Drucella Andersen  
Headquarters, Washington, D.C.                             May 11, 
1994
(Phone:  202/358-4733)

H. Keith Henry
Langley Research Center, Hampton, Va.
(Phone:  804/864-6120)

RELEASE:  94-74

TESTS SEEN AS STEP TOWARD QUIETER HELICOPTER BLADES

 Converting a small area of a helicopter main rotor blade into a 
controllable flap may prove to be an important step toward greatly 
reducing the irritating noise known as "blade slap."

 Tests conducted jointly by NASA, McDonnell Douglas and the U.S. 
Army indicate reductions in blade slap of four decibels in certain 
flight conditions using the concept.  That translates to a 
reduction of 40 percent in the strength of the sound from blade 
slap.  These results were obtained while simulating a helicopter 
flying through its own air turbulence during descent or landing, 
the time when blade slap is the loudest and when the helicopter is 
closest to people on the ground.

 The McDonnell Douglas Helicopter Systems (MDHS) concept -- called 
active flap control -- was tested in a wind tunnel at NASA's 
Langley Research Center, Hampton, Va.  The specially-built rotor 
model and test stand were designed and constructed by the MDHS 
facility at Mesa, Ariz.  The Army Joint Research Program Office at 
Langley teamed with Langley because of an interest in exploring 
performance benefits using the flap control concept.

 Costs for the $2 million project are approximately split between 
Langley and MDHS.  For the tests, the model was fitted with flaps 
on the outer 20 percent of each blade trailing edge.  The flaps 
were actively controlled in an attempt to limit the blade slap's 
sound.  On either side of the model was an array of microphones 
that moved up and down the wind tunnel test section, mapping the 
noise "footprint" under the blades.  To improve the quality of 
data gathered, the walls, ceiling and floor of the 14 x 22-foot 
Subsonic Tunnel were treated with special acoustic foam.

                              -more-



                               -2-

 Flaps on the mode were actuated by a mechanical control system.  
MDHS Project Engineer, Seth Dawson, believes that hydraulic, 
electromechanical or smart material actuators could be used on a 
production helicopter.

 Under some test conditions, blade slap noise levels were reduced 
by six decibels, equivalent to 50 percent reduction in sound 
strength.

 However, the flap's effectiveness is highly dependent upon flight 
conditions and can cause noise increases if deployed at the wrong 
point and time, according to Ruth M. Martin, Rotorcraft Program 
Manager at Langley.

 By varying test conditions, researchers were able to take data on 
use of the flaps for vibration control and for increased 
aerodynamic performance.  These results are still being analyzed.

                              -end-

EDITORS NOTE:  A photograph of the McDonnell Douglas rotor test 
model fitted with trailing edge flaps is available from the 
Langley Research Center.  Contact:  H. Keith Henry at 
804/864-6120.  Photo No. L-94-1851.