Wind Tunnel

BY BARNABY WAINFAN

It’s all about the

angles

.

n a way, the aerodynamics of an airplane are all about angles. Several angles are critical in determining the aerodynamic forces acting on the airplane. This month, we’ll take a look at a few of these.

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Horizontal Reference and Angle of Attack The horizontal reference line (HRL) of an airplane is a line that is used to define the nominal “horizontal” attitude of the airplane. Usually, the HRL is as close as possible to being parallel to the center of the side view of the fuselage. For example, on an airliner that has a cylindrical fuselage, the HRL is the centerline of the cylindrical portion of the fuselage. On a more complex shape, the HRL is a bit more arbitrary, but we try to keep it representative of the centroid of the fuselage. The HRL is used as a reference for many other angles. This is important because airplanes are sensitive to angular dimensions, and small changes in critical angles can make big differences in the airplane’s flying characteristics. The angle of attack of the complete airplane is defined relative to the HRL. Angle of attack is the angle between the approaching airflow and the HRL, as measured in the side view. The angle of attack is positive when the airflow is coming at the airplane from below and rising relative to the HRL as we move aft. For some major components of the airplane such as the wing, tail and fuselage, we also define component

angles of attack or local angles of attack. These are defined as the angle between the relative airflow and a locally defined reference line. For example, the angle of attack of the wing is defined relative to the chord line of the root section of the wing, and similarly, the angle of attack of the tail is defined relative to the root chord of the tail plane.

Incidence and Decalage There are other angles that significantly affect the longitudinal characteristics of the airplane. Unlike angle of attack, which is measured between the airplane and the airflow, these angles are strictly functions of the geometry of the airplane itself. The incidence, or incidence angle, of a flying surface is the angle between the chord line of the root of the flying surface and the HRL of the fuselage. If the leading edge is above the trailing edge when measured relative to the HRL, the incidence of the surface is positive. The difference in incidence angle between the wing and the horizontal tail is referred to as decalage. The term is also used to describe the incidence difference between the wings of a biplane. If the tail is at the same incidence angle as the wing, the decalage is zero. Decalage is positive if the wing incidence is more leading-edge-up than the tail incidence. On a biplane with staggered wings, decalage is positive if the forward wing has more positive incidence than the rear wing. The decalage between the wing

and tail affects the pitch trim of the airplane. Ideally, the decalage is set so that the airplane flies with zero elevator deflection (relative to the fixed portion of the tail plane) in cruise. Typically, conventional aft-tail airplanes fly with slight positive decalage. In this condition, the tail plane is slightly leadingedge-down relative to the wing. Since most airplanes need to fly with some download on the tail to trim properly, the decalage causes the fixed portion of the tail plane to generate this trimming load so the elevator can remain in trail to minimize drag.

Wing Incidence Effects For an airplane flying at a given airspeed, the lift is determined primarily by the angle of attack of the wing. To maintain steady-state level flight, lift must equal weight. Thus, the angle of attack of the wing is uniquely defined for a given gross weight, airspeed and altitude. The attitude, and hence angle of attack of the fuselage, is determined by the combination of the wing angle of attack required to generate the lift to maintain level flight and the incidence of the wing. Positive wing incidence rotates the wing nose-up relative to the fuselage and conversely rotates the fuselage nose-down relative to the wing. Since the wing angle of attack in level flight at a given flight condition (airspeed, weight and altitude) is fixed, increasing wing incidence causes the fuselage to fly more nose-down, and decreasing incidence causes the fuselage to fly more nose-up. We can use wing incidence to minimize cruise drag by orienting the fuselage properly. Ideally the fuselage should fly at nearly zero angle of attack and zero lift. Because of the short effective span of a typical fuselage, fuselage lift generates a lot of induced drag per pound of lift. It is better to fly the fuselage at zero lift and let the wing, which is a much more efficient lifting surface, K I T P L A N E S

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Wind Tunnel CONTINUED

carry the lift. Accordingly, a small positive incidence in the wing can reduce drag by allowing the fuselage to fly level at cruise. From a drag point of view, the worst case is to fly the fuselage too nosedown, causing it to generate negative (downward) lift. This can happen if the wing has too much camber or positive incidence for the cruise flight condition. While designers rarely make this mistake on an initial design, the situation can arise if the airplane flies significantly faster than originally intended. If, for example, we significantly increase power, the airplane will fly faster. At this new, higher cruise speed, the angle of attack of the wing flies as to generate lift equal to weight, and it is lower than before. The whole airplane now cruises at a lower angle of attack, and the fuselage (assuming it was level at the original cruise speed) is now flying nose-down. Reducing the incidence of the wing can cure this and reduce drag. In practice, modifiers rarely alter wing incidence because it is difficult. The wing structural carry-though attachments to the fuselage must be modified, and this is a major undertaking with serious safety of flight implications.

