Aerodynamically, an airplane is more than the sum of its parts. As the air flows over a component of the airplane, the airflow is altered. This affects the flow over other parts of the airplane. These effects vary in importance. For some configurations, the effect is small, and for others it is very large.
The importance of interference effects was first noticed as designers attempted to predict the drag of their creations. In the world before computational fluid dynamics, aerodynamic predictions were made with a combination of first-principles theory and experimental data. NACA and similar research organizations developed large bodies of experimental data on airfoils, wings, fuselages, etc.
For many years designers believed that they could predict the total drag of an airplane by simply measuring (or calculating) the drag of each one of its components (wing, fuselage, tail, landing gear, etc.) separately and then adding up the drag of the components. Unfortunately, it turned out that this method was not particularly accurate. Sometimes the drag it predicted was close to the actual drag of the airplane and sometimes it was not. The reason for this is that the “summation” method of drag estimation ignores the aerodynamic interaction between the various parts of the airplane.
When the wings, fuselage, tail, landing gear, and other components are assembled to form an airplane, the airflow over each component is altered somewhat by the presence of the other components in the airstream. Usually these interactions have undesirable effects on drag.
The drag caused by the interactions of the component airflows is called interference drag, and it can be a substantial portion of the drag of an airplane.
A Critical Junction
One of the most important areas of aerodynamic interference on an airplane is the junction between the root of the wing and the side of the fuselage. A poorly designed junction can cause a large increase in drag and a reduction in maximum lift, which increases stall speed. The drag effect can be particularly severe in climb and economy cruise, where the lift coefficient is high.
In general, junctions on the upper surface of the wing are more critical than lower-surface junctions. The local airflow is moving faster on the top of the wing than on the bottom. The flow over the upper surface of the wing is much more sensitive to interference that might cause premature separation or stall than the flow over the lower surface.
Better wing-to-fuselage junctions are an argument in favor of high-wing airplanes. On most high-wing configurations, the upper surface of the wing flows smoothly into the upper skin of the fuselage. The fuselage side does not cut the upper surface of the wing, and hence does not interfere with the flow over it.
The wing/fuselage intersection on most high-wing airplanes affects only the less-critical lower surface of the wing, provided the windshield and aft fuselage are properly shaped to guide the flow onto and off of the top of the wing.
Although it is more difficult to achieve, with proper design of the fuselage and wing-to-fuselage junction fairing, low-wing airplanes can have interference drag that is no higher than that of high-wing airplanes.
Because the airflow over the upper surface of the wing is so sensitive, the design of the wing-to-fuselage junction area requires careful attention. One popular myth, which appears over and over in the popular aviation press, is that if the angle between the wing surface and the fuselage side, in the front view, is 90 degrees or more, the junction does not require a fillet or any form of fairing. This is most definitely not true. There are many airplanes with 90-degree or near-90-degree wing junctions that could benefit significantly from the addition of a root fairing.
A rule of thumb that is useful is that if the junction occurs at an angle of less than 90 degrees then a fairing is usually necessary. However, this does not mean that a fairing is unnecessary if the junction angle is greater than 90 degrees.
View from the Top
The plan (top) view shape of the fuselage in the area of the wing root has a large effect on the flow over the wing root. On many light airplanes the fuselage sides begin to pull inward rapidly immediately aft of the cabin. Unfortunately, this occurs ahead of the trailing edge of the wing in an area where the surface of the wing is sloping downward and also pulling away from the direction of the airflow. The air may not be able to follow the changing slopes of both the wing and fuselage at the same time in the area of the junction. The flow may separate and cause drag and a premature stall.
Since most wings stall from the root first, a poor root junction design can cause a portion of the wing root to stall at climb angle of attack. This premature root stall will hurt the rate of climb and increase the stall speed of the airplane. This situation is illustrated Figure 1. The separated zone can be eliminated by the use of a proper root fillet, unless the contour of the aft fuselage is too extreme.
Another solution to the junction problem is shown in Figure 2. If the fuselage has its widest point at or near the trailing edge of the wing, then premature flow separation at the wing root is much less likely. The Thorp T-18 and the Questair Venture both use this type of fuselage shaping to minimize separation at the wing-root junction.
