- Oct 11, 2021
I believe the SZD- Foka used tapered pins.
See post #70 above.References/examples?
Every engineering doc I've ever seen related to aircraft says that joint friction is ignored by the designer of structural shear joints.
The few strut braced attachments I've seen use clevis pin type attachments; in some cases using a literal clevis pin & clip, with zero clamping force.
Maybe I'm having trouble parsing this; perhaps it's the added phrase 'without preload'. All my informal training has said that the threads shouldn't be within the shear-loaded area of the joint, so the threads *don't* take any of the shear loads. Then there's the example of strut braced wings using literal clevis pins at their joints. At least in the case of Van's designs, the spec to use a castellated nut on the rear spar/fuselage attach bolt, because of the possibility (expectation?) that the joint can rotate with wing loading/unloading; the same caution given for any joint that can be loaded in rotation (sorry for not having the correct engineering term for that).Even with a close-tolerance bolt without preload, some of the bearing occurs on the threaded portion of the bolt
Yes, that's good practice, and the thread length on AN bolts is designed to accommodate this. A slightly longer bolt and a second washer under the nut can accomplish this. (This is convenient since two washers equals 1/8" and AN bolts come in 1/8" increments--see AC43-13 on stacked washers.) I only mentioned bearing on threads because many bolts have a longer thread length than AN hardware, resulting in either lots of extra thread outside the joint, or someone not knowing any better and leaving threads inside the bearing portion of the joint, which is common practice (at least outside of aviation) on joints not designed for shear.All my informal training has said that the threads shouldn't be within the shear-loaded area of the joint, so the threads *don't* take any of the shear loads.
The castellated nut makes sense to safety the nut given the potential for rotation caused by cyclic load reversal; Van's probably also has included this to ensure not only that the nut stays in place, but so that preload is maintained. I shouldn't really armchair-engineer their design, but if I were designing a similar joint, I would make it preloaded and check for a very clean set of faying surfaces before assembly. (I bet there's a torque value called out on that bolt, and it wouldn't surprise me if they have either a prohibition on, or guidance provided on, faying-surface coatings.)At least in the case of Van's designs, the spec to use a castellated nut on the rear spar/fuselage attach bolt, because of the possibility (expectation?) that the joint can rotate with wing loading/unloading; the same caution given for any joint that can be loaded in rotation (sorry for not having the correct engineering term for that).
I *know* (from direct, personal experience) the clamping idea is ineffective with, for instance, IVO props. Back in the 1990s (my last experience with them) IVO prop hubs had no drive lugs for the blades. The blades had loose-fitting holes for the prop bolts. When assembled to IVO's torque specs, the blades had a pretty strong tendency to move in the hub, to the point that IVO had to issue service bulletins about it, then they knurled the faces of the hub clamping plates, and *still* had blade movement when running on direct drive 4 cyl engines, and eventually quit selling the props for Lycs because they couldn't solve the blade movement issue. Now I realize that this isn't a steel-on-steel or steel/aluminum joint (it's aluminum/fiberglass interface), but is there anyone on this forum who would risk running a constant speed prop on a Lyc, without the drive lugs? Based on the clamping theory, it should work just fine.
Discussion of this 'new' wing probably deserves another thread, but from what I can see, this 'new' wing isn't really achieving anything over the original wing...?Hi PC,
There are spherical rod ends on the studs. The good question or concern is the bending loads imposed when the aircraft is pulled or pushed on the struts when ground handling the aircraft. I have been working on a KF-7 and am aware of the design details. This is why Piper has increased the strut forks to 5/8dia. on the J-3 and all others.
Designing with bolts in shear is optimum for light aircraft. As for the North American AT-6 cantilever wing attach of the outer panels is at the outer most surface of the airfoil, a multitude of bolts in tension on the flanges.
We are currently designing and building a new and more conventional wing for the Aircam that will create an alternative to the factory wing. Here are some photos. Best regards, Rick Berstling
Gets better all the time. I was concerned when someone indicated it MIGHT be okay to have threads in the fitting (?). I don't like this, even on nonaviation applications. I was aware, although it's been a long time that AN hardware was available in 1/8 increments, that there were various thicknesses of washers, that you were limited to a certain number of washers (2), and a certain number of exposed threads (was it 3?). DennisSlight correction here on washer thicknesses... AN/NAS/MS series washers come in at least 5 different thicknesses, depending on size.
Typical engineering standards are for a total of 3 washers on a single bolt or screw, but, there are cases where there are different callouts. MS spline bolts have a radius under the head and must be used with a special radiused washer, so that it sits flat on the surface. Often, this is then combined with up to three washers for the rest of the installation. There is also the case of a type of nut that has a concave faced washer that is used with it to sit flat on a surface, where a bolt or screw hole was not drilled perpendicular to the face. I've forgotten the designation on them, but they allow a bolt to have something like a 10 degree angle thru the structure. Another oddball case, is where the designer calls out a peel washer along with multiple regular washers to act as a gap filler on a non-moving installation. The peel washer is often up to .090" thick and is constructed of .003" laminations. It's peeled to provide the exact thickness required, when standard washer thickness cannot be combined to generate the needed dimension.Gets better all the time. I was concerned when someone indicated it MIGHT be okay to have threads in the fitting (?). I don't like this, even on nonaviation applications. I was aware, although it's been a long time that AN hardware was available in 1/8 increments, that there were various thicknesses of washers, that you were limited to a certain number of washers (2), and a certain number of exposed threads (was it 3?). Dennis
Regular AN bolts and nuts are intended for tightening to a large fraction of shank yield. Regular bolts and nuts have enough material in bolt heads and nuts to carry those large loads that make for friction.
Loss of preload comes from:is loss of preload a result of high stress and deformation????? and that is why we ignore friction in our calculations?
With the way engineers like to calculate and test things I would have though there would be all kinds of charts as regard friction and joint strength.....what charts I have found point to traditional calculation ignoring friction.....perhaps if I factor in fatigue exposure the calculated numbers make more sense?If joints fail, somebody or multiple somebodies failed at their jobs
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