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  #21  
Old 10-11-2018, 10:23 AM
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rmartingt rmartingt is offline
 
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Originally Posted by Toobuilder View Post
Not sure what your pro/con analysis is suggesting here. Building a larger tank a few bays towards the tip is less overall weight than "wetting" a few bays AT the tip, and certainly results in a lower MMOI. The bigger tank also has zero changes to the supply plumbing, which is better than "almost completely stock".

Adding bays to a fuel tank design has some drawbacks, but from a systems integration and weight perspective its about the best way to go.
"Just make the tanks bigger" is a lot more easily done on a slow-build 4 or 6. Much more difficult on a later kit because the spar is pre-punched for rivets outboard.

Separate leadung edge tanks a bit further outboard aren't going to impact the MMOI much worse and they're lot easier (relatively at least) to integrate on a - 7 or later, especially if you don't mind them not being removeable.
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  #22  
Old 10-11-2018, 11:10 AM
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airguy airguy is offline
 
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I went the "extra tank" route on my 9A (a la Pat Tuckey), I kept the original tanks as almost stock and converted the outboard leading edges from structural to tankage. I added a small non-fuel bay about 6" wide between the two tanks, with an access plate, to contain a transfer pump and connections. Then the outside leading edge was turned into a fuel tank by removing 5 structural ribs and installing 7 fuel ribs (same as the inboard fuel ribs) plus end plates and a baffle with Z-plates to the spar in the back. My outboard tanks are not removable - they remain riveted to the spar just as the original plans called for, and if I develop a major leak in them I'll simply abandon them in place and use only the inboards. I use a rocket-style fuel vent at the wingtip that feeds the outboards with air, and the lowest point (fuel pickup) of the outboards is connected to the highest (vent) point of the inboards, to provide flow-through venting and automatically transfer fuel from outboards to inboards as the engine burns it from the inboards. This preserves a two-tank fuel valve in the cockpit to simplify the plumbing. The outboards have a 15.5 gallon each side measured capacity and only add a handful of pounds (11 pounds total, if memory serves, including transfer pumps) to boost total fuel available from 36 to 67 gallons. Being honest about it, I'm going to say this mod (both wings) added 100-125 hours of additional build time.

I do not make landings with fuel in the outboards - the weight of the fuel and the long lever arm increases the bending moment on the wing root and could cause damage there - so I only use the extra tankage when I know I will burn the outboards dry in flight, often landing with full inboards and avoiding a fuel purchase at a high-price location. I have about 330 hours on the airplane now and make a lot of long cross-country trips with it - I probably have close to a quarter of my total time with fuel in the outboards and love the mod.
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Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.

Last edited by airguy : 10-11-2018 at 03:54 PM.
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  #23  
Old 10-13-2018, 10:01 PM
diamond diamond is offline
 
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Can someone explain to me what it is about the hotel whiskey tanks that justifies a $2500+ pricetag? Maybe I need to see a parts list to help me feel good about spending that amount for what seems like a minimal amount of material.
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  #24  
Old 10-13-2018, 10:55 PM
Capt Capt is offline
 
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When one gets older the fuel capacity is directly proportional to ones bladder, for me that's 2 hrs tops
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  #25  
Old 10-13-2018, 11:03 PM
rv7charlie rv7charlie is offline
 
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It's not always about long legs. For me, it's the ability to tanker cheap fuel for round trip range.
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  #26  
Old 10-13-2018, 11:08 PM
rv7charlie rv7charlie is offline
 
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Originally Posted by diamond View Post
Can someone explain to me what it is about the hotel whiskey tanks that justifies a $2500+ pricetag? Maybe I need to see a parts list to help me feel good about spending that amount for what seems like a minimal amount of material.
I wouldn't pay that much. On the other hand, I did pay almost 20 grand for a few thousand dollars worth of parts when I bought my -7 kit. What's your time worth, and do you have the skills/tools to make them yourself?
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  #27  
Old 10-14-2018, 09:12 AM
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airguy airguy is offline
 
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Originally Posted by rv7charlie View Post
I wouldn't pay that much. On the other hand, I did pay almost 20 grand for a few thousand dollars worth of parts when I bought my -7 kit. What's your time worth, and do you have the skills/tools to make them yourself?
Exactly correct. HW designed them, built them, marketed them, and has been at least moderately successful selling them. Some people have more dollars than time and are willing to make the trade. It's called capitalism.

