Influence of miniflaps on sailplane flight characteristics

AbstractThe effect of miniflaps for increasing the L/D ratio and the lift coefficient has been studied on airliners as well as on UAV-s and wind turbines. For sailplanes the lift when Cl>1.0 is of main interest. As the maximum wing loading of racing sailplanes reaches 60–62 kg/m2, it is necessary to achieve a high Cl max (1.7–1.8) in thermals. In this case the decrease in TAS caused by a high Cl max even compensates for the drop of the L/D ratio to a certain extent, as the climb speed will increase when the spiral flight radius diminishes in thermals. To bring the L/D to Cl>1.0, a 2% chord miniflap at a 30° deflection angle was attached to the trailing edge of a Jantar-Standard 3 type sailplane wing (airfoil NN-8). In flight tests it was found that the miniflap increased the sailplane's Cl max to 1.35–1.66, i.e. by 23% (Re 1.0–0.92×106). At the same time the L/D ratio Cl increased by over 1.0. Especially good L/D improvement was noted with Cl at 1.13–1.19. In thermal Cl of 1.57–1.65 the roll control was g...


Introduction
A miniflap (also a microflap, a mini TED a. o.) is a 0.5-4% chord length flap of the trailing edge, meant for increasing the lift coefficient and decreasing the profile drag at higher lift coefficients. Miniflaps may be divided into three classes: a mini plain flap, a mini split flap and a Gurney flap (Gf). The latter is at a 90°° angle with the airflow. The auxiliary device which increases the lift coefficient, first used by the car racer Dan Gurney, was acknowledged in aviation after the publication of (Liebeck 1978). Thus, when a 1.25% high Gurney flap was used, the maximum lift coefficient of the Newman airfoil increased, but, surprisingly, the drag coefficient diminished at the same time. Still, at a Gf height of over 2%, the lift coefficient increased, and the drag began to increase faster.
In his search for the solution of the paradox that arose, Liebeck created a hypothesis according to which the vortices behind the Gf are accompanied by the diminishing of the thickness of the trailing edge boundary layer, thus decreasing the growth of the drag at an increasing lift force (Fig. 1). This hypothesis was confirmed by tests which showed that the NACA 4412 airfoil's boundary layer transition separation location lengthened from 92 to 98% when adding a 1% Gf, and an AoA of 4°° (Jang et al. 1998). At the same time a Cl of 0.818 increased to 1.167 (Fig. 2).
In spite of its small size, the miniflap is a surprisingly effective means for increasing the lift force. Using the same airfoil, it was found that a 4% high Gf increased the lift force more than a 25% chord length plain flap deflection of +9°° (Vlasov et al. 2007). At the same time, the hinge moments caused by the deflected flaps are smaller. At lower miniflap deflection angles, the profile drag diminishes significantly (Bloy, Durrant 1995).
The airfoil NACA 63 2-215 L/D max was retained at 2% C when a 45°° miniflap was used at a higher lift coefficient, Cl. When the flap slope angle was increased to 90°°, the lift force increased but the L/D max decreased. When the miniflap deflection angle is increased by more than 45°°, the lift coefficient growth intensity will diminish (Fig. 3).
The use of a miniflap increases the wing pitching moment, depending on the miniflap's width and deflection angle.
Similarly to the use of the plain flap, the tilt of the miniflap increases the reach of the HQ-17 airfoil's surface part to some extent, as the angle of attack is smaller at the same Cl (Bechert et al. 2001). This diminishes the role of the vortices behind the miniflap for the whole drag (

