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bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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 Posted: Thu Apr 15, 2010 2:09 pm Post subject: Accurate Altitude and Airspeed |
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4/15/2010
Hello Mike, You wrote: "Not sure why this (altitude variation with airspeed)
would be, or more importantly, how to fix it."
Hello Bernie, You wrote: "There didn't seem to be any theory behind it."
This is not a very rare or mysterious phenomena. Inaccurate airspeed
indications can be caused by inaccurate dynamic and static air pressure
forces. Inaccurate altitude indications can be caused by inaccurate static
pressure forces.
AIRSPEED. Let's talk about inaccurate airspeed measurements first. An
airspeed indicator is a balancing mechanism. It balances the difference
between a force created by dynamic air pressure and a force created by
static air pressure. The force from the dynamic air pressure is the result
of the forward movement of the airplane. The faster the airplane moves the
greater the force exerted.
In order to get an accurate measure of that dynamic force one needs to
accurately sense the free stream dynamic air pressure and send it to the
airspeed indicator via leak and kink free tubing. That means sensing the air
movement in the actual direction that the airplane is moving and having that
air movement not affected by some local air flow direction change caused by
the airframe itself. This is why you sometimes see flight test airplanes
with a long boom sticking out forward with a small vane mechanism on the
front of it. This boom and vane mechanism, along with connecting tubing, is
a pitot tube system intended to accurately measure the force from the free
stream dynamic air pressure without any inaccuracies introduced by local
airframe air flow. See Note One below.
So much for the dynamic force side of the airspeed indicator balancing act,
what about the static force side? An accurate static force is provided by a
static port ideally located somewhere on the airframe such that it is
measuring the true static air pressure. But finding that ideal location and
making the perfect static port that does not introduce static air pressure
errors is not always so quick and easy. And what do we do if we discover
that the static port that we have installed is not producing accurate static
air pressure? See Note Two below.
ALTITUDE. An altimeter is also a balancing mechanism. It measures the
difference between the initial altitude setting of the altimeter mechanism
as compared to the static air pressure encountered by that same mechanism
while in flight and displays that difference in some lineal measurement
(usually in feet in our part of the world). If the static pressure provided
to the altimeter via the static port and the connecting tubing is in error
or changes with the airplane's airspeed, when the actual altitude is
constant, then the altimeter's altitude indication will be in error. And
what do we do if we discover that the static port that we have installed is
not producing accurate static air pressure? See Note Two below.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
Note One: Why don't we use these long boom and vane type pitot tubes on our
experimental amateur built aircraft? Because the boom and vane would take a
terrific beating from people walking into them on the ground and because the
dynamic force errors introduced by the type of pitot tubes that we commonly
use are just not great enough to cause us concern. There is not very much we
can do about adjusting the force coming from dynamic air pressure other than
using a pitot tube located a sufficient distance from a disturbing piece of
airframe (usually the bottom surface of a wing) and avoiding a leak or kink
in the tubing going from the pitot tube to the airspeed indicator.
Note Two: So we have built our airplane, installed our static port(s), and
discovered that we are getting inaccurate airspeed and / or altitude
indications and decide to do something about it. We could just go on
installing new static ports in different locations, but that is a lot of
work and we are not assured of better results. So we should do just what the
big boy aerodynamic types do, we fudge or bandaid as needed to get the air
to give us the results that we want.
Let's say that the airspeed indicator is reading too high -- it says the
airplane is going faster than it really is. (See Note Three). The dynamic
air pressure side of the airspeed indicator is providing too much force in
the desired balancing act. How can we counteract that excess dynamic force?
We increase the static force being fed to the airspeed instrument by the
static port by installing a small wedge just aft of the opening on the
static port (thin edge of the wedge facing forward towards the hole). This
small wedge causes air to pile up and increase the static air pressure going
to the airspeed indicator (and also to the altimeter unless you have
provided separate static ports for the two instruments) and give us the
accurate force balance measurement that we want.
Let's say that the altimeter reading goes down 200 feet when you speed up 60
miles per hour (Mike, you did not say which direction your altitude was
changing with the changes in airspeed). This means that the static port is
feeding greater than static pressure as your airplane flies faster. How can
we reduce that undesired increase in static air pressure? We install a small
wedge in front of the hole in the static port (thin edge of the wedge facing
aft towards the hole) to shield it a bit from dynamic air pressure coming
from the front in order to get a more stable and accurate altitude reading.
