delta drag
AQ500,
Generally, in the high-speed case, induced drag is not as big a component of the total drag. As I try to think of a good design example, it occurs to me that all the full sized aircraft that are built with an emphasis on speed are designed for supersonic flight nowadays. The examples we have that were designed for subsonic flight, old fighters, were also designed for maneuvering and range, which significantly affects the wing design. The cruise missile, as I mentioned, has range as a major design goal. Dusteater's goal was to fly fast, with no concern for any other aspect of performance. This implies that we want minimum total drag, and that we don't need a high-lift wing. Certainly, this means that the wing should be small, since this will reduce drag, and getting enough lift at high speed is 'easy'. I don't know where the tradeoff is between induced drag and the other components of drag. I don't think the Tomahawk was designed mainly for speed. Maybe the best real-world example we have is swing-wing fighters. When do F-14 pilots decide to sweep the wings, thereby reducing aspect ratio? I don't know, but I'll see if I can find something on the web. In any case, unless Dusteater ends up with the very highly spept delta that he mentioned on one post, it would be hard to acheive a lower aspect ratio than Diamond Dust.
As I look at the X-15 picture, it does indeed look like a fairly low drag configuration, and I think this is a good illustration of the induced drag vs. speed issue. You are flying a high-speed airframe low speed. To get enough lift, it is probably flying at a pretty high angle of attack, even though the weight is low. In this situation, all the other components of drag are fairly small, and the induced drag is pretty big. A curve of induced drag vs. speed shows that induced drag gets much smaller as speed goes up, while the other components of drag go way up as speed increases. The result of this, I think, is that your X-15 IS a high-drag plane the way you are flying it, but if you put a big engine on it, you would find it to be fast, and lower drag than a Q500 plane, for example, for straight, level flight. As I said, I'm not sure what the tradeoff between induced drag and the other drag components would suggest for aspect ratio, but my gut tells me that the high aspect ratio wing does not compare favorably in terms of drag at high speed. I don't know this for a fact, but intuitively it seems that the higher frontal area presented by the high aspect ratio wing would result in a big increase in profile drag. I'll see if I can easily find something about that on the web as well.
Thanks for the replies. When I finally get time to build my 'cruise missile', and learn to fly something that hot, this discussion will be really useful for me.
Banktoturn