A 
Rotor disk area, ft^{2}. 
a_{0} 
Speed of sound, ft/sec. 
Number of blades. 

bifilar 
Bifilar absorbers are mounted on the UH60A rotor hub and reduce inplane vibratory forces. 
boundary layer 
A thin layer near the airfoil surface where viscosity is important and influences the airfoil forces and moments, primarily drag. Stall is characterized by major changes in the boundary layer, including separation. 
BVI 
An abbreviation for bladevortex interaction. Although the terminology is sometimes applied to vortex and blade interactions regardless of the source of the vortex, more commonly the term refers to the interaction of a tip vortex trailed from a prior blade with a following blade. These interactions are not just with vortices from the immediately prior blade, but may extend well back in terms of the rotor wake history. 
Chord bending moment coefficient, . 

C_{d} 
Section drag coefficient. 
C_{FM}/σ 
Flap bending moment coefficient, . 
C_{l} 
Section lift coefficient. 
C_{lmax} 
Maximum section lift coefficient. 
C_{m} 
Section moment coefficient. 
C_{mmin} 
Minimum section moment coefficient (during dynamic stall cycles). 
C_{TM}/σ 
Torsion moment coefficient,. 
C_{n} 
Section normal force coefficient. 
C_{n}_{max} 
Maximum section normal force coefficient (during dynamic stall cycles). 
C_{P} 
Power coefficient, . 
C_{p} 
Pressure coefficient, . 
C_{T} 
Thrust coefficient, . 
C_{W} 
Weight coefficient, . The weight coefficient and thrust coefficient differ by factors such as download, aircraft lift from the fuselage and stabilator, and for the UH60A, the lift of the canted tail rotor. For flight data, the weight is known accurately, whereas the thrust is not. 
c 
Blade chord, ft. 
A comprehensive analysis developed by Johnson Aeronautics (Johnson 1994). Although the primary purpose of this tutorial is directed to the experimental measurements, comparison with theoretical results is sometime useful. See also CHARM and RCAS. 

Cartesian surface plot 
These 3D surface plots show with a dependent variable, such as normal force or flap bending moment, as a function of both azimuth and another independent variable such as radius or advance ratio. Rotor measurements are, of course, in a polar system, but surface plots in polar coordinates appear less effective in understanding rotor system behavior than the use of a Cartesian grid (Hooper 1984). 
CHARM 
An aeromechanics analysis developed by CDI (Quackenbush et al. 1999). Although the primary purpose of this tutorial is directed to the experimental measurements, comparison with theoretical results is sometime useful. See also CAMRAD II and RCAS. 
chord bending moment 
The rotor blade structural moment in the chordwise direction, measured about the blade's crosssectional neutral axis using a straingauge bridge. The moment is positive when the section's leading edge is in compression. This moment does not exactly align with the section's chordwise axis because of the method of calibration. 
chord force 
Force obtained from integration of airfoil section measured pressures. This force is parallel to the section chordline and does not include viscous drag; it is sometimes called pressure drag. Positive forward. 
critical pressure coefficient. 
The critical pressure coefficient, C_{p}*, marks the boundary between subsonic and supersonic flow on an airfoil. This boundary is sometimes referred to as the sonic line. 
counter 
Term used to describe a test point in flight testing. This derives from the an earlier period when quantitative flight test information was obtained from a photo panel. The photo panel used a camera to photograph instrumentation, usually such things as altimeters, airspeed indicators, and so forth. An incremental counter was included on the photo panel to identify each test point. [Panel 3.8] 
A 3D surface plot where the two independent variables are blade azimuth and cycle (or revolution). In this plot the azimuthal time histories are aligned such that each cycle or revolution is adjacent to the previous one. The adjacent cycles may be from a single counter, in which case they show the steadiness or repeatability for steady counters or they show the effects of maneuvering for unsteady counters. Or, the adjacent cycles may be from different counters, as in the case where individual housekeeping points are compared. This plot is sometimes called a waterfall plot. 