How do we get the right size wedge facing the correct direction to get the
airspeed and altitude results that we want? The same way the big boys do:
TRIAL AND ERROR and MORE TRIAL AND ERROR. Because our airplanes are
experimental, amateur built we are permitted to do just that -- experiment.
Note Three: So now we know how to tweak our static port(s) to give us
accurate airspeed and altitude information, but how do we know that the
airspeed and altitude information is inaccurate in the first place and
during our trial and error efforts how do we know when we have achieved the
accuracy that we are seeking? The answer to those two questions is not as
simple as one might first expect. I won't attempt to answer them here
because the answers are too big and complex to answer in this forum. What I
will suggest is that the reader google "accurate airspeed" and "accurate
altitude" and delve into those subjects to the level desired. Here is just
one source you will find:
http://gpsinformation.net/main/altitude.htm
Note Four: This is a personal view point. There are several methods
available for determining accurate true airspeed. Some rather elaborate --
some use GPS. Just google "accurate airspeed using GPS".
One thing that I've never quite understood regarding these methods is the
focus on precisely determining airspeed accuracy indication in the cruising
airspeed range. If I determine that my airspeed indicator shows 150 knots
indicated airspeed and I determine through some elaborate scheme that I am
actually only going 145 knots through the air what do I do with that
information? Being 5 nautical miles short of my destination after a one hour
flight is a trivial naviagation error contribution compared to all the other
error sources (such as heading, wind, and climb airspeed) that I have to
contend with and should overcome anyway by some means of real time enroute
navigation.
I think that if I were going to invest a lot of time and effort in
determining my exact airspeed error I would be inclined to do that
determination in the approach airspeed arena, not the cruising airspeed
arena. And even then I would not be obsessed with absolute airspeed
accuracy, I'd just want to know what number on the indicator gives me the
right kind of safe approach and landing time after time.
=================================================
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Sam
Joined: 18 May 2008
Posts: 135
|
| Posted: Sun Apr 18, 2010 6:44 am Post subject: Accurate Altitude and Airspeed |
|
|
This is great information about the static ports. I recently switched from steam gauges to a Dynon FlightDek. One of the neatest features is the winds aloft display. Unfortunately, I figured out that it was pretty inaccurate.
One day, I went up and flew four headings and this is what got:
Compass Wind Direction Wind Speed
360 242 20
270 334 18
180 251 31
090 271 08
This told me the display was pretty much worthless. I posted s couple of notes about it on the Dynon factory forum and was told to look after my static ports, but no indication how to go about it.
I will definitely be investigating the wedges at the static ports.
Thanks.
Sam Hoskins
Murphysboro, IL
On Thu, Apr 15, 2010 at 5:06 PM, <bakerocb(at)cox.net (bakerocb(at)cox.net)> wrote:
[quote] --> AeroElectric-List message posted by: <bakerocb(at)cox.net (bakerocb(at)cox.net)>
4/15/2010
Hello Mike, You wrote: "Not sure why this (altitude variation with airspeed)
would be, or more importantly, how to fix it."
Hello Bernie, You wrote: "There didn't seem to be any theory behind it."
This is not a very rare or mysterious phenomena. Inaccurate airspeed
indications can be caused by inaccurate dynamic and static air pressure
forces. Inaccurate altitude indications can be caused by inaccurate static
pressure forces.
AIRSPEED. Let's talk about inaccurate airspeed measurements first. An
airspeed indicator is a balancing mechanism. It balances the difference
between a force created by dynamic air pressure and a force created by
static air pressure. The force from the dynamic air pressure is the result
of the forward movement of the airplane. The faster the airplane moves the
greater the force exerted.
In order to get an accurate measure of that dynamic force one needs to
accurately sense the free stream dynamic air pressure and send it to the
airspeed indicator via leak and kink free tubing. That means sensing the air
movement in the actual direction that the airplane is moving and having that
air movement not affected by some local air flow direction change caused by
the airframe itself. This is why you sometimes see flight test airplanes
with a long boom sticking out forward with a small vane mechanism on the
front of it. This boom and vane mechanism, along with connecting tubing, is
a pitot tube system intended to accurately measure the force from the free
stream dynamic air pressure without any inaccuracies introduced by local
airframe air flow. See Note One below.
