Airspeed Error?

Hello all,

I have a question about errors in the airspeed indicators for RV's. My 9A seems to indicate about 180mph true on my airspeed indicator regardless of power setting as I put in more throttle and rpm. I can watch the groundspeed increase as more power is set, but the airspeed indicator seems to settle in around 180mph true. It does however increase when the plane is nosed over and speed obviously increases from the previous readings.

Last weekend I flew beside my father-in-law for some in flight photos at 4500' and once again my airspeed showed 180/181mph, and his showed 188mph true. However, our groundspeeds were the same. His airpspeed has always seemed right and matches many airplanes he has flown with. He has owned his airplane for approx. 30 years and is sure his is close to right on.

Does anybody have any ideas? It seems like if there was a leak the indicator wouldn't increase when the plane was nosed over. I wouldn't worry about it but during decents I have brought it up to the yellow quite often, when actually I was probably closer to the red. Also, just knowing the real speed of the airplane should be a gimme.

I know it is kind of hard to complain when you confirm that your airplane is about 8mph faster than the guage shows, but now that I'm finished smiling about my new found speed I'd really like to see it everytime I fly.

Thanks,
Bryan Wood

Airspeed error?

Bryan Wood said:

My 9A seems to indicate about 180mph true on my airspeed indicator regardless of power setting as I put in more throttle and rpm. Bryan Wood

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Bryan, you have two issues from what you wrote. Airspeed "stuck" at 180 mph for different power settings and Low indicated airspeed.

If you are reading low check for leaks or a bad airspeed indicator (you did not mention what type). You can check for leaks and a bad gage with a water manometer. After making these checks, if there are no leaks and the gage is good, low airspeed error is often from the static port. Some use aftermarket flush static ports vs. the one provided by Van's kit, a protruding rivet find they have low airspeed indications by as much as 10-11kts. There are fixes for that, which involves one of the following: bonding on a pop rivet head, a raised bump in front of the port or replace it with a different port which is not flush.

You state the airspeed indication seems to be stuck at 180mph. Also you say "indicate about 180 mph true" Is it indicated or true airspeed? Not that it matters for the sake of discussion, unless you are comparing to ground speed.

Than you mention no airspeed change with increased power (throttle/rpm). You should see something, but it may be a very small increase. Airspeed indication remaining the same (180mph) regardless of power setting, may be normal if you are already near full power. Adding a little more throttle may result in no noticeable airspeed increase. From what you wrote it does go up when you descend so it is not stuck. Right?

It sounds like you are reading 8 mph low by your side-by-side check. You did not say anything about your engine HP or if you have wheel fairings. Van's list a 188mph @ 75% power @ 8,000 feet with a 160hp RV-9A. So 180 mph could be low?

Good Luck George

Thanks for your reply. I was talking true with my readings. I put a 170hp 0-320 with a hartzell which yields me around 190mph as best I can tell, but the airspeed indicator reads low. The plane has all its fairings, is painted and runs strong. Again, my groundspeed increases with the added power, but airspeed seems to settle in around 180mph true. I can reach 180 mph true at an rpm setting of around 2400 and full throttle at say 7500' and each 100 rpm affects groundspeed by 3mph or so. With these increases the airspeed still chugs along at 180mph or so true.

Here is the kicker, it didn't used to act this way. I remember during the flight testing getting it up to about 193mph down low on high power before the intersection fairings were on the top of the gear legs. My airspeed indicator read it just fine then. I've also flown trips of a couple of hundred miles and turned around to come right home and have groundspeeds in both directions closer to the 190mph range. I know the airplane is significantly faster than the indications I'm seeing. As for the guage, it is the one sold by Van's, true airspeed, knots inner/mph outer that most of us use.

As for the static ports, yes I used the pop rivets supplied with the kit and placed them where the plans stated. I'm not sure I understood what you meant on adding more rivets in front of the others, but am very interested.

I'm going with your suggestion to check for leaks and then move on if that is not the problem.

Thanks again,
Bryan