Figure 2: The position of the widest point of the fuselage greatly affects flow separation at the wing-root junction.
Even in a situation where major separation is not expected, some filleting is still a good idea. Air flowing in a corner between two surfaces is slowed down by skin friction with both surfaces. This causes the boundary layer to thicken and generate more drag. It also may produce early separation. Radiusing the corner with a fillet accomplishes two things. First, the wetted area in the area of the corner is reduced. Second, the air will not have to rub against both surfaces forming the corner and will accordingly produce less skin friction drag. Fillets are particularly effective in areas where surfaces meet at angles significantly less than 90 degrees.
One common example of this situation is at the wing root of a low-wing airplane with a rounded fuselage cross-section and/or dihedral in the wing. With no fillet, the air must flow through a narrow channel where it is rubbed on by the skin on both sides. Adding the fillet will clean up the flow in the corner area and reduce drag.
Proper design of the fuselage can minimize the interference drag caused by the wing root junction and reduce the size of the root fairing required. A high-wing configuration with some dihedral is an example of a low interference drag design. The upper surface of the wing is uninterrupted and the lower surface joins the fuselage at a large angle, thus minimizing the undesirable corner effects on the airflow.
Here are a few general rules for designing wing root junctions with low interference drag:
1. Keep the included angle between the wing surfaces and the fuselage as large as possible. This is particularly important on the wing upper surface.
2. The sides of the fuselage should not pull in sharply in the top view in the area of the wing-to-fuselage junction. This is illustrated in Figure 1.
Without the fillet, the junction is likely to exhibit separation in the shaded areas at relatively low angles of attack. The airflow may be separated during climb and slow cruise. This early separation will hurt rate of climb and increase stall speed. If other design considerations force the fuselage to have this type of shape, then a large fillet should be used to eliminate the root flow separation.
If the fuselage has a constant or slightly increasing width in the area of the wing, the chances of early flow separation at the root are reduced. A fillet may still be required, but it can be smaller and the penalty for not filleting this type of wing-root configuration will be much lower than it would be for a configuration where the fuselage is pulling in sharply over the wing.
3. The fuselage should have no corners or areas of large curvature in the plan view in the area of the wing. A corner or sudden change of direction in the shape of the fuselage can induce separation, even if the wing isn’t there. The addition of the wing makes the airflow more sensitive and easy to separate, particularly in the region just above the wing. If the airflow does separate at the corner, it will cause separation over the upper surface of the wing in addition to the fuselage side.
If other considerations cause the fuselage to have a break, the break point should be aft of the wing to reduce the chances of separation and interference drag. Corners are never good from the drag point of view but if they are going to be used to reduce construction cost or time then they should be placed where they do the least harm.
4. Avoid gaps between the wing and fuselage: Any gap between the root of the wing and the fuselage should be well sealed. Air leaking through such a gap if it is left unsealed can markedly increase the drag of the airplane. Leakage can produce flow separation on both the wing and fuselage where they join. It can also increase the induced drag of the airplane. The gap acts somewhat like an additional wingtip. This effect decreases the effective aspect ratio of the wing by reducing the span efficiency factor and increases the induced drag of the wing.
5. Avoid situations where the wing is pushing air one way and the fuselage is pushing it the other way. An example of this is an aft-cambered wing mounted low on a fuselage that pulls up sharply just aft of the wing. The air leaving the wing is being deflected down. The air flowing over the bottom of the fuselage, just inboard of the wing, is being deflected up by the fuselage. Clearly, the air right at the junction is likely to be severely confused and will likely separate or form a vortex. This will cause additional drag.
Proper design of the wing-to-fuselage junction is particularly important if the designer chooses to use one of the new generation of aft-cambered airfoils. Because of their camber distributions, these aft loaded sections are likely to be quite sensitive to the perturbation of the airflow produced by the fuselage. A good wing-root junction is essential to prevent separation.
Wing junction design seems to be one of the most neglected areas in light airplane design. Many production and homebuilt airplanes lack any form of root fairing at all, and many others have root treatments that are inadequate to minimize interference drag. What is true of wing-to-fuselage junctions is also true of tail-surface-to-fuselage and wing-to-nacelle junctions on multi-engined airplanes. Cleaning this up provides an opportunity for significant drag reduction on many types.
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.