Helicopter Wake Vortices


  James Stack, Jonathan Colby, William Tsai



  "Experimental Investigation of Rotor Vortex Wakes in Descent,"  AIAA Region VI Graduate Student paper competition, April 2003  (PDF, 600K)

  "Flow Visualizations and Extended Thrust Time Histories of Rotor Vortex Wakes in Descent," submitted to the Journal of the American Helicopter Society, May 2004   (PDF, 1.3M)


    Following on earlier work done in this lab on fixed-wing aircraft wake vortices, we’re now studying the wake vortex structure generated by rotorcraft. While the wake system of fixed-wing aircraft is a highly complicated problem, understanding the dynamics of helicopter wake vortices is especially challenging due to their dependence not only on the blade geometry and loading, but also on the aircraft’s operational state (i.e., hovering, climbing, descending, or maneuvering). Our work is primarily experimental, as we use thrust measurements (from strain gages), flow visualization techniques (injecting dye and air bubbles into the flow from the blade tips) and Particle Image Velocimetry (PIV) to study the wake structure and dynamics of small model rotors.

    Our initial work involved testing our model rotor in a stationary water tank -- i.e. simulating a hovering rotor. The flow visualization and PIV results from these tests have allowed us to gain a better understanding of the evolution of rotor wakes.
    Our primary interest, however, is on the dynamics of descending rotors. When the descent velocity of a rotor approximately matches its wake's velocity, the helical wake tends to roll up into a thick vortex ring that remains near the rotor plane and interferes with the rotor's inflow.  This is known as vortex ring state (VRS) in helicopter lingo. For reasons that are currently unknown, the vortex ring that forms has a tendency to periodically detach from the rotor and convect away. This formation/detachment process can lead to severe loading fluctuations that can catastrophically impact the performance of the rotorcraft. The focus of our more advanced work is on the VRS process, and for this we do our model tests in a 70m long water towing tank.


   In this experiment, a three-bladed, 10" diameter rotor with manually-adjustable blades is used.  Each of the carbon-fiber blades has a small tube embedded in it that allows air bubbles or dye to be leaked from the tips as a means of marking the tip-vortex cores. The blade airfoils are ARA-D 10 and the blades have a root-to-tip twist of 5° - relatively low compared to typical rotorcraft blades.   



  Figure 1.  (clockwise from upper left) Three blade rotor (a) overall, (b) close-up of the variable pitch mechanism, (c) and with dimensions.


      The entire model assembly is shown below  in Figure 2.   The rotor is driven by a digitally-controlled microstepper motor (25,000 pulses/revolution). The one-inch thick mounting plate beneath the motor is instrumented with strain gages that allow us to measure the rotor's thrust during testing.  


full model

Figure 2.  Complete assembly.  Micro-stepper motor (upper left) drives the rotor. 


Flow Visualization - Stationary Tank


     Figure 3 shows the full helical wake structure in three dimensions. For the sake of clarity, only one rotor is injecting air bubbles into the flow. While air bubbles do an excellent job of showing the details of the vortex filament structure in the near-wake region of the rotor, they float to the surface shortly thereafter and so do not show the far downstream region of the wake.


Figure 3.  Single-rotor air bubble run at 4 rev/s with volumetric illumination.


   Figure 4 shows an image taken using sodium fluorescent dye (being leaked by all three rotor blades). A vertical laser sheet is used for illumination here, aligned with the axis of the rotor. This view provides us with a view of a single cross-sectional slice of the flow -- thus we clearly see the vortex cores at the top and bottom of the wake, but none of the helical structure seen in Figure 3. The smoky region at the left is from dye emitted during a previous run. (The tank must be drained and re-filled frequently during dye testing for best results.)    


Figure 4.  Dye flow visualization image taken at 2 rev/s with planar illumination. 

         Flow Visualization - Towing Tank


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