978-1-4244-8551-2/10/$26.00 ©2010 IEEE 317 ICIAfS10

Abstract— Single Rotor based Vertical Take Off and Landing systems are more prone to stability issues than normal aircraft. The main factors causing instability are anti torque, pitching and rolling. Anti torque, which causes the body of the aircraft to turn in the opposite direction to the rotor, is generated as the engine turns the rotor against the air. Pitching and rolling, on the other hand, can be caused due to sudden turbulent air conditions during the flight of the aircraft. Today’s anti torque mechanisms, like the tail rotors of helicopters, cause the overall system to be bulky and complex. In this paper, a compact automated mechanism is presented which can be used to attain stability in VTOL vehicles. Experimental verification of the system is being performed on a modeled aircraft. Analysis using Velocity Triangles and CFD has been carried out on this system. Keywords: ADSC, Anti-torque, CFD, Deflector Fins, Pitch, Roll, Velocity Triangles, VTOL.

I. INTRODUCTION ertical Take Off and Landing (VTOL) aircrafts achieve vertical lift off without the need for a runway or airstrip.

These aircraft usually have fixed wings - some aircraft may also be able to take off in other ways whilst others can only use VTOL modes.

Post World War II, the need for aircrafts with abilities to take off and land from any place and to be as compact as possible became a necessity. Researchers concluded that rotor based VTOL vehicles were the best solution to the problem. Thus was born the helicopter, a type of VTOL in which lift and thrust are supplied by one or more engine driven rotors. In contrast with fixed-wing aircraft, this allowed the helicopter to take off and land vertically, to hover, and to fly forwards, backwards and laterally. These attributes allowed helicopters to be used in congested or isolated areas where fixed-wing aircraft could not take off or land. The capability to hover for extended periods of time, and to do so more efficiently than other forms of vertical takeoff and landing aircraft, allowed helicopters to accomplish important tasks, like search & rescue, surveillance etc, which fixed wing aircrafts could not perform.

II. DISADVANTAGES OF ROTOR BASED VTOL The main disadvantages of rotor based VTOL vehicles are

the stability issues like anti-torque and tipping. In relation to rotor based VTOLs, the engine turns the drive shaft, which turns the rotor. This rotation causes a torque which in turn, creates an anti-torque that causes the body of the aircraft to rotate in the opposite direction. If the aircraft is on the ground the friction between the craft’s landing gear and the surface will prevent the torque force from spinning the body. When the craft becomes airborne, the force of friction is removed and if nothing is present to counteract the torque force, the body will rotate in a direction opposite to that of the rotors. This is in accordance to Newton’s Third Law, the action of the rotors creates an equal and opposite reaction (the body spinning opposite the direction of the main rotors).

III. CONVENTIONAL SOLUTION The most common method to counteract torque is with a

tail rotor. A tail rotor is situated on the tail of conventional VTOL. The purpose of the tail rotor is to reduce the effect of torque and the yaw motions inherent in flight but causes unwanted translational motion [1]. The tail rotor is comprised of two or four small airfoils that the pilot is able to control in the cockpit by manipulating the rudder (anti torque) pedals. Another way of countering the torque produced is by using multiple rotors. A rotor rotating in the opposite direction to that of the main rotor negates the torque produced. This can be either done by placing another rotor on top of the main rotor or by having two rotors side by side [2][3]. This method not only negates the torque but can also negate the pitching caused during flight.

The main disadvantage of the above two methods is the fact that the extra additions cause the system to become more heavy and bulky thereby reducing the speed, payload carrying capacity and runtime of the system thereby drastically reducing the overall efficiency of the system.

Innovative Dynamic Stability Control for VTOLs using Thrust Vectoring

Avinash S. Nair#, Vishnu Aravind, Aditya K.$1, Balakrishnan Shankar$2, Sheryas N.$3 Ananthapadmanabhan J. and and Joshua Udar Freeman* Sai Dinesh P *Department of Electrical and Electronics Engineering, Department of Electronics and Communication Engineering, $Department of Mechanical Engineering,

