Whoa…I guess I never had to think of this before…
This is something I need to deal with soon…so…just thinking (typing) out loud…(helps me think) because…I have not been in the position to “certify” the results of the static/altimeter tests…every airplane I have flown has already been certified/tested so all I need to know when flying the airplane is…has this particular airplane been static checked within the preceding 3 years and is there a logbook entry?
With experimental airplanes here we (I) go…
Let’s say I get the static altimeter inspection/certification guy to swing by and test/certify my altimetry and transponder system(s).
He puts the suction cup-ey looking thingy(s) on the static port(s one the left side and one on the right side) of my airplane and then the testing device produces a 29.92 inches of mercury (InHg) pressure to the ports. If the altimeter is set for 29.92 the altimeter should show 0 feet. The transponder is always is set to 29.92 so the transponder should also indicate 0 feet.
Great…then the testing device reduces the pressure applied to the static ports to 28.856 InHg (standard pressure at 1000 feet MSL) and both the altimeter and transponder each should show 1000 feet MSL. And great they both do…so far so good…
Then…the testing device “sucks” down (I guess “up”) the scale every 1000 foot increment (27.821, 26.817, 25.842, 24.896, 23.978, 23.088, etc.) up to the reasonable altitude that the airplane could fly and with each increment of pressure reduction the altimeter and transponder each show a 1000 foot increase in altitude. Perfect…the testing/certification guy makes the appropriate airframe logbook entry and takes my cash and he goes away.
I am legal right?!?!?
Well…like people are pointing out…the static ports are sensitive to the installed location on the airplane…however how do we (I) know that the altitude indication is indeed accurate based on airspeed?
On a Piper Warrior, the static ports are installed on either side of the fuselage and if they are installed too far forward the propeller “blast” (not much really) can make the static ports sense more outside air pressure. If the static ports are installed too far aft near the concave of the fuselage then as the airplane increases or decreases in speed the “vacuum” created by the airflow rushing past the concave surface of the aft fuselage will change at the square of airspeed and create a slightly different static pressure reading dependent on airspeed.
The Piper Warrior is certified and it because of the limited airspeed/power available it therefore has a very limited envelope of airspeed verses static pressure “schedule” to deal with and the engineers have done a significate number of very accurate tests.
My question is what do we single (one of a kind) aircraft manufacturers do to “certify” that the installation location of our static port(s) and the indicated pressure at various airspeeds/altitudes is accurate?
The GPS altitude comparison is a no-brainer (it is the same thought process as flying past a known altitude object like an antenna).
Fly the airplane at 70 KIAS with the autopilot set to ALT HOLD mode at 5000 feet MSL and note the GPS indicated altitude (lets say the GPS altitude indication shows 5075 feet) and you then accelerate to 90 KIAS with the autopilot still set to ALT HOLD mode. The autopilot will correct the pitch to maintain a 5000 foot MSL pressure (INDICATED) cruise altitude and the altimeter still shows 5000 feet. Heck…if you are running with a Dynon autopilot /servo system it will also manipulate the trim system and trim the airplane for “hands off” as the speed increases.
When the airspeed stabilizes at 90 KIAS record the GPS altitude, then accelerate to 110 KIAS and record the GPS altitude, and repeat again and again until the airplane cannot fly any faster all while maintaining a constant ALT HOLD (sensed pressure altitude).
If the static ports are installed in a location that is not effected by airspeed (meaning the rush of airflow over the fuselage does not affect the pressure sensed by the static ports) then the GPS altitude at each airspeed step will still show 5075 feet MSL because the static source pressure is constant.
So…like…what do we (I) do if the GPS altitude shows something different than 5075 feet MSL during the different airspeed changes?
Slight changes such as 5000 to 5100 feet MSL are expected and no big deal but what is an acceptable limit???
What if the GPS altitude showed something way off like or 5500 feet?
How could this happen?
If the static ports were installed way back on the fuselage and then as the airspeed increases the air mass moving past the fuselage contours would diverge and that divergence of flow would create a lower pressure at the static port and cause the indicated pressure at the static ports to reduce and because the sensed pressure was lower than before the autopilot ALT HOLD mode would say “hey we are climbing because I see a reduction in pressure sensed by my static ports” and the autopilot would then correct more nose down…so as to maintain a level 5000 MSL PRESSURE (sensed) altitude…so then the GPS altitude show lower than the original 5000 feet.
Is there some settings/adjustments available in the Dynon magic that we can “tune” out the differences in static source changes using a “correction table”?
If this same test was done at 7000 feet indicated altitude with different airspeeds could the “correction table” be still accurate or is it based on each point of a alt/airspeed table?
How does one (me) “certify” that my static port location is “clean” and not affected by airspeed changes?
