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Unlike most aircraft, the DC-8 is certified to use reverse thrust in flight (albeit only on its inboard engines). This serves as a very effective airbrake, although it also produces considerable buffeting (due to the large areas of separated and/or reversed airflow it produces over the wing’s surface).

It also, more interestingly, produces a pitch-up moment (at least according to the CAB AAR on the crash of EA304):

[...] The pilots in this case, if the drive system failed during a PIO rather than earlier, had no way of knowing the real reason for its failure to operate in the ANU direction. In the split seconds available to them for analysis they could easily have concluded that the failure was due to heavy stick forces. Reverse thrust, in addition to drag, produces a nose up pitching moment, a fact known to the first officer if not to the captain, and as indicated previously, they did employ this aid. It is also true that during the time, no matter how short, required to go from forward thrust to reverse, the noseup pitching moment of forward thrust has been removed and therefore contributes to the severity of the dive. Small as it may be, this factor becomes more significant at very low initiating altitudes. [Page 25 of the aforementioned AAR; my emphasis.]

For an aircraft with engines mounted below the aircraft’s center of mass, like the DC-8, only the use of forward thrust would be expected to produce a pitch-up moment (due to the torque exerted by the offset net thrustline), and reverse thrust should produce a pitch-down moment (for the same reason):

a diagram to go with the words

Yet, for the DC-8, the use of reverse thrust not only creates a nose-up pitching moment, but, apparently, creates one that is stronger than that produced by forward thrust!

Why?

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What you're missing from your diagram is @JohnK's drawing (which I'll borrow) from his answer to your related question, Why does the use of reverse thrust in flight on the DC-8 cause areas of separated/reversed airflow over the wing?

enter image description here

The use of reverse thrust increases the wing's angle of attack (AOA) more than it does for the tailplane (as opposed to if the whole plane had pitched up). And not only that, the inboard thicker part of the wing now starts with turbulent air, this reduces the amount of lift at the neutral point (aka aerodynamic center), unlike what happens in a normal AOA increase.

Since the AOA of the tailplane hasn't changed much, it can't contribute to the stability, and with the big reduction in lift, the tailplane wins and the plane pitches up.


Think of it as deploying spoilers, but spoilers that affect the wing from before the leading edge:

When extended, wing-mounted speed brakes usually produce a nose-up pitching moment (Boeing).

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  • $\begingroup$ That makes sense, but would that effect really be so strong as to completely cancel out the nosedown pitching moment produced by the engines' thrustline offset, and still have enough noseup left over after that to produce a net noseup pitching moment significantly stronger than that produced by the engine thrustline offset during forward thrust? $\endgroup$ – Sean Dec 14 '19 at 22:41
  • $\begingroup$ @Sean: The tailplane has a much bigger moment arm around the center of mass, and the reverse thrust isn't like takeoff thrust in reverse, more like idle in reverse. $\endgroup$ – ymb1 Dec 15 '19 at 12:37
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    $\begingroup$ @Sean: Also as you've noted, it's only half the engines, so a rough approximation is that the thrust essentially zeroes. $\endgroup$ – ymb1 Dec 18 '19 at 13:02

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