By Ray Prouty | August 11, 2017
Any aircraft flies by pushing air down in most situations; this downwash causes only minor effects. But in the case of a helicopter flying slow and low, the downwash may find dramatic ways to call attention to itself.
One effect is to produce a disturbance at the ground, the magnitude of which depends both on the disk loading (which governs the downwash velocity) and on the gross weight (which governs how much air is being moved). When the rotor is hovering more than about its diameter above the ground, the downwash velocity starts at one value at the rotor disc and then doubles as the wake contracts about a quarter of a diameter downstream.
The higher the disc loading, the higher the initial and the fully developed downwash velocities. Figure 8-2 shows this relationship for some familiar helicopters. As you can see, any helicopter is an effective wind machine. And with the modern trend to higher disc loadings, they have become more effective.
Tests have shown that even when the helicopter is hovering with the rotor two diameters high, the wake reaches the ground with very little loss of energy. With the rotor closer to the ground, the wake does not contract. So the velocities are reduced gross weight (pounds), but the total energy is about the same.
Whether hovering high or low, the wake can do significant “placer mining” of an unprepared surface. Snow, dust and newspapers are easily moved by any helicopter. Those with disc loadings of 10 pounds per square foot or more can also dislodge gravel and high disc loadings of 30 to 60 pounds per square foot associated with the propeller-supported VTOLs of the 1960s. Those could blow the rose bushes right out of your garden.
As the wake reaches the ground, it spreads into an outflowing layer, whose thickness starts out at about 10% to 15% of the rotor diameter. It then gradually gets thinner and slower, until several diameters out it turns up to recirculate back into the rotor. The thickness of the flow has an effect on operations near the hovering helicopter.
For example, tests with a Sikorsky CH-53E — which has a rotor diameter of 79 feet and a disc loading of 14.2 pounds per square foot — showed that not even a strong ground crewman could stand close to the rotor. Since he was completely immersed in the flow and strong random gusts, it made it difficult to brace himself. This is in contrast to the Bell XV-15 tiltrotor with nearly the same disc loading but only 25-foot rotors. The high-velocity ground layer produced by this aircraft is only waist high, and thus not nearly so upsetting.
A possible source of the gusty flow is the action of the blade-root vortices. Cortices generated by blade tips have always gotten a lot of attention, but root vortices also exist. It is conjectured that the rotor vortices from each blade get together to form one big vortex that goes down, strikes the ground and then wanders around in an unpredictable manner to influence the flow throughout the wake.
The top of the rotor disc is a region of low pressure, and its bottom one of high pressure. Since on most helicopters the blades do not extend all the way to the rotor shaft, air will leak upward through this hole in its desire to equalize the pressure. This is especially evident when hovering near the ground.
Many investigators have reported this upflow pattern, which is characterized as a “good news-bad news” situation. The good news is that the upflow reduces the normal download on the fuselage caused by the rotor wake, thus increasing the ability to takeoff heavily loaded. The bad news is that the upflow may bring dust, sand and other such environmentally undesirable material up to the engine intakes.
A lesser, but real, problem can exist when similar debris is stirred up by the outflow along the ground and then rises to eventually be recirculated back down through the rotor. This is the primary sources of blade erosion damage.
Another unexpected effect of the rotor wake has been observed on several helicopters, most recently with the Sikorsky Advancing Blade Concept (ABC) test bed aircraft, which when hovering close to the ground is subjected to random side forces. This aircraft has a nicely rounded fuselage bottom that presents no logical point where the downwash can separate. This wandering separation point produces erratic side forces the pilot has trouble anticipating.
Flying with the cockpit doors open improves the situation by breaking up the aerodynamic shape of the fuselage. A possible fix is the installation of lower fuselage strakes to force the separation at a definite point and eliminate the changing side force.
Although it is convenient to draw the rotor wake as a continuous downflow of air, it is a little more discretely organized than that. At each blade tip, the air, going from the high-pressure bottom to the low-pressure top, generates a tip vortex that, acting like a small whirlpool, induces rotational velocities in the nearby air.
Figure 8-5 shows the pattern that tip vortices reveal as they are made visible by moisture condensation. This vortex organization at the edge of the wake is usually of no consequence. But an exception arises with armed helicopters, when it is necessary to fire unguided rockets from hover or low speed with some degree of accuracy.
A rocket passing just over a tip cortex will be subjected to a different aerodynamic nudge than once passing just under a cortex and will thus be launched into a different flight path and will hit in a different part of the landscape. Since the instantaneous positions of the vortices change with gross weight, proximity to the ground, relative wind and control inputs, there appears to be no feasible way to satisfactorily time the firing so that each rocket passes through the same point of the vortex pattern.