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Raisbeck/Hartzell Swept Blade Turbofan Propellers: Striking Looks And Performance

You can't miss Raisbeck/Hartzell Swept Blade Turbofan Propellers on the ramp. They look like something out of a sci-fi movie, and their performance is just as striking. Raisbeck's engineers gave us an inside view of the design process; you'll find the theory behind their design is as amazing as their looks.


Raisbeck

Performance Benefits You Can't Miss

As part of a complete EPIC package, the Swept Blade Turbofan Propellers seriously boost King Air performance. They're currently available for the King Air 90 and 200 series, and a five-bladed version's in development for the King Air 350. While we're focusing on the propellers in this article, the performance numbers only apply when you have the complete EPIC performance package installed, not just the props.

On the King Air 200, the swept props cut takeoff distance over 50 feet from 3,300' to 2,210' - dropping 1,090'. They increase sea-level twin-engine climb performance by 90 feet per minute to 2,510 at max weight. And, they reduce maximum cruise RPM from 1,900 RPM to 1600-1800 RPM. Check out the full King Air 200 performance charts here.

The swept props give the King Air C90GTx an equally impressive performance gain, which Raisbeck recently certified. The EPIC package cuts 1,260 feet off the factory standard King Air 90GTx' 3,240' takeoff distance - crossing 50 feet in only 1,980'. And, the cruise RPM drops from 1900 to 1750, reducing cabin noise. Check out the full C90GTx EPIC specs here.

The Science Behind The Looks

By using a fourth blade and increasing the diameter to 96", Raisbeck's Swept Blade Turbofan Propellers gain obvious thrust improvements over the factory standard four-blade 93" (King Air 200) or three-blade 90" (King Air 90) propellers. However, the propeller's chord, camber, twist and sweep properties are the real secret behind the massive boost in efficiency.

A propeller is simply a spinning wing, and the sweepback on Raisbeck's propeller has the same effect as sweep on a wing - it lowers the wing's local Mach number and delays high-speed drag.

Airflow moving parallel to the propeller's chord line accelerates as it crosses the blade. When a propeller rotates at high RPM (during takeoff) or in cold air (at cruise altitude, where the speed of sound is slower), air flowing over the propeller's blade moves at transonic speeds. At maximum cruise RPM on a King Air, airflow can reach speeds as high as Mach .92 at the propeller tip.

Airflow speeds at the tip are faster than those at the root because the tip and root both take the same amount of time to complete a revolution. However, the tip travels along a much longer arc. The final airflow velocity seen by the propeller is a vector sum of the cruise airspeed (the airflow generated by the aircraft's forward movement) and the propeller's tangential velocity (the airflow generated by the propeller's rotation.)

As airflow speeds up across a wing, you eventually reach the "divergence Mach number" - where drag begins to increase rapidly with an increase in airflow speed. The drag increase may stem from the formation of a normal shock wave on top of the airfoil as the air reaches supersonic speeds. Airflow separation from turbulent flow on the aft portion of the blade can also cause the jump.

By sweeping the wing, you reduce the component of airflow traveling parallel to the propeller's chord line - which reduces the airflow's acceleration. This delays the divergence Mach number, allowing you to use larger diameters and improving efficiency during high RPM or cold air (cruise) operations.

You'll notice the sweep and airfoil shape change as you move out along the propeller's span. As you move towards the tip, local Mach numbers increase. The airfoil thins to reduce drag, and the sweep increases to lower the Mach number. Raisbeck uses a peak 30 degree quarter-chord sweep on their Swept Blade Turbofan Propeller.

Ram Air Recovery

While the sweep may be the most noticeable part of the propeller's design, Raisbeck has also optimized the root. By altering the blade's shape in front of the turbine inlet, you ram more air into the engine, lowering the interstage turbine temperature (ITT) for a given amount of torque. This allows you to generate more total torque at your engine's maximum ITT, increasing thrust.

The King Air's engines are flat-rated, which means they limit their thrust to a "flat" maximum output. During takeoff, you can't use the additional thrust created by ram air recovery. However, at cruise altitude, the engines operate below their "flat" limitation. Raisbeck's ram air recovery design allows you to extract the most cruise power possible out of your engine.

Advances In Manufacturing Technology

While the benefits of swept-wing aerodynamics have long been known, the ability to practically design and manufacture a swept blade propeller is just starting to appear. Computational fluid dynamics, along with Hartzell's vast database of airfoil design parameters, helped Raisbeck's engineers estimate the thrust, drag and stress along the propeller's blade.

During the development process, Raisbeck passes their propeller design to Hartzell, who evaluates the propeller against their database of performance metrics. Hartzell identifies potential design issues, like flutter, and hands the results back to Raisbeck. This process repeats over-and-over, spanning more than three months for the King Air's swept props.

Manufacturing the Swept Blade Turbofan Propellers also poses a challenge. Hartzell mills propellers out of solid aluminum forgings. The existing forgings allow for very little sweep - especially with the increased span of the swept propeller. To accommodate the significant sweep, Raisbeck and Hartzell teamed-up to design custom forgings - a process that requires a significant investment of time and money.

Increased Power + Reduced Noise + Awesome Looks = A Great Experience

The Raisbeck/Hartzell Swept Blade Turbofan Propellers are a key part of the EPIC performance system, and by far the most striking. They increase torque, boost performance and lower noise, offering performance gains from takeoff to landing. While looks may not be everything - in this case, they come with everything. Learn more at Raisbeck's site.


Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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