Unmanned multi-rotors vehicles are a consolidated reality in the modern aeronautical field. These small helicopters consist in a body “hanged” under a set of fixed pitch propellers each powered by an electric motor. These vehicles have great potentials and the research in this topic is increasing aimed at the reduction of the structure weight, therefore maximizing flight endurance, range and payload. The development of multi-rotor components represents a key challenge both for the structural optimization and the additive manufacturing; they mainly consist in complex shapes where the most important features are robustness and lightness. These parts are usually produced in small series (i.e. eight parts for a single prototype), are subjected to high loads and need to be able to interface different materials. The work here presented reports the research conducted in cooperation between Politecnico di Torino and Altair Engineering to design and optimize two vital components for the structure of a multi-rotor. These parts represent a challenge because of the main need is to interface the arms, consisting in carbon fiber tubes, with the electric motor flange, on one side, and the body frame on the other, both made in 7075 Alloy. In particular, the second part will also have an important role as structural shock absorber in case of emergency landing. The use of topology optimization techniques plays a key role to minimize the weight of the components and to improve the productivity of the machines. Moreover, the fused deposition modeling (FDM) technology, applied in this case, allows producing more parts in less time, improving the cost effectiveness of the project. An important role is played from the Altair tool used for the preliminary design: Inspire. This tool is conceived to quickly and easily generate structural efficient concepts to obtain lighter designs and to eliminate structural design problems, finally providing input files for 3-D printers. Speakers Carlo Ferro, Politecnico di Torino

1. Altair E-ATC 2015

2. SAPR, UAV: Definition and Italian Regulamentation
• ENAC: a SAPR is a aircraft remotely piloted without people
on board not used for recreational use
• The UAV or SAPR used in special operations or in
experimental activities constitutes the system of aircraft
remotely piloted and are submitted to the air law (ICAO)
• The aircraft modelling ARE NOT considered aircraft and are
not submitted to the air law codex. Indeed they can’t be
used in aerial work or in other application except the
recreational use in special areas.
SAPR..
..UAV..
..APR..

3. SAPR, UAV: Italian Classification
SAPR are divided into two main categories:
• MTOW < 25Kg
• MTOW > 25Kg
No critical operations, no densely inhabited areas, no controlled ATZ,
AutoRoute, rail station etc.
V70 flight space (50 m minimum distance from people, 70 m
maximum height over soil, 200 m maximum operative radius from the
pilot, VLOS link)
Only daytime operations.
Project PBK

4. Progetto PBK, il velivolo
Attempt to
establish a
world
record!
• Multi rotor aircraft (8 Propellers);
• Long range / time of flight (more than 3 h);
• Patented power generation system
• High resistance / low weight structure
designed using aluminum alloys, carbon fiber
and additive manufactured abs parts in the
interfaces:
• Carbon pipe – electric motor;
• Carbon pipe – aluminum frame.

5. SAPR & ADDITIVE MANUFACTURING
Additive layer manufacturing (ALM) could bring great
potentials to the design of SAPR due to:
• Great demand of weight reduction to match the
requirements;
• Small series pieces that doesn’t require shells special tooling
etc.;
• Demand to vary the design of the parts customizing the
aircraft
• Time to market as short as possible

6. Additivemanufacturing:scoutingofpotentialtechnologies
• Powder bed (SLM, EBM, DMLS): useful technology
to build metal andplastic part. At the time to
expensive for the UAV market both for machine cost
and part produced via external service.
• Stereo lithography: a photopolymer is hardened via a selective coherent
light deflected in a x-y plane by movable mirrors. 3DSystem was the first to
introduce this technology and is now the leader. Optimal mechanical
properties and dimensional tolerances but cost still unaffordable for the
SAPR market.
• FDM (material extruder): is the most diffused technology on earth and
outside (the machine installed on ISS bt NASA). All derives from the open
source project RepRap.
• More than 21000 printers sold only in 2012 by Stratasys.

7. Progetto PBK: Work Stream
Evaluation of the results
Weights, stress, deformation Modal frequencies etc
FEM analysis with Optistruct
Stress, strain modal
Redesign
Preliminar optimization with Inspire
Objective min mass Boundary constraint max stress and model frequencies
Design of the rough model
design space non-design space
Material characterization at tension / compression
Construction of microtensile specimen Testing Evaluation of material properties

8. Pbk project: material characterization
• Choosing process of the micro
tensile specimens via DOE to
evaluate also the
manufacturing constraints (min
wall thickness, maximum angle
without support etc.)
• Construction of 96 micro-tensile
specimens to extimate the
ϭbrake nd the elongation at
roupture
• Experiment design and post
processing carried out with
hyperstudy
Displacement [mm]
Load[N]

9. Paretodiagramandmaineffectfortheloadatrouptureand
theYoungmodulus*
*growth direction perpendicular to the plate*growth direction parallel to the plate

10. Pbk project: choosing of the parts to be constructed
Two components were
selected to be built
with FDM:
• Interface between
motor and carbon
pipe
• Interface between
carbon pipe and
aluminum alloy
structure
This two components have been selected due to the high
solicitation and because of the simplification of the gluing process.
The original version of the two parts were in 7075 alloy with a weight
of 34,5 grams, the other 31,1 grams
Total weight: 773 grams

11. Pbk project: design and optimization
• Non design space: connections
with the pipe and with the screws
of the motor
• Design space volume: gross space
usable
• Objective function: min of the mass
• Constraint : max stress and first 10th
modal greater than 250 Hz.

12. Pbk project: results obtained
• Boundary constraint satisficed
• Need to add some other
manufacturing constraint
orientation angle, removal of the
support tooling space etc.)

13. Pbk project: re design of the parts

14. Pbk project: validation analysis
• Analysis post the redesign phase: stresses
and strain are under the limit. The first 10
modal are over than 250 Hz.
• Motor support optimized weight
10,2grams
• Pipe support post opt 6,7 grams
• Total weight of the parts: 189,2 versus
773 grams

15. Progetto PBK: Costruction and testing
Both the prototypes were tested
statically in laboratory to verify the
displacement with good
comparison with the expected
results

16. ProgettoPBKNextsteps:integrationofmorecomponentina
singlepartshockabsorber
• Task: obtain one multifunctional
component bult in a single part (shock
absorber and motor support);
• Minimum mass;
• Constructed with FDM
• Both statical and dynamical simulation
with large displacements(shock
simulation executed with RADIOSS)
• Experimental verification on October
2015
1
2
3

17. Risultati preliminari
• Weight: 90.46g
• Good printability results with FDM
• Scale version 1:2 printed for the preliminary statical test
CAD model Prototype Static Test

18. ProgettoPBK:Sviluppifuturi,integrazionepiùcomponenti,
shockabsorber
• Evaluation of other material properties (PLA);
• Testing with hybrid filaments(carbon fiber);
• Study on the fatigue of ABS built specimens via FDM;
• Introduction of the optimization concepts also to other
parts of the UAV
• Numerical simulation and experimental testing of the
vehicle in take off and heavy landing.