I've wondered this question because if the horizontal stabilizer is longer than that means more lift. My guess to this question would be the result of wing tip vortex strength on a longer wing
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6$\begingroup$ What's the question? $\endgroup$– Jamiec ♦Aug 7, 2015 at 15:22
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13$\begingroup$ Do you know that they provide negative lift? They provide stability at the cost of drag. The shape is optimised to provide low drag whist providing enough longitudinal stability $\endgroup$– DeltaLima ♦Aug 7, 2015 at 15:28
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3$\begingroup$ @Ethan: No, drag is caused by generating lift. By any means. The wing tip vortex is also caused by generating lift, though if it is by other means than wing, it is not a wing-tip vortex, but something else-vortex. $\endgroup$– Jan HudecAug 7, 2015 at 15:41
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1$\begingroup$ @DeltaLima, Ethan, the primary source of drag on a lift-generating surface is always going to be induced drag, not profile drag or a wingtip vortex. $\endgroup$– egidAug 8, 2015 at 20:21
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2$\begingroup$ Why are you assuming they can't be longer? $\endgroup$– Manu HNov 9, 2015 at 23:11
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6 Answers
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The horizontal stabilizers can be longer, they just don't need to be any longer than they are.
Each additional square inch will add induced drag and parasitic (form / profile) drag which costs fuel so they're not made any bigger than necessary to provide adequate control of the plane.
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On most aircraft, the horizontal surfaces in the rear are horizontal stabilizers. These surfaces actually provide negative lift, which balances the center of gravity being forward of the center of the lift force. This balance of forces provides natural stability in a simple way, which is why is is the standard design for both large and small aircraft. Of course this negative lift is working against the main wing, which increases the drag, so this surface is kept as small as possible to provide sufficient stability with as little drag as possible.
There is an aircraft layout called tandem wing, where there are two wings in a tandem configuration that both provide upwards lift.
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4$\begingroup$ Ehh, no. These surfaces 'can' provide negative lift which can be desirable when you want to push the tail down (to pull the nose up.) but equally they can generate positive lift to pull the tail up (and the nose down). They are usually designed to be lift neutral is most circumstances to reduce drag and fuel consumption. $\endgroup$ Aug 8, 2015 at 14:05
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6$\begingroup$ @PaulSmith: No, to make the rear wing lift-neutral would be inefficient and produce too much stability. Normally, lift on the rear wing is less per unit of area than on the forward wing, but still positive, even at high speed. Just look at the position of the wings - the cg is between both, so both need to create lift. $\endgroup$ Aug 24, 2015 at 18:44
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1$\begingroup$ Paul and Peter are talking about two different things - Paul's referring to horizontal stabilizers on conventional-layout aircraft (as discussed in the first part of fooot's answer) while Peter is talking about the rear wing of a tandem-wing aircraft. $\endgroup$ Oct 15, 2017 at 6:08
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The tail surfaces of an aircraft are also called the "empennage", a term originating from the French word for the fletching of an arrow. The term thus indicates the purpose. The horizontal and vertical stabilizers are exactly that, stabilizers. Their purpose is to keep the plane's fuselage in line with the relative wind caused by the aircraft moving through the air. Without them, the plane could easily enter a sideslip or a tumble. They also provide pitch and yaw control by redirecting the relative wind upward or downward, having the opposite effect on the airframe (Newton's third law).
They are not intended to generate lift to counter gravity, and in many cases the horizontal stabilizer does just the opposite, providing a downward force on the back of the aircraft through a combination of negative pitch and "downwash" of air from the wings. This keeps the nose up during forward flight, compensating for a slightly nose-heavy weight distribution which in turn provides desirable flight characteristics such as the tendency to nose-down in a stall (if you're going to fall out of the sky, you might as well fall in an attitude that restores a low angle of attack and thus has the potential for you to recover).
Therefore, in a traditional configuration, they aren't any bigger than they are because they don't have to be. A larger horizontal stabilizer will increase drag due to the greater surface area and volume of displaced air, for no real gain. Potentially the control surface area could be increased, but there is a limit to how big they can be before the forces acting on the control surface in a deflected position exceed the materials strength of the control surface or the airframe. Even before that, larger control surfaces make the plane more sensitive to stick/yoke input, which is useful for a fighter or aerobatic airplane, but potentially deadly for a plane designed for use by the "everyman" pilot.
