OK Time will tell
Wanneer het ogenblik aangebroken is dat iemand kan aantonen dat een rompbuis in slechts één richting krachten op hoeft te nemen ben ik de eerste die daarop aanpassingen gaat doen..........................
Ik durf dan ook de stelling aan dat een ovale rompbuis minder is als de ronde doorsnede.
Arno
Time has past............. misschien is dit een verklaring die je over de streep kan trekken, maar misschien ook juist niet
Russ,
If you intended to post some pictures, they didn't make it to the thread.
Now in general about ovalizing...
Ovalizing can be done such that the major axis of the oval is along the force line, which provides greater stiffness and strength for that direction of force, or such that the minor axis of the oval is along the force line, which provides maximal stability for bending in that preferred direction.
Ovalizing in the first form by too great an extent would result in the oval tube trying to twist its way out of the load, so as to bend in its weaker direction. If the loads are balanced, then it becomes a bifurcation problem where it is not predictable which direction it will twist and bend.
Since our fuselages are long and thin, we can only get away with small ovalization in the tailboom. Greater ovalization can be used in the forward part of the fuse as it is fatter and shorter, and therefore far more torsionally rigid.
Torsional rigidity is related more to cross sectional area, and length of the tube, than anything else. For the same wall area, a round cross section will have the greatest torsional rigidity. So that is something we are giving up on when we go ovalized. Therefore it is a good idea to use tails that balance the load around the boom. For instance having the vertical stabilizer be of equal shape above and below the boom. That eliminates torque from aero loads on the vertical.
On launch, the plane is going to be skewed into a sideslip. It will oscillate between left and right sideslip directions, and hopefully dampen out to straight line climb very quickly. But when in a sideslip, a round tube shape provides quite a bit of drag. Horizontally ovalizing the structure should reduce the drag somewhat.
On normal flight, as one changes camber and airspeed, the fuselage will have some small pitch changes with respect to the freestream - the air it sees that it is flying through. Again, having the fuselage not aligned with the freestream results in some drag.
So for launch, horizontal ovalization, to a rather small degree, is likely best. For flight, vertical ovalization to a larger degree is best. Round or near round is not a bad compromise.
Alignment to the streamlines is slightly more complex than it might appear at first glance. Assume we are optimizing for one flight condition and ignoring others. Then the nose of the fuselage should point down the glideslope. But as air approaches the leading edge of the wing, it is sucked upwards somewhat. So the front of the fuselage should curve slightly like a banana, but in the opposite direction of what most fuselages do. This effect is generally ignorable farther than one chord length in front of the wing. In the area of the wing, air is constrained to relatively follow the contour of the wing (at least on the bottom, where there shouldn't be a bubble). Aft of the wing, downwash is in effect. When the plane flies through the air generating lift, the air it flies through is deflected downwards. So the tailboom should be angled down a little compared to the freestream, which is also compared to the nose of the fuselage. And that downwash has a subtle curvature to it.
So generating a minimal drag fuselage can be done, but it will be specific to a chosen flight condition!
So what condition to choose? The places fuselage drag makes the most difference is launch and speed mode. Basically when flying faster rather than slower. Myself, I choose to optimize for speed mode, as a compromise position.
To add to the fun, the fuselage in front of the wing and the area attached to the wing interfere with the wing. That interference should be minimized. Aligning with the streamlines helps a great deal with that, as does having a narrow fuselage.
Gerald
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