What is the weight saving of windowless fuselage for cargo aircraft?

Question

What is the weight saving of a windowless fuselage for cargo aircraft? Does anyone have a rough estimation?

Answer 1

The additional mass due to windows can best be estimated by statistical methods. By comparing the structural mass of windowed and window-less fuselages of otherwise identical aircraft and formulating a numerical equation which fits all cases with the least error, you get formulas like $$n_{Windows}\cdot 4.641\cdot S_{Window}^{0.976}\cdot(1.0 + \Delta p)^{4.945}$$ where $n_{Windows}$ is the number of windows, $S_{Window}$ is their surface area in m² and $\Delta p$ is the pressure differential of the pressurized fuselage in bar. The resulting mass is in Kilos (kg). This formula is from the LTH (Luftfahrttechnisches Handbuch) and based on 16 aircraft. Standard deviation is 20.3%.

Actual values per window are between 3.5 kg in case of the DC-10-10 and more than 7 kg in case of the Boeing 747-200. Sorry, I have no data for the XL-sized windows of the Boeing 787. If we assume 80 windows in total for a Boeing 737-class aircraft, the total mass of the windows is about 300 kg.

Jan Roskam cites a different formula (called the GD method, originally from General Dynamics). Here window mass is $$109\cdot(n_{pax}\cdot\frac{1+\Delta p}{100})^{0.505}$$ Now the pressure differential is in psi, the result in pounds and for an aircraft with $n_{pax}$ = 150 passengers the value is 437 lbs or 198 kg.

The window mass takes account not only of the transparent material, but also of the local reinforcement of the fuselage skin. Either doublers are riveted around the window hole, or the whole skin is chemically etched to reduce its thickness, and the area around the window is masked to keep the original thickness. As a rule of thumb, you should add around the circumference what you take away by cutting the hole. Thus, a well-designed fuselage should have a net mass increase roughly equivalent to the mass of the windows themselves.

The Roskam formula is older, and window size and mass have increased over the last decades. Therefore, the newer the aircraft, the heavier its windows.

Answer 2

Let's take a Boeing 737 as a generic example, and spitball some numbers.
I'll make the post community wiki so someone can come along and plug in more accurate numbers.

Aluminum has a density of 2.7 g/cm³
(For our purposes let's assume the skin alloy is "close enough to pure aluminum" because I'm too lazy to look up the proper alloy density).

Plexiglass (just picking a random variety) has a density of 1.18 g/cm³.

The aluminum skin on a 737 is about 0.038 inches (or 0.1cm) thick.
We know a window on a 737 is thicker than the skin. I couldn't find the number but let's go with 0.25" (0.6cm).

If we have a hypothetical one-foot square window (about 930 cm²), the aluminum skin occupying that area would weigh about 251 grams. The Plexiglass equivalent would weigh about 658 grams (407 grams heavier, little more than 2.5 times as heavy).

Figure there are about 70 side windows on a 737 (quick count from a photo of a Southwest plane, not including exit door windows), so that's 28,490 grams or about 64 pounds, just for the plexiglass.

In addition to the plexiglass there's also supporting structure that needs to be added around the window to hold it in place, sealants to seal it against the skin, etc. - these add additional weight. I've no idea what these components weigh, but let's assume it weighs twice as much as the plexiglass, that's about 1.5 pounds per window of additional structure, or 105 pounds, bringing the total weight savings to just under 170 pounds.

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170 pounds is not a lot of weight (in passenger terms it's not even one person), but it is that much more cargo you can carry. Also when combined with the fact that you no longer have to maintain the windows (fix pressure leaks in the seals, replace cracked plexiglass, etc.) the savings can add up over the life of an airframe.

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Some more spitballing here... please feel free to correct my numbers:

On the subject of the cost savings: a 737 can carry about 27000 lbs payload, and burns about 5500 lbs of fuel / hour during cruise, and about 3000 lbs for a 15 minute climb. Assuming an average flight of 4 hours you get about 25000 lbs of fuel burnt, or almost 1 per pound of weight. This means that the fuel cost of 170 pounds of weight is about 170 pounds of fuel, which today costs (at around \$4 / gallon, and 6.5 lbs/gallon) around \$100. Note that the cost of fuel varies a lot with location, so this is very approximate.

Now multiply this by the typical life of the aircraft - three flights per day, 7 days per week, for 30 years? That would be over \$3M of savings. And all because your parcels can't look out of the window.