B263
OTHER:
Aerodynamics - Definitions & Algorithms|
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Note: The algorithms are in blue
Ref. Symbols are in Times New Roman 2 sizes up from smallest. Subscripts are increased one size.
See: Access database for procedures running the algorithms and for additional information.
There is, currently, duplication on Dynamics, Aerodynamics & Mechanics.
Active Blade Twist: (ABT) [θ1A] [blade & flight control]
In-flight changing of the twist on the blades, to increase thrust, plus reduce vibration and noise.
Advance Ratio: [rotor]
See: Tip Speed Ratio
Advancing Blade Concept (ABC): [rotor]
A coaxial rotor configuration built by Sikorsky where the advancing blades developed more thrust than the retreating blades.
Aerodynamic Center: [blade] [rotor] [craft]
The point about which the resultant lift force acts when the incidence is changed.
Will vary depending on application of cyclic.
The one special point on an airfoil for which it is found that the section moment coefficient [Cm ] is constant (note ~ constant, not zero) and independent of the angle of attack. [Source ~ PHA p.267]
The point along the chord of the airfoil where all the changes in lift effectively take place. If the point about which the moment is taken is properly chosen (the aerodynamic center), the moment coefficient is essentially constant up to maximum lift. The aerodynamic center varies with airfoil thickness, with the exception of the NACA 00xx series. See graphs for various airfoils.
[Source ~ TWS p.182] It is not affected by the camber nor angle of attack. It is located between the 23% and 27% of chord length.It has been estimated that moving the aerodynamic center back 2% of the chord can result in a 10% saving in total blade weight.
[Source ~ RWP1 p.421]|
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Airfoil: |
AC position: %chord x/c |
AC position: %thickness y/c |
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NACA 0009 |
25 |
(1) |
(1) Varies w/ Reynolds Nbr. |
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NACA 0012 |
25 |
0 |
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VR-7 |
27 |
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23012 |
24 |
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Depends on taper, twist, airfoil mix, etc.
See 3rd column in above table
Aerodynamic Precession: [rotor]
The act of the rotor disk flying to position.
The characteristic that causes the rotor disk to react to an aerodynamically applied force at a point between 0-degrees and 90-degress away from the point of application, in the direction of its rotation.
Aerodynamically Variable PropRotor (AVPR):
The <= 50% reduction in the effective aerodynamic disk area during forward flight from that during hovering flight. This is achieved by combining of two rotors, which are aerodynamically independent during vertical flight (side-by-side or fore-aft configurations), into a counterrotating propeller set during forward flight.
The dynamics of bodies moving relative to gases, especially the interaction of moving objects with the atmosphere.
Angle of Attack: [α]
See Angle of Incidence, below.
Angle of incidence is the angle between the blade chord line and the plane of rotation of the rotor system. See: Angle of Incidence ~ by Paul Cantrell. On an airplane it is the acute angle formed between the chord line of the airfoil and the longitudinal axis of the craft on which it is mounted.
Other applications of incidence;
The angle formed by the mast centerline and a line that is perpendicular to the helicopter's body X-axis. iM The body axis system is used on this web site therefore there is no angle of incidence for the mast.
The angle formed by the blade's chord line and the tip path plane. iM
The angle between the horizontal stabilizer's chord line and the and the helicopter's body X-axis.
iHAngle of Zero lift:
[αL=0] [airfoil]........depends on the camber of the airfoil.
Angular Momentum:
See:
A downward slope or bending of the rotor blade, or blade tip. Opposite to
More:
DESIGN: UniCopter ~ Rotor - Disk - Anhedral along Blade SpanMore:
DESIGN: SynchroLite ~ Rotor - Blade - Composite - VR-7b - Anhedral (drooped tip)Aspect Ratio:
[AR]The ratio of the blades length to its chord.
Descent of a helicopter without engine power applied to its rotor. An aerodynamic force causes the rotor to spin.
See:
See:
DESIGN: UniCopter ~ Rotor Disk - AutorotationSee: DESIGN: SynchroLite ~ Rotor Disk - Autorotation
Blade Angle:
[propeller]The angle formed between the propeller's plane of rotation and the chord line of its airfoil section. It is expressed in degrees. Measured at 75% of the blade's radius.
Blade Area: [Ab] [blade]
The sum of the area of all the blades on the helicopter.