Review
In commercial aviation, the miniflap deflection in flight at subsonic speeds, 0.70-0.82 M, enables to decrease the wing drag from CL 0.53 (Richter 2010). The mini split flap is considered to be the most effective type. Various studies have pointed out that, depending on the aeroplane's type, the optimum flap deflection angle is 7.5-22.5° (Gardner et al. 2006). A certain role in the weakening of the drag might also be attributed to a decrease of 2-2.5° of the body slope angle. In wind tunnel tests using the Airbus A340-300 model with a Ma of 0.82, the use of a 2% chord mini split flap at a 7.5° deflection angle increased the aircraft's L/D value by 4.4% at a Cl of 0.65 and by 6.07% at a Cl of 0.67 (Fig. 6). This corresponds It should be pointed out that the use of miniflaps for airliners gives significant results at wing loadings over 600-700 kg/m². The highest fuel economy is achieved with the parallel usage of miniflaps and winglets. If there is need, by changing the angle of the miniflaps during the flight, the load distribution can be changed as well (Gardner et al. 2006), thus decreasing the wing bending moment and the effect of turbulence. A similar effect was achieved when the Trailing Edge Wedge was used for the airfoil S 904 (Bruscoli 2011). At Re 1×10 6 , with a 2% chord length and 0.8% height T. E. Wedge and a Cl of 0.52, the drag coefficient dropped from 0.0083 to 0.0049 and with a Cl of 0.8, from 0.0103 to 0.0068 (Fig. 7). Unlike with other kinds of miniflaps, when the T. E. Wedge was used, the Cl max decreased when compared to the standard profile.
The above mentioned airfoil with a TE Wedge could be used for UAV-s, solar powered aircraft and sailplanes. Fig. 7. The influence of various types of miniflaps on the S 904 airfoil's L/D value (Bruscoli 2011) In the Boeing CLEEN programme (Wilsey 2012), miniflaps were tested on an American Airlines' Boeing 737-800 in September 2012. 3% chord-length mini plain flaps at a 30° angle were used. In addition to reducing the fuel consumption, the miniflaps enabled to reduce noise both at takeoff and at landing, as the necessary airspeed at takeoff is lower and the plane's angle of ascent is greater. On gliders, miniflaps are of interest in the thermal regime, when CL > 1.0. A higher wing load 50-60 kg/ m² used for 15 m and 18 m class sailplanes presupposes an advisable flap lift coefficient Cl of 1.4-1.6 in thermals (Fig. 8). With an increase of the lift coefficient to 1.3-1.5, the Diana 2 sailplane's climbing speed increased by 0.2 m/s in medium wide thermals and by 0.4 m/s in narrow thermals (Kubrynsky 2006). The flap positions of+14° -+28° that were used enabled to raise the CL max to 1.65-1.7, but from a CL of 1.4-1.45 the profile drag starts to increase rather sharply as the boundary layer starts to separate from the flap's surface and also from a CL of 1.5 the roll control starts to degrade.
To investigate the influence of miniflaps on gliders, test flights with a DG-1000 were made on Prof. Joseph Mertens' (Akademische… 2005(Akademische… -2006 initiative in Aachen in 2006. For this purpose, 20 mm miniflaps (2.2%) C were used. 5 test flights were made at flap deflection angles +15°, +30°, +45°, +60° and +90°. In thermal, the best results were achieved at flap deflection at +30° and +45°. Due to greater drag at landing, the most favourable flap deflection was +90°. It appeared that at flap deflection of +60° and +90° the drag increased significantly, but no significant lift increase was added. Unfortunately, due to bad weather conditions, it was not possible to continue with the tests.
As the wing load and aspect ratio of new sailplane models are increasing, the need to use a CL of 1.5-1.7 is increasing as well.
To model the miniflap, the XFoil programme (Drela 1989) XFLR5 v. 696 was used. With the wing section SZD-48-3 Jantar-Standard, 3 similar NN8 airfoils were used on the sailplane, of which the most optimum variant was a 2% c length miniflap at a 30° angle. With the miniflap airfoil NN-8 the drag was lower than CL > 1.02 in the standard variant, also the Cl-max increased (Fig. 9).
To test the theoretical results, miniflap sections at a fixed angle from 1.5 mm thick CFRP with a relative wing chord ratio of 2% incl. ailerons were made at the Department of Aircraft Engineering of the Estonian Aviation Academy. The miniflaps were fixed with a double-sided adhesive tape (Fig. 10).

Methodology
In flight tests, the methodology developed by Johnson was used (Johnson 1989). In addition to comparing the sink speed, the parallel flight method was applied (Hendrix 2011). Before the flight tests, the altimeter and speed indicator were calibrated; the test equipment that was used was the air data test set D. Marchiori MPS 43. Also, the air tightness of the pipes of the gages was checked and additional gages were installed. To calibrate the airspeed indicator in flight, the static probe DFS-60 was used. The measurements were made at an IAS of 21-28 m/s. A table of calibrated speeds was compiled on the basis of the results collected (Fig. 11). The Dynon Avionics equipment D100 was used for measuring and recording, and a Pitot tube was attached to the sailplane for recording the flight parameters. To record the data from mechanical gages and to observe the airflow by tuft, GoPro cameras were used, one camera was attached to the stabilizer and two in the cockpit. In test flights, the parallel flight method with sailplanes of the same type was applied. A miniflap was used for one sailplane, another was used for comparison. Before the test flights the sailplanes were weighed together with pilots and, according to necessity, water ballast was added. Both sailplanes had the same centre of gravity and wing loading (G/S 35.78 kg/m²).
In the early morning, the sailplanes were towed by planes simultaneously to 1,700 m from the ground. During the following glide, the sailplanes flew parallel to each other at a distance of 30-50 m at equal speeds. To find out and compare the sink speed, the flight was divided into separate parts. The same speed was retained for 240 seconds and, at the same time, the sink speed was measured. The measurements were stopped at the height of ca 600 m which was higher than the inversion layer. To find out the angle of attack vs. speed, separate flights were made, as at different angles of attack the speed had to be maintained for no more than 10 seconds running. Each test variant was repeated two or three times. The obtained results were adjusted according to the air pressure and temperature.

