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Why is it that these planes had vertical stabilisers that were noticeably smaller in proportion the aircraft itself to other aircraft? I understand that they had the engines mounted on the tail but couldn't the surfaces have been made larger? Doesn't this make it more difficult to control during flight as the surface is way smaller? Maybe is had some other mechanism in the engine that could compensate for the smaller area of the vertical fin?

I've added some images of the DC10, the TriStar, Boeing 767 and A310 which are aircraft that are sort of comparable in era and age and the most interesting one is the TriStar which so small:

DC10 L1011 Airbus A310 Boeing 767

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Two characteristics drive vertical tail size in airliners:

  • Critical engine out: Here the tail needs to compensate the yawing moment of the live engine on one wing.
  • Yaw damping: This is aided by a yaw damper, but enough damping must remain for the case of a failed damper. Especially the dutch roll eigenmode is driving tail size.

On the other hand the tail should not be bigger than absolutely necessary to keep drag and weight down. On the Tristar, the big intake added so much tail area that a relatively small tail was sufficient. Note that the engine intake on the Tristar is much longer than on the DC-10 because of the S-duct. The flow must be brought down to the engine without internal flow separation, which is made easier when the intake is longer.

Since in the engine-out case only one third of total thrust creates the yawing moment which the tail needs to overcome compared to one half of the thrust in case of the A310, the vertical tails of both the Tristar and DC-10 are smaller relative to that of the A310. The A310 is a shortened version of the A300, so the reduction in fuselage length made an especially large vertical tail necessary. If you look at the vertical tails of the MD-80, the Il-62 or the VC10, they are even smaller since very little yawing moment needs to be compensated in the engine-out case. Adding to this, the T-tail configuration makes the existing tail more effective due to the endplate effect (PDF!) of the horizontal tail on those three types.

Vickers VC10 (left) and Tristar (right) tankers in flight

Vickers VC10 (left) and Tristar (right) tankers in flight (picture source).

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  • $\begingroup$ Am I seeing things, or is that VC10 reversing its outboard engines in flight? $\endgroup$
    – Vikki
    Apr 9, 2021 at 21:12
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    $\begingroup$ Does the length of the lever arm from the CoG have any effect? Is it relevant in this case? $\endgroup$
    – Dan
    Apr 10, 2021 at 13:58
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    $\begingroup$ @Vikki-formerlySean: those openings are fixed, think of them as mixers, when reverse is deployed, internal blocker doors deploy $\endgroup$
    – user14897
    Apr 10, 2021 at 19:07
  • $\begingroup$ @ymb1: Ah, makes sense. $\endgroup$
    – Vikki
    Apr 10, 2021 at 19:53
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Doesn't this make it more difficult to control during flight as the surface is way smaller?

Design

Rudder authority becomes most important (arguably) when one wing-mounted engine fails or is shut down.

To combat this on the DC10

  • The rudder takes up a large proportion of the area of the fin
  • The rudder has dual articulation - there are two vertical hinges. This gives the rudder greater effectiveness.
  • The wing-mounted engines are mounted closer to the fuselage. This reduces the yawing effect of a dead engine.
  • The wing-mounted engines provide only 1/3rd of the thrust instead of half as with a 2-engine aircraft, so the effect of one engine out is less.

The 747SP is another aircraft with dual articulation in the rudder.

Operating Procedures

It may be the case that the DC10 operating procedures had lower limits than some other aircraft for cross-wind on landing.

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    $\begingroup$ Perhaps an addition to this answer: the wing-mounted engines provide only 1/3rd of the thrust instead of half as with a 2-engine aircraft, so the effect of one engine out is arguably less. $\endgroup$
    – Sanchises
    Dec 18, 2015 at 12:28
  • $\begingroup$ @sanchises That's not really arguable at all. - haha - It's definitely less. $\endgroup$
    – reirab
    Dec 18, 2015 at 16:13
  • $\begingroup$ @sanchises: Excellent point, thanks. Added to answer as suggested. $\endgroup$
    – RedGrittyBrick
    Dec 18, 2015 at 17:19
  • $\begingroup$ Also, the DC-10's big tail-engine air intake adds to the effective area of the vertical stabiliser. $\endgroup$
    – Vikki
    Apr 9, 2021 at 21:14

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