The die is cast - now we wait and see - will it or won't it?

[airguy]

Tonight I joined the select band of proseal brotherhood and finished my right main tank. I'll close the right auxiliary tomorrow, and then give them both a few days to cure out in the house (cold here tonight) before pressure testing them.

Anyone care to start a pot on which rivet line leaks?

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[cjensen]

Cool Greg! Don't see too many fully alodine'd tanks or skins...at least that's what it looks like.

No leaks! No leaks! No leaks!!

[airguy]

Yep - I alodined all the fuel-wetted aluminum surfaces (including the pick-ups) since I plan to run E10 mogas for the majority of the lifetime of the aircraft. I didn't have a convenient way to alodine just the internal side, so I dipped the entire skin.

[bullojm1]

Man, that's one thing I wished I did on my tanks -- alodined the inside. It didn't even occur to me until I saw another local builders tank. Anyways, looks GREAT. You should be able to pressure test those puppies 24 hours after you finish them. I did my tanks in the January/February timeframe (read: COLD outside) and let them cure in my basement for a night. Next day I did the test.

Just a word of advice on testing the tanks -- put some fuel lube on the O-ring on the gas caps. Use a manometer. If you have a basement, do the testing there since the temperature in a basement is very stable. Also, note the barometric pressure when you pressurize the tank and when you check it. You don't want to get discouraged if you see the manometer change for the wrong reasons. And lastly.......GOOD LUCK!

[airguy]

I'm happy to report two good tanks now!

The auxiliary tank tested out fine immediately, which surprised me since I have almost double the number of driven rivets in that one (had to close up the unused structural rib rivet holes in the skin and baffle) and I figured my odds of a leak were considerable with all the extra exposure.

The main showed a large leak immediately upon pressurizing, and the soap bubble test showed a single leak coming through the stem hole of one of the sealed-head blind rivets used to attach the Z-brackets to the rear baffle of the tank. Apparently the sealed head cracked when I pulled the rivet, and I stupidly trusted the sealed head to remain sealed - that won't happen again. I pulled a vacuum on the tank and hit that rivet head with loctite, gave it a couple hours to set, and it pressure tested good.

I riveted the aux tank in place, screwed/bolted the main in place, and I'm on to the top skins. Once the skins are riveted on to hold everything square, I'll pull the main tank and fit the transfer pump and plumbing between the main and aux tanks.

[cjensen]

Big congrats Greg!! Looks fantastic!!

Where will your aux tank go? Wingtip? Or is it a tube or something?

[dons]

That's a big step forward Greg, any custom design change takes a lot of time, but when it involves fuel tanks it it even a bigger job. Congrats.

[airguy]

Big congrats Greg!! Looks fantastic!!

Where will your aux tank go? Wingtip? Or is it a tube or something?

The auxiliary tank is the entire outboard leading edge. I kept about 6 inches of each end as non-fuel bays, removed the internal 5 structural ribs and inserted 7 fuel ribs (same ribs as the main tank) and put a cut-down rear baffle on the back. I attached Z-brackets onto the rear baffle to mount it on the spar just the same as the main tank. It's essentially another main tank, just converting the existing outboard leading edge into a fuel tank. Construction methods and quantities are almost identical.

I have a fuel transfer pump to move fuel from the auxiliary tank (outboard leading edge) to the main tank (inboard leading edge) where the engine fuel pump pickup is located. My vent system introduces air into the outboard end of the auxiliary tank, flows throught the aux tank and into the main tank. This mod will almost exactly double the available fuel quantity on board, at a dry weight cost of 11 pounds (total, both wings) counting all parts (removed and added), including plumbing, wiring, pumps, switches, access plates, etc.

Here is a pic of the bottom of the right wing, the main tank is on the right (inboard) and the auxiliary tank is on the left (outboard). The access panel is cut in the short non-fuel bay between the tanks for the fuel transfer pump and associated plumbing/wiring.

[airguy]

That's a big step forward Greg, any custom design change takes a lot of time, but when it involves fuel tanks it it even a bigger job. Congrats.

Thanks - I appreciate that. The expense was minimal, I'm only into this mod about $260 total (counting both wings), but I underestimated the amount of time it would take to do it. I had guessed at about 25 hours per tank to convert them, but by the time I sat down and did the design, planned out the build process so I wouldn't work myself into a corner, figured out the mounting and plumbing, and cut an access plate into the mid-wing fuel pump transfer bay, I was right at 40 hours additional for the right wing and expect that to be 30 hours easy for the left wing later this year.

