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Header tanks scare me...

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smoore

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So do those behind the seat. Am I just being paranoid?

Somehow the idea of 5-15 gallons of gasoline either in my lap or on my back really freaks me out. At first I was convinced an LSA airplane was for me but most of them seem to bundle the fuel in the cockpit.

I know I can build a Zenith LSA and have the tanks in the wings but again, am I just being paranoid?

That fatal Sonex accident this summer where the cockpit burned to slag really slapped me across the face. I look at the pictures in the article below and think, "Would this man still be with us if he had a wet wing?"

http://www.ntxe-news.com/artman/publish/article_39551.shtml
 

Topaz

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The thinking generally goes that if the cockpit section has been deformed enough to rupture the tank, the occupants have far worse problems to deal with than fire. A lot of cars use the same priciple.

This only helps, of course, if the (full or partially full) tank structure itself can handle the G loads up to those that would crumple the fuselage in the cockpit area. Otherwise, the tank splits and you may, indeed, have problems.

As always, it's tradeoff.

That was an ugly-looking crash. :depressed
 

smoore

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OK, I can buy the, "Well if the fuel tank takes a hit you've got worse problems!"

I guess that I'm probably comparing apples and oranges in my head, tube+rag and AL structures that take impact in completely different ways. I assume it would be folly to have a large header tank in a T+R structure because of how I imagine the T+R cage takes a hit. The AL structure should crumple AROUND the tank, where the T+R cage will stress the mount points to possible failure and allow whatever is mounted to careen about in whatever fashion it chooses.
 
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Rhino

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That guy didn't die from the fire. He was killed on impact.

An autopsy was performed on the pilot on August 15, 2007, by the Southwestern Institute of Forensic Sciences, in Dallas, Texas. According to the report, the cause of death was "multiple blunt force injuries."
http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20070821X01218&ntsbno=DFW07LA183&akey=1

That doesn't invalidate the concern, but it is quite common for people to assume a fire is what caused deaths in aviation accidents, when the fire was most often 'post-crash' and not the cause of death. That doesn't mean it doesn't happen. It just doesn't happen as often as a lot of people think.
 

Canadian_JOY

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I have had the same concern about header tanks. I personally don't like the feeling of having 20 gallons of fuel hanging above my feet and legs. But with that having been said, I own and operate an aircraft with its 20 gallon fuel tank located directly above my feet and legs. So how to I justify this to myself? Well, it's both easy and difficult.

The easy justification comes from having owned, for a short while, a wreck identical to my airplane. It hit the ground pretty hard (hit a 5' dirt berm and a tree at something like 75mph). The fuselage did as it was supposed to do, as did the seat. They were badly crumpled, but the pilot walked away with only a facial scar from hitting his head on the instrument panel (another good reminder to have the shoulder belts tightly fastened at all times!). When I finally got around to doing a good close inspection of the fuel tank I found that it had survived impact amazingly well, and still held fuel without leaking. Now I feel much better about a header tank.

Now for the harder sell... Wing tanks are great, but rely on fuel pumps to get fuel to the engine. Normally builders also install on/off selector valves in order to maintain lateral balance as the aircraft burns fuel. The net result is a much more complex fuel system, and one much more prone to mechanical failure. So, while we don't necessarily like having a header tank, I think it's worthwhile to analyze the statistics. While I don't have the ability to do this myself, I would guesstimate that at least as many people are killed or severely injured in crashes because of fuel mismanagement induced by complex fuel systems as are killed or injured as a direct result of having a header tank.

Oh, one other little point... A header tank provides one real advantage - gravity feed. So while we think that high-wing airplanes like a Cessna 185 are safer because they have wing tanks, we should think again. They actually have a small header tank as well. Even certified airplanes have header tanks, so they can't be all bad.
 

pilot103

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I have had similiar concerns. And will concede that by the time the tank gets bent you have far more serious things going on. But another thought is about the carb being on the bottom of the motor and most planes have a sump for water as low as they can be mounted on the firewall. These are 2 of the first things that are going to be broken. Both splashing gas on a hot exhaust.
 

Waiter

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I think "Paranoid" may be the incorrect word.

It shows common sense to ask, "Why am I strapping this highly flammable liquid to my butt?"

The reasons already mentioned are valid, and should prompt a follow up question: "How crashworthy is the fuel tank strapped to my butt?"

The correct answer should be (morbid), "The tank construction is strong enough so that You'll be dead, long before the collapsing, shredding and tearing metal that sliced you into pieces can do the same to the fuel tank."

I can think of two criteria for fuel tank location placement.

