About fuel tank design and CG.

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Piper
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Joined: Fri May 21, 2021 2:08 pm

About fuel tank design and CG.

Post by Piper »

There may be threads here about forward CG issues, but I haven't read one. There are many about aft CG issues. As I've done a mock-up for an E-AB Air Camper my design seems to be teetering on the edge of that rear limit when loaded-- and I'm not that heavy. I’ve seen where other people have extended the nose. But I'd rather eliminate the issue before building it, at least as much as possible.

I have a header tank in my design because I like header tanks. The plans call for a drain, sealed cap, and double vents back to the main tank in the wing’s center. The main 12.5 gallon tank as drawn in my set of official plans will drain from near the center (both fore-aft and left-right almost to empty at normal flight attitudes. But the full tank's CG is behind the empty airframe's CG. With the FC10 airfoil’s undercamber it leaves a large unused empty space near the front spar.

I drew up a few alternative tanks to take advantage of this space, basically centering the sump left to right just behind the front spar which serves to move the center of gravity for the full tank itself about an inch and a half closer to the center of gravity for the empty airplane and away from the aft CG limit.

If you look at a copy of the plans, basically I’m thinking of running the tank's bottom up from below the normal position on the front spar up to the normal position on the back spar in a straight line whose center valley clears the undercamber. The tank will be slightly wider than on the plans and it's bottom will have a shallower ‘V’ shape left to right. The sump will be lower than the height of the support blocks.

After rejecting several ideas I am focusing on two. A 12.5 gallon tank would weigh the same, and the sump would still be the low point as it sat on three wheels on level ground. A 13 gallon tank with a half gallon of unusable fuel (and condensate) would place the barbet at the rear of the tank where it would be easier to access and with the tail in the air add 3 lb of ballast slightly forward of the balance point for the original design.

Each design will drain as well as the original tank and when full move the center of gravity forward about the same small amount. Has anybody done this before, and even more importantly, can anybody think of a really good reason not to do it.
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taildrags
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Re: About fuel tank design and CG.

Post by taildrags »

Let's think about this. If you have 12.5 gallons of fuel up in the wing centersection (which puts it on the CG, for all practical purposes), then it will be feeding into your header tank for a couple of flight hours with relatively no noticeable effect on the CG. I can't sit for more than a couple of hours in my Air Camper, so if I had that setup my header tank would be full almost all the time that I fly the airplane. This can be good or bad, depending on circumstances. In most situations you'd have the weight of the fuel in the header tank to move the CG forward a bit. You didn't say what size header tank you plan on using, but let's say it's an 8 gallon one. The CG spreadsheet calculator for my Air Camper shows that the CG moves forward about 1.1" with 8 gallons of avgas in a header tank (that's where all the fuel is in my airplane). With the header tank full, you'd have 1.1" of CG "in the bank" most of the time.

You could fool yourself into thinking "I'll almost never burn the fuel in the header, so I can figure on at least an inch of CG offset by keeping it full". On my airplane, that amount of offset means the difference between a 160 lb pilot maxing out the aft CG at gross, and a 198 lb pilot maxing it out, and most of the time that would be true. When it might bite you would be if you got into headwinds or had to divert and you burned all the wing tank fuel and got into the header tank fuel. That 1.1" of aft CG would then start putting you in a squirrelly situation as you burned the header fuel, very low on fuel, tired, and anxious to get on the ground. Tight turn from base to final, the elevator is near the point where pulling back on the stick to tighten the turn is getting you into control reversal, you could lose it right there. Adding ballast can cure some ills, but then the airplane is hauling around dead weight all its life. Better to design the airplane to work right without any trickery.

Oscar Zuniga
Medford, OR
Air Camper NX41CC, A75 power
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Piper
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Joined: Fri May 21, 2021 2:08 pm

Re: About fuel tank design and CG.

Post by Piper »

Hi Drags,

Thanks for reponding. (Sorry, I rearranged your quotes, but I didn't change them.) "Better to design the airplane to work right without any trickery." -Amen.
taildrags wrote: Wed Jun 23, 2021 10:46 pm1.1" of aft CG would then start putting you in a squirrelly situation as you burned the header fuel, very low on fuel, tired, and anxious to get on the ground. Tight turn from base to final, the elevator is near the point where pulling back on the stick to tighten the turn is getting you into control reversal, you could lose it right there.
That's exactly the sort of situation that I actively plan to never put myself into.
taildrags wrote: Wed Jun 23, 2021 10:46 pmYou could fool yourself into thinking "I'll almost never burn the fuel in the header, so I can figure on at least an inch of CG offset by keeping it full". On my airplane, that amount of offset means the difference between a 160 lb pilot maxing out the aft CG at gross, and a 198 lb pilot maxing it out, and most of the time that would be true. When it might bite you would be if you got into headwinds or had to divert and you burned all the wing tank fuel and got into the header tank fuel.
I didn't mention the size of the header tank but it's going to be much smaller. So even if empty and I become a draggy glider it won't create an aft CG situation. I always w+b zero fuel and mission fuel drawing a straight line to avoid this situation. Most of the aircraft that I have flown have a sloping forward CG limit. (Higher GW = more aft, lower GW = more fwd)
taildrags wrote: Wed Jun 23, 2021 10:46 pmLet's think about this. If you have 12.5 gallons of fuel up in the wing centersection (which puts it on the CG, for all practical purposes), then it will be feeding into your header tank for a couple of flight hours with relatively no noticeable effect on the CG. I can't sit for more than a couple of hours in my Air Camper, so if I had that setup my header tank would be full almost all the time that I fly the airplane.
The AC's main fuel tank as designed sits in the almost 30 inch space between the main spars. It's arm is about 23.5 inches back from the LE, but the empty aircraft is 13-ish with the driver (this is the more useful number). The desired aircraft CG is about 15 to 20 inches back from the leading edge, redesigning the tank moves the full tank's arm to 22 or so and as it drains the arm moves just slightly forward.
taildrags wrote: Wed Jun 23, 2021 10:46 pm Adding ballast can cure some ills, but then the airplane is hauling around dead weight all its life.

