General question on gross weight

[Bluebird19kf]

I was wondering about how the weight of fuel plays a role in the gross weight calculation. I know you have to add it into the equation of weight and balance but I don’t understand what it would have to do with the weight you can put in the fuselage. It seems like with the fuel tanks being supported by the wing and not be transferred through the lift struts like say a passengers weight would be that it would be less of a factor in figuring max gross weight. Just something we were talking about at the hangar and no one could answer.

[aviator79]

Max gross weight in a typical light aircraft is a structural and performance limit. All of the weight of the airplane is borne by the wings, regardless of where it is located in the airplane. Fuel in the tanks has inertia that will bend/break things in the wings just like people in the seats, and will definitely keep you from achieving adequate performance on a hot day at high elevation.

I suppose it is true that weight at different locations does stress the structure differently, but to figure out an acceptable loading during pre-flight would be cumbersome. You'd have to add the stress contributions from all the loads at each location where they might be concentrated, and then make sure that those structures won't exceed a safety margin in bending, compression, or tension at the certificated g-load limits. It's would be more complicated than simply having different weight limits for the tanks and for the cabin area, and far too cumbersome. It's much easier if the flight envelope only has total weight and CG.

[PapuaPilot]

Let's keep it simple.

The structural part of this question is done by the designer of the aircraft . . . engineers. Pilot's don't need to worry about this, but we do need to keep an aircraft within the limitations given (see Section 2 of any POH for limitations).

The weight and balance is what pilots and mechanics have to do. We need to keep from exceeding the CG envelope limits, zone limits and floor loading limits. That's about all.

For pilots to do W&B we must have the EW and EWCG for the aircraft. Then we add the weights for fuel, the pilot, passengers, cargo, equipment, etc. at the correct stations.

[Bluebird19kf]

Thanks guys. So I guess it comes down to yes fuel in the wings probably has less of an effect on structural integrity but for practical purposes we just treat it like any other load applied to the airframe with regard to its arm location.

[Bluebird19kf]

I just felt like 50 pounds in the tanks would be way different structurally than 50 pounds in the seat.

[PapuaPilot]

For a pilot it doesn't matter. For an aircraft engineer there are structural considerations.

[TJay]

Just my opinion here but I would say when flying lots of fuel just makes you climb out slower. if you have a hard landing full of fuel a small part of that transfers throu the lift struts and the rest goes strait down to the landing gear.

[aviator79]

If you look at frame of a Kitfox, it's not a simple structure. Every force applied anywhere on the airplane will induce reaction forces throughout the entire airplane's structure in ways that are not necessarily intuitive. The modes of structural failure that can result from those stresses are also complicated and nonintuitive.

That's why, as Phil says, the problem needs to be condensed and simplified so that pilots can do it, and do it quickly. Distilling the analysis to total weight and total moment (or CG) makes it possible to plot the envelope on two axes. If you're inside, you're good. If you're not, you're not.

[HighWing]

Just my opinion here but I would say when flying lots of fuel just makes you climb out slower. if you have a hard landing full of fuel a small part of that transfers throu the lift struts and the rest goes strait down to the landing gear.

This is why, with an airliner, dumping fuel in the event of an aborted departure is mandatory. Max take off weight far exceeds max landing weight.

[mscotter]

Fuel in the fuselage versus fuel in the wing tanks right next to the fuselage is not going to make a ton of difference when it comes to the bending of the wing spars and calculating the max load they can support. Now if you were putting the fuel in tip tanks (like on a bonanza) that could be as different story, as the weight of the fuel out at the tips would counteract the bending of the spars (in normal upright flight). Some aircraft even secure gross weight increases when adding tip tanks.

[Bobo]

Regarding the original poster’s comments, large aircraft often have 2 maximum weights the pilot must not exceed; Max Gross Weight (MGW) and Max Zero Fuel Weight (MZFW). I’m not aware of a light GA aircraft that has a MZFW limit.

ZFW is the weight of the aircraft plus any payload carried in the fuselage. MZFW is the max an aircraft can weigh minus the total fuel. This weight does not include the weight of the fuel or wing stores (weight of tip tanks or in military aircraft, weapons on wing pylons) because their weight is supported by the wing as the original poster referenced.

The P-3C aircraft I flew in the Navy had an empty weight of 70,000 lbs, MZFW of 77,000 lbs and MGW of 139,760 lbs. So... the max payload we could carry in the fuselage was 7,000 lbs. All weight above 77,000 lbs had to be either in fuel or wing pylon carried weapons. The weapons carried in our bomb bay also counted towards our MZFW.

I hope my example helps to explain some of the original poster’s questions.

Todd