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Most aircraft maintain longitudinal stability by balancing three forces:

  1. The down force acting through the center of gravity (CG)
  2. The lifting force acting through the center of lift
  3. The down force acting through the center of pressure on the horizontal stabilizer.

Forces of flight

My question is, what are some aircraft designs that deviate from this basic principle? Are there aircraft we see all the time that don't work this way? As a side note, how would weight and balance be affected if this principle isn't used?

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  • $\begingroup$ What makes you think that most aircraft are trimmed for downforce on the tail in level flight? $\endgroup$ – Guy Inchbald Apr 4 at 15:19
  • $\begingroup$ It is valid to choose either the CG or the Center of Lift or any other arbitrary point as the pivot point for a torque balance calculation. This question chooses the Center of Lift as the pivot point. It needs to be noted that the Center of Lift is not fixed-- it moves fore and aft-- so while the force applied at the CG is constant (at least in 1-G flight), the resulting nose-down torque about the Center of Lift is not constant. So the situation is much more complex than the diagram implies. The question could be improved but care would be need to be taken not to invalidate existing answers. $\endgroup$ – quiet flyer Apr 4 at 15:58
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There are multiple configurations which are possible with tail or canard, which based on their locations and whether they produce lift or down-force, results in a stable or unstable aircraft (taking the aircraft center of gravity into account). The figures below show some of the possible configurations.

Stability

Source: f-16.net

In the most general case, there is no need for even a canard or a horizontal tail as long as a (down or up) force is produced in some way. A good example is the Boeing Bird of Prey, where the chines produced (vortex) lift and thus a stabilizing force. The forces acting on the longitudinal axis of the aircraft look something like this, leading to a statically stable aircraft without any lifting surface except the wing.

Boeing Bird of Prey

Profile from bj-o23.deviantart.com, others own work.

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  • $\begingroup$ Where is the case of stable wing-tail with lifting tail? See aviation.stackexchange.com/questions/22087/… $\endgroup$ – quiet flyer Apr 4 at 14:54
  • $\begingroup$ Also I believe there could be a case of a stable canard where the canard is lifting or neutral, not creating a downforce, as long as the canard is meeting the air at less positive angle-of-attack than the main wing. – $\endgroup$ – quiet flyer Apr 4 at 18:10
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Of course, just put the center of gravity back to its rear limit and fly slowly. Then all of them will produce positive lift on their tails.

Stability is not produced by a downforce at the tail. The newest book I read which claimed this was from 1911 (I happened to read the 1913 edition). Stability is produced by making the lift per area of the forward parts higher than that of the rear parts. If the lift on the forward wing is high enough, the rear wing can provide stability with a positive lift.

Canards should be the easiest to prove this: Their main wing produces positive lift, and still they can be made to fly stable.

If you are looking for a design which seems to deviate from this basic principle, maybe the Fauvel AV.36 will do:

Fauvel AV.36

Fauvel AV.36 in flight (picture by Daniel-Wales-Images)

Charles Fauvel designed several flying wing gliders, of which the AV.36 is the most popular. All of them had positive, natural pitch stability. There is only one wing, and it must produce lift, or the plane couldn't fly, right?

The only way to really do away with this principle is to give up natural longitudinal stability. Weight and balance would be affected such that the center of gravity is behind the neutral point, where the angle-of-attack-dependent lift force acts. This kind of airplane, however, would be very hard to fly manually.

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  • $\begingroup$ Is this practical and has it been done either experimentally or in production? Though I guess the question is "does a design exist"... $\endgroup$ – FreeMan Oct 16 '15 at 14:54
  • $\begingroup$ I'm going to do some research on the airplanes I fly to see if the most rearward allowable CG position gets far enough back for the tail to be required to generate upward lift. It seems doubtful that it's allowed to get that far back for normal category flight. $\endgroup$ – ryan1618 Oct 16 '15 at 14:56
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    $\begingroup$ @RyanBurnette I'm fairly sure the center of lift in the PA28 I fly is close to station 95.5 (the rearward CG limit, give or take somewhere around the fuel filler cap) in slow flight -- you could probably fly with zero tail downforce, and maybe even need some upward force if you really hang it on the prop. (Getting your CG that far aft however is left as an exercise for the reader: I expect it would involve adding a good quantity of ballast into the aftmost part of the tail) $\endgroup$ – voretaq7 Oct 16 '15 at 16:15
  • $\begingroup$ @voretaq7 I once hung a lead cannonball-shaped fishing weight (50 pounds as best as I recall) off the tail tie-down eyelet of a Cessna 152 for some CG experiments. Had additional 20-pound bags of lead that I could re-position within cabin by pulling on strings. Verified that takeoff CG was within limits of course but in-flight re-positioning did achieve completely neutral pitch stability (untrimmable). $\endgroup$ – quiet flyer Apr 4 at 18:13
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A tandem wing airplane has two sets of wings, each providing upward lift. One is near the front of the plane, one is near the back, and the center of gravity is between them.

tandem wing

(source: nurflugel.com)

The Rutan Quickie is one such plane:

Rutan Quickie (souce: wikimedia)

This design is actually fairly old; it even predates (successful) heavier-than-air flight, as it was used on the Langley Aerodrome, an experimental attempt at manned flight in the 1890s.