Articulated rotors, such as on the UH60A, require dampers to be mounted at the root of the blade across the effective leadlag hinge. The damper is needed to prevent ground resonance, which is a rotor instability that is inherent in all articulated rotors. The damper loading characteristics are highly nonlinear. 

disk vortex 
Individual blade tip vortices tend to wrap around each other on the left and right sides of the rotor disk. These rotor disk vortices are analogous to the tip vortices of a fixedwing aircraft. The apparent size of the disk vortex core by the time it is intersected by a trailing blade may be as much as ten times the size of the blade chord. Sometimes called vortex bundles or supervortices. See also tip vortex. [Panel 2.12.] 
diving turn 
A steady turn in diving flight. Steady turns in level flight require additional power, depending upon the load factor developed. At some advance ratio, the aircraft will no longer be able to sustain level flight at the specified load factor. Diving turns allow increased airspeed or load factor by converting potential energy to power. For the UH60A, steady turn data are obtained to μ ~ 0.3 (Bousman and Kufeld 2005). 
drag divergence 
Section drag is relatively constant at subsonic Mach numbers, but as Mach number increases, shocks form on the airfoil surface and the drag increases in a nonlinear manner. The nonlinear increase in drag is called drag divergence and the associated Mach number is referred to as the divergence Mach number. This is also called transonic drag rise. 
The load limit where fatigue damage starts to accumulate on a component and affects the safe life. (See also safe life.) 

EPNL 
Effective Perceived Noise Level is a modification to Perceived Noise Level (PNL), which measures broadband noise, to account for duration and the effects of tonal components. 
The ideal power to hover divided by the actual power required, . 

flap bending moment 
The rotor blade structural moment in the flapwise direction, that is, perpendicular to the section's chordline. The moment is positive with the upper surface in compression. It is measured about the blade's crosssectional neutral axis using a straingauge bridge. This moment does not exactly align with the section's flapwise axis because of the method of calibration. 
frequency response 
Frequency response refers to the magnitude and phase of an output as compared to a known input. Pressure transducers typically respond as undamped firstorder systems with a large response at the first natural frequency. This frequency is designed to be much higher than the filter settings that will be used during measurement, such that the inputoutput magnitude is approximately one over the frequency range of interest. But transducers with poor frequency response show a vast range of responses. Some have essentially zero frequency response (dead), whereas others have reasonable frequency response for structural measurements (lower frequencies), but not for acoustic measurements (higher frequencies). 
Gross weight, lb. 

Any time series can be represented as a Fourier series of cosine and sine harmonics of some fundamental frequency. For a helicopter, the appropriate fundamental frequency is 1/rev. The time series and harmonic series are directly equivalent and represent measurements in the time and frequency domains respectively. For a rotorcraft, most loading occurs at multiples of the rotor's 1/rev frequency. As this loading tends to fall off with harmonic number (referring to 1/rev, 2/rev, and so on), the representation of measurements within the frequency domain is often convenient. To look more clearly at lower harmonics, one can eliminate the higher harmonics (a lowpass filter). Or, to look at higher harmonics, one can eliminate the lower harmonics (a highpass filter). If a particular problem covers just a limited range of harmonics, then just these harmonics may be examined (a bandpass filter). 

Crews (1987) has defined an Intrusion Index to represent how the pilot or crew respond to helicopter vibration. The Intrusion Index requires vibration measurements in three orthogonal axes for each location in the helicopter where the index is calculated. The three axes are weighted differently: the lateral axis has an 0.75 weight relative to the vertical and the longitudinal axis has an 0.50 weight. In addition, each component includes a weighting factor that depends upon frequency. The Intrusion Index is the norm of he four largest harmonic amplitudes in each axis and, hence, is composed of 12 harmonic amplitudes. Although the 1/rev amplitude is excluded, all other harmonics up to 60 Hz are included and there is no restriction to include only bN/rev harmonics, where b is the blade number and N = 1,2, . . . 

