Seat belt/harness physiological considerations

[Andrew King]

I'm working on a replica 1913 airplane, in which the pilot's shoulders stick out a ways above the fuselage, so that it's impossible to install shoulder harness with the proper angle without having them outside the airplane. It would be possible to install a second seat belt under the pilot's armpits, across his chest, attached to the upper longerons, to perform the same function as shoulder harness, but are there physiological reasons why this would be a bad idea??



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[steveinindy]

It would be possible to install a second seat belt under the pilot's armpits, across his chest, attached to the upper longerons, to perform the same function as shoulder harness, but are there physiological reasons why this would be a bad idea??

I can probably answer this for you since you're asking about the area in which I work. The answer is unfortunately "no". While it would reduce the likelihood of a pilot flailing forward into the instrument panel, there are a few major problems with the idea.

The first is that you're introducing a stress riser almost directly over the great vessels of the chest (aorta, venae cavae and pulmonary vessels) and heart. Severe compression of the sternum is a significant factor in direct or hydraulic rupture of these structures. Even a person's head flailing down and forward as they decelerate can cause the chin to strike the sternum and inflict serious or lethal injury (this is called the "chin-sternum-heart syndrome" and has been described in aircraft crashes, race car crashes, motorcyclists and parachuting mishaps).

The second issue would be one of comfort and what would happen in a situation where the pilot slides down during a crash (called submarining). This is very common in restrained individuals where there is no "crotch strap". Conceivably, such a scenario could cause dislocations of the shoulders or fractures (due to upward bending of the lateral ends) of the clavicles. Either of those injuries would make it difficult or impossible to exit the aircraft without help. The other theoretical issue with a submarining scenario would be a distraction (pulling) injury to the thoracic vertebrae (the part of the spine to which the ribs attach). I say theoretical because the only use of a transverse chest strap that I have ever heard of was in concert with a lap belt, crotch strap and dual shoulder harness which would eliminate the issues I have mentioned.

The final two points to consider are the comfort of the wearer and the structural attachments. Something abrading or binding the chest and armpits would rapidly become obnoxious and would probably not be used. A good example of this would be to get a length of nylon webbing, sit in a chair and place the webbing across your chest in the way your envisioning and have someone (a wife, etc) move it back and forth a bit.

The limiting point in most light aircraft restraint systems is actually not the webbing or buckles but the points at which they are attached to the aircraft structure. The rate at which restraints fail in survivable crashes is appalling (it's often cited as 25% but a lot of survivability researchers believe this is an underreporting because of the problems the FAA/NTSB have with using a scientifically valid definition of a survivable crash) because the standards for design and manufacture are based around beliefs on human tolerance that were disproved around the time of WWII.

Honestly, ideal option would be simply to modify the design to allow for good quality restraints but it takes away from the look of a classic. Even a slight raising of the area behind the pilot could allow for restraint routing and attachment without excessively modifying the design in most cases.

Also, it sounds like your aircraft is likely to have the same problem as the popular canopy equipped aircraft if/when they turn turtle. Pretty much folks like me wind up helping look for the scattered fragments of the pilots head. The implications of the myth of "see and avoid" and the desire to look like a little fighter plane might well kill more people than the improved visibility saves annually. Sorry....didn't mean to rant a bit there....

[Bill Greenwood]

`Could the replica have a small headrest and fairing just outside and behind the pilots head that can hold the shoulder harness?

I would not compress the chest area, and you don't want to pull down on the spine either.

[steveinindy]

you don't want to pull down on the spine either.

You don't, but I honestly believe that- at least in light aircraft- the shoulder harnesses "pulling down" gets a lot of blame that is probably more appropriately directed at the failure to have an energy absorbing seat and/or just having the seat bolted flat to the floor of the aircraft. It's not a good thing to have the spine compressed from both directions (it's more complicated than that but let's not go there unless you're really interested in hearing it) but the harnesses are the lesser of two evils in that regard.

The biggest danger with "vertical" attachments of restraints (pulling them down over the back of the seat rather than attaching them behind the seat) is actually the attachment failing as the bracket or bolt fails under the torsional load rather than a tensile/longitudinal one.

[Kyle Boatright]

Andrew,

Why not make the shoulder belts removable for static display.

Also, you could use steel cable leaders from near the seat back to the primary structure, so the belts aren't visible in flight, other than from the transition at the seat back down to where they join the lap belts.

Realistically, you get the best of both worlds - aesthetics and protection.

[WLIU]

I will second the idea that you can make up a rig that is removable for static display. It is accepted practice to anchor a shoulder harness something like 36" behind the seat using cable, then run the cable over a hoop structure right behind the pilot. The hoop is higher than the pilot's shoulders so that the shoulder harness restrains the wearer but does not create compression force on the wearer's spine.

I agree that and anti-submarine belt (crotch strap) is a good component of a restraint system.

That said, the assertion that a belt across the front of the pilot from one side of the cockpit structure to the other will create more forces on the internal organs is not correct. Such a belt creates a barrier just like more familiar belt systems but will not create additional compression forces unless it completely encircles the wearer's chest. Is it more or less convenient to use? Maybe, maybe not. Looking at the FAA publications on restraint systems, AC43.13-2B Chapter 9, TSO C114, AS8043, it appears to be possible to design and install a torso restraint system based on the original poster's requirements.

That said, 1913 airplanes were pretty bad airplanes when examined by every modern measure and none of the replica's that I am aware of (Rhinebeck, etc.) fly very often, very far, or very high. I would suggest that a good crash helmet be part of the pilot's flight gear.

Best of luck,

Wes
N78PS

[steveinindy]

I would suggest that a good crash helmet be part of the pilot's flight gear.

In an open cockpit or canopy aircraft, you really need either a helmet that meets the requirements of motorcycle racing associations (or open cockpit racing cars e.g. F1 or Indy) or a helmet intended for helicopter crew. They aren't cheap but my choice would be: http://www.gibson-barnes.com/dept-29...56-Helmet.html

Actually, I should phrase that as "is my choice". I have one on order and I am simply waiting for it to be painted and delivered.

[steveinindy]

Looking at the FAA publications on restraint systems, AC43.13-2B Chapter 9, TSO C114, AS8043, it appears to be possible to design and install a torso restraint system based on the original poster's requirements.

Of course, you have to remember that these are the same people who believe a lap belt is suitable to protect folks in a 140 knot crash landing....just sayin'....

[Andrew King]

Thanks for the info and opinions everybody, lots of food for thought. One thing I am always careful about is good restraint attach points in the aircraft structure, although fortunately I've never had to test one. Also, this 1913 monoplane has a cable braced upside-down V cabane in front of the cockpit for wing brace wire attach, and that should be good rollover protection. Re the practicality of pre-WW1 airplanes, we put about 150 hours on the replica 1911 Curtiss Pusher in 2011, and about 5,000 miles cross country. It is a major effort though, with a lot of planning and support, and requires a little cheating, especially by using a modern engine (we liked to joke that 1947 was "modern"), http://elycurtisspusher.com/
Fortunately the Curtiss had good places to attach belts and harness.

I'll post pictures of the project when it gets far enough along.



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[steveinindy]

One thing I am always careful about is good restraint attach points in the aircraft structure, although fortunately I've never had to test one

That's one of my goals: that no one ever has to test my design's occupant protection systems in a real world crash.