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![Page 1: Aerodynamics & Flight Mechanics Research Group · 2012. 3. 26. · Aerodynamics & Flight Mechanics . Research Group . The Rotor in Axial Flight . S. J. Newman . Technical Report AFM-11/02](https://reader035.vdocuments.net/reader035/viewer/2022071507/6127a069f8300130346982b6/html5/thumbnails/1.jpg)
Aerodynamics & Flight Mechanics Research Group
The Rotor in Axial Flight
S. J. Newman
Technical Report AFM-11/02
January 2011
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UNIVERSITY OF SOUTHAMPTON
SCHOOL OF ENGINEERING SCIENCES
AERODYNAMICS AND FLIGHT MECHANICS RESEARCH GROUP
The Rotor in Axial Flight
by
S. J. Newman
AFM Report No. AFM 11/02
January 2011
© School of Engineering Sciences, Aerodynamics and Flight Mechanics Research Group
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Introduction The following analysis is an appraisal of Actuator Disc Theory and Blade Element Theory applied to a
rotor in axial flight. The solutions to the methods require scrutiny to choose which is correct for the
particular flight condition. The report discusses the manner in which the solutions interact and
roposes a scheme to assemble them together in a unified whole. p
The analysis is then extended to Ann
Nomenclature
ulus Theory.
T Thrust
R Rotor Radius
VT Tip Speed
VC Climb Velocity
VD Descent Velocity
Vi Rotor Downwash Velocity
N No of Blades
C Blade Chord (Constant)
Air Density
a Lift Curve Slope
s Rotor Solidity
0 Collective Pitch
Overall Blade Twist (Linear)
x Non‐ Dimensional Spanwise Variable
z Vertical Advance Ratio
i Non Dimensional Rotor Downwash
CT Thrust Coefficient
BET Blade Element Theory
ADT Actuator Disc Theory
AT Annulus Theory
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Diagrams
Disc Plane
Blade Section
r
iC VV
Diagram 1 ‐ BET for Climb
Disc Plane
Blade Section
r
iD VV
Diagram 2 ‐ BET for Descent
T
VC
VC+Vi
VC+2Vi
Diagram 3 ‐ ADT for Climb
T
VD
VD-Vi
VD-2Vi
Diagram 4 ‐ ADT for Descent
x
dx
Diagram 5 ‐ Annulus Theory
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Blade Element Theory with Momentum Theory
Climb Root BET gives:
dxx
xxaNcRVT izT
1
0
022
2
1
(1.)
which on normalisation becomes:
23
243
75
0
iz
izT
sa
C
(2.)
ADT gives the following expression for rotor thrust:
iizT
iiz
AV
VVVAT
22
2
(3.)
which on normalisation becomes:
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2
4
4
iizT
iizT
C
C
(4.)
Combining equations (2) & (4) results in the quadratic equation:
02348
752
z
zii
sasa
(5.)
i.e.:
02 CBA ii
(6.)
where the coefficients are defined as:
234
8
1
75 z
z
saC
saB
A
(7.)
if we introduce the product sa as a basic normalisation variable, the above equations reduce to:
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2375
2
7575
iz
TT
ii
zz
sa
CC
sa
sa
sa
(8.)
0234
1
8
1 752
z
zii (9.)
02
CBA ii (10.)
234
1
8
1
1
75 z
z
C
B
A
(11.)
It is now possible to solve this problem within a domain where rotor scale factors have been
completely removed.
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The solutions of (10) & (11) are:
2
38
1
8
1
2
238
1
8
1
75
2
75
2
zz
zzz
i
(12.)
These are denoted C+ & C‐
In order for either solution to exist, the following condition must be met:
042
CAB (13.)
i.e.:
038
1 75
2
z
(14.)
which is a parabolic region defined by:
2
75 8
13
z
(15.)
Provided that this region is avoided, the remaining part of the domain will give rise to a positive or
negative solution.
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For a positive solution we have from inspection of (12):
C+
8
1
08
1
z
z
(16.)
OR
8
1
38
1 75
2
zz
(17.)
which on squaring out gives:
z2
375
(18.)
the domain now sub‐divides as shown in Figure 1:
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75
z
C+
+ve solution
-ve solution
no solution
Figure 1 ‐ C+ Solution Space
C
a positive root will be obtained if:
8
1
08
1
z
z
(19.)
AND
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8
1
38
1 75
2
zz (20.)
which on squaring out gives:
z2
375
(21.)
the domain now sub‐divides as shown in Figure 2:
75
z
C-
+ve solution
-ve solution
no solution
Figure 2 – C‐ Solution Space
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Descent Root
BET gives:
dxx
xxaNcRVT izT
1
0
022
2
1
(22.)
which on normalisation becomes:
23
243
75
0
iz
izT
sa
C
(23.)