Takeoff and Landing Incidence also affects takeoff and landing characteristics. For a given fuselage attitude, increasing wing incidence increases the wing’s angle of attack, and hence its lift. During the takeoff roll, some positive wing angle of attack is beneficial for two reasons. First, the lift of the wing reduces the load on the tires as the airplane accelerates, reducing rolling friction and shortening the takeoff roll. Second, the reduced load on the wheels reduces the amount of tail power required to raise the nosewheel and rotate for takeoff. Positive wing incidence also reduces the angle of rotation required to lift off. This reduced rotation angle allows the main landing gear to be shorter, and hence lighter and lower drag. 66 K I T P L A N E S

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It is possible to overdo it and put too much incidence in the wing, even for takeoff. In the extreme case, excessive incidence could stall the wing during ground roll. If the wing is unstalled but the incidence is higher than optimum, then the aerodynamic drag of the wing during the takeoff roll can be greater than the reduction in rolling friction caused by the wing lift.

No-Rotation Takeoffs If we put enough positive incidence in the wing and/or set the airplane nose-up on the landing gear enough, it is possible to take off without first rotating and lifting the nosewheel. This sounds simple and desirable: just accelerate down the runway until you reach liftoff speed, and the airplane will levitate. Unfortunately, no-rotation takeoffs are tricky and dangerous in practice because the airplane might not be in trim as it lifts off. When an airplane rotates for takeoff, the pilot is actively controlling the attitude and pitch rate. When the airplane lifts off, it is essentially in trim and at a low or zero pitch rate. This is not always true for a norotation takeoff. During a no-rotation takeoff, the pilot is not controlling the angle of attack of the airplane actively. Rather, the attitude is controlled by the landing gear interacting with the runway surface. When the airplane lifts off, this interaction disappears, and the airplane is now controlled entirely by aerodynamic forces. If the elevator is not in the right position, the airplane could be drastically out of trim and it could pitch rapidly either up or down— neither of which is a desirable situation close to the ground.

Approach and Landing As we have just seen, the attitude of the fuselage is a function of both the angle of attack and the wing incidence. On landing approach, the airplane is flying slowly and is hence at a relatively high angle of attack. With

the airplane flying in a nose-up attitude, the pilot may have difficulty seeing the runway over the nose. Overnose vision is a particularly acute issue for carrier-based Navy aircraft. Increasing wing incidence can help this situation by reducing the nose-up attitude of the fuselage with the wing flying at the approach angle of attack. Just as we saw with takeoff, this can be overdone. If the fuselage is too nose-down, there is a significant risk of a nosewheel-first landing. It is vital that the nosewheel be significantly above the ground when the mains touch in a normal landing to avoid a dangerous pitch-up caused by a premature nosegear strike. Three-point landings are fine for taildraggers, but they can spell disaster for a trigear aircraft. I speak from experience on this one. Some years ago I was involved in a UAV program where the vehicle had so much wing incidence that it took off and landed in a flat attitude. Two aircraft were built, and both were eventually seriously damaged in landing accidents caused by nosewheelfirst touchdowns.

Setting Incidence The proper wing incidence is, like all things in airplane design, a compromise. In general, it is a good idea to compromise on the side of good cruise performance, since the other considerations we have discussed can be at least partially addressed other ways. One extreme solution is to make the incidence of the wing variable in flight. While it is unlikely this will ever be desirable in a light airplane, at least one operation fighter—the Vought F-8U Crusader—had a variable-incidence wing to meet the requirements of carrier launch and recovery but retain its good high-speed characteristics.

Aerodynamic questions of a general nature should be sent to [email protected]. Use “Wind Tunnel” as the subject line. W W W . K I T P L A N E S . C O M