Blade loading = Gross weight over Blade area. BL = GW / Ab
Consideration reguarding ABC (the 'unloading' of the retreating blades) and slow speed rotor. See; DESIGN: UniCopter ~ Rotor - Disk - Large Chord & Low Tip Speed ~ Blade Loading
Blade Loading Coefficient: [CT/σ] [rotor]
The coefficient of thrust divided by the solidity.
The perturbation on a blade caused by its passage through the vortices of a previous blade.
Camber: [airfoil]
Camberline (mean camber): [airfoil]
A line drawn through a series of points, each of which is located midway between the upper and the lower camber.
Center of ....
Blade: [airfoil]
See: Center of Pressure below.
See DESIGN: SynchroLite ~ Rotor - Blade - Composite - VR-7b - Centers, Radii & Moments
Disk:
[rotor]See DESIGN: SynchroLite ~ Rotor - Disk - Center, Radii & Moments
Helicopter:
[craft]Aerodynamic Center: Center of Parasite Drag is probably the same.
Center of Gravity [CG]; Center of Mass;
The point in the helicopter at which all of the weight is considered to be concentrated. The sum of the moments about the center of gravity is zero. This location is established by the SynchroLite Weight and Balance Calculations and the UniCopter Weight and Balance Calculations .The chordwise distance between the point of application of the normal force and the
The center of pressure moves to different locations on the chord with changes in the angle of attack.
Centrifugal Force: [rotor]
See:
See:
Chord:
[c] [airfoil]An imaginary straight line drawn through an airfoil from its leading edge to its trailing edge.
Chord-axes System:
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Axis |
Orientation |
Origin |
Position: |
Direction: |
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z |
Span |
Center of hub |
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Positive toward tip |
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x |
Leading edge |
x/c |
Positive toward trailing edge |
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y |
Thickness |
Chord |
y/c |
Positive upward |
The blade axes system has the positive x direction along the blades quarter chord line. The positive y and z directions are such that the blade and hub sub systems align when the flapping is zero and the azimuth angle is 180º.
[Source ~ HFD p.179] This appears to differ from Theory of Wing Sections.Circulation: [blade]
The strength of the bound and trailing vortices. Its value is proportional to the lift of the blade divided by the forward speed. This and more; [Source ~ RWP4 p.33]
Schemes to improve the lift of a blade.
Centripetal Force and Centrifugal Force:
An imaginary line drawn through an airfoil from its leading edge to its trailing edge.
Coaxial Rotors: [rotor]
Coaxial rotors of a helicopter are mounted on concentric shafts in such a way so that they may turn in opposite directions to cancel torque.
Coefficient of Drag: [cd] [airfoil]
A dimensionless number used in the formula for finding the induced drag of an airfoil as it relates to the angle of attack.
Drag coefficients are almost always determined experimentally using a wind tunnel.
Coefficient of Lift: [cl] [airfoil]
A dimensionless number relating the amount of aerodynamic lift produced by an airfoil to its angle of attack.
Coefficient of Moment: [cm] [airfoil]
Pitching Moment Coefficient. A dimensionless number used to express the aerodynamic moment in respect to 1/4 of the chord. The Cm is considered positive when pulling up.
Coefficient of Rotor Lift: [cT'/σ]
See:
OTHER: Dynamics - General - Rotor CoefficientsWhat should be used to determine the intermeshing rotor's
solidity ratio?________________________
Coefficient Equations:
'The following 6 are valid equations for fixed wing aircraft. Some of these
C
L = L / ((ρ * A * Vfwd ^ 2) / 2)CD
= D / ((ρ * A * Vfwd ^ 2) / 2)C
y = Y / ((ρ * A * Vfwd ^ 2) / 2) Lateral force coefficient.C
l = l / ((ρ * A * Vfwd ^ 2) / 2) Rolling moment coefficient.C
m = m / ((ρ * A * Vfwd ^ 2) / 2) Pitching moment coefficient.C
n = n / ((ρ * A * Vfwd ^ 2) / 2) Yawing moment coefficient._____________________
Coefficient of Thrust [CT] & [Ct]; [rotor]
Coefficient of thrust is nondimensional. It is the thrust (equal to weight at low speed) divided by the rotor disk area, the air density, and the square of the tip speed.
See:
Coefficient of Power
[CP] & [Cp]; [rotor]See:
Rotor configurations: Single, Coaxial, Intermeshing, Interleaving, Side-by-side, Tandem, Compound etc.
See: OTHER: Aerodynamic - Rotor Disk - Dual Configuration
The single, the compound, the tandem and the coaxial soon to become relegated to history as lateral twin main rotors reclaim the preeminent position that they once held. 
