FEA Final Report - Jawanza Bassue

FEA of High Altitude Research Plane’s Engine Mount

Jawanza Bassue M.S. Candidate, Aerospace Engineering

University of Oklahoma School of Aerospace & Mechanical Engineering

AME 5763: Introduction to the Finite Element Method

May 3nd, 2016

Abstract he Bergey Aerospace High Altitude Research Plane (see in Figure 1), is an aircraft that has been

undergoing design at the University of Oklahoma since the fall semester of 2014. The aircraft is

designed to attain a 60,000 ft altitude, breaking manned-high altitude reciprocating engine

records in its class through the flight profile as well as function as a research platform for the University

of Oklahoma. The aircraft is designed to be as light as possible while using mostly aluminum

construction. The aircraft uses a reciprocating, 6 cylinder horizontally opposed Continental GTSIO 520-H

engine with accessories (propeller, turbochargers, intercoolers, piping, etc…) all weighing 775 lb.

The engine and accessories are to be mounted on a custom designed space frame engine

mount, attached to the aircraft firewall that also carries the aircraft’s fixed main landing gear. This

document outlines the process of conducting a finite element analysis of the engine mount through the

combined use of SolidWorks 2015 for modeling and ANSYS Workbench 16.1 for the analysis. In all

considered loading cases, the design is never to exceed the rated strength of the applied structural steel

and exhibit small displacements of less than one-quarter inch. Finally, this report will make

recommendations for modifications to the design to meet objectives.

T


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Table of Contents Abstract ......................................................................................................................................................... 0

Table of Figures &Tables ............................................................................................................................... 1

Introduction .................................................................................................................................................. 3

Boundary Conditions ..................................................................................................................................... 5

Modelling ...................................................................................................................................................... 6

Analysis ......................................................................................................................................................... 6

Condition One: Ground Stop..................................................................................................................... 7

Tabulated Results .................................................................................................................................. 7

Case Two.I: MAX Thrust Flight Condition ................................................................................................. 8

Tabulated Results .................................................................................................................................. 9

Case Two.II: Crucial Design Adjustment ................................................................................................... 9

Tabulated Results & Improvement ....................................................................................................... 9

Case Three: 3.8G Loading in Flight .......................................................................................................... 10

Tabulated Results ................................................................................................................................ 10

Case Four: One Wheel Stand .................................................................................................................. 10

Tabulated Results ................................................................................................................................ 11

Design Recommendations .......................................................................................................................... 11

Smaller Torsional Members .................................................................................................................... 11

I-Beam to Carry Engine Feet ................................................................................................................... 12

Lower Member Modification .................................................................................................................. 13

.................................................................................................................................................................... 13

Conclusion ................................................................................................................................................... 14

References .................................................................................................................................................. 15

Table of Figures &Tables Figure 1. Bergey Aerospace HARP ................................................................................................................ 3

Figure 2. Engine Mount Sketches ................................................................................................................. 3

Figure 3. Engine Compartment ..................................................................................................................... 5

Figure 4. ANSYS Boundary Conditions .......................................................................................................... 5

Figure 5. Line Bodies with Cross Section ....................................................................................................... 6

Figure 6. Initial Mesh (Default) ..................................................................................................................... 6

Figure 7. Boundary Conditions for Case 1 ..................................................................................................... 7

Figure 8. Location of MAX von Mises Stress Case 1 ...................................................................................... 8


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Figure 9. Location of MAX von Mises Stress Case 2 ...................................................................................... 8

Figure 10. Case 2.II Location of MAX von Mises ........................................................................................... 9

Figure 11. Insertion of 3.8*g Acceleration .................................................................................................. 10

Figure 12. Case 3 Location of MAX von Mises & F.S. .................................................................................. 10

Figure 13. Case 4 Location of MAX von Mises & F.S. .................................................................................. 11

Figure 14. Torsional Cross Members .......................................................................................................... 11

Figure 15. F.S. On Small Cross Members .................................................................................................... 12

Figure 16. Low Stress Members .................................................................................................................. 12

Figure 17. Lower Member Position Modification ....................................................................................... 13

Figure 18. After Lower Member Modification ............................................................................................ 13

Table 1. Case One Results ............................................................................................................................. 7

Table 2. Case Two Results ............................................................................................................................. 9

Table 3. Case Two with I-Beam ..................................................................................................................... 9

Table 4. Case Three Results ........................................................................................................................ 10

Table 5. Case Four Main Results ................................................................................................................. 11


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Introduction Initial drawings of the space frame engine mount design were provided by lead aircraft designer and

owner of Bergey Aerospace Co. Ltd., Karl Bergey and are shown in Figure 2. Note that the engine,

propeller and front landing gear of the aircraft are all supported by the engine mount currently under

investigation. The aircraft will weigh roughly 2900 lbs and has a tail dragger configuration. We assume

the front landing gear take 2/3 of the aircraft weight. It is assumed that the final mount will be made

from 1.5 inch (1.66 inch actual) - 3/16 inch wall tubular steel chrome-molybdenum (4130)1 which is

typical for aircraft mounts with a 0.29 Poisson’s ratio, 29,700 ksi modulus of elasticity and yield strength

of approximately 63 ksi2 or similar. A mock-up engine mount will be initially made of structural steel

(A36 or equivalent) as it is cheaper and easier to access. This analysis will initially cater to this mock-up

design, with the expectation that the final mount will outperform. Being a fair-weather aircraft to be

flown by an experienced pilot, it is being designed to withstand 3.8g loading in flight as prescribed by

the lead designer and standard transport aircraft design measures.

Figure 1. Bergey Aerospace HARP

1 (Experimental Aircraft Info, 2016) 2 (Aerospace Specification Metals, 2016)

Figure 2. Engine Mount Sketches


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The software use in this analysis comprise of SolidWorks 2015 for the modeling and ANSYS Workbench

16.1 for the analysis.


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Boundary Conditions A CAD rendering of the intended engine layout is shown in Figure 3. This figure also outlines some of the

expected boundary conditions to be applied to the ANSYS model. Note that the mounts are mirrored on

the opposite side.

Figure 3. Engine Compartment

Basic Boundary Conditions under “normal”

operating scenarios: (# of places)

Firewall Mounts: (4)

o Zero Displacement

Propeller Thrust: (1)

o 1500 lbf

Engine “Feet”: (4)

o 750 lbf evenly distributed

o 3.8*750 lbf evenly distributed

Landing Gear Mounts (2)

o 1933.33 lbf evenly distributed

MAX Engine Torque (1)

o ~590 lbf-ft

A representation of the model with basic boundary conditions is shown in Figure 4. Note that

tag “E” in that figure represents a “remote load”. This is approximately the position of the propeller hub

and also represents the location of a remote moment added to the model. The remote moment

represents the theoretical maximum amount of torque the engine can produce by following the

following equation typically found in automotive applications:

Landing Gear Mount

Firewall Mount

Four (4) Engine Feet

Propeller Mount

Figure 4. ANSYS Boundary Conditions


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The Continental GTSIO-520-H produces 375 HP at 3350 RPM and would ideally create a

maximum 587.910 lbf-ft of torque. This is an approximation and would have to be validated by dyno

data or manufacturer information. The frame would have to counteract the effect of the torque and is a

justifiable boundary condition.

Other conditions faced by the engine mount would have to be evaluated on a dynamic response

basis as it reacts to impact and is not within the scope of this analysis. The conditions herein represent

only steady state approximations, with attempts to create worst case scenarios.

Modelling Using the initial conceptual sketches, SolidWorks was used to create

both a “3D sketch” and a separate solid body weldment model. Both

the 3D sketch and the weldment model (partially seen in Figure 4)

were retained to be imported as line bodies and solid bodies in

separate “.IGS” and “.STP” files to ANSYS respectively. SolidWorks

offers a more robust and user-friendly modelling experience.

However, it is not well suited for detailed finite element analysis.

Fortunately, ANSYS allows the importation of a long list of CAD file

formats.

For a purposes of validation in this analysis, two differing

types of elements are used – beam elements and 3D elements for the line body and solid body files. At

first, the line body file was imported to a static structural module in ANSYS. The joints where lines meet

were joined using the “Connect” tool to simulate a perfectly welded joint between the lines. The cross

section of 1.66 inch pipe with a 3/16 in wall thickness was defined and applied to all line bodies. The

resulting line bodies are seen in Figure 5 prior to applying loading and boundary conditions. When used,

this model gives the fastest computation time but fails to offer a direct distortional energy failure

criteria set and factors of safety response.

Although this engine mount design is well suited for the 3D beam elements mentioned above, a

second solid body model (created using SolidWorks Weldments) was imported directly into ANSYS and

would require 3D elements for meshing and analysis for more complete results. We cannot utilize

symmetry in this model to shorten analysis time due to asymmetric loading conditions of the engine’s

torque. The above mentioned boundary conditions are later applied to this model and allowed to run

sequentially. An initial run of this model pointed to high stresses located at the cantelivered joint behind

the two rear engine feet. Adjustments to the

mesh are made accordingly.

Analysis This analysis will primarily be done with 3D

elements on the full solid model exported from

SolidWorks.

Figure 5. Line Bodies with Cross Section

Figure 6. Initial Mesh (Default)


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A mesh of 1 inch tetrahedral elements (see Figure 6) was set for the initial runs of this model

under various loading situations. Refinements (sizing/sphere of influence) to the mesh will be made on a

per-needs basis. The entire model did not require dense meshing as this would increase computation

time and the major stress will be limited to only a few areas.

We will use the von Mises yield criterion (maximum distortion energy theory) to address the

factor of safety and failure. Since we would like to construct a low lost steel mock-up mount prior to the

final engine mount, the ANSYS Workbench default structural steel is applied as a replacement to the

stronger (43% stronger in tensile yield – 36 ksi vs 63 ksi) steel chrome-molybdenum the engine mount

will be made from. Acceleration due to gravity is also added to the simulation though it does not make a

significant contribution.

Condition One: Ground Stop This represents the simplest case: the aircraft at rest in a parked configuration. The engine weight and

the weight of the aircraft on the engine mount are the only loads to be considered.

Figure 7. Boundary Conditions for Case 1

Tabulated Results Table 1. Case One Results

MAX Equivalent Stress Total Deflection Downward Deflection Min Factor of Safety

18213 psi 0.13125 in -0.13125 in 1.9908


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Figure 8. Location of MAX von Mises Stress Case 1

We see the bending stress at the cantilevered portion of the engine mount cases great concern. The

design still has a factor of safety of nearly two (lower than expected) and minor deflection.

Case Two.I: MAX Thrust Flight Condition In this case we apply engine weight, remote thrust force and torque (moment) applied to a remote point

to the engine mount to gage its performance in flight conditions.

Figure 9. Location of MAX von Mises Stress Case 2


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Tabulated Results Table 2. Case Two Results


87075 psi 0.64023 in -0.64023 in 0.41642

The results here show a surprising minimum factor of safety and relatively large deflections both

indicating failure of the engine mount and a design change must be recommended. Again the trouble

area appears to be near the cantilevered end. We see a large bending stress and will have to account for

that in a design change.

Case Two.II: Crucial Design Adjustment Since the round pipe is not properly suited for bending, we return to SolidWorks and replace the section

that sees the greatest bending, with a 3x2.25” I-beam, unfortunately accepting the weight penalty.

Running Case Two once more with the new I-beam section shows the following results.

Tabulated Results & Improvement Table 3. Case Two with I-Beam


16862 psi 0.16016 in -0.15976 in 2.1504

80.64% Improved 74.98% Improved 75.05% Improved 416.40% Improved

Instantly we see the difference the I-beam substitution makes. It also shifted the location of the

MAX von Mises stress to the location seen in Figure 10. Because of the favorable results, all high load

tests will be conducted on the I-beam model exclusively.

Figure 10. Case 2.II Location of MAX von Mises


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Case Three: 3.8G Loading in Flight The aircraft is designed to be a fair weather aircraft and piloted by someone highly experienced and

even so, has been designed to withstand a maximum of 3.8g loading (typical for transport aircraft). An

acceleration equal to 3.8 * gravity is applied to the simulation space and the weights on the engine

mounts are also exaggerated 3.8 times.

Figure 11. Insertion of 3.8*g Acceleration

Figure 12. Case 3 Location of MAX von Mises & F.S.

Tabulated Results Table 4. Case Three Results


25533 psi 0.24713 in -0.24672 in 1.4201

Note that the location of the maximum von Mises stress (Figure 12) now shifts to a lower

member of the engine mount. There is also a 1.42 factor of safety (again in Figure 12) that assures us of

the engine mount’s ability to withstand the most intense loading. Deflections are at the maximum

prescribed limit.

Case Four: One Wheel Stand In a situation where the aircraft may have to be serviced or inspected, it may become necessary for a

large load (assumed and exaggerated to be the entire 2900 lb weight of the aircraft) to rest on one front


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wheel strut. This situation is analyzed in a “ground, engine off” configuration. We see the results of this

analysis next.

Figure 13. Case 4 Location of MAX von Mises & F.S.

Tabulated Results Table 5. Case Four Main Results


33931 psi 0.21093 in -0.21055 in 1.0686

Design Recommendations

Smaller Torsional Members For the worst case loading scenario (see Figure 12), we see that the cross members designed to improve

the overall torsional rigidity (highlighted in red in Figure 14 below) do not experience significant stress or

deformation and as a result, their diameter may be reduced to aid in weight savings. The validation of

this design decision is seen in Figure 15.

Figure 14. Torsional Cross Members


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Figure 15. F.S. On Small Cross Members

I-Beam to Carry Engine Feet The determination had been made that the round pipe was not

good enough in bending to avoid failure in flight conditions.

Evidence of this is seen in Figure 9 and Table 2. The decision

was made to change these members to I-beams and the failure

was rectified. However, this I-beam cross section is not

optimized and can be analyzed further to produce weight

savings. Furthermore, lightening holes may be incorporated into

the web of the I-beam used for these members as the webs

carry insignificant stress. Finally, the size and cross section of

members shown as A and B in Figure 16 can be significantly

reduced to promote weight savings. In none of the considered loading cases do they experience

significant amounts of stress.

Figure 16. Low Stress Members

A

B

C


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Lower Member Modification A simple repositioning of the member marked C in Figure 16 (example shown in Figure 17 and results in

Figure 18) would serve to alleviate bending concerns at that point.

Figure 18. After Lower Member Modification

We see that for the same conditions as Case One (outlined above), the maximum von Mises

stress appears in a similar location but is 51.30% reduced (8870 psi vs. 18213 psi). This effect is

significant and should be taken advantage of if the engine feet do not interfere. This would allow an

even smaller (lighter) I-beam or similar bend-resistant member to be used (see I-Beam to Carry Engine

Feet recommendation).

Figure 17. Lower Member Position Modification


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Conclusion After addressing the challenges encountered during the modelling and meshing phase, and defining the

various load cases experienced by the engine mount, this analysis has provided enough evidence for a

favorable recommendation. If this analysis is used and the recommendations implemented in the final

design, the mock-up engine mount will safely hold the engine assembly and accessories. Furthermore,

when structural steel is replaced by chrome-molybdenum steel in the flight ready build of the engine

mount, there will be improved factors of safety above the already passing failure criteria outlined here.

Accepting maximum deflections less than one-quarter inch in the load cases, including a 3.8 g

loading scenario points to the rigid design of this engine mount. It is now appropriate to recommend

that this design (with suggested modifications) be taken into the build phase and used on the

experimental High Altitude Research Plane.


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References Aerospace Specification Metals. (2016, April). AISI 4130 Steel, normalized at 870°C (1600°F). Retrieved

from Aerospace Specification Metals:

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=m4130r

Experimental Aircraft Info. (2016, April). Aircraft Engine Mounts. Retrieved from Experimental Aircraft

Info: http://www.experimentalaircraft.info/articles/aircraft-engine-mounts.php

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FEA Final Report - Jawanza Bassue