Results
With miniflaps of 2% of the wing relative chord at a 30° deflection angle, the sailplane's stall speed decreased from 75 to 66 km/h and, computationally, the Cl max from 1.35 rose to 1.66 (Fig. 12), also taking into account the small growth of the wing area. In addition to this increase, the wing critical angle of attack grew to 1.6°.
The increase of the critical angle of attack when using a Gurney flap has been noted by several authors (Cavanaugh et al. 2007). Usually, it does not exceed 1°, but in one study the critical angle of attack increased to 2.1° (Vlasov et al. 2007). The analysis of the results obtained by comparing the test flights indicates that the use of miniflaps with lift coefficients Cl 0.99-1.21 and 1.32-1.66 decreases the drag of the sailplane in comparison with a normal configuration (Fig. 13).
The decrease of the drag need not be caused solely by the drop of the profile drag, it may also be connected to the decrease of the body drag and the interference drag, as the angle of attack decreased to 2.5° at the same speed. At the airspeed of 81 km/h (Cl 1.13) an anomalous decrease in the sink speed up to 0.63 m/s and an increase in the aerodynamic value at the same speed occurred. The most probable cause for the drag decrease is the thinning of the near trailing edge boundary layer as a result of the vortex appearing on the wing surface behind the miniflap. At airspeeds under 70 km/h the sink speed increased mainly due to the induced drag, which, in its turn, was generated by the relatively small aspect ratio of the wing (20.2) for gliders. At the same time, at airspeeds over 86 km/h (Cl 0.99), the miniflap increased the sailplane's drag. At a further increase of the speed up to 150 km/h, apart from the drag, the loads on the ailerons increased as well. According to the test pilot, these loads exceeded the normal flight forces more than twice. During towing, the necessary airspeed for flight decreased typically for this type from 125-130 km/h to 115-120 km/h. At critical angles of attack, the sailplane roll stability and controllability were retained. During one of the test flights, the test pilot managed to keep the plane at a critical angle of attack for a ca of 10 seconds. The controllability was retained, but the vibration, accompanying stall, increased, as the T-stabilizer was also located in the vortice area. To investigate the sailplane's practical behaviour in thermals, a separate flight was made with the purpose of finding out the changes in the controllability and stability generated by the use of miniflaps. The weather was windy and turbulent. The day was characterized by narrow and intermittent thermals. In spite of the turbulence, the sailplane´s controllability and stability were good and did not differ significantly from the normal configuration. It was possible to retain the airspeed of 76 km/h at a 35° -40° bank angle in a spiral flight, which is significantly lower than the normal airspeed (85-90 km/h) for this plane type at the given regime.

Conclusions
Similarly to modelling in the programme XFLR 5, the flight characteristics of the sailplane Jantar-Standard 3 improved significantly -Cl > 1.0 -when a 2% chord length miniflap at a 30° deflection angle was added. Cl max increased from 1.35 to 1.66. The critical angle of attack rose from 9.6° to 11.2° and the L/D value of the sailplane improved to Cl > 1.0, especially in Cl 1.08-1.19.

Future work
In future research, it would be necessary to test the miniflap at a 45° deflection angle and to find out the accompanying flight parameters. It would be most interesting and necessary to test miniflaps together with the plain flap. The results were promising when considering the modelling in the XFoil programme. Further testing is necessary for the wedge flap. The drag of the tested 2% c wedge flap at a 45° angle with a Cl of 1.0 was approximately 10% lower than with a miniflap at a 45° deflection angle (Fig. 14) (Bloy et al. 1997).
With Cl values in the range of 0.52-0.8, it would be a challenge to test the miniflap variant with a TE Wedge of 0.8% -2%, with which it was possible to decrease the Cd of an airfoil S904 by Re 10 6 more than 1.5 times (Fig.  7) (Bruscoli 2011). The airfoil S 904 of a 2% height Gf was tested -it might be studied for wing tips and also for UAV-s. With a serrated Gf Re 0.4×10 6 at a Cl of 0.71, the drag decreased to 0.00507 which is nearly twice lower than under usual conditions (Jarzabek 2011).  (Bloy et al. 1997)