Mechanical Design Flashcards
What are 3 challenges of mechanical design?
- Dynamics of multiple rotor systems with large gyroscopic couples
- Cold parts are subjected and based on dynamic problems
- Hot parts are limited by fatigue and creep as well as damage tolerance philosophy
What does mechanical design require?
- Determination of physical parameters (temperatures, stress, strain)
- Deep understanding of Material properties and failure conditions (very low safety factor
margins are required) - Interaction and collaboration with other disciplines
What is an example of 2 colliding goals of mechanical design?
- Coaxial Rotors require the bore of the HP discs to allow the passing of the LP shaft – Stress increases with the bore radius = LP (N1) shaft diameter as low as possible
- Critical speeds should be above the range of operational speeds = LP shaft diameter should
be as high as possible
What are the mechanical loads acting on rotor blades?
- Rotating Mass at high speeds -> Centrifugal Loads
- Gas loads
- Vibration phenomena are the main cause of failure (in compressors)
- Natural frequencies (i.e., Critical speeds)
- Mode-shapes and response levels
- Damping levels
- Stability (i.e., vulnerability to flutter phenomena)
What are the 5 adverse conditions for rotor blades?
- Erosion prone
- Damage from foreign objects
- High Temperatures
- Working over melting temperatures
- Lifetime limited Components (Creep, fatigue)
What are the 5 blade cooling methods?
- Film and convection cooling
- Transpiration cooling
- Multiple small hole transpiration cooling
- Water cooling
- Strut insert blade
What does “shrouded” / “shroudless” mean?
- Shrouded: “Eingerahmt”, rotor blades have a frame around them
- Shroudless: No frame
How did rotor blades improve over time?
- Sweep was used for aerodynamic improvements (“geschwungen”)
- On rotors, hardly any dihedral can be used due to structural reasons
- No need for supports between blades, as materials now stronger
What are 7 properties of fixed stator vanes?
- Featuring either a free inner tip or shroud
- Single aerofoils or segments of aerofoils
- Mostly featuring T-shape root for fixing in
the outer casing - Fitted in the circumferential direction
- Dedicated vanes for nominal size, under-size,
locking and boroscope inspection - Can be rotatable about the aerofoil axis
- In most design solutions
supported at the hub and the casing
What defines an outlet guide vane?
- high flow turning, many aerofoils, potentially 3D stacking,
tightly spaced, conventional platform design difficult,
often designed as an integral part with a diffuser - integral ring, cast in one piece, all aerofoils,
some post-casting machining, high roughness, low precision
(high manufacturing and build tolerances) … usually acceptable on this aerofoil
What is the difference between HPT vanes and LPT vanes?
HPT needs cooling (usually film cooling)
What are 2 axial roots?
- Extended root, with full formation of a platform, Blades with extended roots are held in place using retaining plates
- Non-Extended root, featuring no or a partial platform, the resulting gap is filled with segments of the
disc rim to give complete contour
When is an axial root used and what needs to be considered?
- When single-blade weight is high
- Need to be combined with stators
having inner shrouds - Fitting is in the axial direction into the
the rim of the disc
What are circumferential roots and when are they used?
- When single-blade weight is low
- When blade count is high and some
sealing is required at the root to
prevent reverse leakage - Can be combined with stators having
either a free inner tip or an inner
shroud - Fitting in a circumferential direction
into the rim of the disc - Dedicated blades for nominal size,
under-size and locking - Platforms are primarily rectangular,
or sheared or angled if required
What are the 6 load sources in a rotor disc?
- Centrifugal body force of disc material
- Centrifugal load produced by the blades and their attachments to the disc
- Thermo-mechanical stresses produced by temperature gradients between bore and rim
- Shear stresses produced by torque transmission from the turbine to the compressor
- Bending stresses produced by aerodynamic loads on the blades
- Dynamical stresses of vibratory origin
What are the 3 types of rotor construction?
- Drum rotor
- Disc rotor
- Drum-Disk Rotor
What are Blisks?
Integral design of blades and disk
(Bladed Disk = BLISK)
What are the 2 ways of manufacturing a BLISK?
- Milling from solid, point contact milling, or flank milling,
depending on geometry complexity - Attachment of readily prepared blades (without roots) to the
disk by linear friction welding, plus subsequent machining of
the blade hub region
What are some design advantages and disadvantages of BLISKS?
+ low weight, low
manufacturing cost/time, good structural stress
distribution, low assembly effort, no root leakages
- managing vibrational
behavior at low damping level, repair of damage
What are 3 advantages of multi-spool design?
- Higher compression ratios
- Quick acceleration
- Better control of stall characteristics
Draw the schematic dynamic model of the jet engine!
Lecture 5, page 38
How are the structures in an engine supported?
- Main jet engine shafts are supported by a
minimum of two bearings - One bearing has to be a thrust ball bearing
that can take axial and radial loads - Cylinder roller bearings take only radial
loads - For example, rotors (shafts and discs) are
supported by five bearings mounted in two
engine sumps for lubrication
Which materials are used in aeroplane engines?
- Titanium (Fan to IPC)
- Nickel (HPC to Nozzle)
- Steel (Mechanical parts, e.g. shafts)
- Aluminum (Leading edge of the casing)
- Composite (Rest of the casing)