Any advice would be appreciated!!!
This is something I need to deal with soon…so…just thinking (typing) out loud…(helps me think) because…I have not been in the position to “certify” the results of the static/altimeter tests…every airplane I have flown has already been certified/tested so all I need to know when flying the airplane is…has this particular airplane been static checked within the preceding 3 years and is there a logbook entry?
With experimental airplanes here we (I) go…
Let’s say I get the static altimeter inspection/certification guy to swing by and test/certify my altimetry and transponder system(s).
He puts the suction cup-ey looking thingy(s) on the static port(s one the left side and one on the right side) of my airplane and then the testing device produces a 29.92 inches of mercury (InHg) pressure to the ports. If the altimeter is set for 29.92 the altimeter should show 0 feet. The transponder is always is set to 29.92 so the transponder should also indicate 0 feet.
Great…then the testing device reduces the pressure applied to the static ports to 28.856 InHg (standard pressure at 1000 feet MSL) and both the altimeter and transponder each should show 1000 feet MSL. And great they both do…so far so good…
Then…the testing device “sucks” down (I guess “up”) the scale every 1000 foot increment (27.821, 26.817, 25.842, 24.896, 23.978, 23.088, etc.) up to the reasonable altitude that the airplane could fly and with each increment of pressure reduction the altimeter and transponder each show a 1000 foot increase in altitude. Perfect…the testing/certification guy makes the appropriate airframe logbook entry and takes my cash and he goes away.
I am legal right?!?!?
Well…like people are pointing out…the static ports are sensitive to the installed location on the airplane…however how do we (I) know that the altitude indication is indeed accurate based on airspeed?
On a Piper Warrior, the static ports are installed on either side of the fuselage and if they are installed too far forward the propeller “blast” (not much really) can make the static ports sense more outside air pressure. If the static ports are installed too far aft near the concave of the fuselage then as the airplane increases or decreases in speed the “vacuum” created by the airflow rushing past the concave surface of the aft fuselage will change at the square of airspeed and create a slightly different static pressure reading dependent on airspeed.
The Piper Warrior is certified and it because of the limited airspeed/power available it therefore has a very limited envelope of airspeed verses static pressure “schedule” to deal with and the engineers have done a significate number of very accurate tests.
My question is what do we single (one of a kind) aircraft manufacturers do to “certify” that the installation location of our static port(s) and the indicated pressure at various airspeeds/altitudes is accurate?
The GPS altitude comparison is a no-brainer (it is the same thought process as flying past a known altitude object like an antenna).
Fly the airplane at 70 KIAS with the autopilot set to ALT HOLD mode at 5000 feet MSL and note the GPS indicated altitude (lets say the GPS altitude indication shows 5075 feet) and you then accelerate to 90 KIAS with the autopilot still set to ALT HOLD mode. The autopilot will correct the pitch to maintain a 5000 foot MSL pressure (INDICATED) cruise altitude and the altimeter still shows 5000 feet. Heck…if you are running with a Dynon autopilot /servo system it will also manipulate the trim system and trim the airplane for “hands off” as the speed increases.
When the airspeed stabilizes at 90 KIAS record the GPS altitude, then accelerate to 110 KIAS and record the GPS altitude, and repeat again and again until the airplane cannot fly any faster all while maintaining a constant ALT HOLD (sensed pressure altitude).
If the static ports are installed in a location that is not effected by airspeed (meaning the rush of airflow over the fuselage does not affect the pressure sensed by the static ports) then the GPS altitude at each airspeed step will still show 5075 feet MSL because the static source pressure is constant.
So…like…what do we (I) do if the GPS altitude shows something different than 5075 feet MSL during the different airspeed changes?
Slight changes such as 5000 to 5100 feet MSL are expected and no big deal but what is an acceptable limit???
What if the GPS altitude showed something way off like or 5500 feet?
How could this happen?
If the static ports were installed way back on the fuselage and then as the airspeed increases the air mass moving past the fuselage contours would diverge and that divergence of flow would create a lower pressure at the static port and cause the indicated pressure at the static ports to reduce and because the sensed pressure was lower than before the autopilot ALT HOLD mode would say “hey we are climbing because I see a reduction in pressure sensed by my static ports” and the autopilot would then correct more nose down…so as to maintain a level 5000 MSL PRESSURE (sensed) altitude…so then the GPS altitude show lower than the original 5000 feet.
Is there some settings/adjustments available in the Dynon magic that we can “tune” out the differences in static source changes using a “correction table”?
If this same test was done at 7000 feet indicated altitude with different airspeeds could the “correction table” be still accurate or is it based on each point of a alt/airspeed table?
How does one (me) “certify” that my static port location is “clean” and not affected by airspeed changes?
Any advice would be appreciated!!!
