New research suggests that a bird-shaped airplane -- with angled wings and a fatter body -- would make the modern plane more fuel efficient. I spoke recently with Geoffrey Spedding, an engineer at the University of Southern California, about this discovery -- and how it could change the way we fly.
How did you discover that a bird-shaped design would make planes more efficient?
To start at the beginning, Joachim Huyssen [of North-West University] in South Africa was wondering about efficient aerodynamic designs and whether the current dominant configuration is the best available. There are several features of that design that are aerodynamically not convenient and not optimal. The best possible [design] from an aerodynamic point of view is a wing by itself. You would think then the best possible airplane would be a flying wing, and, in fact, flying wings have been made. But the difficulty is two-fold:
- You have to make the wing so big to pack people and cargo in that you end up wasting a lot of space. The waste of space causes aerodynamic drag, the airplane is more heavy than it should be. Your optimal design turns out to be not optimal after all.
- All existing airplanes have this tail plane in back. The purpose of the tail plane is to provide pitch stability. The nose goes up or the nose goes down -- you control that with the tail plane. This comparatively little tail plane sits at the end of a long body on the conventional plane. The purpose of the long body is to carry the tail plane and put it a long way [from the plane's center of gravity], so it has a large effect. But that's a really inefficient way to do business. All the time you're dragging this tail plane through the air, its only purpose is to make sure you don't pitch up or down. It doesn't otherwise do any good and it forces you to have a long, cigar-shaped body, which is not the best shape a body can be.
[Huyssen] thought: Suppose we could pitch up or pitch down using just the wings. He probably did sneak a look at birds -- seeing that they obviously did it. You have to have some forward and backward motion available. That's why he came across this design. If you look at the wing from above, it sweeps forward and then it sweeps back. If you do that and you can control those sweep angles, then that is sufficient to provide stability. You don't need a tail plane -- and you don't need a long body. You can pick the most aerodynamically-efficient body, which is surprisingly a lot fatter than these cigar-shaped things we have now.
It turns out that you can put a stubby little tail on the back of that body and control the flow of air over the body. It acts like part of the wing. Remember, we were saying that the best possible shape is just a wing by itself. We did these comparisons: What does the flow behind the wing look like? What happens when you put a body on that? Plus a tail? You can restore the air flow to just about what it was without the body. It's as if there's no body there, it's just the wing. That has to be more efficient.
We worked together for a year in South Africa thinking about this and developing ways to explain it. But these ideas had never been tested until this summer. [Huyssen] came over to USC and we tested a model he made in a wind tunnel. We made measurements that showed this was indeed the case. That's what we got so excited about.
Did you find out how much more fuel efficient it would be to design a plane like this?
No. To be honest, we can't. Not yet. We have high hopes. I think unless we demonstrate at least 10 to 20 percent, then it's probably not worth it [as far as] the extra costs involved, the redesign. It's going to have to be very clearly worthwhile in order to tempt people to do this.
What would it mean to redesign planes to look like this?
Designing aircraft is a very complicated balancing of many different constraints and requirements. We've thought of as many of the aerodynamic ones as we can. That includes the aerodynamic efficiency and where the control services go and how stable the plane is. This is basically an aerodynamic efficiency argument. If it does work on those grounds, then it's worth taking a look at these things. I think we've demonstrated that it does work on aerodynamic grounds alone. On the aerodynamic side, you win many times over. The wings can now be shorter and lighter. The body can be lighter because it's more volume-efficient.
Would this redesigned plane have any limitations compared with planes today?
Nothing in this argument depends on the size of the plane. According to us, the jumbo jet should look like this and so should the small, single person plane.
Talk more about the test you did over the summer.
We took a simple, straight wing and we put it in the wind tunnel. We can measure aerodynamic forces very accurately and we can measure the flow characteristics very well. That's why Huyssen came all the way over here from South Africa to put his model in our wind tunnel. We conducted a series of comparisons. The first was the wing just sitting in the wind tunnel by itself. We measured the flow field behind the wing, verifying that it looked as we thought it should. Then we added a body to that wing and it messed up the flow field just like we thought it would. The exciting part was when we added a tail to the back of that body. It cleaned up the flow remarkably, so the flow behind the wing-tail-body combination looked just like the flow behind the wing alone. That was the key test we hadn't done before and that showed the basic idea worked.
What are the next steps for this project?
I'm going to go back to the lab and do more tests with different angles between the components, different tail shapes to fill out the table of results. At the same time, we're publishing a paper on this result. Then, I want to find some funding from a commercial airline or NASA or somebody like that. I'm going to work with a professor of architecture here who is interested in new interior designs which are now possible because the shell is a different shape.
Image, top: Bird plane rendering / RJ Huyssen
Image, bottom: Geoffrey Spedding