ASI Issue

[Mona44]

Hello,

I own a new to me RV7 that I recently imported from the USA to England. It's gorgeous in every respect but has one minor niggle that I'm hoping for some advice on. When we did the pitot static tests we found that the ASI under reads by 7 knots. If I open the ALT static source (a valve toggle switch on the panel) it reads correct. Am I correct in thinking this means the regular static source lines may be blocked? All help most welcome!

Thank you,

Monica
G-OGRL (formerly N92LT)

[rjcthree]

Can you detail the systems onboard, please? Also, do you have anything from the previous owner regarding the relationship of IAS to TAS? What you may have, depending on systems, is a system with a calibrated counter-error compensating for position (or?) error in the exterior static system.

I'm in phase 1, and facing a static error generating about 5kt delta, so I've been studying this a bit.

Rick.

[lr172]

Originally Posted by Mona44

Am I correct in thinking this means the regular static source lines may be blocked? All help most welcome!
(formerly N92LT)

No, assuming your altimeter is not frozen/acting like a barometer. The pressure inside the cabin is slightly different than that in the slip stream outside the plane and this is why you see the different ASI readings and altitude readings.

You'll want to test for leaks, but your troubleshooting has already confirmed that you don't have a significant leak. After confirming a leak free static system, you will need to play around with your static source. While everyone says they are dead on when done via plans, many including myself, see a 5-10 kt variance from true with that arrangement. There are several posts discussing strategies to adjust this. It is usually small shims in front or behind the rivet. You'll experiment with different thicknesses.

Larry

[Mona44]

Thanks guys, that's very helpful.

Larry - what you say makes perfect sense - I'll search the forums for relevant threads. Thanks again.

Monica

[BobTurner]

You can do preliminary testing on the ground, with clear vinyl tubing, a glass of water, and tape. Attach the tubing over the pitot (snug fit) and tape it to remove leaks. Tape drain hole. Hang the tubing in a U shape and pour in water. The water at the bottom of the U should be higher on the open side than the pitot side. If it is level, you have a leak.
Repeat for the static port. If you have two ports, tape over one. Again, if the water is level on both sides of the U (after adding enough to completely fill the bottom of the tube) you have a leak.
Once you've found and fixed the leaks, you can check the ASI calibration. With the tube on the pitot, raise the free end, and measure the height difference of the water in the U. Look and see what the ASI is reading. You can find on the internet tables to convert inches of water to indicated airspeed.
Final step is to test fly, carefully. Usually a triangular course with a gps so you can calculate TAS, then with pressure altitude and temperature data, IAS. Any remaing error is probably due to the static port placement, and you will have to experiment with little dams around it, etc. BTW, the Vans rivet seems to work as well as, or better than, some of the fancier looking designs, imho.