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airspeed indicator calibration

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mdnanwelch7(at)hotmail.co
Guest

Posted: Sat Sep 18, 2010 2:41 pm Post subject: airspeed indicator calibration

Kolb guys,

 

  I am in the process of installing all the instruments in my MkIII.  One of the

instruments is an analog airspeed indicator.  It's a standard, typical type of indicator that uses

ram air to determine it's reading, by way of a pitot tube.

 

  Out of the box, I can't imagine it can be very accurate, can it?  I mean, everybody will have 

 their own unique design in installing one of these, so I would think that each installation would 

 be off a little from someone else's installation.  Or, are they much more uniform in their 

 readings?

 

  I have the exact placement in my panel and the routing of the tubing pretty much figured out.

 

  For those guys that have installed an airspeed indicator, did you calibrate it in any way first,

or did you just stick it in the plane and live with whatever reading it says once you start

flying the plane?

 

  I thought what I would do is essentially build the whole system, and get someone to drive 

a car down the road, with me hanging the A/S indicator's pitot tube out the window.  While this 

 may seem somewhat silly, at least I could an idea how accurate it might be.  At least it's a start, 

 right?  Has anybody done this??  Or is it a waste of time?   

  

   For an instrument that has so much riding on it, and if new, hasn't really proven itself for

 accuracy, it seems to me a guy ought to verify in some way it comes "close" to the correct

 airspeed.

  

   Any thoughts or experiences would be greatly appreciated.  Thanks

  

 Mike Welch

 MkIII

  

   

 

 

 		 	   		    [quote][b]

Dana

Joined: 13 Dec 2007
Posts: 1047
Location: Connecticut, USA

Posted: Sat Sep 18, 2010 4:02 pm Post subject: airspeed indicator calibration

The instrument itself should be quite accurate, but pressure 

variations at the pitot and static sources can throw it way off.  The 

pressure field around a moving car will make any such calibration 

meaningless, too.  Put it in the plane.  You can use your GPS to 

calibrate it on a dead calm day, or average readings into and with a 

steady wind.



-Dana



--------------------------------------------------

Mike Welch <mdnanwelch7(at)hotmail.com> wrote:

(09/18/2010 18:35)



 	  Quote: 	
		   

 Kolb guys,

  

   I am in the process of installing all the instruments in my MkIII.  One of the

 instruments is an analog airspeed indicator.  It's a standard, typical type of indicator that uses

 ram air to determine it's reading, by way of a pitot tube.

  

   Out of the box, I can't imagine it can be very accurate, can it?  I mean, everybody will have 

 their own unique design in installing one of these, so I would think that each installation would 

 be off a little from someone else's installation.  Or, are they much more uniform in their 

 readings?

  

   I have the exact placement in my panel and the routing of the tubing pretty much figured out.

  

   For those guys that have installed an airspeed indicator, did you calibrate it in any way first,

 or did you just stick it in the plane and live with whatever reading it says once you start

 flying the plane?

  

   I thought what I would do is essentially build the whole system, and get someone to drive 

 a car down the road, with me hanging the A/S indicator's pitot tube out the window.  While this 

 may seem somewhat silly, at least I could an idea how accurate it might be.  At least it's a start, 

 right?  Has anybody done this??  Or is it a waste of time?   

  

   For an instrument that has so much riding on it, and if new, hasn't really proven itself for

 accuracy, it seems to me a guy ought to verify in some way it comes "close" to the correct

 airspeed.

  

   Any thoughts or experiences would be greatly appreciated.  Thanks

  

 Mike Welch

 MkIII

Richard Pike

Joined: 09 Jan 2006
Posts: 1671
Location: Blountville, Tennessee

Posted: Sat Sep 18, 2010 6:25 pm Post subject: Re: airspeed indicator calibration

IMO, it is almost impossible to calibrate an airspeed indicator unless it is in the airplane, because of the variables involved in each installation. When we first flew the FSII, we had screwy airspeed readings, and it turned out we had two problems: one was with the indicator, the other was the pitot location. Fixed the location of the pitot, but the numbers were till odd, so tried an experiment. 



Strapped a piece of 1 1/2" aluminum tubing onto the roof rack of the Jeep so that the forward end was ahead of and well above the hood (no bow wave allowed) and ran a piece of vinyl tubing to the airspeed indicator. Wrote down the numbers at various speeds, then swapped out the airspeed indicator with another that was known to be good, did it again, and there was quite a disparity.



