C90-Swept-Blade-Turbofan-01Available For All 200-Series and 90-SeriesKing Airs.
Raisbeck Engineering and Hartzell Propeller have teamed to develop and certify the first business aviation turbine propeller using practical swept-wing theory as an integral part of its design.

The effort has culminated in the FAA certification of the Raisbeck/Hartzell Swept Blade Turbofan Propeller for the King Air 200/B200/B200GT and 90 series. The new Swept Blade Propeller has been added to Raisbeck Engineering’s Airplane Flight Manuals, making it compatible with all combinations of Raisbeck performance systems for these aircraft, and carrying with it all these systems’ FAA-approved increased performance levels.

Developed jointly by Raisbeck Engineering and Hartzell Propeller for the C90GTx, C90GTi, C90GT, C90B, C90A, C90 and E90 models.

  • Stunning Ramp Presence
  • 30° Blade Sweep
  • Larger Diameter (96 ̋)
  • More Thrust With Less Noise
  • Measurably Improved FAA-Certified Performance (With The C90 Epic Performance Package) Affordable Aluminum Construction

Raisbeck C90 EPIC* Performance System with New Swept Blade Turbofan Propellers
*EPIC includes 96 ̋ Swept Blade Turbofan Propellers, Dual Aft Body Strakes, 10,500 lb Gross Weight and FAA-Approved Performance

C90GTx EPIC Performance Comparison*

*C90GTi, C90GT similar

TAKEOFF (ISA/SL) Raisbeck EPIC + C90GTx Factory C90GTx IMPROVEMENTS
Maximum Takeoff Gross Weight (MTOW) 10,500 lbs 10,485 lbs 15 lbs more
Takeoff – Flaps Up 2,150 ft 3,000 ft 850 ft less runway
Takeoff – Flaps Approach EPIC uses flaps up for all takeoffs 2,550 ft 400 ft less runway
Accelerate-Go – Flaps U 2,970 ft 4,380 ft 1,410 ft less runway
Accelerate-Go – Flaps Approach EPIC uses flaps up for all takeoffs 3,600 ft 630 ft less runway
Accelerate-Stop – Flaps Up 3,770 ft 4,200 ft 430 ft less runway
Accelerate-Stop – Flaps Approach EPIC uses flaps up for all takeoffs 3,770 ft ——–
Takeoff Climb Gradient – Flaps Up Takeoff Climb Gradient – Flaps Up 3.9% 20% greater
Takeoff Climb Gradient – Flaps Approach EPIC uses flaps up for all takeoffs 2.7% 74% greater
CLIMB Raisbeck EPIC + C90GTx Factory C90GTx IMPROVEMENTS
Single Engine Rate-of-Climb 525 fpm 495 fpm 105 fpm better
Single Engine Service Ceiling (ISA, 10,000 lbs) 24,200 ft 18,100 ft 6,100 ft higher
CRUISE Raisbeck EPIC + C90GTx Factory C90GTx IMPROVEMENTS
Cruise RPM 1,750 RPM 1,900 RPM 150 RPM less
LANDING (Flaps Down) Raisbeck EPIC + C90GTx Factory C90GTx IMPROVEMENTS
Landing Distance – No Reverse 2,120 ft 2,480 ft 360 ft less runway
Landing Distance – w/ Reverse 1,580 ft 2,370 ft 2,370 ft
OTHER Raisbeck EPIC + C90GTx Factory C90GTx IMPROVEMENTS
Propeller 96 ̋ Raisbeck/Hartzell Swept Blade Turbofans 90 ̋ Hartzell/Beech 4-Blade • Exceptional performance
• Stunning ramp presence
C90GTx EPIC Takeoff Comparison

C90GTx Epic Take-Off Comparison
EXAMPLE: An EPIC-equipped C90GTx can operate from mile-high fields as short as 2,600 ft. at its Maximum Gross Weight of 10,500 lbs.

