Replacing an airspeed indicator

On Fri, 07 Mar 2014 13:45:53 -0800, son_of_flubber wrote:

Is there any reason to suspect that an ASI (especially a non-TSO'ed one)
might be more accurate in the middle of it's range?

On this side of the pond the rules say that ASI calibration must be
checked as part of the Annual Inspection and must be within 2kts
throughout its range.

The calibration check applies a known pressure to the pitot inlet and and
records the ASI reading. This is done at 10 kt intervals from the ASI's
highest indicated speed down to 20 kts.

An ASI is a sensitive pressure gauge that happens to be calibrated in kts.
The calibration is independent of the airframe. IOW, if the IAS in flight
shown by a freshly calibrated ASI is not the same as independently
measured TAS then the error is due to the placement of the static vent
and/or the pitot. For example, its well known that pressure under the
wing is above bulk atmospheric pressure, so a static vent anywhere near
the underside of the wing will make the ASI read low. Similarly, a static
vent placed on a convex bulge in the fuselage away from the wing will see
a lower pressure and so will make the ASI read high. That's why the
static vent is commonly halfway along the boom: at that point the boom is
a straight taper and so has minimal curvature in relation to the
slipstream while the vent is placed well away from the flying surfaces.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Friday, March 7, 2014 12:19:16 PM UTC-7, Luke Szczepaniak wrote:
who knows, this glider may go to 160kts, but do I really need to go that fast?



YES!

What he said!!!!

Altitude is safety, speed is life!

Kirk
66
(VNE does not mean "do not approach", it means "do not exceed")

Thank you Martin. Pointing out how the pieces fit together is very helpful..

An annual test of the ASI over the entire range suggests that they have in the past found a few ASI that were not accurate over the entire range and that ASIs might go bad gradually over time. I wonder what those tests find in the field.

(My question may sound like FUD, but I have a habit... people paid me to find defects in their technology.)

Do I have it right that at a given point in time and altitude, the difference between TAS and IAS is constant over the entire range? I'm still figuring out how to use my LXNav V7 to check the calibration of my 15 year old Made in China ASI (and vice-versa, if I find a problem, either one of the devices could be the culprit.)



On Friday, March 7, 2014 7:30:41 PM UTC-5, Martin Gregorie wrote:
On Fri, 07 Mar 2014 13:45:53 -0800, son_of_flubber wrote:



Is there any reason to suspect that an ASI (especially a non-TSO'ed one)

might be more accurate in the middle of it's range?



On this side of the pond the rules say that ASI calibration must be

checked as part of the Annual Inspection and must be within 2kts

throughout its range.



The calibration check applies a known pressure to the pitot inlet and and

records the ASI reading. This is done at 10 kt intervals from the ASI's

highest indicated speed down to 20 kts.



An ASI is a sensitive pressure gauge that happens to be calibrated in kts..

The calibration is independent of the airframe. IOW, if the IAS in flight

shown by a freshly calibrated ASI is not the same as independently

measured TAS then the error is due to the placement of the static vent

and/or the pitot. For example, its well known that pressure under the

wing is above bulk atmospheric pressure, so a static vent anywhere near

the underside of the wing will make the ASI read low. Similarly, a static

vent placed on a convex bulge in the fuselage away from the wing will see

a lower pressure and so will make the ASI read high. That's why the

static vent is commonly halfway along the boom: at that point the boom is

a straight taper and so has minimal curvature in relation to the

slipstream while the vent is placed well away from the flying surfaces.





