Need help for Modified Seat-back and safety harness system

I'm soliciting help in modifying the front seat-back and safety harness system. Specifically, I need assistance in determining what and how much material to use.

 

On the standard Cozy MkIV, the shoulder harness anchor points are attached to a composite cross-beam that extends from one longeron to the other, at shoulder height.

 

I have modified my project, resulting in a shallower fuselage. As a result, my seat backs rise above the longeron level by about 8".

 

My design intention is to keep the longeron-level composite cross beam as the anchor points for the shoulder harnesses, but loop the harnesses up and over the seat backs.

 

I will use an inverted-"U" or "A" shaped seat back, composed of composite box-beams 2.5" wide, 3" deep. The "U" or "A" seat-backs will extend from the floor, up through the longeron-level cross-beam, and up 8".

 

The box-beams will use a 3/8" H-45 (3#) Divinicell foam core, with a 1 ply BID internal layup, and 2 ply BID external layup, at +/- 45 degrees. On top of that I will lay up E-glass roving for a cap, with another 2 ply BID layup at +/- 45 degrees.

 

This is a design that is employed in several car-seats. However, I do not have the experience to determine how thick the composite layers should be. I would like to avoid going overboard on

 

 

Design Loads:

 

I am using the FAR 23 guidelines to determine the static load requirements for the seat.

 

FAR 23.785a specifies a occupant weight of 215 lbs, with a 1.3x safety factor.

 

FAR 23.561 specifies a static load (in the foward direction) of 9bs. Using the 215 lbs passenger w/1.3x safety factor, that corresponds to 215 x 9 x 1.3= 2515.5 lbs.

 

Taking this a step further -- if we presume a 2.0 safety factor to account for homebuilt composite construction variances, and assume that the shoulder harnesses restrain 70% of the load in a forward deccellerating crach, then the static load requirements for the shoulder harnesses (combined) is 215 x .7 x 9 x 2.0 = 2709 lbs.

 

So... The seat back needs to withstand 2709 lbs static load, and each anchor point (2 anchors) needs to withstand a 1354 lb static load.

 

(NOTE: The diagram is simplified to show only one seat, when actually there are 2 seats across.)

I would be very hesitant about this design. When your plane hits something and stops, the pilot would pivot around the seatbelt and his/her shoulders would slide down the shoulder belts, thus compressing the back.

 

Ideally one would like to have a two padded bar shoulder restraint like the ones that you sometimes see in amusement park rides. If you are going to pull a shoulder strap, it would be good to have it on a much lesser than 90 degree angle from the seats to prevent the slide and compress phenomenon.

 

In my Stallion, I took my shoulder straps out to the spar almost vertical and behind. But that was a high wing. Hope this helps.

 

I read an article about this once but don't remember where...

Tom,

 

I've attached two more pictures that show a little more detail. For a person of my height (6'), the shoulder harness will loop over the upper cross beam at 15-20 degrees above the occupant's shoulder.

 

This design keeps the shoulder harness well within the allowable FAA guidelines. FAA requres the shoulder harness to be no more than 30 degrees above the occupant's shoulder, and no less than 5 degrees below the occapant's shoulder.

 

I dissagree though that you want to have the shoulder harness below the occupant shoulder.

 

If the shoulder harness is going to cause a spinal compression injury, it's not because the occupant slides down the seat, but rather because as the occupant decellerates forward, he/she is "squeezed" or "bent backwards by the shoulder harness, if it's located below his/her shoulder height.

 

The worst that will happen with a shoulder harness that is attached above the occupant's shoulder is that he'll slip out of it.

 

(NOTE: Spinal compression injuries can still occur even with a well designed safety harness, due to a excessive vertical speed at impact.)

 

Althouth the shoulder harness is obviously anchored well below the pilots shoulder, the harness is effectively above the pilots shoulder, as it loops through guides in the seat-back cross-beam above the pilot's shoulder.

"I dissagree though that you want to have the shoulder harness below the occupant shoulder. "

 

I don't think that I said this...

