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  #31  
Old 02-16-2019, 12:18 PM
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Hartstoc Hartstoc is offline
 
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Originally Posted by lr172 View Post
You'll have to explain this one for me. An experiment to prove a counter theory to yours. Take a 2x4 and drill a hole in the middle and bolt to wall. Then set it at 5* from level. Take various measurements with a level across the 2x4 and let me know what variation you find at different points on the 2x4.

Larry
If the GMU25 is far off the center of the roll axis, it is describing an arc instead of pure rotation in roll, inducing a complicated and erroneous input of vertical and lateral accellerations when the only real displacement aboutthe 3-DC.G. is rotation about the roll axis. There may be compensating internal math to back that out, but That poor CPU is going to work hard to keep up with the situation! Mine is dead center span-wise and about 2.5Ē below the crankshaft axis. That second number is just a guess, of course, but probably not too far off. Finding the true vertical CG is tricky.- Otis
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  #32  
Old 02-16-2019, 12:33 PM
lr172 lr172 is offline
 
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Originally Posted by Hartstoc View Post
If the GMU25 is far off the center of the roll axis, it is describing an arc instead of pure rotation in roll, inducing a complicated and erroneous input of vertical and lateral accellerations when the only real displacement aboutthe 3-DC.G. is rotation about the roll axis. There may be compensating internal math to back that out, but That poor CPU is going to work hard to keep up with the situation! Mine is dead center span-wise and about 2.5” below the crankshaft axis. That second number is just a guess, of course, but probably not too far off. Finding the true vertical CG is tricky.- Otis
The AHRS will have three discrete sensors for each axis. Not sure how the roll axis could describe and arc, as an arc implies two axis'. The roll sensor will simply read degrees from level.

Does your wing form an arc when you are banked relative to your fuselage? The entire length of the wing is tilted at the same number of degrees from level as the tip of the wing is. Maybe I didn't take enough physics, but I am not understanding your issue here. Leaving dihedral out of this simple example.

A graphical representation would be an arc. However, that is "representation" and not related to a sensor that only capture movement across once axis.

Take my 2x4 example. How does the presence or absence of an arc impact the level's reading? It is either 5* from level or it is not. It is that binary.

Larry
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Last edited by lr172 : 02-16-2019 at 12:37 PM.
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  #33  
Old 02-16-2019, 01:02 PM
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If you have a GMU 22, and you want to ensure that you have heading data if you lose the GSU 25, you can simply add a GMU 11 to your system. The G3X and G5 share a CAN bus, so once you install the GMU 11 on the CAN bus, it could be used by either the G5 or the GSU 25.

Regarding placement of an AHRS unit, the ideal location is in the center of rotation for all 3 axis. However, that is not practical or required for good performance in a small aircraft. When you are not on the center of rotation, in a dynamic maneuver you will add some error to the reading. For instance, if the AHRS were mounted out in the left wing, rolling left would be seen as a left bank but also as a vertical acceleration downward as the wing drops. In a small aircraft the affects are pretty small. The following note in the installation manual is written for this very purpose:

The GSU 25 should be mounted within 13 feet (4.0 meters) longitudinally and 6.5 feet (2.0 meters) laterally of the aircraft CG (center of gravity). In cases where the longitudinal distance from the CG is planned to be greater than 6.5 feet (2.0 meters), it is preferable to mount the GSU 25 forward of the aircraft CG if possible, to enable better acceleration outputs for autopilot use.

Thanks,
Levi Self

Team X Project Manager
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  #34  
Old 02-16-2019, 01:09 PM
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Originally Posted by lr172 View Post
The AHRS will have three discrete sensors for each axis. Not sure how the roll axis could describe and arc, as an arc implies two axis'. The roll sensor will simply read degrees from level.

Does your wing form an arc when you are banked relative to your fuselage? The entire length of the wing is tilted at the same number of degrees from level as the tip of the wing is. Maybe I didn't take enough physics, but I am not understanding your issue here. Leaving dihedral out of this simple example.

A graphical representation would be an arc. However, that is "representation" and not related to a sensor that only capture movement across once axis.