Critical Mach Number: [
Mcr] [rotor]xxxx
Cutout Ratio: [rotor]
The distance from the center of the mast to the blade root divided by the rotor radius.
Damping:
[c]The resisting moment per unit angular velocity of the helicopter.
Density of air:
[ρ] [rho]At sea-level standard conditions
The means of providing yaw on coaxial and most intermeshing helicopters. Yaw is achieved by increasing the thrust and drag on one rotor while decreasing the thrust and drag on the other. The net thrust will not change but the torque of the two rotors will no longer be equal. The higher torque of one rotor will rotate the craft in the opposite direction.
The positive angle formed between the lateral axis of a helicopter and the blades which are flapping up on the side which is leading the sideslip. Dihedral is used to increase the lateral stability of an aircraft. The opposite is
anhedral.Re: Horizontal Stabilizer: Sweeping a wing back also gives a dihedral effect, with about 5 degrees of sweep being equivalent to 1 degree of dihedral.
Calculation of Equivalent Dihedral Angle:
http://www.rc-soar.com/tech/spiral_eda.htmMore:
DESIGN: UniCopter ~ Rotor - Disk - Dihedral at Blade Tip ~ Bladelet
Disk Area: [A] ~ Single rotor [AONE], Multiple rotors [ASYS]; [rotor]
Disk Loading:
[DL] [w] [rotor]The simple concept of disk loading has served as a quick tool to evaluate aerodynamically independent rotors. However, this concept has two major flaws. One is that it assumes that the disk loading is equitably distributed about the disk. The other is that it does not take into account aerodynamic interactions with other items such as another rotor or the fuselage etc.
The up-and-coming rotorcraft will have multiple main-rotors. These rotors will aerodynamically interact. In addition, they will incorporate features such as; Slowed Rotors, Advancing Blade Concept, Reverse Velocity Utilization and Higher Harmonic Control. When all of the above is considered during hover flight, and then totally reconsidered during cruise flight, the concept of disk loading is no longer 'simple'.
Consideration regarding ABC (the 'unloading' of the retreating blades) and slow speed rotor. See;
DESIGN: UniCopter ~ Rotor - Disk - Large Chord & Low Tip Speed ~ Disk LoadingDissymmetry of Lift: [rotor]
The unequal lift across the rotor disc resulting from the difference in the velocity of air over the advancing blade half and retreating blade half of the rotor disc area.
Downdraft: [rotor]
In an intermeshing configuration, the upper blades will pass through the downdrafts created by the lower blades.
Downwash: [rotor]
Air forced down by aerodynamic action below and behind the rotor of a helicopter. The body of the craft may be subjected to this downwash. In an intermeshing configuration, the lower blades will pass through the downwashes created by the upper blades. . The body of the intermeshing configuration may be subjected to the downwashes from both rotors.
Downwash Angle: [ε]
The angle formed between the direction of air movement as it approaches an airfoil and its direction as it leaves.
Drag: [D] [blade][rotor] [craft]
The value of the free stream Mack number at which the drag coefficient increases significantly.
For helicopters ~ The Mach number at which the drag coefficient is twice its incompressible value.
[Source ~ PRW1 p.409]Dutch Roll:
See: More on Dutch Roll
Induced velocity delay [Source ~ RWP4 ch. 27]
[q]The pressure a moving fluid would have if it were stopped [pounds / square foot]
Dynamic Pressure Ratio:
In rotor wake:
Retreating blade stall.
The discharge of exhaust and cooling gases.
The span of the blade is divided into approximately 10 to 15 segments for aerodynamic computations and an element is one of these segments. See Station:
The singe average chord, which would yield the thrust of the actual planform.
For a linear tapered blade, it is the chord at 0.75 Radius.
[De]D
e = D + SHP * 550 / V [D in lbs, V in fps]Equivalent Flat Plate Area: [f] [craft]
The area of a hypothetical flat surface perpendicular to the direction of motion of the body that produces the same opposition to the airflow as the streamlined body. f = D / q [ft2]. f = Σ CD * frontal area [ft2]. This hypothetical flat surface has a drag coefficient of 1 and this should not be confused with Flat Plate Area.
Equivalent Lift to Drag Ratio: [L / De]
A means of evaluating the overall efficiency of a vehicle in horizontal translation.
[Source ~ RWA, Book I, p.135]W/D
e = (W * V) / (550 * SHP) [D in lbs, V in fps]Equivalent Solidity: [σe]
Accounts for the major effects in varying the chord.