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As already noted, they can be, but are not, in order to reduce drag.
In general, the horizontal stabilizers in present generation of aircrafts are smaller than their predecessors. This is a result of the advances in design the aircraft with the introduction of fly-by-wire systems.
The horizontal stabilizers are designed to give stability to the aircraft by giving a negative pitching moment. The aircraft wing, in itself is unstable. As lift is generated, the wing pitches up, which increases the angle of attack, increasing lift. This process goes on till the wing stalls. The horizontal stabilizer is effectively a smaller wing located at the other side of the center of gravity at a longer distance, negating this pitch up moment of the main wing.
So, basically the horizontal stabilizer produces a positive lift, but a negative pitching moment. The bigger the horizontal stabilizer is, more the lift and stability, but also the drag.
One way to reduce the drag is to have a smaller horizontal stabilizer, but this reduces stability, requiring the pilot to continuously adjust the controls to fly the aircraft. However, the introduction of computer controlled controls (fly-by-wire systems) meant that the aircraft could be unstable, with the computer adjusting the controls continuously to achieve stable flight.
As a result, the aircraft designed after 1990's mostly have fly-by-wire control systems with smaller horizontal stabilizers, resulting in less drag and decreased fuel consumption.
As an example, compare the horizontal stabilizers of DC10 and MD11.
Source: Boeing 757 Maya
The MD11 was based on DC10, with stretched fuselage and increased wingspan, however with a smaller tailplane. This was achieved using a (partially) computer controlled horizontal stabilizer. As can be seen from the image, the horizontal stabilizer in the MD11 was smaller than the DC10, though the aircraft was larger.
So, the reason for smaller horizontal stabilizers is to reduce weight and drag and this is achieved mainly through the use of computer controlled control surfaces. Because the smaller stabilizer relaxes the stability, even though it might have enough control due to the longer moment arm:
Relaxed stability designs are not limited to military jets. The McDonnell Douglas MD-11 has a relaxed stability design which was implemented to save fuel. To ensure stability for safe flight, an LSAS (Longitudinal Stability Augmentation System) was introduced to compensate for the MD-11's rather short horizontal stabilizer and ensure that the aircraft would remain stable. However, there have been incidents in which the MD-11's relaxed stability caused an "inflight upset."
answered Aug 8, 2015 at 17:57
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5$\begingroup$ Ist't the tail of the MD-11 smaller because it has a longer lever arm? The tail volume of both aircraft should be quite the same. Also, any FCS cannot help to trim the aircraft over a wide range of cg positions, and it is this trim range which drives tail surface volume. $\endgroup$ Aug 24, 2015 at 19:33
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Concorde designers took a different approach: they removed the horizontal tail planes to decrease drag as much as possible.
Every unnecessary thing (pod/pylon/etc.) on the outside of the fuselage or under the wings add drag, even without generating lift.
Another historical example is the MD-11, evolution of the DC-10. If you notice, the MD-11, even if longer and heavier, has smaller tail planes for better cruise performance.
answered Aug 7, 2015 at 16:23
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$\begingroup$ I notice a slight bulge in the tail of the Concorde, about where the rear horizontal stabilizers would be. I would have guessed that it is there for a similar reason, albeit quite small. $\endgroup$– KRyanAug 7, 2015 at 20:29
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1$\begingroup$ @KRyan what bulge? If you're referring to the two "bulges" on the vertical stabilizer, they're just the fairings for the rudder actuators. If you look at both sides, you'll notice they're not symmetrical: the left one moved the lower part of the rudder, while the right one moved the upper part. $\endgroup$ Aug 7, 2015 at 21:16
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The horizontal tailplane can be longer, in order to keep the tail area constant, the chord would be reduced accordingly. The higher tail aspect ratio would result in a higher root bending moment, therefore a heavier construction.
A higher aspect ratio reduces induced drag, very desirable in the main wing but of secondary importance in the tailplane. Induced drag is proportional to lift, and lift generation of the tailplane is minimised anyway, for minimum trim drag.
