IAS Error

[newt]

Originally Posted by Ironflight

A static leak into the cabin will cause airspeed to read high by maybe 10 knots (I?ve seen as much as 15) at cruise speed in RVs.

Okay, I find that slightly confusing.

I have an alternate static for IFR, which, when opened, vents the static line to cockpit.

There's a bit of instrumentation disturbance when I open it, but after it settles down the airspeed is more or less the same as what it was before I opened it.

(depends on how far open the eyeball vents are, though - there's a knot or two of difference between full-closed and full-open cockpit ventilation)

If a static leak in the cockpit will cause a 10 - 15 knot deviation, why don't I see a 10 - 15 knot deviation with a deliberate full-throated "static leak" in the cockpit?


- mark

[kevinsrv7.com]

If your buddy is using the nylon pitot and static tubing in the picture below, be sure to use Teflon tape, on the nylon threads that screw into an aluminum or brass (back of steam gauge or garmin GSU-25) component. However that hasn't been accomplished yet in this picture. The tape is only necessary where the nylon is screwed into metal.

[Cumulo]

Originally Posted by newt

Okay, I find that slightly confusing.

I have an alternate static for IFR, which, when opened, vents the static line to cockpit.

There's a bit of instrumentation disturbance when I open it, but after it settles down the airspeed is more or less the same as what it was before I opened it.

(depends on how far open the eyeball vents are, though - there's a knot or two of difference between full-closed and full-open cockpit ventilation)

If a static leak in the cockpit will cause a 10 - 15 knot deviation, why don't I see a 10 - 15 knot deviation with a deliberate full-throated "static leak" in the cockpit?


- mark

Good question. It likely indicates a very well sealed cabin area - if we are talking RV/low wing.

The pressure inside the fuselage will be the sum of all the openings to the outside and the pressures that are there. Generally, in a low wing aircraft, much of the leakage/ venting will be dominated be the pressure of the wing top surface which is accelerated flow/low pressure and will effect the airspeed reading accordingly with a high reading from an open static. If that area is well sealed though, openings as far away as the tailcone could be be dominant producing a more neutral pressure.

Ron

[David Paule]

If there are no leaks in the pitot-static system, then that system isn't open to cabin pressure. Unless a cabin air leak changes the airflow around the fuselage enough that it affects the pressure at the static port, which I doubt, how could it affect the indicated airspeed? And if it did, wouldn't adjusting the cabin vet opening affect the indicated speed?

Dave

[GalinHdz]

Originally Posted by newt

I have an alternate static for IFR, which, when opened, vents the static line to cockpit.

There's a bit of instrumentation disturbance when I open it, but after it settles down the airspeed is more or less the same as what it was before I opened it.

Based on this information it appears your static leak is inside your fuselage, the same place your alternate static source is vented to.

Originally Posted by newt

If a static leak in the cockpit will cause a 10 - 15 knot deviation, why don't I see a 10 - 15 knot deviation with a deliberate full-throated "static leak" in the cockpit?

Because in both cases your ASI receives the same static pressure (from inside the fuselage/cockpit) so it displays the same airspeed.

BTW; If this is the case, your altimeter is also reading incorrectly which could be dangerous in actual IMC.

[Bavafa]

When in doubt, test it. An inexpensive vacuum pump and taping over your static ports and using the pump will show if you have any leaks in your static line.
As a reference when I tested my alternate static port (in the cabin) during flight, my altimeter changes instantly by around 200'

https://www.harborfreight.com/Brake-Bleeder-and-Vacuum-Pump-Kit-63391.html?ccdenc=eyJjb2RlIjoiNjMzMTIyODYiLCJza3Ui OiI2MzM5MSIsImlzIjoiMTguMzkiLCJwcm9kdWN0X2lk%0D%0A IjoiMTIyNDYifQ%3D%3D%0D%0A&cid=paid_google|||63391 &utm_source=google&utm_medium=cpc&utm_campaign=&ut m_content=&gclid=EAIaIQobChMI-oWh4bLa5wIVmZOzCh3kTQuDEAQYAiABEgJPcvD_BwE

[Cumulo]

Good diagnosis by GalinHdz. My response would only apply if the static system of Newt' a/c was tight. Galin's assumption is more likely.