But all that did was prove that the original airspeed indicator was bad. Bought a new one and put it in the airplane and flew it, and compared it to the GPS. It was good enough that we left it alone.



Wonder if we hurt it in the wreck? Guess we'll find out in a couple months.



Richard Pike

MKIII N420P (420ldPoops)

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Sun Sep 19, 2010 3:10 am Post subject: Re: airspeed indicator calibration

Another way to test the ASI itself is with a manometer which you can build easily enough but Richard Pike's method was probably more fun.



On my Slingshot, the most accurate airspeed indication was achieved by having two static sources. One is the standard opening in the aft end of the static tube portion of the combo pitot/static tube. The static tube going from there to the back of the ASI also has an open Tee fitting in it to read the ambient pressure in the nose cone. The average of these two soruces give results that are equal to TAS at a density altitude of 1500', which is quite close enoughin my book.



On the Allegro 2000 SLSA I once part owned, I was able to adjust the indication by altering the static pressure perceived by the instrument. In this case there were two static ports, one on each side of the fuselage. From the factory it indicated as much as 13% faster than it should have, which meant that the static pressure was lower than it should have been. Since the static ports were flush with the fuselage, I put a spot of RTV just aft of the static ports creating a slightly higher pressure area at the ports. After a few trial and error modifications of the size of the RTV bump, I got the IAS to read within 2% throughout the normal speed range. 



Getting an accurate airspeed indication is sometimes a bit of an art but mostly just trial and error, one of the joys of experimenting.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

slyck(at)frontiernet.net
Guest

Posted: Sun Sep 19, 2010 5:24 am Post subject: airspeed indicator calibration

 	  Quote: 	
		   On your first ride in your new bird keep one eye on the ASI during takeoff.  The speed you see just

at the point you can coax it off the ground will suffice for an initial stall speed.
	


The actual numbers are not as important as stability and repeatability.  My pitot will sometimes get

a bug in it even though I plug it.  You can tell because the needle will stutter a bit and then suddenly start

indicating again.  I figure mine is off some but that is WAY down on my list of things to do.



It's like your age, knowing it won't change it. 

BB



 	  Quote:

olendorf

Joined: 06 Jul 2006
Posts: 140
Location: Schenectady, NY USA

Posted: Sun Sep 19, 2010 6:48 am Post subject: Re: airspeed indicator calibration

 	  Thom Riddle wrote: 	
		  .., the most accurate airspeed indication was achieved by having two static sources. One is the standard opening in the aft end of the static tube portion of the combo pitot/static tube. The static tube going from there to the back of the ASI also has an open Tee fitting in it to read the ambient pressure in the nose cone. The average of these two soruces give results that are equal to TAS at a density altitude of 1500', which is quite close enoughin my book. 	




That is exactly what I did also.  It happened to be very accurate.  When I take my doors off though it reads about 10mph too low.

_________________
Scott Olendorf
Original Firestar, Rotax 447, Powerfin prop
Schenectady, NY
http://sites.google.com/site/kolbfirestar/

John Hauck

Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

Posted: Sun Sep 19, 2010 7:13 am Post subject: airspeed indicator calibration

 	  Quote: 	
		   It's like your age, knowing it won't change it.

 BB


	




Bob B/Gang:



I like my ASI reading high.  Watching that needle indicating 90 when it is 

flying 60 is exhilarating.



I don't think anyone has mentioned during this ASI thread that is the ASI is 

serviceable, but not calibrated to the aircraft, no matter what the 

indicated airspeed is at stall, it will always stall at that indicated 

airspeed under the same conditions.



I once asked Steve Whitman at Sun and Fun 1993, how he calibrated his ASI, 

what static airpressure source he used.  His replay was, "Right out the back 

of the instrument."  Said he was not concerned with his true airspeed and 

the stall speed was always the same.



He did share with me that the cut the pitot tube at a 45 deg angle.  Thought 

that would give him a better indication at high angles of attack and slow 

airspeeds.



john h

mkIII

Rock House, OR

_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama

mdnanwelch7(at)hotmail.co
Guest

Posted: Sun Sep 19, 2010 7:45 am Post subject: airspeed indicator calibration

> no matter what the indicated airspeed is at stall, it will always stall at that indicated 

 	  Quote: 	
		   airspeed under the same conditions.

 john h

 mkIII

 Rock House, OR 


	


 John, guys,

  

   You're right, John, and this IS an important point.  When the A/S indicator is on it's

 very first test flight, one would likely want to establish it's exact reading while testing 

 an aircraft's stall speed.  We want the "indicated aispeed" (IAS), regardless of whether or 

 not the A/S indicator is accurate for all flight speeds.  If it read 35, or 62, or whatever...(at stall), 

 that's going to be that magic number to stay well above when shooting the first landing.  Yes?

