What other types of variable geometry have been tried?
The idea is to have a larger wing area at takeoff for more lift and a smaller wing area in flight for more efficiency.
The NIAI RK and followup RK-I used two tandem wings that served as rails for an extendable panel that could be rolled out between them.
The project failed because Stalin was so enthusiastic about it he had it use the most powerful engine available which was too unreliable. I can't find a reason why the concept was not tried again.
The German FS-29 glider of 1972 had a different arrangement. It had an outer wing that fit over an inner wing and could unsheathe like a sword. Only one was built.
A normal wing at takeoff then pivots during flight. High-risk idea to make transport aircraft more efficient in the transonic regime.
Lighter and simpler than a swing wing, with no change in the center of lift as the geometry is changed. Disadvantage is that flight characteristics become asymmetric left and right, plus problems with rigidity. See this question for more.
In addition to types already mentioned by other answers:
Was used to allow VTOL operations by tilting the entire wing, as can be seen on the Hiller X-18. The concept was never used outside testing as far as I can tell.
Tested on the X-29 and later on the Boeing X-53, which was based on the F/A-18 Hornet. The idea here is that the wing can be twisted to control roll, giving better control while reducing load on the aircraft. Only used in testing so far.
The concept was that an aircraft could use a rotary wing similar to a helicopter for vertical take-off and landing; once up to speed it would stop the rotor and use it as a conventional wing. Was never tested in VTOL mode and the project was cancelled. See Boeing X-50 Dragonfly for more information.
Variable geometry wingtip
The XB-70 Valkyrie had hinged wingtips which could be angled downwards by up to 65 degrees to improve lift and stability in certain regimes.
Variable incidence, on the F-8 Crusader.
From the Wiki page:
The most innovative aspect of the design was the variable-incidence wing which pivoted by 7° out of the fuselage on takeoff and landing (not to be confused with variable-sweep wing). This allowed a greater angle of attack, increasing lift without compromising forward visibility.
Tiltrotor, on the V-22 Osprey
From the Wiki page:
A tiltrotor aircraft differs from a tiltwing in that only the rotor pivots rather than the entire wing. This method trades off efficiency in vertical flight for efficiency in STOL/STOVL operations.
And of course: rotating wings! Pioneered by Juan de la Cierva for the autogyro, and Igor Sikorsky for the helicopter
Variable Camber Wing
From the late 60s to the early 90s NASA tested experimental variants of their F-111s; at one period they were trying out a "Mission Adaptive Wing":
The second phase called transonic aircraft technology (TACT/F-111A) added an highly efficient supercritical wing and later the third phase applied advanced wing (Mission Adaptive Wing-MAW) flight control technologies and was called Advanced Fighter Technology Integration (AFTI/F-111A). source
The F-111 was already a swing-wing aircraft, but this modification was a supercricital mission-adaptive wing with smooth variable camber, similar to the aero-elastic wing already mentioned.
source In flight - compare with landing wing below.
The Tu-144 had retractable canards, used for good control during landing and takeoff, but retracted during cruise for better streamlined shape.
Edit: I will take "variable geometry" to mean a big change in the wing or lifting surface, or thrust vectoring. This is how I understand the term.
If you took it literally, then a whole host of things come up like retractable landing gear, drag chutes, drop tanks... I don't think this is what the OP had in mind, but correct me if I'm wrong.
And let me just point out the caveat, that conventional control surfaces (ailerons, rudders, elevators) are very much variable geometry in the literal sense. From an aerodynamics point of view, they can be thought of as changing the camber of the surface, which deflects airflow and produces a certain amount of lift.
Now on to the continued list:
- All-Moving Tails (Stabilator). This is where the entire tailplane (horizontal stabilizer) changes its angle, rather than just an elevator. Many fighter aircraft have these, like the F-16.
- Variable Shock Cones, like in the SR-71. These moved backwards or forwards depending on flight speed, to give better shockwave intake geometry to the engine.
- Also, the interior of the SR-71 engines themselves had variable geometry. Parts of the flow shut off so that it became a ramjet, and at low speeds, parts opened again to act more like a turbojet. I don't know the exact details.
- Thrust Vectoring. There's all kinds of thrust vectoring. Some don't even involve variable geometry, but most do.