Take my 2x4 example. How does the presence or absence of an arc impact the level's reading? It is either 5* from level or it is not. It is that binary.

Larry
Larry, During maneuvers, the aircraft rotates about its 3-D CG. Imagine the extreme of installing the GMU25 in a wingtip. During an abrupt roll, it would register the roll, a non-existent sharp climb, and a significant non-existent lateral acceleration. If you can agree that the wingtip would be a really bad place to put the GMU25, it follows that any other location apart from the roll axis is also bad, albiet less and less so as you approach the roll axis. This points pretty convincingly to the roll axis as the best of all possible locations. The same argument applies to both of the other axis, but to a lesser degree because of the much gentler nature of rotation about those axis during maneuvers.-Otis
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RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2019 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"
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  #35  
Old 02-16-2019, 01:24 PM
lr172 lr172 is offline
 
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Originally Posted by Hartstoc View Post
Larry, During maneuvers, the aircraft rotates about its 3-D CG. Imagine the extreme of installing the GMU25 in a wingtip. During an abrupt roll, it would register the roll, a non-existent sharp climb, and a significant non-existent lateral acceleration. If you can agree that the wingtip would be a really bad place to put the GMU25, it follows that any other location apart from the roll axis is also bad, albiet less and less so as you approach the roll axis. This points pretty convincingly to the roll axis as the best of all possible locations. The same argument applies to both of the other axis, but to a lesser degree because of the much gentler nature of rotation about those axis during maneuvers.-Otis
Why would a wing drop be read as a climb. The fact that the wingtip fell relative to gravity has no bearing on the pitch axis. It may mess with the units measurement of lateral G forces, but no bearing on pitch or roll indication. The AHRS would only show a climb if it' relative pitch angle increased and a wing drop in a roll won't do that. As long as the pitch axis is unchanged, a wing drop would not show a climb unless the aircrafts pitch axis also changed along with it's roll axis.

I don't mean to argue and will stop here. I think you and I have different ideas as to what is being measured inside of an AHRS and neither of us has the background to settle that argument.

Larry
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Last edited by lr172 : 02-16-2019 at 01:29 PM.
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  #36  
Old 02-16-2019, 01:42 PM
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Quote:
Originally Posted by g3xpert View Post
If you have a GMU 22, and you want to ensure that you have heading data if you lose the GSU 25, you can simply add a GMU 11 to your system. The G3X and G5 share a CAN bus, so once you install the GMU 11 on the CAN bus, it could be used by either the G5 or the GSU 25.

Regarding placement of an AHRS unit, the ideal location is in the center of rotation for all 3 axis. However, that is not practical or required for good performance in a small aircraft. When you are not on the center of rotation, in a dynamic maneuver you will add some error to the reading. For instance, if the AHRS were mounted out in the left wing, rolling left would be seen as a left bank but also as a vertical acceleration downward as the wing drops. In a small aircraft the affects are pretty small. The following note in the installation manual is written for this very purpose:

The GSU 25 should be mounted within 13 feet (4.0 meters) longitudinally and 6.5 feet (2.0 meters) laterally of the aircraft CG (center of gravity). In cases where the longitudinal distance from the CG is planned to be greater than 6.5 feet (2.0 meters), it is preferable to mount the GSU 25 forward of the aircraft CG if possible, to enable better acceleration outputs for autopilot use.

Thanks,
Levi Self

Team X Project Manager
Levi- I missed this response prior to posting just below. Glad to hear confirmation that the center of rotation is the ideal, but surprised by the generous off-center allowances. And my bad, GSU25 not GMU25. I guess the fact that it cross-references pitot , static, and AOA with accelleration data makes this possible. Iím still going for the center!

Your teamís active participation in this forum is very much appreciated by all!- Otis
__________________
Otis Holt-
RV-7A (bought)
Built Monnett Moni
Frmr Test Pilot/Author CAFE APR's:
RV-8A, S-7C, Europa, Glastar.
-2019 VAF donation!!-
"RV-Fun is inversely proportional to RV-Weight!"
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  #37  
Old 02-16-2019, 02:10 PM
Mancival Mancival is offline
 
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Quote:
Originally Posted by g3xpert View Post
If you have a GMU 22, and you want to ensure that you have heading data if you lose the GSU 25, you can simply add a GMU 11 to your system. The G3X and G5 share a CAN bus, so once you install the GMU 11 on the CAN bus, it could be used by either the G5 or the GSU 25.

