Control Surface Design: Keeping them Balanced

When an aircraft designer is faced with the problem of tailoring control surface hinge moments as well as the effectiveness of and force required to operate those surfaces, she or he has several available design tools. Two of the most effective are types of aerodynamic balances—the horn and the external-airfoil balance.

The horn balance is a section of the control surface at the tip that extends forward of the hinge line. It extends outboard of the tip of the fixed surface the control surface rides on, hence the term "overhanging or overhung balance."

There are two types of horn balances. The first type, shown in the introductory photo, is the exposed horn balance. In this type of balance, the leading edge of the balance area is exposed directly to the airstream and has no fixed surface ahead of it. This type of balance was popular for ailerons during World War I and for about 15 years thereafter. Simple horn-balanced ailerons, sometimes called "elephant-ear" ailerons were a feature of the Fokker Dr.I triplane and the Fokker D-VII, as well as the post-WWI Travel Air airplanes. The horn-balanced ailerons on the Travel Air gave it a superficial resemblance to the Fokker D-VII. Since Travel Airs were easier to get than war-surplus Fokkers, they appeared as "German fighters" in several movies, leading to a nickname of "Wichita Fokker" for the Travel Air.

The second type of horn balance is the shielded horn balance. This has a fixed surface ahead of the moveable balance surface. Shielded horn balances are common on the tail surfaces of modern light airplanes.

Aerodynamically, horn balances behave very much like offset-hinge-line balances (the hinge axis is placed aft the leading edge of the moveable control surface) with the exception that they have a much larger effect on the floating tendency of the surface. The exposed horn balance has a greater effect on floating tendency than the shielded balance because the leading edge of the exposed balance is directly exposed to the airstream, and can develop significant aerodynamic force in response to changes in the airplane's angle of attack. The fixed surface ahead of the balance area on the shielded balance protects the control surface leading edge from the free stream at low control deflections, reducing the balance's effect on floating tendency.

Like offset-hinge balances, horn balances will stall if the control surface is deflected too far. Unlike offset-hinge balances, the stall is not total. Only the surface near the balance horn stalls. The rest of the surface is likely to remain effective. Although the pilot will feel sudden changes in control forces and buffeting, he will probably retain control over the airplane. The effect of stall of horn balances is further reduced by the fact that the exposed balance tends to be a relatively low aspect ratio, so it can get to a higher angle of attack before stalling.

Horn balances are most commonly used on tail surfaces (horizontal and vertical) where the reduction of the floating tendency is highly desirable because of the detrimental effect elevator and rudder float have on the stability of the airplane. Horn-balanced ailerons are rarer since the floating tendency of ailerons is of less concern than the floating tendency of elevators or rudders. Since ailerons move antisymmetrically, that is one up and one down, the floating tendency of the ailerons puts symmetric loads into the control linkage to the stick, and the two ailerons balance each other out. Horn-balanced ailerons have been used in the past, particularly on early airplanes, but are no longer common.

The horn balance has two primary advantages over the offset-hinge-line balance. The first is that the horn balance reduces the floating tendency more than the offset-hinge balance while having about the same effect on the restoring tendency. This means that a horn-balanced surface will float less and therefore destabilize the airplane less than a surface which has no balance or an offset hinge line. This is particularly important for elevator design.

The second advantage of the horn balance is that it is easier to build and hinge a horn-balanced surface than to provide an offset hinge line. A horn-balanced surface can be hung on simple hinges, while a surface with an offset hinge line must have more complicated hinges that are cantilevered a significant distance away from their mounting point.

A third advantage of horn balances is that the balance area can also house the mass-balance weights used to prevent the control surface from fluttering. This is very common practice in the design of light airplane elevators and rudders.

The primary disadvantage of the horn balance is that it is vulnerable to damage. The horn is exposed at the tip of the control surface and is therefore the first thing to be hit during the seemingly inevitable ground handling mishaps that cause hangar rash. This is a problem particularly if horn-balanced ailerons are used since wingtips are the first areas to get hit. The exposed nature of horn balances also increases the possibility that the surface can be jammed by debris lodging in the fore-and aft slot between the balance and the tip of the fixed surface. The fixed surface ahead of the balance on the shielded balance greatly reduces the chances of this happening.

A second disadvantage of horn balances, particularly if they are large, is that since they are cantilevered off of the end of the control surface they can produce relatively large structural loads on the rest of the surface. The designer who uses horn-balanced controls must take this into account when designing the movable surfaces.

An external-airfoil balance is a small surface mounted to the main control surface on struts so that the aerodynamic center of the smaller surface is ahead of the hinge line of the control. This type of balance has become very common on aerobatic airplane ailerons as designers attempt to reduce the aileron forces to improve roll performance.

The most common design is a flat metal plate suspended ahead of the aileron on a single supporting tube or strut. This type of balance is sometimes called a paddle balance, but are usually referred to as "spades" because of their resemblance to small shovels.

The spade balance affects the control surface hinge moments very much like the simple, unshielded horn balance. While spade stall will cause an increase in hinge moment, it will not significantly affect the control power of the main control surface it is attached to.

The primary advantage of the spade balance is that it is easy to modify to tailor the control surface's hinge moment to the desired level. Both the size and planform of the spade affect how it changes hinge moment over the full range of control surface deflection. Modifying spades is one of the primary tools used to fine-tune competition aerobatic airplane roll performance.

The primary disadvantages of spade balances are their vulnerability and the drag that they produce. Because spades are exposed, they are relatively easy to damage in ground handling. This is offset by the fact that they are easy to replace or repair.

Spades add wetted area to the airplane, and the struts supporting the spade can produce significant drag. Because of this, spades are rarely seen on airplanes which are designed for fast cross-country performance. They are, however, quite common on aerobatic airplanes where proper tailoring of the aileron forces is much more important to good performance of the airplane mission than a small reduction of drag.

A secondary advantage of spade balances is that the struts supporting the spade can be used to hold mass-balancing weights to prevent aileron flutter.

When designing an airplane with balanced controls, proceed with caution. In general it is more dangerous to have control forces which are too light than too heavy. An airplane with heavy controls may not be particularly pleasant to fly, but unless the forces are so heavy that the pilot cannot move the controls, it will be safe. An airplane with excessively light controls will be prone to pilot-induced oscillations, and it will be easy for the pilot to exceed the airplane's structural limits by an abrupt control input.

Another danger that must not be overlooked is control-surface instability, sometimes known as overbalance. If the aerodynamic balance is too large, or behaves nonlinearly, the control surface may have an unstable restoring tendency. This means it will tend to go hard over when deflected. If this happens, the pilot must fight the control to keep it in the neutral position. When this happens on ailerons it is known as "aileron snatch". Control-surface overbalance is extremely dangerous and has been responsible for several fatal first-flight accidents. An airplane with overbalanced controls is, at best, difficult to fly and may prove to be uncontrollable.

Before deciding to balance the controls aerodynamically, the designer should have some idea of what the control forces on an unbalanced control surface would be. In many cases, aerodynamic balance is unnecessary. In situations where control forces will be too high, or floating tendency excessive, aerodynamic balancing offers a way to get satisfactory characteristics.

Barnaby Wainfan is a principal aerodynamics engineer for Northrop Grumman's Advanced Design organization. A private pilot with single engine and glider ratings, Barnaby has been involved in the design of unconventional airplanes including canards, joined wings, flying wings and some too strange to fall into any known category.

This article originally appeared in the January 2016 issue ofKitplanesmagazine.

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