Series-5 Seat Pan Cracking

Just thought I'd post a comment about my seat pans, after my recent
re-upholstery project. When I stripped the seat pan I found significant cracking
along the front lip, and the lip at the top of the seat back. I have since
significantly reinforced the seat, but it's something people should look at
carefully at annual.

I added about 6 layers of glass in the cracked areas, and .025 aluminum
doublers which are riveted and bonded in place. I also upgraded to 3/8 wide
cable ties.

Just for what it's worth I weigh 170 and typically fly solo, so the seat is not
overloaded. I do have under the pilot seat a safety tunnel to keep the seat
off the controls as well if the seat should somehow bust loose.

Here's a pic with the upholstery installed, and even with the very big cable
ties in place they are hidden beneath the carpet which is velcro backed
under the seat cushions.

Jeff Hays

Good info. Jeff. Thanks.

Hey Jeff do you have any pictures of your repair. It might be prudent for me to beef mine up before I start flying mine.

No pics, but if you can visualize a bit, here is what I did:

where the seat lip goes over the lower front tube, and behind your back
the upper tube, I scuff sanded, and cleaned the top and bottom of the
pan with MEK and layed up about 6 layers of additional glass cloth. I used
polyester Resin, as I think the seats are polysester resin based. In any case
it sticks well.

With the seat pan in the plane I marked with pencil where the frame tubes
were located. I then removed the pan, and cutout 3" wide strips of .025
aluminum (6061) the full width of the flat area of the pan (under your thighs,
and behind your shoulders). I did an inside and outside piece for the four
flat sections of the pan (8 pieces of aluminum). These were located right
behind the pencil marks. I painted them with polyester resin, and thru
riveted them in place sandwiching the seat pan between them. The rivets
I used are 5/32 Avex every 40mm around the periphery of the aluminum.
(I'm also a Zenith builder so I have a ton of these rivets on hand)

Next through all this mess I figured out the best places for my cable ties to
go and drilled holes all the way through. So the ties are going through the
upper aluminum, seat pan, and lower aluminum and around the frame tube.

So imagine under your thighs there is a strip of aluminum left to right directly
behind the frame tube, both above and below the seat pan, through which there are six
3/8 cable ties. Same behind your back, and copied on the CP
side.

This might be an enormous amount of overkill, but I do NOT want my seat
to break loose.

I will also say that I even notice that the seat feels firmer .... Something I
previously thought was the foam in the seats.

My seat install originally was exactly per Skystar's version of the instructions
with no deviation, which like a lot of things I now think (after ten years
flying it) was inadequate. Like I said I'm about 170, and have only ever had
1-2 passengers over 200 so the seat has never had a heavy load. It just was
not strong enough by design over the frame rails.

Regards,
Jeff

Thanks, I can understand what you did. Sounds like a good idea, I weigh around 210 pounds.

Thanks for the details and ideas Jeff. I think this issue has been mentioned a few times with some people installing timber supports under the seats to stop the seats falling onto the control rods under them in the event of cracking. I wonder if anyone knows if the design ( shape/strength etc) has been changed for the model 7 or is it the same as the 5???

thanks again for the really practical post.
cheers

ross

Ross- the seat has been changed since the model 5. One obvious example, the seat tool boxes have been incorporated into the seat. No cutting of the seat is needed. Just make a lid and hinge. This retains, in my mind anyway, lots of initial strength. Cable ties are no longer used to retain the seat although I'm not quite far enough along to tell you exactly what the final retaining system is but I know there are welded attach points now. Likely changed not because of weakness but knowing John, more to make building and service easier.
I've seen many earlier models with too much fiberglass material removed from the top and bottom seat lips by builders. It makes you wonder how the seat stays on the tubing even during a normal landing or minor turbulence.
Zip ties lose their strength over time so relying completely on those keeping the seat off the controls with 2 big guys in the seat seems optimistic to me, although at least once a year those ties should be replaced with new if following the conditional inspection check list.

I've seen many creative ways to prevent seats from slipping off the support tubing to prevent control binding. Early model owners should be using at least one of those methods to prevent this problem.

DF 4 is totally up on the current configuration - but then I guess he has one of the new kits!.;)

I had one of the early S-7 kits prior to John's ownership of the company and I can confirm that the new seat strucure which I have seen on newer kitfoxes is great.

Although I had the old seats with our kit, I did get the aftermarket under seat buckets from John to make the storage and provide support to the seat (and keep my tochous off the control rods if the seat didn't hold). There were some weak spots on the original seat and I also did some minor fiberglass work to fix that.

Although our kit did not have the welded attach points that the new kits have (great improvement), I never liked the zip tie thing and got some clamps so the seat is clamped firmly in place....a little putzy to work with the nuts/washers on back to take the seat out, but I like that better than the zip ties. To keep the cushion thickness at a minimum I used plain clamps with a strip of hockey stick tape around the steel tubes to prevent contact between the clamp and steel tube.

