[Charliesixtysix]

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Sorry, I still might be missing something here - Have you tested the ASI against a calibrated unit or not?

If so , great - It is unlikely to be the problem and you need to look into the plumbing for the cause.
If not, please do so before running it in parallel with another ASI of unknown accuracy.. ...if you get differing readings, how will you know which one ( if any) is correct....... ( unless of course you already have that covered with your mate's being a calibrated unit, or a new one with Form 1?).

Next, please consider the advice given regarding piping the static system to the ASI - it is unlikely that you will see true speeds using cockpit static, for all the reasons listed above.

Actually, belay that - If I may be blunt, I tend to think it is time you sought advice from an inspector/ engineer to get this sorted at the airfield where he can see and test the systems properly for you.
Once again, that offer is open here if it might help.

[Ben Twings]

It's easy to test the accuracy of the ASI with a simple manometer made out of a length of clear tubing and a piece of board to tape it onto. Plenty of detailed instructions on the web.
Assuming your ASI is reading correctly, then, as others have stated, the static is the likely cause of error.
As for why that has changed over time. If, as you say, you are using cabin static, then it suggests something may have changed the cabin static pressure in flight.

Have you had any changes to canopy sealing and ventilation?
Does airspeed change when you use cabin heat or open cold air vents?
Do you see the altimeter change as you accelerate or decelerate on a level runway?

An external static will give you a more consistent reading not subject to errors when you change ventilation etc.. It might still be incorrect if you don't locate the vents correctly, but at least you can calibrate out the errors.

[Flyin'Dutch']

Good replies all- thank you.
The ' facts' as presented are;
The previous owner with the very same kit saw a very different stall speed.
I'm getting 43 knots indicated for an unaccelerated stall when he got around 30 indicated , possibly a smidge lower.
He got a cruise speed of 90/95 knots (C90-14F) , I see 110 ish (125 at full chat)
His numbers match everyone else's with a D117, mine seem a bit fanciful for a 90 hp
The only reason I'm going on about it is because I can't understand why it's higher- lower I can see reasons for.
A friend is bringing another ASI along this weekend, maybe we will get to the bottom of it.
I have changed one parameter during the week, re- fitting the spats. Hopefully that's not it, that would be an anticlimax.

Ignore the previous owner's statement, if you didn't see it then it wasn't like that!

I think you need to follow C66's advice and get someone in the know to help sort it. As an aside, well not really, I would be worried about flying something with an known fault for which the cause is not established and which may well vary at some point.

Also thought the LAA insisted on doing the old fashioned performance check flights? When was that done for the last time and what were the values? Did they match 'book'

[Flying_john]

Just be careful with a home made manometer (water column) as any mistakes or slip ups could send water into the ASI.

[ChrisRowland]

A constant difference in airspeed seems more like an offset in the instrument than some sort of pressure induced error. Pressure induced errors tend to be a function of airspeed, increasing as the speed increases.

Could the needle have moved? Does it read zero on the ground. There's usually a dot to mark the zero point.

Ask around, I would expect that most airfields have an ASI calibration manometer.

[thegasguy]

Wow! Loads of advice - I'll need a while to chew through that lot.
Taking things in no particular order;
It's not feasible to run two ASI's in parallel - how could I do the plumbing without butchering the existing setup and introducing unknown factors, plus I have no idea how I could mount it in such a small space.
Which would be correct? Who could tell?
What is a "proper bench set-up" ASI tester ? I don't have one and it won't test mine in flight.
The plumbing has been tested for leaks and the instrument reads correct on the ground.
I'm not worried about mine being "out" as I don't know that it is really - are other Jodel flyers claims of 28 knot stall just fisherman's tales? It weighs 617 kilos so I find that speed very very unlikely.
I had a big kite as a kid that needed that much to fly, 5kg max.
Cruise indicated isn't a safety issue and I take whatever stall speed it says on the clock, x 1.3 to be ok for approach speed.
Besides, how it feels/ sink developing is far more important for getting into my ( B difficult) home strip than a mark on a dial.
As for ' book figures' - no it doesn't agree, but it's 59 years old , who is to say it should? It's undoubtedly heavier than designed and probably a funny shape in the aerofoil department after thousands of grass field landings all over Europe.
It's LAA inspected and permitted, flies like a dream and I take safety very seriously despite an irreverent approach to posting here !
I just wish I could find an answer to why it reads high, any other fault I could argue away.
Keep giving me things to look at and I'll keep looking.
Ps, I'm going to fit an Avmap Ulta EFIS soon, these connect to pitot/ static as well as having GPS ground speed so maybe that will give me accurate speed data to compare. Or totally confuse the issue.

[Genghis the Engineer]

The racetrack method in this paper will allow you to do a full range calibration in an hour or so against GPS - just needs accuracy in flying (and if you've any sense, another pair of eyes in the cockpit to lookout whilst you're concentrating on flying ultra-accurately and taking numbers down).

http://bura.brunel.ac.uk/bitstream/2438 ... 0FINAL.pdf

This is a simplified calculation cribsheet (not including the small corrections for a rate of descent at the high speed end), compatible with the method.

http://www.bmaa.org/files/aw_043_asi_calibration.pdf

G

[thegasguy]

Genghis, thank you- that is exactly what I need. Maths and flying, that's my idea of a perfect day.