 

Your new pix make me worry less about your design...

Your new pix make me worry less about your design...

...... however, the webbing used in your restraint system has a certified strength rating that does not take into account the pressure points you will create by running it over the seat back. It may not be an issue but I know from my experiece with parachute harness systems that what seems insignificant to the casual eye can bite you.

The other Tom.

...... however, the webbing used in your restraint system has a certified strength rating that does not take into account the pressure points you will create by running it over the seat back.

I hadn't considered this. The seat back corners are well radiused, and the "hole" it will run through is radiused on the corners as well. That should help. I'll have to investigate this aspect though.

 

I know car seat belts typically pass through a steel "D" ring bolted to the A-pillar, which has a much tighter radius than I'm using, with a smaller, more concentrated pressure point. While they may be designed for this, I'd have to see if there is any difference in material or dimensions between car & aircraft shoulder harnesses.

I just happened to read this this AM, I think it may lend some information useful to the safe design of your harness system. I doubt you will be F-1 racing but this information is offered with safety as a primary objective.

 

"2.7 Safety Harness - It is recommended that each mounting point be designed to support at

least a 12g load or 1300 lb. load in the direction of the belt (180 lb x 12 g x 60%). The rules

require a shoulder harness. The shoulder straps should be designed to meet the shoulders at

an angle of 0 deg. to 30 deg. above the horizontal to prevent download on the spine in an

accident."

 

from International Formula One Design Guide 2002, www.if1airracing.com/IF1DesignGuide2002R1.pdf

 

Also refer to Mil-HDBK-17 for information pertaining to composite structures design, I think you need Vol. 2F, and maybe volumes 1 and 3 as well. I read these and MilHDBK 5 when I have trouble falling asleep.

That's interesting, because SCCA and FIA (if I remember correctly) require that the shoulder belt run from +10 or 15 to -35 degrees or so, from the shoulder top to the fastening or brace bar point.

Gontech. Thanks -- I was looking at racing seats and rules earlier, to see what design insights they might have. Too bad they generally design with steel / aluminum roll cages vs. composites.

 

I've been using an online beam calculator to perform some simplistic calculations on the composite beams. I used (http://www.wdv.com/Software/BeamCALC/BeamCALC.xls)

 

 

For simplicity sake, I calculated the load capacity of one (1) of the two vertical beams that make up the seat back.

 

Box Beam:

I used an 8" cantilever, hollow box beam, 3" x 2.5". (8" corresponds to the unsupported top of the seat back, from the seatbelt anchor to where it loops over the back of the cross beam.)

 

Materials:

I modified the tools internal material properties data, selecting a manual layup 1543 S-Glass, and reducing the properties to 65% to account for the reduced strength of E-glass spar tape.

 

Layups:

I used the equivellent of 3 layers UNI tape for the for and aft faces, 2 layers for the sides. (This calc. ignores the 4 layers BID sheer web).

 

Results:

Based on this rather simplistic analysis tool, I show that this beam should support a force of 4,400 lbs, concentrated at the end of the beam.

 

 

 

 

Way forward:

Using two beams, that should more than exceed the the required (approx) 2709 lbs.

 

Next step is to calculate the load on the cross beam (a simple supported beam).

 

Following that, I think I'll make a protype seatback, build a test rig, and weight it to 3000 lbs. Not sure where I'll get 3000 lbs of weight, or how to build a test rig that will hold that weight yet though...

That's interesting, because SCCA and FIA (if I remember correctly) require that the shoulder belt run from +10 or 15 to -35 degrees or so, from the shoulder top to the fastening or brace bar point.

seeing that this is not for the SCCA or FIA I would check with an aircraft seat belt manufacture like Hooker Harness. they have done a few more aircraft installs then the SCCA has.

Very good prices for the amount of safety

their belts are made for aircraft and designed to not clutter up the cockpit like the racing seat belts do. they use 1 3/4" web not the bulky 3" stuff which is very bulky in a cozy

http://www.hookerharness.com/aviationgallery.php

seeing that this is not for the SCCA or FIA I would check with an aircraft seat belt manufacture like Hooker Harness.

