Effect of Tail Dihedral Angle on Lateral Directional Stability due to Sideslip Angles

AIAA 2015-1498 AIAA SciTech 5-9 January 2015, Kissimmee, Florida 53rd AIAA Aerospace Sciences Meeting

Effect of Tail Dihedral Angle on Lateral Directional Stability due to Sideslip Angles Nur Amalina Musa∗, Shuhaimi Mansor†, Airi Ali‡, and Mohd Hasrizam Che Man‡, Wan Zaidi Wan Omar§ Downloaded by UNIVERSITI TEKNOLOGI MALAYSIA on February 3, 2015 | http://arc.aiaa.org | DOI: 10.2514/6.2015-1498

Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Skudai, Johor, 81310, Malaysia

This paper will describe the aerodynamic characteristic of complete aircraft equipped with conventional tail and V-tail configurations. Based on lateral-directional stability, the introduction of tail dihedral angle can significantly affect the yaw stability derivative of an aircraft. The wind tunnel test was conducted for sideslip angle, from -25◦ to 25◦ and the results were used to verify the CFD works. Good agreements were achieved at a lower sideslip angle which is below 15◦ . Then, CFD will be used to study the flow field around the tail region and figure out the effect on the directional stability. This study found that during sideslip condition, conventional tail stall at a lower sideslip angle and only effective at lower sideslip condition. This study shows that the V-tail configuration provide an advantage at higher sideslip condition. The interactions of V-tail vertical tailplane slightly increase rolling stability thus causes the reductions in yawing motion.

Nomenclature β b Lf Sw c¯ bt c Cl Cy Cn Cnβ Cyβ Clβ CF D T 35 ◦

Sideslip angle Wing span Length of fuselage Wing surface area Wing mean chord Tail span Tail chord Rolling moment coefficient Side force coefficient Yawing moment coefficient Yawing moment due to sideslip Side force die to sideslip Rolling moment due to sideslip Computational Fluid Dynamics Conventional tail Degree

I.

Introduction

ecently, aviation industry progressively seek for aircraft designs which not only look good but better in R performance. This leads to the introduction of unconventional design in order to improve aerodynamic characteristics. One of them is tail part. The tail provides stability and control to aircrafts and it has the ∗ PHD

student, Department of Aeronautics, Automotive & Ocean Engineering and AIAA member Professor, Department of Aeronautics, Automotive & Ocean Engineering ‡ Research Officer, Aeronautical Laboratory § Senior Lecturer,Centre of Electrical Engineering Systems † Associate

1 of 12 American Institute of Aeronautics and Astronautics Copyright © 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Downloaded by UNIVERSITI TEKNOLOGI MALAYSIA on February 3, 2015 | http://arc.aiaa.org | DOI: 10.2514/6.2015-1498

ability to restore the aircraft from perturbation in pitch, yaw and roll. It is vital that the aircraft is stable in handling the moments created from various disturbances while maintaining the body under control [1]. Meanwhile, aerodynamicist believes that tail surfaces add wetted area and structural weight hence they often sized as small as possible. Although in some cases this is not optimal, the tail is generally sized based on the required control power [2]. There are attempts to completely remove tails from aircraft in order to reduced aerodynamic drag and weight, however, this result in poor handling qualities which affects safety. But the development of fly by wire flight control system technology made it possible to design a tailless aircraft like B-2 [3]. Based on that, lots of unconventional empanage have been design including v-tail configuration which removed the vertical tail. Basically ,vertical tail provides lateral directional stability, yaw damping and effective directional control [3]. In performance point of view, the disadvantages of vertical tail are increased in aerodynamic drag and weight penalties [3]. While from the stability and control point of view, the disadvantages of vertical tails is the reduction in their directional stability as the effectiveness of yaw control contribution was reduce at the higher angle of attack. In this case, V-tail need to cater the role provides by vertical tail [3]. The control surface for the V-tail aircraft is known as ruddervator where the combination between rudder and elevator work to control the pitching and yawing motions of the aircraft. The first aircraft which introduce the V-tail design was Beech Model 35 Bonanza which was first produced in 1947 but was grounded due to safety reason. At this moment, lots of unmanned aerial vehicle used V-tail as it provides fascinating features. Nowadays, unmanned aircraft is widely developed for aerial observation and scientific research. Various types and unmanned aircraft configurations were developed in order to fulfill their specific operations and mission requirements. The required flight performance of an UAV is the ability to fly for a very long endurance. The important criteria needed to satisfy this requirement is to design UAV which have high lift to drag ratio and low trimmed drag. Some studies have suggested that a V-tail configuration has low trimmed drag due to the reduction in number of parts and wetted area compared to conventional tail [4]. Since less parts are required for V-tail, its help reduce the weight of aircraft [5]. Furthermore, it is also reported that the V-tail configuration may reduce radar detection[5].Beside the advantages mentioned, there are frequent reports that there are problems associated with stability and control of a V-tail aircraft[6]. V-tail configuration for unmanned aerial vehicle (UAV) design produces cross-coupling effect between yawing and rolling moment causing UAV lateral stability and directional control to be unsatisfactory[4]. There are many cases of pilot complaints on the difficulties on flying the V-tail aircraft (Beech Model 35 Bonanza) using a ruddervator control input. Several fatal accidents had been reported due to loss of ruddervator control of the V-tail aircraft especially on lateral motions[7]. It is clear that the application of V-tail for unmanned aircraft may cause similar issues on flying and handling qualities on lateral motions especially in dutch roll mode [6].The problems may even be worse if the aircraft is flying under gusty conditions.

