NASA CASE NO.

PRINT FIG.

E_)_T_ICE

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to

grant licenses to commercial concerns. Although NASA encourages nonexclusive

licensing to promote competition and achieve the widest possible utilization, NASA

will provide the necessary incentive to the licensee to achieve early practical

application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 143, Hampton, Virginia 23681-0001.

Serial No.: 08/238,044

05/02/94 LaRC

(NASA-Case-LAR-I4569-]) FORNARD N94-36767

SWEEP, LOW NOISE ROTOR BLADE PatenL

Application (NASA. Langley

Research Center) 8 p Unclas

G3/05 0019645
AWARDS ABSTRACT 

FORWARD SWEEP, LOW NOISE ROTOR BLADE 

The present invention relates generally to a helicopter rotor blade and more particularly 

to a forward sweep, low noise rotor blade. 

According to the present invention, a low noise rotor blade includes a straight inboard 

section, a forward-swept outboard section and an aft-swept section located between 

the straight inboard section and the forward-swept outboard section. The forwardswept outboard section measures between about 20% and 55% of the span of the rotor 

blade, and preferably measures 30% of the span of the rotor blade. The forwardsweep angle of the outboard section is between about 25" and 45", preferably, 35". 

The inboard section measures between about 25% and 50% of the span of the rotor 

blade, and preferably measures 45% of the span of the rotor blade. The aft-swept 

section measures between about 15% and 40% of the span of the rotor blade, and 

preferably measures 25% of the span of the rotor blade. The aft-sweep angle of the 

aft-swept section is between about 15" and 40°, preferably, 20". 

Novel aspects of the present invention include providing a rotor blade which reduces 

rotorcraft noise, in particular, blade-vortex interaction (BVI) noise. The present 

invention is passive in nature and is applicable to standard helicopters and tiltrotors. 

Inventor: Thomas F. Brooks 

Address: 1 16 Chisman Landing Road 

Seaford, VA 23696 

Employer: 'NASA LaRC 

Initial Evaluator: John S. Preisser 

Serial No.: 08/238,044 

Filed: 5/02/94 
LAR 14569-1 -1- PATENT APPLICATION

FORWARD SWEEP, LOW NOISE ROTOR BLADE

Origin of the Invention

5 The invention described herein was made by an employee of the

United States Government and may be manufactured and used by or for

the Government for governmental purposes without the payment of any

royalties thereon or therefor.

10 Background of the Invention

15

1___. Technical Field of the Invention

The present invention relates generally to a helicopter rotor blade

and more particularly to a forward sweep, low noise rotor blade.

2__=. Discussion of the Related Art

Blade-vortex interaction (BVl) noise is one of the most objectionable

types of helicopter noise. This impulsive blade-slap noise can be

20 particularly intense during low-speed landing approach and maneuvers.

Over the years, a number of flight and model rotor tests have examined

blade tip modification and other blade design changes to reduce this noise.

Impulsive rotor noise is generally caused by localized aerodynamic

events on the blade. At high advancing tip speeds, it is due to the

25 presence of transonic flow and associated shocks about the blade tip

region. For this, blade tip speed and thickness effects are the most

important parameters in defining the noise mechanism and thus its control.

At lower speeds, where BVl noise dominates, the number of important

parameters increases because the complexity of the rotor wake and its

30 embedded tip vortices becomes central to the problem.
LAR 14569-1 -2- PATENT APPLICATION

The important interactions on the advancing side of the rotor occur

in the first quadrant (between azimuth angles of • = O to 90 deg.) where

blades interact with tip vortices which were shed upstream. The resulting

BVl noise is most strongly directed upstream on the advancing side below

5 the rotor. Strong BVl can also occur on the retreating side in the fourth

quadrant, with noise directed below and downstream of the rotor. The

local parameters which govern the intensity and directivity of BVl noise

include the strength and core size of the tip vortex at the time of

interaction, the interaction geometry (angles and "miss distances") of the

11] blade and vortex and the interaction Mach numbers. Of course, in turn,

these parameters are determined by operational requirements, such as rotor

thrust and flight conditions, and design parameters, such as rotor speed,

number of blades, and blade section, planform, twist and tip shape.

One early BVl noise reduction approach taken was to modify the

15 blade tip vortex structure by tip shape design changes. While this

approach was unsuccessful, general lessons learned through theoretical

and experimental work were used to improve the blade designs. The

newer design rotors were generally four-bladed with lower tip speed,

thinner blade sections and swept tips. The effect was to decrease

21] thickness and high speed impulsive noises and, to a lesser extent, the BVl

noise. Much of the progress in recent years has been in areas other than

blade design. For example, flight operation procedures, such as changes in

landing speed and descent rate for fixed glide slope approach, were found

to reduce main rotor impulsive noise levels some 2 to 5 dB. Rotor tip

25 speed reduction was used successfully on a light helicopter to reduce noise

3 to 4 dB by reducing rotor speed to 90 percent of standard rpm. Another

method employed higher harmonic control (HHC) of blade pitch to reduce

BVl noise. Results have shown up to 6 dB BVl noise reduction depending

on operating condition and control phase. Although the progress of these

30 studies is impressive, the operational and active control methods may be
5

10

15

LAR 14569-1 -3- PATENT APPLICATION

difficult to apply in practice. Also, for many rotorcraft, they may be less

effective than passive blade and tip design changes.

