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Shoulder Harness attachment

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Viper_on_line

Joined: 27 Apr 2008
Posts: 3
Location: Maitland Australia

Posted: Wed Sep 10, 2008 2:21 am Post subject: Shoulder Harness attachment

I am used to a bit of security when I sit in a plane and am going to install a harness. Has anyone out there installed a 4 point harness rather than the Zenith lap and sash belt in their 701 or 801?

Now I am actually at that  stage I find myself designing a roll cage suitable for a racing car rather than an attachment that would suit the 701. 

Where do you secure the shoulder straps without over engineering the baggage shelf?



Thanks

_________________
701 scratch builder
one day it will fly

lrm(at)skyhawg.com
Guest

Posted: Wed Sep 10, 2008 6:33 am Post subject: Shoulder Harness attachment

Yep, mine is a four point.  I used a piece of aluminum flat stock, 3" X 1/8" I think.  Run it all the way across the front of the baggage shelf.  Then take another piece of the same and "T" it from the front center to the rear and bend it 90 degrees up for an inch or two.  Of course you do this underneath the baggage shelf.    Rivet or bolt it in several places.  Then bolt your belts down.   I feel a lot better with my setup than the original design of 3 points. You can get your aluminum flat stock at Lowe's or Home Depot.  I always thought that their aluminum was cheap stuff.  But one time when I was in the store I had them call the supplier and check, it is 6063 T5.   It's good all around aluminum with plenty of strength for general use. 

 

 Take care, Larry N1345L

 

 Viper_on_line wrote: [quote]    	  Quote: 	
		  --> Zenith701801-List message posted by: "Viper_on_line" <randnc(at)bigpond.com> (randnc(at)bigpond.com)



I am used to a bit of security when I sit in a plane and am going to install a harness. Has anyone out there installed a 4 point harness rather than the Zenith lap and sash belt in their 701 or 801?

Now I am actually at that  stage I find myself designing a roll cage suitable for a racing car rather than an attachment that would suit the 701. 

Where do you secure the shoulder straps without over engineering the baggage shelf?



Thanks  



--------

701 scratch builder

one day it will fly





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lgold(at)quantum-associat
Guest

Posted: Wed Sep 10, 2008 12:32 pm Post subject: Shoulder Harness attachment

Scratch,

The only change I made to my harness design to strengthen it was to add

plates inside/under the center seat belt attachment points to which I

attached 1/8" stainless cables to the opposite corners of the bottom of the

fus about 2' behind the seats. I have seen a situation where a very sudden

stop caused the center part of the seat to buckle, so I knew it needed to be

reinforced. My 701 kit was from 2004. I think that a rear plate for the

center seat belt attachment point now comes with the newer kits, but still

feel that this assembly needs to be anchored to the corner of fus to help

prevent it from buckling.



Regards,

Les

701-N67MG    



[quote] --

n85ae

Joined: 14 Mar 2007
Posts: 403

Posted: Thu Sep 11, 2008 11:54 am Post subject: Re: Shoulder Harness attachment

Something to think about, because Zenith sure did not do their engineering

calculation with regard to the seatbelts.



I calculated the stock 801 seat belt tabs at 200lb.s at 10G's in shear, if

indeed the rivets don't pull out before that. So I replaced them with 4130

steel the same thickness which I bolted with AN3's to the fueslage longerons. I also added structure to the center console.



Crash impacts can go up to 40/50G's and be survivable, so I'm not sure

what they were thinking when the designed the stock ones. But they were

not designed to handle more than the bare minimum. 



I suppose if I crash that hard I might not survive anyway, but I sure don't

want to leave on account the seat belt tabs sheared.



Jeff

Viper_on_line

Joined: 27 Apr 2008
Posts: 3
Location: Maitland Australia

Posted: Thu Sep 11, 2008 4:19 pm Post subject: Re: Shoulder Harness attachment

Thanks Guys,

The load dissipation of the seat belt mount can become intricate when you can't get to a longeron as in the case of the shoulder harness. And then as has been said is the 0.063 going to be going to be strong enough to prevent the bolt ripping out if you make the mounting structure too strong? I think it is better to have the seatbelt support structure crumple but keep you at least partially secure than have the belt bolt rip out. 



