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My Armrest support idea

Looks great,
I have seen another post on this where the builder used a piece of the angle aluminum and dimpled the arm rest and machined the angle
A little support is good. I have considered this but have not got it done yet.
 
Depends on your point of view.
From lightness aspect, nothing is best, J channel is next, angle is worst.

From stiffness aspect, the exact reverse is true.

Edit*** Um, stiffness comment not exactly correct, see my post further below.
 
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I did something similar:

Trimmed (then filed) one leg of some 3/4 x 3/4 x 1/16 angle down:
20090615-01-tn.jpg


Radiused the corner to fit in the bend radius:
20090615-03-tn.jpg


Drilled both legs of the angle; countersunk angle; dimpled arm rests:
20090628-02-tn.jpg


Primed and riveted (visible surfaces will be painted later):
20090702-05-tn.jpg


...of course, flipping the angle so the horizontal leg was spaced away from the arm rest (instead of riveted to it) would have been a lot stronger, but it also would have been harder to rivet.

Another improvement would be to bend the vertical leg of the angle 90 degrees at each end and rivet to the F-704 and F-705 bulkheads. I took the easy way out.
 
Me, too!

I did just like Mike did. It's a stiffener that is riveted to the underneath side of each arm rest. From what I read, the arm rest can be easily bent or damaged by persons who aren't familiar with the aircraft getting in and out of the cabin. All it took to convince me that this is a good idea was to climb inside Van's factory demo RV-7A and look at the armrests. The damage is apparent at a glance. So I cut two lengths of .063 x 3/4 x 3/4 aluminum angle, clamped, measured and drilled them to the arm rests, and then did the usual deburring, dimpling, countersinking, scuffing, cleaning, and priming. The vertical side of each angle had to be trimmed down to about 3/8" so it wouldn't show below the arm rest flange. My bandsaw took care of that task easily.

100_6704%20(Small).JPG


Here they are riveted together, showing the primed bottom of the right armrest and the unprimed top of the left one. The rivets ended up being about 7/8" apart.

100_6705%20(Small).JPG


Here's one of them, clecoed to the center section. It's a highly-recommended modification!

100_6706%20(Small).JPG
 
I opted not to put any reinforcements in the arm rests when building the 7A, in deference to Van's admonishment to keep it light.

I'm pretty glad. The one thing I found in flying so far is I don't NEED reinforcement. The only thing the arm rests supports is my elbow. I know people think people are going to try to ease themselves out of the plane by pushing on the armrests, but this notion is quickly dispelled and for me, it didn't take long to develop a good habit of how to exit the plane properly.
 
I cut the width of the armrest in 1/2. And then added the angle. Placed the angle on top of the cut with the flange of the angle down.

Plenty wide enough to rest my elbow on, and gave a few extra inches in width to move around.
 
Throwing the gauntlet

This common modification has been bugging me for a long time. Mainly because it is a poor use of metal via the addition of too much weight for the stiffness gained.

This thread made me go ahead and do the assembly area moment calculations of the different methods to see what is best. The dimensions I used were approximate, but consistent through this analysis.

Original Armrest Ixx= 0.002 in^4 Comparative Weight = 0

Added 3/4 x 1/16 angle Ixx = 0.006 in^4 Comparative weight = 2

Added 3/4 x 1/8 angle Ixx = 0.010 in^4 Comparative weight = 4

Added "J" channel to make "C" section Ixx = 0.008 in^4 Comparative weight =1

When designing beams, which this is, it is more important WHERE the metal is rather than how much is added. The distribution of metal around the bending axis makes a BIG difference in effectiveness. A "C" shape is WAY better than an angle. EDIT ** For non-engineers, the "area moment" numbers designated by Ixx are an indication of how stiff a beam will be. When loading scenarios and material properties are the same, Ixx is the only difference. The larger the Ixx number, the better for stiffness.

You can see that the J channel weighs half of the 1/16 angle and is actually stiffer. It is almost as good as the 1/8 angle at 1/4 of the weight. The image below shows how to install it.



13314031004867999ed0f7e.jpg
 
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J channel

Where did you purchase the J channel? I remember seeing some somewhere.

Thanks,

Dennis Conner
RV7A
 
I used leftover J-channel from the fuselage kit. I didn't go full length of the armrest, stopped maybe three inches short of each end. Dimpled and back riveted them on the horizontal surface, not the vertical flange like the previous post. It looks great and made the armrest much stiffer. The weight add is pretty small. I have pictures on my kitlog if you want to see it.
 
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It?s really funny how much interest this little mod has raised! Okay, I get the ?light is right thing? but, I have owned a couple of birds and I know how passengers and especially kids treat things! My idea was to build a support that would rivet to the bulkheads on each end and also support the middle. If enough weight is put on the support alone, it will give in the middle and since it is also riveted to the side skin? you get the idea and I do not believe the J channel will protect you from something like that. I really do not want the Classic Aero Side Panels so; I have a weight trade off. For those that do not understand how I made the support.. angle cut at a 45, matched to the bulkhead and TIG welded.
 
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