Here we go again. Folks at MIT are proposing a low-drag low-boom supersonic aircraft, but this is a head scratcher to me:
Through calculations, Busemann found that a biplane design could essentially do away with shock waves. Each wing of the design, when seen from the side, is shaped like a flattened triangle, with the top and bottom wings pointing toward each other. The configuration, according to his calculations, cancels out shock waves produced by each wing alone.
However, the design lacks lift: The two wings create a very narrow channel through which only a limited amount of air can flow. When transitioning to supersonic speeds, the channel, Wang says, could essentially “choke,” creating incredible drag. While the design could work beautifully at supersonic speeds, it can’t overcome the drag to reach those speeds.
If the design “lacks lift” (which it does — that’s the problem with a Busemann biplane) how does it “work beautifully at supersonic speeds”? What holds the airplane up?
Anyone? Anyone? Bueller?
Jeez, I know of some people who have an even better solution to the problems of the Buseman Bi-plane… 😉
At high speed lift is not really an issue. Just a fraction of degree of angle of attach of the fuselage could provide all the lift needed.
The V^2 term in the lift equation can hide a lot of sins….
Now how do you make wings big enough to lift at take off and landing and at the same time not generate sonic booms and huge drag at speed, that is the problem.
If the angle of attack is non-zero, then the wing starts to generate shocks, big time. And if the fuselage is providing that much lift at zero AOA for the wing, it’s going to be generating a lot of shock boom and drag. You can’t solve the problem with simple geometry.
High speed does not automatically translate to high qbar; depends on the altitude. Also, the lift curve slope decreases significantly as Mach number increases.
An aircraft could fly at supersonic speed with far less sonic boom and drag if it were a flat plane, which has no internal volume. The genius of the Busemann Biplane is a structure with substantial internal volume producing a sonic boom similar to a flat plane.
An aircraft could fly at supersonic speed with far less sonic boom and drag if it were a flat plane, which has no internal volume. The genius of the Busemann Biplane is a structure with substantial internal volume producing a sonic boom similar to a flat plane. The conceptual drawing in the linked article defeats that with the bulge fuselage.
An airplane could pass through the air at supersonic speed with far less boom, etc, if it were a flat plane with no internal volume. It could do this because it would be passing through the air without disturbing it.
In order to fly, it has to generate lift. In order to generate lift, it has to exert a net downward force on the air it passes through, which disturbs its (lack of) motion. Such a disturbance, propagating supersonically, results in a shock and thus a boom.
Creating a structure which has the appearance of a wing or two, but which carefully refrains from exerting a net downward force on the air, doesn’t help. Saying that the angle of attack will be just a fraction of a degree, also doesn’t help. Nothing in the usual description of the Busemann Biplane allows it to generate any amount of lift without losing its shock virginity and generating about the same sort of sonic boom as an ordinary wing generating the same amount of lift.
If the folks at MIT have come up with something that results in lift but no boom at supersonic speed, they have come up with something extraordinarily useful and perhaps ought to describe it.
Sorry, any shape with nonzero thickness will generate a shock wave, even a flat plate at zero angle-of-attack. And there will be a pressure discontinuity across any shock wave. You might want to review your compressible flow theory.
You have drag/sonic boom due to lift, and parasitic drag/sonic boom. The former can’t be completely eliminated, though it can be reduced some. Drag due to lift is reduced by deflecting a greater mass of air a lesser amount. Sonic boom intensity is reduced by spreading the lift over a greater length along the axis of flight. In short, a large wing area as a trade off with skin drag and thickness needed for structure. Parasitic drag can be practically eliminated by clever geometry.
The Busemann Biplane is clever geometry which captures mach waves in a way to cancel them.
To quote South Park “Simpsons Did It!” Now to quot eth eSimpson’s Episode ‘Lisa’s Wedding’:
“I
love these new planes,” exclaims Lisa. “Yes,” concurs Hugh, “it’s a
good thing they reevaluated those wacky old designs.” The camera pulls
back to reveal the plane they’re in has six sets of wings on it.
Since the drag increases as it approaches supersonic it got me imagining a parabolic flight we’re they had to go to negative g’s to get past the barrier. Of course they’d have to corkscrew to keep the passengers in their seats. It would also help if Mr. Magoo was the pilot and Bozo the navigator. Ah, those crazy MIT kids.
There was an enjoyably humorous story in Analog a while back about the ‘supersonic blimp.’ Lift was supplied by helium.
OK, the article says that a Japanese team is working at having wings change shape–that seems the obvious solution. One configuration for high lift takeoff and landings, another for sub- through trans-sonic flight, another for supersonic cruise.
Jeff:
IIRC, JP Aerospace have some mad-science plan for two-stage blimps to orbit, with some kind of (ion?) engines on the second stage.
Eureka! Maybe
The wiki on the Busemann Biplane mentions that it’s been tested with projectiles (ammunition, I assume), and from looking at the shock pattern I infer that it works just as well as a cylinder with a flat outer wall and tapered inner walls. I would also assume that it fails as a good projectile because it won’t have much mass because it’s a thin-walled hollow tube.
But the other day we had a thread about a magnetic launch system, where the problem was getting the muzzle high out of the atmosphere because of the drag at lower altitudes. What if the vehicle was shaped as a Busemann tube? Much lower drag, so it could launch from lower in the atmosphere, and a greatly reduced sonic boom (which would become an issue if the muzzle was anywhere except far out to sea).