At this stage the plan is to increase the shoulder harness attachment brackets to 0.093 and using Larry's idea of a cross fuselage support at the front/rear baggage shelf junction and the zee channel all secured with A5 rivets. To spread the load I am using .040 L's from the centre of the cross fuselage harness support (long name?) to the back corners of the rear baggage shelf. Thus creating a truss of the rear baggage shelf and spreading the load hopefully over the whole area.



Does that sound suitable to the more engineering minded of you?

_________________
701 scratch builder
one day it will fly

cal.riggs

Joined: 17 May 2008
Posts: 3

Posted: Mon Sep 15, 2008 9:44 am Post subject: Re: Shoulder Harness attachment

Slamming the engineering is rather arrogant unless you are a structural engineer and even then there are better ways to discuss the strength of a system. Since CH has experience in structural engineering, I'd be more likely to ask the question to him rather than making negative comments as to competency on the list.



I've attached a photo of the field tested seat belt attachment. The collision with the terrain resulted in crumpled distortion of the fuselage NOT the bolt pulling through the attachment. I'm more likely to assume that the structural engineer has calculated considerations of the strength of the fuselage and its characteristics under heavy loads. This crumpling seems to have some good features in that the pilot receives less bodily damage due to an immovable seat belt system.



Regards,

Cal

 	  n85ae wrote: 	
		  Something to think about, because Zenith sure did not do their engineering

calculation with regard to the seatbelts.



I calculated the stock 801 seat belt tabs at 200lb.s at 10G's in shear, if

indeed the rivets don't pull out before that. So I replaced them with 4130

steel the same thickness which I bolted with AN3's to the fueslage longerons. I also added structure to the center console.



Crash impacts can go up to 40/50G's and be survivable, so I'm not sure

what they were thinking when the designed the stock ones. But they were

not designed to handle more than the bare minimum. 



I suppose if I crash that hard I might not survive anyway, but I sure don't

want to leave on account the seat belt tabs sheared.



Jeff

seat_belt_attach.jpg

Description:

Crumpled seat belt attach point.

Filesize:

41.13 KB

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seat_belt_attach.jpg

Description:

Crumple at seat belt attach point.

Filesize:

41.13 KB

Viewed:

422 Time(s)

n85ae

Joined: 14 Mar 2007
Posts: 403

Posted: Mon Sep 15, 2008 11:23 am Post subject: Re: Shoulder Harness attachment

Cal -



Glad you qualified that with "unless you are", because I hate being called

arrogant.



Jeff

 	  Quote: 	
		  Slamming the engineering is rather arrogant unless you are a structural engineer and even then there are better ways to discuss the strength of a system. Since CH has experience in structural engineering, I'd be more likely to ask the question to him rather than making negative comments as to competency on the list. 



I've attached a photo of the field tested seat belt attachment. The collision with the terrain resulted in crumpled distortion of the fuselage NOT the bolt pulling through the attachment. I'm more likely to assume that the structural engineer has calculated considerations of the strength of the fuselage and its characteristics under heavy loads. This crumpling seems to have some good features in that the pilot receives less bodily damage due to an immovable seat belt system. 



Regards, 

Cal

n85ae

Joined: 14 Mar 2007
Posts: 403

Posted: Mon Sep 15, 2008 11:28 am Post subject: Re: Shoulder Harness attachment

Meaning, yes I am an engineer.



Jeff

cal.riggs

Joined: 17 May 2008
Posts: 3

Posted: Tue Sep 16, 2008 5:08 pm Post subject: Re: Shoulder Harness attachment

Jeff,



I figured you were... an engineer. What branch? AE?



I thought that showing the photo of the deformation of the fuselage at the shoulder harness attachment would show that maybe a calculation of shear force on an anchored piece of aluminum may not be an accurate representation of the complete picture. Being that I'm not a structural engineer (or even a mechanical engineer), I got the impression that your calculation only included the aluminum attachment piece, the bolt, and 200 lbs at 10Gs shear. If you, in fact, included force absorption due to the deformation of the fuselage structure, then I'd say we may have to consider replacing our stock seat belt tabs. However, I can't seem to get past the evidence that in an actual crash, the tab was stronger than the fuselage. At what point, with the inclusion of structural deformation, does the scenario of tab, bolt, 200lbs body take over?