Lifttodrag ratio. 

lag damper 
Rotors with a first inplane nondimensional frequency less than one, either articulated or soft inplane, require damping in the rotor blades to prevent either ground or air resonance. The UH60A has lag dampers near the blade root and these are effective when the blade moves inplane around the effective hinge point of the rootend elastomeric bearing. These damper forces were measured during the Airloads Program experiments. 
lift 
The lift on an airfoil section is defined to be perpendicular to the free stream. But for flight test measurements, the angle of attack is unknown and the lift cannot be obtained using pressure measurements. See normal force. 
limit dive speed 
Airspeeds above the maximum level flight speed, which is power limited, can be obtained in diving flight. But as loads increase in diving flight a speed is reached which is described as the limit dive speed or 'neverexceed' speed. This is not a power limit, but a load limit condition. 
load cancellation 
Not all vibratory loads in the rotor pass through to the fuselage to cause vibration. If all blades are perfectly identical, then flap bending and torsion moments will be transmitted only for vibratory loads at frequencies of N_{b}, 2N_{b}, 3N_{b}, and so on (N_{b} refers to the number of blades). Thus, for the UH60A, vibratory loads are transmitted for 4/rev, 8/rev, and so on. But for vibratory chord bending moments, transmission occurs at frequencies of N_{b}±1, 2N_{b}±1, 3N_{b},±1, and so on, which for the UH60A means transmission occurs at 3/rev, 5/rev, 7/rev, 9/rev and so on. 
load factor 
In normal level flight there is no acceleration force on the aircraft except for that of gravity, in which case the load factor, n_{z}, is one (1g). In accelerated flight during maneuvers, the load factor may increase and in severe maneuvers may exceed 2g's. If flight data are shown on a plot where the product of load factor and weight coefficient are the ordinate and advance ratio is the abscissa, then both steady and maneuver conditions may be compared. 
LRTA α 
Large Rotor Test Apparatus. Rotor test stand developed to test UH60 class rotors in the NFAC wind tunnels. 
Mach number, , velocity divided by the speed of sound. Mach numbers below one refer to subsonic flow, whereas those above one refer to supersonic flow. 

M_{dd} 
Divergence Mach number. 
Tip Mach number, . 

M_{ζ} 
Chord bending moment, inlb (or ftlb). 
M_{b} 
Flap bending moment, inlb (or ftlb). 
M_{q} 
Torsion moment, inlb (or ft.lb). 
M^{2}c_{m} nondimensional pitching moment 
The pitching moment coefficient, c_{m}, is nondimensional, but is referenced to the local geometric velocity, which depends on radius and azimuth. By multiplying by M^{2}, the reference velocity becomes the speed of sound rather than the local geometric velocity and is invariant with radius and azimuth. 
M^{2}c_{n} nondimensional normal force 
The normal force coefficient, c_{n}, is nondimensional, but is referenced to the local geometric velocity, which depends on radius and azimuth. By multiplying by M^{2}, the reference velocity becomes the speed of sound rather than the local geometric velocity and is invariant with radius and azimuth. 
M^{2}c_{p} nondimensional pressure coefficient 
The normal force coefficient, c_{p}, is nondimensional, but is referenced to the local geometric velocity, which depends on radius and azimuth. By multiplying by M^{2}, the reference velocity becomes the speed of sound rather than the local geometric velocity and is invariant with radius and azimuth. 
McHugh stall boundary 
McHugh (1978) reported the results of a wind tunnel experiment that used a rotor model designed and constructed so that the aerodynamic limits of the rotor could be determined. Thus, during testing at one advance ratio for example, the collective pitch was increased until the rotor thrust reversed (because of stall). The limit conditions obtained were aerodynamic limits as opposed to structural limits as occurs in normal rotor tests. 
miss distance 
When a rotor blade passes close to a tip vortex from a previous blade that interaction can radiate acoustic energy to the far field, which is highly annoying. The severity of the radiated acoustics depends upon a number of factors, one of the most important is how close the vortex is to the rotor blade during the interaction. This distance is referred to as the miss distance. See also BVI. 
M/R 
Main rotor. 
Vibratory forcing is canceled in the rotor for some frequencies and transmitted to the fuselage for others. N per rev forcing, where N refers to the number of blades, refers to those frequencies that are transmitted to the fuselage and cause vibration. Normally, vibratory loads in flap are transmitted to the fuselage for frequencies of N/rev, 2N/rev, 3N/rev, and so forth. Vibratory loads in chord are transmitted at frequencies of N±1/rev, 2N±1/rev, 3N±1/rev, and so forth. Thus, for the UH60, 4/rev, 8/rev, and 12/rev loads in flap bending are important for vibration and 3/rev, 5/rev, 7/rev, and 9/rev loads in chord bending are important. 

