The Incredible B-2 Wing Flutter Video and What It Means.

Dec 29 2017 - 14 Comments
By Tom Demerly

Test Video Shows Interesting Structural Capabilities of Advanced Aircraft Like B-2.

This remarkable video of a Northrop Grumman B-2 Spirit stealth bomber undergoing wing flutter testing on June 14, 1995 is fascinating for a number of reasons.

All aircraft wings have a remarkable capacity for flex. But one of the most significant changes in aircraft engineering in the last three decades has been the addition of composite materials, especially carbon fibers, into aircraft structural design. In many cases these composite materials have replaced metal alloys in structural components on advanced aircraft.

The B-2 Spirit is approximately 80% composite, mostly carbon fiber. Some of the structural framework internal to the B-2, especially where the wing blends into the fuselage and the largest fuel tanks are located, is titanium and aluminum. While part of the reason for this is structural, another reason is that composites, being made up of a number of different elements, can have radar absorbent materials included in their manufacture or “lay-up” during the process of combining materials into a composite.

There are many reasons composite materials, or materials made up of a combination of advanced materials including metals, polymers (plastics) and carbon atoms at the most elemental level, have become so common in aviation engineering.

Two of the chief reasons are illustrated in this video.

Composites Are Anisotropic.

Anisotropic materials can be engineered to transmit energy differently in different directions. Metals, including metal alloys like aluminum made from the ore bauxite, are isotropic, they only transmit energy in one direction. That means a composite like carbon fiber, being anisotropic, can be engineered to be stiff along one axis or direction of stress, but flexible in another. In the case of this B-2 video, the carbon-fiber composite material in the wings is engineered to absorb energy from aerodynamic stress and bend significantly in the vertical axis, with little change in flight attitude during the flexing. The wings do not twist or begin a similar leading edge to trailing edge vibration, which would change the aircraft’s angle of attack.

Structural elements of a B-2 Spirit. (Photo: Northrop Grumman)

Composites Have Different Fatigue Characteristics.

Eventually all materials fail. The number of normal fatigue cycles a material can endure prior to failure is part of its fatigue limit. In general, isotropic materials like metals and metal alloys have a shorter, lower fatigue limit or number of fatigue cycles. Composite materials like carbon fiber can be engineered at the molecular level to have much longer fatigue limits, enduring more fatigue cycles. And when they reach their fatigue limits, composites tend to fail differently than metals, with cracks propagating through the laminated material very differently than through the often more linear grain structure of a metal or metal alloy. As a result, carbon fiber structures in aircraft can be more durable than alloy structures, one of the reasons for the growing use of carbon fiber composites in advanced aircraft like the B-2 and in future aircraft like the B-21 Raider.

B-2s also have a system called the Gust Load Alleviation System that looks like the aircraft’s beaver tail. The GLAS counters the rolling impact or resonance to smooth out the ride of the B-2 in turbulent conditions and extend the aircraft’s fatigue life. The GLAS also smooths the ride of the B-2 in low altitude flight, even though the B-2 is predominantly designed for the high-altitude flight regime.

Air Force test data from tests like the one in this video and through finite elemental analysis (FEA) modeling suggest the B-2 will remain structurally sound to approximately 40,000 flight hours. This analysis also revealed that the rudder attachment points at the B-2’s wingtips are the highest structural stress areas and will be the first to fail. B-2 Spirits have not implemented an Aircraft Structural Improvement Program (ASIP) as we have seen on the primarily alloy B-52 heavy bombers. Some sources suggest this may make it more difficult to predict an economic service life and attrition rate according to author Don Greer. Given the current engineering limitations established in tests like these, the current B-2 Spirit force will fall below its requirement of 19 aircraft (of which less are combat capable) by the year 2027, making the new B-21 Raider even more important.

A graphic showing the wing loading and stress vectors on a B-2 wing. (Photo: Northrop Grumman)

  • leroy

    Very interesting article. I can’t wait to see how they incorporate this composite material into the upcoming B-21.

    I will make this statement with 100% certainty regarding the new American bomber. No nation on Earth will manufacture anything comparable for at least 50 years. B-21 will rule the skies – with a little bit of help from some friends (F-22, F-35, MQ-25C and our future 6th gen total spectrum air dominance tail-less stealth PCA fighter). The future of air warfare will change forever, and the U.S. and NATO will dominate it!

    • With Russia now transitioning to the late 70’s in aircraft design, you can bet it will be decades before we see anything comparable anywhere.

      • Furrmin The Cat

        The Soviet SU-35 is a beast of a 21st century aircraft. But in all honesty you must understand that most of out aircraft designs are from the 70’s and 80’s. The stealth aircraft are all 80’s designs. The B2’s had enormous problems when they were being tested. The first ones could not fly in the rain.