II.

Methodology

Developing an aircraft requires an accurate aerodynamic data, therefore lots of numerical studies and experiments were conducted throughout the research. This helped to reduce potential mistake which can affect the design process [8] and predict the performance of the aircraft itself. Computational Fluid Dynamic (CFD) simulation offer lower cost to evaluate and optimize the design compared to wind tunnel testing but the accuracy of the data only valid for lower condition and it is proven through this study. In this research, wind tunnel test data was used to correlate and validate CFD results. The simulation settings such as meshing technique and turbulence model which gives good agreements with the experiment were retain to study the flow field around the tail area. The CFD also helped to visualize and capture the specific aerodynamic phenomena around the tail region. A.

Wind Tunnel Test Model

A one fifth Scale UTM-UAV model constructed using fiberglass was used in the experiment. The tail part is changeable in order to test different tail configurations. There are three sets of V-tail with different dihedral angle (35◦ , 47◦ and 55◦ ) and one conventional tail set. The dihedral angle is measured from horizontal plane to the tail chord plane and the selection of angle were based on NACA Report [9]. The span for vertical and horizontal stabilizer for the conventional tail were derived from 35◦ V-tail projection areas to the horizontal

2 of 12 American Institute of Aeronautics and Astronautics

and vertical planes, hence, the total references area would be the same[4]. This is due to the fact that, based on isolated tail theory, in order V-tail to have the same stability parameters as the conventional tail, they must have equal areas [10]. Selection of dihedral angle starts at 35◦ and the value is arbitrary [11]. V-tail with 35◦ dihedral angle was taken to be the baseline tail part for the projected conventional tail. The main wing of the aircraft was equipped with 1◦ anhedral angle. The parameters of the model are listed in Table 1.


Table 1. Summary of Model Parameter

Parameter Wing span,b (m) Length of fuselage,Lf c (m) Wing surface area,Sw (m3 ) Wing Mean chord,¯ c Tail span,bt (m) Tail chord,c

V-tail (35◦ ) 0.791 0.51 0.067432 0.0852 0.282 0.062

V-tail (47◦ ) 0.791 0.51 0.067432 0.0852 0.282 0.062

V-tail (55◦ ) 0.791 0.51 0.067432 0.0852 0.282 0.062

T-tail (35◦ ) 0.791 0.51 0.067432 0.0852 0.231 0.062

(a) V-tail with 45◦ dihedral angle

(b) V-tail with 55◦ dihedral angle

(c) Conventional tail with Projected Area from 35◦ dihedral angle

(d) V-tail with 35 dihedral angle (Baseline)

Figure 1. Configurations of wind tunnel test models


B.

