Adjusting airspeed indications

[RRHaldeman]

By flying with other Champs, I am pretty sure my airspeed indicator reads consistently higher than actual airspeed - by at least 10 mph. I believe there is an adjustment (nut? "thingamajig"?) at the end of the static line near the left wing strut, but I have found no written reference or guidance on how to adjust it. It "looks like" there is a set screw on the "thingamajig" and detents in front of, and in back of, it's current location. I would like to adjust the airspeed but would like some guidance. Help.

[Roger Anderson]

I bought a Champ once that would indicate a 105 mph cruise. Champs don't go that fast. While looking at the pitot tube, I noticed a paint mark scuffed into it either ahead of or behind where the round nut looking device was presently located, can't remember which. Anyhow, I loosened the set screw, slid the "nut" to what appeared to be a previous location, and the Champ there after indicated a more realistic 80 mph. Try moving it one way or the other and see what you get. On a calm day, you could use a GPS GS reading to get it fine tuned. At the high end (crusing speed) you really don't care what it reads though. My Chief now is about 10 mph too fast, reading a cruise of around 100 mph when my actual true is 87.

[rkittine]

I had the same issue when I bought my Champ. Read consistently 15 MPG faster then averaging (360 turn with 4 stops) on the GPS. Clean out the static line and adjusted the nut and it was right on. I since have purchased a new TSO'd and marked Airspeed Indicator and it out of the box is right on.

[RRHaldeman]

Thanks Roger and Robert,

I believe that the "thingamajig" is held on with a set screw and I do see detents in front and back of its current location. I was hoping someone could confirm that moving it fore or aft was the right thing to do. Armed with your comments, I'm going to experiment with it. BTW, I did use a GPS on a calm day so I know that, at cruise, the Champ does 78 knots (about 90 mph). But remember, I have a new C85 with the O-200 conversion.

[Rich Valler]

I'm in the UK with different legalities to the US, but would suggest that if you can do so legally you could try simply removing the disc completely.

On the first Champ which I shared some years ago the ASI over-read badly, indicating 120mph or so in the cruise (I wish!) and a stall in the 60s. The ASI was replaced with a calibrated example but it made no difference. After much head-scratching we came up with the theory that the disc could be creating a vacuum at the static holes, and therefore a negative datum for the ASI (as opposed to a 'null'), leading to over-reading at the needle. The theory may be suspect, but we removed the disc anyway and the ASI immediately read 90mph in the cruise with stall in the low 40s. Pretty well spot on what would be expected.

My own Champ was without the disc when acquired 10 years ago, and still is without, and indicates 85-90mph at cruise and just under 40mph at the stall.

So, empirical testing (ok, of a sample of just two...) shows a 100% result! Worth a try?

[RRHaldeman]

Thanks Rich.

[MikeB]

I always thought (and I could be wrong) that the disc was blocking the air flow at the static port which allows the calibration of the airspeed indicator. Sort of crude but seems to work Ok when everything is set right. I pretty much don't worry too much about the air speed other than approach to landing and the stall point, of course.

MikeB

[CptKelly]

You mean all you guy's Champs don't cruise at 100?

Mike

[Rich Valler]

Mine doesn't, not with Avgas locally at $12.74 per US gallon !!

Rich

[rkittine]

I just watch the cars pass me and know that I am probably doing the 75 that I am indicating.

[CptKelly]

Holy Moley, Avgas at $12+ per gallon? Dang, I thought it was expensive at $5.25/gallon???

Mike

[Roger Anderson]

Mine doesn't, not with Avgas locally at $12.74 per US gallon !!

Rich

WOW!!! That would put me out of the airplane business I'm afraid...even at 4 gph. I hate to even do the math on a Baron or such.

[RRHaldeman]

OK, this thread has been officially hijacked. BUT to close this thread, at the price of gas today, perhaps I should not adjust my airspeed indicator at all. I'd rather be going 10 mph faster, if I have to pay that much for gas.