To evaluate aerodynamic characteristics, it is important to determine how the aerodynamic coefficients change with respect to the airplane attitude, i.e., the angle of attack, yaw angle and roll angle, for a given flight condition. The aerodynamic force balance is a widely used method to directly measure the forces and moments experienced by a model. From the measured forces and moments, as well as the airflow temperature, static pressure and total pressure, the aerodynamic coefficients can be obtained for several angles of attack and yaw angles.

It is possible to obtain the aerodynamic characteristics of a full-scale object by testing a small-scale model, provided that the dynamic similarity condition is met and the appropriate corrections are applied. In the case of an incompressible steady flow, the relevant similarity parameter is the Reynolds number based on a proper reference length.

For a low speed airplane, like the DC-6B, the aerodynamic characteristics can be measured in a small, low-speed wind tunnel since it is possible to match the Reynolds number for the same flight conditions. Under these conditions, one can obtain the dependence of drag and lift on the angle of attack, α. This dependence on alpha can be used to evaluate the airplane performance.

Once the aerodynamic coefficients are measured for several conditions and configurations, for example using two different tail geometries, the stability derivatives (dC_M/dα, dC_N/dβ), lift slope (dC_L/dα), maximum lift coefficient, maximum lift-to-drag ratio, and other aerodynamic characteristics can be found. From these aerodynamic coefficients, the effect of modification or design choices on the airplane stability and performance can be determined.

The stability derivatives indicate whether the aircraft is stable or unstable. For example, if the angle of attack of the aircraft suddenly increases due to a gust of wind, the response of the aircraft characterizes its stability. If the angle of attack continues increasing indefinitely, the aircraft is said to be unstable. However, if the angle of attack returns to its initial value, the attitude before the gust, the aircraft is said to be stable. The same is true for the directional stability; if the tendency of the aircraft is to return to its initial yaw angle after a sudden change, the aircraft is said to be directionally stable.

In this demonstration, the aerodynamic force balance for force and moment measurements in a wind tunnel, will be introduced. To remove the contributions of the supporting struts and the model's weight, the balance will be tared to ensure the final results on aerodynamic force and moments are due to the aircraft only. Additionally, this demonstration illustrates the effect of a tail in a conventional airplane design and its importance in longitudinal and lateral aircraft stability.

In this demonstration, the performance and stability characteristics of a DC-6B model in two configurations were measured. In one configuration, a conventional airplane tail was attached to the model (tail-on), and in the second configuration, the tail was removed and replaced with a cone (tail-off). For each configuration, the variation of lift coefficient and drag coefficient with angle of attack was determined (Figure 3). The variation in pitch moment coefficient and yaw moment coefficient with respect to angle of attack and beta was also investigated (Figure 4).

The results show the aerodynamic effects of the tail. In Figure 3, although the tail increases the maximum lift and the drag, overall the tail decreases the aerodynamic performance. When the tail is off, the model is longitudinally and directionally unstable (Figure 4). Therefore, the airplane tail is necessary to achieve stability, even though it could result in reduced aircraft performance.

Figure 3. Performance evaluation curves for tail-on and tail-off configurations. A) Lift coefficient vs α; B) Drag coefficient vs α; C) Drag polar; and D) L/D vs α. Please click here to view a larger version of this figure.

Figure 4. Performance evaluation curves for tail on and tail off configurations. A) Pitch moment coefficient vs α; B) Yaw moment coefficient vs β. Please click here to view a larger version of this figure.

Testing a small-scale model using an aerodynamic balance in a wind tunnel enables the determination of major aerodynamic characteristics of an aircraft. A 6-component balance measures three force components, lift, drag, and lateral forces, and three moment components, pitch, yaw, and roll moments.

When the dynamic similarity between the full-scale object and the model is achieved, for example the Reynolds number is the same for the case of incompressible steady flow, then the aerodynamic coefficients obtained using the small-scale model are applicable to the full-scale object and aerodynamic characteristics, such as performance and static stability, can be determined.

Force and moments measurements by an external balance in a wind tunnel have several applications. This method is widely used in the aerospace industry; however, it has been applied successfully in research and development in many areas, for example in naval engineering, automotive industries, and civil engineering.

There are several applications in naval engineering. For example, sail boats and race boats are significantly affected by aerodynamic forces, and their effect on the vessel needs to be considered to optimize performance. For low-speed ship design, aerodynamic forces should be considered to reduce fuel consumption and improve overall performance.

Another industry that benefits from wind tunnel testing is the automotive industry. Wind tunnel testing is used to determine the drag forces, lateral forces and moments experienced by a car. This is now a standard practice for the development of new cars since this technique leads to more competitive and efficient designs.

Wind tunnel testing for force measurements is not restricted to performance optimization. In the modern civil engineering industry, wind tunnel tests are used to increase safety. There are tall and slender skyscrapers which are subject to strong wind gusts. These wind gusts generate high loads that need to be accounted for in building design to avoid building collapse. This also applies to bridges, which must be tested in wind tunnels to ensure safety.

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