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Gear Tower Modification

M

mlwynn

Well Known Member
Hi folks,

I have been reading about what I guess is a relatively common modification: building a removable cover for the gear towers for the RV 8. The following picture and brief thread is most of what I find in terms of documentation:

http://www2.mstewart.net:8080/super8/geartower/index.htm

I wondered what Van's had to say about this, as several people had asked questions about the structural safety of this modification. Their answer is as follows:

"We have not evaluated alternative methods in this area. A removable
cover should be OK, assuming it is attached with screws at a similar
spacing to the rivets. It is more work and weight, but if you think
you'll need better access for retorquing the gear bolts, it may be
worth it."

I suppose that is as clear an endorsement as we are likely to see.

I am planning to go forward with this modification. I have searched the archives here, at Rivetbangers and on the Matronics site. There is really sparse documentation. If anyone has any additional pictures, comments or observations of potential pitfalls, please share them. I will try to document my modification as thoroughly as possible and post it here for future reference.

Michael Wynn
RV 8 Fuselage
San Ramon, CA
 
I can assure you that this mod is of no factor on my fat super 8 that routinely crash lands at 2000lbs of weight.

The gear bolts are still a pain, but access to wire bundles, vent lines, heat conduit etc, makes this a must have simple modification.

Best,
 
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<<If anyone has any additional pictures, comments or observations of potential pitfalls, please share them. I will try to document my modification as thoroughly as possible and post it here for future reference.>>

Ditto.

I elected to remove only the web between the two existing lower access holes. Nutplates and #8 screws fasten a solid cover plate, which should be at least as strong as the web. Were I slow-building, I would probably do the more extensive mod. This one is fast and easy on a QB fuselage.

Vent%20Handle.JPG
 
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DanH said:
<<If anyone has any additional pictures, comments or observations of potential pitfalls, please share them. I will try to document my modification as thoroughly as possible and post it here for future reference.>>

Ditto.

I elected to remove only the web between the two existing lower access holes. Nutplates and #8 screws fasten a solid cover plate, which should be at least as strong as the web. Were I slow-building, I would probably do the more extensive mod. This one is fast and easy on a QB fuselage.
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Good morning, Dan -

That plate is loaded almost entirely in shear, and just connecting the lightening holes takes away a considerable amount of strength. If it were me - which I may make that mod one of these days - I'd go with a tighter pattern on your screws. Just based on intuition, I'd probably go double the fasteners you have.

Thanks for sharing your info.
 
<<That plate is loaded almost entirely in shear, and just connecting the lightening holes takes away a considerable amount of strength.>>

Let's run some simple numbers and see what we have. My web is already gone. Could you measure yours and tell me the width of the web at the narrow point between the two lower holes?
 
DanH said:
<<That plate is loaded almost entirely in shear, and just connecting the lightening holes takes away a considerable amount of strength.>>

Let's run some simple numbers and see what we have. My web is already gone. Could you measure yours and tell me the width of the web at the narrow point between the two lower holes?
Click to expand...
I will do that next time I'm at the hangar. Unless anyone is already near their plane or part and can measure the component.
 
<<2 5/8" at the narrowest point, and it's 0.040" material.>>

2.625 x .040 is 0.105 in^2. At a shear strength of 40,000 psi we have 4200 lbs.

Doubt there is any published shear rating for a #8 MS35206 screw. For a rough estimate let's assume 1/2 tensile for shear (60,000/2). A #8 has a minor diameter of about .125", so (r^2 x pi) x 30,000 is 368 lbs per screw.

12 screws total, 2 of which are doing nothing, so 5 per side x 368 is 1840 lbs.

Uh oh. Strictly a quick and dirty approach, but it sounds like I do indeed need more fasteners, or stronger fasteners.

Gear box access is a worthwhile mod, so let's hash this out. I flat couldn't get my hands in through the round holes.
 
Let's back up slightly and re-calculate.

