I am confident I have seen an article couple years ago about a "static surface aircraft" (or at least a concept of it) with no control surfaces, which uses a "wet mass" to adjust its roll and pitch.

The container within its CG is filled with a fluid which is then transferred (or sucked) to the wing tips (to control roll) and along the fuselage (to control pitch). I'm not sure about the yaw control - it should be elevon-like.

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Having spent half a day online, I have been able to only find a DEMON UAV from BAE systems, a "flapless" aircraft which uses flow injection above the control surfaces for maneuvering. That is not the same.

My question - is there something fundamentally wrong with a concept I have described? Were there any experimental aircraft made?

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    $\begingroup$ The Demon UAV does not use wet mass, it uses compressed air in the place of controls. It's not even close to this concept. $\endgroup$
    – GdD
    Commented Jul 27, 2017 at 8:13
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    $\begingroup$ I found this link, about a person who's investigating it for a UAV, but it doesn't look like they're much further than fancy renderings $\endgroup$
    – ROIMaison
    Commented Jul 27, 2017 at 8:19
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    $\begingroup$ And I know there are aircraft that have a fuel tank in the tail plane to trim the aircraft, like the Concorde But that is for slow, small, long term balancing control forces, not for rapid, I-need-to-avoid-those-trees-control forces $\endgroup$
    – ROIMaison
    Commented Jul 27, 2017 at 8:29
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    $\begingroup$ I really wonder what problem they would be trying to solve. $\endgroup$
    – Koyovis
    Commented Jul 27, 2017 at 8:54
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    $\begingroup$ Weight shift hang gliders come to mind, but that's probably not what you're looking for. $\endgroup$ Commented Jul 27, 2017 at 13:07

3 Answers 3


The aerodynamic forces resulting from surface deflections are orders of magnitude larger and faster (in their rate of change) than the gravitational forces obtainable by shifting the centre of mass.

Additionally, as OP seems to realize, yaw control would not be feasible with this method.

So overall it would not be impossible, but it has no clear advantages to outweight the heavy disadvantages. I am also unaware of any prototypes or tech demonstrators using this method.


So far, only pitch. Fuel is pumped between tanks to trim the aircraft. This is especially helpful for supersonic aircraft, because their center of pressure moves by a quarter of the wing chord between sub- and supersonic flight. The prime example would be the Concorde:

… during flight fuel is transfered from tank to tank to maintain trim and balance of the aircraft as it does not have a full tailplane which would be used on a subsonic airliner to perform this task. Also for supersonic flight the Center of Gravity is critial and required to be moved for different speeds.(source)

Before take off and during the acceleration through Mach1 to an eventual Mach 2, fuel is pumped out of the forward trim tanks to the rear trim tanks and the collector tanks in the wings. Around 20 tons of fuel is moved in the process and results in a rearward shift of the CoG by 6ft (2 meters.)

Concorde fuel tanks and transfer scheme

Concorde fuel tanks and transfer scheme (picture source)

Airbus is using a similar scheme for finely adjusting the center of gravity for optimum performance.

I expect that especially for roll, fluid pumping will not give sufficiently fast response. When flying through gusty air, it is very helpful to have responsive roll control.

If you accept a solid mass rather than a fluid, then the gliders of Otto Lilienthal qualify as the first aircraft which used weight shifting for control, both in pitch and roll. Following his footsteps, every hang glider and trike is using this technique until today.

Otto Lilienthal in his Normalflugapparat

Otto Lilienthal in his Normalflugapparat, the first serially produced airplane (picture source). Here he throws his legs back in order to pitch up.

Another first used weight shifting, albeit only for pitch control: The very first Zeppelin had a 100 kg weight suspended between its fore and aft gondola which could be shifted fore and aft to adjust the center of gravity.

From www.airships.net:

Pitch was controlled by a sliding weight suspended under the hull which could be shifted fore and aft; there were no elevators for pitch control, or fins for stability.

The weight proved insufficient, jamming on occasion of the first flight. Later is was increased to 150 kg. Elevators and fins would had helped less because the two 14 hp gasoline engines never allowed the ship to fly fast enough for effective aerodynamic control.


Nasa's Space shuttle (among many other missile systems) uses pumped liquid to control Pitch, Roll and Elevator

Not in the way that you diagrammed it, though. Instead of using hydraulic control surfaces, it uses thrust vectoring to control all three angles. By sending the Fuel out at a slight difference in angle, or by using RCS to change angles.

Thrust vectoring works by changing the angle that fuel is dejected from the nozzle of the rocket, an thus changing the direction of the thrust. This can change the pitch and yaw of an aircraft (for a rocket, pitch and roll). Controlling roll is another aspect that requires two or more thrusters, which apply a torque to the structure, similar to the way one spins a top.

  • $\begingroup$ The question specifies moving fuel "within the aitframe." Thrust vectoring isn't what's being asked about. $\endgroup$
    – Ralph J
    Commented Jul 28, 2017 at 4:43
  • $\begingroup$ @RalphJ rockets still do exactly what op is talking of today, to cancel out torque within the frame while thrusting. Though it's done in space, not in the air. $\endgroup$
    – tuskiomi
    Commented Jul 28, 2017 at 13:39

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