Kevin Murphy has some thoughts about supersonics, based on my previous post. He's skeptical.

Given that he's not stooped to calling me a scientific lightweight, and incapable of understanding mathematics, that's fine, but he doesn't really understand the whole picture, which is understandable since I haven't really presented it. This is a matter of some frustration to me, but one that I can do little about until I can persuade the company involved to put up information on the web, so that it can be critiqued and reviewed.

Regardless, I'll try to respond to his comments as best I can under the circumstances (which include limited time on my part).

...even if you have the same drag coefficient at supersonic as you do at subsonic -- your drag, and thus fuel consumption, will increase substantially.

The key clause here is "if you have the same drag coefficient at supersonic." At least for the wing, it's actually possible to do better, at least in terms of induced drag (an effect of the end of the wing, which makes it greater than two-dimensional) which is actually improved at higher speeds. The notion, right or wrong, postulates that supersonic L/D for aircraft designed under this theory will be similar to that of subsonic aircraft, so it offers the potential (if not promise) of airfares comparable to subsonic fares for the same routes.

With regard to his comments on angle of attack, they're not relevant, because any angle of attack that is non-zero will dramatically increase wave drag and induce shock waves. The aircraft's nominal design condition is zero AOA. Takeoff and time to cruise aren't an issue, either (as isn't the engine) because we can get rid of the extreme sweep that has always been associated with supersonic aircraft (a design strategem that was always a kludge to come up with a way of minimizing wave drag without solving the fundamental problem).

Something like the SR-71 engines are a likely solution, in terms of the inlet, but that's not a problem because they'll be optimized for fuel economy at cruise speed (which will constitute most of their operating time), not takeoff/landing. Also, we're not proposing anything as fast as the Blackbird--Mach 2.4 will probably be adequate.

But here is really the crux of the issue.

The claim is that with enough leading edge sharpness and the proper contouring behind, you can fly supersonically without shockwaves, except circulation (flow around the airfoil) which produces lift elimates the shockless effect. Why would this be? Well, without lift on a sharp symmetric airfoil the stagnation point would the the leading edge. If you add circulation, perhaps you move the stagnation point so that it is no longer on the leading edge. Could this be the problem? The flow splits at the stagnation point (that's where it stops), and if it isn't sharp where it splits, you get a shockwave? If that is the case, well, we're screwed. No amount of adding in balancing circulation downstream will matter, and adding it to the flow over the wing to cancel it out will mean an end to the lift from the wing. Now you could make an unsymmetrical airfoil such that at the cruise condition the stagnation point is on the sharp point of the airfoil, but you'd have shockwave drag getting to that point (or if you had to fly off design point.)

The proposal is not to build a symmetric airfoil. Stagnation points really aren't relevant.

Imagine a Busemann biplane, which is really a DeLaval nozzle inside two wings. The top of the upper wing is flat, as is the bottom of the lower wing. That allows the airflow to move past without shock. The ramping occurs within the two wings. Now, Busemann showed that this will have a shock-free flow, but because of the symmetry, it has no lift. Now imagine that the lower wing is dynamic--it's actually a supersonic airflow coming from a non-shocking duct, with a flat lower surface. The lower surface of the "biplane" (after a short ramp) is a stream of higher-energy air (to satisfy Crocco), that mixes the total flow to provide the anti-circulation to balance the wing circulation.

The idea is to provide that balance to eliminate the need for the highly entropic downstream vortices, that require far more energy than that required to simply provide that balance. It spreads the residual shocks over a much larger footprint, reducing almost to insignificance the PSF on the ground, and essentially eliminates the wave drag.

Bottom line: if this works (and I don't claim that it will--only that it's not obvious to me that it won't), this means wide-body supersonic aircraft, at non-ozone-eating altitudes, at ticket prices comparable to subsonic ones. It means obsolescing the current subsonic fleet in the same way that prop-driven airplanes were put out of business by jets, other than niches.

I think that it's worth spending a tiny fraction (how about a percent of one year's budget?) of the billion-plus dollars that NASA wasted on the High-Speed Research program, but NASA didn't agree in the late nineties, even when Congress specifically appropriated it.

I forgot about the previous discussion of the situation from last week, and I figure this should be a good place to respond... I brought up the Buccanneer not because it's achieving the same thing as you're suggesting, but that it might have a lot of the necessary hardware for running experiments of this type built in already.

I know it doesn't do what you were thinking of, but I think it might be a useful testbed. I'm sure you can find some still-flyable examples somewhere in the world...

I think the proper attitude when anyone claims they have something revolutionary is skeptical; though that doesn't mean I should reject it out of hand. And I do appreciate that people who are actually working in a field have limitations on what they can say -- those who know the most about a subject usually can say the least about it.

Clearly some of my speculations ran off in the wrong direction; it will take some time to digest your new info.

I wholeheartedly agree that too often the government starves promising programs while pouring money into giant projects. Somehow it eludes the budgeteers that amounts of money the giant project doesn't even notice can fund a lot of innovative ideas. So what if not all (or most) of the innovative ideas pan out - enough will and its not like all (or most) of the giant projects pan out either.

I have no problem with skepticism--it's my natural take on most things. The problem is when it's automatically rejected without analysis or consideration. One has to do an expected value analysis. Even if the probability that this works is very low, the payoff is so huge that it makes sense to spend a little bit of money on it. After all, NASA spends money on things like anti-gravity. Why not something that doesn't require a new understanding of the laws of physics, but is merely an engineering problem that could be resolved with some good CFD?

Oh, and Phil, as to using the Buccaneer as a test bed, to test this concept would require a completely different wing chord (flat on top with a razor leading edge, and a properly contoured lower surface). You might be able to keep the spar, but little else. And we don't really know how to design the wing, and won't without a lot of parametric studies with CFD. By the time we figure that out, it's unlikely that using an existing aircraft will make much sense.

How little money can it take to test the science? How little money can it take to look at some designs using said tested science?

A few million at most to at least see if it's theoretically feasible, a few tens of millions to see if it's practical.

Rand,

Can you draw a picture of this? I've only had one fluid mech. course and I'm having a little trouble visualizing all this.

The term "NDA" comes to mind ...

> A few million at most to at least see if it's theoretically feasible,

Why? SW isn't free, but computer time almost is.

> a few tens of millions to see if it's practical.

Tens of millions for a wind-tunnel test?

Last time I checked, people who do CFD analysis weren't free, and determining the conditions under which it will work in anything approaching an optimal fashion will require a lot of analysis...

And it will take a lot more than wind-tunnel tests to determine whether or not the technology is practical.

"In theory, theory and practice are the same, but in practice, they're different."

Someone is going to have to pay for the development of at least an X-plane before we can assess whether or not it's practical.

> Last time I checked, people who do CFD analysis weren't free, and determining the conditions under which it will work in anything approaching an optimal fashion will require a lot of analysis...

I was under the impression that the relevant folks claimed to know the relevant shapes and mechanisms. If that's true, the question is testing whether said shape and mechanism has the desired properties. In that case, only "optimization" is in getting useful answers cheaply out of whatever test mechanism you're using, be it CFD[1], wind tunnel, or cleverly shaped projectile fired through a photo booth.

I can see how things get expensive if that's false.

[1] I can easily imagine that CFD isn't up to the job or this is a hard case for CFD, but got the impression that those who know don't share those concerns.