Any mod takes longer than you think it will - but I considered this one to be well worth the time and effort. For 11 pounds of empty weight I now have twice the fuel capacity anytime I want to use it.

[4kilo]

Greg,

A lot of stuff in that mod sure looks familiar.

Pat

[airguy]

Greg,

A lot of stuff in that mod sure looks familiar.

Pat

It should - I've got a copy of the modified blueprint you did for your -8 tacked up on my shop wall! Imitation being the sincerest form of flattery, and all that...

I had been wanting more fuel for a long time and was thinking about just making the entire leading edge one piece, and all fuel tank, but was not sure how to go about doing that. When I saw the mod you did for N804PT, I really liked it, and changed it only slightly with respect to adapting it to the 9 wing. The rib locations are different with different spacing due to the different wing planform, and there are a couple other minor differences, but it's essentially the same idea.

Did you put transfer pumps between the tanks on yours? I'm using the Facet pumps for a carbed engine, low pressure and plenty of flow in a small package.

[Wicked Stick]

So, how much fuel do you now have in each wing ?
So, how much weight when full does the aux tank now hold ?
So, did you strengthen or re-inforce the stock wing spar to compensate ?

I know each person has their own mission and options.
I was just curious if your concerned at all about any overstress to the wing and spars if you have full tanks all around and encounter any turbulence or in-advertantly get the G forces up there.

Are you considering lowering the G-rating when fuel is in the aux ?
Or perhaps lowering the max control deflection manuevering speed ?

Just food for thought. After all, you can build your airplane anyway you like.

[airguy]

I haven't measured the fluid volume available, but quick back-of-the-envelope calculations show it to be about 17 gallons, very close to the stock tanks. That being the case, the weight should be about 102 pounds fuel plus a couple for the added material, call it 110 additional on each wing when they are filled. I did not do any additional strengthening to the wing spar for this weight, but the tank itself does have closer rib spacing than the main tank, plus some internal stiffeners.

Airborn G-rating and turbulence will have zero effect on this mod - which is why I chose not to do any stiffening to the spar. The aux tank is a lifting body tank - meaning that any lift created by airflow must first support the weight of the wing (and the fuel contained in it) before it can put force on the airframe to achieve lifting of the fuselage. In flight, any G's (whether pilot-induced or turbulence-induced) will actually result in LESS stress on the wing spar with the outer tanks full than with the outer tanks empty, since the total weight will be higher with full outers (resulting in lower total acceleration for a given gust factor) and the wing load is self-supporting. This same idea applies to aerobatic aircraft - fuel load in the wing is not counted against the total aerobatic weight, because the lift generated by the wing supports that load FIRST before putting any force on the rest of the aircraft. The one major limitation for fuel in the outer while airborn is that it may make the aircraft difficult to get out of a spin, due to centrifugal force throwing fuel out to the wingtips and increasing the rotational moment - but the 9/9A is a non-aerobatic aircraft in the first place, and paying careful attention to manuevering flight with fuel in the aux tanks should suffice.

The only real drawback as far as strength is concerned is ground ops. When operating on the ground with fuel in the outer tanks, there will be an increased bending moment on the spar root. This means ground ops (taxi, takeoff, landing) with fuel in the outers must be done very gently, and landings with fuel in the outers will be the worst-case scenario and should be avoided any time possible. Pat has a very good examination of this posted on his website, discussing the physics involved and the limitations imposed due to the extra outboard weight. He's done a good job of looking at the implications of the mod, and I have to agree with his conclusions after looking at the numbers myself.

Bottomline - this is somewhat of a purpose-built aircraft, I will use it as a daily flyer but I built in the long range tanks to enable me to do a round-the-world flight someday. With the wing tanks full and a 30-gallon ferry tank in the passenger location, I can make the worst-case longest leg distance. That would California to Hawaii, westbound, against 95% probability headwinds, with one hour reserve. If I can make that leg, I can make any leg around the rest of the globe. The rest of the time, with those outboard tanks empty, it's just a normal 9A with 11 more pounds of structure than normal.

