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  #41  
Old 05-08-2014, 08:57 AM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by krw5927 View Post
I agree with you. However there may be hundreds of -9's flying with this installation per the Dynon instructions. We seem to be on a path of suggesting that the manufacturer's installation instructions and parts are inadequate, which may not be the case. The OP has a non-standard installation that will produce more than a 50% increase in force over the standard installation, using the same hole in the servo arm. Given the same geometry and test routine, would the SV32 have been capable of damaging the steel brackets? Who knows. But thankfully this is the first case we've heard of.
I think that the drawings are based upon Bill's setup...

In any case, this design ignores some fundamental mechanical design elements. Is the stiffness of the system "adequate" for an SV32? Perhaps, but it might also be right on the edge. But clearly it's not adequate for a SV42.

At any rate, there are two absolute truths in any servo installation:

1. A servo can not be allowed to bend any part of the structure or linkage through output power;
2. In the case of servo lockup and subsequent pilot override, the shear pin or other failsafe must give way before the linkage or structure.
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Last edited by Toobuilder : 05-09-2014 at 09:25 AM.
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  #42  
Old 05-08-2014, 12:44 PM
BenNabors BenNabors is offline
 
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Default Bending load vs shear or tension

Quote:
Originally Posted by Toobuilder View Post
There are some scary installations out there and sorry Bill, but yours looks like one of them.
There are so many issues with this design; it is hard to know where to begin. Primarily, always minimize bending loads, design for the loads to be passed in shear or in tension. These induced bending loads should be designed out both for the bolt and the bolt's interface. This arrangement performed exactly as would be expected, which is not the way it was intended.

I looked for a good basic reference on joint design and found this one: https://www.fastenal.com/content/fed...t%20Design.pdf . It is not all you need to know, but it is a start.

From a design stand point; the structure should be able to withstand all applied loads plus margin. This would include, servo, pilot input, friction, and wind gusts for starters. To say it another way, the full force application of the servo should not be able to deform the joint or bracket and should have a safety factor or 2 for a skilled kinematic and stress analysis, or S.F of 4 for a hand analysis.

I would expect that this deformation is repeatable. Put a new bracket on, apply the load and watch it bend. I believe this is a faulty design.
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  #43  
Old 05-09-2014, 09:48 AM
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Quote:
Originally Posted by Walt View Post
Oops, my bad, I was thinking TT.

However looking at the servo drawings show:
TT arm height is .768
Dynon arm height .750

Thats not much difference, the TT installs on top with plenty of clearance so maybe the bearing is differnent?
The Dynon arm height is .75, the arm itself is .12 thick, so the nominal distance between the body and arm is .63.

I have a bunch of rod ends laying around and the thickest one I have measured .495. The stacked height of a Hi-Loc (used instead of an AN bolt) and an AN970L washer is a scant .070 for a total of .565. This total stack up clears the body of the servo by .065, though I'd like to see a thinner rod end used just to allow as much clearance as possible for eventual gear train wear, slop and possibility of trapping FOD.

Flipping the rod end (and significantly reducing/eliminating the spacer on the bellcrank) would definitely improve the geometry of the -9 installation, but people need to keep in mind that the rod end/bolt is running at significantly reduced clearance to the servo body and the possibility of a hard jam is increased. I think it's a good way to go, but in the interest of full disclosure it should be noted that if the bolt backs out a little or something gets stuck in there, then the shear pin won't help.
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Last edited by Toobuilder : 05-09-2014 at 10:52 AM.
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  #44  
Old 05-09-2014, 10:27 AM
Bavafa Bavafa is offline
 
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Quote:
Originally Posted by N941WR View Post
This makes me wonder how many other AP servos from all the manufacturers have been installed w/o a limiter.
Which one is the limiter, looking at the picture/drawing I can not figure out which one is the limiter of the arm.
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  #45  
Old 05-09-2014, 10:40 AM
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Mike S Mike S is offline
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Quote:
Originally Posted by Bavafa View Post
Which one is the limiter, looking at the picture/drawing I can not figure out which one is the limiter of the arm.

Quote:
Originally Posted by N941WR View Post
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!
Sounds like it was not installed at all, so probably not in the photo.
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  #46  
Old 05-09-2014, 10:45 AM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by Mike S View Post
Sounds like it was not installed at all, so probably not in the photo.
...and the limiter is also not shown in the drawing linked in post #13.
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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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  #47  
Old 05-09-2014, 06:28 PM
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Bill,
Thanks for bringing this to our attention. Clearly we're concerned to hear about any failure like this.

As has been identified, this is why we recommend the over-center limiting bracket on every install. Even in an install that appears that it can not go over-center, the bracket is a great backup to unexpected failures. While it isn't in the drawing, it is in the text requirements for the install and included with every servo we ship.

Our mechanical team at Dynon is looking over the install and doing some of our own tests to see if we can re-create this issue using only the strength of the servo (about 4 lb/ft of torque).

A few bits of misunderstanding to clear up.

1) This was not in a test mode. You can engage the AP on the ground, by design. At zero airspeed, the AP can be engaged. This allows you to still use the AP if your pitot system fails and goes to zero. So when you engaged on the ground, you engaged it in a standard mode and it attempted to roll the plane to the heading, track, or course you had selected. The AP will disengage when the airspeed hits 20 knots, so you can't take off with it on accidentally.

There is a self-test mode which can only be accessed via menus in the system, not in any kind of normal operating mode. This does not cause the controls to travel to the limits.

2) The shear screw does not exist to limit the force of the servo. If it did, you could break the shear screw with your knee when you reached over to grab your lunch. The shear screw is stronger than the servo motor and is only there to protect from a geartrain failure. In normal operation, the servo motor itself is limited in torque, and can be overridden without breaking the shear screw. Just grab the stick and override it. So it is not unexpected that the shear screw didn't break.
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  #48  
Old 05-09-2014, 07:07 PM
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N941WR N941WR is offline
 
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Stand by for a fix.

I am going to work up a fix and will report back. (As well as install the limiter.)

Give me a week or two to fabricate, install, test, and document the fix.
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  #49  
Old 05-09-2014, 07:10 PM
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Dynon,

Regarding activating the AP with zero airspeed, you also have the GPS ground speed. So, why allow the SV to activate, if both are zero?
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  #50  
Old 05-09-2014, 08:12 PM
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Quote:
Originally Posted by dynonsupport View Post
...Our mechanical team at Dynon is looking over the install and doing some of our own tests to see if we can re-create this issue using only the strength of the servo (about 4 lb/ft of torque)...
Are you considering the output of the servo with the load only at 90 degrees, or through the full range of motion? It looks like with the linkage geometry of this particular installation, the force applied rapidly increases as the servo reaches the end of travel due to the effective reduction of output arm length. Depending on where the aileron linkage hits it's hard stop, the servo could be applying 10 (or more) times the force it does in level flight.
__________________
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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