Alleviation of Helicopter Tail Boom Loads Through Passive Venting
Patent Applied For
Objective
The National Aeronautics and Space Administration seeks companies to license its Alleviation of Helicopter Tail Boom Loads Through Passive Venting technology (patent applied for). The passive venting technology was developed by NASA to counteract adverse effects of sideward and downward loading on helicopter tail booms caused by the complex flow fields from the main rotor and tail rotor wakes, the forward fuselage wake and the free air stream. Mitigating these adverse effects results in improved helicopter control and a reduction of required engine power.
Product Profile
The tail boom of a single rotor helicopter is subject to a complex flow field generated by the main and tail rotor wakes, the free airstream flow, and the wake from the forward fuselage. Hover and sideward flight present the most critical flight regimes in terms of adverse side and down loads on the tail boom. To overcome this adverse loading, additional engine power is required in many flight conditions. This in turn results in reduced payload, performance, and available yaw control margins. In practice, a good tail boom design, from an aerodynamics standpoint, should minimize the down load and the adverse fuselage yawing moments. Some tail boom designs actively control the circulation around them to minimize adverse forces. Such active control systems must be driven, in part, from engine power. Other designs attempt to disrupt the adverse circulation about the boom by using stakes or spoilers.
In recent research conducted by NASA for fixed-wing aircraft, a porous surface was used to alter surface pressure distribution. NASA researchers then postulated that, by passively venting portions of a helicopter tail boom (generally representative of a blunt body shape), the pressure distribution, and therefore, the loading on the boom could be modified in a favorable way. Various venting schemes are possible and they can potentially be used in conjunction with other methods, such as tail boom strakes. Some of the venting schemes include the use of porous material on all or parts of the boom connected to a specific plenum or open to the boom cavity, and the use of doors, grilles, slots, or other openings. Furthermore, it may be useful to capture a portion of the relatively high velocity, high pressure downwash from the main rotor as it impacts the upper surface of the boom and channel that flow to another, low pressure area on the boom.
Benefits
- Alleviates some of the adverse side forces generated in hover
- Reduces downward tail boom loads
- Reduces engine power required to control helicopter
- Increases performance, payload, and yaw control margins
- Allows the pilot better control for increased safety
Potential Commercial Uses
NASA's Alleviation of Helicopter Tail Boom Loads Through Passive Venting
technology is suitable for the following commercial applications:
Technical Basics
NASA conducted a wind-tunnel investigation to determine the effectiveness of passively venting a helicopter tail boom model to reduce side force and down load during simulated hover and sideward flight conditions. A two-dimensional tail boom model with two different cross-sectional shapes was used. One was an oval cross-sectional shape representative of a UH-60 tail boom, and the other was a trapezoidal cross section representative of a low-observable tail boom. The models were 1/2-scale cross sections of the mid boom. The shape that was tested most extensively was that of the UH-60. Various venting schemes were investigated with a porous skin. The porous skin was partially covered in some cases to simulate venting in various locations as well as the completely solid (totally covered) and completely porous cases. Some of the venting schemes were also investigated in conjunction with single and double strakes. The aerodynamic forces, including down load and side force, were measured at freestream dynamic pressures up to 30psf and full-scale Reynolds numbers, and at angles of wind incidence from -90 to +90 degrees to simulate left and right crosswinds. Calculations were made using the normal-force and side-force coefficients, to determine the approximate trends of the aerodynamic forces on the boom of a full-scale UH-60 helicopter and to estimate the main-rotor and tail-rotor power needed to trim these forces in the presence and absence of venting.
The test results indicate that passive venting can reduce some of the adverse side forces created on the tail boom at conditions representative of hover and sideward flight. Most of the venting schemes created additional down load at low incidence angles. However it was found that for the same loading, it takes more power to overcome a side load than a down load because the tail rotor is much more heavily loaded and less efficient than is the main rotor. Thus the penalty in down load was found to be less than the beneficial reduction in side load. Overall, the results indicated reductions in the adverse forces and the calculated power demand, similar to reductions achieved by the application of strakes.
The down load penalty may be attributable to the additional skin friction created by the porous skin used. By using a porous skin more optimally designed for this purpose or by employing venting schemes that do not use a porous surface, it may be possible to gain the benefits of venting in side force benefits with minimal increases in down load.
Technology Commercialization Status
NASA Dryden currently seeks to license the Alleviation of Helicopter Tail Boom Loads Through Passive Venting technology to U.S. companies interested in developing commercial applications. NASA has applied for a patent on this technology.
Contact
NASA Far West RTTC
3716 S. Hope St #200
Los Angeles, CA 90007
Phone: (213) 743-2927
Fax: (213) 746-9043
Email: zeller@usc.edu
Licensing opportunities are available.