[thegasguy]

Not done Genghis's maths yet, but I do have some more data to further muddy the water regarding my over- reading ASI.
To test if ' suction' in the cabin, in flight, is causing the error, I have flown with a spare ASI beside me.
This has the pitot inlet plugged and the static open.
The theory is any drop in pressure will read as an increase in indicated speed on it.
It read zero on the ground to start with.
The expectation was to see a few knots.
I flew at various speeds level at 1000' and noted the readings.
It basically followed my own ASI, lagging by 10 to 15 knots up until a max speed of indicated 120 knots when it read 98 knots.
Opening the vents etc had no effect at all.
I don't really understand any of this, but, apparently this is massively more than expected.
Pretty odd that it should be much closer to book figures than my built in one, and yet it wasn't connected to anything !
The more technically minded among you may like to comment? Any variation of the test you can come up with is welcome.

[GrahamB]

Your spare ASI behaved in the way that I would expect; the one thing you have learnt though is that opening vents etc. has no general discernible effect on static pressure inside the cabin.

[T67M]

So a known-good ASI with a blocked pitot and a static exposed to cabin pressure behaved very similarly to the one installed in your instrument panel...

Hmmm.

Does this mean that the one in your instrument panel ALSO has a (partially) blocked pitot and a static exposed to cabin pressure?

At least the lack of change when you open the windows suggests that you already have "maximum vacuum" in the cabin during flight - but that doesn't mean it's exposed to static pressure.

Out of interest, how does your altimeter compare to a GPS altitude readout, or another plane flown alongside? And how does your airspeed indicator compare to GPS ground speed, allowing for head/tail wind? How does the VSI behave? Looking at this reference, and especially at row four of the table, "Using alternate cockpit static air: Airspeed reads high; Altimeter reads high; VSI shows momentarily in a climb."

[Flying_john]

(With hindsight)

I dont think this is a valid test - the ASI with blocked off pitot, just exposed to static is acting as a (leaky) altimeter. Reducing the pressure on the static side of the aneroid capsule as you increase you altitude will make the needle go up the scale.

I think the only option is, difficult as that may be, to get behind your panel and substitute the ASI and check / replace the tubes while you are in there.

[ChrisRowland]

If an ASI has a plugged pitot then the pitot pressure will not change. The static is open so as the altitude increases the static pressure reduces. An ASI measures the difference between pitot and static pressure so as the altitude increases the difference between the fixed pitot and the reducing static pressure will increase so the "ASI" reading will increase. It will act as an altimeter. I don't think there should be any leakage in an ASI.

I once took off with a blocked static. Everything looked normal on the take off but as I climbed the indicated ASI reduced. This is the reverse situation, the static had a fixed pressure so initially the pitot pressure made it read correctly but as I climbed the dynamic pitot pressure was offset by the reducing altitude pressure. Released, landed back and spent the day cleaning the static tubes.

Then there was the occasion where the rear ASI was reading increasingly low as the airspeed increased. The front ASI was fine. Investigated to find that the static was connected to a line that was open to the cockpit on one end and to a spare tube pointing forwards on the other and so was delivering a mix of cockpit and pitot pressure. Connected the static correctly and it was fine.

It's going to be tricky to do a valid test with a second ASI without plumbing it into at least a reasonable source of pitot pressure. A piece of tubing taped to the side of the fuselage pointing forward probably won't work.

[Mavis Enderby]

Alignment error.
Has the pitot been accidently bent or displaced so that it is not in the same alignment to the airflow as it was? All the above tests and advice could be perfectly valid but if the Pitot & ASI system is not measuring the same airflow the numbers can only be different.

[thegasguy]

Hi Mavis, thanks for the reply.
I agree that pitot alignment may be a source of error ( more later!)
However, can you explain 'why' it would result in an increase in reading ? i.e. The mechanical reason?
I understand the various faults- blockage; leakage, etc etc but, they all result in either a frozen reading, slow reading change or a lower reading.
A higher reading requires an anomalous higher pressure differential between pitot and static.
No one has come up with a good way that can happen - as yet, remembering that the static port is open to cabin atmosphere and nothing I do in the cabin affects it.
How can the pitot ram pressure be " too high?"
In mathematical terms, a pitot tube angular displacement from straight ahead should result in a 'decrease' in reading in proportion to the value of the tangent of the angle between straight ahead and the pitot alignment ( I think !)
Anyway - a decrease, not an increase.
Right- now more weird science.
We have removed the automatic "flip up" pitot tube blocker device and " bent" the pitot a bit.
Now it's much better! OMG as the kids would say !
It's drivng me mad. If I can get a ' higher' ram air pressure by having a tube ' not' pointing into wind then I'll point it sideways and I have invented infinite free energy.
You heard it here first.
Please keep answers coming, remembering that ' get it looked at' has been done to death. I really really have. Honest.