It is accordingly an object of the present invention to provide a rotor

blade which reduces rotorcraft noise.

It is a further object of the present invention to provide a rotor blade

which reduces blade-vortex interaction (BVI) noise.

It is a further object of the present invention to provide a rotor blade

which reduces BVI noise in a passive manner.

It is a further object of the present invention to provide a rotor blade

which is applicable to standard helicopters and tiltrotors.

It is yet another object of the present invention to accomplish the

foregoing objects in a simple manner.

Additional objects and advantages of the present invention are

apparent from the drawing and specification that follow.

Summary of the Invention

A rotor blade is described which includes a straight inboard section,

a forward-swept outboard section and an aft-swept section located

20 between the straight inboard section and the forward-swept outboard

section. The forward-swept outboard section measures between about

20% and 55% of the span of the rotor blade, and preferably measures

30% of the span of the rotor blade. The forward-sweep angle of the

outboard section is between about 25 ° and 45 ° , preferably, 35 ° . The

25 inboard section measures between about 25% and 50% of the span of the

rotor blade, and preferably measures 45% of the span of the rotor blade.

The aft-swept section measures between about 15% and 40% of the span

of the rotor blade, and preferably measures 25% of the span of the rotor

blade. The aft-sweep angle of the aft-swept section is between about 15 °

30 and 40 ° , preferably, 20 ° .
LAR 14569-1

Brief Description of the Drawing

-4- PATENT APPLICATION

Fig. 1 is a top view of the rotor blade.

5 Detailed Description of the Invention

Figure 1 shows a rotor blade 10 having an inboard section 12, an

aft-swept section 14 and an outboard forward-swept section 16. In a

preferred embodiment, the inboard section 12 measures 45% of the span

10 of the blade 10, the aft-swept section 14 measures 25% of the span of the

blade 10 and the forward-swept section 1 6 measures 30% of the span of

the blade 10. The span of the blade 10 refers to the distance between the

center of the rotor hub, upon which the blade is mounted, and the blade

tip. The aft-swept section 14 is swept back at an angle 18 of 20 ° from

15 the inboard section 12. The forward swept section 16 is swept forward at

an angle 20 of 35 ° from the inboard section 12.

The actual dimensions of the straight inboard section 1 2 and the aftswept section 14 are dependent on aerodynamic and aeroelastic

calculations after establishing the spanwise extent and sweep angle 20 of

20 the forward swept section 16. Aerodynamically, the design should set the

location of the effective lift center to be close to the blade patch axis to

minimize the control loads. The lift from the aft-swept section 14 and that

portion of the forward-swept section 1 6 which is located aft of the 1/4

chord projection line should nominally balance with the outboard forward

25 blade portion. Iterations with standard aeroelastic calculations are then

required to establish structural design details.

The purpose of this rotor blade is to reduce the noise of rotorcraft,

including both standard helicopters and advanced systems such as

tiltrotors. The primary noise reduction feature is the forward sweep of the

30 planform over a large portion of the outer blade radius. The purpose of the
LAR 14569-1 -5- PATENT APPLICATION

aft-swept region is to provide a partial balance to pitching moments

produced by the outboard forward-swept portion of the blade. The aftswept section permits both an aerodynamic and dynamic force balance for

the blade along the 1/4 chord line, and its projection, of the straight

5 inboard section. This allows hub-hinge forces and moments to remain

within practical bounds. The aft section also provides the stabilizing blade

forces and moments to counteract any forward-sweep-dependant

instability which occurs during the blade's rotation. The noise source

showing maximum noise reduction is BVI noise, as the forward-swept

10 section intercepts the advancing side tip vortices at skew angles to the

blade rather than at near parallel angles, causing the BVl occurrences to

lose much of their impulsive character, thus reducing noise. High speed

thickness noise is also significantly reduced due to favorable surface

thickness gradients for this planform. Compressive noise sources are

15 reduced and high advance ratio performance is improved over standard

planforms due to the "forward-sweep" relief of the tip region compressive

effect. The forward sweep maintains this benefit into the forward rotor

quadrant on the advancing side and does not suffer from an unsweeping

effect which may occur for swept back designs due to locally parallel

20 vortex orientations in that quadrant. Cabin vibration and loading noise are

reduced due to a "load averaging" effect of the double sweep planform.

Although my invention has been illustrated and described with

reference to the preferred embodiment thereof, I wish to have it

understood that it is in no way limited to the details of such embodiment,

25 but is capable of numerous modifications for many mechanisms, and is

capable of numerous modifications within the scope of the appended

claims.
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LAR 14569-1 -8- PATENT APPLICATION

FORWARD SWEEP, LOW NOISE ROTOR BLADE

Abstract of the Disclosure

5 A forward-swept, low-noise rotor blade includes an inboard section,

an aft-swept section and a forward-swept outboard section. The rotor

blade reduces the noise of rotorcraft, including both standard helicopters

and advanced systems such as tiltrotors. The primary noise reduction

feature is the forward sweep of the planform over a large portion of the

10 outer blade radius. The rotor blade also includes an aft-swept section.

The purpose of the aft-swept region is to provide a partial balance to

pitching moments produced by the outboard forward-swept portion of the

blade. The noise source showing maximum noise reduction is blade-vortex

interaction (BVl) noise. Also reduced are thickness, noise, high speed

15 impulsive noise, cabin vibration and loading noise.
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