meditations on wing assembly

[nichzimmerman]

Here are some ideas ive had for lightening my 4 place bearhawk wings. This is my first airplane build, so I speak with little authority. Fuel system: convert main and aux fuel tank bay into integral tanks by adding forward false spar (and rear false spar on aux bay) and using solid center ribs on either side and copious amounts of sealant. This should be lighter than the 2 tank system as designed. I have run the details of this plan by the Bob and he approved. Two fuel lines would run from each aux tank, and one vent line for the standard clear tube fuel gage. All four tanks would be plumbed as completely separate systems down to the fuel selector. Fuel tubing would pass through ribs using nylon inserts from ACS. Im considering the use of 1/2" tubing if it would feed an O-540 without requiring an aux fuel pump...? I would love electricity to not be essential. Electrical: in an effort to design a system lighter than the 14' PVC pipe, I am considering routing wiring and the pitot tube hose through holes in the rear portion of the center ribs which have nylon inserts in them for protection. A short aluminum tube will have to be used in front of the aileron since no access will be available through the lightening holes in the rear spar. All comments are welcomed. Please educate me! Nic BH1217 almost ready for wing assembly

[DesertBearhawk]

As someone who works on aircraft that have integral fuel tanks I can tell you that everything is all well and good until you get a leak, and you will get a leak. By the time you make all the requisite structure and seal everything I bet you weight savings is nill at best and you might even be heavier. You wanna save weight, build composite tanks....for my money, there are much better ways to save a pound or two, add in a first time builder and I think I would build it as per the plans. YMMV....

[Craig C.]

Nic: Having built wet structure for a living....over a thousand F-16 aft and center fuselage tanks and 70+ F-22 center fusealage tanks, I'd advise you to go with the original design. Doing wet structure is not a big hassle for someone that has considerable structural experience and has enough help to get the job done within the sealant worklife. It takes a considerable amount of prep work on the structure as well as setup hardware and knowing the necessary rivet lengths by location prior to even mixing sealant. Doing wet work is messy, smelly and you are going to get it on you and anything within 10 feet of you. Once you start, you can't stop for anything. Make a mistake and you can't take more than a moment to figure out what is wrong and how to correct it, or you might as well stop and drill it all back apart and start cleaning the sealant off. Been there before....

Cost is going to be the same or greater than using the original design. You can figure that you will use at least 10-12 of the 3.5 oz tubes per tank at $20+ each.... Now add in the domed nutplates for all the inspection/access panels and you have another big expense. NAS1474's are a typical nut plate and they run at least $3.50 each and you will need at least 8 per panel and two panels per bay...so that's another $58 per bay to add to your costs.

Now add up the additional time requirements....you've got to build extra ribs, make access cutouts, the doublers for the access, the cover plates, rivet the doublers on along with the domed type nut plates, seal these independantly and then make form in place gaskets for the access panels. Once that's all complete, then you can think about riveting the bottom skin on.

We reskinned the center section on a friend's BT-13. All involved were intimately familiar with wet structure work and between us, had about 35 man years of wet structure work. It still took us about 10 hours to shoot the bottom skin on each side, while using a B-6 sealant and a heck of a lot of setup hardware...ie we had to fill almost every hole with a cleco or setup bolt before we started to even shoot rivets. Even with all that skill, we chased pinhole leaks for a couple of weeks before the tanks would hold pressure for more than a few hours.

I'll suggest that going with the stock design is going to save you a massive amount of frustration as well as cost in the long run, and you will be in the air a whole lot sooner too. BTW...you can buy the stock tanks for less than the additional materials cost to go to a wet wing.

[Craig C.]

By the way...going to a wet tank setup will also require building the spars to wet standards....ie fully wet assembled to eliminate inducing additional stress concentrations due to changing from a wet to dry stackup. It also means that all rivets have to go in wet and then be oversealed after completetion to eliminate possible leak sources.

[bestbearhawk1231]

I second Dave on the fuel tanks. As an prior avionics guy, I can't tell you how many times their fuel leaks have saved the avionics guy's butt come time for the test flight or delivery day. Don't get me wrong, it is entertaining as hell watching them defuel the plane, reseal the tank and access panels, refuel the plane, find a new leak or miss the original leak, defuel the plane...you get the point. Fuel tank sealant is some nasty stuff to work with, not so much toxic as it is just nasty. It will stick to everything, I mean everything. Oh ya, it reeks also just to add to the enjoyment factor. Do a web search for RV7 or 8 and read what people have to say about the fuel tank assembly on one of those. By the time an amateur (no offense) has a wet wing tank sealed up to the point where they are confident it won't leak it is heavy. Also, fuel tank sealant is pretty pricy. If you have never used it before, I would recommend using the semkit that has the sealant and activator in one tube. Basically, you use the pushrod to shove the activator into the sealant then chuck it into a drill (since I am assuming you won't have the actual semkit mixer, the drill adapter can be bought from the yard store for about 5 bucks) and SLOWLY mix it. If you mix too fast you will get considerably less working time since it heats up and heat cures the sealant. My experience is pretty much limited to 890b2 and 890b1/2. I have used others and from what I remember, they are similar. We used it to seal pressure bulkheads after running wires.

