Originally Posted by
ProPax
I was watching a documentary on YouTube about a company called "Omega" (I think) that provides refuelling services to USAF (I think). One phrase caught my attention. The narrator said that if there was no fuel in the wings, the loaded plane would simply snap them off with its own weight. The channel in question used to describe the process of starting a 777 engine as "the compressed air pushes against the piston making the crankshaft rotate", so I'm not sure whether I should believe them.
So, is that true? Is that true for just KC-135 or for other planes, too? For example, can a Boeing 777-200F loaded with 110 tons of cargo take off with nearly empty wing tanks?
This sounds like pseudo engineering fiction. Every bit of weight that you add to an airplane will impact the loading of the structure. The manner in which weight added impacts loading depends on where that weight is added and how the airplane is being supported. You can imagine that the distribution of loads is quite different when sitting on the gear on the ground vs. flying with lift being generated by aerodynamics. The main reason that main landing gear are mounted near the wing root on most configurations is to keep the difference in the distribution of loads between in air and on ground as small as possible. As I hope you can imagine, some of the airplane structure sees maximum loading when on ground while other parts of the structure experience maximum loading when in air.
To speak directly to the question posed, any weight added to the airplane at any point within the wings or in the fuselage will cause loads within the structure to go up. The distribution of those increased loads will depend on where that weight is added. Similarly, taking weight out of the airplane will lower loads. It is pure fancy to think that structure will be at greater risk from a static loading perspective with less weight than it will be with more weight. There are also balance considerations with regard to cg location and structural flutter, but those points will have to wait for another lesson.
In general, it is easiest to design structure if the payload is located near where the supporting lift is generated. For that reason it makes sense to put as much of the fuel as you can in the wings as carrying the same amount of weight in the fuselage would be much harder on the structure in flight - particularly during an elevated g maneuver. Many airplanes are volume constrained within the wing cross section and thus cannot fit all of the fuel they need to carry in the wings so they also have fuselage tanks. Usually the fuselage tank is the last one that you would fill only using it if the wings are already full and you need more fuel for the mission. Similarly, the fuselage fuel is usually the first to be burned. One of the vicious design cycles that you can find yourself in is that if the range of the airplane is not quite what you had for a target you will have to add fuel and likely will have to add that fuel by adding capacity to the fuselage tank. The penalty that you pay is increased structural loads as that fuel does not sit near the source of the lift that must carry it. The wing root loads go up, add more structure, that increases weight, fuel burn goes up, and now you need to add even more fuel. As you can imagine, most new airplane designs go through some serious weight reduction efforts to get the design to close meeting the payload / range targets they are after.
One of the interesting optimizations that takes place on modern commercial transports that are simultaneously optimized for fuel efficient cruise and minimum structural weight is management of lift during flight. For minimum drag the optimum spanwise wing loading is essentially elliptical with a good deal of lift generated on the outer portion of the wing. This lift distribution, however, generates very large bending moments at the wing/body joint - particularly when maneuvering at elevated load factors. These high bending moments would require heavy structure in that portion of the airframe. For lower wing bending moments and thus to enable lighter weight structure it would be desirable to have wing spanwise loading the has more of the lift on the inboard wing and less out near the wing tips. Starting with 787 Boeing has incorporated wing maneuver load alleviation into its control systems whereby the spanwise lift distribution is managed to be that for efficient, low drag operation when at 1g cruise and more inboard during elevated g maneuvers.
Time for the end of this lecture - FCeng84 signs out (gums is rubbing off on me!)