Yeah spam cans. Thin sheet alloy lousy in stiffness and alloy has a known fatigue limit.
FRP does not have a known fatigue limit as no one has done the testing to the finite limit yet. Big difference is while the FRP has a lower TS it is layered thicker to accommodate that and thus the panel is stiffer and thus less likely to buckle from shear loads on a rivnut. Yes there is the issue of the compressive strength but in this case a minor factor. Nige. From: [email protected] [mailto:[email protected]] On Behalf Of Michael Stockhill Sent: Monday, September 4, 2017 11:32 PM To: [email protected] Subject: RE: RE: Re: [DOG mailing list] Fuel tank removal Thanks Nigel for your informative thoughts. My reservations come from the use of rivnuts in what seems to be a structural application, but I am far more used to working on spam cans than on plastic birds. Michael On Sep 4, 2017 7:33 AM, "Nigel Baker" <[email protected] <mailto:[email protected]> > wrote: Hi All. I am answering several mails in one hit so excuse if disjointed. This mail is probably the best description of the procedure so far. I like the reference to wiggle as it is the best description. There are 4 variants of the tank fitted to the H36. The original 60 litre. Then a 70 Litre. And finally an 80 Litre with all variants being in FRP. Then the alloy version which bears no resemblance to the previous 3. They are all different but the wiggle principle works for me. There is a good explanation of the technique of removal in the maintenance manual if anyone cares to read it. Removal of the canopy ( a small job as Ian mentioned) is essential. The front retaining flanges mounted to the rear of the spar bridge are indeed removable as previously mentioned in the later variants but fixed in the earlier versions. While there is a distinct version I and version II there is a whole heap of variations in between them all that is not reflected in the MM or certification of the type. Things obviously worked differently in Austria under the LBA at the time compared to now under EASSA. As for strength for retaining consider the following. The Tail boom of the H36 is 4 layers of 92-125 and 1 layer of 92-110 with 4 roving stringers to add extra strength in 2 directions for ground loop/heavy landings. The cover panel for the inspection portal at the rear of the luggage shelf is from memory made of 3 layers of 92-125 and held in by 4 X M5 bolts going into rivenuts mounted in the bulkhead. The bulkhead is quite heavy for torsional stiffness so the panel is the week point. The tank weighs about 12KG again from memory. Assuming constant mass over length which is conservative given its shape this means a load about its Centre to create a moment resulting in a load of 6KG at its aft point. Assuming a fuel load of 80litres (Tank choker block) this adds 57.6 KGs to the load. Given the shape of the tank it is safe to assume about 1/3 is aft of Centre. Thus working on the same moment principal the fuel load at the aft point will be 19.2 KG’s So total load at the aft point would be around 25.2. Now turn the aircraft upside down at minus 6g. Load is now 151.2KG. So do I reckon me and the Mr’s standing on the tank inverted at the rear edge held up by 3 layers of 92-115 in an angle shape as a support close to the tank edge. Yep I do and more. The reason the H36 is 100K lighter than a G109 is because of attention by Wolf Hoffmann to keep the weight down in all areas of design and construction. And it works. None have failed in the air except for the prototype and it simply couldn’t handle the abuse it was enduring at the time. Every aircraft has its limits. Respect them or suffer the consequences Cheers. Nige. From: [email protected] <mailto:[email protected]> [mailto:[email protected] <mailto:[email protected]> ] On Behalf Of Greg Wilson Sent: Monday, September 4, 2017 11:52 AM To: [email protected] <mailto:[email protected]> Subject: Re: RE: Re: [DOG mailing list] Fuel tank removal Removing the tank is tricky. After removing the angle flanges and the luggage compartment base: Remove the fuel gauge sender and earth wires Remove the fuel intake pipe off the tank and fuselage Lift the rear of the tank until the tank is vertical Reach over and under to disconnect the fuel hose and fuel drain hose Rotate the (vertical) tank 90 degrees to the left (I think this is correct -rear should now be at the left. Once I worked it all out I wrote a diagram & instructions on the top of my tank in texta.) Lift and wiggle tank up and top forward to remove it through cockpit Good luck, Greg. Greg Wilson ---- On Mon, 04 Sep 2017 11:17:43 +1000 Michael Stockhill < <mailto:[email protected]> [email protected]> wrote ---- Thanks all. Whoever installed my tank didn't install any sort of flanges or other restraining devices at the rear bulkhead to secure the baggage compartment covers, so their back edges and the tank are floating. If anyone gets a chance I would appreciate a photo or two so I can fabricate something. There are two fiberglass angles creating a flange at the front of the tank under which the covers fit. These are attached to the back side of the seat bulkhead with rivnuts and machine screws. Naturally the machine screws were seized or the rivnuts spun so I spent an hour or two grinding off the screw heads so I could remove the flanges. I have a hard time believing that rivnuts are a solution for a structural application. Looking forward to pulling the tank tomorrow. May be an issue due to some fiberglass buildup behind the shoulder harness fittings from someone's indeterminate repair. Currently an interference fit. I hate cleaning up other people's messes on airplanes. Thanks for all of your insights. M On Sep 3, 2017 6:08 PM, "Ian Williams" <[email protected] <mailto:[email protected]> > wrote: The reason I suggested to remove the canopy is that it is real easy to remove and yes, I did crack the rear window !@#@#$ . Best regards Ian W. From: [email protected] <mailto:[email protected]> [mailto:[email protected] <mailto:[email protected]> ] Sent: Monday, 4 September 2017 10:44 a.m. To: [email protected] <mailto:[email protected]> ; Mike Collins Subject: Re: Re: [DOG mailing list] Fuel tank removal I forgot to mention that we have a fibreglass tank that replaced an ally one and same method worked with both. On Monday, 4 September 2017, 8:28:14 am AEST, Mike Collins <[email protected] <mailto:[email protected]> > wrote: If you have a new aluminium tank it just rests very snugly on (from memory) five or six fiberglass covered supports that are attached to the fuselage. The passenger luggage panels are what hold the tank in place together with the fuel filler system - there are no bolts! Mike _____ From: [email protected] <mailto:[email protected]> <[email protected] <mailto:[email protected]> > on behalf of Michael Stockhill <[email protected] <mailto:[email protected]> > Sent: 03 September 2017 18:52 To: [email protected] <mailto:[email protected]> Subject: [DOG mailing list] Fuel tank removal It looks like I need to pull my Dimona fuel tank to route ADS-B antenna coax. For the life of me I can't see how the tank is secured to the airframe. Where are they hiding the bolts etc.? Thanks M Stockhill Polson MT
