CHAPTER 3PROBLEM FORMULATION31 MotivationIn design of an aircraft

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CHAPTER 3
PROBLEM FORMULATION

3.1 Motivation

In design of an aircraft we are always looking for an optimum design which will fulfil all the mission requirements. Design can be optimum with respect to what we want to optimize in the aircraft e.g. minimum weight, minimum drag, maximum endurance etc. Designing a wing is the primary task in the aircraft design process. In context of flight, wing is arguably the most crucial component. Whereas the airfoil to be used on a new aircraft was once chosen from a catalog of possibilities as the compromise which most closely matched the design requirements, the state of airfoil design is now at such a level that each new vehicle should have an airfoil tailored specifically to the intended mission.

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The role of the airfoil designer in this case is as it has always been, that is, to achieve the required lift for the least possible drag. It should not be inferred, however, that the best airfoil design is accomplished by maximizing the section lift-to-drag ratio.

Instead, by making use of modern airfoil design technologies, the designer arrives at the most suitable airfoil for a particular aircraft by trading off the conflicting goals. These types of trade-offs make it clear that in order to achieve the highest levels of aircraft performance possible, the airfoil design process should be integrated.

The flight profiles of high-altitude long-endurance unmanned air vehicles (HALE UAVs), at high altitudes and moderate flight speeds, represent low-Reynolds number operating conditions. The required lift coefficient is high when compared to more conventional lower-altitude aircraft, due to reduced air density.

Therefore, the aerofoil section needs to be optimised to generate high lift coefficients with a profile drag as low as possible. The pitching moment (Cm) should also be considered especially for the flying wing configuration with no tail. An additional constraint in the selection of suitable aerofoil sections is the thickness required to hold fuel tanks or batteries and the required structural elements for adequate stiffness. The design process is an interdisciplinary approach, and includes selection of wing section, aerodynamic optimisation of swept wing/unswept, stability analysis, weight balance, structural and ?utter analysis, many on-board redundant systems, reliability and maintability analysis, safety improvement, cost and performance optimisation. Presented paper focuses mainly on aerodynamics related to performance and geometry of wing design (airfoil).

3.2 Objectives:

Objectives are dictated by the mission specifications. The main objective of this work is to design a airfoil for an UAV which is capable of achieving cruise altitudes in range of 20km or 60000ft with long endurance of greater than 24hrs . In contemporary scenario any Military oriented HALE system should be capable of carrying all basic payloads like SAR, SIGINT, FLIR etc apart from specilalised payloads for specific missions. That makes the system should be capable of carrying a payload in range of 1000kg. Keeping these three parameters as primary mission requisites i.e payload, altitude and endurance, our objective is to design airfoil for such system. The objectives of the work are the following-

1. Understanding key features of airfoils and study of the equations and mathematical models used to determine their characteristics.

2. Development of a new airfoil.

3. Testing of airfoil in multiple softwares.

4. Ensuring that its feature is better than the pre-existing models.

5. Fixing parameters keeping in mind the economic constraints.

6. Learning the proper use of the softwares and the nature and effect of changes of shape of an airfoil on lift and other defining parameters.

The objectives will be achieved in following steps.

· High altitude atmosphere study.

· Initial UAV sizing.

· Engine selection.

· Weight estimation.

· Empirical selection of airfoil and validation of parameters.

· 2D analysis of airfoil using various automation tools.

· 3D analysis of airfoil using ANSYS.

· Calculation of coefficients and lift.

· Calculation of wing dimensions.

· Taper, swept back and washout inclusion and effects on performance at all altitudes.

· Calculations of most suitable parameters for optimized flight in all segments of mission.

· Constraint Analysis

· Finalisation of airfoil and wing design for optimized flight.

3.3 Design requirements for flight profile

Design Requirements and the mission specifications for a military HALE system

are:

· Endurance 24 hours+ under specified weather condition

· Weight Class 15000 kilogram AOW

· Cruising Altitude 60000ft

· Rate of Climb 20 m/s

· Cruising velocity >650km/s

· Payload 1000kg

CHAPTER 4

4.1 Atmosphere Study at High Altitudes

HALE UAVs are high-flying aircraft or airships, which will operate from altitudes of 17 to 24 km. The main reason for this operating altitude is that numerous wind-speed measurements show that the slowest wind speeds occur at these heights, extending above the tropospheric jet-stream wind altitudes. Less power is therefore required for the system’s stabilization. A second minimum wind zone occurs at around 90 km altitude, but this is less advantageous HALE operation. Parameters of a standard atmosphere are depicted below.

Numerous temperature measurements show that the temperature decreases with altitude, and reaches a local minimum near the tropopause layer, around an altitude of 18 km. The average temperature is quite constant, around -56 degree C, at this lower stratospheric layer where HALE UAVs will operate. And change of viscocity and density with temperature is mentioned in below figure.

4.2 Initial UAV Sizing

UAV sizing is the synthesis of multiple design disciplines to de?ne the major attributes of the design. A rigorous analysis of the UA using detailed methods is necessary to ensure that the solution is compliant with the system requirements. However, simple methods can be used to gain visibility into the design space and better understand characteristics of the UA solution. Flexibility is important for early sizing of unmanned aircraft that can span several orders of magnitude in weight and size.

UAV takeoff gross weight WTO is a major parameter that largely de?nes the vehicle class and from which much can be inferred. Therefore, the sizing discussion begins with intuitive methods for determining this important parameter. The ?rst objective is to derive rapid, closed-form methods for estimating WTO that are suitable for all classes of unmanned aircraft, from MAVs to very large vehicles. While enabling broad applicability and expediency might compromise accuracy, these methods are suitable for many applications. Some example uses include initially assessing new requirements, determining technology impacts, evaluating the veracity of competitor’s marketing claims, or providing a “sanity check” for the outputs of more detailed methods. The simple weight sizing relationships are well suited to scaling aircraft within a family of UAV.

Initial UAV Sizing process

4.3 Selection of propulsion system for HALE UAVs

At high altitude, the atmospheric pressure is low, but the air density is still sufficient for activating aircraft propellers operated by electrical motors from solar energy or fuel. The relatively smooth-flowing air stream, combined with state-of-the-art propulsion, aerodynamic, thermodynamic, and material designs, should provide a stable and controlled flight. This will result in accurate position maintenance and minimal axial rotation. The rate of change of the velocity of stratospheric winds is well within the capability of the propulsion and control systems of HAPs for maintaining the desired position and heading. There are two categories of HALE UAVs. One is Lighter-than-air (LTA) HALE that are usually balloon aerostats, or airships filled with helium gas. The lighter-than-air HALE need less energy for launching and stabilization over a fixed spot. Heavier-than-air (HTA) are those HALE systems which uses fuel or electric system for propulsion. An adequate forward thrust is provided by propellers activated by electric motors, jet engines, or other thrusters.

It is widely acknowledged that the propulsion system is the most difficult technical challenge. Whether it is manned or unmanned, an aircraft designed to fly sub sonically >60 kft for >24 hrs will require a propulsion system that is quite different from existing systems. Because of payload carrying capability, range and flight duration, air breathing propulsion becomes a mandate. In this flight regime, however, air breathing propulsion is difficult to achieve. The difficulty arises from the exponential lapse of air density and pressure with altitude. At 60 kft the ambient air density and pressure are about 1/28th of sea level values. Therefore, turbomachinery is needed to supply most of the intake pressurization required to compress ambient air into a power plant working fluid of reasonable density.

To pressurize the intake to 1 atm at 60 kft, an overall pressure ratio better than 33:1 is required. Several turbomachinery stages are needed for intake pressurization, which translates to larger turbomachinery diameters. Pressure ratio requirements also increase with altitude, which translates to more turbomachinery stages. Since power is proportional to airflow for any air breathing engine, the machinery size required to process airflow for a given rated power will grow with altitude. Because of the increased size and weight of the air handling, thermal management and thrust delivery components, a propulsion system designed for high altitudes is significantly larger and heavier than its low altitude counterpart. Further complicating matters, the high altitude aircraft will need more power to stay aloft because of the faster flight speeds necessary (to maintain dynamic pressure and support its weight in low density air). The propulsion system grows in both rating and in specific weight, which tends to claim greater and greater fractions of the airplane’s gross weight. This of course runs counter to the airplane’s ability to carry the weight.

The weight penalty associated with air handling and thermal management becomes a major discriminator when choosing propulsion for the high altitude aircraft. There are two power plant candidates to consider, turbine engines (i.e. a turbojet/turbofan) and turbocharged reciprocating engines. The turbojet/turbofan engine would be associated with a relatively high speed (0.7

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