NFAC  National Fullscale Aerodynamic Complex 
Largescale wind tunnels at the NASA Ames Research Center, including the 40 by 80Foot and 80 by 120Foot wind tunnels. 
noise footprint 
Measurements on the ground of the perceived noise levels of a helicopter approaching a landing area can be used to create a contour map of the noise heard by ground observers. Based on a specific noise level, the resulting map describes a 'footprint' of annoyance. The extent of the footprint can be affected by approach speeds and descent angles. 
normal force 
The normal force is obtained by integrating the airfoil section measured pressures. It is defined perpendicular to the airfoil chordline. Integration of pressure measurements can provide the normal force, but not the lift as the actual angle of attack is unknown. Positive up. See also lift. 
Oneengineinoperative. 

offset plot 
This plot uses offsets in the zeropoint of the ordinate when plotting multiple curves that represent a second independent variable. For example, if the blade normal force is shown as a function of blade azimuth at nine radial stations, the curves will overlap each other using a standard plot, obscuring the behavior. But with an offset plot, the curves' ordinates are displaced by a sufficient amount to reveal the relationships. 
Rotor power, Hp.  
P_{H} 
Rotor power in hover, Hp. 
p 
Pressure, lb/in2. 
p_{∞} 
Freestream (static) pressure, lb/in^{2}. 
parasite power 
Power required to overcome the drag of the aircraft. 
pitching moment 
Moment obtained by integrating pressures with respect to the section quarter chord, positive nose up. 
profile power 
Power required to overcome the drag of the airfoil sections. 
Dynamic pressure, , lb/in^{2}. 

quadrant 
The rotor disk is commonly divided into four quadrants when describing rotor behavior. The zero degree reference is when the blade is over the tail. The first quadrant is from 0° to 90°, the second from 90° to 180°, and so on. The advancing side of the rotor contains the first and second quadrant, whereas the retreating side has the third and fourth quadrants. 
Rotor radius, ft. 

r 
Radial location on blade, ft. 
RCAS 
An aeromechanics analysis developed by the U.S. Army and Advanced Rotorcraft Technology (Saberi et al. 2004). Although the primary purpose of this tutorial is directed to the experimental measurements, comparison with theoretical results is sometime useful. See also CAMRAD II and CHARM. 
reversed flow 
As forward flight speed increases, the local velocity at a blade section increases on the advancing side and decreases on the retreating side. At a sufficiently high speed, the local velocity goes to zero inboard on the blade and then reverses as flight speed increases. The dynamic pressure, q, is generally low in the reversed flow region, so the loads are moderate. The largest effect seen is normally for the section pitching moment, since the lift on the airfoil occurs at the 0.75c rather than at 0.25c as in normal flow. This lift offset can cause large pitching moments. 
RVDT 
Abbreviation for Rotary Variable Differential Transformer. The RVDT is a transducer that is able to make accurate measurements of angular deflections about the instrument's axis. 
Fatigue damage occurs when the loads exceed the endurance limit. The damage is cumulative over time. A 'safe life' interval requires that a component be replaced before fatigue damage can cause component failure. The safe life interval is normally determined based on an assumed loading spectrum and fatigue testing of the component. Conservatism is built into the safe life interval so that the probability of component failure prior to replacement is extremely low. (See also endurance limit.) 

