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The Comet's horizontal tail has a noticeably extreme amount of dihedral compared to that of, say, a 707:

Dan-Air Comet (foreground) and 707 (background), showing the very large dihedral of the Comet's tail

(Image by MilborneOne at Wikimedia Commons.)

Note how the Comet's tail has about twice as much dihedral as that of the 707 in the background, especially considering the Comet's quite low wing dihedral.

Why such a large amount of dihedral on the Comet's tail? Does this have anything to do with the Comet's tail being unswept (whereas most later jetliners have horizontal stabilizers that have almost as much sweep as the wings, if not more)?

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    $\begingroup$ The Comet's engines are in the wing, higher than the 707's pod-mounted engines so I'd guess the dihedral is to keep the elevators out of the engine wake $\endgroup$ – Dave Gremlin Jan 13 at 10:19
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As mentioned in this question, the two major factors are:

  • Lifting the tail plane out of the jet efflux area. Placement of the stabiliser root is structurally optimal when at fuselage mid, dihedral can then lift the surface out of the efflux stream. Note that the Comet has a relatively straight aft fuselage cone, which is better for low drag. And like @DaveGremlin mentions in his comment: the jet engines on the Comet are higher than those of the B707 and need to be lifted up higher.
  • Reducing sudden main wing wake effects: with a pronounced dihedral, there is always some of the tailplane sticking out into a less affected streamline.

The tailplane dihedral does contribute to roll stability, like main wing dihedral does as mentioned in this answer.

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The Comet first flew in 1949, and went from 20,000 lbs TOTAL thrust (4x5000) to 40,000 lbs in the Comet 4 and an increase of MTOW from 100,000 lbs to 150,000 lbs, making it a good study in power and weight evolution for a type.

The buried jet engines, though structurally more complex than pods, offered a better centered thrust line. The higher exhaust plume was avoided with greater Hstab dihedral, which also contributed to stability.

This work could be possibly continued by eliminating the rather anemic V stab and creating a Vtail Beechcraft Bonanza style. Overall, a very solid design.

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  • $\begingroup$ "Overall, a very solid design" Which one, the structural-failure-prone v-tail Bonanza or the fatigue pioneer dH Comet? $\endgroup$ – AEhere Jul 31 at 9:51
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    $\begingroup$ @AEhere I said design, not how they were built. The square windows were fixed. If you look closely, many modern fighters have V tails, the accompanying Hstabs make a "quad" tail needed for additional maneuverability. Pure research is best focused on function, and a good design can be improved. $\endgroup$ – Robert DiGiovanni Jul 31 at 11:59

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