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If you look at this picture, you'll see that the horizontal stabilizer has a very pronounced dihedral in comparison to the wings. Could somebody explain in a simple way why? I understand that a dihedral wing makes an aircraft more stable on the roll axis, but why on the horizontal stabilizer, which produces a downforce?

This question is not just about the DC-10, because there are also other planes that are not trijets and have this feature, I just used the DC-10 as an example

See also this schematic:

enter image description here

Image excerpted from original, by Julien.scavini, used under CC BY-SA 3.0

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    $\begingroup$ Speculation: the weight of the third engine (and plumbing) combined with a source of thrust so far up and aft might require more stabilization. Dihedral is cheaper that adding more hardware for a damping system. The L1011 has relatively little dihedral but also seems to have a much larger vertical fin. $\endgroup$ – acpilot Mar 19 '16 at 21:58
  • $\begingroup$ Just conjecture by it might have something to do with getting the HS out of the downshift downwash from the wings. $\endgroup$ – TomMcW Mar 20 '16 at 0:30
  • $\begingroup$ the stability provided by diedral is coming from wing, not from stabilizer (less load). Looking at the plane one reason might be avoiding the wake of the twin engines in high angles of attack (where the stabilizer is dimensioned), I do not know for sure but likely is the reason $\endgroup$ – Trebia Project. Mar 20 '16 at 11:12
  • $\begingroup$ I noticed at the airport today (with this question in mind) a bunch of 737s, a 787, and to a lesser extent A330s with a very pronounced dihedral too. $\endgroup$ – Ben Mar 21 '16 at 9:15
  • $\begingroup$ At the airport you're only seeing the dihedral at rest. Once in the air, the stab will have air loads, and that will bend it downward somewhat. Remember stabs fly upside down, generally pushing downward on the tail $\endgroup$ – Harper - Reinstate Monica Jun 14 at 2:49
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In a general sense, there are multiple reasons for the horizontal stabilizer to have dihedral/anhedral.

  • To clear the tail from the downwash/propwash. A good example is the Cessna 425, which was developed from the Cessna 421. The 421 had a straight horizontal stabilizer. However, with the addition of dihedral, the tail is kept out of the propwash, improving ride quality. From a report citing Cessna engineers:

Cessna engineers say the dihedral improves Dutch roll qualities and moves a major portion of the tail out of the prop wash, thus providing a smoother ride.

The Dassault Falcon 50 had anhedral so as to clear the tail from the wing's downwash. Another good example of this is the F-4 Phantom. The prototypes had a straight tail. However, the anhedral tail was adopted so as to keep it clear of the disturbed air from the wing at high angles of attack while at the same time keeping it out of engine exhaust.

The anhedral in BAE Hawk is explained by the same reason. From The Hawk Story:

This model gave the first indications of high speed problems due to interference between the high intake and the low wing, and the position of the tail.

This resulted in the adoption of a low intake, ... .The rear fuselage was cambered downwards to bring the tail down, and the tailplane was given pronounced anhedral for the same purpose.

  • Stability- The dihedral/anhedral in the stabilizers affects the aircraft's stability. In short, for a conventional tail, an anhedral improves the yaw stability of the aircraft while reducing the pitch stability (slightly), while a dihedral will reduce the tail's contribution to both pitch and yaw stability.

    However, the use of dihedral in the horizontal tail influences static roll stability in the same way as that of dihedral in the wing. Dihedral affects the roll response in the same way as that of the wing- it causes the lift on the right and left semispans to respond differently to downwash.

    A good example of this is the Vickers Viscount, which had a dihedral horizontal stabilizer, which improved its stability.

    Due to this, it is better for the designer to add a dihedral to the horizontal tail rather than wing especially if problems are found in later stages of design as the impact will be lesser.

From Roskam's Aircraft Design Vol III, Chapter 5.1:

The overall lateral stability is usually dominated by the inherent lateral stability designed into the wing. Wing sweep, dihedral angle and (wing) location on the fuselage dominate the magnitude and the sign of the stability derivative. Since the wing is designed by performance and operational considerations, the empennage is often used to fine tune the lateral stability of the aircraft. Examples include the F-4 and the AV-8B.

