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  #1  
Old 05-07-2014, 09:28 AM
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N941WR N941WR is offline
 
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Default RV-9 with Dynon AP servo beta test

Quick note, this is a new thread split off from http://www.vansairforce.com/communit...d.php?t=112766 due to the extreme thread drift, and yet a valuable bunch of info that deserves its own thread. MJS

Two weeks ago I installed the two new Dynon control heads, which are a great addition to the SkyView!

However, there is a potential problem with the Auto Pilot that I never experienced with all the beta testing I did on the AP.

At the top of the AP control panel is a big button labeled “AP”. After I installed the control head and powered everything up I pressed that “AP” button and the controls started banging around, especially in roll.

At the time, I didn’t think much of it. However, two days later when I was preflighting, I noticed the aileron would only travel in one direction. Meaning I could have flown right turns or straight. Obviously not a good situation.

After looking in the cockpit to make sure nothing was jamming the control stick, I removed the inspection panel in the right wing, below the roll servo.

I found that the AP test function bent the attach plate and allowed the roll servo to go over center. (See pictures below.) I contacted Dynon support and was told that is why you are supposed to install the "Range of Motion Limiting Bracket". Dynon’s installation manual clearly states that they “recommend” the installation of these brackets and you can bet, as soon as I find mine, they will be installed!

My recommendations to Dynon were to fix the software so the thing won't beat the controls during the test and to redesign the attach bracket with small tabs that run along two sides to add strength and prevent the bracket from bending.

One other thing that might help is to put a large area washer between the aluminum spacer tube and the steel plate to help spread the bending load.

The roll servo arm in the "normal" position. Note the bent control attach plate.


The roll servo arm in the "reversed" position, jamming the controls.


The bent control attach plate.
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Bill R.
RV-9 (Yes, it's a dragon tail)
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Last edited by Mike S : 05-08-2014 at 10:58 AM. Reason: Added comment about the large area washer.
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  #2  
Old 05-07-2014, 10:23 AM
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Kevin Horton Kevin Horton is offline
 
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Quote:
Originally Posted by N941WR View Post



The roll servo arm in the "reversed" position, jamming the controls.


The bent control attach plate.
Bill,

Is the servo installation exactly as recommended by Dynon? That long bolt that attaches the autopilot push-pull rod to the bellcrank provides a huge amount of leverage to bend the attach bracket. I'd like to see a setup that had the push-pull rod using a very short bolt.

In any case, the structure must be strong enough to withstand full servo output, without any damage. You either need a weaker servo, or a vastly beefed up way to connect the servo to the bellcrank.

You are very lucky to have caught this now.
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  #3  
Old 05-07-2014, 10:42 AM
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Toobuilder Toobuilder is offline
 
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I've seen poor servo geometry lock up the controls. My Rocket did this even without the servo engaged. There are some scary installations out there and sorry Bill, but yours looks like one of them. There are some pretty extreme forces involved here due to leverage and fairly complex kinematics that many people don't understand all that well. I see several areas of Bill's that are just begging for trouble.

It's a lot more than just output torque and shear pins. Be careful with these things!
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1984 L39C

Last edited by Toobuilder : 05-07-2014 at 10:45 AM.
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  #4  
Old 05-07-2014, 10:59 AM
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Regarding Bill's post and pictures... As someone who barely survived an A/P malfunction years ago, this gets my attention. It was a non-RV, certified A/P in a certified airplane and it still very nearly killed us.

Never - EVER - trust electronics, period. Trust electronics controlled by microprocessors even less. Program-set limits are convenient but absolutely not something you can bet your life on. You absolutely must have mechanical limits to keep hardware and firmware malfunctions from killing you. If it's physically possible for the servo arm to go over-center and jam the controls, you have to assume that it will happen at some time, and build in mechanical means to prevent it from happening. In this case you have to take into account the force a runaway servo can put on the end of that nice long AN3 bolt. It doesn't surprise me that the bolt could provide enough leverage to bend the attachment plate. You'd think the shear screw would have done its job before that happened, but obviously you can't depend on that.

