Prop Tip Speeds Create Majority of Noise

You Can Help by Flying Friendly
By Syl Heumann
Do you know the diameter of your propeller? If you don?t, read on, because it has a very important effect on noise outside your aircraft. We are continually faced with noise complaints at airports, and some of them are warranted too. As the tip speed of your propeller approaches the transonic range, the noise level outside the aircraft takes a tremendous jump. The transonic range begins about 625 MPH, or Mach .85, and continues until the speed of sound which is 736 MPH (Mach 1) at sea level. This increase in noise is not apparent inside the aircraft (as outside). Beginning at approximately 625 MPH, the tip of the propeller is going fast enough to cause compression of the air and is causing at least two mini sonic booms per revelation.

No doubt you have heard aircrafts take off and noticed the noise level increases greatly just as the aircraft is opposite of your position. The reason is that the tips are at transonic speeds, and the sound is radiated on a path perpendicular to the path of the aircraft. The sound appears to drop off as soon as the aircraft has passed, but that disk of noise is following the aircraft, and will continue until the prop tip speed is reduced. It is important to understand that once the threshold of transonic speed is approached, any increase in RPM causes a dramatic increase in noise outside the aircraft.

To determine if your aircraft has a potential for excessive noise look at the chart on this page. Generally speaking, aircraft with 250 horsepower and above with two bladed props are the biggest offenders. Look closely at the chart to understand how important a 100 RPM reduction can be. A reduction of 100 RPM will usually reduce the excessive noise factor, and a reduction of 200 RPM is even better. Plan on reducing RPM as soon as you feel it is safe, or by 500? above ground level (AGL). The calculations do not include the forward speed of the aircraft because the effect is negligible.

The next time you your aircraft is in annual don?t forget to have the tachometer calibrated. A glance at the chart will show how important 100 RPM will be in reducing noise. If all pilots consider these suggestions, we will have fewer noise complaints which will make airport neighbors and pilots much happier.

Propeller Tip Speed- MPH
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Propeller

RPM’s

Length

2400

2450

2500

2550

2600

2650

2700

2750

2800

74

528

539

550

561

572

583

594

605

616

75

535

547

558

569

580

591

602

614

625

76

543

554

565

577

588

599

610

622

633

77

550

561

573

584

596

607

619

630

641

78

557

569

580

592

603

615

627

638

650

79

664

576

588

599

611

623

635

646

658

80

571

583

595

607

619

631

643

654

666

81

578

590

602

614

627

639

651

663

675

82

585

598

610

622

634

646

659

671

683

83

593

605

617

630

642

654

667

678

691

84

600

612

625

637

650

662

675

687

700

85

607

620

632

645

657

670

683

696

708

86

614

627

640

652

665

678

691

704

710

87

621

634

647

660

673

686

699

712

726

88

628

641

654

668

681

694

707

720

733

89

635

649

662

675

688

702

715

728

741