Do jumbo and smaller airliners' horizontal stabilizers provide any lift? Apart from maintaining the aircraft's angle of attack, can they perform the dual function that includes adding some lift as well? Are there any aircraft designs that use horizontal stabilizers for lift as well, apart from delta wing aircraft?
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asked Mar 16, 2019 at 17:48
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$\begingroup$ I don't know about airliners, specifically, but I am just about certain that I have seen it stated that a stabilizer may produce lift at low speed, particularly with the C of G aft (but still within limits), but I cannot find the reference. The context was with respect to sailplanes: the author was explaining that this is not the benefit it might seem (and certainly not worth aiming to achieve an aft C of G for) because this lift comes at the cost of a higher drag than if it were generated by the wing. $\endgroup$– sdenhamMar 16, 2019 at 18:25
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1$\begingroup$ This question cannot be answered without leading into debate. Even if you ask different university professors you get different answers. Most people think the stabilizer only creates down forces (as you can see down below), which is what they have been told again and again but this doesn't make it any more or less correct. All I know is that the math also allows for the stabilizer to create an upwards lift with the aircraft still being stable and controllable and depending on the flight situation you can have significant positive lift. $\endgroup$– JanMar 17, 2019 at 19:22
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$\begingroup$ @sdenham: If the lift is in proportion to the respective aspect ratios, moderate tail lift will indeed produce the lowest overall induced drag. That is what good competition glider pilots aim for. Your run-of-the-mill weekend glider pilot will be more comfortable with neutral to slightly negative lift on the tail, however. Here is where you might have read about positive tail lift. $\endgroup$– Peter KämpfMar 18, 2019 at 20:38
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$\begingroup$ @PeterKämpf Thanks for the link, though that's not the reference I am thinking of. I think it was from a blog post from, or hosted by, a German sailplane manufacturer - maybe Glaser-Dirks, and it might have been a predecessor to this one: dg-flugzeugbau.de/en/library/optimum-cg-sailplanes $\endgroup$– sdenhamMar 18, 2019 at 22:00
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$\begingroup$ @PeterKämpf would competition canard glider pilots (if these even exist) aim for positive or neutral lift? $\endgroup$– jkztdMar 19, 2019 at 3:26
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5 Answers
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The horizontal stabilizer provides lift, but usually in the negative direction. Could one provide positive lift?
The answer depends on cg location.
Forward CG, the answer is no.
Aft CG, the answer is yes but only if the CG was aft of the center of lift.
In other words, the more forward the CG is compared to the center of lift, the more downforce the tail must provide. The lift from the horizontal stab in the "wrong direction" must be countered by the wing, and thus there is more induced drag. For a given aircraft gross weight and thrust/power, there will be a higher cruise speed with a more aft CG.
There are practical limits to extreme fore and aft CG positions, and so each aircraft has an allowable "envelope" for loading for safety margin.
Aircraft are designed to have a range of allowable CG locations to make loading easier before the flight (especially for cargo aircraft), and also to allow for passengers & crew to move about the cabin in flight.
Horizontal stabilizers could be sized smaller but that would decrease loading flexibility, decrease passenger comfort, and decrease aircraft handling performance; often all airfoils and control surfaces are designed with many factors in mind, for good control harmony, good ride in rough air, for stability on instrument approaches, for flexible loadings of various belly and main deck cargo holds / passenger configurations, etc.
Flying wings and delta wing designs come to mind as aircraft without horizontal stabilizers, such as the Northrup Spirit or Convair Delta Dart.
Your other question, would manufacturers design horizontal stabilizers to provide lift? Hopefully you can see by now this is not a design goal. Wing design is so advanced that designers are making amazingly beautiful and high-performance wings that are most efficient at creating lift. Look at a Gulfstream V wing or Boeing 747 wing. It is not efficient to create a wing that doesn't work great and then needs an additional surface to help make more lift. Exceptions to that might be bi-planes or tri-planes, as they used several shorter wings instead of 1 longer wing, but that is not practical in air transport. How many Boeing Biplanes and Airbus Triplanes do you see traveling the globe? It's simply better to put all the lift needed into a single wing design.
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$\begingroup$ "The horizontal stabilizer provides lift, but usually in the negative direction. Could one provide positive lift?" The answer depends mostly on intended pitch attitude, before considering center of gravity location in straight and level flight conditions. $\endgroup$– jkztdMar 19, 2019 at 3:15
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$\begingroup$ To increase pitch, the tail provides a certain amount of down force (lift vector pointed down toward Earth). To decrease pitch, the tail provides less down force, but it is still negative lift. For a transport category aircraft whose load factor limits are +2.5g and -1.0g, there is hardly a need to have positive lift coming from the tail. $\endgroup$ Mar 19, 2019 at 17:51
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As a general rule, if the small surface is forward of the main wing, it lifts up. If it's aft of the main wing, it lifts down (most of the time).
This is a gross oversimplification, but it's very useful to help with imagining what is going on in your mind.
An airplane with a normal tail is like a seesaw, except the pivot is not a solid point, but the middle of the lifting force generated by the wing. One end of the seesaw has the center of gravity trying to tip it forward, like a kid sitting on the seat, and the other end is you pushing down to hold the kid up, or in the airplane's case, the horizontal tail pushing down against the moment of the center of gravity acting forward of the center of lift, to keep it in balance. If you broke off the horizontal tail, the airplane would point straight at the ground, as if you let go of the seesaw seat on your end and let the kid drop.