F-force: [F]
The sideward-pointing component of rotor force, perpendicular to the control axis. [Source ~ AH p.183] Is this the same as Y-force?
The action which changes the pitch angle of helicopter rotor blades by rotating them about their feathering axis.
Figure of Merit: [FM]
A measure of the efficiency of a hovering Helicopter.
FM = Ideal power required to hover / Actual power required to hover. [Source ~ PHA p.46]
Fineness Ratio:
The ratio of length to breadth of a streamlined shape , is called the fineness ratio of a streamlined body. For best results it should be about 4 to 1 (NACA 0025}, but it really depends on the air speed; the higher the speed, the greater should be the fineness ratio, but experiments show that there is not much variation in the drag for quite a large range of fineness ratios.
Blow Back, Back Flap: [rotor]In forward flight, the rotor disk has a natural tendency to tilt back (longitudinally) because of the dissymmetry of lift that would be produced if the advancing blade was not allowed to reduce its angle of attack and the retreating blade increase its angle of attack.
Flapping:
[rotor]The vertical movement of a rotor blade about its delta, or flapping, hinge.
Longitudinal: See Flapback.
Lateral: This effect arises because of coning. Note that in a hypothetical case with no coning (UniCopter - Absolutely Rigid Rotor), the blade sees the same increase in angle of attack at ψ = 0º and 180º and there will be no lateral tilt.
Flapping to Equality:
The flapping or teetering of the rotor's blades so as to overcome any dissymitry of lift between two sides of a disk.
Flat Plate Area: [A]
The area of a flat plat placed normal to the air stream.
Geometric Pitch: (Propeller Pitch)
The distance that a propeller will move forward in one revolution. This is based on the propeller blade angle at 75% blade station. It is theoretical in that it does not take into account any losses due to inefficiency.
Ground Effect: (Ground Cushion)
"The [ground] effect has long been recognized but the aerodynamics are still not fully understood." ~ Leishman
For additional information see; [Source ~ RWA Book II, p.44]. Outside web page: Ground Effect
Gurney Flap:
An aerodynamic 'dam' on the trailing edges of helicopter stabilizing surfaces
Part 1 by Prouty: http://www.avtoday.com/reports/rotorwing/previous/0200/02rwaero.htm
Part 2 by Prouty:
http://www.avtoday.com/reports/rotorwing/previous/0300/03rwaero.htmGust: (wind)
A temporary increase in the speed of the wind. A gust lasts for a very short period of time, and it is usually followed by a wing whose speed is lower than normal.
H-force: [H]
The rearward-pointing component of rotor force, perpendicular to the control axis. [Source ~ AH p.183]
Higher Harmonic Control: (HHC)
An oscillatory modulation is superimposed on the basic pitch control of the swashplate. (The swashplate is comprised of two parallel plates, providing the means by which collective and cyclic pitch of the blades is achieved [Seddon, 1990].) Thus, all the blades are affected equally despite inherent differences in (the time dependent inputs to) each blade. This is unrelated to Active Blade Twist.
Hyperbole:
A lot of hot air. It's good for giving lift to balloons but not for helicopters. 
Ideal (hyperbolic) Twist:
Will theoretically give uniform inflow distribution along the span of the blade, with constant chord blades.
θ = θ
TIP (R /r): [Source ~ AH p.57]Incidence:
Angle of attack. See: Angle_of_Incidence
Independent Root & Tip: (IRAT) [blade & flight control]
A project to improve the L/D ratio and Figure of Merit of rotors; by providing a much greater rate and amplitude of pitch change than that of current active blade twist methods;
Individual Blade Control: (IBC) [rotor]
A method by which the pitch of each blade is controlled individually. This is unrelated to Active Blade Twist.
Induced Power: [Hpind]
The power required overcoming the drag produced by an airfoil when it is producing lift.
The downward air velocity generated in the process of developing rotor thrust.
NACA Report ~ 1954 ~ #1184 Have hard copy.
Inflow Angle
[φ] [rotor]Defined by the two mutually perpendicular velocities. φ=v1/Ωr
Inflow Ratio:
[λ] [rotor]Dimensionless velocity normal to the reference plane;
For small disk inclination;
λ = μ * α + λiInterference-induced Power Factor:
[κint] [rotor]For Coaxial Rotors.
