Airspeed indicator always reads low

[NJMike]

I am flying a Quicksilver Sport 2S. In May I installed the big bubble windshield. My ASI was consistently reading low, never getting above 38.

I thought it might be bad so I purchased a new one. This one does not read above 30!

I am sure that I am doing something wrong....fundamentally wrong with the installation...but don't know what it is.

I have moved the pitot tube from in front of the windshield out on the struct, but no change.

I have changed the tubing from an I.D. of 3/16" to 1/4", no change.

The ASI might read normal below 30mph, but that is hard to determine because that only applies when accelerating for takeoff.

My latest thought....there is a fitting on the back to which the tubing attaches. If the fitting is not in tight enough, could there be a leak which prevents it from reading above a certain limit?

I also have a Hall ASI on the strut and it reads about 20mph higher...

Any help or ideas would be greatly appreciated.

[FastEddieB]

Well, airspeed indicators measure the difference between dynamic pressure (at the pitot tube) and static pressure (normally at the static port).

Since both are important, I'm wondering why you did not mention your static port and how it's plumbed.

And, yes, any leak in the pitot lines would cause a decrease in the pressure in the line and a low reading, as would any blockage.

[NJMike]

You're right, fasteddie, I was focused on the pitot tube. The primary reason is that the altimeter works perfectly. The static port is on a tube that is mounted in front of the windshield.

And I do understand how the pitot static system works. Been flying for a long time. What I lack is on the mechanics/maintenance understanding.

I read online that the fitting on the back of this ASI should be about 45 lbs. I have no idea how to measure that since the fitting cannot be tightened with a socket wrench.

Another thing I did not mention is this: at first the pitot tube was also in front of the windshield with the static port/tube. It had the same problem in that location. I moved it out to the strut and test flew it, same problem. During the flight I noticed the angle that the pitot tube made to the relative air, and it did not look like it was pointed directly into the oncoming air. It looked like it was pointed upward - above the relative air. I landed, repositioned the pitot tube downward quite a bit, then test flew again. This time the angle looked right but there was no change in the indicated airspeed.

So I made a number of significant changes trying to cause a corresponding change I the IAS. Since nothing worked, I'm trying to think "outside the case", to understand more.

It's easy enough for me to tighten up the fitting on the back. And my gut feeling is that it is not anywhere near 45 lbs.

[FlyingForFun]

Delete

[NJMike]

If there were a blockage in the static system wouldn't the altimeter NOT work?

[FlyingForFun]

Delete

[FastEddieB]

45 ft/lbs of torque sounds huge for a simple hose fitting. Could it be 45 in/lbs?

I'd wonder about the static port picking up some dynamic pressure. Try one flight with the static line disconnected at the case. If nothing else, that will eliminate the static system as a contributing factor.

Any shop should have the ability to do a "leak down" test of the pitot/static system. That would be my next suggestion.

[NJMike]

It could be 45 in lbs.

It is very easy to disconnect the static tube at the ASI so yes, I can try that.

[NJMike]

It was the static source. It works when disconnected from the metal static tube located in front of the windshield. I am leaving the static source vented right at the ASI, where it is behind the windshield and not being hit by any wind.

[FastEddieB]

Bingo!

Now, just remember that the pressure in the cabin will be less than static pressure, thanks to Bernoulli.

This will cause the airspeed to read higher than it should, and to make things worse, it will change as you open vents and the like.

Airspeed is kind of important, so I wouldn't play too fast and loose with this - an artificially high reading is especially dangerous.

[NJMike]

It's a Quicksilver - totally exposed to static pressure, no vents, etc. so I don't think Bernoulli will be an issue. You agree?

I compared it to my Hall ASI. They read the same at low speeds. It read less than the Hall at cruise speeds. I had suspected the Hall was reading low so this just confirmed it.

[FlyingForFun]

Delete

[CharlieTango]

Quicksilver Sport 2S

[FastEddieB]

It's a Quicksilver - totally exposed to static pressure, no vents, etc. so I don't think Bernoulli will be an issue. You agree?

Agree!

[FastEddieB]

It's hard to see in the photo where the airspeed indicator is.

I would be inclined to not just leave an unplugged hole in the back of it, lest air currents cause dynamic pressure. And it would leave it open to dust and insects and the like.

My thought would be a length of hose, maybe and inch or so long, plugged at the end with lots of small (1/8"?) holes drilled all around its circumference - or something along those lines.

And be sure to do some stall series to see at what IAS the stall does occur in level flight.

Good luck.