Test Set up

This research measured the aerodynamics characteristics from a wind tunnel test and the prediction of vortex structure using computational fluid dynamic (CFD). The wind tunnel tests for V-tail configurations were conducted at the speed 40m/s for various sideslip angles, β from -25◦ to 25◦ with 5◦ of increment. CFD analysis results were validated for V-tail configurations with the same conditions with the wind tunnel test. CFD analysis were then carried out for V-tail and T-tail configurations for sideslip angle, β = 0◦ , 5◦ , 10◦ , 15◦ , 20◦ , 25◦ , 30◦ and 35◦ .


1.

Computational Fluid Dynamics (CFD)

CFD analyses for both configurations (V-tail with dihedral angle, 35◦ and T-tail) were performed using a combination of structured and unstructured meshing methods. The structured meshing method produced an initial layer with controlled size of less than 6.125 mm high over the model surface. The CFD analysis was solved using κ − ω SST turbulence model. 2.

Wind Tunnel Testing (WT)

The static wind tunnel tests were conducted in the 1.5 m × 2 m × 6 m closed circuit Universiti Teknologi Malaysia Low Speed Tunnel (UTM-LST).This facility is capable to provide maximum wind speed of 80 m/s with maximum turbulence intensity approximately 0.01 % across the test section. The model was mounted on a single strut support while the model angle of attack was fixed to zero degree. Forces and moments sensed by the model were measured using JR3 Six Component Balance. This sensor is capable to return three aerodynamic forces and moments and it is placed under the test section floor. The Balance Moment Center (BMC) is located at the center of this sensor. The sideslip angles were changed by rotating the tunnel turn table and all data have been corrected for tares caused by model strut support.

III.

Results

All results presented here are referred to the aircraft center of gravity (CG). The forces and moments result related to lateral stability are presented in body axis. A.

Reynolds Sweep Test

In order to determine the suitable test speed, Reynolds sweep tests were conducted at various wind speeds for zero angle of attack and zero yaw angles. The drag coefficient is then evaluated to find a range of wind speeds where the drag coefficient is independent of the wind speed. The wind speed was varied from 10 m/s to 50 m/s with 5 m/s increments. The Reynold sweep results is presented in Figure 2, from which it is found that at 30 m/s and above the drag coefficients were almost identical. The test speed can be selected from any wind speed fall within this range. However, it is still dependent on the sensitivity of the sensor. Since the model is small, the force sensed by sensor will be small at the lowest speed in the range. The effect due to any small changes in the model may not be detected if the wind speed is too low. The selection should be made by taking this consideration into account. Hence, wind speed of 40 m/s which correspond to Reynolds number of 0.2064×106 based on wing model chord was selected to be a test speed throughout this study.



Figure 2. Uncorrected coefficient of drag with variation of wind speed in Reynolds sweep test

B.

CFD Result compares with Wind Tunnel Test Data

All CFD analyses have been carefully post processed. The coefficients of side force, yawing moment and rolling moment from numerical and experimental study were plotted for comparison and validation purposes. Result from the numerical for side force and rolling moment show a good agreement with experimental data for sideslip angles up to 25◦ while for Yawing moment, the result is matched only up to 15◦ . Side force and yawing moment coefficients for conventional tail are almost identical compared to 35◦ V-Tail from the sideslip angle of 0◦ to 15◦ . Table 2 shows the derivatives related to lateral stability calculated for low sideslip angle (0◦ to 15◦ ). Compared to experimental data, numerical method is able to predict the derivative of this model with maximum error of 6.5%. As to understand the V-tail with wing fuselage combination, additional factors such as downwash and sidewash associated with the wing fuselage vortex must be considered [9].

Figure 3. Yawing moment coefficient against sideslip angle



Figure 4. Side force coefficient against sideslip angle

Figure 5. Rolling moment against sideslip angle Table 2. Lateral Stability Derivative for Numerical and Experimental data

Derivative(Deg−1 ) C nβ Cyβ Clβ

CFD Conventional Tail 0.0546 -0.3432 -0.1173

CFD V-Tail 0.0595 -0.3721 -0.1628

Wind Tunnel V-Tail 0.0622 -0.3496 -0.1553

V-Tail Error(%) -4.37 6.42 4.82

As the relative strength of aircraft directional stability and dihedral effect will determine several lateraldirectional characteristics of the aircraft itself. V-tail contributes to strong directional stability as compared to Conventional tail but at the same time V-tail also generate higher rolling moment and this will lead to the cross-coupling problem when aircraft start to yaw. Conventional tail has a less directional stability and it is likely to induce spiral instability which caused the roll and yaw motion to be slightly decreased due to damping effect cause by vertical tailplane . Referring to aerodynamic principal which indicates the changes in dihedral angle can alter the angle of attack along span wise tail section while aircraft in sideslip mode, this leads to change in lift distribution along span wise section of the tail which altered the lateral directional aerodynamic derivatives [12].



