3
$\begingroup$

Is there a reason (except aesthetics) behind the horizontal oval form of the windows of some private jet/small aircraft?

As far as I'm aware this shape should be rather counterproductive. According to the following diagram, a lying oval is the worst stress-wise and a vertical oval (similar to commercial aircraft) would be better:

Y-axis depicts stress-factor

enter image description here

A higher stress-factor results in higher structural weight.

The diagram assumes monoaxial stress. I think the stress in the tangential direction of the hull should be much greater than the stress in the longitudinal direction.

E. g. a Cessna 300 Series aircraft:

enter image description here

$\endgroup$
6
  • 4
    $\begingroup$ Y'know what would be even safer? NO windows at all! $\endgroup$ Commented Feb 13 at 15:15
  • 4
    $\begingroup$ In a fuselage that small, a tall window might have to go through a longeron, or be so small as to be not worth the trouble. $\endgroup$ Commented Feb 13 at 16:02
  • 1
    $\begingroup$ Can please show a reference for the image? Keep in mind that your diagram is only valid for stresses in the horizontal plane. In case of vertical stresses, you flip the image 90 degrees, and the horizontal windows are suddenly the best. $\endgroup$
    – ROIMaison
    Commented Feb 14 at 8:35
  • $\begingroup$ "A higher stress-factor results in higher structural weight." Yes, because you have to put enough material to resist the stress arising from the high stress factor. $\endgroup$
    – EarlGrey
    Commented Feb 14 at 8:40
  • $\begingroup$ "I think the stress in the tangential direction of the hull should be much greater than the stress in the longitudinal direction" it strongly depends on the fuselage shape, so saying anything in general is difficult. $\endgroup$
    – EarlGrey
    Commented Feb 14 at 8:43

2 Answers 2

2
$\begingroup$

You are reading the graph the opposite way you should read it. In the extreme case of a "horizontal flat" windows (i.e. a window with zero thickness, i.e. no windows as funnily but correctly suggested by @A.I.Breveleri), the stress concentration factor (/y-axis) is 1, i.e. the maximum stress is the same as the far field stress acting on the intact fuselage.

For an elipsoid elongated in direction of the maximum tensile stress, the stress factor is always smaller than the stress concentration factor of a rounded or eliptical-in-the-other-direction windows (exactly the line you point in the graph is always lower than thew other two).

The reason maybe is that for small plane the maximum stress is due to the fuselage itself carring the tail/nose load (therefore horizontal), while for big planes it may be due to pressurization (therefore radial i.e. "vertical")?

The stress direction is probably changing along the fuselage. too, and it may even change due to dynamic conditions during the life of the plane.

If you look carefully at the Cessna 400 windows, you will see that the shape is slowly changing with distance from the wings (see photo here), probably to minimize the fuselage material required to support stress factor from varying loads, as well as to optimize for the stresses (towards the tail they may come from all directions, so a rounded shape is a one-size-fits-all, while close to the wings the stress are more close to horizontal, so one can save fuselage material and weight by having horizontal elipsoid).

In short: these windows are extremely ugly, no aesthetics whatsoever can be possibly involved, it is just engineering to provide the maximum windows area possible without needing to add two engines to fly the resulting reinforced brick.

$\endgroup$
1
  • $\begingroup$ Regarding the windows getting smaller towards the end, isn't that just a consequence of the fuselage tapering and thus the 'roof' coming down? If you would keep the same size windows, you would run into the tail ventral fin $\endgroup$
    – ROIMaison
    Commented Feb 15 at 8:40
1
$\begingroup$

Humans are naturally more interested than scanning left and right along the horizon than scanning up and down.

We inherently tend to disregard or mentally minimize objects that are located far from the horizon. That's why the full moon looks huge when it is just above the horizon, and tiny when it is high in the sky.

The window designers designed the windows to give the humans the largest view of the part of the visual field that the humans were most interested in.

By shaping the windows to prioritize the part of the visual field that inherently seems "biggest", the illusion is created of a greater overall field of view.

Also, by avoiding tall windows, the sun is less likely to shine into the passenger's eyes.

$\endgroup$
2
  • 2
    $\begingroup$ These reasons also apply to airliner windows, but those are tall not wide, so something structural not just physiological must be guiding such decisions. $\endgroup$ Commented Feb 16 at 16:39
  • $\begingroup$ Just about the only windows where visibility is the key requirement are the ones up front. Aside from very specific cases, the rest are more about aesthetics and efficiency. $\endgroup$
    – fooot
    Commented Feb 16 at 22:04

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .