Report SA-1000-05
National Center for Atmospheric Research
Blunt body Aerosol Sampler (BASE)
Structural Analysis SA-1000-05
NSF C-130 Aircraft
Revision:
Date:
3.0
April 14, 2011
Clarkson University
Potsdam, NY 13699
Prepared By:
Arash Moharreri and Suresh Dhaniyala
Mechanical and Aeronautical Engineering
Clarkson University, Potsdam, NY 13699
And
Dave Rogers
Research Aviation Facility
National Center for Atmospheric Research
© 2011 University Corporation for Atmospheric Research
Disclosure of information contained herein may only be made to UCAR or FAA personnel and/or
Designees to support this certification effort.
Disclosure to other persons requires the express written consent of UCAR Legal Counsel.
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Revisions
Revision
original
2
Date
3/29/11
4/12/2011
3
4/14/2011
Description
Inlet structural analysis
Flow misalignment loads added
to analysis
Flow misalignment loads
corrected
Pages
4-6
4
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TABLE OF CONTENTS
REVISIONS ………………………………………………………………………..….. ii
1.0
INTRODUCTION ................................................................................................... 1
1.1
Discussion ............................................................................................................ 1
1.2
Applicable Drawings ............................................................................................ 1
1.3
Minimum Margins of Safety ................................................................................ 1
2.0
Blunt body Aerosol Sampler (BASE) assembly ...................................................... 1
2.1
Description ........................................................................................................... 1
2.2
Geometry .............................................................................................................. 1
2.3
Weights................................................................................................................. 2
2.4
Loads .................................................................................................................... 2
2.5
Stress Analysis ..................................................................................................... 3
3.0
SUMMARY ............................................................................................................. 7
4.0
APPENDIX .............................................................................................................. 8
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1.0
INTRODUCTION
1.1
Discussion
Clarkson University is installing a sampler on the aircraft skin, called the Blutn
body Aerosol Sampler (BASE) and the associated instrumentation for aerosol
measurements. BASE is a heated inlet with pressure measurement ports and has a
support structure similar to NCAR HIMIL and is mounted on a standard 15”
diameter plate, under the belly of aircraft.
1.2
Applicable Drawings
Drawings are included in the Appendix.
1.3
Minimum Margins of Safety
Item
Load Condition
Strut
Drag + Gust load +
ultimate handling
load (lateral &
vertical load)
Screws attaching different Drag + Gust load +
parts of assembly together
ultimate handling
load (lateral &
vertical load)
2.0
Stress Condition
Bending moment
+ shear
MSmin
High
Bending moment
+ shear
High
BLUNT BODY AEROSOL SAMPLER (BASE) ASSEMBLY
2.1
Description
The BASE consists of six main parts that are shown in Fig. 1: Blunt body
housing: inlet side and strut side, inner tube, interstitial inlet, strut, and base plate
(not shown here). The base plate will be mounted on 15” plate provided by
NCAR, under the belly of aircraft. The vertical criteria of 10” maximum length
has been considered and applied in design of this assembly.
2.2
Geometry
The geometry of sampling assembly illustrating different parts of the system is
shown in Fig. 1.
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Blunt Body
Housing –
Inlet Side
Interstitial
Inlet
Inner Tube
Blunt Body
Housing –
Strut Side
Additional
Interstitial
Inlet – not
shown in the
assembled
view
Strut
Fig. 1 Blunt body Aerosol Sampler Assembly, exploded and assembled views. Inlet aligns
with local airflow along the axis of symmetry, and flow is left to right.
2.3
Weights
Component weights are:
2.4
Component
Weight (kg)
Blunt Body Housing – each side
0.82
Strut
0.97
Base Plate (not shown in Fig. 1)
~1.0
Loads
As recommended in NCAR C-130 handbook (chapter 5), following loads are
assumed in stress analysis of the system:
I.
Airspeed – 250 kias at sea level conditions
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II.
In Flight Gust Loads – 8.25g down and 5.25g up
III.
100 lb ultimate vertical handling load
IV.
100 lb ultimate lateral handling load
2.5
Stress Analysis
Strut
Drag Load: Fluent simulations were used to obtain an estimate of the pressure
and viscous drag acting on the inlet (table 1). The coefficient of drag can be
calculated from this computed drag by rearranging the drag formula.
Cd 
2  Drag
AV 2
(1)
Table 1: Drag data from Fluent and drag coefficient
Surface
Total Force
Projected Area
Velocity
Air Density
Cd
N
m^2
m/s
kg/m^3
Strut
2.85
0.008001
128.61
1.23
0.03573
Blunt Body
Housing
66.38
0.007109
128.61
1.23
0.91793
Using the obtained coefficients, the aerodynamic forces acting on the inlet can be
estimated using the standard drag formula:
Drag  1 C d AV 2
2
(2)
According to recommendation given in NCAR investigator handbook, an airspeed
of 250 kias at the sea level was assumed for drag calculations.
Using the worst-case scenario for drag, we can calculate the maximum deflection
and maximum shear stress in the strut by approximating it as a cantilevered beam.
The second moment of inertia of the strut with respect to the principal axis that is
perpendicular to both direction of deflection and longitudinal axis of the strut is
2.595x10-6 m4 (calculated for the exact structure by ProE software). We can
assume that this axis is close to the neutral line of the strut. The maximum
deflection for the strut can then be calculated as:
y max 
 Drag  Length 3
3EI
(4)
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where E is the modulus of elasticity (Young’s Modulus), which is about 70GPa
for aluminum. The maximum deflection calculated for our strut is 7.14 x 10-7 m,
i.e ~ 2.81 x10-5 in.
The maximum stress is calculated by approximating the total drag force exerted
on the whole assembly with a point force acting at the interface of the strut and
the blunt body housing:
 max 
Drag  l  c
I
(5)
where c is the distance from neutral axis to extreme fiber (edge). The calculated
maximum stress is 0.275 MPa. The yield stress for the 6063-T6 aluminum alloy
used in the fabrication of this inlet assembly per Federal Specification QQ-A200/9C is 25 ksi (172 MPa). The calculated stress for the strut element is only
0.16% of yield stress for this aluminum alloy.
Flow Misalignment Loads: In addition to drag loads, the aerodynamic loads
caused by misalignment of the probe with the free stream air are also considered
for load analysis. The normal force on an object due to aerodynamic loading is:
Fn  C n V 2 Sref / 2
(6)
For this case, a 10° misalignment between the flow and structure is considered.
The lift coefficients considered for this case are 0.11/deg for the strut and
0.8Sin(10°) for the blunt body housing (as per communications with Mark Lord at
NCAR/RAF). The forces on the strut and housing (250 kias at sea level) are
given in the following table:
Table 2: Normal forces
Surface
Cn
Reference
Area
Velocity
Air Density
Total Force
m^2
m/s
kg/m^3
N
Strut
1.1
0.032
128.61
1.23
358
Blunt Body
Housing
0.14
0.024
128.61
1.23
34
The total aerodynamic normal force is 392N (88lbf). Due to the shorter moment
arm the lateral handling force will produce higher stresses in the strut and reaction
forces in the attachment fasteners. This load case is not critical.
Gust Load: For the vertical downward load, the NCAR investigators handbook
recommends that we consider a gust load equal to 8.25 times of the weight. For
maximum moment at the base plate, the vertical force is considered to act at the
leading edge of the blunt-body housing. As the moment at the interface of the
leading edge of the strut and the base plate is in the same direction for both the
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vertical and drag forces, a combined net stress is calculated considering the
simultaneous action of both forces.
The mass of the blunt body housing together with the strut is 2.6kg. Thus, a
vertical force equal to 210.42N (=8.25*210.42*9.81) is exerted at the leading
edge of the blunt-body housing, which is at a distance of ~ 0.22m. The net
moment at the strut-base plate intersection from this load is 10.64 Nm.
Considering this load and moment, the maximum tensile stress resulting from
both drag and gust force at the intersection is 1.3MPa. For the strut (considering
the yield strength of the Al alloy used) this results in safety factor of ~ 132.
Handling Loads: In order to account for handling loads, two cases are
considered: (i) 100lb vertical force and (ii) 100lb lateral force, both applied at the
leading edge of the blunt body housing.
The effect of vertical handling load is similar to that of gust load with a difference
in the magnitude of exerted force. This force will result in a maximum tensile
stress of 2.2MPa in the strut and consequently a safety factor of 96.
We can consider the effect of lateral handling force acting at the leading edge of
the blunt body by replacing it with a lateral force and a torque applied at the
interface of the blunt body housing and the strut. The lateral force will try to bend
the strut in the lateral direction and the torque will have a torsion effect. The
appropriate second moment of inertia for this case is 1.55x10-7m4. Considering
the c in Eq. (5) to be half of the thickness of strut cross sectional profile (18 mm)
and the arm l being ~18 cm, the maximum tensile stress caused by the bending
effect is 9.2MPa and the corresponding safety factor will be 23.
Maximum shear stress caused by the torque can be approximated by considering
the strut as a thin rectangle, using the following equation:
 max 
T 
t 
3  1.8 
2 
wt 
w
(7)
where T is torque and w and t are width and thickness of the rectangle,
respectively. Calculated maximum shear stress using the above equation is
1.8MPa. Hence, the safety factor here is 68.
Screws
Joining the Two Halves of Blunt Body Housing
In order to join the two parts of blunt body housing together, four 6-32 and six 440 screws are used. These screws should be able to bear the drag force exerted on
each half of the blunt body. The drag force on one half of the housing will be
33.19N. Assuming that only 6-32 screws will be carrying the load from drag
force, the shear stress on each screw will be about 1.65 MPa. Thus, the MS16997
screws (Ptu = 6.3kN) factor of safety will be 450. It was assumed that the
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maximum shear strength of the material is equal to 0.58 times of the tensile
strength.
If we want to find the factor of safety with the gust load as the design force, we
should consider a force equal to 8.25 times weight of half the blunt body. Each
half of the blunt body weighs ~ 0.82 kg and therefore the downward force that the
screws between the two housing parts should support will be 66.36 N. This results
in 3.28MPa tensile strength on each screw (considering just the four 6-32 screws),
results in a factor of safety equal of 225.
Joining the Blunt Body Housing to the Strut
Two screws of size 10-32 and two 6-32 screws are used to attach the blunt
housing to the strut. The 10-32 screws are MS24694 screws and the 6-32 screws
are NAS601. As one of the 10-32 screws (the forward one) has lesser material
around it, we have ignored this screw in the load calculation. Considering just the
other three screws, their loads will be similar to that acting on the screws joining
the two halves of blunt body, with the difference that the forces are doubled as the
entire blunt body is considered as one unit. Then the factor of safety considering
the drag and gust loads will be 160 and 280, respectively.
If we consider the specified lateral and vertical handling loads, the corresponding
safety factors will be 6 and 10.
Joining the Strut to the Base Plate
The strut is supported at the base plate using eight 10-32 screws (MS24694).
These screws should tolerate forces similar to those exerting on the strut. The
worst case scenario considered here is a situation where the drag force and the
gust load act to tip the assembly over a point on the foot of the strut with largest
distance from the front (upstream) edge of the housing in the horizontal direction.
We assume that only the screw at the front side of the base plate tries to
compensate for the moment about the pivot point. This way, the front screw
should withstand a force equal to 475N to balance the moment from the drag and
the gust force. This will result in a tensile stress of 40.7Mpa for this screw and a
factor of safety over 21 (for this worst case scenario). Consider the other screws
(total 8), the strut to base plate assembly has a much larger safety factor than this
value.
The vertical handling force will have an effect similar to that of the gust load,
with the magnitude of the force being 2.11 times greater. This will result in a
safety factor of over 9 when considering this force as design load.
The lateral force will try to tip the strut in the lateral direction and therefore the
acting arms of forces are different at this case. Assuming that the two screws
which are the farthest the pivot point will prevent it from tipping, the tensile stress
on each one of these screws will be 60MPa and the safety factor associated with
that will be 14.
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Joining the Base to the 15” Plate
There will be 8 screws of size 10-32 attaching the inlet base plate to the aircraft
15” base plate, with their pattern being offset ~ 1.0” towards the outside of the
strut. The factor of safety for these screws will be similar to that calculated in the
above case (strut-to-base-plate).
3.0
SUMMARY
In summary, the structural analysis of the inlet assembly showed that all
components have substantial safety margins - mostly over 100. Therefore, it can
be concluded that in terms of static structural concerns, the assembly is
completely safe.
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4.0
APPENDIX
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