Zero Fuel Weight

[javelin]

Got into a discussion the other day, neither of us are current or clever enough to resolve it, so over to you young bucks.

As we are mainly ZFW limited (don't go down the mass argument, I'm British and old) and we never have a situation when we don't any fuel in the aircraft, why do we get limited and why can't we apply a factor to allow for the fuel in the wings.

[Mad (Flt) Scientist]

It's not immediately obvious to me what your question is....

If you are asking why you can't apply the inertia relief of wing fuel to the ZFW limit:

Assuming that the max ZFW limit is being imposed by structural considerations, perhaps wing bending loads at the root, then adding wing fuel will not reduce the requirement to carry the fuselage weight throught the root. The load at the root will not go up (because the additional mass has been added to the wings as fuel, and so inertia relief applies to that added mass) but neither will it go down - the weight of the fuselage at 1'g' is still the same, and must still be transmitted through the same load paths.

Inertia relief from wing fuel enables you to carry a higher total aircraft mass than would be the case were the fuel carried in the fuselage; it does not transfer the loads for the fuselage into the wing 'for free'.

Regarding the statement of 'never having no fuel in the wing'. If you were to use the reserve wing fuel and assume it was always in the wing, to meet some loading restriction, then you should not be counting it as reseve fuel - essentially that has to become unusable fuel. Otherwise you may get into a situation where you need to use the reserve to divert or hold, but can't use it for fear of some other problem (weight and balance, say).

Only if you absolutely NEVER NEVER will ever have to use that fuel can you consider it to be in the nature of ballast.

(Someone might tell me operationally that's wrong, but from an engineering/design approach, we can't assume that reserve fuel will not be used one day)

[john_tullamarine]

If my recollection is correct and adding to MFS' post, a number of aircraft (freighter conversions are the only ones I know of) over the years have been recertificated to higher MZFW on the basis of mandated minimum ballast fuel loads in the wing tanks.

[javelin]

Ok so far, now lets assume a ZFW of 61 tonnes, which we load up to with a 'minimum fuel' in the wing tanks of say 3 tonnes. Now I can put 13 tonnes of fuel on (16 tonnes in all) before I gross at 77 tonnes, 12.6 will go in the wings and the rest will go in the fuselage tank - i.e. inboard of the undercarriage and presumably adding to the loadpath as previously described.

Your serve !

[bluskis]

Don't understand the question? I don't understand the answers to the questions either. I think a simple answer is possible and I await it with expectation.

[Old Aero Guy]

javelin,

In the case cited in your latest posting, you need to think about the sequence for burning fuel.

Adding fuel to the wing tanks does relieve the wing root bending moment so more fuel can be added to the body.
I believe you are equating the fuel added to the body being the same as adding cargo.

This isn't valid because, as stated in M(S)F's reply, the airplane still needs to be good for 2.5 g's Flaps Up with no fuel on the airplane.
That's why the first fuel burnt is the body fuel.
As the body fuel burns off, the wing root bending moment reduces.
As the wing fuel burns off, the wing root bending moment increases, with the max. bending moment reached at MZW.

As an example: (values are notional)

Load Condition/ Weight/ Wing Root Bending Moment
MZW/ 61 tonnes/ 661 meter-tonnes
MZW+Wing Fuel/ 73.6 tonnes/ 547 meter-tonnes
MZW+Wing+Body Fuel/ 77.0 tonnes/ 581 meter-tonnes
(It's hard to do a good table)

For this example based loosely on the numbers you provided, the highest bending moment occurs at MZW .
If the gear could stand it and tail loads were O.K., you could argue for a higher gross weight and more body fuel to increase range, but MZW would still be limited to 61 tonnes if the wing root limit was 661 meter-tonnes.

Of course, other loading conditions might be designing MZW, such as fatigue.

[john_tullamarine]

One of the analogies which I use in training to hide the numbers (big numbers and, even worse, equations, often tend to frighten people who think that engineering is somehow related to beating your head against a brick wall) is to liken the wing to a rule (as in the office stationery measuring device around 12 inches typical length).

If you can bear imagining the situation without a picture .. it probably would help if you actually did the exercise with a rule .. and taking the simple situation where the problem is one of simple bending limits.....

If one holds the rule so that a central load can be applied downwards and tipwards loads up, then the situation is similar to a wing if one thinks of the central load as being the fuselage and the tipwards loads being the net of (wing lift - wing weight) where wing weight includes wing fuel.

For a constant fuel load, as we pull some G, or increase the fuselage weight (mass if you prefer), or some combination of the two, the extent of bending load necessarily increases and the rule deflects to form a nicely curved shape. Some basic structural analysis results show that the extent of bending is related to the maximum stresses experienced in the structure, so we are entitled to think in terms of more curvature = closer to the structural limits.

At some point, as the load/G increases and the bending increases, we reach a stage where enough is enough and any more curvature would cause an unacceptable risk of damage to the wing ... either static or fatigue.

Now, if we increase the quantity of fuel in the wings, maintaining all the other things constant, the additional fuel provides a mass, and hence force, downwards on each half wing ..... tending to flatten or straighten out the curvature. Now it is feasible to increase the fuselage load (i.e. more centre tank fuel) which, naturally, will increase the curvature of the rule.

If we consider a few different fuel weights it is not too difficult to see that the worst (i.e. most critical) case for allowable fuselage load will occur with no wing fuel load... hence MZFW. As it would be a bit difficult to schedule a sensible way of making use of these effects, the certification rules simply look at the worst case and we end up with a single figure for MZFW.

The same logic applies to the various freighter recertifications which trade mandatory outboard wing fuel against an increase in MZFW. This is one way in which a benefit can be obtained. However the exercise is an expensive and involved one as it involves a recertification of the aircraft .... we have all heard the old joke that certification only occurs when the substantiating paperwork weighs considerably more than the aircraft.

Similarly, the manufacturer can gain a structural fatigue benefit by following the same principles ... load the wings first, then the centre fuselage tank and fuselage aux tanks .... and use the fuel in the reverse order.

[javelin]

Thanks chaps, it is what I had suspected - it comes down to paperwork every time !