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TieDown/Jack Point Engineering

Tag

Tag

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I just want to understand/confirm something. I'm currently working on my slow build RV8 wings and was concerned about the Wing Tiedowns. Looking at the construction, it seems like the tiedowns are only being supported by 4 AN3 bolts. Is it true that these 4 AN3 bolts (and possibly some support from two of the spacer screws) are strong enough in shear strength to support nearly 1/2 the weight of the aircraft when it is up on jacks using the tiedowns as jack points? Or is there another source of support for the weight?
 
I'm in the camp of "never use the wing tie down sockets to jack up the plane". Others say airplanes have always been jack like this - but I note a few where the jack point slipped and the jack does a nice job ventilating the bottom of the wing.

It is simple to just put a 12" or so padded 2x6 on the end of a long neck jack bottle. The 2x6 has a pipe flange attached with a piece of 1/4" piece of aluminum under it such that the jack rests against the aluminum and inside the floor flange. The advantage here is the 2x6 can rotate to be flush against the bottom of the wing centered on the wing spar. The piece of aluminum is there so that the jack does not work a hole in the 2x6.

Carl
 
Structural strength...

Yes Carl, I understand your viewpoint; that very thing happened to a neighbor's plane - it slipped off the jacks and punched holes through the wings... Good idea with the 2x6's.

But I just wanted to ensure that those Tiedowns; installed with 4 AN3 bolts were designed to support the weight of the plane (as Jackpoints)?
 
It's been over 20 years since I owned one, but IIRC, four 1/4" bolts in shear keep the fuselage of a T-18 attached to the wing. Don't remember ever hearing of one separating from the wing, and many have been built much heavier than the design point.

Remember, you're not looping the plane while it's on jacks. Those bolts are pretty strong; shear strength is ~2100 lbs per bolt.
http://skybolt.com/catalog-an-ms-hardware.pdf

Feel better?
 
Tag said:
Yes Carl, I understand your viewpoint; that very thing happened to a neighbor's plane - it slipped off the jacks and punched holes through the wings... Good idea with the 2x6's.

But I just wanted to ensure that those Tiedowns; installed with 4 AN3 bolts were designed to support the weight of the plane (as Jackpoints)?
Click to expand...


Well, If you don't trust the tie down supports to jack your airplane, how can you trust them to keep it tied down?

~Marc
 
Yes, Much better now. Just wouldn't have figured that from looking at them, thanks!
Marc, I guess I was questioning both functions....
 
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I use one of these

http://bogertaviation.com/products/rv-jack-pads

on a bottle jack (in a three-legged support base, etc.) on one side at a time, sloooowly and carefully, and never for longer than it takes to get the tire off or on (just set the axle back down on a 4x4 block as soon as the tire is off.

Be sure to chock the other wheels while jacking.

With non-retractable gear, I can't think of any reason to leave a plane on jacks for any length of time.

Another option, and one I have, is to use the modified axle nuts if you have the round gear legs, which used to be sold by Avery. Don't know if anyone has them nowadays, but I think ACS sells the jack and you could easily fab up the modifications to the nut.
 
The shear strength of a steel AN3 bolt is 2,125 pounds, so 4 bolts should certainly do the required job. :)
 
rv7charlie said:
It's been over 20 years since I owned one, but IIRC, four 1/4" bolts in shear keep the fuselage of a T-18 attached to the wing. Don't remember ever hearing of one separating from the wing, and many have been built much heavier than the design point.

Remember, you're not looping the plane while it's on jacks. Those bolts are pretty strong; shear strength is ~2100 lbs per bolt.
http://skybolt.com/catalog-an-ms-hardware.pdf

Feel better?
Click to expand...

Per the chart, an AN4 bolt would have a single shear of 3680 lbs. If there are 4 acting in single shear along the same axis then the rated load would be 3680 X 4 =14,720 lbs to fail the bolts. Lets assume a .5 factor based on the material the bolts go through and it should hold your roughly 1100 lb airplane ok.
I use these points routinely for jacking with no problems.
 