    • really?

      propaganda much? someone paying you for this constant stream of effluent?

    • Shlomo Yaalon

      Not worth the imaginary money that pays for any of it

      • FelixA9

        The butthurt is strong with this one.

    • Yes, I totally agree, and this has been our Think Tank’s conclusion on our advances in military aerospace

  • Gindylow

    It’s all in the structural monitoring be it civilian or military things flex and eventually are degraded.

    I don’t hear of many emergency landings due to wing failure, other than by impact damage from either munitions or other objects like heavy birds.

    It reminds me of the helicopter blade stories from the Vietnam era about Huey’s being used to clear a circle of foliage.

    • Furrmin The Cat

      Those stories were likely lies but I think the Apaches did have “Jesus” nuts holding the rotors on.

  • juretrn

    It’s actually quite amusing to see those wings flex, maybe it thinks it’s a swallow!

  • Gyoz

    AH AH AH

  • Furrmin The Cat

    Amazing! But the wings of every aircraft ever made must do this. They have to flex or break. I saw the wings on a B-767 makes these videos look like these wings were hardly moving. The old B-52’s were famous for this and needed wheels attached to the ends of the wings to allow the plane to take off under a load. It is frightening to look out the window on a commercial flight and see this. But it actually happens on every flight. Just not a lot like this.

  • Just Little ol’ Me

    It’s difficult to know what to say about this article. There’s so much wrong and silly here, it’s not funny.

    First, all structures, and that includes metal and concrete can and do exhibit these properties. It’s called “aeroelastic flutter”, it typically limits your maximum speed in an aircraft — unfortunately, its typically neither a function of indicated airspeed, or true airspeed, but rather somewhere in between. Searching on Youtube for this will reveal a NASA video of a Cessna whose horizontal stabilizer looks like its about to break into multiple pieces — much more violent than the B2. Similarly, there are a few videos of gliders, both metal and plastic exhibiting this during flight tests. The Tacoma Narrows Bridge is essentially an example of aeroelastic flutter (although in this case the bridge wasn’t moving but there was strong wind).

    Second, isotropic doesn’t mean it transmits strain in direction, it means all directions. What we’re really talking about is that the modulus (stress / strain — the amount of deformation per unit of pressure attempting to compress or stretch something) is not uniform in all directions. Often times something is only going to get stressed in one direction, if that’s the case you can optimize it to have a very high modulus in one direction at the cost of the other directions.

    Third, composites have been used in production aircraft since the early 60s, and carbon fiber has been used for over 40 years…. So the 3 decades thing is a bit off.

    Fourth, the use of the term “composite” is a bit off. Typical composites consist of a plastic resin (epoxy) and a reinforcement (glass, carbon, or kevlar). You can also various core materials (balsa, foam, nomex, aluminum honeycomb, etc) to get extra stiffness, but typically when someone is talking about “composites” they aren’t talking about mixing different types of epoxies or different types of reinforcements, although the latter is done sometimes, but simply the mixture of epoxy with a reinforcement….

  • Andy Blackburn

    Wow, as an aerospace engineer I can’t even begin to list how much is factually incorrect about this article. One of my NASA friends described it as “written by a middle school student”. Another aircraft designer said “I felt myself getting dumber as I read this”.

    First, carbon fiber reinforced composites have about twice the Young’s modulus at about half the weight of aluminum alloys – that is, they are stiffer and lighter. They are not designed to “flex more” – they flex less. Anyone with a carbon fiber golf club knows this.

    The video does not show the that the B-2 wing can “absorb energy from aerodynamic stress and bend significantly in the vertical axis”. Why is happening is called ‘body-freedom flutter’. It can be characteristic of tailless airplanes. It is also characteristic of airplanes with relaxed stability, which use high-frequency control inputs to stabilize the airplane. It happens when the frequency of the control inputs is the same as one or more of the structural resonant frequencies. This is called aeroservoelastic behavior. It has virtually nothing to do with the use of composites – it is a feedback between the resonant frequencies of the structure and the frequency response of the control system.

    Also, the B-2 is not stealthy because of the “layup” of the carbon fiber. Carbon fiber is a conductor and is about as visible to radar as metal construction – layup has nothing to do with it. What makes the B-2 stealthy is it’s shape, which is reflects radar in every direction other than back at the radar station (or nearly so). Stealth aircraft also have special (not carbon-fiber) radar-absorbing material (RAM) coatings that are radar-absorbent. They tend to be made from dielectric composites and metal fibers containing ferrite isotopes.

    I know engineering is a complicated subject, but jeez, try to get the basics right.