Air speed indicator

Tom-D

Tom-D

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Tom-D
How to calibrate ?

I have a completely new system, How can I verify it is correct prior to flying it?

I know how to calibrate the gauge.
 
If the pitot tube is aligned with the direction of flight and the static pressure port is indeed in a static pressure condition the speed will be close to the pressure associated with the airspeed . I don't even think that perfect alignment of the pitot tube with flight is required. The final testing/check may have to be done in flight (In no wind conditions) or a wind tunne. lIn reality it will only be correct when the pitot cover is on and dirt daubers have sealed the static port and the gage reads "0"

disclaimer: this is the opinion of an uninformed, uneducated,unwashed redneck hillbilly.
 
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i believe there is an Advisory Circular on testing air data systems, also, i sometimes look in FAA TSOs for reference to design/performance specifications to better understand things.




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If you're talking about a Cessna 170 it's a fixed installation, there is no adjustment. It's not as if you've designed a new airplane here.
 
Silvaire said:
If you're talking about a Cessna 170 it's a fixed installation, there is no adjustment. It's not as if you've designed a new airplane here.
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That is assuming Tom rebuilt that airframe to the manufactures original specifications....

Surely a ace, top notch A&P /IA like Tom is, does not have a thing to worry about.......................:confused:....:confused:...
 
N801BH said:
That is assuming Tom rebuilt that airframe to the manufactures original specifications....

Surely a ace, top notch A&P /IA like Tom is, does not have a thing to worry about.......................:confused:....:confused:...
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There was a home made heated pitot system installed, plumbed with a plastic tube, I removed it and returned it to the Hard line. like it came from the factory.

The pitot tube sticking out of the wing is curved to point directly into the wind, how would you insure it is?
 
Silvaire said:
If you're talking about a Cessna 170 it's a fixed installation, there is no adjustment. It's not as if you've designed a new airplane here.
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You can bend the tube, and throw the airspeed off. the pitot tube does not come out of the wing sticking directly into the wind. It actually comes out below the leading edge and is bent. there is no template for aligning it to the wind.

you must eye ball it, how do you know if you got it right?
 
As mentioned earlier it is not super critical, not until you deviate from alignment to a point where the airflow would be at an angle across the tube opening that would create a shear and reduce the pressure in the pitot line.

There, did that sound like I knew what I was talking about, because I pulled it right out of my......:rolleyes2:
 
Tom-D said:
There was a home made heated pitot system installed, plumbed with a plastic tube, I removed it and returned it to the Hard line. like it came from the factory.

The pitot tube sticking out of the wing is curved to point directly into the wind, how would you insure it is?
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Beats me..... I am just a dumb experimental builder..:goofy:

Your best bet is to ask Tom, the top notch A&P- IA that hangs out on POA.. He knows EVERYTHING.... ;)..........:rolleyes2:..

ps.... What does the Cessna factory manual say for aligning the pitot tube... Cause that is what you are required to do....
 
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The bottom line is that because the pitot tube is fixed to the aircraft there is no position that will always be directly aligned with the relative wind because the wing is going to be at a different angle of attack for every specific speed you are flying. Do you want it to be perfectly aligned at cruise or perfectly aligned at stall? Because you can't have both.
 
kontiki said:
i believe there is an Advisory Circular on testing air data systems, also, i sometimes look in FAA TSOs for reference to design/performance specifications to better understand things.
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http://www.casa.gov.au/rules/1998casr/021/021c40.pdf

This is more than I wanted to know. It's like Pi to 700 decimal places.
This is what made us invent GPS and AOA indicators.
 
When we did RVSM checks on LR45's there was a fixture with a scale that was installed on the bullhead where the radar was attached to. On the pitot tube a different fixture with a laser pointer was attached. The laser pointed at the scale and you could be off only so much.
 

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I'm pretty sure a C-170 is not that critical. An LR45 at 40,000 ft is operating in an environment with pretty narrow margins.
 
Load to gross and see check stall speeds (at altitude) at bottom of green and white arc. Those would be the ones I'd want to be accurate? :)
 
You can use the LR45 methods to copy a known good set up of a similar aircraft and use that reading to set yours. Just a thought.
 
Tom-D said:
There was a home made heated pitot system installed, plumbed with a plastic tube, I removed it and returned it to the Hard line. like it came from the factory.

The pitot tube sticking out of the wing is curved to point directly into the wind, how would you insure it is?
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Strap a bubble level onto it and go fly it.
 
edessa said:
You can use the LR45 methods to copy a known good set up of a similar aircraft and use that reading to set yours. Just a thought.
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Actually a digital protractor or smart level is all you'd need. Compare the aircraft level reference with the pitot tube on both aircraft.
 
i was hoping that one of the folks here would come up with a good way to get the pitot tube at the right angle.