I am not that heavy either but I understand I might stress the seat a bit more in a steep bank:confused:

Sincerely,

Dave S
KF7 Trigear - Flying
912ULS Warp Drive
St Paul MN

I made some clamps out of stainless steel, then secured them to the tubing with Hysol. I also secured them in the "clamped" position with a couple of flush s.s. pop rivets. I then installed 10-32 steel nutserts (TricNuts) in lieu of nutplates. On the seat, I epoxied AN970 large area washers at the 8 locations where the clamps & screws will secure the seat.

I have reinforced my seat with a couple of layers of carbon. I also have fitted the underseat stages but only the p2 has the lid the pilot one helps keep the seat stiff. I have also fitted carbon and fibrelam bum blocks to protect the control push rods.

Nice install!

John

Looking at those clamps, you must do sheet metal work for a profession, those are NICE.

Paul

John,

I think I just found the way to beef mine up.;)

Hi Guys,

Hmmm..... you've got me thinking now, We have a Mk IV and it may be worth installing this magic piece of wood under the seat. Does anyone have photos of their install under an older style seat?

Thanks

Jono

John, I can really tell from this post and others before that you are building a quality Kitfox. You have put a lot of thought into many aspects of the process. Can't wait to see the finished product . Keep up the nice workmanship. Bruce N199CL

Quote:

---

Originally Posted by jmcaliece

Hi Guys,

Hmmm..... you've got me thinking now, We have a Mk IV and it may be worth installing this magic piece of wood under the seat. Does anyone have photos of their install under an older style seat?

Thanks

Jono

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Nothing fancy here, I just contoured a block of scrap wood for the seat pan to rest on.

Fantastic, Thanks Mate! A picture is worth 1000 words

http://www.teamkitfox.com/Forums/sho...ght=butt+block

For a few other ideas and conversations on the topic.

My seat pan is cracked and I have the added experience of my former Kitfox seat pan breaking in flight so, I'm going to reinforce this current one however, I do not have experience with Carbon/Kevlar and am wondering if this would be a good product for reinforcing. I also don't know how many layers of this fabric I would need to use.
https://www.ebay.com/itm/Carbon-Fibe...eb99%7Ciid%3A1

I've been reading up on it and will likely use a West Systems Epoxy resin. I planned to put reinforcements across the lip in the front and the lip at the shoulders as well as along each side (this is were the current one is cracked). Any advice would be greatly appreciated.

Hopefully Jeff sees this, (jmodguy), he has done alot of this kind of thing lately.

Don't forget to put supports under the seat; they may possibly be more effective than reinforcing the seat.
Attachment 22268Attachment 22267

Ok here’s what I have picked up during my cowl redesign/layups.
Epoxy and polyester resins are VERY different.
The tensile strength of epoxy resins are much stronger than polyester and much less brittle.
Gel coat is a polyester product and will not adhere to epoxy well and vice versa.
Gelcoat weighs about 10lb/gal!
Parts made with polyester resin are typically heavier due to the Gelcoat, added resin and matting added for strength.
Epoxy resin is waterproof. Polyester not so much
Epoxy will not shrink when curing. Polyester typically will, especially in larger quantities.
carbon fiber requires special fasteners (titanium). Aerospace cf layups typically have barriers between the cf and what it is contact with.
There are 3 basic types of fiberglass cloth.
E glass - this is the every day cheap stuff
S glass - this glass is stronger than E glass and costs a bit more
S2 glass - this is an aerospace grade glass and has tensile strength close to cf but is not as stiff. This is nearly as expensive as cf. This is what I used for my cowl.
carbon fiber is strong, but brittle.
some cf fabrics are interwoven with Kevlar to mitigate the brittle properties of the cf. I do not know how well these fabrics wear. With epoxy they should wear very well.
I will be making a seat pan using epoxy resin and either S2 glass, or the cf/Kevlar cloth I the near future. It will be less brittle, and stronger/lighter than the polyester seat pan. I’ll let everyone know what I learn!
I am NOT an expert in composites. I have learned a bit about composites over the last year or so...
So take this for about what you paid for it...!

what a piece of kit is what I kept saying to myself as the weeks passed while trying to come up with something better than little blocks of wood placed over little horizontal tubes that were not intended to take vertical loading ..
to get clearance for the control rods under the pilot, and more shoulder room at the same time, the seat pan has been cut in two and supported by two bands of 2inch velcro, fully lapped, and the pilot seat being raised higher is taken up on the top mounting of the seat pan, thereby creating some additional shoulder room ...
forget about the little curved lip of the original seat pan, what kind of structural fastening can happen with that, sikaflex glue an aluminum strip and bolt onto some adel clamps ..
it takes a bit of trial and error to set some tension / load bearing on the velcro, but IMHO far better than trying to bond some additional strength onto a gel coat polyester glass fractured piece of kit

Quote:

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Originally Posted by jmodguy