service ceiling 
The maximum altitude at which a helicopter can fly, which occurs at its minimum power speed. The service ceiling for OneEngine Inoperative (OEI) is critical for twin or multiengine aircraft. 
soft inplane 
The first inplane frequency (leadlag frequency, chordwise frequency) of a rotor is either above 1/rev (stiff inplane) or below 1/rev (soft inplane). Articulated rotors, typically, have a first inplane frequency just above 0/rev. But hingeless rotor must be designed to be either stiff inplane (no ground or air resonance instabilities) or soft inplane (normally requiring some form of damper). 
sonic line 
This line marks the boundary between subsonic and supersonic flow on an airfoil. The pressure coefficient at this boundary is referred to as the critical pressure coefficient, C_{p}*. Note that C_{p}* is not invariant on a helicopter airfoil section but instead depends upon blade azimuth. 
stiff inplane 
The first inplane frequency (leadlag frequency, chordwise frequency) of a rotor is either above 1/rev (stiff inplane) or below 1/rev (soft inplane). Articulated rotors, typically, have a first inplane frequency just above 0/rev. But hingeless rotor must be designed to be either stiff inplane (no ground or air resonance instabilities) or soft inplane (normally requiring some form of damper). 
Rotor thrust, lb. 

tip vortex 
A concentrated vortex forms near the tip of each rotor blade and dominates the rotor's wake. The core of this vortex is about a tenth or twentieth of the blade chord when it leaves the blade, but it thickens with wake age. The apparent size of this core when it is intersected by the next blade may be as much as a blade chord. See also disk vortex. 
torsion moment 
The rotor blade structural moment about the quarter chord, positive nose up. 
transition 
Describes the airspeed range from hover to the minimum power speed. Most helicopters pass through the transitional range fairly quickly and may not attempt steady flight in this speed regime. There is a peak in vibration that is normally encountered in this speed regime, either on approach or takeoff. 
transonic drag rise 
At small angles of attack, airfoils normally show low drag coefficients, but as Mach number increases, at some point the drag coefficient will increase in a nonlinear manner as supersonic flow and shocks form on the upper and lower surfaces. For rotor airfoils, this drag rise is sometimes referred to as drag divergence and the associated Mach number is referred to as the divergence Mach number. 
A terrain avoidance maneuver was made part of the performance specification for the U.S. Army's procurement of the Utility Tactical Transport Aerial System (UTTAS) program in the early 1970s. This procurement, which funded prototype aircraft from two manufacturers, led to the development of the UH60A. The UTTAS maneuver was defined in two parts: a pullup and a pushover. The specification for the UTTAS pullup was that the maneuver was to be entered at maximum level flight speed and the pilot was to pull the aircraft's nose up to quickly obtain a load factor of 1.75g, and the load factor was to be held for three seconds without loosing more than 30 knots of airspeed. Two UTTAS pullups were flown during the Airloads Program. Counter 11029, a 2.1g UTTAS pullup, was the second most severe maneuver in the flight program, while Counter 11031, a 1.8g pullup, was the tenth most severe maneuver (Bousman and Kufeld 2005). 

Vertical rate of climb. 

V 
forward flight speed, ft/sec. 
V_{h} 
Ideal induced velocity in hover, , ft/sec. 
V_{ROC} 
Vertical rate of climb, ft/sec. 
V_{T} 
Rotor tips speed, ft/sec. 
Azimuthal time histories are aligned in a 3D Cartesian surface plot such that they show the evolution of sequential events. 

Xlocation on airfoil section, measured from leading edge. Nondimensionalized by section chord. 

Angle of attack, deg. Angle between the airfoil section chordline and the freestream. 

Advance ratio, . Normally, the is defined relative to the control axis, hence the term . But for all flight test data, this angle is assumed to be zero. 

Ρ 
Density of air, slugs/ft^{3}. 
σ 
Solidity, . 
Ω 
Rotor speed, 1/sec. 