Amrita School of Engineering, Amrita School of Engineering Kerala, INDIA Kerala, INDIA

email: #avinashnair@ieee.org email: $1u4mee08020@studenstmail.amrita.ac.in

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IV. PROPOSED SOLUTION In this paper we are proposing an innovative system to

counteract the torque created by the propeller, using Deflector Fins. This system is light in weight, compact, easy in functionality & handling and is more cost effective than the systems present in today‘s market. Thus it significantly reduces the cost and increases the efficiency of the system in which this is incorporated in. The system mainly consists of four rectangular flaps, called Deflector Fins, placed symmetrically at the bottom of the aircraft, directly beneath the propeller. The fins are attached to servo motors which are used to adjust the angle of attack of the fins. To ensure proper automated functioning and dynamic stability the system has an electronic Automatic Dynamic Stability Control (ADSC) system. The system adjusts the angle of the deflector fins to ensure the aircraft does not torque, pitch or roll.

V. THEORETICAL ANALYSIS In a rotor based VTOL aircraft it is imperative that we

calculate the thrust and torque generated by the propeller in a fluid medium to understand the forces acting on the body of the aircraft. The thrust and torque of the system depends on the engine and propeller. Two methods that can be used to calculate the thrust generated by the propeller are described below.

Initially, the wax model of one of the blades of the sample APC propeller with dimensions (0.512x0.2048) m (propeller diameter x propeller pitch) is made. This wax model is then cut into sections and the airfoil profile of each section is plotted through the engineering drawing software Solid Edge (V16).

Fig. 1 Wax model of sample propeller (above) and its corresponding

engineering drawing (below) The following are valid assumptions that can be made for

the analysis of the system [4][5] 1. The air flow has attained steady state. 2. The air is incompressible. 3. Losses are assumed to be negligible. 4. Flow velocity is fairly uniform except near the center

of the hub and trusses holding the deflector fins.

A. Analysis of Propeller through Velocity Triangles The legend for the following section is as follows:

N – RPM (Revolutions Per Minute) U – Rotational velocity of the propeller cross-section h – Height of the propeller cross-section

b – Breadth of the propeller cross-section r – Radius from hub to propeller cross-section D – Diameter of the propeller F – Thrust P – Power

– Mass flow rate of air – Density of air (1.225 kg/m³)

A – Area of the inlet i.e. Propeller swept area V – Absoluter air velocity Vf – Flow velocity Vw –Whirl velocity Vr – Relative air velocity β – Angle of relative velocity with blade velocity α – Angle of absolute velocity with blade velocity Subscripts 1and 2 added to the symbols above denote inlet and outlet respectively.

The following steps were implemented for performing the analysis:

The absolute air velocity at the inlet at steady state is to be measured using an anemometer. The inlet and outlet blade angles (β1 and β2) are measured for various radial distances (‘r’) from the hub to section being analyzed. These values are in turn used to calculate the absolute air velocity at the outlet and the thrust exerted by the propeller by drawing the velocity triangle for each ‘r’ value. The velocity triangle drawn for one such section and the equations associated with it are given below.

Fig. 2 The initial input values for a propeller cross-section at a radial

distance r.

Fig. 3 Velocity Triangle Diagram

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(1) m (2)

(3)

(4)

(5)

(6) sin (7)

(8)

(9)

(10) . (11)

Using the above relations (1 – 11), we calculate the final theoretical power and thrust achieved through the velocity triangle method. The net thrust is the sum total of the values obtained for all the sections. Twice this value is the final theoretical thrust due to the presence of two blades.

B. Analysis through CFD method The CFD analysis is mainly implemented through the

software Gambit and Fluent. Initially, the mesh for each of the sections is generated in Gambit and the pressure plots are generated in Fluent by defining the appropriate boundary conditions. The pressure plots are then analyzed and the average pressures on the top and bottom regions of the airfoil are noted. Then using the formulas defined below, the total power and thrust produced by the propeller for the given RPM is calibrated by integrating the thrust obtained for each section over the entire length of the propeller. Here, depending on the Mach number, we include a correction factor which is inclusive of the Prantld tip loss factor, the correction due to shocks produced at higher Mach numbers etc.

