It tells us why engines are placed slightly in front of wings, and why the fuselage of supersonic planes narrows along the wings,

and why redesigning the hump of the 747, to be longer, not only allowed it to carry more passengers but also increased its cruise speed and decreased drag.

This simple rule is called the area rule, and it tells us the ideal distribution of cross sectional area along the length of a transonic or supersonic plane will look something like this.

That includes the wings, engines, everything.

Because a gradual change in cross-section results in a reduction in wave drag, which is the drag that arises from shockwaves forming over the plane as it approaches the speed of sound.

In reality, planes never achieved this ideal distribution.

The Concorde's distribution looked similar to this, and could have been closer to the ideal had they tapered the fuselage over the wing—but more on that later.

The rule was discovered as planes started to edge ever closer to the speed of sound,

and designers started to notice that adjusting the position and shape of features on the plane had significant effects on performance.

Take this patent from the Junkers Company company, in 1944, showing obvious attempts to experiment with the distribution of cross-sectional area along the length of the plane.

With the cross-sectional areas highlighted here.

This ultimately lead to the bizarre-looking Ju 287.

The effect was discovered again by Richard Whitcomb when testing models in a wind tunnel in the 1950s.

His discovery was used to redesign the F-102 Delta Dagger, which was originally designed with a straight fuselage.

The redesign tapered the fuselage over the wings and allowed the distribution of cross-sectional area to more closely resemble the ideal.

This small change, along with slightly more powerful engines, brought its max speed from 0.98 Mach to 1.22.

Sometimes it's not possible or does not make economic sense, as was the case for the Concorde, to narrow the fuselage over the wing.

Instead, it makes more sense to make the fuselage before the wing larger to make the change in cross-section less dramatic.

The 747 800 featured a longer hump for exactly this reason, which allowed its cruise speed, passenger capacity and fuel economy to rise.

This rule even affects the placement of engines.

Most large planes mount the engines slightly forward of the wing and not directly under, as that would compound the wings' addition to the cross-sectional area.

And in narrow-body planes, the engines are often mounted above and behind the wing for the same reason.

There are countless other examples of this rule that you can find in the real world and I challenge you to name some of them in the comment section below.

I'll pick up my favorite one and send you a free Real Engineering t-shirt.

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There's been a slight delay for my next video on ICBM defense, and I've committed to posting videos at least every second Friday.

So please forgive how short this video is!

Thanks for watching, and thank you to my Patreon supporters for helping this channel exist.