(a) Wake region at rudder for Conventional tail configuration at sideslip angle 15◦

(b) Streamwise vortex generated by fuselage crossflow for V-tail configuration at sideslip angle 30◦

Figure 6. Non-linear response of Conventional tail and V-tail configuration

The non-linear response of Conventional tail configuration is starts at sideslip angle, 15◦ due to the stall of vertical tail (rudder) as shown in Figure 6(a). On the other hand, Figure 6(b) shows the non-linear response of V-tail configuration start at sideslip angle of 30◦ due to the stream wise vortex generated by the fuselage cross flow [3].

Figure 7. Static pressure distribution at sideslip angle 15◦

As shown by numerical analysis, the front wing fuselage generated asymmetric downwash airflow to the rear tail. This effect will destroy the vortex formation on the V-tail [5]. C.

Effect of Tail Dihedral Angle on Directional Stability

National Advisory Committee for Aeronautics (NACA) have conducted a lots of wind tunnel test as to gain understanding in aircraft directional stability but they dealt with geometries which differ from the typical civil aircraft. This is due to the fact that all the test were motivated by World War II where the result were used to design new fighter aircrafts [13][14].The understanding was basically developed based on certain geometry but yet not accurate to be applied to any typical civil aircraft. In this research, a typical civil aircraft with different V-tail dihedral angle were used to obtain a basic understanding on aircraft directional stability characteristics. Considering special case during wind tunnel test, when the direction motion remains unchanged but aircraft is yawed, hence the sideslip and yaw angle are related by relation, ϕ = -β. Due to response of sideslip motion in wind tunnel test, typically both yawing and rolling moment are created [15]. The aircraft said to have a stable in roll if Clβ < 0. Roll moments are created when the aircraft starts to sideslip and 7 of 12 American Institute of Aeronautics and Astronautics


depends on the arrangement and design of vertical tail. The rolling moment produced by vertical tails tends to bring back the aircraft to the wing level attitude. The introduction of tail dihedral angle were also contributes to the production of side force during sideslip and created an aditional effect in directional stability.

(a) Yawing Moment

(b) Rolling Moment

Figure 8. Experimental results of comparison between yawing and rolling moment characteristics

Figure 9. Experimental results of side force versus yaw angle

A conventional tail configuration is added in the experiment to see how V-tail will perform compared to