1) Put it in a location where the tank is protected for crash worthiness, and,

2) Put the fuel at the operating CG location.

Unfortunately, these also coincide with the best location for the Pilot.

If the tank ruptures during a crash, it really doesn't make any difference where's its located, 15 gallons of fuel being dispersed by random G forces.
The safest place to be in this case would be a 1/4 mile away.

Waiter
 

Peter V

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These are 2 of the first things that are going to be broken. Both splashing gas on a hot exhaust.
Actually there little harm in gas hitting exhaust manifolds. It evaporates quicker than it can heat to a combustible temp. This happens to me a bit too often when I'm half asleap fueling my motorcycle. It's air cooled, so gets mighty hot. Fuel running over the engine gets instantly vapourized into a hydrocarbon fog. Scares the crap out of car drivers!! :gig: All that hissing and spitting makes me nervous too. But the biggest danger is oil. That's what causes engine fires. So keep your engines clean!
 

Kmccune

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Also with wing tanks, the hoses run through the cabin.

Kevin
 

addaon

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Is there a rule of thumb for how far above the carburetor the fuel tank has to be in a purely gravity fed system? Does it mostly depend on carburetor style? (I'm particularly interested in a Jab 2200 with a standard Bing carburetor).
 

gschuld

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I have been fairly concerned with having fuel in the cabin as well. I have been reminded several times to avoid fuel, especially electric fuel pump pressurized fuel in the cockpit if at all possible. As I am building a KR-2s, I plan on having the main tanks in the stub wing area, about 12 gallons each. For extra protection, I made provisions to route the fuel pump pushed fuel along the outside of the fuselage and into a forward blended wing root fairing that allows the braided fuel line to make it straight into the lower engine cowling corners without ever having any fuel, or fuel line in the main fuselage at all. Nothing is foolproof, but I am happy so far with this arrangement. I will be running an Ellison carb so I need presurized fuel anyway, gravity feed won't work. I figure a separate fuel pump just outside each tank in the stub wings allow a fair amount of redundancy, as well as isolation in the event of an abrupt unscheduled stop:ermm:.

George
 

GESchwarz

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Excellent photos Smoore.

"Multiple blunt force entries" were obviously caused by all the Light Sport airframe pieces that used to be the cockpit. Obviously the designer of this plane was not thinking about crashworthiness. Notice how good of shape the rest of the airframe was in.

To have fuel inside the cockpit or anywhere between you and mother earth in a crash or forced landing is just plain risky. My tanks are protected somewhat by the strength of the wing's main spar. The tanks are aft of the main spar and outboard of the main gear, and so what if I'll need redundant fuel pumps.

The idea that if an impact is bad enough to rupture a fuel tank, and that spilled fuel is the least of your problems should, is crazy to me. The crashworthiness of the fuel tank and the cockpit cage should be of first design priority, but obviously it is not on most GA aircraft. Anyone who is serious about crashworthiness will make the compromises that are necessary.

Most GA aircraft are no safer than the Ford Pinto. Aluminum airframes perform terribly in a crash. Aluminum is great whe loads are applied according to design, but in a crash, it's stress riser city everywhere. Aluminum cracks and tears with ease. Take notice of how they do it in auto racing where crashes are planned for...it's all welded steel.

See my post titled "Crashworthiness" for additional thoughs on this unpopular subject.

Smoore, you're right on target.
 
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Topaz

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Is there a rule of thumb for how far above the carburetor the fuel tank has to be in a purely gravity fed system? Does it mostly depend on carburetor style? (I'm particularly interested in a Jab 2200 with a standard Bing carburetor).
You have to be able to develop whatever necessary head pressure is required to flow fuel into your carburetor when the airplane is at maximum (1G stall) angle of attack. Since the fuel tank is generally aft of the carburetor, a positive AoA will reduce the height of the tank relative to the carb. That's what generally sets the height of the tank in the airframe for gravity-feed systems. The head pressure, I believe, simply needs to be enough to reliably overcome the pressure drop due to the fuel lines and valves. If you can get a flow rate greater than your engine's maximum-power consumption rate at any AoA, you should be fine.

This gets a lot of people into trouble. I've read of quite a few crashes in new-design airplanes because the engine starved for fuel during the first climbout. The designer/pilot forgot that the airplane isn't always level with the ground, when they were designing/installing the fuel system, or forgot about the pressure drop across the lines and valves, and the engine couldn't draw enough fuel from an otherwise adequate system.
 

orion

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This is my paranoia also, despite plenty of evidence that the header tanks are actually a lot safer than first glance might suggest. But despite the statistics, my goal in designing products for my customers is simply to not store any fuel in the fuselage unless circumstances dictate no alternative (like a small jet airplane with marginal wing volume).