AN3 Bolt Strength

Great info, thanks. I just recall how easy it was snap the head off of an AN3 bolt once while applying too much torque to it. That experience really made an impression on me, even realizing it's much different from shear forces.
 
Ignore the AN3 bolt strength. How confident are you about those course cut threads in the soft aluminium tie down bracket?

Another consideration. Unless you were careful to make the tie down bracket flush with the bottom of the skin, any fixture you screw into the bracket will deform the skin until the skin touches the bracket.

Just use the padded 2x6 like I suggested and avoid all this.

Carl
 
Aluminum Threads

Another good thought Carl; Seems there should be a minimum number of "engaged threads" in the tiedown eyelet/device to support the weight - meaning don't use one with a very short threaded section. Wow! So much to think about; I'm just thinking out loud. Maybe you have the right idea and avoid all of this. A couple of my friends jack their aircraft using the gear, or an engine hoist to lift the entire front end... depending on what they're working on.
 
Another option is the fuse. Whenever I lift my 6A, I use the tall bottle jack, but insert a small, padded 2x6 between the jack and the fuse bottom. I apply the jack force where the spar runs in the fuse. It's less stable there than the wing, but doesn't require me to remove my tie down rings or create a damage risk to the wing skin.

Larry
 
lr172 said:
Another option is the fuse. Whenever I lift my 6A, I use the tall bottle jack, but insert a small, padded 2x6 between the jack and the fuse bottom. I apply the jack force where the spar runs in the fuse. It's less stable there than the wing, but doesn't require me to remove my tie down rings or create a damage risk to the wing skin.

Larry
Click to expand...

Is there actually anything backing up the center section flange in that area? I don't have any pictures handy, but thought there was an air gap between the center section flange and the spar itself. I'm not sure that flange is designed to take point (or nearly point) loads.
 
The safest solution would be to avoid jacking the wing at all, and jack up the end of the gear leg instead. The problem is the lack of an obvious way to do this on an RV-8. Has anyone found a good solution (that doesn't require taking off the gear leg fairing)? Maybe this?
 
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Tie down bracket

Carl Froehlich said:
(snip)
Another consideration. Unless you were careful to make the tie down bracket flush with the bottom of the skin, any fixture you screw into the bracket will deform the skin until the skin touches the bracket. (Snip)
Carl
Click to expand...

I installed mine flush for that very reason.
 
Same here.

And the Handy Jack concept isn't difficult to make, either. I made one using 1" galvanized pipe & fittings I had on hand; the bend was done on a HF hydraulic pipe bender.

Charlie
 
Carl Froehlich said:
How confident are you about those course cut threads in the soft aluminium tie down bracket?

Another consideration. Unless you were careful to make the tie down bracket flush with the bottom of the skin, any fixture you screw into the bracket will deform the skin until the skin touches the bracket.
Click to expand...

No deformation of the skin if the bolt is bottomed out in the bracket regardless of whether bracket is flush with skin. In addition it's then the AN3 bolts, not the aluminum threads that are carrying the weight.

Still, being careful, however you do it, is always prudent.
 
You can argue all day long but the fact of the matter is the tie downs where designed by Vans for jacking purposes. The few accidents where a result of improper technique and hardware. This could apply to any method used.
Do what makes you feel comfortable but there isn't any issue with using the tie downs.
 
Kyle Boatright said:
Is there actually anything backing up the center section flange in that area? I don't have any pictures handy, but thought there was an air gap between the center section flange and the spar itself. I'm not sure that flange is designed to take point (or nearly point) loads.
Click to expand...

Yes. The structural bulkhead that the wing spar bolts to. You are correct that the bottom of the spar does not contact the skin or the lower portion of the bulkhead. However all the weight of the spars is carried by this support member, as this is the only thing that the wing is attached to. The lower skin is rivetted directly to this support member.

I agree that a point loads would would likely be a problem for this .063" member. That is why I put a 2x6 between the jack and the bottom to spread the load across it. I have used 1/2" plywood successfully as well.

Larry
 
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