- Tiltjets. Someone mentioned Tiltrotors, but not tiltjets. They seem to be much rarer.
- Variable Pitch Propellers. There is definitely some varying geometry here, but not sure if everyone would call this thrust vectoring. But if thrust is a vector, and a vector has both direction and magnitude, then I'd say it qualifies.
- Certain kinds of gliders. In hang-gliders, the pilot can move his weight around to control direction. The wings also flex in response to this. In paragliding, the pilot pulls on ropes that adjust the geometry of the "sail", controlling direction to a certain extent.
I have seen other experimental stuff, e.g., this pdf, but they seem to be unmanned so far. Not that that disqualifies anything, but being man-rated or put into production is a big milestone.
Oh, and how could I forget ornithopters!
Ornithopters are machines that actually flap their wings like a bird, providing lift and thrust all in one.
Common Variable Geometries
These are so successful to have become mundane.
- Control surfaces. Rudder, ailerons, elevators, etc.
- Movable flaps and/or slats. Increase lift (and drag) at low speed.
- Spoilers and air brakes.
- Retractable landing gear. Does change shape and performance of aircraft.
- Drop tanks, drag chutes, bomb doors. Debatable, including for completeness.
Yet another extending wing design, the Makhonine Mak-10.
Some early attempts at VTOL involved changing the vector of the thrust, not by tilting the engines or wings but by using other engines.
Doriner DO-31 used two Bristol Pegasus engines (same as the Harrier) for forward flight, and six smaller vertically oriented engines for vertical takeoff/landing. Originally, the Pegasus engines were to vector downward for takeoff/landing, but this was never tried. The DO-31 was canceled by NATO in 1970... the vertical engine pods increased drag to the point where useful payload was very low.
A successful effort to use vectored thrust in unique ways to increase lift is the Shin Meiwa PS1/US1 flying boat. Aside from the four turboprops, the PS1 also has a single GE T58 turboshaft engine mounted behind the cockpit to power a fan to blow air across the flaps for increased lift at very low speed for shorter takeoff and landing. A few years ago, a US1 landed in 15 foot swells to pick up a F16 pilot who had ejected over the ocean. China recently debuted what appears to be a copy of the Shin Meiwa, the AVIC AG-600. but it does not appear to use blown flaps. Its capability to deliver large numbers of troops quickly, without needing a runway, might come in handy should China decide to invade a certain large island.
The Boeing YC-14 transport also used blown flaps for STOL, by directing the output from the center mounted turbofans over the flaps. While the YC-14 never entered production, the very similar Antonov AN-72 did enter production, and has been quite successful.
Would a glider that can drop its motor below the fuselage for takeoff, then re-stow it for gliding fall into this category?
The engine, with electric starter for air starting, erects from and retracts into a bay in the forward fuselage by means of electro-hydraulic power.
I can't find a picture with motor extended.
Don't forget hang gliders. In addition to the weight-shift (movable pilot) aspect mentioned in another answer, modern hang gliders have a feature called "variable geometry" or "variable billow". When the system is engaged by tensioning a cord or lever, the sweep angle of the leading edges is decreased by few degrees. The purpose is to increase the length of the trailing edge, as seen in plan view (from above). This tensions the fabric of the whole wing, decreasing "billow" and twist (washout), which increases the L/D ratio and glide ratio and decreases the sink rate, especially at higher airspeeds, but at the cost of also making the glider less responsive in roll and thus harder to maneuver. An additional side effect of most vg systems is a change in the anhedral angle of the leading edges.
You could also say that the "speed bar" system of a modern paraglider is a type of variable geometry. As for that matter so are the basic steering controls of a paraglider which are a form of wing warping.
Also, a little-known fact is that the weight-shift roll inputs of a hang glider pilot actively pull the moveable "keel tube" (a structural element on/ near the glider centerline) in the intended direction of turn, thus actively warping the entire wing. If you immobilized this tube and also sprayed the whole wing fabric with shellac to stiffen it in a fixed position- even if while placed in a wind tunnel to "inflate" the fabric to an optimum shape- the glider would become extremely "stiff" and unresponsive to roll control inputs. This in fact is the fundamental reason that engaging the VG system makes the glider harder to turn- the keel tube has less freedom to move from side to side.