An airplane with a canard surface at the front and a large wing at the back is more like a dump truck. There is a big axle at the back holding most of the weight, and a little axle at the front holding up a minority of the weight.
Both axles are holding up - and both the canard airplane's surfaces are lifting, but the ones at the back are doing most of the actual work. The center of gravity is close to the end with big axle (the main wing), and the axle at the front end (the canard surface) only doing a small portion of the total holding up (or lifting) because its job is mostly about control (control over pitch and speed in the airplane, control over steering and braking in the truck).
Airplanes with three surfaces, both a tail and the back and a canard on the front, are kind of like a seesaw but with a little support stick under the kid to help hold him up so you don't have to push down as hard at your end (the tail). Kind of a seesaw/dump truck hybrid you might say.
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$\begingroup$ is it not possible to have adesign that moves the wings a bit forward to the point the horizontal stabilizers are not necessary? $\endgroup$ Mar 16, 2019 at 19:08
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1$\begingroup$ Yes, sort of...they are called flying wings and they use special airfoil shapes that are naturally stable in pitch by generating the "seesaw" balancing downforce right at the trailing edge. It's the same balance of forces, just all done within the wing chord. Since the seesaw balancing act is being done over an extremely short arm - the trailing edge is a lot closer to the center of lift than a tail out back, the center of gravity range is very restricted and they tend to be very touchy to control in pitch. That's why you don't see them all over the place. Good configuration for gliders tho. $\endgroup$– John KMar 16, 2019 at 19:22
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1$\begingroup$ An aircraft with a conventional (tail) horizontal stabilizer is designed with the CoG forward of the centre of pressure - a nose-down force. The stabilizer provides a 'downward'-acting force on the tail (opposite to the lift generated by the wing) to oppose and balance the nose-down force. This provides pitch stability: it returns the aircraft to its original pitch when it has been perturbed by a nose up- or down- force such as would result from turbulence. Without this damping mechanism the plane would be pitch-sensitive to the point of being unflyable. $\endgroup$ Mar 17, 2019 at 15:32
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Any canard design will use the horizontal stabilizer for lift.
Airliners historically used downforce on their tails in order to have sufficient static longitudinal stability, but the more recent designs use a small amount of lift in cruise and keep stability up by artificial means. A tail tank is sometimes used to shift the center of gravity sufficiently back.
Tail lift in trimmed flight varies when a cambered airfoil is used. Wings using cambered airfoils will usually result in positive tail lift at high angle of attack and moderate to rear center of gravity locations. Usually, the loading of the tail should be low in order to allow for enough margin for maneuvering.
Note that the negative airfoil camber at the root of a tailplane helps to keep the isobars on the sweptback surface parallel up to the root, so this is not immediately an indication of predominantly negative tail loads. Negative nose camber on the tail also helps to delay flow separation on the tail at high negative loads, which are needed to trim the aircraft with full flaps.
answered Mar 18, 2019 at 21:26
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The opposite. Generally the tail pushes down on the aircraft. While the wings push the fuselage up, the horizontal stabilizer pushes the tail down. Yes, this does increase load on the wings.
Look closely at the airfoil on a horizontal stabilizer, you can see it is inverted.
Also, if you look at the range of motion on fully movable horizontal stabilizers on jetliners, the vast majority of their range is negative AoA.
answered Mar 16, 2019 at 17:58
Harper - Reinstate MonicaHarper - Reinstate Monica
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1$\begingroup$ Lift is lift. It acts roughly perpendicular to the chord of an airfoil; to which side of that chord line depends only on the airfoil and angle of attack. What you call 'anti-lift' is lift generated by the horizontal stabilizer that acts 'downwards' (toward the lower surface of the airframe) in conventional flight. The lift is not affected at all by the aircraft's orientation relative to gravity. $\endgroup$ Mar 17, 2019 at 15:26
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For stability reasons tails generate a down force.
It would in theory be possible to have a horizontal stab generate lift (upward force) by compensating for the resulting instability with a fly by wire system, but the wing/tail system would always be less efficient at generating lift then just making the main wing a little bigger.
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1$\begingroup$ This is an old and hard to dispel misconception. I had airliner pilots insist on it, but it is still wrong. Stability with positive tail loads is possible - all you need is less lift per area on the rear surface. $\endgroup$ Mar 19, 2019 at 19:56
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$\begingroup$ I agree that for longitudinal stability that the CG must be forward of the center of pressure CP. But you also need for stability that when the angle of attack of the main wing = 0, the moment about to the CG must be positive. Otherwise you have a lawn dart. To get this, you need downward force on the tail when the main wing is at zero lift. For sure the tail will be generating downward force in a stable configuration for at least some flight conditions. Haven't thought it through beyond that. $\endgroup$– MikeYMar 19, 2019 at 20:23
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$\begingroup$ Of course you have a downforce on the tail if the plane is stable and the wing is at zero lift. How else would you satisfy the condition of less lift per area on the tail? But most of the times the wing produces considerable positive lift - that is what it is designed for. This leaves enough margin for some positive lift on the tail, too. And if the center of gravity is ahead of the center of pressure, the aircraft will pitch down. Both have to be at the same lengthwise location for straight flight. $\endgroup$ Mar 20, 2019 at 4:59