I do plan to put an A/P in my plane, but will do so with the assumption that it will at some point become possessed and actively try to bring about my demise.
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  #5  
Old 05-07-2014, 12:35 PM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by Kevin Horton View Post
Bill,

Is the servo installation exactly as recommended by Dynon? ...
Yes, that is per Dynon's installation and matches Tru Track's installation.

Quote:
Originally Posted by flightlogic View Post
One additional thing to note. The software in some installations that prevents servo activation on takeoff is dependent on GPS data. If a good lock on GPS is not present... the safety feature will not work.
....
The Dynon EFIS / Auto Pilot is driven off of air data, not GPS. It will not let you engage the AP above or below set air speeds that are set by the installer based on the designer's speed limitations.

The kicker for me was hitting the AP test button on the ground which simply "exercises" the AP as a pre-flight check.
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Bill R.
RV-9 (Yes, it's a dragon tail)
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Build the plane you want, not the plane others want you to build!
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  #6  
Old 05-07-2014, 12:59 PM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by DaleB View Post
Regarding Bill's post and pictures...

...You'd think the shear screw would have done its job before that happened, but obviously you can't depend on that...
One thing to keep in mind with "lever type" servos is that the output force applied to the control system can be variable. Further, when the output arm is 90 degrees to the work, the force is at it's minimum because the effective length of the arm is at it's maximum. As the arm travels through its arc, the effective length gets shorter (and leverage applied higher), ultimately resulting in a zero length arm (and ulimited leverage) at the 180 degree point (over center). So depending on where this arm is in its arc, the shear pin may be perfectly within its design load, yet the output arm is producing destructive force due to geometry/leverage.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #7  
Old 05-07-2014, 01:04 PM
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Quote:
Originally Posted by N941WR View Post
Yes, that is per Dynon's installation and matches Tru Track's installation...
Bill,

While similar, the execution between your pictures and the drawing provided have some subtle, yet important differences. Without seeing the actual dimensions of the two, the spacers are much shorter and larger diameter on the Tru Trak drawing, and the rod end is on the inside of the servo arm. Both contribute heavily toward increasing stiffness of the system.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
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RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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  #8  
Old 05-07-2014, 01:09 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by N941WR View Post
Yes, that is per Dynon's installation and matches Tru Track's installation.
Actually it doesn't match.
Look closely at the diagram.
The push/pull rod is supposed to attach to the servo arm on the side closest to the servo. This will allow a much shorter standoff to be used on the bellcrank.
I think a larger diam. spacer (than what is shown in your photos) is also used to help reduce the bending moment of the bolt.

On our trutrak servo installations, the spacer used at the bellcrank attach point is only about 25% as long as what you are using.

edit - looks like Michael got a post off before me...
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  #9  
Old 05-07-2014, 01:19 PM
gereed75 gereed75 is offline
 
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It has been hinted here but I think it needs to be emphasized. It appears from the photos that this servo installation is capable of creating an over center lock condition. It appears that the length of the actuation rod is too short. The geometry of the set up must preclude over center lock. It appears this one does not

I do not think that there is an electronic mode that can lock the servo. The lock up is electro magnetic and there is nothing that can increase the strength of the magnets to the point that they can not be over ridden.
This a wrong and dangerous installation!!
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Last edited by gereed75 : 05-07-2014 at 01:33 PM.
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  #10  
Old 05-07-2014, 01:36 PM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by Toobuilder View Post
Bill,

While similar, the execution between your pictures and the drawing provided have some subtle, yet important differences. Without seeing the actual dimensions of the two, the spacers are much shorter and larger diameter on the Tru Trak drawing, and the rod end is on the inside of the servo arm. Both contribute heavily toward increasing stiffness of the system.
You are correct. I will see if I can cange that tonight.

Thanks Gang!
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Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
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