One design point I might share is that there are quite a few Glasairs flying that do incorporate a header fuel tank in front of the instrument panel, up high on the firewall. This was an option for those who wanted to fly aerobatics but apparently quite a few opted for the option. But despite the Glasair designs' long history, I don't think there is any direct evidence that the header tank resulted in an increased level of risk to the occupants.

Out of all the Glasiars built, I can think of only one particular case where the crash could have caused tank damage and have been a contributing cause of the occupants' death, but despite the severity of the accident, the header tank still survived. It was an airframe that was converted to using an Allison 250. The folks doing the build however did not take sufficient care in the weight and balance aspect of the resulting configuration (I spoke with the pilot about a week before his fatal accident and based on his description of how the airplane flew, advised him to ground the airplane until he was able to strap about 50# of mass to the engine mount), which caused the pilot to over-rotate on take-off (he was a bit of a hot dog) and essentially climb out on the prop. The resulting torque simply twisted him into the trees that paralleled the runway, killing him and his wife.

I don't recall the specifics of the post crash investigation but I think I remember the resulting fire was from the rupturing of the main wing tank but not the header.
 

addaon

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Responding to Topaz's post:

Okay, makes sense. And I can mount the carb lower, relative to the engine, if necessary to increase the head...

This comes up because I'm looking at going to a single fuselage tank for my pusher design (simplicity, plus not needing to deal with tanks in wings with anhedral). If I go this route, the most natural position for the tank is about three feet in front and one foot above the carburetor at cruise; but of course with a pusher my head increases as angle of attack increases.

Based on what head requirements I can find, this is definitely possible with an aerocarb, and probably possible with the standard Bing carb. So I'm planning on going this route, now that my engine is so low.
 

Topaz

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...The idea that if an impact is bad enough to rupture a fuel tank, and that spilled fuel is the least of your problems should, is crazy to me. ...
Most cars take this approach today. My old Honda CRX Si had the fuel tank right behind the passenger seats, under the front of the cargo area. It was probably the safest spot in the whole vehicle, and CRX's are not known to be fire-hazardous. The Pinto, on the other hand, had the fuel tank exposed at the rear of the vehicle (and outside the passenger compartment!), with about the same level of exposure as is generated by putting the fuel in the wings.

While I applaud your attitude towards safey, recall that all the old rag-wing airplanes (that generally put the fuel right behind the instrument panel) do not have a known history of predominantly killing passengers in a crash-induced fire. Largely because that position, while in the cockpit, is about the least distorted in any wreck the occupants can survive. A sheared-off wing will spray wing-root-tank fuel all over the outside of the aircraft, which will make exit nearly impossible, especially if the aircraft has overturned in the process. You'll burn in an airplane that you can't get out of. I'm not saying that wing tanks are less safe than cockpit-tanks, but rather that each have their own characteristics, and types of wrecks where they may or may not be better than the other.

There is no "safety" in the absolute sense, and there is no "absolutely safe" place to put fuel, except to not put it into the airplane in the first place. You can't build an airplane like a tank, or it'll be unsafe for other reasons. You have to manage risk, and accept that yes, the airplane can crash, and yes, there will be some situations where the fuel might ignite. If those where the fuel is likely to ignite generally require enough distortion of the structure to kill the occupants, fuel ignition really isn't much of an issue anymore. The people are already dead.
 

Topaz

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... but of course with a pusher my head increases as angle of attack increases.
A plus for you, but now you've got to watch out for the case where you're on an extended steep final under full flaps, then have to execute a rapid full-power go-around. You'll want full-power flow rate immediately, and you may be able to generate a pretty steep nose-down deck angle under full flaps.
 

addaon

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Yep. I'm not thrilled that my design leads to favoring putting the fuel in the fuse, and it's on my short list of definite changes when building rev2 (where I have to be responsible for a passenger's safety as well); but, as with anything else, it's about trade-offs.
 

addaon

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Another benefit of putting the tank in the fuse for my design is that I have way too much space there (balance reasons) right now... so I can play games with the tank geometry, like make the bottom slope upwards towards the front at greater than my max nose-down angle, so that the fuel port should never be sucking air. This plus checking height at the critical condition you mention (yep, I can get to about 4° nose down, in theory) should be enough to make me comfortable. I really like the idea of single tank, single sump on the tank, no selector valve; just seems like less to screw up with the first design.
 
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