Features

Drone aircraft and their uses have been evolving quickly, supported by a great deal of ongoing research. One area of increasing interest is the decoy drone, designed to mimic the radar and heat signature of an actual aircraft. These drones are intended to confuse or mislead anti-aircraft defense systems. If operating as designed, one or more drones are launched from an actual aircraft as it enters airspace monitored by anti-aircraft systems. The system picks up the signature of the drones and attacks them while the actual aircraft can be hidden with the swarm of drones.

Figure 1. A baseline high-speed decoy design CAD model

In this study, computational fluid dynamics (CFD) embedded in CAD software was used to optimize the aircraft design and test the aircraft performance during different operations such as cruise, maneuverability, and maximum speed. The mission requirements for the high-speed decoy were a maximum altitude of 15,000 feet with maximum speed of 450 knots and an endurance of at least one hour.

The highly agile decoy UAV with high maneuverability capability was designed to launch from a pneumatic catapult and land via a parachute. This aircraft design had a 6-g sustained and 9-g instantaneous load factor. The required payload capacity was 22 lb, consisting of a smoke dispenser, a passive radar cross section augmenter (luneberg lens), a chaff and IR dispenser, and a miss distance indicator.

Wing Geometry

After considering the catapult, maneuver, and cruise constraints, the wing loading at takeoff condition was calculated as 23.209 lb/ft2, which is the maximum that it should experience. From this, the advantages and disadvantages of high-wing, mid-wing, and low-wing configuration types were considered. The mid-wing configuration was selected because it had the properties of high- and low-wing; it also had the lowest drag from wing-body interference.

Figure 2. Drone test configurations

The wing incidence angle was 0 degrees for this decoy design because the wing incidence angle is generally set at 0 degrees for mid-wing jet fighter aircraft. The aspect ratio (AR) of the high-speed decoy wing was 5, and taper ratio value was 0.36 from other successful aerial target designs.

If an aircraft's maximum speed is less than 0.3 Mach, wing sweep is not recommended. However, wing sweep angle is used for high-speed aircraft. Wing sweep helps to protect from shock formation by increasing the critical Mach number. The leading-edge sweep angle value increases as the aircraft maximum speed increases. After considering the decoy's maximum speed requirement, leading edge sweep angle was chosen as 30 degrees.

Wing dihedral angle gives lateral stability to an aircraft; however, too much reduces rolling controllability. Wing sweep and high-wing configuration gives naturally positive dihedral, whereas, low wing gives naturally negative dihedral effect. Considering aircraft wing sweep selection, wing configuration and aerial target requirements of the dihedral angle was 0 degrees for this design.

Fuselage

Figure 3. Pressure contours 0.13m from centerline for high wing, mid-wing and low wing angled at 6°

For an initial guess, fuselage length was initially estimated by using the following formula assuming jet fighter coefficients:

where a is the speed of sound, and Wo is the maximum takeoff weight.

However, the actual length of the aerial targets is longer compared to the calculated values. Examining other aerial target designs, the average length difference was calculated as 26%. Therefore, for the calculated Wo, the aircraft length was calculated as approximately 9.03 ft.

Another important parameter for fuselage design is the slenderness ratio value (f). This is the ratio of fuselage length to the maximum diameter of fuselage:

A slenderness value was chosen as 11 from previous successful aerial target designs with similar design requirements. Slenderness value of 11 is also close to jet fighter designs.

Tail Geometry

Figure 4. Mach number contours 0.13m from the centerline for high wing, mid-wing, and low wing angled at 14°

The tail has three main functions: stability, control, and trim. Trim refers to generation of the lift force; by acting through some tail moment arm about the center of gravity, it balances some other moment generated by aircraft. Different tail configurations were considered. The T-tail configuration was selected because of its simplicity. T-tail provides a wake-free horizontal tail and a heavy vertical tail structure to carry the horizontal tail.

A horizontal tail generates aerodynamic force to trim the aircraft longitudinally; in other words, it is responsible for balancing the moment by the wing. The horizontal tail chosen was a movable tail. Leading-edge sweep was 35 degrees, which was 5 degrees more than the wing sweep to ensure the critical Mach number would not lose elevator control from shock formation. The thickness ratio of the airfoil (t/c) section was thinner than the wing t/c to reduce the flow Mach number at the tail section. The aspect ratio (AR) of the horizontal tail was lower than the wing to improve the stall characteristics. Horizontal tail AR was estimated to be two thirds of the wing aspect ratio.

Vertical tail generates aerodynamic force to trim the aircraft directionally. Rather than (yawing) directional stability, the rudder is a movable part of the vertical tail. Therefore, directional control and maneuvering of the aircraft is done by the vertical tail. The vertical tail and horizontal tail combination should be designed so that at least a third of the rudder should be out of the wake for spin recovery.

Like a horizontal tail, the vertical tail also should have a high sweep angle to increase Mcrit and avoid problems from shock formation. The vertical tail airfoil section t/c ratio was selected the same as the horizontal tail to reduce the vertical tail Mach number. A high lift curve slope airfoil was selected because the directional stability derivative is directly related to the lift curve slope of the airfoil of the vertical tail.