The legend for the following section is as follows: Pup - Average pressure on upper face Pdown - Average pressure on lower face Fup – Force per unit length exerted on the upper face Fdown – Force per unit length exerted on the lower face Lup - Length on upper face Ldown - Length of lower face FT - Total thrust per unit length P - Power

(12) (13) (14) (15)

2 (16)

Fig. 4 Sample pressure plot for a given propeller section

C. Analysis of the Deflector Fins Once the exit velocity of the air from the propeller and the

thrust generated is calculated, the torque generated by the deflector fins can be determined. Applying conservation of momentum:

(17) Where m1 – Mass of incoming air m2 – Mass of the aircraft Vi1 – Initial velocity of air Vf1 – Final velocity of air Vf2 – Final velocity of the aircraft r – Perpendicular distance from the center of the body to

the center of mass of the deflector fin. Since losses are assumed to be negligible, . The effective mass flow rate of the air acting on each

deflector fin ( ) is the total mass flow rate times the horizontal component of the deflector fin’s area divided by the total area.

(18) where l and b are the length and breadth of the fin, A is

the total circular area at the inlet (see Fig.5) and θ is the angle the deflector fin makes with the horizontal (see Fig.4).

Considering the components of the momentum along the horizontal axis, we get the horizontal force acting on each deflector fin as cos (19)

where the negative sign indicates that the force is acting in the direction opposite cos . The horizontal components of the force on two diametrically opposite deflector fins form a couple producing a torque given by 2 cos (20)

So the net torque created by the deflector fins is given by 4 V cos (21)

VI. CONSTRUCTION OF FINS • Four similar rectangular sheets/flaps placed at the

bottom of the vehicle and whose positions will be controlled by servos.

• To calculate the dimensions of the deflector fins, the exit velocity from the propeller should be calculated by the methods described in subsection A of section

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V. The maximum torque exerted by the propeller on the body of the aircraft can be measured using a dynamometer. This will give the torque that must be generated by the deflector fins. The dimensions of the deflector fins can then be determined using (18) and (21). The dimensions of the deflector fins must be such that it can generate the required amount of torque to counter the torque exerted on the body of the aircraft and at the same time the net thrust must be enough to lift the aircraft.

• Another, more accurate, method to determine the dimensions of the deflector fins is to perform CFD analysis similar to that for the propeller.

• It is recommended to use stators below the propeller to reduce the swirl of air and regulate the airflow for better efficiency.

Fig. 5 2D Model of Deflector Fins in Auto CAD

Fig. 6 3D Model of Deflector Fins designed in Solid works

VII. WORKING OF DEFLECTOR FINS • The deflector fins are used to vector the thrust

produced by the rotor to counteract the torque exerted on the body. The fins are oriented such that the out flowing air from the engine-rotor system impinges on the fins and creates an opposing force [7] to the torque thereby restricting the body from turning.

• The angle of attack of the fins must be varied depending on the rpm of the engine thus creating the appropriate force to negate the torque. Since the rpm

is controlled by the throttle, the deflector fins should vary its angle of attack in accordance with the throttle. In the absence of instabilities due to pitch and roll the deflector fins needs to counter only the torque exerted by the propeller. In such a situation all the deflector fins will have an equal angle of attack which ensures that the body does not tip.

• In case of pitching and rolling, the appropriate flap can be moved such that more air is allowed to eject as compared to other parts of the system thereby raising that particular side; For example, if the forward portion is pitching, the deflector fin must be adjusted to allow more ejection of air in the front and less in the rear thereby, pushing the forward portion into the stable level at the same time also preventing the body from yawing.

VIII. AUTOMATIC DYNAMIC STABILITY CONTROL (ADSC) As mentioned before, the required angle of attack (AOA)

of the deflector fins depend on the rpm of the engine, controlled by adjusting the throttle. The main component of the ADSC is a processor which calculates the AOA of each deflector fin and sends corresponding signals to the actuators controlling the deflector fins. The processor needs a predefined set of deflector fin AOA’s corresponding to various engine rpm’s, stored in tabular form. This table can be obtained either through simulation or experimental calculations based on the aircraft. The processor calculates the rpm using the throttle control signal from the receiver and compares it with the data in the table. When a match or a near match of the rpm with the tabled values is obtained, the processor records the corresponding angle from the table. Then it adjusts the angle of attack of the deflector fins by sending appropriate signals to the actuators.