Regarding placement of an AHRS unit, the ideal location is in the center of rotation for all 3 axis. However, that is not practical or required for good performance in a small aircraft. When you are not on the center of rotation, in a dynamic maneuver you will add some error to the reading. For instance, if the AHRS were mounted out in the left wing, rolling left would be seen as a left bank but also as a vertical acceleration downward as the wing drops. In a small aircraft the affects are pretty small. The following note in the installation manual is written for this very purpose:

The GSU 25 should be mounted within 13 feet (4.0 meters) longitudinally and 6.5 feet (2.0 meters) laterally of the aircraft CG (center of gravity). In cases where the longitudinal distance from the CG is planned to be greater than 6.5 feet (2.0 meters), it is preferable to mount the GSU 25 forward of the aircraft CG if possible, to enable better acceleration outputs for autopilot use.

Thanks,
Levi Self

Team X Project Manager

Thanks, do you think that the 3d vision will come back once I have installed the GMU11, when I pull the GSU25 breaker?

Last edited by Mancival : 02-16-2019 at 02:54 PM.
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  #38  
Old 02-16-2019, 07:38 PM
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Originally Posted by Mancival View Post
Thanks, do you think that the 3d vision will come back once I have installed the GMU11, when I pull the GSU25 breaker?
If the G3X Touch has reverted to using the G5 ADAHRS data because the GSU 25 is powered down/failed and you have a GMU 11, the G3X Touch Display will show the G5 Attitude and Air Data with Synthetic Vision.

Thanks,
Levi Self

Team X Project Manager
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  #39  
Old 02-16-2019, 07:56 PM
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g3xpert g3xpert is offline
 
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Larry and Otis,

You would both be correct IF your assumptions of what is happening inside of the little black box (GSU 25) were correct.

There are 3 axis gyros and accelerometers inside the GSU 25 which are used in complex algorithms written by some of the smartest engineers (not me ) to output an attitude solution as well as the turn rate indicator and slip skid indicator.

Because acceleration data is used by the system, an erroneous acceleration due to GSU placement would have a negative affect on performance of these indications. As stated in an earlier post, the G3X Install manual covers what locations are acceptable. If you follow this guidance, you will not have an issue.

Another simple way to think about this is to take the example of the slip skid indicator, which is a measure of lateral acceleration. If the GSU was in a wingtip and you did an aileron roll, you would measure lateral acceleration, but you could still be perfectly coordinated.

Thanks,
Levi Self

Team X Project Manager
__________________
Garmin G3X Support
g3xpert@garmin.com
1-866-854-8433 - 7 to 7 Central Time M to F
Please email us for support instead of using Private Messaging due to the limitations of the latter.
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  #40  
Old 02-16-2019, 09:36 PM
lr172 lr172 is offline
 
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Originally Posted by g3xpert View Post
Larry and Otis,

You would both be correct IF your assumptions of what is happening inside of the little black box (GSU 25) were correct.

There are 3 axis gyros and accelerometers inside the GSU 25 which are used in complex algorithms written by some of the smartest engineers (not me ) to output an attitude solution as well as the turn rate indicator and slip skid indicator.

Because acceleration data is used by the system, an erroneous acceleration due to GSU placement would have a negative affect on performance of these indications. As stated in an earlier post, the G3X Install manual covers what locations are acceptable. If you follow this guidance, you will not have an issue.

Another simple way to think about this is to take the example of the slip skid indicator, which is a measure of lateral acceleration. If the GSU was in a wingtip and you did an aileron roll, you would measure lateral acceleration, but you could still be perfectly coordinated.

Thanks,
Levi Self

Team X Project Manager
Thanks for clarifying and helping us better understand what is happening inside the black box.

Larry
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