Airfoil affects the aircraft performance such as cruise speed, stall speed, handling qualities, and overall aerodynamic efficiency. The airfoil can be defined as the 2D profile of the wing. Optimum pressure distribution can be achieved on the upper and lower surfaces by choosing the right airfoil. The right airfoil can be chosen if the design lift coefficient Cl(ideal), Cl(max), operating Reynolds number (RE), and design Mach number, are known.

The NACA 63-412 airfoil was selected. NACA 63-412 airfoil has a maximum thickness at 34.9% of the chord and is 2.2% maximum camber at 50% of the chord. For the horizontal and vertical tail, a symmetric airfoil NACA 0009 smoothed was selected.

Figure 5. Pressure contours 0.13m from the centerline for high wing, mid-wing, and low wing angled at 14°

CFD Analysis

Because the aim of this study was to design an optimized high-speed decoy that surpasses its predecessors, the FloEFD CFD tool was used to achieve the high-speed decoy configuration that resulted in the best aerodynamic performance. Baseline design and other configurations were created according to their vertical wing and tail geometry designs. All models were created in a CAD environment and analyzed for different flow regimes and envelopes. Finally, configuration was selected based on various design and performance criteria.

Figure 6. Mach number contour cut plots 0.231m from centerline of T-tail, cruciform tail, and conventional tail at a 14° angle

Once the baseline design of the UAV was created in CAD, nine variations were generated with the different wing/tail design combinations as shown in Figure 2. CFD analyses of each of these combinations were executed to find the optimum combination that would best meet the mission requirements of the drone. Nine design variations were compared from three wing and three tail options. The following design aspects were considered in the analyses:

  • Wing vertical location affects the performance directly; it alters the C.G. of the aircraft and therefore, the stability.

  • Baseline high-speed decoy UAV was designed as mid-wing because of reasons stated previously.

  • Low wing has less ground clearance and is not as laterally stable, but enables better lateral control. It also produces less lift and induced drag. It has less downwash on the tail, thereby making the tail more effective, and finally, it is structurally lighter than a high wing configuration.

  • High wing has the most ground clearance and is the most stable laterally, though it has less lateral control. It also tends to produce more theoretical lift and, therefore, more induced drag. Plus, it is structurally the heaviest of all the designs.

  • Mid-wing, as the name implies, is in between both the high and the low design with their associated characteristics.

  • Conventional tail has a vertical tail that is the lightest structure of all three tail combinations because the vertical tail does not need to carry the horizontal tail. The wing wake can disturb the horizontal tail in this configuration, especially with the high wing combination.

  • T-tail offers the advantage to have a wake-free horizontal tail because it is positioned the furthest distance vertically from the wing in any configuration. The downside is that it requires a heavy vertical tail structure to support the horizontal tail.

  • Cruciform tail is the combination of the T-tail and the conventional tail. The cruciform tail enables a lighter vertical tail and helps prevent deep stall.

For the initial analyses, high wing, mid wing and low wing configurations were compared with the tail configuration kept as T-tail. CFD analysis showed that the fuselage effect negatively affected the low wing and forced the flow to separate, making the low wing prone to the stall and reducing its lift efficiency — the worst wing position among all configurations. High wing and mid-wing showed similar performance but the high wing was better for high angles of attack.

Figure 7. Pressure contour cut plots 0.231m from centerline of T-tail, cruciform tail, and conventional tail at a 16° angle

FloEFD revealed that for low angles of attack, the mid-wing configuration had lowest drag and highest lift-to-drag (L/D) values. Whereas for higher angles of attack, the high wing configuration yields the lowest drag coefficient, highest maximum lift coefficient, and gave highest L/D values. Unexpectedly, the high wing configuration model yielded the lowest drag at higher angles of attack compared to other configurations. Therefore, the high wing design was selected for the wing configuration.

Figure 8. 3-D flow trajectories of the T-tail configuration at a 16° angle

Three different tail configurations were then analyzed while holding the high wing configuration constant. This showed that the T-tail seemed to be wing wake free and provided the most lift. Because deep stall is an important phenomena, the stall angle of 16° needed to be analyzed in more detail to prove that deep stall would not occur.

To see the occurrence of the stall phenomena completely, 3D flow trajectories were plotted; 200 pipe lines were used to show the flow trajectories through the wing. 3D flow trajectories confirm that the T-tail configuration's horizontal tail tips were not significantly affected by the stall wing wake.

Finally, the decoy UAV was tested with the CFD tool at maximum, corner, and cruise velocities. The results showed that the aircraft design would be able to fly at the required maximum velocity without a strong shock occurrence. Optimum cruise velocity was 0.38 M in the drag polar curves. Then, optimum corner velocity was found from the CFD result CL max. Wind tunnel testing is considered indispensable for getting the most accurate aerodynamic performance. However, creating prototypes of every configuration and testing them in a wind tunnel is too time consuming and expensive for a designer. The FloEFD CFD tool embedded in CATIA was helpful to reduce the number of prototypes.

This article was written by Umut Baycara, Aeronautical Safety Assistant Expert, Middle East Technical University (Ankara, Turkey) and Mike Croegaert, Senior Industry Manager of Military and Aerospace Technology, Mentor Simulation and Test Solutions (Wilsonville, OR). For more information, visit here.