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Sunday, April 1, 2012

Tail Feathers and the Gurney Flap

By Frank Lombardi

Traveling the floor at the 2012 Heli-Expo, I had to make a second and third visit to many a helicopter on display in order to absorb more of what my eyes were taking in, beyond the glitz of the latest gee-whiz cockpits. Many waited for their turn to sit in the driver’s seat. I slowly circled each aircraft from tip to tail, trying to pick up on as many ingenious design points as I could. One of the things that I noticed, was that no matter what degree of automation may have been in the cockpit, the old tried and true engineering fixes to aerodynamic issues were still just as prevalent today as ever. Such is the case of the Gurney Flap.

A Gurney Flap “bends” the airflow around a surface in such a way that the surface acts as if it were a different size or shape.
The Gurney Flap was named after racecar driver Dan Gurney, who devised it to increase the aerodynamic downforce helping a racecar hold the road. It’s nothing more than a small tab, angled at 90 degrees to an airfoil and mounted at its trailing edge (see Figure 1). Although seemingly insignificant, its effect can be great. A Gurney Flap simply “bends” the airflow around a surface in such a way that it makes the surface act as if it were a different size or shape. This of course changes the aerodynamic characteristics of that surface, but without the need for a complete structural redesign; something manufacturers like to avoid.

When a Gurney is placed on only one side of a surface, the flow acts as if a flap or an aileron was deployed along that side. When installed on both sides of a surface to form a “T,” it tricks the airflow into thinking the airfoil is longer to travel over. A larger surface area equates to more lift production. More on that in a minute.

Designers beware. A Gurney Flap can add few percent increase in drag, depending on how tall it stands. The height is usually kept within 1-3 percent of the length of the airfoil’s chord. Keeping it short allows it to remain in the boundary layer (the layer of air very close to the airfoil surface that is slowed due to viscosity/skin friction). Here, its drag will be negligible.

Due to the craziness that is helicopter aerodynamics, it can be difficult to explain why Gurney Flaps end up on certain designs without knowing a bit of history. However there are some common reasons they are used.

When it’s discovered that the aircraft doesn’t fly at the optimum cruise angle, adding a Gurney Flap to the horizontal tail will change this “balance” angle for the same airspeed. It’s like changing the angle at which the horizontal tail was mounted to the aircraft, but much easier to do.

The tail surfaces of a helicopter provide stability. They provide a balance force to resist a change in trim, and a restoring moment to help bring us back to trim. To maintain stability, it is important that the airflow remains attached to the tail without stalling. Because helicopters climb at an almost level fuselage attitude, the negative angle at which the air hits the horizontal tail is very high and can approach stall (that’s why most horizontals are cambered, and mounted “upside down”). Attaching a Gurney Flap to part or all of the upper surface can delay the stall even more when flight testing discovers the initial design is not enough, or when it’s found that the left side of the horizontal sees a different flow than the right.

Thick airfoils that end in pointy trailing edges also tend to stall from side to side at shallow angles as the air tries to rejoin at the trailing edge, creating a directional instability. Adding a “double” Gurney gives the flow a defined separation point.

Lastly, as increases in gross weight or CG are made with subsequent model variants, a once-adequately-sized tail can now prove to be too small to maintain proper stability margins. Adding a double Gurney can generate the larger lift forces required to regain stability without redesigning a new tail.

In short, a Gurney Flap is a quick and easy fix to solve minor problems in flight test. They can cost you some drag, but they are as simple and reliable as it gets. Walk around your own aircraft and see what you find!

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