Table 3. Yawing and Rolling Moment Derivatives for Different Dihedral Angle


Tail Configuration V-tail V-tail V-tail Conventional tail

Dihedral Angle (deg) 35 47 55 0

Cnβ 0.0016 0.0032 0.0041 0.0025

Clβ -0.0028 -0.0061 -0.0062 -0.0041

the conventional tail. However a fair comparison can only be achieved if these two tails configuration have the same projection area. This is due to the fact that a V-tail of same span but with different dihedral angle will produce different projection area; hence, the side force is expected to be much larger at high dihedral angle. Since the yawing moment is a product of the side force, one can expect that higher V-Tail dihedral angle will provide better lateral stability. The projection area for the conventional tail used in this experiment is equivalent to the baseline configuration (35◦ V-tail). The gradient of yawing moment due to sideslip angle, Cnβ for Conventional tail is higher than baseline configuration which indicates that V-tail is less stable directionally than conventional tail. However, this is only true for low sideslip angle (-10◦ ≤ β ≤ 10◦ ). Referring to Figure 8, at sideslip angle, β = ± 15◦ , the yawing moment for V-tail aircraft still in linear region while yawing moment for conventional tail start to flat out indicating the aircraft is going to stall. This is due to the fact that conventional tail is always positioned in serious asymmetric downwash region created by wing fuselage, which will not create any additional lateral forces [5][14][6][16]. The roll stability is also reduced in V-tail configuration; however, the linear region is still present in conventional tail for higher sideslip angle. Based on Figure 9, V-tail aircraft is slightly more sensitive to the side flow as the horizontal components of lift on the two surface combine to produces a net side force to that opposes the sideslip motion and is proportional to the sideslip angle [4] and this will make the aircraft more vulnerable to turbulence especially side gust at lower sideslip angle. It can be conclude that if dihedral angle is too small; it will generate less yaw stability effect during flight while if it is too large, it will affect the longitudinal stability but it also can make directional stability recovery moment too excessive causing the aircraft to lose the power to control the rudder during sliding motion. Because of this, aircraft could start to rotate and enter the tail spin condition [5].On the other hand, too much dihedral angle can cause safety issues in Dutch roll modes especially during high speeds as in the case of F4U Corsair and the V-tailed Beechcraft Bonanza are famous victim of this effect. Based on NACA Report, 47◦ dihedral angle provide a better longitudinal and lateral stability as compared to Conventional tail [9]. This is due to the decrease in the rate of change of effective downwash with angle of attack due to the high tail position and the favorable effect of sidewash at the tail. V-tail must replace horizontal stabilizer as close as possible as a reservation for static stability and control in extreme situations in order to avoid sudden stall [10] .



Figure 10. Variation of Directional Stability with Tail Dihedral Angle

Figure 11. Eigenvalues for different V-tail configuration

Neglecting the Dutch roll problem at high speeds, tail dihedral angle boosts the yawing stability of the aircraft. Figure 10, shows that, the Cnβ value increased as higher dihedral angle were applied due to the reason that higher dihedral angle provide better directional stability. This is true as the eigenvalues for all V-tail configurations are located at the left side with negative real parts of the S-plane plot which lies in stable region. Note that the eigenvalues for V-tail with 55◦ dihedral angle is located further left from the imaginary axis compared to those with lower dihedral angles, indicating stable condition. As the dihedral angle reduces, the eigenvalues moves towards positive side, indicating less stable conditions. These shows that the V-tail with greater dihedral angle will have greater degree in directional stability as it eigenvalues are moves away from unstable region as shown in S-plane plot. Higher yawing moment provides a better stability and controllability during landing and takeoff in crosswind conditions [17]. In terms of lateral handling qualities, the important parameters are the vertical tail surface and the dihedral angle as the changes of these two parameters will imply positive changes to the stability derivatives Cnβ and Clβ ; hence, satisfy the Spiral and Dutch roll mode based on Routh conditions [18].


D.

Study Tail Contribution Only


In this section, the test was made in such a way as to measure the moments contributed by the tail surface itself.

(a) Yawing moment

(b) Rolling Moment

(c) Side force Figure 12. Tail contributions to directional stability


Table 4. Lateral Stability Derivative for Numerical and Experimental data

Tail Configuration V-tail Conventional tail

◦

Dihedral Angle, deg ( ) 35 0

Shaded Area, deg(◦ )based on Yawing Moment Plot (Positive Region) 0.447705 0.425456


Figure 12 show that tail plays a vital role in determining the stability and performance of the aircraft. The graph compares the two types of configurations which is wing-fuselage without tail and with the complete aircraft configurations as to determine the tail contribution to directional stability of the aircraft. It is found that V-tail contributes 5% to directional stability to the aircraft compared to conventional tail.

IV.

Conclusion

The static wind tunnel test focusing on effect of directional stability was used to verify the CFD simulation. The results agreed with the experiment up to 15◦ sideslip angle and the simulation was used to analyze flow field around the tail area. V-tail with 35◦ dihedral angle was chosen as the baseline configuration. This paper investigates the effect of different tail dihedral angles and tail type to the aerodynamics characteristics of the same wing body fuselage. The conventional tail was constructed based on projection area of the V-tail. Introducing dihedral angle in tail design caused the increment in rolling moment. Wind tunnel test has been done as to eliminate the effect of interaction between wing-body fuselage and found that V-tail produce a better directional stability.

