12
$\begingroup$

This is the Beluga:

enter image description here

It's used for carrying pieces of a fuselage (of much larger planes) from one place to another so they can be fitted together to make a plane.

As I recall, it's based on an Airbus A300, and then they added on the huge bulging fuselage piece so it could carry more stuff and moved the cockpit down a bit to make way for the huge door at the front of the bulge.

My question is what did they do in terms of wing/control surface design to keep the plane from becoming unstable. Clearly when this plane is fully loaded the CG is much much higher than a regular A300. Which seems like it would make it more likely to want to tip over in some fashion. What did they do to counter-act this?

$\endgroup$
  • $\begingroup$ added roll stability in the envelope protection. $\endgroup$ – ratchet freak Aug 6 '15 at 13:31
  • $\begingroup$ @ratchetfreak lol, right, but how? $\endgroup$ – Jay Carr Aug 6 '15 at 13:44
  • 4
    $\begingroup$ the payload is only a third of the MTOW (47t/155t) It's mostly for large but light cargo. $\endgroup$ – ratchet freak Aug 6 '15 at 13:53
  • 1
    $\begingroup$ if the empty CoM aligns with the floor of the cargo hold then in worst case (uniform cargo filling up the entire hold) the CoM would move up 2.3 m. I'd have to boot up my KSP to find out how that affect handling. $\endgroup$ – ratchet freak Aug 6 '15 at 14:16
  • 1
    $\begingroup$ @ratchetfreak: Did A300-600ST get envelope protection retrofitted? Because A300 did not have them (they were introduced in A320). $\endgroup$ – Jan Hudec Aug 7 '15 at 11:55
8
$\begingroup$

As long as the aircraft flies without much sideslip, the vertical position of the center of gravity is not important.

It is a common misconception that the aircraft could "tip over" when banking due to a high location of the center of gravity, or even that dihedral would right the aircraft up because the lower wing will create more lift. This is all wrong!

To understand why, look where gravity is pointing in banking flight. By gravity I mean the total acceleration felt by the pilots and passengers, the apparent vertical direction.

A320 front view straight and banked with force vectors

A320 front view straight and banked with force vectors in the aircraft's frame of reference.

The force vectors in banked flight don't differ from those in level flight, only their magnitude grows. In both cases the apparent vertical is parallel to the vertical plane of symmetry and will go straight through the center of gravity, regardless of its vertical location. Therefore, it has no lever arm and produces no momentum around the center of gravity.

To put it another way, the aircraft banks in a coordinated turn to keep the apparent vertical in its vertical plane of symmetry, now that a horizontal force needs to be added to change the aircraft's angular momentum.

Therefore, neither the wing nor the control surfaces need any special adaptation to the huge fuselage. Only the vertical tail needs to be increased to compensate for the increased side area ahead of the center of gravity. In case of the Beluga, two vertical fins were added and the strake enlarged. This is a very common procedure for modifications of existing aircraft, because it is easier than designing a new vertical and improves handling compared to a new, scaled-up fin.

Side view of the Airbus Beluga

Side view of the Airbus Beluga (Picture source)

The fuselage has its aerodynamic center ahead of the center of gravity, so increasing its size will also increase its destabilizing yawing moment in a sideslip. This requires more stabilizing area at the rear, which can be kept relatively small since its aspect ratio is much higher than that of the fuselage, making it more responsive to sideslip angle changes. Both together will create a side force in sideslip which will be substantially bigger than that of an unmodified A300 in the same conditions. This sideslip-induced side force and the drag which comes with it will make the flying characteristics of the Beluga different to that of an A300. Since it acts quite a bit above the center of gravity, it will add a rolling moment, increasing the dihedral effect. I expect that it will be harder to build up a sideslip angle in the Beluga, and if the pilot crosses controls, the aircraft will slow down more quickly than a regular A300.

When designing the Beluga, it was clear from earlier planes (Super Guppy, 3M-T or BM-T Atlant) that the oversized cargo bay would not make flying impossible.

3M-T in flight

The 3M-T/BM-T modification for rocket stage airlifting. Picture source.

$\endgroup$
  • $\begingroup$ Peter nice answer but there is one thing that totally confuses me: The vector of centripetal(?) force drawn to the outer opposite of the turn. Shouldn't it point to the direction of turn like it's shown here? $\endgroup$ – Stelios Adamantidis Aug 7 '15 at 8:59
  • 2
    $\begingroup$ @SteliosAdamantidis: The shown vector is that of centrifugal force, which is the apparent force in accelerating (rotating) reference frame that behaves similarly to gravity. $\endgroup$ – Jan Hudec Aug 7 '15 at 9:52
  • $\begingroup$ It would be nice to add what happens if there is a side-slip as it explains why it needs the additional vertical stabilizers (the question says control surfaces, not specifically ailerons) $\endgroup$ – Jan Hudec Aug 7 '15 at 9:55
  • 3
    $\begingroup$ @SteliosAdamantidis: In the rotating frame of reference (the one relative to the aircraft) the inertial (centrifugal) force exists and must be included in calculation and the result is that the forces are in balance and the aircraft is not accelerating, which relative to itself it isn't. In the inertial frame of reference there is no centrifugal force and the centripetal force (which is the horizontal component of the lift force) causes the plane to turn. The picture shows the forces as appropriate for the airplane-relative, rotating, frame of reference. $\endgroup$ – Jan Hudec Aug 7 '15 at 12:57
  • 1
    $\begingroup$ @foot: There are many minor differences, like the axis of roll which is above the cockpit. This creates a sensation of shifting sideways when the ailerons are deflected and the aircraft rolls. But the same is true for the 747, where the pilots sit above the axis of rotation. It does not make flying the Beluga fundamentally different. $\endgroup$ – Peter Kämpf Aug 7 '15 at 20:20
2
$\begingroup$

Aircraft Stability

As we can see in the above picture, an aircraft's center of gravity (CG) always lies at the intersection point of the three axes. So whenever CG is shifted along one axis, it affects the other two also.

When CG is shifted vertically (along vertical axis), it affects directional stability. In such case, rudder is needed to ensure directional stability.


The unusual bulge in the fuselage of an Airbus Beluga may make one think it can carry more weight than the A330, but that is not correct. MTOW of A330 is between 364,000-378,500 lbs, however Beluga's MTOW is 341,713 lbs.

Since CG does get shifted upwards in Beluga, it affects the aircraft's directional stability. It has been mentioned at several places (Wikipedia and here and here) that the tail of Airbus Beluga has been enlarged and strengthened to maintain directional stability.

Beluga Tail
Image Source

$\endgroup$
  • 3
    $\begingroup$ I don't think the CG is affecting the directional stability, it's more the increased surface area forward of the CG, and reducing the effectiveness of the stabilizer behind the enlarged compartment. $\endgroup$ – fooot Aug 6 '15 at 20:26
  • 1
    $\begingroup$ The tail surface has clearly been upsized, see this shot of several Airbus's allaboutguppys.com/beluga/ai-fama.jpg with A320, A321, A310, A300-600, A330-200, A340-300, and the A300-600ST "Beluga". More info here including "The fin was also changed to maintain stability in flight. " allaboutguppys.com/beluga/600stf.htm $\endgroup$ – CrossRoads Jan 2 at 17:18
  • $\begingroup$ @CrossRoads: According to Wikipedia, the Beluga's vertical tail is a stretched A340 model rather than that used on the stock A300. $\endgroup$ – Sean Sep 20 at 4:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.