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Zero Fuel Weight

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paulsalem

Well-known member
Joined
Dec 17, 2003
Posts
1,234
Can anyone explain the reasoning for Zero Fuel Weight?

Is it an inflight limit, or ground limitation or both.

Thanks
 
Could be both, however most of the stuff I think is an inflight structural limitation.

I have seen a couple that were limited due to gear loading(Wing spar issues at the gear attach points) with empty tanks on the ground.
 
Ever watch gymnast on the rings? The real hardcore guys do the "iron cross" thing, where they suspend themselves between the rings with their arms horizontal and body vertical?

With fuel generally being stored in the wings, it's like adding weights to their arms and wrists...doesn't have as big an effect on their ability to perform the routine.

Non-fuel items are generally put in the fuselage, which would be like adding ankle weights to our gymnast, severely limiting his ability to perform an iron cross.

The bottom line is the amount of lift that has to be transferred through the spars and wing connections to support the fuselage, as opposed to the amount of lift that's "used up" where it's generated in the wing.

Hope this helps!

David
 
paulsalem said:
Can anyone explain the reasoning for Zero Fuel Weight?

Is it an inflight limit, or ground limitation or both.

Thanks
I'm not quite sure what you're getting at, since Zero Fuel Weight is a term that has a definition, not a limit. If it were MAX ZFW or MIN ZFW, it would be a limit, and we might discuss why.

Zero Fuel Weight is the sum of the weights of the empty airplane, the crew, hydraulic fluid, flight attendants, passengers, bags, cargo, and other furnishings. Basically, it's the weight of everything but the Usable Fuel.



If you could shed some more light on why you speak of it as a limit...





.
 
Well in a 310 there is a zero fuel limit in the Limitations section. I understand you can't exceed it but I'm trying to understand why its a limitaion. (Ie what happens if it is exceeded)


We've all seen the video of the 777 with the wing being bent 30 feet or so above horizontal before it snapped.

So if there was fuel in the wings, could it have bent further up? That is what doesn't make sense to me
 
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paulsalem said:
We've all seen the video of the 777 with the wing being bent 30 feet or so above horizontal before it snapped.

So if there was fuel in the wings, could it have bent further up? That is what doesn't make sense to me

Nope...if there was fuel in the wings, and the total weight was the same, they would have bent less...on the other hand, if you added fuel to the wings you'd get about the same amount of bend in the wings for the heavier weight.

Just to throw a little more confusion into the issue, on Beechjet 400's, fuselage fuel is actually INCLUDED in zero fuel weight. Again, this fuel is in the fuselage, increasing the bending moment on the wings in flight. On the 400A's and Hawker 400XP's, they changed the way the fuel is burned, so fewer Beechjet pilots accidently violate their limitations section in them ;)

Fly safe!

David
 
paulsalem said:
Ok, I just can't wrap my head around it.

We've all seen the video of the 777 with the wing being bent 30 feet or so above horizontal before it snapped.

So if there was fuel in the wings, could it have bent further up? That is what doesn't make sense to me

Think of it as a 12 foot long 2 x 4 laying across two saw horses. The middle of the 2 x 4 is where the Cabin would be and the board is the wing spar. In flight all of the weight is being supported by the wing spar.

If you push down on the middle of the board it will flex without too much pressure. If you press down further out on the board (wing) it does not flex as much as it does in the very center.

In flight the aircraft is rated for a given amount of g force. Typically +2.5 to 4 or so and -1.0 to 1.5 or so in a transport catagory airplane. As long as the weight is evenly distributed across the entire wing it will support the load. However if you concentrate too much weight in the middle (you sit or jump on the board) it will not be able to support near as much total weight as it could if it were spread out across the entire length of the board.

Basically it is the structural weight limit for the wing roots. Kind of the opposite of holding your arm straight out and putting 10 pounds on your hand or putting 10 pounds just off your shoulder. The 10 pounds in your hand feels like more than the ten near your shoulder. Same principle as a lever. You can exert more force with a longer lever than you can with a short one.

Another way to look at it would be to tie a rope as tight as you can between two trees trunks about 3 feet high. Sit or stand on the rope close to the tree and you only sag a little bit. Climb on to the middle of the rope with the same weight and the rope will probably sag all the way to the ground. Again it works much like a lever.

Not a very scientific answer, but hopefully it gets the point across.
 
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