the IAS was about 10kts slow based upon GPS - stall speed and altitude. I cut the head off of 2 Vans recommnded pop rivets and went for a flight to document my results, landed and super glued on the pop rivet heads over my expensive factory static ports and repeated the test. My variance droped to within 1~2kts. Nothing new here, just rehashing what many already know. Now I will drill the center of my factory static ports to accept a press fit of the pop rivet stem. Sometimes "better" isn't better.

Ironflight · #7 06-11-2011, 08:03 PM

As has been hashed out many times, the Piper "blade" Pitot (any of the models) works great for Pitot on an RV - and miserably as a Static source. I've been using it on the RV-8 for 1350 hours with no problems as a Pitot source, with Van's standard static ports on the fuselage. Doing the same thing on our RV-3 project.

As for the ailerons, that isn't flutter - it is a well-known "burble that everyone gets at full deflection with RV ailerons. Experience (tens of thousands of hours in many airplanes) shows it's not an issue, other than scaring you the first few times....

Paul

cdr.rschultz · #8 06-12-2011, 09:36 PM

Thanks to all for your thoughts and experience. Very happy to suspect the standard static port should do the trick. Cheers, Bob

douglassmt · #9 06-12-2011, 11:05 PM

Maybe it would make a difference if you flew right side up more often

Seriously, if it's just the static port, it should be a simple fix. Cut the static line at the wing root and install a new one where Van's recommends and tie into the cut line at the wing root. Of course, you'll have to crawl in the tail cone to put the new one in

I'd be happy to help if you need a hand or a tool...