Fiberglass, or carbon fiber fuel tanks?

[Barney]

Currently, I'm flying a VariEze, had it for five years.  My other plane is an RV-3.  The RV-3's fuel tanks are TU, having been fixed many time, and both still have bad leaks.  The RV forum is "awash" with leaking fuel tanks (aluminum), however, I've never read of leaks being a problem with Rutan aircraft here.  

Van's doesn't supply an RV-3 wing fuel tank kit, and Evan Johnson (good reputation for building, sealing RV tanks) would prefer not to build from scratch, as we're going to have to do.

Wing fuel tanks are about 48" long, 8" high, and approx. 12" wide.

Please advise any problems with Rutan aircraft and fuel tank leaks, and offer whatever advice you can on building fiberglass wing fuel tanks for my RV-3.

I have already talked with Robert Harris at EZJets and he thinks this project is do-able.

Thanks for any help,
Barney

[N31WJ]

G'day,

There are hundreds of VariEze and Long-EZ aircraft out there with 10-20 years service and no leaks.  I've rarely seen or heard of a leak in Rutan aircraft, and I suspect some leaks are a result of substandard construction or perhaps a hard landing.  There have also been leaks atributed to using auto gas containing ethanol, which can degarde some epoxies.

There is no advantage in using Carbon Fiber to construct fuel tanks, unless they're an integral part of the structure.  It looks cool but it's expensive and if nobody is going to see it, why bother?  Since RV tanks are not structural members, fiberglass or Kevlar are adequate materials.  My first choice would be Kevlar 17x17 crowfoot weave (for its impact resistance) and my second choice would be S-Glass 18x18 plain weave.  My third choice would be E-Glass in plain or crowfoot weaves.  S-Glass offers higher strength and stiffness than E-Glass -- useful in flat panel construction -- but costs 2-3 time more.  The cost delta over E-Glass makes Kevlar more favorable.    

Numerous materials are suitable for the core, including marine plywood, but for light weight I'd recommend either Last-A-Foam (6 lbs/CuFt in 1/4" thickness) or Polyurethane foam.  If you use Polyurethane, you'll probably have to go with 1/2" thickness.  Both foams are fuel resistant and easy to fabricate.  As to epoxies, there are several excellent choices.  Do not use polyester resins.  It's important that your construct the tanks using Fiberglass/Kevlar & foam core panels.  I've built many composite tanks for motor racing applications (subject to Gs and extreme vibration) and I've never has a failure.

I don't know where you're located, but there's a vast network of composite builders probably willing to give you a hand.  You're tanks sound like a weekend's work.  Let me know if I can do anything else.  

Good luck!

N31WJ

[Bruce Hughes]

Hi Barney

Making fuel tanks should be a very easy, successful project.  I
believe that N31WJ (why don't people sign their names?) has much
more experience than I do.  

I just have one comment:   Why consider any foam besides the
thinnest that you can buy?   Last-a-foam can be bought in 1/4"
pieces and it is cheap.   I see no advantage to using thicker foam.

If you use kevlar, there are 3 problems:   You have to spend
about $60 for special scissors and  it is hard to cut.   I have been
there and done that.

And there is a possibility of getting delamination.    If you are
careful to cover all of the cut ends with epoxy delamination will
not happen.

S glass or E glass should work well.   Carbon fiber is way too
expensive and not needed.

There is a possibility of pinholes.   You can deal with those.
Gasoline does not dissolve Last-a-foam.

Get an expert to help!   I never had one and still do not do good
glass work.  
 

Bruce Hughes  

[djrise]

There are many company's that make custom blow molded fuel tanks, some even with baffles. If the dimensions were just under the plans size tanks. I would think that would be the way to go. I'm surprised Vans has not looked into this for their kit builders. Build all except top. Install plastic tank, plumb, and screw on top skin instead of rivet.
Could be made for Long-EZ, Cozy fuel strakes too. Only problem not a very large production run for them.
These tanks are widely used on everything from Weed Whackers, Motor cycles, to Bulldozers. inexpensive, and complex shapes don't seem to be a problem.

[cloudman]

I expect that it would be a good idea to remember to add some baffling to slow the fuel movement in the long direction. The Varieze tanks have one in each tank. The baffling has the two bottom corners cut off per the original plans.
My Varieze has been flown since 1976 and did use a lot of auto fuel from the local gas station. In more recent years have stuck to aviation 100LL.
So no problems or leaks with fiberglass and epoxy tanks. Rutan recommended that the inside have a wet coating. I have heard about a few people having leaks, but builders  workmanship varies.
Also, no water condensation....... ever. Only time I had water in the tank was when I used a hose on the gas cap area.
I would like to have an all fiberglass RV-3 !!
cloudman

[N31WJ]

Barney,

Another member made a good point -- baffles.  I didn't say anything because I assumed you'd employ them during design and fabrication.  But since it came up, I recommend you consider open cell foam (Decacell is a leading manufacturer's brand name).  Open cell foams are available in gasoline only, alcohol/gasoline, or multi-fuel tolerant formulations.  These foams consume about 3% of their enclosed volume -- a small price to pay for 'infinite' baffling, and have the added advantage of explosion resistance if things get ugly.  The additional cost is not significant, particularly if you use blocks positioned throughout the fuel cell rather than completely filling the tank.  

Whether you use foam blocks or fill the tank(s), you should plan to add the foam after the cell is finished and pressure tested @ 7-8 psig. If you have a leak, use a spray bottle with slightly soapy water to pinpoint the source. Bubbles will immediately form at the leak site(s) and since the tank's internally pressurized, there is no opportunity for soap spray to contaminate the leak path.  If porosity or lap-joint leaks are present, thoroughly clean the area with acetone or similar solvent and apply epoxy to the affected area while pulling a small vacuum on the inside of the tank.  The vacuum will pull the epoxy into the porosity/lap-joint... forming a bond within the composite structure rather than merely on the outside surface.  Repeat until the tank is pressure tight.  

Open cell foam blocks are available from the following sources:
1 - http://store.atechmotorsports.com/egnsearch.asp?N=400379+301310+115&autoview=sku
2 - http://store.summitracing.com/partdetail.asp?part=SUM-290190

Installing safety foam after fabrication and pressure testing requires an inspection port large enough to accept the foam (which can easily be compressed but quickly expands to its original size).  I've seen foam installed during and after fabrication, and both approaches work, but I prefer post-fabrication installation because foam can easily be  removed if necessary in the future... without cutting the tank open.   The inspection port must be large enough to get your hand and forearm through, and can be part of your filler port assembly or a separate end plate.  The inspection port is usually a gasketed aluminum plate attached to the tank via a bonded backing-ring with sealed nutplates.  This may sound complex but it's not.

As a final thought on composite or metal fuel tank construction, it's probably a good idea to coat the inside of the tank with a sealer designed for the task.  The following website offers one alternative: http://www.wicksaircraft.com/catalog/product_detail.php/pid=9129~subid=2651/index.html

The foregoing may seem like overkill, but when you've flown your airplane for many years without a fuel leak, you'll realize it was time and money well spent.

I hope this is useful.

Wayne

[Barney]

Thanks to all for your suggestions.

Plans are in the works to construct composite wing tanks for the RV-3.

More as the plan progresses.

Barney