[4kilo]

Greg,

My transfer pumps are the Facet pumps, and they are installed in the bay between the tanks. I always turn them on for take off, but so far the transfer has always worked fine even without the pumps running.

Dave,

Actually, in flight having extra weight carried in the wing REDUCES stress on the spar (a little counter intuitive I know). There are still several problems to be addressed however. First, the stress on the spar is greatly increased when the airplane is on the ground. My take on this is that for take off this is not much of a problem, and I never land with fuel in the aux tanks. The second problem is handling. With fuel in the outboard tanks, the moment of inertia about both the roll and yaw axes is greatly increased. This could have major ramifications on spin recovery, as well as causing difficulties with low speed (landing) handling. Again, with fuel in the outboard tanks, my flight manual forbids aerobatics and landing may only be made in an emergency. A third problem is the tank structure itself. I am not familiar enough with the -9 wing to address Greg's situation, but in my -8 the skin is thinner in the outboard LE than the original tank skin. This is more of a limitation on the G-loading than the spar considerations, since the weight of the fuel on the tank bottom skin needs to be carried by a thinner skin. In my case, I decided though comparitive analysis that the max G-loading with fuel in the outboard tanks is 4.5 G's. A fourth problem is brake energy. For my airplane, the max take off weight was increased so that I could take off with fuel in the outboard tanks. With this extra weight, an aborted take off would exceed the maximum brake energy rating of the standard brakes, so I put heavy duty brakes on my airplane. There are some other considerations, but they are minor compared to the above.

Pat

[airguy]

A third problem is the tank structure itself. I am not familiar enough with the -9 wing to address Greg's situation, but in my -8 the skin is thinner in the outboard LE than the original tank skin. This is more of a limitation on the G-loading than the spar considerations, since the weight of the fuel on the tank bottom skin needs to be carried by a thinner skin. In my case, I decided though comparitive analysis that the max G-loading with fuel in the outboard tanks is 4.5 G's.
Pat

My tank skin thickness is the same as yours, .032" on the main and .025" on the converted outboard leading edge. I did some rough number crunching of the weight, material strength, fasteners, and moment arms for the attach points, and came up with a rough number of 6.5-7 G's for failure of the leading edge when full - which is close to what you computed if memory serves. This is well above what the rest of the airplane is meant to see for flight service, which agrees well with your 4.5 G in-flight limitation, though in-flight loading is likely not where the problem will arise. I think a lot of people would be shocked to see how many G's are pulled during a hard landing - and with the additional bending moment from outboard fuel, this is where the danger lies. Like you, I intend to make landings with outboard fuel a strictly emergency procedure, with an overweight landing inspection mandatory for each occurrence.

More than likely, these outboard tanks will see fuel in them MAYBE 20 times during the life of the airplane after Phase I, other than a round-the-world attempt. Anytime I put fuel in them, I'm mentally taking myself back to Phase I operational procedures since this is a little bit outside the proven envelope.

[4kilo]

Greg,

I think that you will find the aux tanks give you even more operational flexibility than you think. If you find fuel cheaper somewhere than at your home field, you top off the mains and put enough in the outboards to land with full main tanks. This can be a real money saver.

Pat

[Wicked Stick]

Thanks for your responses. It tells me you have taken the time to at least research the mod, and considered all the pros and cons.

I keep forgetting about the "no factor" in flight rule of weight added to the wings.

You both brought up good points about landing with full fuel out there on the ends, and about the outboard skins being thinner and I completely understand your reasons for adding the extra fuel storage.

Hope everything works as intended once you are flying.

[airguy]

Thanks for your responses. It tells me you have taken the time to at least research the mod, and considered all the pros and cons.

I don't know about ALL of the pros and cons - but I'd like to think that I've covered the major ones, at least enough to avoid the nasty surprises. I invite commentary from any and all on the mod, because sooner or later someone will come up with an angle that I hadn't considered, and might just be important. Most will tell me I don't need to do it at all - but I don't NEED to fly around the world, or even have an airplane at all - NEED has nothing to do with it.

[Wicked Stick]

I would never be the one to tell a builder how they should build it.

I was simply curious if they carefully considered all aspects of their mods and if they factored in any circumstances or consequences both good and bad.

Looks like you've done your homework and it sure sounds like it will fit your mission profile.

Keep bang'n those rivets ! It looks like it's coming out nicely.