Need for speed

March 17, 2010 by Tim McAdams

Flying a rotor system edgewise through the air creates a problem known as dissymmetry of lift. One side of the disc advances into the wind (headwind) while the other side is retreating (tailwind). For a fixed angle of attack, the lift on the advancing side is greater creating a lift imbalance that increases with airspeed. Early engineers came up with a way to equalize the lift by changing the angle of attack (flapping) as the blade moves around the rotor disc. This required increasing the angle of attack on the retreating side as the helicopter gained airspeed. As with any airfoil, the angle of attack can only increase so much before it stalls. Referred to as retreating blade stall, this problem limits a helicopter’s forward speed.

Ever since the development of the helicopter engineers have tried to figure out ways to get around the rotor system’s speed limitation. Although there are several ideas for this, one is a co-axial rotor design. This arrangement uses two stacked rotor systems with the same axis of rotation, but turn in opposite directions. This way when the retreating blade is losing lift the blade above (or below) it is advancing and gaining lift. This helps equalize lift across the rotor system. (There are other advantages as well like increased lift ability and no need for a tail rotor.)

In the 1970s Sikorsky demonstrated high speed flight with the S-69 Advancing Blade Concept (ABC) system. The S-69 used two rigid counter-rotating rotor systems with an auxiliary propulsion arrangement. Today, Sikorsky Aircraft is incorporating decades of research and development from the S-69 into its X2 technology concept demonstrator. The X2 aircraft uses two rigid counter-rotating rotor systems and a pusher propeller for auxiliary propulsion. Additionally, Sikorsky has included new technologies including an integrated fly-by-wire system, high lift-to-drag rigid blades, low drag hub fairings, and active vibration control.

The X2 is currently being test flown at Sikorsky’s West Palm Beach, Florida, development flight center. In October 2009 the X2 achieved a speed of 106 knots. This milestone moves the company a step closer to its stated goal of demonstrating that a helicopter can cruise comfortably at 250 knots while retaining such desirable attributes as excellent low-speed handling, efficient hovering, and a seamless and simple transition to high speed.

If Sikorsky is successful, the next 10 years could bring a new era of high speed helicopter flight.

  • Bob McKillip

    It’s great to see a manufacturer doing such R&D to push the technology forward. The coax rotor, like all configurations, though, is no “free lunch” – the interference of the wake from the upper rotor on the lower creates losses on the order of what one sees with other configurations, be they tandems or main/tail rotor systems. Johnson (Helicopter Theory) discusses this aspect in his chapter on design.

    The ABC has extremely stiff blades to prevent their flapping into each other at high speed. This was done to allow for a smaller hub length that separated the two rotors, which is important as hub drag is a BIG contributor to parasite power required at high speed.

  • Justin

    I hate to post a dumb question, but why don’t they just spin the standard rotor faster and decrease the angle of attack? Does it have something to do with what Mr. McKillip was saying about hub drag?

  • Rich

    Justin, good question. The advancing blade has a speed limit which is generally the speed of sound. The speed of the advancing blade is the forward speed of the aircraft plus the rotor speed. As this approaches the speed of sound (a flight regime for which it’s not designed) an increase in drag occurs as well as problems with shock waves among other things. Also with the increase in drag comes an increase in power required, eventually something has to give.

    Why not design the rotors to go faster? Basic aero problems, there is a reason the wing shapes of high speed aircraft and low speed aricraft are different.

  • Ehud Gavron

    I hesitate to sound like an Airwolf enthusiast… but there are three states of the MR’s lift:
    1. The advancing blade is generating lift. The retreating blade is generating LESS lift.

    2. The advancing blad is generating a LOT of lift. The retreating blade is not moving fast enough with respect to relative wind to create lift.

    MIRACULOUSLY [by today’s technology, but think JATO] the aircraft is accelerated MUCH FURTHER beyond point 2…

    3. The “retreating blade” generates lift. It can do so either through going slow but being pitched opposite its normal direction, or it can do so through being very quick, and not having any part of it exceed supersonic speeds.

    Personally I think that one day we will see MR systems where the blade on the not-in-the-direction-of-airspeed side (also known as the retreating blade) can be pitched AWAY from the direction it’s moving, and because the relative speed between it retreating and the advancing relative wind will be positive… it will have lift.


  • Jared

    Why don’t we just buy a $150,000 fixed wing plane. I’ll never understand rotor heads. I work on them for a living and we all make fun of them. unless you are military, logging, or hauling injured people from accidents there is no need for a helicomputer.

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