C90B EPIC Performance Comparison
TAKEOFF (ISA/SL) Raisbeck EPIC + C90B Factory C90B IMPROVEMENTS
Maximum Takeoff Gross Weight (MTOW) 10,500 lbs 10,100 lbs 400 lbs more
Takeoff – Flaps Up 2,150 ft 2,710 ft 560 ft less runway
Accelerate-Go – Flaps Up 2,970 ft 3,650 ft 680 ft less runway
Accelerate-Stop – Flaps Up 3,770 ft 3,600 ft 170 ft more runway @ 400 lbs heavier
Takeoff Climb Gradient – Flaps Up 4.7% @ 10,500 lbs 4.8% @ 10,100 lbs Virtually the same @ 400 lbs heavier
CLIMB Raisbeck EPIC + C90B Factory C90B IMPROVEMENTS
Single Engine Rate-of-Climb 525 fpm 495 fpm 30 fpm better
Single Engine Service Ceiling (ISA, 10,000 lbs) 14,500 ft 13,600 ft 900 ft higher
CRUISE Raisbeck EPIC + C90B Factory C90B IMPROVEMENTS
Cruise RPM 1,750 RPM 1,900 RPM 150 RPM less
LANDING (Flaps Down) Raisbeck EPIC + C90B Factory C90B IMPROVEMENTS
Landing Distance – No Reverse 2,120 ft 2,290 ft 170 ft less runway
Landing Distance – w/ Reverse 1,580 ft 2,130 ft 550 ft less runway
OTHER Raisbeck EPIC + C90B Factory C90B IMPROVEMENTS
Propeller 96 ̋ Raisbeck/Hartzell Swept Blade Turbofan 96 ̋ Raisbeck/Hartzell Swept Blade Turbofan • Exceptional performance
• Stunning ramp presence
C90B EPIC MTOW Comparison

C90B EPIC MTOW Comparison
EXAMPLE: Sea Level, 3,000 ft. runway, Accel-Go, Flaps Up

TECHNICAL OVERVIEW

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.

The C90s, although slower by a bit than the 200s, turn their propellers at higher RPMs during takeoff, climb and cruise. The resulting propeller C90 tip Mach numbers are right up there with the 200. While the new C90GT-series incorporates an engine which turns slower on takeoff, the climb and cruise RPMs are the same as the earlier versions.

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 ; on the C90B at 2200 RPM, it’s .88 Mach.

These takeoff, climb and cruise conditions are encountered on almost every King Air in flight, and they push the propeller blades significantly into the transonic drag rise, the same as 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.

Typical commercial airplane quarter-chord sweep angles are the Douglas DC-8 at 32 degrees and the Boeing 737 and 757 with 25 degrees sweep. Boeing’s biggest sweep ever built into a Boeing commercial airplane is the 747 with 371⁄2 degrees of quarter chord sweep which cruises over Mach .9 when pushed.

With a jet airplane, its entire wing is at the same freestream 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 flowing 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 engine. 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. As a matter of fact, air flowing over the convex lifting blade surfaces will frequently be supersonic.

Introducing blade sweep to the blades can largely overcome these drawbacks. One can never get rid of noise, but sweep does allow an increased propeller diameter for greater performance without paying the normal noise and drag penalties.

Brief History of Wing and Blade Sweep

The next question is, if the swept wing has been around since Willy Messerschmitt put it on the ME 163 in 1943, and that work was discovered by George Schiarer from Boeing after the War in 1945 and was first employed in 1947 on the B-47 swept-wing bomber, why hasn’t anybody designed swept propeller blades until now?

Actually, there have been some successful attempts to design and build true swept propellers. The European A400M cargo plane has swept wings and swept propellers (above right). It is designed to fly at Mach .72 and airspeeds to 421 knots. The propeller has 8 blades and is very costly for general aviation consideration.

There are other examples such as the C130J (above right), but they are all very expensive and usually on military airplanes. inherently don’t qualify for markets such as the King Air.

But to the layman, it is not intuitively obvious that sweeping the blades of a propeller installed on a King Air that incorporates no wing sweep itself and flies at cruise Mach numbers well below the transonic drag rise, makes any sense.

And since no one has gone there in propeller design for airplanes like the King Air, no market has ever been developed. Market is what drives research, technology and their results—new products which satisfy a new market need.

