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Horizontal Tail (for a twin boom pusher) - In propwash or Out?

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DaveD

Well-Known Member
Joined
May 28, 2010
Messages
375
Location
Perth, Western Australia
I guess this is really an 'H' tail vs inverted 'U' tail question.

First up there seem to be both types out there: The Sadler Vampire has most of it's HT in the propwash as does a Cessna 337. The Scaled Composites (& Northrop Grumman) Firebird has a inverted 'U' tail clear of the propwash as does the adam A500...Theres also the Anderson Greenwood 14 solution of putting the HT half way up the VTs

Anyhow, I've been trying to make an informed decision on this for a while but there seems to be a lot of confilcting views so I've put together a "Pros & Cons" list. I'd love to hear everyone's thoughts... or if I've missed antyhing. Does anyone out there have experience flying twin boom pushers?


'H' tail with HT in Propwash

PROS

  • Better control responsiveness under power (also good for STOL)
  • Lighter structure than 'U' tail
  • 'End Plate" effect from Vertical Tails
  • simpler/more direct control linkages
  • More asthetically pleasing (OK this one is subjective!!)
CONS
  • Buffeting of elevator due to propwash (would presumably also mask stall buffet)
  • loss of control responsiveness when blanketed by windmilling prop (bad for flare/engine out)
  • higher interference drag (twice as many corners)
  • higher skin friction drag
  • reduced longitudinal stability (or larger surface required)
  • Trim also affected by changes in power setting

Inverted 'U' tail with HT out of Propwash

PROS

  • No buffeting elevator due to propwash
  • No power related trim changes (assuming thrust line aligned with C of G)
  • Lower interference drag
  • Lower skin friction drag
  • bettter longitudinal stability (or smaller surface area)
CONS

  • Heavier structure than 'H' tail (worse for flutter?)
  • More complex control runs
  • possible masking of HT by main wash when stalled (probably not in my case at the main wing is positioned high)
  • Less attractive?
It looks like the inverted 'U' tail is might have the edge?
 
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