References 1 Carrier, G. and Gebhardt, L., “A Joint DLR-ONERA Contribution to CFD-based Investigations of Unconventional Empennages for Future Civil Transport Aircraft,” CEAS/KATnet Conference on Key Aerodynamic Technologies, 2005, pp. 1-8. 2 Roskam, J., Aiplane Design (Part III:Layout Design of Cockpit, Fuselage, Wing and Empennage: Cutaways and Inboard Profile), DARCorporation, Kansas, USA, 2002, pp. 249-279. 3 Colgren, R. and Loschke, R., “To Tail or Two Tails? The Effective Design and Modeling of Yaw Control Devices,” AIAA Modelling and Simulation Technologies Conferences and Exhibit, August 1997, pp. 1-13. 4 Phillips, W.F., Hansen, A.B., and Nelson, W.M., “Effects of Tail Dihedral on Static Stability,” Journal Aircraft, Vol. 43, No. 6, Nov. 2006, pp. 1829-1837. 5 Zhang, G.Q., Yu, S.C.M., Chien, A. and Xu, Y., “Investigation of the Tail Dihedral Effects on the Aerodynamic Characteristics for the Low Speed Aircraft,” Advance Mechanical Engineering, Vol. 2013, 2013, pp. 1-12. 6 Rao, K.D., “Modelling Nonlinear Features of V tail Aircraft using MNN,” Transactions Aerospace and Electronic Systems, IEEE, Vol. 31, No. 2, 1995, pp. 841-845. 7 Hoover, K., Fowler, W.T. and Stearman, R.O., “Studies in Ethics, Safety, and Liability for Engineers,” University of Texas, URL: http://www.tsgc.utexas.edu/archive/general/ethics/ [Accessed 01 Apr 2013]. 8 Pettersson, K., “Scaling Techniques Using CFD and Wind Tunnel Measurements for use in Aircraft Design,” Universitetsservice US AB, Stockholm, 2006. 9 Polhamus, R.C., and Moss, J., “Wind-Tunnel Investigation of the Stability and Control Characteristics of a Complete Model Equipped with a Vee Tail,” NACA TN-1478, Washington,DC, 1947. 10 Purser, E.P. and Campbell, J.P., “Experimental Verification of a Simplified Vee-Tail Theory and Analysis of Available Data on Complete Models with Vee-Tails,” NACA TR-823, Washington,DC, 1944. 11 Greenberg, H., “Comparison of Vee-Type and Conventional Tail Surfaces in Combination with Fuselage and Wing in the Variable-Density Tunnel,” NACA TN-815, Washington, DC, 1941. 12 Song, L., Yang, H., Zhang, Y., Zhang, H. and Huang, J., “Dihedral influence on lateraldirectional dynamic stability on large aspect ratio tailless flying wing aircraft,” Chinese Journal Aeronautic, Vol. 27, No. 5, Oct. 2014, pp. 1149-1155. 13 House, A.O., and Wallace, R., “Wind-Tunnel Investigation of Effect of Interference on Lateral-Stability Characteristics of Four NACA 23012 Wings, an Elliptical and Circular Fuselage and Vertical Fins,” NACA TR-705, Washington, DC, 1941. 14 Nicolosi, F., Della Vecchia, P., and Ciliberti, D., “An Investigation on Vertical Tailplane Contribution to a Aircraft Sideforce,” Aerospace Science Technology, Vol. 28, No. 1, Jul. 2013, pp. 401-416. 15 Nelson, R.C., Flight Stability and Automatic Control, 2nd ed., McGraw-Hill International Editions, New York, 1998. 16 Abzug, M.J., “V-Tail Stalling at Combined Angles of Attack and Sideslip,” Journal Aircraft, Vol. 36, No. 4, Jul 1999, pp. 729-731. 17 Sadraey, M. and Colgren, R., “A Systems Engineering Methodology for the Design of Unconventional Control Surfaces,” January 2008, pp. 1-19. 18 Teo, P., “Preliminary aircraft design: lateral handling qualities,” Aircraft Design, Vol. 4, No. 4, 2001, pp. 63-73.


Effect of Tail Dihedral Angle on Lateral Directional Stability due to Sideslip Angles

Recommend Documents