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Hypothetical Asymmetric Aircraft Design

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niccof

Well-Known Member
Supporting Member
Joined
Jan 18, 2022
Messages
61
I was looking through some past design logs and found Topaz's excellent log of his single seat motor glider. One of his concepts was an asymmetric design similar to the BV 141 [wikipedia] from WW2. I loved the idea and while I'm not trying to actually design and build something like that I thought it was an interesting concept and wanted to explore the pros and cons further.

The BV 141 was designed in response to a specification for a single engine recon aircraft. Keeping the recon mission in mind, it makes sense that the designers opted to keep the fuselage in line with the engine: it provided the crew with unobstructed views forward and aft of the aircraft as well as above and below. My theory, however, is that if one were to change the mission to a cross country cruising aircraft it would be preferable to have the engine in it's own pod to reduce the wetted area in the prop wash and move the fuselage and empennage behind the passenger compartment. One thing I initially considered as a non-starter for this design was the fact that both the thrust from the propeller and the drag produced by the fuselage would tend to yaw the aircraft right, but this could be counteracted by an asymmetric vertical tail which produces a yawing force in the opposite direction. No doubt this is another source of drag, but given this is a cruising aircraft it could be optimized for cruise speed and then trimmed for flight outside of that regime (as one would have to do with a conventional aircraft anyway).

I do see some drawbacks to this configuration though. Here's a quick list of pros and cons I've come up with so far:

Advantages:
  • theoretically less skin friction drag because there is less wetted area in the prop wash
  • no induced yaw from prop wash on vertical tail
  • opportunity for improved structural efficiency in wing since two separate masses can provide some bending relief
  • potentially improved forward visibility as the nose could become glider-esque
  • possibly quieter in the cabin (although supposedly the OV-10 Bronco cockpit is exceptionally loud due to the near supersonic propeller tips right next to the canopy)
Disadvantages:
  • vertical and horizontal tail are not in prop wash which likely reduces or delays control authority on takeoff and possibly in other flight regimes
  • lateral weight and balance likely more complicated than conventional layout due to distance of passengers from aircraft centerline
    • e.g. two pax side-by-side was the design point but aircraft must still be balanced for solo flight
  • related to the above, spin recovery could be more difficult or require larger control surfaces because the moment of inertia of the aircraft is higher due to the engine and passengers being further from the CG (I know some aircraft are simply identified as "not allowed to spin", but it seems prudent as a designer to at least consider the tradeoffs required to make spin recovery possible)
  • additional intersections when compared to conventional which could increase intersection drag
  • increased frontal area when compared to conventional layout because the engine and passenger compartments are separate and no longer share a common cross section
I'd love to hear some more thoughts on this as I'm sure I'm missing some good and bad aspects of this configuration. Am I missing something that makes this a DOA concept? To reiterate: I propose this as a thought exercise and not some project I'd like to chase down. I haven't designed an aircraft before and aviation is not a place for running before walking...
 
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