So much for the dynamic force side of the airspeed indicator balancing act,
what about the static force side? An accurate static force is provided by a
static port ideally located somewhere on the airframe such that it is
measuring the true static air pressure. But finding that ideal location and
making the perfect static port that does not introduce static air pressure
errors is not always so quick and easy. And what do we do if we discover
that the static port that we have installed is not producing accurate static
air pressure? See Note Two below.
ALTITUDE. An altimeter is also a balancing mechanism. It measures the
difference between the initial altitude setting of the altimeter mechanism
as compared to the static air pressure encountered by that same mechanism
while in flight and displays that difference in some lineal measurement
(usually in feet in our part of the world). If the static pressure provided
to the altimeter via the static port and the connecting tubing is in error
or changes with the airplane's airspeed, when the actual altitude is
constant, then the altimeter's altitude indication will be in error. And
what do we do if we discover that the static port that we have installed is
not producing accurate static air pressure? See Note Two below.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
Note One: Why don't we use these long boom and vane type pitot tubes on our
experimental amateur built aircraft? Because the boom and vane would take a
terrific beating from people walking into them on the ground and because the
dynamic force errors introduced by the type of pitot tubes that we commonly
use are just not great enough to cause us concern. There is not very much we
can do about adjusting the force coming from dynamic air pressure other than
using a pitot tube located a sufficient distance from a disturbing piece of
airframe (usually the bottom surface of a wing) and avoiding a leak or kink
in the tubing going from the pitot tube to the airspeed indicator.
Note Two: So we have built our airplane, installed our static port(s), and
discovered that we are getting inaccurate airspeed and / or altitude
indications and decide to do something about it. We could just go on
installing new static ports in different locations, but that is a lot of
work and we are not assured of better results. So we should do just what the
big boy aerodynamic types do, we fudge or bandaid as needed to get the air
to give us the results that we want.
Let's say that the airspeed indicator is reading too high -- it says the
airplane is going faster than it really is. (See Note Three). The dynamic
air pressure side of the airspeed indicator is providing too much force in
the desired balancing act. How can we counteract that excess dynamic force?
We increase the static force being fed to the airspeed instrument by the
static port by installing a small wedge just aft of the opening on the
static port (thin edge of the wedge facing forward towards the hole). This
small wedge causes air to pile up and increase the static air pressure going
to the airspeed indicator (and also to the altimeter unless you have
provided separate static ports for the two instruments) and give us the
accurate force balance measurement that we want.
Let's say that the altimeter reading goes down 200 feet when you speed up 60
miles per hour (Mike, you did not say which direction your altitude was
changing with the changes in airspeed). This means that the static port is
feeding greater than static pressure as your airplane flies faster. How can
we reduce that undesired increase in static air pressure? We install a small
wedge in front of the hole in the static port (thin edge of the wedge facing
aft towards the hole) to shield it a bit from dynamic air pressure coming
from the front in order to get a more stable and accurate altitude reading.
How do we get the right size wedge facing the correct direction to get the
airspeed and altitude results that we want? The same way the big boys do:
TRIAL AND ERROR and MORE TRIAL AND ERROR. Because our airplanes are
experimental, amateur built we are permitted to do just that -- experiment.
Note Three: So now we know how to tweak our static port(s) to give us
accurate airspeed and altitude information, but how do we know that the
airspeed and altitude information is inaccurate in the first place and
during our trial and error efforts how do we know when we have achieved the
accuracy that we are seeking? The answer to those two questions is not as
simple as one might first expect. I won't attempt to answer them here
because the answers are too big and complex to answer in this forum. What I
will suggest is that the reader google "accurate airspeed" and "accurate
altitude" and delve into those subjects to the level desired. Here is just
one source you will find:
http://gpsinformation.net/main/altitude.htm
Note Four: This is a personal view point. There are several methods
available for determining accurate true airspeed. Some rather elaborate --
some use GPS. Just google "accurate airspeed using GPS".
One thing that I've never quite understood regarding these methods is the
focus on precisely determining airspeed accuracy indication in the cruising
airspeed range. If I determine that my airspeed indicator shows 150 knots
indicated airspeed and I determine through some elaborate scheme that I am
actually only going 145 knots through the air what do I do with that
information? Being 5 nautical miles short of my destination after a one hour
flight is a trivial naviagation error contribution compared to all the other
error sources (such as heading, wind, and climb airspeed) that I have to
contend with and should overcome anyway by some means of real time enroute
navigation.
