Boing! Boing! Boeing! Airliner related questions

Factual Questions
1

Today was a flyday, wherein I got up at an ungodly hour in order to fly to another far away city to advance my company’s cause, and then flew home.

With mission accomplished, my fellow traveler and I got to musing about stuff. One thing we’d noticed was that the first two airlegs of our trip were on Southwest’s (fairly) new Boeing 737-700s, with the Blended Winglets. Southwest’s website has this to say:

Anybody up for an explanation of how these winglets accomplish the stated performance improvements?

They are zippy-lookin’.

We then meandered off, in recognition of the fact that neither he (a geologist) nor I (a geophysicist) will ever tire of looking out of airplane windows. That activity often provides a wonderful regional perspective on the earth surface, but today most of East Texas and Oklahoma were socked in. So we spent a bit of time swapping made-up on the spot hypotheses about why clouds act like they do.

This lead to questioning why airliner windows are the way they are. Specifically, why are they so small, and mounted at a level that makes one squnch down to really look out of them? My friend offered the guess that they are primarily there to offer some indication of exterior conditions, but that it’s not expected that too many people will really want to look out of them. Vaguely remembering something to the effect that the shape and placement of the DeHavilland Comet’s passenger windows were suspected of contributing to metal fatigue that lead to catastrophic fuselage failures, I offered the guess that a midline placement might have something to do with maintaining the structural integrity of the fuselage.

But we agreed that we don’t really know.

Before we depart the airliner windows segment of this question, we’d also like to know something about their design, which seems to be almost universal. The interior surface (that you can thump your finger against) seems to be quite flimsy, perhaps plexiglass. It looked to me today that there are two more transparent surfaces exterior to that, one of which has a small hole near the bottom. My best guess was that the hole is there to accomodate interior/exterior pressure differentials.

But I don’t really know. Anybody up to explaining common elements of airliner passenger window design?

Lastly, we started to wonder if it’s ever been recorded who might have been the first aviator to surmount a cloud cover and see a sunny day while all below stood under the grey?

2

I’ll tackle a couple of the questions:

A full explanation would be quite complicated. Burt Rutan is the originator of this simplified explanation for what winglets do: they make the air think the wing is longer by about 2/3 their height, while making the plane’s structure think the wing is longer by only about 1/3 their height. (Longer wings are more efficient, but also impose greater loads.) All the benefits that Southwest claims stem from the greater efficiency of the wing with its winglets.

I’m not sure if it’s well recorded, but it is surely a balloonist, probably before 1800. And since hikers on mountains had been doing this long before that, it probably came as no big surprise. (But that doesn’t mean it’s anything less than spectacular.)

3

Airline pilot, not engineer …

The very simple version of “why winglets?” is that a typical wing creates a lot of turbulent flow as the air from the bottom of the wing curls around the end of the wing to get to the top. A swept wing is worse for that than a straight wing. A sizable fractin of the total wing-induced drag comes from the vortex generated by that mixing at the tip. The winglet acts like a plate on the end of the wing which reduces the amount of bottom air mixing with top air, hence reducing turbulence, hence drag.

Also, all else equal, the longer a wing, the more efficient. The winglet is a way to lengthen a wing without taking up more room on the ground, and since it isn’t producing lift, it can be made lighter than an equally-long horizontal tip extension. The up-tile makes it less helpful than a straight tip extension of the same size would be, but it still gives some gain.

The gain in fuel efficiency in cruise is a couple percent. There is a cost in fuel efficiency in climb and also in fuel burn throughout the flight due to carrying the extra weight. There’s also the financial burden of their initial purchase cost. The traditional rule of thumb is it takes about an 800 mile flight for them to break even financially, all things considered.

Southwest in fact decided not to buy them some years ago since then they did almost all short-haul flying. As their average stage length increased, and as fuel has become relatively more expensive compared to capital, the breakeven stage length has gotten shorter & Southwest is embracing them, as are most other new-model 737 operators.