It should be noted that, as with any other design consideration, the dihedral of the horizontal stabilizer depends on the design choices made and perfomance characteristics required; as a result, the configuration required will change from aircraft to aircraft.

In case of DC-10, the main reason appears to be to keep the stabiliser out of the wing influence, especially when high flap settings are used. The more pronounced dihedral (compared to, say Boeing 737) appears to be used as the stabilizer is mounted lower due to the presence of engine in the tail.

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  • $\begingroup$ Now, when you say dihedral is that realtive to the lift produced? Since the horizontal "lifts" down is dihedral the correct term to describe what we see in the DC10 or is it actually anhedral? $\endgroup$ – acpilot Mar 20 '16 at 22:02
  • $\begingroup$ @acpilot the terms dihedral and anhedral refers to the angle (of the wing/stabilizer) to the wing. So, if the stabilizer is at an angle so that the tip is above the root, it is dihedral. DC-10 has a dihedral in its stabilizer, while F-4 has anhedral. $\endgroup$ – aeroalias Mar 20 '16 at 23:02
  • $\begingroup$ I don't understand your answer. How can you measure the dihedral of a wing against itself? I think you are asserting that di/anhedral is measured realtive to the airframe "top" without regard to anything else and has nothing to do with which side of the surface is actually creating "lift," correct? For example, wings can be angled "up." To achieve the same effect on the horizontal you'd need to angle them "down." In this case, does the wing have dihedral and the horizontal have anhederal or, since both surfaces are angled in the direction of lift, do they both have dihedral? $\endgroup$ – acpilot Mar 20 '16 at 23:11
  • $\begingroup$ @acpilot I'm not measuring the wing di/anhedral against itself. I'm measuring it against a horizontal plane passing through the wing/stabilizer root(s). Yes, it has nothing to do with which side is creating lift and is purely a function of geometry. If the stabilizer is angled down, it has anhedral, not dihedral. $\endgroup$ – aeroalias Mar 20 '16 at 23:41
  • $\begingroup$ Thanks. I'd never thought of it in terms of lift and it's never come up in training. Good to know! $\endgroup$ – acpilot Mar 21 '16 at 0:03
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Most airliners with low horizontal tail surfaces have dihedral on them, for two reasons:

  1. To lift the horizontal surface above the wing wake.
  2. To lift the swept tail above the ground, so it will not be damaged in a tail strike. Note that the tail is roughly horizontal at the tail strike attitude to have enough clearance for full down elevator deflection.

Boeing 747-400 tailstrike

Boeing 747-400 tailstrike (picture source)

Scrapped tail of a VC-10

Given enough angle of attack, a swept, un-dihedraled tail surface will touch the ground with the tips first (picture source)

In a T-tail, both reasons don't exist, so T-tails are straight in most cases. A notable exception would be the Handley-Page Victor, where a noticeable dihedral was added late in the design process to increase the projected side area of the tail section. Thanks to @ymb1 for finding this article which includes a detailed description of the Victor.

Vertical area is always too small initially, just witness the tail growth in the BAC Lightning. Interestingly enough, the article says that the vertical on the Victor was shortened after flight test. This is an extremely rare occurrence.

Handley-Page Victor

Handley-Page Victor (picture source)

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The horizontal tail has pronounced dihedral in almost all low tailplane placements. 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. If the fuselage aft cone is lifted up already there is less requirement for tailplane dihedral.
  • Reducing sudden main wing wake effects: with a pronounced dihedral, there is always some of the tailplane sticking out into a less affected streamline.

Note that a stabilising roll moment from tail plane dihedral would have to be transferred through the fuselage: the torque difference between main wing and tail wants to twist the fuselage like a wet rag. Not super consequential for structural loads, but the yield may cause difficulties with the flight control cable run. Wide body aircraft are least affected by this.

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