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Building and finishing my 2+2/PA 14

Seatbelts are important. A friend of mine survived a crash but the seatbelts broke and he sustained serious facial injuries.
 
Jim,
The seat belt tabs are .090 4130. I bought them from Javron to save some time fabricating them. I have decided to wrap them around the cross tube, the surface of the tab will be parallel with the floor and angled up about 45 degrees. I think this will work easily. Pictures of the tabs are on another post, http://www.supercub.org/forum/showthread.php?49410-How-do-I-weld-in-my-seat-belt-tabs

Marty

Seatbelts are important. A friend of mine survived a crash but the seatbelts broke and he sustained serious facial injuries.
These two comments are leading me to question how much thought is going into the seat belt installations. Not just Marty and Jim, but everyone. This might be a good exercise for a retired engineer's evening project. He could calculate the crash loads of a heavy human body against the seat belt. The angle these loads apply to the seat belt anchors. The size of the bolt which holds the belt to the anchor. Is this bolt in shear or bending? Is the bolt in a single (bending) or double shear installation? The tear out loads of the bolt in the hole of the anchor strap. What is the direction of the load on the welding of the anchor strap? Are there enough inches of weld for the applied load? Will the .090" by x" wide strap handle these loads? How far around the fuselage tube should the anchor strap be wrapped? Will the anchor strap tear off in a crash? What alloy should the strap be? Will the fuselage tube support all of the above loads? All of these questions, and perhaps others, need to be answered with certainty. Whichever of those questions produces the least strength is the location that will fail first.

Darrel Starr, are you there?
 
If it's up to me and I'm free stylin' on seat belt mounts on an experimental I always go with .125 material in double shear. If you shear a 5/16 or 3/8 bolt in double shear you are probably not salvageable. As a former sprint car driver I have taken a few extreme G rides and have the mended collar bone to remind me.
 
I'm a bit stuck with the seat belt install and the seat's for that matter. After installing the seat tracks on the shims I'm having difficulties with the seats rolling easily. The new Piper seat track is taller and that might eliminate the issue. I will likely need to toss the tracks I have and get the newer ones from McFarland. The issue is that they are 2.3" wide at the base vs the current ones I have at 1" wide at the base. This adds to the issues with the seat belts.

First, a current picture of the tracks and seat with out the floor. Remember, anything can be changed; I need to get this right.

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Another view:

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You can see some of the issues here for the seatbelt install; where will they fit? I have two options right now to consider. This first option is to simply weld the tabs to a newly welded cross tube; the tabs would be between the seat rails. This would be easy but is it ok? I just don't have enough information to go on. Are the tabs too close, is the orientation ok for the seat belts? The belts would either run between the seat back and the butt cushion or around the seat. Here's a couple pictures of option one:

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The next option is to add a another tube to the right of the seat track and locate right tab on that tube and the left on the side of the fuselage above the ruder cable pulley. Is the left tab location in this option strong enough? It's just 3/4, maybe .035? Here's a picture of option 2:

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I can shorten the tracks and get rid of the wood shim under the track if necessary; those are there so I can slide the seat back further for easier entry and exit. As for thickness of tabs, I can go with any thickness at this point; not restricted to the tabs I have.

I'm really stuck here with figuring this out. Needless to say, there is nothing in the plans except to use a narrowed Cessna 172 seat and tracks with no indication of mount ing the tracks and no mention of the seat belts at all. This phase of the project is very slow and difficult when I have to be the designer and builder; not being very comfortable with the design part. I am open to any suggestions and changes here; different seats, tracks, other options on the seat belts, etc. I can't move forward on anything until I solve this problem. Thoughts???

Thanks,
Marty
 

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Marty, some random thoughts.
This link shows a recommended 55* angle to the horizontal for the lap belt. Figure this from your normal seat location.
https://www.faa.gov/pilots/safety/pilotsafetybrochures/media/seatbelt_web2.pdf

Dave Thurston's book "Design for Flying" suggests that the anchor location for the shoulder harness be able to support a 4000 lb load. Other publications suggest that the angle of the shoulder harness be such that there be no down load on the shoulders. 20* above comes to mind?

CAR 3.386(a) states that the belt attachments be able to withstand a load of 3.0g upward, 9.0g forward and 1.5g sideward. Since the belt is at an angle of 55* it will take some geometry to get the correct answer. The equation "the square of the hypotenuse is equal to the sum of the squares of the other two sides" will work.

I would chose to place the seat belt outboard of the seat frame in order to prevent crushing loads on your hips.