I think that if I were going to invest a lot of time and effort in
determining my exact airspeed error I would be inclined to do that
determination in the approach airspeed arena, not the cruising airspeed
arena. And even then I would not be obsessed with absolute airspeed
accuracy, I'd just want to know what number on the indicator gives me the
right kind of safe approach and landing time after time.
========================
---
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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_________________
Sam Hoskins
www.samhoskins.blogspot.com |
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floyd wilkes
Joined: 11 Jun 2007
Posts: 39
Location: spring branch, tx
|
| Posted: Sun Apr 18, 2010 7:06 am Post subject: Accurate Altitude and Airspeed |
|
|
Sam,
In order for the Dynon to get the correct wind, it must have a accurate indicated airspeed.
I do not know what you are flying, but the Zenith 601XL with standard pitot static setup is very inaccurate at cruise speeds.
Check what your indicated vs calibrated speeds are and If possible adjust to get the indicated very close to calibrated at your cruise speed.
Floyd Wilkes
601XL
From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Sam Hoskins
Sent: Sunday, April 18, 2010 9:43 AM
To: aeroelectric-list
Subject: Re: Accurate Altitude and Airspeed
This is great information about the static ports. I recently switched from steam gauges to a Dynon FlightDek. One of the neatest features is the winds aloft display. Unfortunately, I figured out that it was pretty inaccurate.
One day, I went up and flew four headings and this is what got:
Compass Wind Direction Wind Speed
360 242 20
270 334 18
180 251 31
090 271 08
This told me the display was pretty much worthless. I posted s couple of notes about it on the Dynon factory forum and was told to look after my static ports, but no indication how to go about it.
I will definitely be investigating the wedges at the static ports.
Thanks.
Sam Hoskins
Murphysboro, IL
On Thu, Apr 15, 2010 at 5:06 PM, <bakerocb(at)cox.net (bakerocb(at)cox.net)> wrote:
--> AeroElectric-List message posted by: <bakerocb(at)cox.net (bakerocb(at)cox.net)>
4/15/2010
Hello Mike, You wrote: "Not sure why this (altitude variation with airspeed)
would be, or more importantly, how to fix it."
Hello Bernie, You wrote: "There didn't seem to be any theory behind it."
This is not a very rare or mysterious phenomena. Inaccurate airspeed
indications can be caused by inaccurate dynamic and static air pressure
forces. Inaccurate altitude indications can be caused by inaccurate static
pressure forces.
AIRSPEED. Let's talk about inaccurate airspeed measurements first. An
airspeed indicator is a balancing mechanism. It balances the difference
between a force created by dynamic air pressure and a force created by
static air pressure. The force from the dynamic air pressure is the result
of the forward movement of the airplane. The faster the airplane moves the
greater the force exerted.
In order to get an accurate measure of that dynamic force one needs to
accurately sense the free stream dynamic air pressure and send it to the
airspeed indicator via leak and kink free tubing. That means sensing the air
movement in the actual direction that the airplane is moving and having that
air movement not affected by some local air flow direction change caused by
the airframe itself. This is why you sometimes see flight test airplanes
with a long boom sticking out forward with a small vane mechanism on the
front of it. This boom and vane mechanism, along with connecting tubing, is
a pitot tube system intended to accurately measure the force from the free
stream dynamic air pressure without any inaccuracies introduced by local
airframe air flow. See Note One below.
So much for the dynamic force side of the airspeed indicator balancing act,
what about the static force side? An accurate static force is provided by a
static port ideally located somewhere on the airframe such that it is
measuring the true static air pressure. But finding that ideal location and
making the perfect static port that does not introduce static air pressure
errors is not always so quick and easy. And what do we do if we discover
that the static port that we have installed is not producing accurate static
air pressure? See Note Two below.
ALTITUDE. An altimeter is also a balancing mechanism. It measures the
difference between the initial altitude setting of the altimeter mechanism
as compared to the static air pressure encountered by that same mechanism
while in flight and displays that difference in some lineal measurement
(usually in feet in our part of the world). If the static pressure provided
to the altimeter via the static port and the connecting tubing is in error
or changes with the airplane's airspeed, when the actual altitude is
constant, then the altimeter's altitude indication will be in error. And
what do we do if we discover that the static port that we have installed is
not producing accurate static air pressure? See Note Two below.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
Note One: Why don't we use these long boom and vane type pitot tubes on our
experimental amateur built aircraft? Because the boom and vane would take a
terrific beating from people walking into them on the ground and because the
dynamic force errors introduced by the type of pitot tubes that we commonly
use are just not great enough to cause us concern. There is not very much we
can do about adjusting the force coming from dynamic air pressure other than
using a pitot tube located a sufficient distance from a disturbing piece of
airframe (usually the bottom surface of a wing) and avoiding a leak or kink
in the tubing going from the pitot tube to the airspeed indicator.