About the conduit in the wings...just put it in. I know is seems like a good place to go cheap and use weight savings as the excuse but it is not worth leaving out. You may never need to add or replace wiring and pitot/static lines but I guarantee that if you do you will regret leaving it out. It adds very little weight and it will save hours and hours of kicking yourself in the butt and cursing should you need or want to replace wiring, add equipment, or replace pitot/static lines. The electrical trade has a product called ENT, it is a blue corrugated lightweight plastic conduit that is cheap and light. You could run it through the holes in the back of the rib or make a small aluminum stand off every other nose rib to tywrap it to. The plumbing industry has a lightweight pvc pipe that is designed for irrigation that is also cheap and light...I think it is schedule 10 pvc. A 1/2" or 3/4" conduit weighs very little and will save your butt in the long run especially since you are most likely not planning on running spare wire out to the wing tips. If you use the nylon pitot/static tubing, you can also run those lines in your conduit. Make sure you add a pull wire or pull string as you pull the wire bundle through. If you run the pitot lines through there, make sure you pull a pull string to the access hole near the pitot tube.

[bestbearhawk1231]

Also, if you use 890b the 2 or 1/2 signifies the expected working time. 2 is a 2 hour work time, 1/2 is 30 minutes. There are others out there also but they are not as prevalent such as b4 and b1/4 with a 4 hour and 15 minute work time. The work time is how long you can do anything with it and have it work for you. You will know when that work time is up, it is pretty obvious. The time that is takes to set up is considerably longer before it can be put into service. About the only time I saw b1/4 used is on test flight/delivery day when the mechanics were running around like chickens with their heads cut off trying to get the leaks sealed. The work time starts from the moment the activator makes contact with the resin, kinda like epoxy. If I remember right, the humidity in the air affects the work time also. I would stay away from the pint or quart can kits since it can be a pain to get the contents measured out correctly. The ratio is something like 100 parts resin to 1 part activator but I would have to look it up. If you don't get the mix right or don't get it mixed up all the way, it won't set up and seal correctly.

[Mark Goldberg]

I do not have the work experience of the above posters - but I did build the wet wing tanks on my RV8. Never had a leak for the 5 years I owned the plane. Having said that - it was nasty work. RV builders call the sealant (Proseal) the "black death". But my suggestion to you is to think about how much you want to exercise your creative urges versus actually getting a plane built. If your goal is to see how much you can "improve" Bob Barrows design work - then have at it. That is your right as a builder of an experimental plane. But if your goal is to get a flying airplane within a reasonable time frame - build as designed.

How much time do you think Bob spends trying to keep his designs as light as possible? LOTS of time because building light is a BIG deal to him. Mark

[bestbearhawk1231]

Nic, another option is to rivet and proseal the original design. That is actually my back up plan once I get to that point. If aluminum welding doesn't pan out for me, I will be using the original design and adding access panels to the top so I can get in with a bucking bar and sealant. That is not my first choice though and I am really hoping that AL welding works out for me. Plan C (if the other 2 plans fail) is to buy the tanks from Mark. I talked to a guy a couple years ago at Osh who had riveted and prosealed the original design so I know it can be done without too many changes.

[John Rheney]

Nic: I'm a first-time builder too and with the experience I've gained in the fuel tank building, I can now really appreciate what it would take to do a wet skin tank. I did do the rivet and seal method of the separate tanks (as has at least one other that I know of.) It seems to have worked well since I have found no leaks when testing the tanks. The sealant IS very messy even when you are trying to be careful. Bulk quantities of examination gloves are your friend. Pre planning and cleanliness are essential. If you want to see what what I did to give you some idea of what you are getting into see: http://home.mindspring.com/~rheney/h...uel_tanks.html

[nichzimmerman]

Thank you all for sharing your experience. I will order some of this mystical black plague stuff and build a simple representative structure to better convince myself to not use it. I would still like to know if experience has revealed a way to feed the O540 from an aux tank without an electric fuel pump. Will 3/8" lines provide adequate fuel flow? 1/2"? Thanks.

[Craig C.]

Nic: Go with something like the 2.5 oz version of this for practice: http://www.aircraftspruce.com/catalo...?clickkey=4275
For a shop at about 70F and 30-50% relative humidity, a beginner will have a worklife of about 90 minutes from mix to cleanup. There are kits that range from B-1/2 all the way to B-12, based on how big of an area that needs to be worked and the allowable worklife. B-2 is a good compromise for experimenting. It will take about 116 hours to cure complete. Here is a link to a little primer on tank sealants. One thing I will advise against, is the thought of thinning ANY two part sealant. The introduction of MEK or other thinners can and will change the properties or the adherence of the sealant.

[jaredyates]

I'm curious, though not curious enough to calculate it myself, as to whether you'd be able to save more weight by just eliminating the aux tanks. And the need to pump fuel out of them. And the lines to pump the fuel through. And the corresponding access panels, nutplates, screws, fuel caps, quick drains, etc. How is that for not answering the original question? As to fuel line diameter, I don't think the wings are where you'll find the flow restrictions. Keep in mind that you have two lines from the two tanks, which will be consolidated into one single line at the firewall. Even in the worst case climb scenario, you still have two lines flowing up until you get to the fuel selector.

[nichzimmerman]

Aux tanks are a must for me. I have owned 3 airplanes before, each with 3 hours of fuel plus 1 reserve, and I always wished I had a little more. Not because of an iron bladder, but because I hardly ever land at patch of dirt with a gas pump on it. Sent from my SCH-I535 using Tapatalk