The system also incorporates a feedback circuit that ensures the dynamic stability of the aircraft. The feedback circuit consists of two main components, a gyroscope and a two axis tilt-meter, which sends the feedback to the same processor. A gyroscope is a device used to measure the angular rotation of a body. So, ideally, it should be placed at the axis of rotation. The gyroscope reading will indicate if the body is rotating. If it does not rotate, the net torque on the body is zero. Otherwise, there is an imbalance of forces and so correction is required. Thus the processor will use the gyroscope reading to ensure that the body does not rotate by making small corrections to the AOA of the deflector fins. The two axes tilt-meter, as the name suggests, gives the amount of tilt along each axis that is used to calculate the pitch and roll. The deflector fins are adjusted based on the tilt-meter reading to prevent the aircraft from pitching and rolling. Both the gyroscope and tilt-meter outputs are passed through low pass filters to avoid noise due to vibrations. The output of the low pass filters are then given to the Analog to Digital Convertor (ADC) of the processor. The processor compares the digital values of the gyroscope and tilt-meter readings with pre-defined values to determine the pitch, roll

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and yaw as well the angle at which each deflector fin must be placed. Thus the feedback system ensures quick and efficient stability control for the aircraft. This system ensures dynamic stability as the sensors described above sense any instability due to turbulence or any other dynamic changes in center of mass resulting in pitching, rolling or yawing. The processor immediately sends the corresponding signals to the deflector fins so as to maintain the stability of the aircraft.

Fig. 8 Working of ADSC

Fig. 7 Block Diagram of Automatic Stability Control System

IX. EXPERIMENTAL SETUP The deflector fins have been tested on a prototype single

rotor VTOL unmanned aerial vehicle (UAV). The prototype uses a single cylinder internal combustion engine which powers the main rotor. The deflector fins are attached to support trusses on the body via hinges. The angle of each deflector fin is independently controlled by a servo motor.

The prototype has been tested only on manual control without ADSC implementation using a remote control. The servo motors that control the angle of the deflector fins were coupled to 2 channels of the remote controller. At constant engine RPM, activation of the deflector fins have shown convincing results.

Fig. 9 Deflector Fins implemented in a Single Rotor VTOL

X. ADVANTAGES • The system is very light and compact thereby reducing

the overall size of the full system and increasing the pay load carrying capacity of the system.

• The system can single handedly solve a variety of stability issues like anti-torque, pitching and rolling.

• Automatic Dynamic Stability Control takes care of all the stability issues, thus providing easy flight control.

• The system has a fast response time enabling the system to provide efficient dynamic stability.

• ADSC also incorporates a closed-loop system which means this system can be used in various designs allowing for auto-correction based on the feedback loop.

XI. APPLICATIONS This design can be used to solve the problem of anti-

torque produced by the main rotor of single rotor based vertical take-off and landing (VTOL) aerial vehicle designs. The compact nature of the design paves way for VTOL aircraft designs without the tail rotor.

XII. CONCLUSION A new, innovative system using deflector fins that can be

used to solve stability issues like anti-torque, pitching and rolling found in rotor based VTOL aircraft has been developed and applied to a single rotor aircraft. The

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experimental setup, still in the early stages, has shown promising results. As an extension, this system can be modified to use the deflector fins for horizontal motion as well as through the concept of thrust vectoring. The system can then be used as a full-fledged control mechanism for VTOLs.

ACKNOWLEDGEMENT The authors would like to thank Amrita Vishwa

Vidyapeetham for its continued support towards research activities by students. We would like to thank Mr. P. Eshwar of E.N.R. Model Aircraft and Mr. Kiran D. of Conceptia Software Technologies for their guidance and support. We would also like to express our gratitude to Mr. Pramod S., Mr. N. Thiagarajan, and Mr. Dhananjay R. and Mr. Mohan,

Department of Mechanical Engineering, Amrita Vishwa Vidyapeetham for their time and valuable advice.

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[3] Mathew Uttley, Westland and the British Helicopter Industry, Frank Cass Publishers, 2001.

[4] Yunus A. Cengel and John M. Cimbala, Fluid Mechanics,3rd ed., Tata McGraw-Hill, 2008.

[5] Frank M. White, Fluid Mechanics, 5th ed, McGraw-Hill, 2003. [6] David F. Anderson and Scott Eberhardt, Understanding Flight,

McGraw-Hill, 2001.