A suggestion. If the straps which you show clamped to the tubing were in the shape of a triangle, they would distribute the loads across a larger area of the tubing thus reducing any tendency to buckle the tube. And also would give a greater seat position range for optimum strength.
 
Skywagon,
The Shoulder harness is pretty easy; either the "X" brace can be used or the cross tube at the rear wing attach fitting location. As for the 55* for the seat belt I need to see where that puts the attach point relative to the seat when it is forward. As I have been doing some research into this, one of the issues came up with moving the anchor points to the fuselage frame from the seat frame. With the seat frame attach points the seat belt stays at the optimum angle and is snug in all seat positions; the floor mounted anchor points vary the angle and the seat can not be slid forward after buckling in. The sliding issue is not a big deal because the should straps would need to be adjusted when sliding anyway. As for the size of the tabs; the ones I have are the same size as the STC kit that moves the seat belts from the seat to the floor in the PA-18. Regarding the formula you gave me; what is that used for? I'm not sure what is being calculated using the formula?

The last two pictures give me the best option to have the seat belts at 55* where I will most often have the seat. Take a look at the two pictures below. I think the left side tab needs to be attached to the side of the fuselage to avoid crushing of the hips. I could weld a tube from the longeron to the diagonal that forms the door opening. In the picture I'm holding a piece of strap steel just for location. The tab could than be welded in the wrap method so it angles away from the side wall enough for bolt clearance. The last two pictures I have in previous post show a good location for the right tab; that gives me 55* also. Thoughts on this idea?

Marty

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Marty,

I used the same seats and rails as you. Instead of the wood shim I just bent the rail to the needed angle since that area is only used when removing the seat.
 
Jim,
I tried the bend but the seat would not go past the point of the bend so I tried the shim. I have given up on the shim idea; too much flex in the floor with the shim and the seats hang up to much. With the shim removed and the tracks cut down the seats slide much better. I can't move the seats back as far as with the shim but I'll get used to it and get the hang of getting in and out. I also decided to buy new track material from McFarland. The holes in the tracks I have are pretty worn so I'll replace them. I need 25" pieces so the cost isn't too bad for four new ones. I'm getting closer on the seat belt plan. The seats you build were for a 2+2 or PA-14? How did you do the seat belts?

Marty
 
I'm certainly not the expert here but I did a single shear and figured if my butt shears a 3/8" AN bolt I'm toast anyway...but then it can't hurt to double the tabs. I attached mine on the longerons at a cluster with a gusset using 0.100 CM but it's attached to 0.035 in the longeron so I put in a gusset to distribute the load. My weak point is the shoulder straps which are attached at "V" in the tubes but even so they can bend...without adding a lot of heavy metal I'm not sure how to make it stronger in the overhead.
 
Another reason for double shear attachments is to eliminate bending stress on the tabs, caused by eccentric loading.
 
I guess my question is do I build the seat belt tabs up stronger than certified Cubs? I understand the greater strength of the double shear vs single but is it really necessary? Below are a couple pictures from the STC that Atlee Dodge has available. Are there issues with this design? Like many issues in building a Cub, strength and weight have to be managed accordingly and it is very easy to over build. Has any one known of this type of floor mounted seat belt set up to fail in an otherwise survivable crash? I would think that to receive an STC proof of satisfactory performance needed to be met. Or, does this set up simply match the strength of seat mounted belts and they don't have to provide any greater performance for the purpose of the STC? Gordon, how are the seat belt's attached in your '12? I need to do the back seat also so I would like to have a better understanding of the certified Cubs before I do any welding. Below are a the screen shots from Dodge's web site.

Marty

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The first Atlee picture shows the tab to be welded under the tubing, good installation. This allows the tab some flexibility without tearing out loads on the weld. Naturally a double shear installation would be stronger though in this circumstance is not necessary. In this example it appears that the weakest point would be the edge distance of the hole in the tab.

This is a collection of handy bolt reference charts. http://www.skybolt.com/catalog-an-ms-hardware.pdf Look on the second page at the single shear strength of the AN4 bolt, 3680 lbs. More than enough for the seat belt application.

Just make sure that there are no notches in the edge of the tab which would be a stress riser. A crack or tear could start at this location. Particularly if the notch is next to the end of the weld. For this reason you want the end of the weld to be behind the tubing away from the flexing portion of the tab.
 
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When I made the post about doing tabs in double shear, it was only an opinion. I do it when it will work. .090 in single shear is dam strong too. Seat belts stretch in accidents, a lot more than you would imagine. An installation with proper geometry is as important as the material.
 
