Inlets - 2D vs 3D?


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13 February 2008
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In the recent F-23 EMD thread someone was talking about inlets -- 2D vs 3D?

From what I remember he was saying that 3D was better than 2D compression efficiency (citing how an oval or circle takes up less area/volume than a rectangle/square shaped vehicle), but 2D was better for thermal efficiency. If so, considering the waverider-designs use the same principle for compression-lift as they do for the inlet-compression, how does a 2D inlet work better? (Unless you call the area where the delta wings join the 2D wave-riding section part of an "oval" cross-section)

Kendra Lesnick
3D inward turning axisymmetric compression inlet is stronger physically by design so it requires less support than 2D = lighter weight.

3D is mechanically simpler no moving ramps, activators etc.. Again less weight.

3D has less wetted area = less friction heat etc...again less weight.

3D allows for more conventional rounded designs.

In some cases the differences is small others much larger.

2D planar was easier to sim with 20th century comp capabilities. 3D conical owes much of its resurgence to increased sim, cfd and computer capabilities.

Billig, Molder, Kothari all contributed to recent 3D Busemann inlet resurgence. Billig was working with LockMart just prior to his passing.

Does the inward turning compression inlet contribute to the compression lift effect as well as the 2D types? I'm just asking as older waverider designs seem to have a much longer compression and expansion ramp which sounds more conducive to producing more lift and greater lift/drag ratios

Kendra Lesnick
Compression lift is one advantage of the 2D planar design; at high mach it is quite substantial. However one could still incorporate it into the design of a 3D inlet fuselage lower section. That would be an interesting experiment to compare lift /drag of a 2D planar inlet design utilizing external compression to generate lift vs. a 3D inlet waverider fuselage design.
At least one design that I saw (assuming it wasn't a cut-away) if the duct throughout the whole length was partially exposed on the bottom might have similar area (on the bottom) to be able to produce compression-lift. Still because it's not all flat and thus all of the pressure might not be able to exert a lifting force, it might not work as well (although it's possible that the extra compression from the 3D-design might make up for it).

Still, not all Busemann inlets are designed like this, including the Blackswift -- (at least from the drawings I've seen) does not appear to significantly use the inlets for compression lift-effects (At least apparently to the same extent as 2D-waverider-designs), nor do they seem to use a largely divergent aft-area as a gigantic nozzle/expansion-ramp (Like 2D-waverider designs). It is possible, I suppose that the small external section of the duct could provide more compression lift than expected due to advanced aerodynamic-design, and that the underside of the plane (while not contributing to the inlet) might produce some compression-lift effects of it's own. As for the expansion-ramp on the aft part of the fuselage (aft of the engines) that is not always an important component to a wave-rider design (The XB-70, and this McDonnell hypersonic-design which strongly resembled the so-called "Aurora" except it had an XB-70 style wedge-splitter instead of a full-compression ramp, both had engines that extended all the way to the back).

Kendra Lesnick
(BTW: Yes, I re-wrote a lot of what I typed to improve accuracy -- I hope I'm not wrong)

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