For coaxial rotors in close proximity, the value is
√2 = 1.41For coaxial rotors with the lower one in far-field wake, the value is
1.28. Note ~ My calculations, using Prouty's blade element theory, of a two 2-blade rotors vs. one 4-blade rotor gave a 28% increase in power for the 4-blade rotor. Is this coincidental? ~~~ See Figure 3.14 in RWA. ~ Then see Figure 26 in Stepnieski's ABC Synchropter. ~ Then see notes in HT, middle of p. 119. ~ re source of 0.56 value.See;
Required Power Comparison. Also a coaxial helicopter will have a total of six blades. [See ~ PHA p.71]I think that this is similar to the Overlap Interference Factor: [
kOV] (Induced Power Overlap Correction Factor) for tandem and side-by-side rotors.Interleaving Rotors: [rotor]
Two rotor disks which are located in the same horizontal plane and where the stagger is greater than the radius of the disk, but less the diameter of the disk. Examples; Landgraf H-4 and Mil Mi-12 (Homer). Active Blade Twist is an essential prerequisite for this configuration.
Intermeshing Rotors: [rotor]
Two rotor disks which are located in different planes and where the stagger is less than the radius of the disk.
International Standard Atmosphere [ISA]:
Sea level: Temperature 59 deg F or 15 deg. C; Pressure 2116.7 lb/ft^2, Density 0.002378 slug/ft^3
Leading Edge: [airfoil]
The edge of a blade that reaches a point in space ahead of the rest of the blade. An exception to this is root end of a retreating blade, which is in the reverse velocity region.
Lift [
L]: [airfoil]An aerodynamic force caused by air flowing over a blade element. Normal to the resultant velocity at this element.
[rotor]
The vertical component of a rotor's thrust vector. The horizontal component being Propulsive Force.
For an airplane wing:
LW = (ρ / 2) * V2 * S * CL . Where S is the area of the wing.Lift Overshoot:
See; Vertiflite, Winter 2001 ~ 'Dynamic Lift', p. 30, by Prouty.
Adaptive Airfoil Dynamic Stall Control
Lift-Curve Slope: [a] [airfoil]
The ratio of change in lift coefficient to change in angle of attack.
Lift-to-Drag Ratio ~ Airfoil: [cl/cd] [airfoil]
"To obtain the best hover performance, the airfoil should be flown at the angle of attack that gives the highest lift-to-drag ratio." [Source ~ RWP5 p.23]
"... suggests that the emphasis in airfoil design should be for good L/D, while the maximum lift corvfficient performance is less important" ~
Airfoil Design and Rotorcraft Performance Excellent paper. Have hard copy.Outside web page on
Lift and Drag CurvesLift / Drag Ratio ~ Airfoil ???:
[cl3/2/cd] [airfoil]. What is the proper name for this one and where did I see this mathematical expression?A measurement of the efficiency of an airfoil section.
[Source ~ XXX p.xx]Lift-to-Drag Ratio ~ Rotor:
[cL/cD] [rotor]For proposed methods of improving L/D click on title.
Mean Camber (Mean-line): [airfoil]
A line that is drawn between the upper and lower camber of an airfoil section.
Mean Thrust (Mean Rotor Loading):
The amount of rotor thrust that the undersling in a teetering rotor is optimized for.
Momentum Theory: [rotor]
Allow one to derive a first-order prediction of the rotor thrust and power.
Non-uniform Inflow: [rotor]
See; [HT, section 13-2]
Operational Flight Envelope (OFE):
The limits to the operational capability. The combination of airspeed, altitude, rate of climb/descent, sideslip, turn rate load factor and other limiting parameters that bound the vehicle dynamics, required to fulfill the user's function.
Opposed (Differential) Lateral Cyclic: (Lateral Displacement ~ Sikorsky ABC)
The application of left cyclic to the CCW rotating rotor and right cyclic to the CW rotating rotor in a helicopter with two laterally disposed main rotor, during horizontal flight. For additional information see; DESIGN: SynchroLite ~ Rotor - Disk - Opposed (Differential) Lateral Cyclic Same as above ~ Lateral Displacement
Oswald's Efficiency Factor:
[e]Derived from: Tip speed ratio
[μ], Blade twist [θ1], Angle of attack in tip path plane [αTPP] and Coefficient of thrust over Solidity of rotor (CT/σ). ~~ Maybe e = L^2 /Sum {A^2} [ov] [rotor]A value between 0 (no overlap and 1 (total overlap). It is a linear consideration and does not equate with the 2-dimentional change in disk area.