Note Two: So we have built our airplane, installed our static port(s), and
discovered that we are getting inaccurate airspeed and / or altitude
indications and decide to do something about it. We could just go on
installing new static ports in different locations, but that is a lot of
work and we are not assured of better results. So we should do just what the
big boy aerodynamic types do, we fudge or bandaid as needed to get the air
to give us the results that we want.
Let's say that the airspeed indicator is reading too high -- it says the
airplane is going faster than it really is. (See Note Three). The dynamic
air pressure side of the airspeed indicator is providing too much force in
the desired balancing act. How can we counteract that excess dynamic force?
We increase the static force being fed to the airspeed instrument by the
static port by installing a small wedge just aft of the opening on the
static port (thin edge of the wedge facing forward towards the hole). This
small wedge causes air to pile up and increase the static air pressure going
to the airspeed indicator (and also to the altimeter unless you have
provided separate static ports for the two instruments) and give us the
accurate force balance measurement that we want.
Let's say that the altimeter reading goes down 200 feet when you speed up 60
miles per hour (Mike, you did not say which direction your altitude was
changing with the changes in airspeed). This means that the static port is
feeding greater than static pressure as your airplane flies faster. How can
we reduce that undesired increase in static air pressure? We install a small
wedge in front of the hole in the static port (thin edge of the wedge facing
aft towards the hole) to shield it a bit from dynamic air pressure coming
from the front in order to get a more stable and accurate altitude reading.
How do we get the right size wedge facing the correct direction to get the
airspeed and altitude results that we want? The same way the big boys do:
TRIAL AND ERROR and MORE TRIAL AND ERROR. Because our airplanes are
experimental, amateur built we are permitted to do just that -- experiment.
Note Three: So now we know how to tweak our static port(s) to give us
accurate airspeed and altitude information, but how do we know that the
airspeed and altitude information is inaccurate in the first place and
during our trial and error efforts how do we know when we have achieved the
accuracy that we are seeking? The answer to those two questions is not as
simple as one might first expect. I won't attempt to answer them here
because the answers are too big and complex to answer in this forum. What I
will suggest is that the reader google "accurate airspeed" and "accurate
altitude" and delve into those subjects to the level desired. Here is just
one source you will find:
http://gpsinformation.net/main/altitude.htm
Note Four: This is a personal view point. There are several methods
available for determining accurate true airspeed. Some rather elaborate --
some use GPS. Just google "accurate airspeed using GPS".
One thing that I've never quite understood regarding these methods is the
focus on precisely determining airspeed accuracy indication in the cruising
airspeed range. If I determine that my airspeed indicator shows 150 knots
indicated airspeed and I determine through some elaborate scheme that I am
actually only going 145 knots through the air what do I do with that
information? Being 5 nautical miles short of my destination after a one hour
flight is a trivial naviagation error contribution compared to all the other
error sources (such as heading, wind, and climb airspeed) that I have to
contend with and should overcome anyway by some means of real time enroute
navigation.
I think that if I were going to invest a lot of time and effort in
determining my exact airspeed error I would be inclined to do that
determination in the approach airspeed arena, not the cruising airspeed
arena. And even then I would not be obsessed with absolute airspeed
accuracy, I'd just want to know what number on the indicator gives me the
right kind of safe approach and landing time after time.
========================
---
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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_________________
Floyd Wilkes |
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khorton01(at)rogers.com
Guest
|
| Posted: Sun Apr 18, 2010 7:43 am Post subject: Accurate Altitude and Airspeed |
|
|
On 2010-04-15, at 18:06 , <bakerocb(at)cox.net> <bakerocb(at)cox.net> wrote:
| Quote: | One thing that I've never quite understood regarding these methods is the
focus on precisely determining airspeed accuracy indication in the cruising
airspeed range. If I determine that my airspeed indicator shows 150 knots
indicated airspeed and I determine through some elaborate scheme that I am
actually only going 145 knots through the air what do I do with that
information? Being 5 nautical miles short of my destination after a one hour
flight is a trivial naviagation error contribution compared to all the other
error sources (such as heading, wind, and climb airspeed) that I have to
contend with and should overcome anyway by some means of real time enroute
navigation.
