Raisbeck Engineering Swept Blade Turbofan Propellers for King Air
Available for all 200/B200 and 90 series King Airs. Installation by Cutter Aviation Technical Services.
BENEFITS FOR 200 PROPS
- Quieter cruise and cabin
- Needs 1,090ft less runway
- Premium alternate to power props
- Stunning ramp presence
BENEFITS FOR 90 PROPS
- Stunning ramp presence
- More thrust with less noise
- Measurably improved FAA-certified performance
- Affordable aluminum construction
- Increased takeoff (10,500 lbs.) and landing (9,700 lbs.) weights
- Great payload/fuel/range capability from shorter runways and high-hot scenarios
WHY SWEEP THE PROPELLER BLADES?
The newer King Air 200s can cruise as fast as .52 Mach (MMO) at 28,000 feet. At a propeller RPM of 1800, the propeller tip Mach number is over .9 at cruise.
This same high-Mach phenomenon is also very much present during takeoff at low forward airspeeds but higher prop RPM. As an example, at 120 Knots during initial climb at 2000 RPM, the propeller tip Mach is an astonishingly high .8.
These takeoff, climb and cruise conditions are encountered on almost every King Air 200 flight, and they push the propeller blades significantly into the transonic drag rise for airfoils and unswept wings.
As a comparative example, commercial airliners and business jets typically fly around Mach .79 to .82, and some of them are pushing .90 (747) and even as high as .92 (Gulfstream 650 and Cessna Citation Ten). The wing sweep on these airplanes varies from 30 to 40 degrees. All one has to do is look at the top view of any of these aircraft to see how dramatic the sweep is (see figures above right).
Typical commercial airplane quarter-chord sweep angles are the Douglas DC-8 at 32 degrees and the Boeing 757 with 25 degrees. Boeing’s biggest sweep ever built into a Boeing commercial airplane is the 747 with 37½ degrees of quarter cord sweep which cruises over Mach .9 when pushed.
With a jet airplane, its entire wing is at the same Mach number. However, with a propeller blade, the farther out on its diameter, the higher the Mach number. Adding additional diameter to a propeller adds to its tip Mach number, which in turn adds unwanted additional transonic drag and noise. This of course detracts from the other desirable performance increases resulting from such an increase in diameter.
Now, air over an airfoil doesn’t know if that airfoil is part of a wing going straight through the air, or a propeller blade being whirled around in a circle by its propeller hub. The air reacts the same to increasing Mach number. Merely adding propeller diameter doesn’t necessarily add proportionate performance improvement and it can be measurably noisier because of high Mach effects at the outer parts of the blades.
Introducing blade sweep to the blades can largely overcome these drawbacks. You are never going to get rid of noise, but blade sweep does allow you to increase diameter to increase performance without paying the normal penalties.