Regards,

Cal

n85ae

Joined: 14 Mar 2007
Posts: 403

Posted: Wed Sep 17, 2008 5:41 am Post subject: Re: Shoulder Harness attachment

Cal -



I would think your common sense would be telling you that, having your

seatbelt attached to something that might break is a bad thing.  If you 

really think it is plenty strong, then you're more than welcome to to build 

it per the plans.



Regards,

Jeff

randy(at)rjhebertassoc.co
Guest

Posted: Wed Sep 17, 2008 7:04 am Post subject: Shoulder Harness attachment

Cal and Jeff

I too am a structural engineer.

In buildings and general structures.



In my experience when my client perceives that something is inadequate,

no matter how hard I try to convince them that I am correct, in their

minds it is inadequate.



Perception plays a big part in this discussion.



Jeff can probably relate to this.



Although I am not an Aeronautical Engineer, Structural Engineering

relates to calculation of loadings and stress analysis in any structure.



Loads in these aircraft are of a small magnitude thus aluminum

structures.



Jeff the high G loading are probably out of the design criteria of these

aircraft.



It is like designing a building here in S Louisiana for a 150mph wind

when the statistical wind speeds are 110 mph.  (90mph thanks to Gustav)



The problem I see here is that more is not always better.  Increasing a

parts strength can move a failure regime to another part causing other

types of failures to occur.  



Thanks to both of you guys for bringing this issue up however, because I

will now look more closely at this area when I get to it.



Randall J Hebert

Randall J Hebert & Associates, Inc

Consulting Engineers

Ph 337-261-1976  Fx 337-261-1977



Cal -



I would think your common sense would be telling you that, having your

seatbelt attached to something that might break is a bad thing.  If you

rvickski(at)yahoo.com
Guest

Posted: Wed Sep 17, 2008 10:31 am Post subject: Shoulder Harness attachment

Gents, I am not an engineer but have a fair understanding of physics and many other things. This pertains to the 701, as I have no experience with the 801. The 701 was designed for the standard pilot, 5'6" 165 lbs or something close to that. Being 6'2" and 220 lbs puts me somewhat outside the design parameters of the restraint system ( a 150 mph breeze on a structure with 110 mph hurricane ties). With this in mind I am abandoning the three point harness in favor of a four point, my idea is to install a .025 hat section in the roof of the baggage area from front to rear intersecting at the center of the rear wing attach top channel, incorporating shear doublers at the harness attach point, this should address upper torso restraint adequately in conjunction with the existing upper harness mount. To a degree this will lessen the load on the three existing load points, however the weak link is still the tunnel. Zac addresses this with a doubler design for

 the tunnel. While this may be perfectly adequate I think it can be improved upon by an .040 attach point riveted to the top of the gear channel and tying into the existing tunnel restraint attach with a 3/16 cable. The outer lap belt restraint point can be doubled to the cabin side wall. I am hesitant to attach anything to the gear structure in the event it shears and departs, so the size and number of rivets need to be minimum for the gear channel modification I propose. I envision this type of failure mode (gear departing) to place the gear channel rivet heads in bearing stress while a frontal impact would place these same rivets in shear. By rough estimation not including the harness it would add less than 2 pounds.



Indeed more may not be better and may indeed move a failure regime to another part causing other types of failure but in the event these proposed modifications ever come into play, failure of some sort, has already occurred. The last thing I want to do is to design in a fatal flaw or perish because of an inadequately sized restraint system.



All responses welcome.

 

Roy Szarafinski

701 plans

roysgarage.com

---

n85ae

Joined: 14 Mar 2007
Posts: 403

Posted: Wed Sep 17, 2008 11:03 am Post subject: Re: Shoulder Harness attachment

It looks weak to me, so I'm making it stronger.



Jeff

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