The Area Rule and Aircraft Design Delta Dart » Jet Owner Group

The Area Rule and Aircraft Design

I’m a bit of a biomimicry fan. I may enjoy permaculture gardens and design, but I love smart aircraft design. And I’m even more impressed when good design borrows from birds and nature.

But going really fast isn’t a permaculture kind of thing and it certainly isn’t for birds. A hawk might hit 250 mph in a dive, but that’s it. They simply don’t have the problems jets have.

Going transonic speeds between Mach 0.7 and Mach 1.2 is a different story. And the story begins in 1943 when German designers began to see problems “at shpeed” as you might say to yourself in a German accent. As wings and bodies went faster through the air—drag properties changed. And that happened abruptly.

Shock waves formed well below the speed of sound (Mach 0.70).

This was a problem. They interfered with control surfaces, dramatically increased drag and were an obstacle in the Post WWII race to break the sound barrier.

When an airplane went this fast things changed. Shock wave formation made everything crazy. Think Chuck Yeager pushing his Bell X-1 through the sound barrier for the first time.

Bad design does terrible things when you push it too fast through the sound barrier. But who knew? This was all new territory circa 1947.

Sure, things were hard to control . . . and there were unholy vibrations. But why?

Next, test pilot malaise set in: Why am I doing this? And WTF is going on at at .8 and .9 Mach? (Remember, poor Chuck was essentially flying a fast WW2 fighter with a rocket engine.)

Business Jet Aircraft Design

If you were a business jet, you’d want a smooth ride and experience at near Mach 1 numbers as possible. And, lucky for us, math and aerodynamics combined to offer a solution to the transonic flow problems. And since a business jet spends most of its life just below the speed of sound, how air flows around the wings and fuselage is important.

What Richard Whitcomb discovered was that the easiest way to delay the onset of these shockwaves, was to smooth the body. But smoothing wasn’t enough, the shockwaves persisted, particularly at a specific point where the aircraft’s shape, but more importantly total volume (or cross sectional area) changed.

Aircraft design is similar to good stealth design, you want to think about eliminating “sudden transitions.” But the transitions weren’t the only sticking point. The eureka moment came when Whitcomb realized a transition related problem. If you had a cylinder and you stuck wings on it, no matter how graceful the transition from fuselage to wing, the volume (or cross sectional area) jumped when you added wings. When you add wings, you are adding more cross sectional area:

The Area Rule and Aircraft Design Unifilar Drawing » Jet Owner Group

. . . and this is when you might need to consider how that impacts the total frontal area you are trying to jam through the air.

The total area being jammed through the air was not just the fuselage, but the entire area, including the wing. The shock waves, it turned out, became a problem when this “area rule” was not respected.

Lesson #1 of going fast: Respect “The Area Rule”

The discovery led to “waisting” that we see in so many airplanes today. In other words, the total area being pushed through the air in the front, should gradually (not suddenly) increase other places that pass through the air.

Perhaps the most typical image of this corset effect is the Delta Dart and its tightened mid section.

Next time someone asks you why there is a dimple underneath the engine pylons or why are there such big fairings trailing the wing of that airliner you are riding on . . . calmly explain:

Adding or removing volume (or frontal / cross sectional area) has a real impact on everything.

Those big engines at the back of your business jet increase the total area moving through the air at Mach 0.80. And you don’t want that. So some abrupt waisting (think Lear 60) or dimples (Phenom 300) solve this problem elegantly.

That dimple, or calculated waisting, is exactly the amount of drag surface created by pylon and engine.

Sears-Haack Bodies

Another way to think about the area rule, is to visualize the Sears-Haack body.

This shape—the bullet that dominated science fiction writing in the 1950s is what you want. Want to go through the air fast? And trouble free? Then make like a bullet and Sears-Haack yourself.

Make sure you look like the image below. A mathematical bullet:

The Area Rule and Aircraft Design Sears Haack » Jet Owner Group

But since we build flying things, we need wings and engines.

Now pretend you are child that has just been given a lump of clay shaped like this pointy double ended cigar.

And . . . it was part of their aircraft design class. If this body were clay and you could mush out the wings you would do it in a way if adhering to the area rule. From nose to tail you would be sure that the cross sectional area only grew and tapered gradually (as Sears-Haack body does) so that there were no abrupt changes in area.

Pleasant and efficient transonic flight requires that aircraft design respect the area rule.

The Sears-Haack body can be a helpful guide in remembering to keep the cross section area constant.

But, reality is a tough customer. Ultimately we need engines, control surfaces, and more. And while the design engineers concentrate on that, they do so with an eye towards doing it as intelligently as possible.

The challenge is identifying aircraft that have the deepest respect for the area rule and how their operational history reflects successes in terms of charter affinity and specific range relative to their peers.

Next steps

Does aircraft evolution, value, and design stuff get you out of bed in the morning? Me too. Don’t hesitate to contact me via email at adam@jetowner.com or text me at 617 901 3245. We develop financial, acquisition and management models for flight departments of all sizes.

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