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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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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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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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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

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