I think that if I were going to invest a lot of time and effort in
determining my exact airspeed error I would be inclined to do that
determination in the approach airspeed arena, not the cruising airspeed
arena. And even then I would not be obsessed with absolute airspeed
accuracy, I'd just want to know what number on the indicator gives me the
right kind of safe approach and landing time after time.
|
As you note, an accurate airspeed indication is not really that important, looked at in isolation. Even if there is a significant error at approach speeds, it is not so important as long as the error is the same on every flight. Determining what IAS our aircraft needs for a safe, effective approach and landing is one of the many things we will do during the flight test phase.
But, with modern avionics, many EFIS systems calculate TAS based on IAS, altitude and temperature, and use that in conjunction with GPS track and groundspeed to calculate wind. They then put a nice little wind vector on the display, and it bugs the heck out of folks if the wind info is wrong.
The most important aspect of static system errors is not the effect on IAS. The important aspect is the effect on the altimeter accuracy. It is quite possible to have static system position errors that will give over 100 ft error in the altimeter at cruise airspeeds. If we are flying at our VFR cruising altitude, and there is IFR traffic that is supposed to be 500 ft above or below us, errors in our static system can become significant. If we have 100 ft of static system error, plus each aircraft has 50 ft of altimeter instrument error, plus perhaps we are not on quite the right altimeter setting because we aren't using flight following, and each pilot is 50 ft high or low from his nominal indicated altitude, these errors may add up. It is quite conceivable that aircraft that are supposed to be 500 ft apart may turn out to be a have less than 200 ft of altitude separation. If we are flying IFR approaches in IMC, the effect of static system position error on our altimeter is important to know.
--
Kevin Horton
RV-8
Ottawa, Canada
http://www.kilohotel.com/rv8
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bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
|
| Posted: Mon Apr 19, 2010 3:13 am Post subject: Accurate Altitude and Airspeed |
|
|
4/19/2010
Hello Sam Hoskins, Thanks for your kind words.
You wrote: "I will definitely be investigating the wedges at the static
ports."
You will have lots of fun doing that. Attached are pictures of my two static
ports, one left and
one right on the forward sides of my KIS TR-1 fuselage.
One has a wedge on it the other one does not. I went through a long series
of trial and error alternately blocking off ports individually and wedging
them in various ways until I became satisfied with my airspeed indication in
the 70 KIAS arena.
The IAS may not be absolutely correct, but I know that when I fly a landing
approach at 75 KIAS and a touch of engine power above idle I should be able
to make a decent flare and touchdown (if the gods are smiling).
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
===============================================
Time: 07:44:15 AM PST US
Subject: Re: Accurate Altitude and Airspeed
From: Sam Hoskins <sam.hoskins(at)gmail.com>
This is great information about the static ports. I recently switched from
steam gauges to a Dynon FlightDek. One of the neatest features is the winds
aloft display. Unfortunately, I figured out that it was pretty inaccurate.
One day, I went up and flew four headings and this is what got:
Compass Wind Direction Wind Speed
360 242 20
270 334 18
180 251 31
090 271 08
This told me the display was pretty much worthless. I posted s couple of
notes about it on the Dynon factory forum and was told to look after my
static ports, but no indication how to go about it.
I will definitely be investigating the wedges at the static ports.
Thanks.
Sam Hoskins
Murphysboro, IL
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bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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| Posted: Mon Apr 19, 2010 3:41 am Post subject: Accurate Altitude and Airspeed |
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4/19/2010
Hello Kevin Horton, You wrote: "The important aspect is the effect on the
altimeter accuracy."
Thank you for your most pertinent observation.
I wrote in my original post: "...but how do we know that the airspeed and
altitude information is inaccurate in the first place and during our trial
and error efforts how do we know when we have achieved the accuracy that we
are seeking? The answer to those two questions is not as
simple as one might first expect. I won't attempt to answer them here
because the answers are too big and complex to answer in this forum. What I
will suggest is that the reader google "accurate airspeed" and "accurate
altitude" and delve into those subjects to the level desired."
