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EPIC King Air: Raisbeck's Dual Aft Body Strakes

Look closely at the rear of a King Air on the flight line, and you'll often find Raisbeck's Dual Aft Body Strakes replacing Beechcraft's single fin. This striking modification adds directional stability and cuts drag. It's a key part of Raisbeck's EPIC performance package - increasing safety, economy and dispatch reliability.

Raisbeck Engineering

The strakes primarily add directional stability. In the T-tailed King Air variants, they also raise the aircraft's yaw-damper limit altitude. And, they cut drag and reduce noise - giving you up to an extra 2 knots in cruise.

Additional Vertical Surface Area Carries A Big Stability Increase

Raisbeck's Dual Aft Body Strakes add 10% more vertical surface area to stabilize your aircraft, without increasing empty weight.

Reduced Dutch Roll At High Altitude = Higher Yaw Damper-Inoperative Altitude

The Dual Aft Body Strakes' boost in directional stability also works to limit Dutch roll. During certification, the pilot executes a rudder doublet - essentially kicking the rudder in one direction and releasing it. The aircraft begins to yaw and roll, entering a Dutch roll oscillation. The FAA requires the aircraft to stabilize at 10% of the initial oscillation amplitude within 7 cycles - without pilot intervention.

While a factory-standard King Air can easily meet the requirement at sea level, it's more of a challenge at higher altitudes. As an aircraft climbs, the aircraft's natural directional stability decreases. As the air becomes thinner, the airflow exerts less pressure against the vertical stabilizer, reducing its stabilizing effect.

If the aircraft's natural stability doesn't reduce the Dutch roll fast enough, manufacturers use a yaw damper system to reduce the oscillations. The limit isn't a concern when your yaw damper's operational. But, if it's inoperative, you need to descend below the limit altitude. That's a real issue if you're flying in high terrain or operating into high density-altitude airports.

The extra stability created by Raisbeck's Dual Aft Body Strakes significantly raises the yaw damper inoperative altitude; and in many cases, eliminates it. On the King Air 300/350 series, they'll prevent a costly diversion. On the other T-tailed King Airs, they'll save you money by keeping you at a higher - and more efficient - altitude.

Taming Vortices To Reduce Drag And Noise

The Dual Aft Body Strakes also cut drag on the King Air by up to 2 knots. At first glance, you may suspect the intersections between two strakes and the fuselage would increase interference drag over a single fin. But, in this case, the strakes keep airflow attached underneath the aft fuselage and reduce form drag.

High pressure air flowing underneath the King Air's fuselage interacts with the low pressure air flowing over the top of the wing. A vortex forms at the aft wing root and runs along the fuselage, moving and separating uncontrollably around the vertical fin. The separation causes drag and an "aft body rumble" in the back of the cabin.

The Dual Aft Body Strakes capture these vortices in-between each surface, tucking the flow closer to the fuselage. This relieves the problem of separation, or drag and, in effect, produces thrust.

In the images below, the distance between each streamline shows energy loss. The streamlines on the King Air with Dual Aft Body Strakes remain tightly grouped as they leave the fuselage. The streamlines on the King Air with the factory-standard fin are spread farther apart, which shows energy loss through drag.

What Is Coanda Effect?

Coanda effect helps explain why the vortices attach to the Dual Aft Body Strakes. Henri Coanda, a Romanian aeronautical engineer, discovered the effect while testing out what was possibly the world's first jet powered airplane in 1910. When a jet of air passes through slower moving ambient air, the two normally mix through the turbulent boundary between the jet and the free air.

However, if the jet (in this case, the vortex) borders a surface (in this case, an Aft Body Strake), its low pressure, fast-moving air sucks the jet towards the surface.

Eventually, the suction attaches the jet to the surface and the jet follows the surface, even if it curves. In the case of a King Air with Dual Aft Body Strakes, the vortices now remain attached to the aft fuselage instead of separating uncontrollably, reducing drag.

Significant Improvements With Little Aerodynamic Cost

They say nothing's free in aerodynamics; but in this case, the Dual Aft Body Strakes may be. They decrease drag and increase stability, and they don't increase empty weight over the factory-standard single fin. In aerodynamic terms, that's quite a bargain.

Joao Carlos Medau / Flickr

Next Thursday, December 18th, we'll cover Raisbeck's Swept Blade Turbofan Propellers. They may be the most striking part of the EPIC package, and their performance boost is incredible. Sign up for our email below to get notified when it releases.

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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