I think that we're getting a bunch of "apples and oranges" mixed into the calculations here. First of all, the rivets holding the existing gear tower cover plate on are loaded in shear, but the plate itself is almost certainly loaded in tension (with compression being the other possible choice). The 2-5/8" height of the area between the lightening holes doesn't really factor into the calculation - the cross sectional area in tension at that point is the width of the plate times the thickness. The weakest points (tensile) in the existing plate are at the horizontal centerlines of the holes, where you have only the two strips of material left at the sides of the plate. Your removal of the web between the holes should not reduce the tensile strength of the part at all. I believe that the quote from Van's referred to making the entire part removable, and their suggestion to use screws at current rivet spacing would have you replacing aluminum fasteners in shear (the rivets) with steel fasteners in shear (the screws) - a conservative substitution, and probably the only reason Van's was willing to put it in writing. Removing the web between the holes would, however, make the plate weaker in its ability to resist fore and aft buckling loads of the structure, so putting the cover plate on is certainly a good idea. Bottom line - I believe that if you get the total shear strength of your screws up to equal-or-greater-than the total shear strength of the rivets down the sides of the existing structure, you'll have regained all the effective strength there is to be regained.
 
William Slaughter said:
I think that we're getting a bunch of "apples and oranges" mixed into the calculations here. First of all, the rivets holding the existing gear tower cover plate on are loaded in shear, but the plate itself is almost certainly loaded in tension
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I see this plate being loaded in a combination of shear and tension. As for how much of each, I have no idea. Intuitively, it i believe shear would be the concern with the fore-aft loading of the landing gear acting to twist the box.

The 2-5/8" height of the area between the lightening holes doesn't really factor into the calculation
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Under pure tension, I agree. But it w would if shear of the plate is a consideration, and I believe it should be as the plate acts much like a shear web. If the fwd side of the box is in tension and the aft in compression, the plate would see shearing load. I believe this would make the lightening hole arrangement a consideration. This shear was the driver for my commenting on the fastner spacing.

The weakest points (tensile) in the existing plate are at the horizontal centerlines of the holes, where you have only the two strips of material left at the sides of the plate. Your removal of the web between the holes should not reduce the tensile strength of the part at all.
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I agree.

....I believe that if you get the total shear strength of your screws up to equal-or-greater-than the total shear strength of the rivets down the sides of the existing structure, you'll have regained all the effective strength there is to be regained.
Click to expand...
I would buy that too.

Regardless - these are some good ideas on how to make this structure more managable. Thanks.
 
<<"apples and oranges"....plate itself is almost certainly loaded in tension >>

Good point.

Consider a vertical load at the axle, aka a drop load. The inboard end of the gear leg would try to pivot downward, the inboard gear box web would be in tension (as you say), while the fore and aft webs would be in shear. The rivets at the corners are in shear.

Now consider an aft load at the axle. The gear leg tries to rotate in it's mounting. The forward gear box web is in tension, the aft web in compression, and the subject inboard web is in shear. Again all rivets are in shear.

An accurate analysis would include the combination...

I believe you're right about the tension load. Removing the section between the two lower holes makes no difference at all. The shear case (aft axle load) is the one I was thinking about, Bryan too I imagine.

<<I believe that if you get the total shear strength of your screws up to equal-or-greater-than the total shear strength of the rivets down the sides of the existing structure, you'll have regained all the effective strength there is to be regained>>

Hmm....works for me. All the rivets down the sides, or just the rivets in the vicinity of the two connected access holes?
 
When we left off last, a quick and dirty analysis said I needed more screws. Sooooo, today I got a chance to install them.

I elected to insert eight flush #8's between the existing screws down the sides of the cover plate. Eight more gets us up there to a sensible strength level by rough analysis, or kinda matches the number of rivets in the adjacent tower corner; whatever floats your boat. I did them as flush because I suspect nested dimples add some shear strength to a screw-fastened joint.

