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Clearance hole in skin for tie down ring

R

roee

Well Known Member
I'm finishing up the aft bulkhead... ready to drill a clearance hole in the bottom skin for the tail tie down ring, but I'm not sure what size hole to drill. The manual says 5/8". But the diameter of the "stem" of the tie down rings from Van's is bigger than 5/8". Is the intention that the tie down ring will bottom out against the skin? Or should it clear through the skin and bottom out against the extruded block?

Likewise, the holes in the wing skins for the tie down rings are only approx 3/8" on my QB wings. So here again, the tie down rings would bottom out against the skin.

In both cases there's a gap between the skin the extruded block, so screwing in the tie down ring would tend to bend in the skin. That doesn't seem right.

Those who have been here before, please impart your wisdom!

Thanks,
 
I opened the hole in the wings as to allow the ring to seat against the block, same for the tail. An option would be to put in a set screw of the proper length to bottom out on the unthreaded portion of the block, then the ring would bottom on the set screw. You really don't want the ring bottoming on the skin in my opinion, especially the tail ring which can be left in permanently as a tail skid.

Larry
 
HA! A set screw is a good solution!

I was thinking of putting a spot of weld on the threads and grinding it round and smooth afterward.

Either way is good!

:rolleyes: CJ
 
Jamb nut

I ended up opening the hole to 3/4" to allow the nut to be inserted. But because the holes didn't line up initially, the nut still could not be inserted unless I made the hole even bigger. Then I had a moment of clarity and decided instead to buy a jamb nut (ie, flatter than a regular nut) from Lowes and slide it in from the rear. I am not planning on removing the tie down ring, so this works fine.
 
Home Depot to the rescue!

I decided to replace the tie down rings from Van's with something that doesn't have that shoulder, and could get through an (approx) 3/8" hole and then bottom out against the block at the end of the threads. A quick trip to Home Depot yielded the perfect solution. 3/8" x 4" stainless steel eye bolts, rated for a safe working load of 350lb. I'll have to shorten them by about an inch to the ideal length, but that's no problem. The eye bolts are manufactured by The LeHigh Group, product #7134, UPC code 071514002385. At Home Depot they were located near the ropes and chains, NOT near all the bolts and general hardware. They were $1.99 each (Van's tie down rings are $4.25 each).

Pictured: Van's tie down rings on the left, Home Depot on the right and center.
20060701_2.jpg
 
P.S.

P.S. I'm not planning to leave mine there in flight. If I did, I'd want some secure means of keeping it in there, like a jam nut or Larry's set screw (cool idea!) or maybe just locktite. Just so it doesn't fall out in the air and kill somebody on the ground. Anyway, we RV guys go to great pains to keep drag to a minimum, so taking the rings out before flight is an easy one.

P.P.S. Larry, are you sure the ring will act as a tail skid? My plane is not on the gear yet, but looking at the plans I think the tip of the rudder will still hit the pavement first.
 
Does anyone know what the recomended breaking strengths should be in the tie-down system? When I made some tie-down stakes according to the EAA plans, I was looking at kits in catalogs, and the rope breaking strengths were in the neighborhood of thousands of pounds, so I used some climbing line I had laying around. I rationalized it by figuring that a good blow (I am in hurricane country) could easily generate flying speed winds over the wings, and therefore I should have a tiedown capable of holding the weight of the plane.

I'm sure someone has studied this before...care to share?

The mention above of a 350 lb rating on the eyes reminded me of this question....

Paul
 
How Strong?

How much lift can the wing generate?
At takeoff speed and angle of attack, it's around 600 to 900# per wing.
If you have a tail dragger the angle is greater; it's at stalled configuration until the wind speed is higher, then at a greater angle.
Your maximum could be [empty-weight x 9 / 2 wings]. I would ignore the opposite role of the tail plane and just do the gross analysis. The answer from this equation is unrealistically high, since if the wind velocity is at, say, aerobatic speeds, you have a really big problem. Even most hurricanes don't go over 130 mph.

Just an approach - I have no idea what the "correct" answer is.
 
Here is another question on the general topic: How many times can you thread the ring in and out before the aluminum threads are severely worn? IE. steel eye bolt threaded into and out of a threaded aluminum block with coarse threads, 4-5 times per week. It seems like you would wear out the aluminum threads before too long?
 
Re: strength

The strength question is more complicated than that. Trying to calculate lift generated by the wings assumes a straight headwind. Seems that a crosswind picking up a wing would be a worse and more likely scenario.

Also, strength of the tie down rings is somewhat ambiguous (thanks for not letting me get away with that :) ). I'm not sure whether that 350lb "rating" refers to strength in tension (i.e. along the axis of the bolt), or any torsional forces. (Likewise, how "strong" are the mounting points on the airplane that the tie down rings bolt into?) In general, loads on the tie down rings will have a torsional component depending on where the anchors are located on the ramp.

Anyway, I'd consider the tie downs as a means of securing the airplane in reasonably fair weather. They should withstand the occasional 40kt gust of wind (from any direction) or the prop blast from some inconsiderate pilot. I wouldn't recommend leaving your plane un-hangared during a hurricane.
 
Re: wearing out the thread on the aluminum block

Scott raises a good point. Has anyone already flying and taking the rings in and out known this to be or not to be an issue?

I'm thinking that keeping the threads lubricated should make this work a good long time. Or better yet, maybe some steel threaded inserts in the aluminum blocks.
 
roee said:
In both cases there's a gap between the skin the extruded block, so screwing in the tie down ring would tend to bend in the skin. That doesn't seem right.
Click to expand...
I noticed that too. My plan (similar to other solutions mentioned) is to find some rubber tubing at the hardware store that will fit over the threads snugly. I will cut myself a piece 3/8" long or so (give or take), and push it down the threads so it rests against the base of the tie-down ring. When threading in the ring, the tube should go through the hole in the skin and when it comes in contact with the extruded block I will feel resistance and stop screwing it in (before it contacts the skin). I haven't taken then time to try it yet, but it seems like it could be a simple and effective solution.
 
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roee said:
Likewise, how "strong" are the mounting points on the airplane that the tie down rings bolt into?
Click to expand...

I noticed you had q-build wings, take a look at the drawings, I think you'll break something else before the structure inside the wings gives way. Certainly more than 350lbs. As far as tiedown rings go, Vans says you can use hardware store bolts, just weld the loop closed.
 
Origionally posted by Roee
P.P.S. Larry, are you sure the ring will act as a tail skid? My plane is not on the gear yet, but looking at the plans I think the tip of the rudder will still hit the pavement first.
__________________
Roee Kalinsky

Nope, not sure Roee, I had the rudder mounted but did not specifically measure this (the Rudder is off right now). In any event, a busted Rudder tip is better than a busted tail cone, although I would never drag the tail. :rolleyes:

Larry
 
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