I’d bet at todays cost factors the breakeven is more like 500 miles. then again, when buying something with a 20-year lifespan, you can’t always make a smart decision just looking at the data for next month.
Windows: The small hole in the inner pane is more for preventing condensation & fogging than pressure equalization. The flimsy innermost layer is just a low-cost replaceable scratch shield. The next layer is structural & can hold the pressure load if the outer pane fails (net of a slow leak through small hole). The outer pane carries all the load under normal conditions. The two of them are there for redundancy.

The size is intended to be big enough to see out of, but small enough that an adult won’t go out the hole and the airplane can maintain at least partial pressurization with one missing.

The Comet’s windows were quite square, which led to a significant stress concentration at the corners, which lead to cracking which lead to structural failure. The modern windows have rounded corners with a large radius for that reason.

As to why they’re mounted so low, I have no clue.

FYI, Cockpit windows are 2 or 3 layers of glass and/or plexiglass with embedded-wire heaters and thin sheets of plastic, like car safety glass, bonded in between the panes to prevent shattering if struck. They’re about 1-1/2" thick and stunningly expensive.

They’re much heavier than side windows because they’re a lot larger and because they have to withstand bird impacts at speed.

4

Ok, Hubby works at a large airline manufacturing company. Yes, that one.

Winglets: They decrease the turbulence as air passes over the wing, causing lift. They also channel the eddies off the tips of the wings so lift is more efficent.

Windows: They aren’t low, the seats are high. Most of the world is populated with tiny people, we Westerners are huge, in comparison. Since redesigning the aircraft for different size passengers is impractical, the seats are adapted, thus, your big American seat is high enough that your knees aren’t keeping your ears warm , and you have a place to stow that laptop during takeoff and landing.
The windows are multiple layers, the hole is to allow pressure equalization. Those windows are anything but flimsy. They are about 8" thick.
And last, who saw the sun? That would have been the brothers Wright, they zoomed high enough to breach the ground fog, to see the smiling sun. In fact, that was one of their selling points.

5

It may be a nitpick, but essentially all winglets (including those on a 737) do produce lift (inward).

6

Airliner passenger windows are 8" thick? Wow… that sure must add a lot of weight to the plane. Why do they need to be so thick?

As for why the windows are so low, I always assumed its becuase most of the time you want to look out a jet’s window you want to look down. I think the windows are at just the right level-- except when you’re on the ground, then you have to sort of duck down to see through them. Think if they were at eye level–you’d have to stand up to see the ground below (while cruising).

7

I did a quick google to make sure that the blended winglets were what I thought they were, and I came across this spiffy website describing them. Enjoy!
My WAG as to why the windows are too low: most of the interesting things to see out of an airplane window are at a down-angle. If the windows were higher, the average person would have an easier time seeing things straight out towards the horizon, but would have to sit up uncomforatably straight to see ground features closer to the plane. Plus, think of the children! Lower windows means that the little tots can see out easier, keeping them entertained and less likely to kick your seatback for hours on end. :smiley:

8

Aren’t they two layers of (?) plexiglass with air between? Doesn’t the thickness count the air layer?

9

I’m pretty sure 8" was a typo as I don’t even think the entire section is that thick let alone any of the individual panes. I don’t think Shamu’s tank is 8" thick and it has to withstand much more pressure than an airliner cabin.

Funny but I was paying special note of the windows on a plane today, a 737 but I did note the dash number - Southwest plane with the really tall winglets, as ice had formed on the inside of the window and I have never seen that before. It was snowing in Columbus but I don’t think the ice formed until we were at altitude. Just a small strip near the lower edge no more than 1/16" thick what wiped away easily.

10

A real genuine Boeing airplane assembly mechanic here. In fact, I will be working on a Southwest 737-700 today, line number 1673. The explanation of the winglets was answered above, no need to go into that anymore. As for the windows, they are not 8 inches thick. At the most they are 3/4" thick and they are made of reinforced lexan. It is made of 2 pieces with an airgap between to prevent possible fogging. If you look at the bottom center of the window, you will see a small vent hole. That hole is on the inside piece only. The window is wrapped in a one piece rubber seal and is held in place by 12 clips. What makes it appear to be thicker is the sidewall panel on the interior side of the airplane. The window shade and boot add a couple inches of thickness.