One of the web sites that I had specifically in mind when I wrote that was
your superb treatment of the subject at this web page created by you.
http://www.kilohotel.com/rv8/rvlinks/ssec.html
You most modestly did not mention it, but I would like to draw the reader's
attention to it and thank you for your efforts on our behalf.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
=============================================================
Time: 08:43:36 AM PST US
Subject: Re: Accurate Altitude and Airspeed
From: Kevin Horton <khorton01(at)rogers.com>
On 2010-04-15, at 18:06 , <bakerocb(at)cox.net> <bakerocb(at)cox.net> wrote:
| Quote: | One thing that I've never quite understood regarding these methods is the
focus on precisely determining airspeed accuracy indication in the
cruising
airspeed range. If I determine that my airspeed indicator shows 150 knots
indicated airspeed and I determine through some elaborate scheme that I am
actually only going 145 knots through the air what do I do with that
information? Being 5 nautical miles short of my destination after a one
hour
flight is a trivial naviagation error contribution compared to all the
other
error sources (such as heading, wind, and climb airspeed) that I have to
contend with and should overcome anyway by some means of real time
enroute
navigation.
I think that if I were going to invest a lot of time and effort in
determining my exact airspeed error I would be inclined to do that
determination in the approach airspeed arena, not the cruising airspeed
arena. And even then I would not be obsessed with absolute airspeed
accuracy, I'd just want to know what number on the indicator gives me the
right kind of safe approach and landing time after time.
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As you note, an accurate airspeed indication is not really that important,
looked
at in isolation. Even if there is a significant error at approach speeds,
it is not so important as long as the error is the same on every flight.
Determining
what IAS our aircraft needs for a safe, effective approach and landing
is one of the many things we will do during the flight test phase.
But, with modern avionics, many EFIS systems calculate TAS based on IAS,
altitude
and temperature, and use that in conjunction with GPS track and groundspeed
to calculate wind. They then put a nice little wind vector on the display,
and
it bugs the heck out of folks if the wind info is wrong.
The most important aspect of static system errors is not the effect on IAS.
The
important aspect is the effect on the altimeter accuracy. It is quite
possible
to have static system position errors that will give over 100 ft error in
the altimeter at cruise airspeeds. If we are flying at our VFR cruising
altitude,
and there is IFR traffic that is supposed to be 500 ft above or below us,
errors in our static system can become significant. If we have 100 ft of
static
system error, plus each aircraft has 50 ft of altimeter instrument error,
plus perhaps we are not on quite the right altimeter setting because we
aren't
using flight following, and each pilot is 50 ft high or low from his nominal
indicated altitude, these errors may add up. It is quite conceivable that
aircraft
that are supposed to be 500 ft apart may turn out to be a have less than
200 ft of altitude separation. If we are flying IFR approaches in IMC, the
effect of static system position error on our altimeter is important to
know.
--
Kevin Horton
RV-8
Ottawa, Canada
http://www.kilohotel.com/rv8
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kuffel(at)cyberport.net
Guest
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| Posted: Wed Apr 21, 2010 1:28 pm Post subject: Accurate Altitude and Airspeed |
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Sam,
<< Dynon FlightDek ... winds aloft display ... pretty inaccurate .. look after my static ports ... investigating the wedges at the static ports. >>
If you wish, I have an Excel spreadsheet which will help you measure the accuracy of your pitot/static system. For various indicated airspeeds and pressure altitudes (set your altimeter to 29.92) you fly an approximate box course noting the GPS track and groundspeed. (Also note temperature for later use.) Enter the 4 data points and the spreadsheet calculates 4 true airspeed and wind values. The 4th, redundant, datapoint allows an internal quality check since the 4 calculated air and wind speeds should be very close to each other. Discard any measurement which isn't.
Knowing true airspeed, pressure altitude and temperature do a "reverse" E-6B calculation to determine what your indicated airspeed *should* have been. Graphs of errors for constant indicated airspeeds vs altitudes and constant altitudes vs airspeeds will give you an excellent view of system/instrument errors. Not only can you determine even subtle effects of any changes to your pitot/static system but when you are done you have accurate graphs of true vs indicated airspeeds.
This is an exercise everyone should do as part of any homebuilt test period.
Email me direct off-list for a copy of the spreadsheet: kuffel(at)cyberport.net (kuffel(at)cyberport.net)
Tom Kuffel
EAA Flight Advisor
[quote][b]
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