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    $\begingroup$ This would be great answer if you expanded it to include some actual examples (Rutan Quickie comes to mind). $\endgroup$ – Jan Hudec Oct 16 '15 at 20:36
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    $\begingroup$ @JanHudec Confession: I only really know the tandem wing is a thing, I don't actually know much about it. :) I'd be happy to mention the Rutan Quickie, though other than that I can only really crib examples from the wiki page. Is that acceptable behavior? $\endgroup$ – yshavit Oct 16 '15 at 20:44
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    $\begingroup$ . . . First thoughts seeing that thing: "Yup, that's a Burt Rutan design alright!" $\endgroup$ – voretaq7 Oct 17 '15 at 3:22
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This answer will approach the question by focusing on a more narrow case:

"In a conventional wing + tail configuration, is it possible for an aircraft to be stable even if the tail is creating an upforce rather than a downforce?"

The Center of Lift (or Center of Pressure) is the point where we can treat the wing's lift vector as acting, without having to also apply an additional pitching moment to account for a cambered airfoil's inherent nose-down pitching tendency.

The Center of Lift is not fixed-- a point that is not obvious from the diagram in the original question.

If we choose to use the Center of Lift as the pivot point in a pitch torque calculation (which is a valid choice), we are relieved of having to directly account for the wing's lift force in our calculation, but we do have to recognize that the Weight vector, while constant in magnitude (at least if we confine our calculations to the 1-G case), does not act at a fixed distance from the Center of Lift, and thus the nose-down pitching moment due to Weight is not constant. Therefore the situation is much more complex than the diagram in the original question implies.

The Center of Lift moves forward as we increase angle-of-attack and moves aft as we decrease the angle-of-attack. The Center of Lift can even be located beyond the physical edges of the wing itself.

This makes it more intuitive as well as more convenient to treat the wing's lift vector as acting at a fixed point called the Aerodynamic Center of the wing, while also applying an additional nose-down pitching moment which is airspeed-dependent (i.e. the pitching moment coefficient is fixed but the resulting nose-down pitching moment increases as airspeed increases.)

But since the original question takes the Center of Lift approach, we'll do the same in this answer.

Since the Center of Lift migrates forward as we increase angle-of-attack and aft as we decrease angle-of-attack, the aircraft will tend to pitch up as we slow down and will tend to pitch down as we speed up. This is destabilizing-- it is the reason that we need a horizontal tail. But must the tail always create a downforce in order to allow stable flight?

At any given airspeed, if the CG is behind Center of Lift of the wing, the tail must create an upforce rather than a downforce. Since the Center of Lift moves forward as we increase angle-of-attack, it follows that as we experiment with increasingly aft CG locations, we'll see the tail create an upforce at low airspeed before we see the tail create an upforce at high airspeed.

The Center of Lift of the wing should not be confused with the aircraft's Neutral Point. The Neutral Point is the point where the aircraft has no static pitch stability. If the CG is ahead of the Neutral Point, the aircraft is statically stable in pitch, and if the CG is aft of the Neutral Point, the aircraft has statically unstable in pitch. The latter case is basically only the realm of computer-stabilized fly-by-wire aircraft that sacrifice stability for enhance maneuverability or enhanced cruise efficiency.

In contrast, many aircraft have lifting tails. For example, many vintage "free flight" model airplanes have large, positively cambered tails. (See for example this image.) Obviously these aircraft cannot be statically unstable in pitch. The CG must still be ahead of the Neutral Point, even though it is behind the wing's Center of Lift.

An argument can be made that a large lifting tail is generally not as efficient as a small neutrally-lifting or slightly down-lifting tail. That argument is beyond the scope of this answer, so we should save further exploration of that for another ASE question.

Also, plenty of aircraft have a tandem wing configuration, in which the tail is actually a large lifting wing.

Here is a good description of an experiment that verified that the horizontal tail was creating an upforce in flight with a Cessna 172 with a CG that was within, but near the aft edge, of the prescribed envelope -- see last two paragraphs of section-- https://www.av8n.com/how/htm/aoastab.html#sec-pitch-equilibrium

Here is another ASE answer that gives a good treatment of the difference between Center of Pressure (or Center of Lift), Aerodynamic Center, and Neutral Point-- What is the difference between centre of pressure, aerodynamic centre and neutral point?

In summation, the Neutral Point is not the point where the aircraft will balance with the tail creating no upforce or downforce. Rather, the Center of Lift or Center of Pressure of the wing (or to be as precise as possible, the Center of Pressure of the entire aircraft other than the tail, including effects from offset thrustlines, drag from floats, etc) is the point where the aircraft will balance with the tail creating no upforce or downforce. It is very possible for the CG to be behind the wing's Center of Lift (or the entire aircraft's Center of Pressure) but ahead of the aircraft's Neutral Point, thus causing the tail to generate an upforce, yet still allowing positive static pitch stability.

The basic requirement for static pitch stability is simply that the tail is flying at a less positive angle-of-attack, or at least a less positive lift coefficient, than the wing. This includes, but is not confined to, cases where the tail is flying at a negative angle-of-attack and creating a downforce. You can read more about it in this section of an outside website-- https://www.av8n.com/how/htm/aoastab.html#sec-teeter .

This related ASE answer is also enlightening-- Do any airplane designs exist that don't involve a flight surface that provides downforce?

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  • $\begingroup$ Obviously adding floats affects the location of the neutral point (and changes the stabilizer trim required to hold a given angle-of-attack of the wing)-- $\endgroup$ – quiet flyer Apr 4 at 16:25
  • $\begingroup$ (future edit-- could note that first linked ASE answer has a diagram that assumes tail is uplifting not downlifting) $\endgroup$ – quiet flyer Apr 4 at 18:09
  • $\begingroup$ possible future edit-- remove quotes from second sentence $\endgroup$ – quiet flyer Apr 5 at 10:22
  • $\begingroup$ possible future edit-- introduce the idea of the center of pressure of the entire aircraft (excluding the tail) earlier in the answer. $\endgroup$ – quiet flyer Apr 5 at 10:30

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