You don't have to buy them, you can always build your own.

This is exactly the same reason we are paying north of $25,000 for a O-360 series new engine today, as well. What can Lycoming say that justifies the price? They don't have to say anything to justify it - because we keep buying it.
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Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.

Last edited by airguy : 10-14-2018 at 09:14 AM.
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  #28  
Old 10-14-2018, 10:33 AM
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RWoodard RWoodard is offline
 
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I’m thinking I can enclose the space in the wingtips and make a composite tank. Shouldn’t be more than about $50 in fiberglass and resin (have most of it on hand anyway), another $150 for some Jeffco sealant, a couple of fuel caps, some fuel line, etc.

All told, less than $500 in materials. If I don’t use them (probably 90% of the time), they just sit there empty. When I do use them, I’d certainly want to limit my flying to airliner-style gentle because of the additional loads and stresses.

Wetted tips just seem like a simple, non-intrusive option. With the help of a good friend who’s an expert with composites, I fabricated a 15 gallon header tank out of pre-preg fiberglass. It wasn’t terribly complicated or terribly difficult.

Honestly, my biggest fear is removing the wing tank to provide access to install a port for inbound fuel transfer. The tanks have been off my RV-3 at one point or another, I just haven’t been the one to do it. The flathead screws aren’t painted, so aside from concerns of scuffing the adjoining skins, I am hoping to do it without damaging anything.

As I get older, my range is becoming more and more bladder limited, but I’ve known folks to use a relief tube with some success, so I’m considering that option, too. None of this stuff is high on my list of priorities, so it may never happen. Just always brainstorming ideas to create the perfect plane!
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  #29  
Old 10-14-2018, 11:00 AM
rv7charlie rv7charlie is offline
 
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The tip is quite floppy until it's screwed to the wing, and as strange as it sounds, it can be 'rotated' around the end rib quite a bit while fitting it (see the many threads on lining up the tip trailing edge to the aileron trailing edge).

What stopped me on the wet tip idea was the need to keep the flange area perfectly shaped to fit the wing. The only way I could see to do that was to buy a couple of extra wing ribs, skin, etc to make a jig, or to attach the tip to the wing and build a jig around the tip. Seemed like a lot more work/expense than just wetting the leading edge. If the wing had already been built, that would have changed the equation a bit.

My total expense for two leading edge aux tanks was the fuel fill caps/housings and the plumbing hardware. Scrap aluminum sheet, rivets and proseal (already on hand) were all I needed for the tanks themselves.

Last edited by rv7charlie : 10-14-2018 at 11:03 AM.
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  #30  
Old 10-14-2018, 11:20 AM
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Toobuilder Toobuilder is offline
 
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Quote:
Originally Posted by rv7charlie View Post
My point was, that if it's an acro-capable a/c, I want to keep it as close to 'stock' configuration as I can for acro. By keeping the aux fuel in separate tanks, with minimum achievable weight gain to make the tanks, I'd hope to preserve acro ability (with empty aux tanks) and still have the option of extra range and/or tankerage.

Everything's a compromise; I accept the compromise of an extra position on the fuel selector, to keep acro capability.

For non-acro planes, I like the idea of just making the tanks bigger, as I said in the 1st paragraph of my earlier post.

Does that help?
Makes sense. I think I glossed over your concerns about maintaining acro capability. Roll rate will slow with bigger tanks and spins would have to be fully explored again, but beyond that I'm thinking the typical RV style sloppy/ fun acro would remain essentially intact. I certainly get upside down with the Rocket on virtually every solo flight and I intend to keep doing the same when I install the big tanks.

Of course when they are full, then that will define the "airliner" profile. Which brings up another point with my particular mission. When goofing off, I typically fly with min fuel. If I manage to land at home with near full tanks, I will usually offload most of the fuel if I know my next few flights will be local.
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