Fuselage tubing size - shoulder harness analysis

[M Towsley]

Re: Fuselage tubing size - shoulder harness analysis

What belts/shoulder harnesses did you guys obtain? Thanks.

[skip]

Re: Fuselage tubing size - shoulder harness analysis

I don't know about the other guys, but the engineer in me LOVES it. I just looked at the tube in bending with the loads applied the distance in from a fixed end. I don't even remember the numbers now, but you DO need to remember that you will NOT be applying your full weight to the shoulder straps! Your CG is a couple of inches above your belt buckle, not at the base of your sternum like most people believe. The huge majority of the load will be taken out by the lap belt with the shoulder harness only taking the load applied at your collar bone from you bending at the waist. I think the tube was good for much more than 6G with those assumptions since the force it would actually see was much less.

Hank,

Thanks for the feedback. I agree that in reality, the shoulder strap and tube probably doesn't take that much. That's one good reason to keep telling everyone we can to put something in even if it just wraps around the tube. But I am kind of thinking of being really foolish and giving my numbers to the FAA on a 337 just to get them out as approved data. The basis that I have been told that they will use is in AC 21-34 that specifies (6.c, page 31) "A forward load distribution 40% to shoulder straps and 60% to lap belt. In addition, lap belt should be able to carry 100%..." Then in the next paragraph 6.d they say that when doing stress analysis on an airframe without original design data, conservative assumptions should be used. So if I go that way, I need to use the 40% load distribution.

I wish I had the papers from when we did the GA crash worthiness tests at NASA but they are all boxed up in the hangar. If you are anywhere close I would be happy to go out with you and dig through the boxes. LOTS of interesting stuff from the AGATE project. I would love to see your numbers, but if you think they may be too close to the margin, why not just put a split tube around the spreader to reenforce it? Simple, easy and will open up as it fails, but if you are getting that much force, it will never have to be loaded twice.

Hank

Unfortunatly, I am out in New Mexico. Hmmm, I have been suggesting to my wife that I should show her the National Gallery and the Air and Space Museum on a vacation...

[Bravo 6]

Re: Fuselage tubing size - shoulder harness analysis

Skip,

You certainly have done an admirable amount of homework! Thank you for sharing and asking the question(s). I too just purchased new harnesses. My BC12-D has never had shoulder harnesses and I am about to install them as well. Please share any more info as you work through your process. I am very interested in what you come up with.

[Hank Jarrett]

Re: Fuselage tubing size - shoulder harness analysis

I don't know about the other guys, but the engineer in me LOVES it. I just looked at the tube in bending with the loads applied the distance in from a fixed end. I don't even remember the numbers now, but you DO need to remember that you will NOT be applying your full weight to the shoulder straps! Your CG is a couple of inches above your belt buckle, not at the base of your sternum like most people believe. The huge majority of the load will be taken out by the lap belt with the shoulder harness only taking the load applied at your collar bone from you bending at the waist. I think the tube was good for much more than 6G with those assumptions since the force it would actually see was much less.
I wish I had the papers from when we did the GA crash worthiness tests at NASA but they are all boxed up in the hangar. If you are anywhere close I would be happy to go out with you and dig through the boxes. LOTS of interesting stuff from the AGATE project. I would love to see your numbers, but if you think they may be too close to the margin, why not just put a split tube around the spreader to reenforce it? Simple, easy and will open up as it fails, but if you are getting that much force, it will never have to be loaded twice.

Hank

[skip]

Does anyone know the tubing wall thickness of the 7/8" rear-spar carry-through tube just behind the seat?
The fuselage drawing seems to have its Taylorcraft part number as A-1815 at station 4 on the drawing, but the parts list here on the web site only lists completed fuselage frames as parts and doesn't go down to the individual tubes welded together. I can't find the specification anywhere.

My purpose is that I am adding shoulder harnesses. I really want shoulder harnesses. I have a nice new set of FAA/PMA'ed shoulder harness that match my seat belts.

I know that ACE-00-23-561-01 lets us retrofit add on shoulder harness as long as the harness is PMA'ed (mine is), the plane is old enough (CAR4-1938 certainly is), and there is no drilling or welding or cutting on the frame (Thus, a bolt-on split-tube system). This just requires an A&P sign-off instead of a 337 based on the argument that the seat belts alone meet the certification requirements, so add-on shoulder harnesses are minor mods. The ACE says that 337/field-approvals are preferred, but simple sign offs should not be denied.

So it is clear that I can wrap the belts around the tube and be legal, my goal is not just to be legal but to actually keep my face off the instrument panal in a realistic set of potential crashes. Now, I know that some folks out there have just wrapped the harness around the tube, and are a lot safer than with nothing at all. Good for them! I applaud you all for adding whatever you can in the way of shoulder straps. I believe that a good number have escaped bad injury because they added something. But the anal-retentive engineer part of me wonders what the failure modes and strengths really are and what wll really keep my face from the panel in the widest range of incidents. So with some help from a good mechanical engineer, we have been doing some calculations.

I have the 337 here on the web site for the split 1" X .058 tubing that goes over the rear-spar carry through, and am exploring using a similar system but not exactly the same. For example, I want to have a fitting for the harness attach so that the harness can rotate and not have the potential for side loads on the harness when it wraps around the tube.

I have AC21-34 that lists the strength requirements for all the different certification basies out there. CAR 4-1938 only requires 1000 lbs 45 degrees up. That is ridiculously low! it will keep my face out of the panal on a 25 mph ground loop, but most any more substantial crash will exceed that load easily. Also, it wasn't until CAR3 in 1946 that they started requiring G's rather than just load. A much more realistic case.

I am using a pilot load of 200 lbs rather than the FAA standard 170 to be real, and I have assumed that the tube is probably 7/8 X .058 because that's about right for a spar carry-through tube. But to trust the calculations, I really need to know the wall thickness. 7/8 X .049 gives much lower numbers. Thus this request.

Running the numbers with the harness wrapped around the tube just outside the short diagonals shows the tube failing at a fairly miserable 6 Gs or so. That is a fairly low-speed ground loop running into a fence post or something. I have helped my situation in a lot of crashes, but anything serious will still fail the tube and I may eat the instrument panel. So what can I do to make it better?

I looked at a 5/32 swaged cable from the rear spar carry through back to the next station to spread the load back, but there is little room back there and it is really hard to crawl back into the fuselage unless I really need to. I reall don't want to cut and patch fabric for this.

Adding a middle piece of split tube at the center of the spar carry-through tube and a set of 1" X .090 flanges welded to the front of the split-tube bolt-on fittings angled down toward the pilot's shoulders at 45 degrees stiffens the whole thing a lot and also gives a good place to run an AN5 bolt to attach the harness fitting. I don't want to add stuff at the back because of the aileron cables running by there and I don't want to poke up toward the fabric. The flanges down to the front just touch theheadliner in the cabin. The calcs seem to get this up to about 16Gs. That covers most accidents short of flying into a shear cliff at cruise.

Anyway, as with a lot of my previous posts, I have dumped out a lot of my thinking here. Any corrections or additions are welcome.
So does anyone know the wall thickness of the tube?