Gear%20Tower%20Access.JPG


P7300002.JPG
 
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In English, it looks good!

Dan. Yours is the best looking, job on this mod I've seen yet. I'll be copying it, with permission, in the next month or so.
 
<<How thick is your cover plate material?>>

0.040" 2024-T3, same as the original web it replaced.
 
Looks really nice Dan - with no calculations at all (just eyeball engineering) it looks pretty good to me. Makes me wish I had the mod - maybe sometime when I have a long downtime.... I figure If I pulled the instrument panel, access to do it as a mod would be pretty good.

BTW, I see you've drilled the three holes for engine control cables. I'll be curious to know if you make that work - I had to basically take out a triangular chunk (with the three holes as the corners( in order to get enough room to install and remove cables. Especially if you have to change them once the plane is complete.....

Paul
 
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Ironflight said:
BTW, I see you've drilled the three holes for engine control cables. I'll be curious to know if you make that work - I had to basically take out a triangular chunk (with the three holes as the corners( in order to get enough room to install and remove cables. Especially if you have to change them once the plane is complete.....Paul
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Three holes worked fine for me. I installed/removed them several times and didn't have any problems... of course the entire throttle quadrant wasn't installed yet...

Karl
 
RV8N said:
Three holes worked fine for me. I installed/removed them several times and didn't have any problems... of course the entire throttle quadrant wasn't installed yet...

Karl
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Glad you didn't have any problems Karl....but have you had to try and remove them yet? I know a number of folks with the triangular-shaped hole. The problem is the rigid portion of the end of the cable - as you try and tilt the quadrant shelf out of the way, they hang up. But....whatever works, works!

Paul
 
I want to do this mod. Can you guys that have done this tell me where you routed your brake lines? Thanks.


DanH said:
When we left off last, a quick and dirty analysis said I needed more screws. Sooooo, today I got a chance to install them.

I elected to insert eight flush #8's between the existing screws down the sides of the cover plate. Eight more gets us up there to a sensible strength level by rough analysis, or kinda matches the number of rivets in the adjacent tower corner; whatever floats your boat. I did them as flush because I suspect nested dimples add some shear strength to a screw-fastened joint.

Click to expand...
 
Great Idea

I'm looking at this and thinking:
That's a great idea, and if I ever have to go back and work in that area again, I'll do it.
Building this thing has been one of the biggest challenges. I've made a few mods too, but I decided recently to finish the plane a 'stock' as possible so that I actually DO finish it. :rolleyes:
I'm also going to remember the opening access panel in the back of the fwd baggage hold, the access panel by the fuel pumps next to the left rudder pedal, and the air exit vent in the tail cone to reduce air noise in the cockpit.
These will be based on an actaul demonstrated need.
However, fitting my hand inside that tower was a bummer. If I had heard of it sooner...I'm sure I would have done it.
I love this forum.
 
Just for argument's sake

I am building and have been thinking about doing this modification, too. However, I was thinking...wouldn't it be just as strong as the original if you cut out the portion between the lightening holes, as we have seen here, but only put back the same size piece that was removed, using three countersunk screws on each side? (Using a joggle or lip on the removeable piece.)
Seems to me that would be a lot easier, retain the original look of the RV8 kit, and still be just as strong.
Any comments?
 
When I built my towers many years ago, nobody (to my knowledge) had figured out the need for this access panel. It's no big deal to reach through the existing holes to attach nuts and stuff.

However, as most of us have already discovered, once those towers are stuffed full of tubing, wires and control cables, it's nearly impossible for a human hand to move inside those towers.

I just installed rear seat rudder pedals, and the way the plans call for attachment, it required replacing one of the bolts 'way down in the inside corner to attach the mounting bracket. IMPOSSIBLE.

Moral of the story: Consider this mod to facilitate future maintenance. And, if you're planning on installing rear seat rudder pedals, do it when you're building your gear leg towers!
 
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