ADT gives the following expression for rotor thrust:
iizT
iiz
AV
VVVAT
22
2(24.)
which on normalisation becomes:
2
4
4
iizT
iizT
C
C
(25.)
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Equations (23) & (25) combine to give:
02348
752
z
zii
sasa
(26.)
i.e.
02 CBA ii
(27.)
where the coefficients are defined by:
234
8
1
75 z
z
saC
saB
A
(28.)
normalisation using sa gives:
0234
1
8
1 752
z
zii
(29.)
i.e.:
02
CBA ii
(30.)
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where:
234
1
8
1
1
75 z
z
C
B
A
(31.)
It is now possible to solve this problem within a domain where rotor scale factors have been
completely removed.
The solutions of (30) & (31) are:
2
38
1
8
1
2
238
1
8
1
75
2
75
2
zz
zzz
i
(32.)
These are denoted D+ & D‐
In order for a solution, the following condition must be met:
042
CAB (33.)
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i.e.:
038
1
0238
1
75
2
75
2
z
zz
(34.)
This is a parabolic region, but defined by:
2
75 8
13
z
(35.)
Provided that this region is avoided, the remaining part of the domain will give rise to a positive or
negative solution.
For a positive solution we have from inspection of (32):
D+
8
1
08
1
z
z
(36.)
OR
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8
1
38
1 75
2
zz (37.)
which on squaring out gives:
z2
375
(38.)
the domain now sub‐divides as shown in Figure 3:
75
z
D+
+ve solution
-ve solution
no solution
Figure 3 ‐ D+ Solution Space
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D a positive root will be obtained if:
8
1
08
1
z
z
(39.)
AND
8
1
38
1 75
2
zz (40.)
which on squaring out gives:
z2
375
(41.)
the domain now sub‐divides as shown in Figure 4:
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75
z
D-
+ve solution
-ve solution
no solution
Figure 4 – D‐ Solution Space
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Combined Solution
It is now possible to determine a calculation strategy permitting climb and descent with positive &
negative pitch angles.
The following procedure is proposed:
The C+ & D‐ solutions form the basis. (The C‐ & D+ solutions are not appropriate.) These are shown
together in Figure 5:
75
z
C+
+ve solution
-ve solution
no solution
75
z
D-
+ve solution
-ve solution
no solution
Figure 5 – Comparison of C+ & D‐ Solution Spaces
for any point in the domain of z & 75 75 a choice of C+ or D‐ solution is required.
The tables show the choice order.
Select 1st Choice if it exists
Otherwise
Select 2nd Choice
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The choices are as follows:
75 z 1st Choice 2nd Choice
=>0 C+ C+
=>0
<0 D‐ C+
=>0 C+ D‐
<0
<0 D‐ D‐
This can be simplified to:
z 1st Choice 2nd Choice
=>0 C+ D‐
<0 D‐ C+
Extension to Annulus Theory
The thrust over an elemental annulus is given by BET:
x
aNcdrrdT iz 2
2
1
(42.)
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Momentum Theory Climb
from momentum theory over the annulus:
Equating (42) & (43) gives:
From which we have:
Hence:
Normalizing on sa gives finally:
iiC VrdrVVdT 22 (43.)
xa
R
xVNcVVV izT
iiC
2
8
1
(44.)
iziiz xsa 8
(45.)
088
2
zzii x
sasa
(46.)
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08
1
8
12
zzii x
(47.)
Momentum Theory Descent
The thrust over an elemental annulus is given by BET:
iiD VrdrVVdT 22 (48.)
Noting that:
CD VV (49.)
from momentum theory over the annulus:
iiC VrdrVVdT 22 (50.)
Equating gives:
xa
R
xVNcVVV izT
iiC
2
8
1
(51.)
From which we have:
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Hence:
Normalizing on sa gives finally:
Comparison of Actuator Disc Theory (ADT) & Annulus Theory (AT)
The quadratic equations for ADT & AT are presented below:
Climb
ADT
AT
iziiz xsa 8
(52.)
088
2
zzii x
sasa
(53.)
08
1
8
12
zzii x
(54.)
0234
1
8
1 752
z
zii
(55.)
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08
1
8
12
zzii x
(56.)
Descent
ADT
0234
1
8
1 752
z
zii
(57.)
AT
08
1
8
12
zzii x
(58.)
Inspection of these equation pairs shows the following equivalence between the local pitch angle
(AT) and the pitch at 75% radius (ADT):
753
2 x
(59.)
In other words, the AT solution can be achieved using the same choice of solution developed for the
ADT.
The thrust variation can then be evaluated by:
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dxx
xaNcRVT izT
1
0
22
2
1
(60.)
whence:
dxx
xsa
C izT
1
0
2
(61.)
where finally:
dxx
x
sa
CC
iz
TT
1
0
2
2
(62.)