The team of Hartzell Propeller and Raisbeck Engineering have combined for the last three decades to push back the dual boundaries of technology and market. The latest of these efforts is the Raisbeck Swept Blade Turbofan Propeller System (SBTP).

Application of Wing Sweep to the King Air Propeller

Surveying a number of recent general aviation airplane propellers, they at first appear to have swept blades. But they don’t. Several examples exist like the Hartzell Scimitar propeller.

One of the latest of these is the Composite Scimitar Propeller for the King Air 250.

The blades on these propellers have cut-back leading edges, but the trailing edges remain unswept. Quarter chord sweep (the sweep of the 25% chord) is what the air responds to and calibrates well with drag rise at higher Mach. Practical structural constraints, such as bird strikes, limit the aft sweep of composite blades, and thus their ability to achieve the positive effects of true blade sweep.

The graph below compares the quarter-chord sweeps of the blades on our Raisbeck Turbofan Power Prop (introduced in 1985) with our new Swept Blade Turbofan Propeller.
C90 Turbofan Power Prop Graph
When viewed side by side (see at right), the visual effect of the blade sweep stands out in a crowd.

Development and FAA certification Flight Testing — King Air 200 Swept Blade Props

First conforming propellers with the new blades were delivered to Raisbeck’s flight-test facilities In June 2012, following 21⁄2 years of CFD analyses and resulting studies and trade-offs. Configuration had been frozen in February 2012, and Hartzell had then begun manufacture of the pre-production blades.

During development flight testing, three different propellers were evaluated on a fully instrumented King Air B200; the current Hartzell OEM propeller for the B200GT (93” diameter); the current Raisbeck Turbofan Power Prop (94” diameter); and the new Swept Blade Turbofan Prop (96” diameter). Incremental increases in performance between these three were documented, and fell roughly as expected, with performance following increased diameter in each case.

With performance well documented in-house, Raisbeck’s engineers went forward with full FAA certification. FAA flight testing was completed in August, and all submittals were made except the new Airplane Flight Manual Supplements containing the performance. These were submitted two months later and accepted shortly thereafter.

The propeller itself was separately Type-certificated by Hartzell to add to their Type Certification Data Sheet, a necessary step toward Raisbeck certification. Details of the certified King Air performance comparisons are available in separate documents at www.raisbeck.com, Raisbeck Engineering’s website.

Manufacturing Considerations

The large sweep on the new propeller in turn required new aluminum forgings for quantity production. New forgings cost money; they are provided to Hartzell by Alcoa in this case. Long lead times for new forgings are typical.

As a result, the blades used for flight testing and FAA certification were machined from large aluminum blocks. Three complete propellers were required; two for performance, stability and control, governor pressure, takeoff, landing and taxi characteristics; the third Swept Blade Propeller was fully instrumented and put on one side only, to document stress, strain, loads, vibration and dynamic response.

Second Generation Swept Blade Propellers for the King Air C90 Family

With success of the Swept Blade Turbofan Propeller program for the King Air 200-family assured, Raisbeck engineers turned to the King Air C90, still in continuous production since 1971.

Preliminary design studies revealed that further blade sweep could result in quieter sound levels for FAA and European (EASA) noise certification. Increased sweep also allowed for a propeller diameter increase of a full 6 inches over the OEM propeller, from 90” to 96”.

With newly-earned experience and the availability of Swept Blade aluminum forgings, the C90-family program went considerably faster. A King Air E90 was used for development and certification since the PT6A-28 engine is certified for both 2200 RPM and 1900 RPM takeoff settings in one airplane. Certification flight tests were conducted during the early summer of 2013, and successfully completed on August 14th. C90 Swept Blade Turbofan Propeller production is under way at Hartzell Propeller.

What’s Next?

The engineers and marketers at Raisbeck are looking at where the new technology should be applied next, and on what model aircraft. In keeping with Raisbeck Engineering’s long-standing policy of proving our new technology before talking about it and offering it for sale, any discussion of what tomorrow brings will have to wait until tomorrow.

Download the C90 Swept Blade Turbofan Propeller Brochure