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Puzzled???

N

N661DJ

Well Known Member
I have a friend that owns an RV6 that I maintain. He complained that recently the Air Speed indicator (Vans Steam) reads 15 mph high. I removed the AS ind. and checked it with a manometer, read correctly, checked the static system, thinking a pluged system could cause the error as the aircraft climbed, no problem found, finally found a significant Leak in the Pitot system, repaired the leak, (bad plastic tubeing to the autopilot pitot port), AS now reads correctly. I don't understand how a leak in the pitot system could cause the AS to read high, I would think it would read low???? Is there something about fluid dynamics that I don't understand??
What am I missing???
Dick
 
N661DJ said:
I have a friend that owns an RV6 that I maintain. He complained that recently the Air Speed indicator (Vans Steam) reads 15 mph high. I removed the AS ind. and checked it with a manometer, read correctly, checked the static system, thinking a pluged system could cause the error as the aircraft climbed, no problem found, finally found a significant Leak in the Pitot system, repaired the leak, (bad plastic tubeing to the autopilot pitot port), AS now reads correctly. I don't understand how a leak in the pitot system could cause the AS to read high, I would think it would read low???? Is there something about fluid dynamics that I don't understand??
What am I missing???
Dick
Click to expand...

The leak was probably a static leak inside the fuselage, which tends to be a low pressure area. That would cause a high AS reading.
 
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Does sound odd. Increasing pitot pressure = increasing IAS. A leak should be lower pressure. If it was a static line it might make sense, but not pitot.

You sure it was the pitot line with the leak?
 
Below is a link as to how the pitot static system measures asi

http://www.calpoly.edu/~kshollen/ME347/Laboratory/Pitot_Tube_Calc.pdf

A leak in the pitot side should reduce the indicated IAS in an amount directly proportional to the magnitude of the leak.
The pitot side is very tolerant of slight alterations in pressure, This is why it remains accurate at many different angles of attack (relative wind)

The static side is highly sensitive to leaks and errors causing the airspeed to vary inversely relative to the square of the magnitude of error.

My guess is you inadvertently fixed a static system problem while checking his system.
 
N395V said:
Below is a link as to how the pitot static system measures asi

http://www.calpoly.edu/~kshollen/ME347/Laboratory/Pitot_Tube_Calc.pdf

A leak in the pitot side should reduce the indicated IAS in an amount directly proportional to the magnitude of the leak.
The pitot side is very tolerant of slight alterations in pressure, This is why it remains accurate at many different angles of attack (relative wind)

The static side is highly sensitive to leaks and errors causing the airspeed to vary inversely relative to the square of the magnitude of error.

My guess is you inadvertently fixed a static system problem while checking his system.
Click to expand...

Not sure I agree with this. The ASI measures the pressure difference between the pitot and static lines. So it's equally sensitive to errors in either. But unless you have a pressurized cabin, a static leak is limited to the amount cabin pressure differs from outside true pressure.
Pitot pressure is relatively insensitive to angle of attack because it varies like the cosine of the angle, and around zero the cosine varies slowly. But at high angles of attack, near stall, it does make IAS and CAS noticeably different.
IAS varies as the square root, not the square, of the pressure difference across the instrument.
 
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