Fluid reaction surfaces (i.e. – impellers) – Specific blade structure – Tined or irregular periphery
Reexamination Certificate
1997-09-08
2001-05-15
Lopez, F. Daniel (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Specific blade structure
Tined or irregular periphery
C416S235000, C416S237000
Reexamination Certificate
active
06231308
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rotor blade for rotary wing aircraft such as a helicopter, and particularly to a rotor blade for rotary wing aircraft having a special blade tip planform shape.
2. Description of the Related Art
FIG. 8
is a view showing rotor aerodynamic environment of a helicopter in forward flight. As shown in
FIG. 8A
, when a helicopter
1
flying at forward speed V with a rotor having radius R which rotates at angular speed &OHgr;, the relative airspeed varies significantly between an advancing blade where the angular speed &OHgr;R of the rotor is added to the forward speed V and a retreating blade where the forward speed V is subtracted from the angular speed &OHgr;R of the rotor.
At a position where azimuth angle &PSgr; (angle measured counterclockwise from the rearward direction of the helicopter
1
) equals to 90°, the airspeed of the advancing blade reaches a maximum and the airspeed of the blade tip becomes &OHgr;R+V. At a position of azimuth angle &PSgr;=270°, on the other hand, the airspeed of the retreating blade reaches a minimum and the airspeed of the blade tip becomes &OHgr;R−V. The airspeed of an intermediate portion of the blade takes a value obtained by proportional distribution of &OHgr;R+V and &OHgr;R−V. For example, when &OHgr;R=795 km/h and V=278 km/h are assumed, the airspeed at a position of about 35% from the root end of the retreating blade becomes zero, as shown in FIG.
8
A.
When a helicopter flies at high speed, in particular, the airspeed at a tip of an advancing blade reaches a transonic speed resulting in a strong shock wave. A noise generated by the strong shock wave is called high-speed impulsive noise. A phenomenon called delocalization in an ultrasonic region takes place at this time in a coordinate system viewed from the rotor blade which is in rotational motion. The shock wave generated is transmitted through the delocalized ultrasonic region over a great distance, making a high noise to be heard at a distance.
Since the airspeed of a retreating blade is significantly lowered, the angle of attack a of the blade must be greater in order to produce a lift similar to that of the advancing blade, and it is common to use a cyclic pitch control wherein the pitch angle of the blade is controlled in accordance to the azimuth angle &PSgr;. While the pitch angle of the blade is controlled by means of sine wave of which amplitude is minimum at azimuth angle &PSgr;=90° and maximum at azimuth angle &PSgr;=270°, the angle of attack &agr; of the blade in this case varies in the direction of span as shown in
FIG. 8B
due to flapping of the blade itself. For example, when &PSgr;=90°, the angle of attack &agr; of the blade becomes about 0° at the root end and about 4° at the tip end. When &PSgr;=270°, the angle of attack &agr; of the blade becomes about 0° at the root end and about 16 to 18° at the tip end, thus exceeding the stalling angle. When the angle of attack &agr; of the blade exceeds the stalling angle, lift coefficient Cl and pitching moment coefficient Cm change rapidly, causing to violent vibration of the helicopter structure and a high fatigue load being applied to the pitch link.
Design items used for evaluating the characteristics of an advancing blade include high-speed impulsive noise and those for evaluating a retreating blade include maximum lift coefficient Clmax and stalling angle. The maximum lift coefficient Clmax is defined as the maximum value of lift coefficient when the angle of attack &agr; of a blade having a particular aerofoil section is just before the stalling angle. A blade is considered to be better blade when the high-speed impulsive noise and the absolute value of pitching moment coefficient Cm are smaller, and the values of the maximum lift coefficient Clmax and stalling angle are greater. Applying the blade tip portion with a sweptback angle is an example of reducing high-speed impulsive noises. The blade tip with the sweptback angle mitigates the shock wave and somewhat decreases the noises, though the delocalized supersonic region itself remains and the noise is still at a significant level. Moreover, in the case a large sweptback angle is given to the blade tip, blade tip stalling occurs at a smaller angle of attack, resulting in rapid change in the pitching moment coefficient Cm and a decrease in the maximum lift coefficient Clmax.
SUMMARY OF THE INVENTION
An objective of the invention is to provide a rotor blade for rotary wing aircraft capable of eliminating delocalization in a supersonic region and reducing high-speed impulsive noises.
Another objective of the invention is to provide a rotor blade for rotary wing aircraft capable of increasing a stalling angle to provide a good flight performance.
The invention provides a rotor blade for rotary wing aircraft comprising:
a root end portion attached to a rotor head for rotationally driving,
a central portion having aerodynamic characteristics depending on leading and trailing edges linearly extending in parallel from the root end and a chord dimension therebetween, and
a blade tip portion having a planform shape defined by a first leading edge extending forwardly as a distance from an outboard end of the leading edge of the central portion outwardly increases, a second leading edge which is swept rearwardly as the distance from the outboard end of the first leading edge outwardly increases, a side edge and a trailing edge,
wherein distance R1 from the center of rotation of the rotor to outboard end point P of the first leading edge normalized by the blade length satisfies the following conditional relationship (1):
0.88≦R1≦0.92 (1)
According to the invention, delocalization in the supersonic region can be eliminated and the high-speed impulsive noises can be greatly reduced. With a rectangular blade or a tapered blade of the prior art, a region where the speed of the air exceeds the speed of sound is generated at the forward area of the blade tip portion viewed from a rotating blade coordinate system, and moreover a steep air speed gradient in the supersonic region likely to cause large shock waves and the supersonic region tends to extend further and delocalized, with a result of high noises being easily transmitted over a great distance. According to the invention, in contrast, such an extension of the leading edge is provided as an apex of the extension or outboard end point P is located at 0.88 to 0.92 so that shock wave occurring at this position is mitigated and delocalization is eliminated, and therefore the high-speed impulsive noises can be reduced.
The invention also provides a rotor blade for rotary wing aircraft comprising:
a root end portion attached to a rotor head for rotationally driving,
a central portion having aerodynamic characteristics depending on leading and trailing edges linearly extending in parallel from the root end and a chord dimension therebetween, and
a blade tip portion having a planform shape defined by a first leading edge extending forwardly as a distance from an outboard end of the leading edge of the central portion outwardly increases, a second leading edge which is swept rearwardly as the distance from the outboard end of the first leading edge outwardly increases, a side edge and a trailing edge,
wherein the trailing edge comprises a first trailing edge extending forwardly as a distance from the outboard end of the trailing edge of the central portion outwardly increases and a second trailing edge which is swept rearwardly increasingly as the distance from the outboard end of the first trailing edge outwardly increases.
According to the invention, providing the forward extension on the leading edge of the blade tip portion increases the stalling angle and the maximum lift coefficient Clmax, and making a forward extension in the trailing edge in correspondence to the forward extension on the leading edge makes it possible to have chord dimension of the blade t
Kondo Natsuki
Tsujiuchi Tomoka
Yamakawa Eiichi
Advanced Technology Institute of Commuter-Helicopter, Ltd.
Baker & Botts L.L.P.
Lopez F. Daniel
Nguyen Ninh
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