Designing a Smaller, Lighter Airplane Tail

With engineers from Caltech, Boeing and NASA, Israel Wygnanski is ushering in a new era of fuel-efficient airplane design

Wind Tunnel Testing
A full-size Boeing 757 tail equipped with sweeping jets was tested in a wind tunnel at the Ames Research Center. Courtesy NASA

Israel Wygnanski has been obsessed with flight since his childhood. An amateur pilot, he first soloed at age 16. Now at nearly 80, he still flies and shows no signs of stopping. Over the course of his 50-plus year career, Wygnanski, a professor of aerospace and mechanical engineering at the University of Arizona, has studied how to manipulate airflow and turbulence to make airplanes more efficient.

Next year, the fruit of his work will fly on Boeing’s test plane, the 757 ecoDemonstrator. The project focuses on a major source of in-flight inefficiency: the airplane’s tail. The new tail employs a series of 37 small sweeping airjets that help control steering at low speeds or in the event of an engine failure, when a rudder is necessary to keep the aircraft on course. The design, tested in partnership with Boeing, NASA and Caltech, could lead to smaller, lighter tails and more fuel efficiency in the coming decades. The team received a Group Achievement Award from NASA in October.

The demonstration model you’ve created shows that plane tails are larger than they need to be. Why is that?

The vertical tail is very large; it's almost, in some instances, as large as half a wing. In essence, if an airplane goes through its entire life cycle, say, 25 years, and never loses an engine—that happens, because engines are very reliable today—it essentially carried this large vertical stabilizer throughout its life for no good reason. Think of its weight, its drag. It contributes quite a lot to the fuel consumption of the airplane. It's always used, to some extent, but not to its entire potential. If an airplane doesn't lose an engine, the tail is not a critical control surface.

Earlier this year, you put a full-size tail equipped with your sweeping jets through wind-tunnel tests. How did it go?

Originally, there were 37 [sweeping jet] actuators embedded in this vertical tail. It turned out that even one actuator could improve the efficiency of the tail by almost 10 percent. The area of this one actuator jet, one-eighth of a square inch, can affect the flow over the whole wing, which is 370 square feet. That was an amazing result. I think it will be tested and flight proven.

So just how much smaller can an airplane tail be?

The results show, immediately, that we can shrink it by 30 percent. That's substantial. If you save on fuel consumption in the order of one percent, think of what it means over the life of an airplane. The whole experiment here was to prove a technology and to get our foot in the door, so that the industry will be aware that there is a potential here that they never used. In other words, there is a tool in the toolbox that can change the way airplanes are designed.

Israel Wygnanski
Wygnanski is a professor of aerospace and mechanical engineering at the University of Arizona. Courtesy NASA

So by making a small tweak in the airflow, you’re able to affect the outcome of, say, steering or lift. It seems like a simple concept. What makes achieving it so difficult?

The Achilles heel in this whole problem was the complexity of the actuators that provide the flow control. We initially used electromagnetic ones. People have used piezoelectric ones. Either they are heavy or hard to maintain. Then came this other idea of using a small oscillating jet actuator, which is a device that needs compressed air. It doesn't have any moving parts, and it can be, essentially, etched into the surface of the wing.

And you’ve previously tested this concept on other types of planes?

Yeah. We started investigating some relatively fundamental flow patterns, like mixing of two air streams, which is something you can see in the exhaust of jet engines. That led to larger and larger applications of that idea. For example, in 2003, we tested it together with Bell Helicopters and Boeing, on an airplane that was the technology demonstrator for the V-22 Osprey. What we predicted in the laboratory worked.

It’s a big jump from a V-22 to a passenger jetliner. How did you transition into commercial flight?

We thought, ‘What would be a control surface that is not flight critical?’ In other words, if something happens to that control surface, the airplane can still fly. A typical tail on a commercial airplane is one such surface. Let's say, one engine on an airplane quits. In that case, the tail makes sure the plane will still be able to fly straight, in spite of the fact that the thrust is no longer symmetrical.

Could the system of airjets be used in places other than the tail?

Oh, yeah. Exactly. [This demonstration] was just to convince people it's something we can try. It may do a lot for the future design of airplanes. It can possibly sweep the wings further to the back, and that may increase the speed without an increase of drag. Imagine that you cross the Atlantic with an airplane that consumes the same amount of fuel, but you save an hour and a half of flight. Except for the Concord, we have been stuck with the same speeds for 50 years.

Commercial airliner companies are conservative, with good reason. So the rate by which new technologies are adopted is relatively slow.

Very, very slow. If you are not an expert, you look at the airplanes today and you look at the commercial jet airplanes that flew in the late 1950s, and you'd be hard pressed to see anything very different. It has been more than 100 years since the Wright Brothers. In the first 50 years, there was tremendous change, from the Wright Flyer to the 707. From the 707 to today, yes, there is an improvement in terms of aerodynamics, but it's not very obvious. Today, we fly the same speed we were flying in 1960. There's fuel efficiency, and so on, but, fundamentally, people do say, ‘Well, aeronautics is a sunset science. We don't see anything new anymore.’

And here, you believe that you have something new?

I believe that we do.

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