How do flying wings, like the B-2 Stealth bomber, actually keep themselves from yawing out of control without a vertical stabilizer?

For the record, I assume this has to be a simple mechanics process. Why? Well flying wings go all the way back to the 30s. One of the earliest (and my personal favorite) is the N-9M, which was a scale model of the XB-35, a prototype bomber for the allies during WWII.

They didn't have flight control computers back then, and the only control complaints I recall them having is that early versions had a tendency to flip over backwards when approaching stall speeds, well, that and the ground effects were pretty strong. But, no mentions of going into flat spins when going into hard maneuvers (that I recall).

So how do they control that Y axis on flying wings? Are they perhaps more susceptible to a flat spin than a regular design (even if those risks can be kept to an acceptable minimum)?

up vote 4 down vote accepted

Theres two different approaches, passive and active stability on all three axis.

X axis (roll):

In the X axis, flying wings are stabilized the same as any other plane. Passive stability is achieved through a slight upwards dihedral between left and right wing. As such the "lower" wing creates slightly more lift in upwards direction and allows the plane to roll back to horizontal flight. Active stability is achieved through ailerons near the wing tips which create a controlled differential lift allowing the pilot or flight computer to control the roll velocity.

Y axis (pitch) Passive stability in the Y axis in traditional planes is achieved through the high-leverage corrective force of the horizontal stabilizer (with the exception of canard type planes where the main wing itself takes this role) Active stability is achieved through elevator controls on this stabilizer, which increase or decrease the vertical force in one or the other direction.

In a flying wing this stabilizer is not present. Instead the rearward area of the main wing takes this role. For this to work, a stable flying wing needs to have wings that are significantly swept backwards in such a way that the overall torque force at both positive and negative high angle of attack is always decreasing this angle of attack. Active stability is achieved through control surfaces at the very back of the wing. These are often the same control surfaces also used for Roll control, but deflecting in unison instead of opposite directions (Elevons)

Z Axis (yaw) Similar to pitch, the passive and active yaw control on conventional planes is done with a stabilizer fin with a rudder at the tail.

In true flying wings this is not present (Some don't even have winglets) But the high wing sweep (usually the wings of a flying wings meet at roughly 90° angle at the tip) causes a higher drag force at the forward facing wing if the craft yaws to one side, which enacts the required corrective force. Active control is achieved by causing additional drag through some sort of braking flaps near the wing tips (the actual implementations vary - split elevons - separate break flaps - ...) This can also be augmented by using differential thrust. (Note: The corrective force is much lower than that caused by a tail fin. As such a powered flying wing should have its engines relatively close to the center, otherwise a one-engine-out scenario would cause an uncorrectable yawing moment. This however limits the effectiveness of differential thrust if the craft is supposed to stay flyable in such a condition)

Speed and stall stability: A speed stable plane has its nose pitch up at higher speeds and pitch down if the craft gets slower, thanks to the center of gravity being in front of the center of lift, which causes the nose to drop. This is balanced out by a (usually parasitic) downward force from the horizontal stabilizer acting at the tail of the plane. This arrangement of forces should also cause the nose to drop and the angle of attack to reduce in a stall situation.

As a flying wing does not have this stabilizer, a similar balance of forces needs to be achieved through a careful design of wing profile and twist. If you look at Horton flying wing glider designs for example, they have a "thick" profile creating the majority of lift in the center (which thanks to the wing sweep is also the front) Towards the end and back of the wings, the profile becomes thinner and more symmetric. At the same time, the wings are slightly twisted, so the intrinsic angle of attack is a few degree lower at the tip (and as such at the rear of the wing) than near the nose. At high speeds, the front of the wing creates more lift than the back and rises the nose. This part has the highest wing load and will stall first, while the swept back outer wing area - thanks to its profile, lower AoA and lower load will not have stalled yet. The nose drops, and the control surfaces in this rearward part stay functional.

It should be noted that not all flying wing designs did achieve a safe stall behavior. Some, like early 1940's and 1950's American designs failed to implement stall stability correctly, while others (like the B2 bomber) the plane was deliberately designed aerodynamically unsafe to faciliate other features (stealth) which would have been compromised by a "twisted" wing.

  • Are there any flying wings, past or present, that implement this twist wing design? Can you add a picture or illustration to show what that looks like (bonus points if it shows how the center of lift is moved viz a vi airspeed)? Fascinating stuff... – Jay Carr Jul 31 '15 at 12:52
  • the "lower" wing creates slightly more lift in upwards direction and allows the plane to roll back to horizontal flight - can we please unlearn this nonsense? This has often been claimed and never been true. Dihedral will not right up an airplane. – Peter Kämpf Aug 1 '15 at 5:36

Sweepback helps a lot, and the rest is achieved with drag devices near the wingtips.

Did you notice that the B-2 has split ailerons, and they are partially opened during flight? Their drag pulls the wingtip back, and in combination with the sweep the forward wing in yaw will have a longer lever, so the same drag produces a bigger stabilizing yawing moment.

Differential thrust is not available on gliders, and there are lots of flying wing gliders. Horten III and IV, Fauvel AV-36 and the SB-13 are some examples. The Hortens used speed brakes near the wingtips, and the SB-13 has rudders with differential deflection (70° outwards, 20° inwards) on its winglets to increase the yawing moment.

Directional control is no problem with flying wings. Flutter and a low $c_{L max}$ are the real drawbacks. Ground handling, landing characteristics and payload space are others, but can be overcome somewhat.

And flat spins are not possible with flying wings. Their spin modes are all fairly steep due to the lack of a strong inertial moment from the lengthwise distribution of masses.

  • Fantastic, I've been hoping someobody would swoop in and answer this one! Can you expand a bit on flutter and CLmax (not sure what that is)? Or should I just ask a separate question for that? – Jay Carr Apr 18 '14 at 18:23
  • Sure: $c{_L max}$ is the maximum coefficient of lift. A flying wing needs an airfoil with a low pitching moment, and those create less lift with the same wing area than regular airfoils. A low pitching moment is achieved with reduced or even negative camber in the rear part of the airfoil (google "reflex airfoil" for more info and pictures). Flutter is an interaction of aerodynamic and elastic oscillations which looks from the outside as if the aircraft flaps its wings. It scares the heck out of the pilot, because most flutter ends in the wings (or whatever else which is involved) breaking off – Peter Kämpf Apr 18 '14 at 18:36

Flying wings use differential thrust for yaw control instead of vertical stabilizers and rudders. Some also have small outboard rudders or spoilers for this purpose.

  • Jet engines have very slow response to thrust setting. So you can use differential thrust for gentle turn, but there's no way you to recover stall, or even damp dutch roll with it. – Jan Hudec Mar 14 '14 at 19:11
  • 2
    The B-2 uses differential thrust in stealth mode, and also uses outboard spoilers or "decelerons" when not in stealth mode. – xpda Mar 14 '14 at 22:45

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