A multi-rotor interference factor. This factor is calculated as the ratio of the induced power required for a multi-rotor system relative to the induced power of a system of an equal number of isolated single rotors, when operating at the same disk loading. See the appendix of Dual Rotor Interference, [PHA, p.71] and [RWA Book 2, p.188]
I think that this is similar to the Interference-induced Power Factor: [
κint] for coaxial rotors.Overlap Ratio: [m']
The percentage of the overlapped area to that of the total area of the two rotor disks. See the appendix of Dual Rotor Interference.
[rotor] The term P-factor is defined to mean ``asymmetric disk loading''. It is an extremely significant effect for helicopters. When the helicopter is in forward flight, the blade on one side has a much higher airspeed than the other. If you tried to fly the blades at constant angle of attack, the advancing blade would produce quite a bit more lift than the retreating blade.
[propeller] Asymmetric loading is caused by the resultant velocity of the propeller in its plane of rotation and the velocity of the air through the propeller disc. With the airplane at positive angles of attack the right, or down swinging blade has a higher velocity than the left, or up swinging blade. Since the propeller blades are in themselves airfoils increased velocity results in increased lift, and the increased lift on the right blade tends to yaw the airplane to the left. |
Asymetric propeller loading, P factor (Animation)Phugoid:
See:
Pitch:
[θ] [blade (rotor)]Pitch angle [
Pitch: [propeller]
The forward distance theoretically traveled by a propeller in one revolution. Measured at 75% of the blade's radius.
Pitch-Diameter Ratio: [propeller]
The relation between the propeller pitch and diameter expressed as a mathematical proportion, as 1.5 to 1, and so on.
Pitch Distribution: [propeller]
The twist in a propeller blade along its length.
Pitching:
A pitching motion is a periodic variation of the angle of attack.
Pitching Moment:
See; Coefficient of Moment:
Plunging:
A plunging oscillation is a periodic translation of the airfoil in a direction normal to the free stream.
Profile Power
[HPOH]The power required overcoming the air friction drag of the individual blade elements.
See:
Propulsive Force:
The horizontal component of a rotor's thrust vector. ~&~ The horizontal component of a propeller's thrust. Lift being the vertical component of both thrusts.
Pull: [rotor]
The horizontal component of a rotor's thrust vector. It is opposed by profile drag; and perhaps the H-force drag.
Push: [thruster]
The horizontal force of a pusher device, such as a propeller. It is opposed by profile drag; and perhaps the H-force drag.
Radial Flow Effect:
See Swirl
Reverse Velocity (Flow) Region:
[rotor]The area on the retreating side of a rotor disk abutting the center of rotation where the speed of flight exceed the velocity of the blade. The diameter of this region = μ * R
A rotor that is designed to operate at high cruise velocities where a portion or all of the retreating blade near azimuth 270º is immersed in reverse airflow.
Reverse Velocity Utilization: (RVU) (Reverse Velocity Rotor), [rotor]
To causes the portion of a retreating blade, which at high forward velocities is experiencing an airflow from the so called 'trailing edge' to the leading edge', to have a negative pitch, so that this reverse airflow will create lift.
Reynolds Number: [Re] [RN] [airfoil]
Reynolds number = chord (ft) x speed (mph) x 9360. Re = c x MPH x 9360
Root Cutout:
[x0]The blade root cutout. Expressed as a fraction of the disk's radius.
See also;
DESIGN: UniCopter ~ Rotor - Blade - General - Root Cutoutxx
Rotor Coefficients:
Rotor Configurations - Twin Rotors:
See: OTHER: Aerodynamic - Rotor Disk - Dual Configuration
Two rotor disks which are located in the same horizontal plane and where the
stagger is greater than the diameter of the disk.Sideslip angle: [ß]
The angle that the fuselage makes with the air in the plan view.
Sidewash angle: [η]
The angle of lateral air flow induced by the main rotors and by the fuselage in sideslip.
Slant-line Distance: [dR]
The diagonal distance between rotors. (
dR = root(g^2 + ds^2) )The strong flow of air moved rearward by a propeller.
Solidity Ratio: [σ] [sigma]
The solidity represents the ratio of the lifting area of the blades to the area of the rotor(s).
Solidity is the total
area of the blades divided by the area of the rotor disk(s).Span Efficiency Factor: [δ] [delta] Oswald Efficiency Factor??
See [Source ~ RWP1 figure 8.17]
The span efficiency factor, e, does not include the viscous drag terms. It is the ratio of the induced drag of an elliptically-loaded wing of span, bref, to the induced drag of the lifting system under consideration when both have the same total lift.