  

   Having never "test" flown anything, I would think the very first thing a guy would do is 

 take the airplane to a safe altitude (2500'+ agl) and cafefully find the stall, or possibly the 

 beginning of the stall, and note what his A/S indicator is reading.  The FAA reg that Rick G. shared 

 with us seemed to be very good advice.  At least to me, I would think before a guy tried to land 

 his newly completed plane, he would want to know the indicated airspeed (IAS), and do his 

 darndest to stay above it.  I have heard from some that 1.5 X IAS (at stall) would be a good 

 choice in the early stages of flight testing.  

  

 Mike Welch

  

   

 		 	   		    [quote][b]

williamtsullivan(at)att.n
Guest

Posted: Sun Sep 19, 2010 7:45 am Post subject: Airspeed indicator calibration

  Just an opinion from an observer with no experience.  It doesn't matter if the indicator is exact, as long as it's consistant.  You will still have to determine take-off speed, stall speed, and safe landing speed based on the readings that the instrument is giving.  The indicator should be reliable, consistant, and reasonably quick to respond.  If it gives the same readings every time, it only applies to your plane, anyway.  Other than for navigation purposes, and comparison purposes, I don't think it matters.  Somebody correct me on this.  And, there is always a Hall meter if you want to check.  $20, I think.

 

                                             Bill Sullivan

                                             Windsor Locks, Ct

John Hauck

Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

Posted: Sun Sep 19, 2010 8:11 am Post subject: airspeed indicator calibration

Mike W/Gang:

  

 Yes, about as basic as you can get to keep on  flying.

  

 I said earlier, I periodically stall my mkIII, in  different configurations, to keep me informed of the current stall  speed.

  

 Homer Kolb recommended in the 1984 Ultrastar  Builders Guide to climb to 1,000 feet AGL on the first flight and check stall  speed.

  

 Stalling a Kolb is about as simple a maneuver as  one can perform in an airplane.  Bring the power back to idle, bleed off  airspeed until it stalls.  Probably all you will get is a little burble as  it reaches mush/stall speed and continue flying, either with a nug of the  forward stick, or nothing at all.  About the only way I can get my Kolbs to  do a classic drop the nose and fall out of the sky type stall is chop power,  pull the nose up as high as I can until the airspeed bleeds off to about zero  and the Kolb falls out of the sky.  Even then, a little forward stick  and she will fly immediately.

  

 I believe one of the problems with inadvertently  stalling a Kolb close to the ground is the conditioned reflex to pull the stick  back to make it fly, because pushing the nose down close to the ground is  unnatural.

  

 Enjoying some cooler weather and overcast sky for  the first time at the Rock House.

  

 john h

 mkIII

 [quote]     At least to me, I would think before    a guy tried to land 

his newly completed plane, he would want to know the    indicated airspeed (IAS), and do his 

darndest to stay above it.  I    have heard from some that 1.5 X IAS (at stall) would be a good 

choice in    the early stages of flight testing.  

 

Mike Welch

 

[b]

_________________
John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama

slyck(at)frontiernet.net
Guest

Posted: Sun Sep 19, 2010 8:40 am Post subject: airspeed indicator calibration

 	  Quote: 	
		  

There are two methods of take off:  the zoomie-zoomie type that RVs use on paved runways and



	
the coax it off that old taildraggers use on grass.  The zoomie guys like a (10%?) margin before they rotate like the

big commercial tin cans.   Due to circumstances the minimum/at stall method is more appropriate for

grass strips, especially if you have limited horsepower.

Not so fancy strips may be a little bumpy which is to your advantage.  That last bump will launch you into the

barely flying zone.  Now you can pick up a little speed in ground effect before resuming a climb.  

This is especially important when flying out of deep or wet snow.  -otherwise you may remain ground bound.

Also a great technique for escaping from that plowed field that you had to plop into when the noise maker stopped.

I hate those big rocks.

It's really quite harmless to stagger the bird off the ground.  Remember the earth is still only a few inches away.

Can't get hurt.

