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The McDonnell XV-1 could slow its rotor from 410 to 180 RPM

Slowed rotor is a concept in designing and flying certain rotorcraft. Reducing the rotational speed of the rotor reduces the drag, enabling the aircraft to go faster and/or fly more economically.

Background

Rotors of conventional helicopters are designed to operate at a fixed RPM^[1][2][3] (within just a few percent),^[4][5][6] causing suboptimal operation in large parts of the flight envelope.^[7]

Two main issues restrict the speed of rotorcraft:^{[6][8][9][10]}

• 
  Retreating blade stall. As forward speed of the helicopter increases, the airflow over the retreating blade becomes relatively slower, while the airflow over the advancing blade is relatively faster, creating more lift. If not counteracted by flapping,^[11] this would cause dissymmetry of lift and eventually retreating blade stall,^{[12][13][14][15][16]} and blade stability suffers as the blade reaches its limits for flapping.^[10][17]
  


• 
  Transonic drag near the rotor blade tip. The faster-moving advancing blade tip may begin to approach the speed of sound, where transonic drag begins to rise steeply, and severe buffeting and vibration effects can occur. This effect prevents any further increase in speed, even if the helicopter has surplus power remaining, and even if it features a highly streamlined fuselage. A similar effect prevents propeller-driven aircraft from achieving supersonic speeds, although they can achieve higher speeds than a helicopter, since the propeller blade isn't advancing in the direction of travel.^{[12][13][16][18][19][20]}
  

These (and other)^[21][22] problems limit the practical speed of helicopters to around 160–200 knots (300–370 km/h).^{[16][20][23][24][25]} At the extreme, the theoretical top speed for a rotary winged aircraft is about 225 knots (259 mph; 417 km/h),^[22] just above the current official speed record for a conventional helicopter held by a Westland Lynx, which flew at 400 km/h (250 mph) in 1986^[26] where its blade tips were nearly Mach 1.^[27]

Theory

Rotorcraft Aspect ratio (mu) diagram

For rotorcraft, advance ratio (or Mu, symbol μ{displaystyle mu }) is defined as the aircraft forward speed V divided by its relative blade tip speed.^[28][29][30] Upper mu limit is a critical design factor for rotorcraft,^[17] and the optimum for traditional helicopters is around 0.4.^[8][20]

The "relative blade tip speed" u is the tip speed relative to the aircraft (not the airspeed of the tip). Thus the formula for Advance ratio is

μ=Vu=VΩ⋅R{displaystyle mu ={frac {V}{u}}={frac {V}{Omega cdot R}}} where Omega (Ω) is the rotor's angular velocity, and R is the rotor radius (about the length of one rotor blade)^[31][17][32]

Effect of blade airspeed on lift on advancing and retreating side, when aircraft speed is 100 knots.

When the rotor blade is perpendicular to the aircraft and advancing, its tip airspeed V_t is the aircraft speed plus relative blade tip speed, or V_t=V+u.^[10][33] At mu=1, V is equal to u and the tip airspeed is twice the aircraft speed.

At the same position on the opposite side (retreating blade), the tip airspeed is the aircraft speed minus relative blade tip speed, or V_t=V-u. At mu=1, the tip airspeed is zero.^[25][34] At a mu between 0.7 and 1.0, most of the retreating side has reverse airflow.^[32]

Drag type curves as a function of airspeed (simulated)

Although rotor characteristics are fundamental to rotorcraft performance,^[35] little public analytical and experimental knowledge exists between advance ratios of 0.45 to 1.0,^[32][36] and none is known above 1.0 for full-size rotors.^[37][38]Computer simulations are not capable of adequate predictions at high mu.^[39][40] The region of reverse flow on the retreating blade is not well understood,^[41][42] however some research have been conducted,^[43][44] particularly for scaled rotors.^[45][46] The US Army Aviation Applied Technology Directorate runs a supporting program in 2016 aiming at developing transmissions with a 50% rotor speed reduction.^[47]

The profile drag of a rotor corresponds to the cube of its rotational speed.^[48][49]
Reducing the rotational speed is therefore a significant reduction of rotor drag, allowing higher aircraft speed^[32] or lower power consumption.^[7] A conventional rotor such as the UH-60A has lowest consumption around 75% rpm, but higher aircraft speed (and weight) requires higher rpm.^[50]

A rotor disk with variable radius is a different way of reducing tip speed to avoid compressibility, but blade loading theory suggests that a fixed radius with varying rpm performs better than a fixed rpm with varying radius.^[51]

Aircraft

Traditional helicopters get both their propulsion and lift from the main rotor, and by using a dedicated propulsion device such as a propeller or jet engine, the rotor burden is lessened.^[16]
If wings are also used to lift the aircraft, the rotor can be unloaded (partially or fully) and its rotational speed further reduced, enabling higher aircraft speed. Compound helicopters use these methods,^{[12][13][23][8]} but the Boeing A160 Hummingbird shows that rotor-slowing is possible without wings or propellers, and regular helicopters may reduce turbine RPM (and thus rotor speed) to 85% using 19% less power.^[7] Alternatively, research suggests that twin-engine helicopters may decrease consumption by 25%-40% when running only one engine, given adequate height and velocity well inside the safe areas of the height–velocity diagram.^[52][53][54]

As of 2012, no compound or hybrid wing/rotor (manned) aircraft has been produced in quantity, and only a few have been flown as experimental aircraft,^[55] mainly because the increased complexities have not been justified by military or civilian markets.^[56] Varying the rotor speed may induce severe vibrations at specific resonance frequencies.^[6]

Contra-rotating rotors like on Sikorsky X2 solve the problem of lift dissymmetry by having both left and right sides provide near equal lift with less flapping.^[10][16] The X2 deals with the compressibility issue by reducing its rotor speed^[16] from 446 to 360 RPM^[32][57] to keep the advancing blade tip below the sound barrier when going above 200 knots.^[58]

List of slowed rotor aircraft

Sorted by year. Click <> to sort by other parameters.

• Visual comparison of slowed rotor aircraft


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  Venn diagram of cruise combinations for rotor power, propeller and wings.
  

  
  

  
  


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  Pitcairn PCA-2
  Unpowered rotor, tractor propeller, wings.
  

  
  

  
  


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  McDonnell XV-1
  Optionally powered rotor, pusher propeller, wings.
  

  
  

  
  


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  Fairey Rotodyne
  Optionally powered rotor, tractor propellers, wings.
  

  
  

  
  


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  Lockheed AH-56 Cheyenne
  Powered rotor, pusher propeller, wings.
  

  
  

  
  


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  Bell 533
  Powered rotor, jets, wings.
  

  
  

  
  


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  Boeing A160 Hummingbird
  No wings, no propeller.
  

  
  

  
  


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  Sikorsky X2
  Powered rotor, pusher propeller, no wings.
  

  
  

  
  


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  Eurocopter X3
  Powered rotor, tractor propellers, wings.
  

  
  

  
  


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  Carter PAV
  Unpowered rotor, pusher propeller, wings.
  

  
  

  
  

See also

• Gyrodyne


• Convertiplane

References

Notes

Citations

Bibliography

External links

External image

Some previous attempts at high-speed VTOL only works in Microsoft Internet Explorer