--

martin@ | Martin Gregorie

gregorie. | Essex, UK

org |

On Monday, March 10, 2014 9:11:34 AM UTC-4, son_of_flubber wrote:
Thank you Martin. Pointing out how the pieces fit together is very helpful. An annual test of the ASI over the entire range suggests that they have in the past found a few ASI that were not accurate over the entire range and that ASIs might go bad gradually over time. I wonder what those tests find in the field. (My question may sound like FUD, but I have a habit... people paid me to find defects in their technology.) Do I have it right that at a given point in time and altitude, the difference between TAS and IAS is constant over the entire range? I'm still figuring out how to use my LXNav V7 to check the calibration of my 15 year old Made in China ASI (and vice-versa, if I find a problem, either one of the devices could be the culprit.) On Friday, March 7, 2014 7:30:41 PM UTC-5, Martin Gregorie wrote: On Fri, 07 Mar 2014 13:45:53 -0800, son_of_flubber wrote: Is there any reason to suspect that an ASI (especially a non-TSO'ed one) might be more accurate in the middle of it's range? On this side of the pond the rules say that ASI calibration must be checked as part of the Annual Inspection and must be within 2kts throughout its range. The calibration check applies a known pressure to the pitot inlet and and records the ASI reading. This is done at 10 kt intervals from the ASI's highest indicated speed down to 20 kts. An ASI is a sensitive pressure gauge that happens to be calibrated in kts. The calibration is independent of the airframe. IOW, if the IAS in flight shown by a freshly calibrated ASI is not the same as independently measured TAS then the error is due to the placement of the static vent and/or the pitot. For example, its well known that pressure under the wing is above bulk atmospheric pressure, so a static vent anywhere near the underside of the wing will make the ASI read low. Similarly, a static vent placed on a convex bulge in the fuselage away from the wing will see a lower pressure and so will make the ASI read high. That's why the static vent is commonly halfway along the boom: at that point the boom is a straight taper and so has minimal curvature in relation to the slipstream while the vent is placed well away from the flying surfaces. -- martin@ | Martin Gregorie gregorie. | Essex, UK org |

Use of a simply made manometer should clarify the delemna of comparing 2 instruments, both of which have unlnown accuarcy.
UH

On Friday, March 7, 2014 6:30:41 PM UTC-6, Martin Gregorie wrote:
IOW, if the IAS in flight shown by a freshly calibrated ASI is not the same
as independently measured TAS then the error is due to the placement of the
static vent and/or the pitot.

This is true only at sea level, standard day conditions in the airspeed range we are interested in. At 5000 feet, IAS and TAS are NOT the same.

For example, its well known that pressure under the wing is above bulk
atmospheric pressure, so a static vent anywhere near the underside of the
wing will make the ASI read low.

Disagree with this. Look at the airspeed corrections on a Schempp-Hirth sailplane using the underwing statics. The Ventus A that Dick Johnson tested is a good example. Goes from reading 4 knots too slow at low speed to 10 knots to fast at high speed.

If you are curioius about the calibration of your indicator, the following should be of interest to you:

http://www.rst-engr.com/rst/articles/KP89JUL.pdf

Steve Leonard

An ASI is a sensitive pressure gauge that happens to be calibrated in kts.
The calibration is independent of the airframe.

Thank you Martin for this answer.
That is what I needed to know.

On Mon, 10 Mar 2014 06:26:26 -0700, unclhank wrote:

Use of a simply made manometer should clarify the delemna of comparing 2
instruments, both of which have unlnown accuarcy.
UH

Yep, my club uses a water-filled manometer with the vertical tube being
cut from a roll of soft plastic tube - most likely its the same tubing
you'd use to connect the ASI to static and pitot.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Mon, 10 Mar 2014 06:49:19 -0700, Steve Leonard wrote:

On Friday, March 7, 2014 6:30:41 PM UTC-6, Martin Gregorie wrote:
IOW, if the IAS in flight shown by a freshly calibrated ASI is not the
same
as independently measured TAS then the error is due to the placement
of the static vent and/or the pitot.

This is true only at sea level, standard day conditions in the airspeed
range we are interested in. At 5000 feet, IAS and TAS are NOT the same.

Yeah. Treat that as a rather bad short-hand. A fully qualified
explanation would be quite long by the time you factor in the different
IAS errors on a different type of glider flying with the one that's
trying to check his ASI. Most people don't own the same sort of trailing
bomb device that Dick Johnson used.

For example, its well known that pressure under the wing is above bulk
atmospheric pressure, so a static vent anywhere near the underside of
the wing will make the ASI read low.

Disagree with this. Look at the airspeed corrections on a Schempp-Hirth
sailplane using the underwing statics. The Ventus A that Dick Johnson
tested is a good example. Goes from reading 4 knots too slow at low
speed to 10 knots to fast at high speed.

The general effect of the wing is like I said, but it can obviously be
affected by factors specific to some airframes such as turbulence round
the root or kicked off by the nose shape. Most gliders have, at best,
quite rudimentary root fairings. I imagine Will Schueman would be quite
unimpressed if he saw them.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |