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NAVAL AIR TRAINING COMMAND NAS CORPUS CHRISTI, TEXAS CNATRA P-401 (REV 09-00) INTRODUCTION TO HELICOPTER AERODYNAMICS WORKBOOK AERODYNAMICS TRANSITION HELICOPTER 2000 DEPARTMENT OF THE NAVY CHIEF OF NAVAL AIR TRAINING NAVAL AIR STATION CORPUS CHRISTI, TEXAS 78419-5100 N3143 1. CNAT P-401 (Rev. 9-00) PAT, Introduction to Helicopter Aerodynamics Workbook, Aerodynamics, Transition Helicopter, is issued for information, standardization of instruction and guidance of instructors and student naval aviators in the Naval Air Training Command. 2. This publication will be used to implement the academic portion of the Transition Helicopter curriculum. 3. Recommendations for changes shall be submitted to CNATRA Code N3121. POC is DSN 861-3993. COMM (512) 961-3993/ CNATRA FAX is 861-3398. 4. CNAT P-401 (Rev. 9-99) PAT is hereby canceled and superseded. Distribution: CNATRA (5) COMTRAWING FIVE (Academics) (395) ) Plus Originals, Code 70000) iii STUDENT WORKBOOK Q-2A-0015 INTRODUCTION TO HELICOPTER AERODYNAMICS WORKBOOK Prepared by COMTRAWING FIVE 7480 USS ENTERPRISE ST SUITE 205 MILTON, FL 32570-6017 Prepared for CHIEF OF NAVAL AIR TRAINING 250 LEXINGTON BLVD SUITE 102 CORPUS CHRISTI, TX 78419-5041 SEPTEMBER 2000 iv LIST OF EFFECTIVE PAGES Dates of issue for original and changed pages are: Original...0... (this will be the date issued) TOTAL NUMBER OF PAGES IN THIS PUBLICATION IS 82 CONSISTING OF THE FOLLOWING: Page No. Change No. Page No. Change No. COVER 0 3-22 0 LETTER 0 4-1 – 4-12 0 iii -ix 0 4-13 (blank) 0 x (blank) 0 4-14 0 1-1 - 16 0 5-1 – 5-14 0 2-1 – 2-14 0 5-15 (blank) 0 3-1 – 3-20 0 5-16 0 3-21 (blank) 0 0 v Change Record Number Description of Change Entered by Date vi TABLE OF CONTENTS STUDENT WORKBOOK TITLE PAGE .................................................................................... iii LIST OF EFFECTIVE PAGES ..................................................................................................... iv CHANGE RECORD........................................................................................................................v TABLE OF CONTENTS............................................................................................................... vi WORKBOOK PLAN TERMINAL OBJECTIVE........................................................................................................... viii INSTRUCTIONAL MATERIAL................................................................................................ viii DIRECTIONS TO STUDENT .................................................................................................... viii AUDIOVISUAL.......................................................................................................................... viii WORKBOOK TEXT CHAPTER ONE - THE ATMOSPHERE OBJECTIVES............................................................................................................... 1-1 ATMOSPHERIC PROPERTIES.................................................................................. 1-2 ATMOSPHERIC PRESSURE ..................................................................................... 1-2 ATMOSPHERIC DENSITY AND POWER REQUIRED .......................................... 1-2 REVIEW QUESTIONS................................................................................................ 1-5 REVIEW ANSWERS................................................................................................... 1-6 CHAPTER TWO - ROTOR BLADE AERODYNAMICS OBJECTIVES............................................................................................................... 2-1 DEFINITIONS.............................................................................................................. 2-2 THEORIES OF HELICOPTER FLIGHT .................................................................... 2-4 AIRFOILS..................................................................................................................... 2-6 PITCHING MOMENTS............................................................................................... 2-6 ROTOR SYSTEMS...................................................................................................... 2-9 REVIEW QUESTIONS.............................................................................................. 2-12 REVIEW ANSWERS................................................................................................. 2-13 CHAPTER THREE - HELICOPTER POWERED FLIGHT ANALYSIS OBJECTIVES............................................................................................................... 3-1 POWER REQUIRED ................................................................................................... 3-3 POWER REQUIRED AND POWER AVAILABLE................................................... 3-5 TORQUE ...................................................................................................................... 3-6 STABILITY AND CONTROL .................................................................................... 3-7 vii VORTICES................................................................................................................... 3-9 GROUND EFFECT.................................................................................................... 3-10 GROUND VORTEX .................................................................................................. 3-12 TRANSLATIONAL LIFT.......................................................................................... 3-12 DISSYMETRY OF LIFT ........................................................................................... 3-12 PHASE LAG............................................................................................................... 3-13 BLOWBACK.............................................................................................................. 3-14 TRANSVERSE FLOW AND CONING .................................................................... 3-15 BLADE TWIST.......................................................................................................... 3-16 CENTER OF GRAVITY............................................................................................ 3-17 REVIEW QUESTIONS.............................................................................................. 3-21 REVIEW ANSWERS................................................................................................. 3-22 CHAPTER FOUR - AUTOROTATION OBJECTIVES............................................................................................................... 4-1 FLOW STATES AND DESCENDING FLIGHT ........................................................ 4-2 AUTOROTATION....................................................................................................... 4-3 AUTOROTATION ENTRY......................................................................................... 4-4 CUSHIONING THE TOUCHDOWN.......................................................................... 4-5 AIRSPEED AND ROTOR SPEED CONTROL .......................................................... 4-7 HEIGHT-VELOCITY DIAGRAM ............................................................................ 4-10 REVIEW QUESTIONS.............................................................................................. 4-13 REVIEW ANSWERS................................................................................................. 4-14 CHAPTER FIVE - FLIGHT PHENOMENA OBJECTIVES............................................................................................................... 5-1 RETREATING BLADE STALL.................................................................................. 5-3 COMPRESSIBILITY EFFECT.................................................................................... 5-5 VORTEX RING STATE .............................................................................................. 5-6 POWER REQUIRED EXCEEDS POWER AVAILABLE ......................................... 5-8 GROUND RESONANCE ............................................................................................ 5-9 DYNAMIC ROLLOVER ........................................................................................... 5-10 MAST BUMPING...................................................................................................... 5-12 VIBRATIONS ............................................................................................................ 5-14 REVIEW QUESTIONS.............................................................................................. 5-16 REVIEW ANSWERS................................................................................................. 5-17 GLOSSARY ......................................................................................................................... G-1 viii NAVAL AIR TRAINING COMMAND ADVANCED PHASE DISCIPLINE: Aerodynamics COURSE TITLE: Aerodynamics (Transition Helicopter) PREREQUISITES: None TERMINAL OBJECTIVE Upon completion of the course, "Aerodynamics, Transition Helicopter," the student will possess an understanding of aerodynamics as applied to helicopters, to include the effects of atmosphere. The student will demonstrate a functional knowledge of the material presented through successful completion of an end-of-course examination with a minimum score of 80%. INSTRUCTIONAL MATERIAL To implement this learning session, the instructor in charge must ensure that one copy of the NATOPS Flight Manual, Navy Model TH-57B/C Helicopter, NAVAIR 01-110-HCC-1, be available to each student. When the material listed above has been assembled, the student will proceed in accordance with the following directions: DIRECTIONS TO THE STUDENT STEP 1 Complete each chapter of the course workbook text. STEP 2 Take the review test for each chapter. STEP 3 Attend aero review before exam. STEP 4 Take the end-of-course examination. Remedial sessions prescribed if necessary. STEP 5 End of this course of instruction. AUDIOVISUAL Stock No. Minutes Chapter 1 Atmospheric Density and Helicopter Flight 4B88/5 19:30 Chapter 2 Rotor Blade Aerodynamics - Part 1 4B88/1-1 Rotor Blade Aerodynamics - Part 2 4B88/1-2 12:00 Chapter 3 Helicopter Powered Flight Analysis - Part 1 4B88/2-1 14:00 Helicopter Powered Flight Analysis - Part 2 4B88/2-2 19:50 Chapter 4 Autorotational Flight 4B88/3 13:00 ix Chapter 5 Helicopter Flight Phenomena - Part 1 4B88/4-1 26:30 Helicopter Flight Phenomena - Part 2 4B88/4-2 14:00 Helicopter Flight Phenomena - Part 3 4B88/4-3 12:00 x THIS PAGE INTENTIONALLY LEFT BLANK THE ATMOSPHERE 1-1 CHAPTER ONE THE ATMOSPHERE TERMINAL OBJECTIVE 1.0 Upon completion of this chapter, the student will define density altitude, the factors affecting it, and the effect density altitude has on aircraft performance. ENABLING OBJECTIVES 1.1 Recall the main gases of the air. 1.2 Recall the effect of pressure, temperature, and humidity on the density of the air. 1.3 Define pressure altitude. 1.4 Define density altitude. 1.4.1 Recall the effect of temperature and humidity on density altitude. 1.4.2 Compute the density altitude using a density altitude chart. 1.4.3 Compute the density altitude using the rule of thumb formula. 1.4.4 Recall the relationship between helicopter performance and density altitude. CHAPTER 1 HELICOPTER AERODYNAMICS WORKBOOK 1-2 THE ATMOSPHERE THE ATMOSPHERE ATMOSPHERIC PROPERTIES Helicopter aerodynamics is the branch of physics dealing with the forces and pressures exerted by air in motion. The atmosphere, the mass of air, which completely envelops the earth, is composed of varying and nonvarying constituents. The nonvarying constituents include oxygen (21%) and nitrogen (78%). The varying constituents include CO2, argon, hydrogen, helium, neon, krypton, and water vapor, which will vary from negligible amounts to approximately 4% by volume (100% relative humidity). Air is a fluid and is affected by changes in temperature, pressure, and humidity. ATMOSPHERIC PRESSURE Atmospheric pressure at any altitude is a result of the downward pressure exerted from the mass of air above that altitude. The air at the surface of the earth will be under a greater pressure than air further up a given column of air. Pressure altitude is defined as an altitude corresponding to a particular static air pressure in the standard atmosphere. The standard atmosphere corresponds to the temperature and pressure of the standard day (15° C, 29.92 or 10MB, 14.7 psi at sea level). Therefore, the pressure altitude of a given static air pressure corresponds to the actual altitude only in the rare case where atmospheric conditions between sea level and the aircraft's altimeter correspond exactly to that of the standard atmosphere. ATMOSPHERIC DENSITY AND POWER REQUIRED Atmospheric density is also greatest at the earth's surface and the atmosphere becomes less dense, or contains fewer molecules per unit volume, as distance from the earth's surface increases. Atmospheric density also decreases with an increase in temperature or humidity. Heated air expands, causing the air molecules to move farther apart, thus decreasing air density per unit volume. As relative humidity increases, water vapor molecules, which have a smaller molecular mass than oxygen and nitrogen molecules, displace some air molecules in a given volume, creating a decrease in density in a given volume. Density altitude is the altitude in the standard atmosphere corresponding to a particular air density. It is pressure altitude corrected for temperature and humidity. Air density affects the aerodynamic forces on the rotor blades and the burning of fuel in the engine, affecting both power required and power available. For a given set of atmospheric conditions, the total power required to drive the rotor depends on three separate requirements, which have a common factor -- rotor drag. Each power requirement is considered separately, and will be discussed in greater depth in a later section. 1. Rotor Profile Power (RPP). This is the power requirement to overcome friction drag of the blades. RPP assumes a constant minimum pitch angle and a constant coefficient of drag value. As density altitude increases and air density decreases, drag, and therefore RPP, will decrease. However, blade stall begins sooner, so more of the blade is in stall, increasing profile power. HELICOPTER AERODYNAMICS WORKBOOK CHAPTER 1 THE ATMOSPHERE 1-3 2. Induced power. This is the power associated with producing rotor thrust and must be sufficient to overcome the induced drag which increases proportionally to thrust. In order to maintain rotor thrust as air density decreases, angle of attack (AOA) must be increased by increasing pitch on the rotor blades. The resulting increase in rotor drag requires an increase in induced power to maintain a constant Nr. Increased density altitude affects induced power significantly. 3. Parasite power. This is the power required to overcome the friction drag of all the aircraft components, rotor blades being the exception. Parasite drag is constant for a given IAS. As density altitude (DA) increases, TAS increases, and parasite drag will decrease slightly. The combination of these ups and downs result in greater power required at a higher density altitude. Power required, the amount of power necessary to maintain a constant rotor speed, is adversely affected by increased DA and decreased rotor efficiency. The pitch angle of the blades must be increased to increase the AOA during high DA conditions in order to generate the same amount of lift generated during low DA conditions. Increased pitch angle results from an increased collective setting, which demands more power from the engine. DA also affects power available, or engine performance. Turbine engine performance will be adversely affected by an increase in DA. As DA increases, the compressor must increase rotational speed (Ng) to maintain the same mass flow of air to the combustion chamber; and the bottom line is, when maximum Ng is reached on a high DA day, there is a lower mass flow of air for combustion, and therefore (because of fuel metering) a lower fuel flow as well. Thus, with increased DA, power available from a gas turbine engine is reduced. Since DA affects helicopter rotor and engine performance, it is a necessary consideration for safe preflight planning. It can be determined in two ways: deriving a value from NATOPS charts (figure 1-1) or a “rule of thumb” which can be used in the aircraft when no chart is available (see figure 1-2). CHAPTER 1 HELICOPTER AERODYNAMICS WORKBOOK 1-4 THE ATMOSPHERE Figure 1-1 Density Altitude/Temperature Conversion Chart Increase DA 100' for each 10% increase in relative humidity Figure 1-2 HELICOPTER AERODYNAMICS WORKBOOK CHAPTER 1 THE ATMOSPHERE 1-5 CHAPTER ONE REVIEW QUESTIONS 1. Oxygen comprises approximately ________ percent of the earth's atmosphere. 2. Air density changes in direct proportion to ____________ and inverse proportion to____________, ____________, and ____________. 3. Compared to dry air, the density of air at 100% humidity is a. 4% more dense. b. about the same. c. decreased 1 percent per 1000'. d. less dense. 4. The altitude of a given static air pressure in the standard atmosphere is_______________. 5. Density altitude is pressure altitude corrected for _______________ and _______________. 6. When relative humidity is 50%, the moist air is half as dense as dry air. ____(True/False) 7. As temperature increases above standard day conditions, density altitude increases/decreases and air density increases/decreases. 8. Using the Density Altitude Chart on page 1-4, find the density altitude for a pressure altitude of 3500', temperature of 240C, and relative humidity of 50%. ______________ 9. Using the rule of thumb formula, calculate the density altitude for a pressure altitude of 6000', temperature of 170C, and relative humidity of 50%. __________________________ 10. An increase in humidity increases/decreases density altitude, which increases/decreases rotor efficiency. 11. State the effects that increased density altitude has on power available and power required. ______________________________________________________________________ CHAPTER 1 HELICOPTER AERODYNAMICS WORKBOOK 1-6 THE ATMOSPHERE CHAPTER ONE REVIEW ANSWERS 1. 21 2. pressure . . . altitude . . . temperature . . . humidity 3. d 4. pressure altitude 5. temperature . . . humidity