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10-25-2011, 02:54 PM
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Join Date: Apr 2010
Location: Apple Valley, ut
Posts: 35
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Impact-resistant, rotationally molded, cross-linked polyethylene
This is what I would like to see in the RV-12 custom molded with all the fittings in the right place.
With the number of 12s that Van's is selling this would ad just a few dollars to to the kit.
Impact-resistant, rotationally molded, cross-linked polyethylene fuel tanks.
Most if not all new cars and trucks use this type of tank.
http://www.westmarine.com/webapp/wcs...classNum=10444
__________________
#120367 started 4/21/10 first flight 10/20/10 with 10 weeks of down time waiting for parts.
Last edited by rschy : 10-25-2011 at 02:59 PM.
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10-25-2011, 03:35 PM
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Join Date: Dec 2009
Location: Boulder, CO
Posts: 4,435
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An alternate approach to reducing the risk of the current system would be to make appropriate modifications that add strength to the center section to prevent it from rotating aft.
Dave
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10-25-2011, 03:38 PM
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Join Date: Feb 2010
Location: Perth, Western Australia
Posts: 1,647
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Quote:
Originally Posted by E. D. Eliot
In my opinion, a safe fuel tank is paramount to the safety of the RV-12. After what we have learned about the current tank, I don't think that the fabricated aluminum tank is safe.
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There is no evidence to suggest that the current tank is inherently unsafe. The problem was caused by the way the tank was rigidly fixed to the channel, and I'm sure Vans will come up with better tank fittings. Chances are an alternative tank fixed in the same way would also be damaged.
I'm still a bit puzzled by the lack of any full depth ribs or stiffeners in the tunnel under the channel to help prevent the kind of severe rotation caused by a heavy rearward impact on the main gear as shown in the photos. The whole channel assembly appears to rely almost entirely on the cockpit sides for support. There is no attempt to use the floor ribs to share the load or to generally stiffen the struture in that area.
Granted that Van's may have very good reasons for deciding to leave the tunnel open (such as ensuring that excessive loads are NOT transferred to the cockpit floor under the occupants in the event of a crash), but otherwise this seems to be an area where the strength of the structure could be improved.
__________________
rgmwa
RV-12LR 912ULS
120346
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10-25-2011, 03:39 PM
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Join Date: Oct 2010
Location: La Feria Texas
Posts: 3,822
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It will take a lot of time I suspect, this is a change to an SLSA plane, so it is not as easy as the old days of kits, it will have to go through lots of testing and approvals. I am not so sure the tank itself is faulty, no problems have been encountered (once the leaks are fixed) in lots of hours. It was in this case not the tank, but the location of the attachment that caused the problem, and that can be changed or improved easily. I think the smart money is on the people currently flying that have come up with an easy effective fix while the gears turn at Vans.
Last edited by DonFromTX : 10-25-2011 at 03:42 PM.
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10-26-2011, 09:33 PM
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Join Date: Oct 2011
Location: Dayton, OH
Posts: 15
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Crash load design
Unless you have aircraft structural engineering design knowledge, I think you should not be redesigning the attachment structure for this tank. I am a retired (Air Force civilian) structures engineer and know a little about design for crash conditions.
The controlling design loads for crash is 16 G forward, 9 G lateral, and 3 G vertical. According to Mil-Std-1290A design shall be computed with a 75% full tank. The cg of the tank with fuel would be elevated over the aft floor. In a forward crash the tank would tend to rotate (****-toe action) about its forward edge. From the pictures first shown in this thread, it appears that the current attachment did its job in retaining the tank to the airframe and only the forward attachment was compromised due to roll of the carry-through structure. I would not be altering the forward attachment bolts. If they are there to carry the shear loads in case of a vertical acceleration (tip over onto tail) I would want all of the bearing area of those bolts in those holes, even if the carry-through rolls.
As to John Bender's design change, it does not look adequate. It looks like he is planning on taking the any vertical tank loads through two tie-wraps holding down the over-the-tank strap hook. The tie-warps would easily rupture and if the forward attachment (now pin) slips out of the hole you are now depending upon the floor attachments to withstand the 3 G vertical load. In addition, what structure is he tying his bracket to under the floor? Assuming the strap worked, could the floor attachment fitting separate from the floor?
What do I think Vans Aircraft might do? I think it is a given that he will eliminate the tie to the carry-through and then perhaps beef-up the floor structure attach points and change the fastener size and/or fastener pitch.
That's my educated two cents worth
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10-27-2011, 05:06 AM
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Join Date: Mar 2008
Location: Texas
Posts: 1,378
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As someone who sounds like he can answer this question....at what point is a compromised fuel tank considered part of a crash? This tank was tested to ASTM standards. Won't they all be destroyed at some point? I don't want fuel in the cockpit...but then I don't want to hit the pavement at 600fpm either. Thank you for the G force ratings...I'm just not able to relate them though I understand 16G's is a bunch.
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10-27-2011, 06:15 AM
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Join Date: Oct 2011
Location: Dayton, OH
Posts: 15
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I understand your concern about fuel in the cockpit during a crash condition. I am just trying to convey to the community that caution is in order when making mods to this important structural tie to the airframe. I think it would best be left up to the designers to make the mod since they understand the load paths and can best make a redesign that is both safe and efficient. I put my background into the posting so that you know I have some knowledge of what I am talking about.
George Dowell
Dayton, OH
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10-27-2011, 06:47 AM
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Join Date: Nov 2007
Location: Jesup, Iowa
Posts: 1,657
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Hey George - -
Appreciate your comments. Not that this makes a big difference, but there are 4 heavy tie-wraps on each side. The slots were smoothed out fairly well. As has been stated, this is designed to be temporary. I feel my risk factors are less than the original design factors. Larry posted this to make people aware of the potential SERIOUS problem. I chose this for a temp fix. Each person should decide their own risk and go accordingly. The accidents we have seen so far have been similar in nature. Not possible to design for every situation.
John Bender
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10-27-2011, 06:57 AM
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Join Date: Feb 2005
Location: Huskerland, USA
Posts: 5,862
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Quote:
Originally Posted by C120driver
I understand your concern about fuel in the cockpit during a crash condition. I am just trying to convey to the community that caution is in order when making mods to this important structural tie to the airframe. I think it would best be left up to the designers to make the mod since they understand the load paths and can best make a redesign that is both safe and efficient. I put my background into the posting so that you know I have some knowledge of what I am talking about.
George Dowell
Dayton, OH
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First and foremost Welcome to VAF.
Thanks for the good info. Keep it coming.
I tried to convey your message in my first post. I agonized for several weeks about posting this info after contacting Vans. I tried to do it in an informative and cautious way. In the end, we fly experimental aircraft. We must all decide for ourselves what is our level of comfort, but my first responsibility (IMHO) was to let the RV-12 community know of this one and only incident so they can decide what to do for themselves. Like I said, a good option is to do nothing.
__________________
RV-7 : In the hangar
RV-10 : In the hangar
RV-12 : Built and sold
RV-44 : 4 place helicopter on order.
Last edited by Geico266 : 10-27-2011 at 08:00 AM.
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10-27-2011, 07:03 AM
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Join Date: Oct 2011
Location: Dayton, OH
Posts: 15
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Thanks John . . .
Thanks for the follow-up comments. I am sorry to come down hard on you in my discussion, but I wanted to make it clear as to what you are "messin' with" here. I am glad to learn that you smoothed out the edges of the pass-through slots in your attach fitting. I was concerned not only about the strength of the tie-wraps themselves but the potential knife edge of the slots.
George Dowell
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