 

I have Hooker Harness belts, but for the MkIV. Very sharp looking.

 

When I talked to them re: my seat design they recommended the FAA +30, -5 degree angles.

 

Steve

seeing that this is not for the SCCA or FIA I would check with an aircraft seat belt manufacture like Hooker Harness. they have done a few more aircraft installs then the SCCA has.

Please note that nowhere did I state that there should be any applicability of the SCCA or FIA regulations to aircraft design. I was only noting the differences of the seatbelt specs, for two applications that share similarities.

Any suggestions re: the math?

Perhaps your diagram is just too simplified at this stage to depict your plans in this regard, but something to stop the straps slipping off the 'shoulder' of your beams would probably be required.

I hadn't considered this. The seat back corners are well radiused, and the "hole" it will run through is radiused on the corners as well. That should help. I'll have to investigate this aspect though.

 

I know car seat belts typically pass through a steel "D" ring bolted to the A-pillar, which has a much tighter radius than I'm using, with a smaller, more concentrated pressure point. While they may be designed for this, I'd have to see if there is any difference in material or dimensions between car & aircraft shoulder harnesses.

a-piller ? you holding in the engine:p you mean B as in #2 upright post from the fw

and you mite think on putting in some truss like chunks in side your box

as you add wight i like to try to guess what will give next and so-on.

what i see in the pix is a tube that if added lots of wight it would tear from the longs or bend or collapse. 6 bid taper to longs, trussed, and the outer bid as you described with a smooth edge(d-ring)

mine is allot like the one you are doing, i have a d-ring at the top of my seat back.........do you think we can see a pix of the plane, it sounds like you have moved forward a lot?? plz

Perhaps your diagram is just too simplified at this stage to depict your plans in this regard, but something to stop the straps slipping off the 'shoulder' of your beams would probably be required.

Yeah the drawing is simplified. The straps actually pass through openings in the top of the seat (above the upper cross beam). They won't slip off unless they can tear through 3 layers of BID -- unlikely since the forces will be down, not lateral.

 

See the picture in the next post...

a-piller ? you holding in the engine:p you mean B as in #2 upright post from the fw

Yes, what you said.

 

 

and you mite think on putting in some truss like chunks in side your box

That doesn't add much weight and improves the box beam. I don't know how many to add, or where though...Originally I was going to dispense with the foam core. But it only adds 2lbs per beam, so it's back in.

 

as you add wight i like to try to guess what will give next and so-on.

what i see in the pix is a tube that if added lots of wight it would tear from the longs or bend or collapse. 6 bid taper to longs, trussed, and the outer bid as you described with a smooth edge(d-ring)

My guess is it will tear the seatback forward, away from the longerons, rollbar, and sidewalls.

 

When I do destructive testing on the prototype seat, the seatback will be attached to a half (left or right side) of the seat back. The seatback will attach to the testing rig -- that's probably where the failure will be.

 

But where do I find 2,700lbs of weight for static testing???

 

do you think we can see a pix of the plane, it sounds like you have moved forward a lot?? plz

 

Ask and you shall recieve -- see attached.

i once had a dreem.... but Steve is the smartest guy i know !

it is looking like you could pull this off:o

i will be the first to tip my hat.!

 

you could try a portapower from Haber frate. they are sold as a 3 ton or a 7 ton (10). but what if you got a bottle jack that could only lift 2500 Pd's

then just rig it to push or get a 5000k and your best guess at haft of what its worth ? there are pulling gages and scales. rig a beam across a bathroom scale so it has a 10 to 1 push ex... 4 foot 4x4,one end on the scale the other end on a small block, you stand in the center and the scale reads 1/2 you wight and so on. find the sweet spot for 10 to 1 and push on your seat back 250 Pd's with the bottle jack (or screw jack) and wala...sota:rolleyes: