Advanced rotor designs break
conventional helicopter speed restrictions
A fast future awaits for rotorcraft with coaxial rotor systems and pusher propellers.
By Jay Chandler
ATP/Helo. Learjet, Shorts 360, Sikorsky S54, Boeing Vertol 234
Benefiting from Sikorsky's advancing blade concept (ABC) and X2 prototype research, the S97 Raider will have rigid coaxial rotor systems capable of high-G maneuvers, quick tactical bursts of speed and a significantly lower noise signature in flight.
There has been a buzz in the helicopter world over the past year, with many helicopter manufacturers showcasing new designs for "high-speed" helicopters that break the bonds of speed for rotorcraft of the future. Examples include the Sikorsky X2 exceeding 250 KIAS, Eurocopter's X3 with speeds above 232 KIAS, and radical new designs from AVX Aircraft with proposed designs that exceed 250 KIAS.
If you're not a helicopter pilot, you might be asking, "What's the big deal? My Learjet goes over 500 kts." The fact is, the Learjet is quite fast but it can't hover—which always has been and always will be the jewel of helicopter flight.
Over the years, incremental achievements in airspeed have been accomplished by rotor blade design, sweeping the tip to the rear or changing the shape of blade tips to reduce the noise signature—but no real breakthroughs in speed limitation have occurred until recently.
To better understand the limitations of high-speed helicopter flight, we first need to review basic helicopter aerodynamics. All airfoils, whether fixed or rotating, produce lift in much the same way—increase the angle of attack and lift increases. Also, if you remember your lift equation, airflow (V) is squared, which means airflow has a remarkable effect on the production of lift. When airflow increases over an airfoil, lift increases.
Likewise, if airflow decreases, lift decreases—squared. No big news yet. Put the airfoil on a stationary helicopter with no wind, and there are still no changes, because the airflow is equal no matter where the blade is in its rotation on the rotor disc. The rub starts as the helicopter moves forward. The rotor system now has an advancing blade and a retreating blade in relationship to the aircraft's forward movement and the relative wind.
Dissymmetry of lift
With advancing blades on both halves of the rotor disc and feathering the retreating blade's angle of attack, the X2 coaxial system avoids retreating blade stall and attains speeds in excess of 250 KIAS.
As the helicopter moves forward through the air, the relative wind passing through the main rotor disc is different on the advancing side and the retreating side. For this discussion, as the pilot sits in the aircraft the advancing blade is the one on the right moving forward from the pilot's right rear to right front.
As the blade rotates from the nose of the helicopter to the pilot's left side and left rear it is called retreating. Airflow passing over the advancing blade has the relative wind from the rotation of the blade plus the relative wind from the aircraft's forward movement, thus increasing lift.
The retreating blade has the same rotation speed as the advancing blade—however, the relative wind generated by the forward movement of the aircraft decreases the total relative wind of the airfoil and therefore loses lift on the retreating blade.
This phenomenon—termed dissymmetry of lift—is compensated for by a mechanism called a swashplate, which miraculously corrects for this and a host of other aerodynamic issues caused by a rotor system which exhibits the characteristics of a gyro—but that would require another article.
On conventional helicopters with 1 rotor system, dissymmetry of lift is compensated for by decreasing the angle of attack (AOA) on the advancing side and increasing the AOA on the retreating side to keep the lift equal across the rotor disc in forward flight. (This has to be done, because otherwise the helicopter would roll uncommanded to the retreating blade side—which is frustrating at best, assuming you want to get from point A to point B.)
As the helicopter accelerates, the difference in relative wind between the advancing and retreating blades increases, as does the AOA on the retreating blade. This can only continue to a certain point until the critical AOA is exceeded on the retreating blade and the airfoil stalls.
You don't need to be a helicopter pilot to know that any time an airfoil stalls it's a bad thing. Helicopters experiencing retreating blade stall will pitch violently up and roll to the retreating blade side. This aerodynamic limitation is called retreating blade stall and limits helicopters to a relatively low speed regime.
Vietnam-era helicopters cruised around 90 KIAS, while helicopters of today cruise between 120 and 140 KIAS. Much faster than that and the retreating blades approach stall speed, or the rotor blade tips approach the speed of sound due to the rotational tip speed and forward speed of the aircraft. This is called compressibility, and it can lead to severe blade vibration and destruction of the blade tips.
Thus the conundrum—how do you create a rotorcraft that maintains its vertical lift characteristics and has significant speed increases without the blades entering retreating blade stall or the blade tips approaching the speed of sound?
Tandem vs coaxial
The US military's proposed future vertical lift (FVL) rotorcraft demands vertical takeoff from a hover and cruise speeds far exceeding those of today's conventional helicopters. (See "Radical helos, V/STOL vehicles and UAVs," Pro Pilot, May 2012, pp 112–115.)
Now that we've reviewed conventional helicopter aerodynamics, let's discuss the different rotor designs available and the pros and cons of each.
Tandem rotor heads made their debut on the Piasecki H21, appearing later on the British Bristol Belvedere and the US Boeing CH46 and CH47. Tandem systems provide 2 rotor heads for increased lift area, but at a price—a strong and sturdy structure must support the 2 rotorheads, the transmissions and the loads between them.