Stall: [airfoil]
Stall Development Time: ?? [c/V]
The approximate time it takes for a particle of air to travel from the leading edge to the trailing edge of the airfoil.
[Source ~ RWA II p.108]Station:
[blade]A point on the span of the blade. See Element:
States of Flow:
The flow of a fluid in which there is no turbulence. All particles of the fluid move in continuous smooth lines.
A small secondary airfoil attached to the tip of the rotor blade. The sub-wing generates a secondary vortex, slightly offset from the primary one. Interference between the two vortices tends to defuse both very rapidly,mitigating the outcome of blade-vortex interaction.
An airfoil designed to delay the onset of wave drag in the transonic speed range. Supercritical airfoils are characterized by their flattened upper surface, highly cambered (curved) aft section, and greater leading edge radius as compared to traditional airfoil shapes. The Supercritical airfoil shape is incorporated into the design of a supercritical wing.
Sweep Angle: [Λ]
The sweep angle is the angle at which the aircraft's wings are angled back from the position of perpendicular to the fuselage. A sweep angle of zero corresponds to wings oriented straight out from the fuselage. Actually, the sweep angle is (usually) measured along a theoretical line at 25% of the chord of the wing.
Swirl: (Wake swirl) (Slipstream rotation) (Radial Flow Effect) [rotor][propeller]
A small swirl component of the velocity in the rotor wake, induced by the spinning rotor and propeller.
A tab can be used on portions of the trailing edge of an airfoil to help negate pitching moment. [See ~ PHA p.272]
Two rotor disks which are separated by both stagger and gap.
Taper Ratio: [blade]
The ratio of the blade's root chord to the tip chord.
For more see: DESIGN: UniCopter ~ Rotor - Blade - General - Taper
Teetering Hinge:
Used at the center of a semi-rigid, 2-bladed, articulated rotors. This hub operates on the principle of a universal joint. The teetering hinge constitutes one of the axes of the universal's X-yoke and the feathering hinges constitutes the other axis. I think.
See:
OTHER: Aerodynamic - General - Semi-RigidBlade slap, which occurs at high advancing tip speeds.
Thickness Noise of Helicopter Rotors At High Tip Speeds Have hard copy.
[T] [rotor]Aerodynamic force created by a rotor disk. The thrust line (thrust vector) is usually normal to the rotor disk (tip path plane).
T = 2 * ρ * V
2 * A, [in pounds]. Thrust can be resolved into 'Lift' and 'Pull'.Thrust Weighted Solidity: [σT]
Equivalent Thrust Weighted Solidity: [σe]
The aerodynamic biasing toward the tip, if the blade is tapered. [Ref. ~ AH p.86; PHA p.110: RWP1 p.17]
For a linearly tapered blade, the effective chord [
ce] is the chord at 0.75 of radius [0.75*R]: σT = (b * ce ) / (π * R)Ref:
FORM: Blade Taper & Function tapered_blade ()See also:
OTHER: Aerodynamics - General - Coefficient of Blade Loading [CT/σ]Control in yaw achieved by means of rotor blade tip-mounted air (drag) brakes. Movable tip brakes are located at the tips of all blades on both rotors. They provide, by their deflection, positive directional control in all conditions of flight by creating an unequal torque distribution in the rotor system. A left turning moment results from starboard rotor tip brake deflection while a right turning moment results from starboard rotor tip brake deflection. In the neutral condition, both sets of tip brakes are undeflected. For more information and pictures see; Gyrodyne
Used to take into account that the blade has drag but no lift at its tip.
B=1-(ct/2R); B=1-√(2CT)/b; B=0.97
[ΩR] [OmegaR] [blade]The speed of the blade tip [ft/sec]. Rotational speed of rotor [
Tip Speed Ratio (Advance Ratio): [
μ] [mu] [rotor]The relationship between forward speed and rotor's tip speed.
Dimensionless velocity parallel to the reference plane; μ = (Vfwd * cos(α)) / ΩR.
For small disk inclination;
μ = Vfwd / ΩRTip Weight:
[blade]The weight that is located in the tip of each main rotor blade. Increasing the weight will delay the rotor decay at the start of autorotation, provide more energy for the autorotative flare and cause responses to cyclic input to be a little 'softer'. In addition, it will also reduce overall vibration slightly because the coning angle will be smaller. It increases the centrifugal loading on the rotor components.
Torque Offset:
Blade/Hub Geometry. To allow the blade centrifugal force to compensate for part of the rotor torque. See 'Even More Helicopter aerodynamics; chapter 32'.
