An airplane-helicopter hybrid called RACER is a new convertiplane design from Airbus

This rapid and cost efficient rotorcraft offers a fresh approach to vertical and horizontal flight aiming to have more speed and range then conventional helicopters.

By Nihad Daidzic, PhD
Pres, AAR Aerospace Consulting
Professor, Minnesota State Univ

Box-wing, pusher propellers, and no tail-rotor on Airbus RACER defines a compound helicopter capable of airplane-like horizontal flight.

Airbus recently unveiled an airplane-helicopter hybrid design named Rapid and Cost Efficient Rotorcraft (RACER). It has a main rotor like a conventional helicopter but no tail-rotor. It also has a horizontal stabilizer with elevators and 2 vertical stabilizers with rudders on each fin providing yaw control in horizontal cruise.

A short box-wing has tip-mounted engines with pusher propellers for anti-torque control during hover and low forward speeds or as chief propulsive devices for fixed-wing horizontal cruise mode. In this article, I'll explore the aerodynamics that make such a design possible.

Despite great utility, edgewise rotors have a major limitation in rather low forward speeds. A designer of a conventional helicopter can make some trade-offs between good hovering characteristic (low disc loading with longer blades) or faster cruise speeds (higher disc loading with shorter blades), but cannot achieve both without radical designs. Common solutions to this issue introduce airplane-helicopter hybrids.

The history of convertiplanes and compound helicopters goes back at least to the early 50s, as helicopters were just gaining acceptance. It was quickly realized that conventional helicopters are rather slow with limited operational ranges. To improve performance, 3 experimental-vertical designs were proposed: McDonnell's winged XV1, Sikorsky's XV2 stopped-rotor design, and Bell's XV3 tilt-rotor (predecessor of XV15 and V22). One experimental design that was successfully flight-tested was the 1960s winged Lockheed AH56 compound helicopter design.

Dissymmetry of lift

Early helicopters having relatively stiff cantilever blades could hover in zero wind condition. However, when starting to "fly" or hover in significant wind, things drastically changed. Advancing side blades (ASBs) experience higher relative speeds due to combined translation and rotation, hence producing more lift. Conversely, retreating side blades (RSB) produce less lift, resulting in edgewise rotor's dissymmetry-of-lift phenomenon. Unequal lift production between ASBs and RSBs resulted in helicopters rolling over as soon as there was translational motion through the air.

While different historical accounts exist, the credit for solving the lift dissymmetry problem is given to Spaniard Juan de la Cierva, a structural/civil engineer who worked with pinned frameworks. He devised a horizontal hub (flapping) hinge that allowed his autogyro blades to move out of plane (flap up and down as needed).

Radial blade stiffness in rotary-wing aircraft is ensured by centrifugal force acting on rotating negatively-twisted blades. For example, the helicopter's operational centrifugal forces are about 10 to 30 times stronger than lift on individual blades (blade loading) resulting in rather small 1g in-flight conning angles (2–6 degrees).

Centrifugal force depends on the square of the angular speed causing the coning angles to increase significantly even for small RPM reductions. Due to vertical force difference in edgewise rotors and with a flapping hinge, RSBs now flap-down and the local angle of attack (AOA) increases. Higher dynamic pressures and lift/thrust on the up-flapping ASBs causes reduced local AOAs and the accompanied reductions of the lift-coefficient (CL).

The faster the helicopter flies, the more RSBs flap down reaching the maximum rate at the 270º point and the maximum blade-down deflection at the 0º (360º) point. This results in increasing local AOAs, CL on RSBs and an approach to the aerodynamic-stall limit. And the higher the helicopter speed, the slower the RSBs are, hence requiring larger AOAs. A reverse flow region develops on the retreating side which is a direct function of the advance ratio m (aircraft forward speed divided by blade tip speed).

As a byproduct of vertically flapping blades, blowback or flapback occurs, which is corrected by forward cyclic until the physical control limit is reached. However, RSB stall is not the only factor limiting helicopter forward speed. As forward airspeed increases, ASBs experience high combined rotational-translational TAS with Mach numbers venturing into the transonic region. Blade stability problems occur as the drag-divergence regime and shock-stall (high-speed buffet limit in airplanes) is approached.

Helicopters at high speeds also experience their own "coffin-corner." Rotating blades are "squeezed" between the aerodynamic-stall and the compressibility limits. Similar to airplane wings, blade tips can be swept and high-speed thin thickness-to-chord ratio (t/c) supercritical airfoils can be used to increase the critical and drag-divergence Mach numbers. This delays shock formation and reduces wave-drag at high forward speeds. The 1986 Westland Lynx helicopter set a 216-kt speed record using British Experimental Rotor Program (BERP) blades, a transonic design.

Unlike with fixed-wing, rotary-wing blades transition between the 2 aerodynamic extremes on a time scale of about 100 milliseconds at 300 RPM. Maximum TAS and/or the maximum transonic Mach numbers limit the ASB's speed at the 90º position. In the next 100 milliseconds or so, and on the retreating side (270º position), blades will encounter slowest local airspeeds while approaching the high-AOA aerodynamic stall.

Vortex lift and unsteady aerodynamics

Dissymmetry of speeds and dynamic pressure as the helicopter transitions from the no-wind hover to progressively faster cruise. Not to scale.

A straightforward application of the steady-state aerodynamics would predict RSBs stalling as they flap extremely downward to increase the local CL. For example, the blade of a 300 RPM conventional edgewise rotor in forward motion will spend 100 milliseconds on the advancing side and another 100 milliseconds on the retreating side, resulting in continuously varying tip speeds, Mach numbers, AOA, CL, CD, etc.

Fortunately, each rotor blade spends only 10-20 milliseconds in the critical high-AOA zone on the retreating side and does not stall. In fact, the CL can significantly increase, extending the flap-down movement range. The answer is in unsteady aerodynamics and vortex-lift dynamics.

The separation of the boundary layer (BL) occurs in a short but finite time. Just as the BL would start separating, the blade has already left the high-AOA region. Additionally, the leading-edge vortex detaches and rolls over the blade, providing low or suction pressure over the upper blade surface.

This results in nonlinear vortex (dynamic) lift. The faster the helicopter moves forward, the lower dynamic pressure and higher-AOA on the retreating side and more down-flapping is required. Clearly, there is a limit to this and is 1 of the reasons for the VNE speed. Paradoxically, and unlike in fixed-wing airplanes, the faster a helicopter flies, the closer its retreating blades are to the high-AOA stall.


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