I do this on Part 103 builds: I weigh 125 pounds and might have 25 pounds of CertifiedRealArticlesofthePilot, so when I build a craft limited to 254 pound empty I'm really modifying the design for between 434 and 404 pounds as the fuel burns off. So that someone who weights more can safely fly it I include a metal water (ballast) tank usually attached to the engine mount (because the mount was designed for an 85 pound engine and mine weights 45). I've worked out a chart for each build to show how much water to carry as ballast to compensate for a heavier pilot.
taildrags wrote: Wed Jun 23, 2021 10:46 pmBetter to design the airplane to work right without any trickery.
Using the LE as a datum some builders have moved the fuselage forward to compensate-- BHP did this with the Sky Scout-- some people have extended the motor mount. While the difference between weight in the pilot's seat will have less effect on a 600 pound frame than a 254 pound frame I may end up putting a ballast tank under the header tank on the firewall since I will be building it for the person who will fly it 95% of the time.

Thanks

Jackie
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Richard Roller
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Location: Olathe, Ks.

Re: About fuel tank design and CG.

Post by Richard Roller »

Piper.
I've been reading the various posts you've put up on building your Pietenpol. You seem to be overthinking this process.

Maybe a little more information would help us all. Are you attempting to build an ultra-lite version? What engine? How much horsepower? Fuel consumption. If an ultra-lite you can't legally carry over 5 gal. of fuel in any case.

If not an ultra-lite, once again, engine, horsepower, fuel consumption? To quote you: "I didn't mention the size of the header tank but it's going to be much smaller. So even if empty and I become a draggy glider it won't create an aft CG situation." If you don't need this small amount of fuel and burning it off doesn't impact on your c.g. why carry it? Why bother with the extra weight, complication and possible leakage of a system you don't need?

Admittedly there are Piets flying that use a header tank combined with a center section tank. In talking with the builders of these aircraft they have told me that they felt they needed the extra fuel due to horsepower installed, or they wanted part of the center section for storage, or range was a factor in the areas they flew in. It would seem from your writings that you don't really need it. It has also been a trend in some builds, lately, to put a 3 foot span center section on the aircraft so a larger tank can be fitted.

More information please.
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Piper
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Re: About fuel tank design and CG.

Post by Piper »

Richard Roller wrote: Thu Jun 24, 2021 12:31 pm Piper.
I've been reading the various posts you've put up on building your Pietenpol. You seem to be overthinking this process.

Maybe a little more information would help us all. Are you attempting to build an ultra-lite version? What engine? How much horsepower? Fuel consumption. If an ultra-lite you can't legally carry over 5 gal. of fuel in any case.
Basically, I live with my sidekick across the street from the grounds of a little daylight only grass strip where I have a repair station in a hangar. Over the years we've built an E-AB Roger Mann RaggWing 26 biplane and several Part 103 "vehicles," including a Heath, a Duster and a Church from Roger's collection, a Kimbrell Banty, and a Bensen autogyro when we were not busy working on somebody else's airplane. I have the fuselage of a scaled down Part 103 Sky Scout (in my personal hangar space, not the station's, to any FAA employees reading this) that I am working on right now.

This Part 103 build is scaled to 22/24 the size of the official 1933 Sky Scout plans, although I adjusted the pilot seat forward to fit my physical dimensions (5'5", 125#) and eliminated the flop. It has space for a nautical five US gallon QD fuel tank right behind the firewall and is powered by a 28 HP, 40 pound, Hirth F-33.
Richard Roller wrote: Thu Jun 24, 2021 12:31 pm If not an ultra-lite, once again, engine, horsepower, fuel consumption? To quote you: "I didn't mention the size of the header tank but it's going to be much smaller. So even if empty and I become a draggy glider it won't create an aft CG situation." If you don't need this small amount of fuel and burning it off doesn't impact on your c.g. why carry it? Why bother with the extra weight, complication and possible leakage of a system you don't need?
My E-AB AC exists only on paper and in my synapses. I do possess a disassembled, hot-tanked, but un-rebuilt Ford model 'A' engine that I have accurate weights for. (The fact that a 40 HP 'A' will haul a Piet around where a 65 HP VW won't is a wonderful example of the limitations of HP as a useful measurement.) I like having a sealed header tank with independent left/right venting and pressurized by fuel from the main tank located near the carburetor for reliability on a gravity feed system. And I prefer gravity feed. Maybe that's just personal preference, but non-existent fuel pumps never fail.

Because it will always be full, a header tank does move the CG forward. Even though I am not weight limited in E-AB, "light and simple" is the way to go, but everything is a trade-off. An extra static item that one-for-one replaces a mechanical item seems keeping with "simple."
Richard Roller wrote: Thu Jun 24, 2021 12:31 pm Admittedly there are Piets flying that use a header tank combined with a center section tank. In talking with the builders of these aircraft they have told me that they felt they needed the extra fuel due to horsepower installed, or they wanted part of the center section for storage, or range was a factor in the areas they flew in. It would seem from your writings that you don't really need it. It has also been a trend in some builds, lately, to put a 3 foot span center section on the aircraft so a larger tank can be fitted.
One of my rejected plans was for a smaller say 10 gallon main tank with a 1.5 gallon header, but figuring out the empty weight it was the same as a 12.5 + 1.5. I can always just put 10 gallons in the main and if the bottom of the tank slopes sharply forward from the rear the arm is almost the same.
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