Trailing Edge Angle: [τ] [airfoil]
Included angle between the tangents to the upper and lower surfaces at the trailing edge of the airfoil.
Transverse Flow Effect: (Inflow Roll) (Rotor Upwash) [rotor]
See; http://www.copters.com/aero/transverse.html , [See ~ RWP3 p.28] , [See ~ RWP4 p.34]
________________________
In forward flight, air passing through the rear portion of the rotor disc has a higher downwash velocity than air passing through the forward portion (nonuniform
induced velocity). This is because the air passing through the rear portion has been accelerated for a longer period of time than the air passing through the forward portion. This increased downwash velocity at the rear of the disc decreases the angle of attack and blade lift, hence in combination with gyroscopic precession, causes the rotor disc to tilt to the right (the advancing side). The lift on the forward part of the rotor disc is greater than on the rearward part. According to the principle of gyroscopic precession, maximum deflection of the rotor blades occurs 90° later in the direction of rotation. This means that the rotor blades will reach maximum upward deflection on the left side and maximum downward deflection on the right side. This transverse flow effect is responsible for the major portion of the lateral cyclic stick control required to trim the helicopter at low speed.More:
http://www.dynamicflight.com/aerodynamics/transverse_flow_eff/Transverse flow effect is greatest between 10 and 20 knots thus greatest vibration due to unequal drag in the fore and aft portion of rotor disk. Rotor Upwash appears to be another way of explaining Transverse Flow Effect
[Source ~ RWP4 ch 9]. The Transverse Flow Effect diminishes at higher forward speeds. See; [Source ~ RWP4 Fig. 9-4]Related info:
DESIGN: SynchroLite ~ Rotor - Disk - Opposed (Differential) Lateral Cyclic________________________
Roll during forward flight is also the result of the coning angle presenting a greater angle of attack at the front of the disk then at the rear of the disk. This effect from the coning angle increases as the speed of forward flight increases
. See also; DESIGN: SynchroLite ~ Rotor - Disk - Opposed (Differential) Lateral Cyclic________________________
My thoughts re the UniCopter with its Absolutely Rigid Rotor; The helicopter will be probably subjected to Rotor Upwash but it will not manifest itself as Transverse Flow effect or Inflow Roll. This is because, unlike the above, the aerodynamic precession will be close to 0º not 90º. I think that forward cyclic will be required, not left lateral cyclic. It appears that the lack of sufficient cyclic on Sikorsky's ABC was a lack of forward cyclic.
[Source ~ RWP4 p. 35] Later, move this paragraph to a UniCopter page and link from here?Blade twist is the difference in a blade's pitch between its root and tip. The tip has less pitch and the amount is referred to as a negative value, in degrees. For more see:
The blades on a gyrocopter have a small amount of positive twist, because they operate full-time in autorotation.
See also;
Ideal Twist:Upwash - Rotor:
[rotor]See Transverse Flow Effect above, specifically as related to an Absolutely Rigid Rotor. [Source ~ RWP4 p.33]
Variable Geometry Airfoil:
[blade]The use of trailing edge flaps, tabs or adjustable contour, and/or leading edge slats, pivoted nose droop or adjustable contour, the position(s) of which can be controlled in flight.
Vibration:
OTHER: Aerodynamics - Vibration - Rotor Induced
OTHER: Flight Dynamics - Definitions & Algorithims ~ Vibration:
The reference to the following page may be from a different location; later
OTHER: Aerodynamics - Vibration - Rotor Induced - Blade Vortex Interaction (BVI)Small low aspect ratio airfoils mounted on the upper surface of wings. The air spilling over their upper ends forms swirl or vortices. Vortex generators prevent the air separating from the surface of the wing.
Vortex Ring State: [rotor]
VRS is strictly defined by the ratio of the descent rate to the downwash velocity of the specific helicopter. VRS is unlikely vertical speed ratios of less than .5 and greater than 1.5
For more information see; Vortex Ring State Discussion by Nick Lappos
The ratio of the radius of the far wake to the radius of the rotor. Momentum theory calculates it at 1/√2 = 07.07, but experimentally it has been found to be only about 0.78.
Washout: (Twist) [blade]
A twist in the blade between the rotor's center of rotation and the blade's tip.
An out-of-plane surface extending from a lifting surface.
Y-force: [?]
The sideward-pointing component of rotor force, perpendicular to the control axis. [Source ~ AH p.183]
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Latest revision; August 18, 2008