BB

[quote] 	  Quote: 	
		  



href="http://www.matronics.com/Navigator?Kolb-List">http://www.matronics.com/Navigator?Kolb-List

href="http://forums.matronics.com">http://forums.matronics.com

href="http://www.matronics.com/contribution">http://www.matronics.com/contribution



	


[b]

rrh(at)rrhoyt.com
Guest

Posted: Sun Sep 19, 2010 1:12 pm Post subject: airspeed indicator calibration

Here is a paper on calibrating Air speed. For initial flights the other 

comments on watching for the stall/takeoff are all you need.



Ron



On 9/18/2010 5:35 PM, Mike Welch wrote:

 	  Quote: 	
		  

 Kolb guys,



    I am in the process of installing all the instruments in my MkIII.  One of the

 instruments is an analog airspeed indicator.  It's a standard, typical type of indicator that uses

 ram air to determine it's reading, by way of a pitot tube.



    Out of the box, I can't imagine it can be very accurate, can it?  I mean, everybody will have

 their own unique design in installing one of these, so I would think that each installation would

 be off a little from someone else's installation.  Or, are they much more uniform in their

 readings?



    I have the exact placement in my panel and the routing of the tubing pretty much figured out.



    For those guys that have installed an airspeed indicator, did you calibrate it in any way first,

 or did you just stick it in the plane and live with whatever reading it says once you start

 flying the plane?



    I thought what I would do is essentially build the whole system, and get someone to drive

 a car down the road, with me hanging the A/S indicator's pitot tube out the window.  While this

 may seem somewhat silly, at least I could an idea how accurate it might be.  At least it's a start,

 right?  Has anybody done this??  Or is it a waste of time?



    For an instrument that has so much riding on it, and if new, hasn't really proven itself for

 accuracy, it seems to me a guy ought to verify in some way it comes "close" to the correct

 airspeed.



    Any thoughts or experiences would be greatly appreciated.  Thanks



 Mike Welch

 MkIII

TrueAirSpeedUsingGPS-DGrayMethod.doc

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Richard Pike

Joined: 09 Jan 2006
Posts: 1671
Location: Blountville, Tennessee

Posted: Sun Sep 19, 2010 5:39 pm Post subject: Re: airspeed indicator calibration

 	  rrh(at)rrhoyt.com wrote: 	
		  Here is a paper on calibrating Air speed. For initial flights the other 

comments on watching for the stall/takeoff are all you need.



Ron  		 	   		



	




Great paper. It had a url for a downloadable Excel spread sheet but the link was broken. By any chance do you have a copy of that spread sheet? And if so, can you make it available? That would be a great resource.



Richard Pike

MKIII N420P (420ldPoops)

George Alexander

Joined: 10 Jan 2006
Posts: 245
Location: SW Florida

Posted: Mon Sep 20, 2010 3:10 am Post subject: Re: airspeed indicator calibration

 	  Richard Pike wrote: 	
		   	  rrh(at)rrhoyt.com wrote: 	
		  Here is a paper on calibrating Air speed. For initial flights the other 

comments on watching for the stall/takeoff are all you need.



Ron  		 	   		



	




Great paper. It had a url for a downloadable Excel spread sheet but the link was broken. By any chance do you have a copy of that spread sheet? And if so, can you make it available? That would be a great resource.



Richard Pike

MKIII N420P (420ldPoops) 	




Richard et al:



See if this has what you are looking for..... seems they moved it around.



http://www.ntps.edu/information/downloads

_________________
George Alexander
FS II R503
E-LSA N709FS
http://www.oh2fly.net

rrh(at)rrhoyt.com
Guest

Posted: Mon Sep 20, 2010 11:51 am Post subject: airspeed indicator calibration

I think this is the spread sheet for the airspeed calculator.

Ron

On 9/19/2010 8:39 PM, Richard Pike wrote:

 	  Quote: 	

 rrh(at)rrhoyt.com wrote:

> Here is a paper on calibrating Air speed. For initial flights the other

> comments on watching for the stall/takeoff are all you need.

>

> Ron  		 	   		

 Great paper. It had a url for a downloadable Excel spread sheet but the link was broken. By any chance do you have a copy of that spread sheet? And if so, can you make it available? That would be a great resource.

 Richard Pike

 MKIII N420P (420ldPoops)

 Read this topic online here:

 http://forums.matronics.com/viewtopic.php?p=312985#312985

TrueAirSpeed-GPS_PEC.xls

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