Ok here’s what I have picked up during my cowl redesign/layups.
Epoxy and polyester resins are VERY different.
The tensile strength of epoxy resins are much stronger than polyester and much less brittle.
Gel coat is a polyester product and will not adhere to epoxy well and vice versa.
Gelcoat weighs about 10lb/gal!
Parts made with polyester resin are typically heavier due to the Gelcoat, added resin and matting added for strength.
Epoxy resin is waterproof. Polyester not so much
Epoxy will not shrink when curing. Polyester typically will, especially in larger quantities.
carbon fiber requires special fasteners (titanium). Aerospace cf layups typically have barriers between the cf and what it is contact with.
There are 3 basic types of fiberglass cloth.
E glass - this is the every day cheap stuff
S glass - this glass is stronger than E glass and costs a bit more
S2 glass - this is an aerospace grade glass and has tensile strength close to cf but is not as stiff. This is nearly as expensive as cf. This is what I used for my cowl.
carbon fiber is strong, but brittle.
some cf fabrics are interwoven with Kevlar to mitigate the brittle properties of the cf. I do not know how well these fabrics wear. With epoxy they should wear very well.
I will be making a seat pan using epoxy resin and either S2 glass, or the cf/Kevlar cloth I the near future. It will be less brittle, and stronger/lighter than the polyester seat pan. I’ll let everyone know what I learn!
I am NOT an expert in composites. I have learned a bit about composites over the last year or so...
So take this for about what you paid for it...!

---

Good info. Only thing I'd add is that you do not have to use gelcoat with any of the fiberglass's nor CF. It's purely cosmetic.

CF is not always the best thing for the job. It's greatest strength is it's stiffness when properly paired with the correct resin, but a lot of other materials are close with the same resin.

Being parts crack, a material such as Kevlar might be a better choice as it's more resilient and therefore does not crack as easily. In boats we layer CF and Kevlar to gain stiffness and some flexibility without failure. Meaning a pure CF product will break whereas a combination will flex and not break at the same loads.

Epoxy resins and chop fiber glass do not mix as the binder in the chop fiber is not compatible with epoxy.

As you touched on, polyester resins and gelcoat will not cure (kick off) when applied over epoxy, therefore a boundary layer must be put down first. Can be as simple as a layer of primer.

Finally, CF is no lighter than anything else out there. It's the biggest joke of a statement I read all the time. A 16 oz cloth of CF has the same weight as a 16 oz cloth of E Glass. I't just a matter of can you use less of one over the other and that's the key as sometimes the answer is no so the weight is the SAME.

S2 is damn amazing just not as attractive as CF but is definitely a swapable choice.

Hey Rik
I agree with you on the gelcoat. I won't use it if the part is to be painted. I guesstimated the weight on an average kitfox cowl and came up with about 5 or 6 lbs. I could be way off...
I used kevlar in some of my parts. I think it is best to get it in between the laminates. The kevlar I put on the edges of my boot cowl peeled off like peel ply.
My boot cowl and upper cowl were a layup of 5 layers of 8.6 oz S2 glass. After the epoxy resin was cured they became very stiff! I made a test layup (8x12) of 3 layers and the results were amazing! My lower cowl has a lot of compound curves so I stopped at 3 layers and added a 4th to the frontal areas. I shaved about 6 lbs off the weight of a factory cowl.
THanks for the other info. I was aware of some but it is good to get the info out.

Cheers

Without the Gelcoat you will have some pin holes that need to be filled. You can primer the mold prior to laying up the first layer and it will help with the pin holes.

Structurally speaking, the only areas that are prone to cracking are the mounting and any area where there's a hole. You can beef just those area's up and leave a single or fewer layers everywhere else in order to save weight.

With boats, we use a peel ply and then use bubble wrap to squeeze the resin under vacuum in order to get the resin out as it's the majority of the weight.

jmodguy, you wrote, "Gel coat is a polyester product and will not adhere to epoxy well and vice versa." ​

My question is regarding the adherence of the epoxy to the polyester resin. The Gel Coat is on the top surface of the seat pan. On the bottom there is no Gel Coat so, will adding strips of Kevlar/Carbon Fiber weave added to the bottom of the seat pan with epoxy adhere or am I just spitting in the wind?

If any release agents, wax, etc. are removed and the surface is roughed up, epoxy will bond to it just fine.
Polyester over epoxy, no.

Quote:

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Originally Posted by Maverick

jmodguy, you wrote, "Gel coat is a polyester product and will not adhere to epoxy well and vice versa." ​

My question is regarding the adherence of the epoxy to the polyester resin. The Gel Coat is on the top surface of the seat pan. On the bottom there is no Gel Coat so, will adding strips of Kevlar/Carbon Fiber weave added to the bottom of the seat pan with epoxy adhere or am I just spitting in the wind?

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Like John said, if you rough it up well with 80 grit paper you should get an acceptable bond. I re-inforced the edges of my seat pan with cf and west system epoxy resin after it sagged and got stuck under the door frame 😤