Here's how you do it, you simply eye ball it straight with the wing bottom surface, tie a piece of yarn to it, and go fly. the yarn should be parallel to the tube in level flight.

It pays to be a member of the type club :)
 
Tom-D said:
... the yarn should be parallel to the tube in level flight...
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At what speed? and why level flight? You mean it's not going to be accurate in a climb or descent?
 
A halfway decent pitot tube will be accurate with the flow +/- as much as 20 deg off-axis (ref: USNTPS FTM-108 or any fluid dynamics textbook) so you might not have as much to worry about as you think. Trailing bombs and cones and stuff don't tell you anything about the total pressure error, they're to get an accurate static pressure reading *but* that's where most of the error is in a typical system anyway. GPS courses have made any other means of airspeed calibration for everyday GA use pretty much obsolete. Search for "GPS airspeed calibration" or "static source position error correction" for more details. Heck, you can probably find FTM-108 online which will tell you more than you need to know.

Nauga,
the tower flyby guy
 
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nauga said:
A halfway decent pitot tube will be accurate with the flow +/- as much as 20 deg off-axis (ref: USNTPS FTM-108 or any fluid dynamics textbook) so you might not have as much to worry about as you think. Trailing bombs and cones and stuff don't tell you anything about the total pressure error, they're to get an accurate static pressure reading *but* that's where most of the error is in a typical system anyway. GPS courses have made any other means of airspeed calibration for everyday GA use pretty much obsolete. Search for "GPS airspeed calibration" or "static source position error correction" for more details. Heck, you can probably find FTM-108 online which will tell you more than you need to know.

Nauga,
the tower flyby guy
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Tom is working on a CERTIFIED plane....

He is required to follow Cessna's manual for aligning the pitot tube...
 
N801BH said:
And those methods are ???:dunno:...:confused:
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Specifically? I don't know what they are, I'd have to look if there is a standard generic procedure as with many airframe repairs. One can always ask a DER to write a procedure, and with their stamp, it's good.
 
N801BH said:
Tom is working on a CERTIFIED plane....

He is required to follow Cessna's manual for aligning the pitot tube...
Click to expand...

OBTW the 170 does not have a maintenance manual. it was built and dropped from the line prior to the 68 publication date of the 100 service manual.

So... we just muddle along :)
 
N801BH said:
He is required to follow Cessna's manual for aligning the pitot tube...
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I didn't tell him how to align it, I told him the likely error source and how to find out how to measure that error. Cessna has no say in what numbers you write down in flight.

Nauga,
focusing on the issue at hand
 
Henning said:
Specifically? I don't know what they are, I'd have to look if there is a standard generic procedure as with many airframe repairs. One can always ask a DER to write a procedure, and with their stamp, it's good.
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you'll never get that done.
 
Did you relocate or change the static ports or anything around them?

Nauga,
under pressure
 
nauga said:
Did you relocate or change the static ports or anything around them?

Nauga,
under pressure
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I didn't. they were moved to the right side of the aircraft when the fuselage modification was completed long prior to me owning the aircraft. The heated Pitot was a part of that mod. all instruments are now hard plumbed. and do not leak.

My only concern is how to position the pitot tube after it protrudes from the wing.
 
Tom-D said:
I didn't. they were moved to the right side of the aircraft when the fuselage modification was completed long prior to me owning the aircraft. The heated Pitot was a part of that mod. all instruments are now hard plumbed. and do not leak.

My only concern is how to position the pitot tube after it protrudes from the wing.
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I would just tape a bubble level to it and fly the plane at cruise speed, level it to that, and call it good, it's not really all that bloody critical.
 
Tom-D said:
My only concern is how to position the pitot tube after it protrudes from the wing.
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Sorry, I thought your concern was how to calibrate your airspeed indicator as you stated in your first post. The angle of the pitot tube is almost irrelevant as long as it's close to level. If, as you imply, the static source has been relocated or changed from the production location regardless of the state of the pitot tube then any airspeed calibration or static source position error correction in the POH (some have 'em, some don't) is questionable and a calibration should be flown. I gave you some search suggestions that should lead to procedures.

Static source position error also affects the altimeter, by the way.

Nauga,
live from the AIMS range
 
nauga said:
Sorry, I thought your concern was how to calibrate your airspeed indicator as you stated in your first post. The angle of the pitot tube is almost irrelevant as long as it's close to level. If, as you imply, the static source has been relocated or changed from the production location regardless of the state of the pitot tube then any airspeed calibration or static source position error correction in the POH (some have 'em, some don't) is questionable and a calibration should be flown. I gave you some search suggestions that should lead to procedures.

Static source position error also affects the altimeter, by the way.

Nauga,
live from the AIMS range
Click to expand...
Did you really read the first post?
 
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