Newt's Q: ~Hardly any change when the alt static is opened to cabin press. Why?:
GalinHdz: ~maybe it is already open/leaky, could give faulty altitude IFR

The OP 's plane: too high air speed reading.
Thread consensus: likely an open/leaky static system.

Two different a/c.

Ron

[Cumulo]

Originally Posted by Bavafa

When in doubt, test it. An inexpensive vacuum pump and taping over your static ports and using the pump will show if you have any leaks in your static line.
As a reference when I tested my alternate static port (in the cabin) during flight, my altimeter changes instantly by around 200'

https://www.harborfreight.com/Brake-Bleeder-and-Vacuum-Pump-Kit-63391.html?ccdenc=eyJjb2RlIjoiNjMzMTIyODYiLCJza3Ui OiI2MzM5MSIsImlzIjoiMTguMzkiLCJwcm9kdWN0X2lk%0D%0A IjoiMTIyNDYifQ%3D%3D%0D%0A&cid=paid_google|||63391 &utm_source=google&utm_medium=cpc&utm_campaign=&ut m_content=&gclid=EAIaIQobChMI-oWh4bLa5wIVmZOzCh3kTQuDEAQYAiABEgJPcvD_BwE

Thanks Bavafa

That HF vac pump should work very well, maybe better than a football pump that I reversed the valve and piston in. It is good to have a reservoir. I made a vacuum tank from a large tomato can. First I punched two small holes in the can, drank the tomato juice, rinsed the can several times, soldered some copper tubes on, got some tubing and needle valves from a local aquarium supply store and had a 1st class leak tester. Very practical.

Ron

BTW You MUST hook the pitot system to the static system FIRST before you pull any vacuum on the static or you will very likely damage any airspeed, G5, etc device installed.

[GalinHdz]

Originally Posted by RV7A Flyer

Flying next to a friend in his new-to-him RV-6A (I believe he has an O-320 FP prop), I asked why we were going so damned slow...I was showing 100-110 KIAS, and he was showing 20 knots or greater *higher*. I *know* my airspeed is correct, by comparison with numerous other RVs in formation flights, at all sorts of altitudes and power settings. So this is a problem with his airspeed. But what?

BOTH his mechanical ASI and his D-10A were showing the same IAS. He has just had a pitot/static/XPDR cert done and passed, but I know this won't detect installation error that can only be found in flight.

He performed some stalls in order to find the *indicated* stall speed, for safety of flight on T/O and LDG, and says he stalled at around 65-70 KIAS, clearly way too high.

What could cause this? FWIW, his static ports are flush rivets, but I've only heard of problems with flush rivets causing an IAS that is too *low*, and it's almost impossible to think of an installation error with the *pitot* tube that would make it read that high.

Anyway, looking for troubleshooting advice to help out a buddy...TIA!

If it has been this way since originally built and there are no leaks, the problem is in the static source installation. A common problem in home built aircraft. If it is a static source installation problem no amount of ground testing will expose it, only proper flight testing will expose it. There have been many threads explaining this particular issue here and how to identify/fix it.

Here is a recent DYNON Forum thread about this issue in an RV-14 and the multiple problems that were found.

[DanH]

It's natural to attribute a problem to a single cause, but often a particular result is the sum of multiple factors. Check for leaks? Sure...tick off everything on the possibles list. Worth the time, because 20 knots error is a lot.

Assuming no leaks or instrument issues, yes, consider changing the static port. Plenty of old discussion in the archives. Short version: flush isn't likely to be accurate. Start with a dome head, and fly a three leg using the NTPS method. If airspeed indicates high, apply some masking tape to protect paint around the static port, and slide a smooth file across the port to flatten it just slightly. Fly another NTPS 3-leg. Repeat as needed.

Be sure to convert IAS to TAS. If EFIS equipped, calibrate the temperature probe before relying on a TAS indication.