[thegasguy]

Some of you may remember me posting previously about my Jodel stalling speed seeming to be a lot higher than expected
Further to this: I have had the ASI checked by calibration machine attached via the pitot, and it reads as being remarkably accurate right through the speed range.
The previous owner, who flew her for 30 years, has looked her over and confirmed nothing has physically changed as far as can be seen externally.
He is adamant, and I totally believe him, that she stalled about 15 knots less and cruised about 15 knots less than the indications I now get ( same rpm)
Obviously the instrument or tubing or pitot head or something may have deteriorated since then, but, how do you explain a ' rise' in readings when the check box says its accurate on the ground?
I guess the stall speed may go up a bit as the airframe ages but surely the cruising speed can't, and besides the gps says it's about 90 kts- not 105 to 110.
Any ideas?

[aerofurb]

GPS results aside, have you checked the static side of the pitot static system?

If the static system is sensing cockpit static for instance, which tends to be lower than 'ambient' static, it would cause the ASI to overread in flight.

It happened to a friend (also in a Jodel!) and they subsequently found the static pipe disconnected from the altitude encoder.

[4535jacks]

Maybe the issue is in the pitot tube. I can't off hand think what would cause a pressure change though and the flow will already be turbulent.

A small obstruction would not cause a pressure drop across the whole system due to benoullis law of continuity. Maybe check the static and dynamic pitot tubes for cracks or damage to see if this is affecting the pressure differential due to the pressure inside the cabin or airframe.

[Charliesixtysix]

Agree with Aerofurb,

Chances are you have a problem with the static side of the system - you say you have checked pitot to ASI against a calibrated unit, so have eliminated that section as the cause of your over reading.

Try connecting to a static port after sealing all other ports and gently apply suction with a calibrated (or known good) ASI in the test line to check the two ASI readings match. This will also allow you to check any leaks in the static system if you nip off the test line downstream of the test ASI. (ASI readings would drop if a leak).

Hopefully this might help you get to the bottom of the problem.

[thegasguy]

The plot thickens !
There has never been a static connection according to previous owner.
Apparently the static port on the back of the ASI is just 'open'.
I've never been back behind there to verify, but have absolutely no reason to doubt him.
No one has altered anything on the system so it must be a 'fault' that has developed over the 3 years or so she sat at the back of the hangar.
So- I am told that a leak caused by a split pipe, loose connection etc on the( only pipe) pressure side would result in a low reading. It's the opposite of that.
A constriction in the pipe/ tubing would cause the reading to be slow to react. It's not at all slow and is nicely damped- no wobbling needle etc, just too high.
The pitot has the little 'flip up' cover and flips up just fine on the ground and I get airspeed indication immediately I start my take off run starting at about 20 knots.
Anyone know if crud in the static port side of an ASI causes a high reading ? That is eminently possible as she sat about for a long time in an open fronted hangar with a loose stone floor.

[johnm]

Blocked static will underread in the climb and overread in descent iirc

[aerofurb]

Most GA aircraft 'cabin' pressure is below ambient pressure.

Any holes in the fuselage tend to have air drawn out of them creating the relatively low pressure.

Some aircraft may behave differently when windows, vents etc are opened. It may be mentioned in POHs when there is an alternate static installed.

The relatively low pressure results in a greater pressure differential between pitot and static - hence an ASI over read.

Perhaps the aircraft has a gap/hole so where that it never used to have and that's why the discrepancy has appeared.

[thegasguy]

Possibly a hole has appeared that wasn't there before, but, I hope I might have noticed, say, a drone shaped hole somewhere !
Seriously though, I have tried several ideas that seem to rule out the possible variations of actual static pressure as the cause.
I've tried cruising at steady speed and height and then using the cabin heat - no change even though a zephyr of (luke warm) air appears round my feet.
Opening, and rotating into wind, the circular air vents in the canopy introduces enough freezing ram air to blow everything about a bit - no change!
I do think the problem may be static pressure related though, but somewhere in the workings of the instrument maybe rather than in reality.
If the Static inlet is partially blocked, would the instrument ' see' a bigger difference between that and dynamic pressure ? For the life of me I can't see why it would in a steady state cruise condition.
Can the needle slip on the pivot pin and simply be 'offset' from the number it's supposed to point at ?
If so, then I could stick new numbers to the glass I suppose, or just do what I do now and use it as a 'don't fly slower than that mark' instrument.

[Charliesixtysix]

Early in this thread you said the ASI had been checked as accurate against a calibrated unit.
If that is correct, it is unlikely to be the cause of your problem.

[thegasguy]

Yes I agree that seems likely, but, I'm running out of ideas.
The calibration box was used in the hangar, stationary, and the error is in the air- in motion.
So that is one difference between testing and use that's difficult to allow for.
My idea of the needle slipping on its spindle isn't so silly now I consider it- if it 'slipped' 15 knots higher up the graduations but sustained no damage- that fits the bill quite neatly; as long as there is enough 'slack' in the mechanism to allow it to still fall back to zero ( it does).
Is this possible - any instrument experts out there?

[ChrisRowland]

Could the whole system now be correct but previously it wasn't and that accounts for the discrepancy?

Difficut to find a problem when you don't have one.

[Charliesixtysix]

Yes I agree that seems likely, but, I'm running out of ideas.
The calibration box was used in the hangar, stationary, and the error is in the air- in motion.
So that is one difference between testing and use that's difficult to allow for.
My idea of the needle slipping on its spindle isn't so silly now I consider it- if it 'slipped' 15 knots higher up the graduations but sustained no damage- that fits the bill quite neatly; as long as there is enough 'slack' in the mechanism to allow it to still fall back to zero ( it does).
Is this possible - any instrument experts out there?

You would be very welcome to pop up here if you wish and we can run my test kit on the aircraft to see if that helps get to the bottom of this one.

BTW what is unaccelerated stall speed that you are currently seeing (1kt/sec deceleration, wings level, idle power)?

[Flyin'Dutch']

Yes I agree that seems likely, but, I'm running out of ideas.
The calibration box was used in the hangar, stationary, and the error is in the air- in motion.
So that is one difference between testing and use that's difficult to allow for.
My idea of the needle slipping on its spindle isn't so silly now I consider it- if it 'slipped' 15 knots higher up the graduations but sustained no damage- that fits the bill quite neatly; as long as there is enough 'slack' in the mechanism to allow it to still fall back to zero ( it does).
Is this possible - any instrument experts out there?

This reads like a problem with the pipework.

1. Calibrate the ASI on a proper test bench
2. Pressure test the system using the appropriate kit for it
3. Put another ASI on the piping at the same time as your ASI and test it in situe both on the ground and in the air. If it is a piping/system error they will show the same readings, if it is the ASI there will be a difference.

[riverrock]

You say that the static input on the ASI isn't attached (and never has been attached?) so is open to the air behind the panel.
Do you have any behind-panel cooling?
Could that be changing the air pressure in that area? Does something (relative to airspeed) cause a mini-vacuum behind the panel to draw air though it?

[thegasguy]

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.