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Gordon, how are the seat belt's attached in your '12?
Marty, in light of this discussion I'm a little embarrassed to admit that mine are the factory arrangement. However - the factory arrangement did serve me well when I wrecked my plane 25 years ago. I hit a large upturned spruce stump with enough speed to tear the engine mostly off of the fuselage, knock the radio out of the panel and destroy one wing. But the seat belt (and shoulder harness) held and I was unhurt except for a dislocated toe where the firewall folded over my foot. But despite that success, I wish I had routed the front seatbelt to the floor.
 
I own the original 2+2 N65WA and am just finishing a restoration. My seat belt mounts (the original configuration) consist of 3/16 " cable wrapped around the fuselage tubes. The belt clips around the cable. I will post some pictures tomorrow. Not sure how they will hold up in a crash but that is how they were done.
 
Great discussion here. I like the idea of the double shear attachment. I think I could make that attachment pretty easy for the back seat as it wraps around the rear cross tube. Same could be done for the inside tabs for the front seats. The tabs that attach to the side might be a little more difficult to do a double shear. I did come across a nice attachment from Airframes Alaska. They use a welded lock nut on a tab welded at the cluster where it would be difficult to get to the back for a nut. I also like the way the tab is tied to each of the tubes next to the cluster.

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For the sake of discussion of ideas; here's another one. Since I am welding a cross tube between the "V" on the floor, I could slide the below tab on the cross tube and weld in place between the cross tube is welded to the "V". I would still weld the short tubes between the new 3/4" .065 cross tube and the existing cross tube to keep the new tub from bending in a crash. Single tab could be use or two tabs for each belt could be added for a double shear is preferred. I need to add the short tubes in that area anyway for an attach point for the floor mounted flap handle I still need to add. Here's a drawing of this attach tab idea.

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In the picture below, this new tab idea could be welded to the new cross tube before welding to the "V". I would place the tab just to the right of the inside seat track so the seat belt would be placed just outside the seat. Thoughts on this idea?

Marty

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Marty, Is this your fuselage or just an example of the use of the steel nut? The reason that I ask is because I am questioning the weld joints of the two flat tabs, with the large burned out notch to the vertical tab and wondering what is the purpose of that cross tube. What is the load path through both? If that upper tube is part of the primary structure the tube should have it's load path centered in the cluster not in a bending location.
 
The picture looks like it is left side of PA18 3rd seat seatbelt mod. Dogleg tube on right side and gingerbread for extended baggage.
DENNY
 
DENNY is correct; this is from a PA-18 third seat. This is from the Airframes Alaska web site. I posted this as a possible example of an alternate tab; specifically just the tab with the welded nut. A strap with a welded nut might come in handy in other tab locations where getting a nut on the seat belt would be a problem. I'm more interested in what others think of the drawing I suggested.
Marty
 
Other than the requirement of sliding it over the tube before the tube is welded in place, it is fine. You might wait to weld it until after the tube is welded in order to be able to fine tune it's alignment. Also it could be strong enough with just some tacks with no need to weld all the way around. Alternatively rather than a hole to slide over the tube it could be a "U" shape. Then it could be made stronger by heating and bending over the edge after welding the curved section. Then welding that bent over section to the tube.
 
I think sliding it over the tube is a great plan and as skywagon8a said wait to weld until fine tuned. Do you have elevator cables running under the floor in that area? Just something to check before final weld. I think the high heat nuts are one of the best things you can do to save yourself a lot of grief once the plane is covered. I see way to many cubs built/restored that people get in a hurry and don't do the little things that will make it easier for the next guy that has to work on the plane.
DENNY
 
Just one more thought about the nut. Over time the locking feature can become unreliable particularly since it has been welded upon. So it may be a good idea to have an alternative method of locking the bolt. Like perhaps a hole in the head for safety wire. I would hate to have a seat belt bolt (or any bolt) fall out just because it was not easily seen.

Another thought about the double sheer attachment. You could weld a second triangular tab over the first one leaving a space for the seat belt attachment to fit in between. Make the base of the second triangle about 3/16" less that the first. This will help to distribute the weld rather than concentrate it in one place.
 
DENNY,
No cables in the center area of the fuselage in that location. There is a push/pull tube that connects the sticks to a bellcrank further back. I'm beginning to like the idea of the slip on tabs. I could just as easily weld it after everything is in place to fine tune the angles; good idea. Other than purchasing the tabs from Airframes Alaska, I can make the tabs my self but need to find a weldable lock nut that can be gas welded (not stainless steel).

Marty
 
I hate the clip on nuts/tabs I have screw tabs some behind the side panels in my cub and they never seen to line up right.
DENNY
 
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