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Light Aircraft manufacture and materials
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Comparison of materials, specific buckling stiffness.
WG
FR
P
UD
GF
RP
WC
FR
P
UD
CF
RP
Ep
oxy m
atr
ix
Bir
ch
ply
Ba
lsa
Sp
ruce
Fir
Ste
el 8
.8
60
82
allo
y
70
75
allo
y
Tita
niu
m a
llo
y
E/ro^2
E/ro
0
50
100
150
200
250
E M
PA
/g
/cc
Comparison of materials for specific stiffness (E/ro)
column buckling ( E/ro^2) and panel buckling ( E/ro^3)
E/ro^2
E/ro^3
E/ro
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Specific strength of materials UTS/ro
0
50
100
150
200
250
300
350
400
450
500
WG
FRP
UD
GFR
P
WC
FRP
UD
CFR
P
Epo
xy m
atrix
Birc
h ply
Bal
sa
Spr
uce Fir
Ste
el 8
.8
6082
allo
y
7075
allo
y
Titani
um a
lloy
sp
ecif
ic s
tren
gth
MP
a/g
/cc
UTS/ro
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Relative cost of materials cost/kg
0
10
20
30
40
50
60
70
80
WGFR
P
UDGFR
P
WCFR
P
UDCFR
P
Epo
xy m
atrix
Birc
h ply
Balsa
Spr
uce Fir
Ste
el 8
.8
6082
allo
y
7075
allo
y
Titanium
allo
y
£/k
g
cost/kg
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Considerations
● Metals – design for fatigue, corrosion
● Isotropic - best for 3D loads
● Bolted and rivetted joints good in metals
● Wood – drainage/ventilation, considerations apply as for composites
● Composites – Impact, drainage ( esp. foam),service temperature, repair schemes
● Wood and composite Orthotropic – best for 1D or 2D loads
● Rivetted joints unsuitable for thin composite or wood panels
● Fibres must be aligned with primary load paths
● Through-thickness peel and shear loads must be carefully considered
● Slot together joint features best – avoid peel loaded joints
● Carbon composite strain limit around 0.4% to allow for joints, repairs and to avoid damage growth after impact
● Special factors applicable for temperature degradation and variability of production ( see CS-VLA AMC 619)
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Design process
● Sketch ideas for layout, calculate flight envelope, weights, loads
● Simple hand or XL spreadsheet calculations
● Optimise layups using laminate analysis program to obtain A,B,D stiffness matrices
● Refine calculations and weight estimates
● Apply laminate analysis to FE program e.g. Nastran
● Check answers are in line with initial hand calcs
● Optimise FE design
● Build it and load test unless experienced with very similar previous analysis models
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CFM Shadow uses many different materials to good effect
● Fibrelam fuselage structure
● Extruded light alloy boom
● Foam ribs
● Ply leading edges
● Extruded spar caps
● Pultruded U/C legs
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Classic Cessna 150 light alloy semi-monocoque. Wing has few ribs and uses stiffeners, postbuckling at ultimate load. Poor
payload/empty weight compared to Jodel.
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ARV super 2 – all light alloy, with super plastic formed fuselage belly pan. Now called Opus in USA with Rotax 912 engine.
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The Jodel with wood construction is light and efficient
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Columbin MC30 Luciole – even lighter!
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Columbin Luciole uses wood, foam and carbon pultrusions.
● 97kg empty, 200kg max
● 25bhp Briggs and Stratton V twin
● 100kt on 5 litres/hr
● Wing uses wood spar with carbon pultruded spar caps and foam ribs
● Wing is quickly detachable from fuselage
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Spar construction
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Wing skin bonding under partial vacuum
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Ply skinned wood fuselage
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Fuselage with wing connection
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Spar to fuselage carry-through structure
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V twin Briggs and Stratton conversion, direct drive
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The Mosquito used innovative design with wood to achieve excellent performance, pioneering modern composites design practice
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Mosquito construction
● ply/balsa/ply fuselage sandwich monocoque, with reinforcements built into the core
● Fuselage 80% fitted out in 2 halves, hydraulics in one side, electrics in the other
● ply/spruce stiffeners/ply wing skin sandwich panel production
● Laminated spruce spar caps with plywood shear webs
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Composites allow smooth aerodynamic shapes and laminar flow
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Glass/Epoxy UD fibres make an effective spring undercarriage
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The A.I.R. Atos has a cantilever carbon leading edge D box with folding ribs for transport, flaps for takeoff/landing, 15m span, 40kg,
20:1 glide and 150fpm sink rate
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Firewall uses reflective and intumescent coatings e.g. Technifire
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Storch uses mixed construction with extruded light alloy tailboom, strutted wing can be folded
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Prepreg/autoclave – high cost/high performance process with some problems.
● De-bulking required every 4 plies
● High energy usage
● Thermal stresses/warping
● Expensive equipment
● Traditional high-end aerospace process
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Vacuum bag processes
● Wet-preg/vac bag process gives better properties than bucket and brush
● Widely used for gliders, motorgliders, microlights
● Messy process
● Resin infusion process setup and process control more complex
● Clean process
● Low void content
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Stabilator sandwich skin vacuum bagged
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RFI and RTM
● RFI similar to resin infusion but allows toughened resins
● Easier to get uniform infusion over an area
● Vacuum stack compacts during cure
● RTM requires precision closed tools
● Good for small parts e.g. Propeller blades
● Process control and preform production critical
● Good finish with tight tolerances on all surfaces
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Winding, Wrapping and pultruding
● Filament winding or prepreg tape wrapping for torque tubes and pressure vessels
● Pultrusion and pulwinding give high quality constant section parts at low process cost
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Finishing composites
● Polyester gel coats favoured over epoxy for UV resistance – but check compatibility with epoxy gelcoat
● Brushed gel coats typically 1mm thick
● Spray gel coat 0.5mm thick, more uniform colour dispersion and metallic effects possible, air fed mask and fume extraction required
● Structural composites generally white to minimise surface temperature ( 54C)
● Lightest finish is to prime/surface then 2K acrylic paint – but very labour intensive
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CT production
• CT rear fuselage is monocoque sandwich
• Aramid used on inside skin
• Reinforcing bulkheads carbon with ply inserts
• Finish is by primer/filler then sanding and painting
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Slotted H-joint features in the Beech Starship
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Wing construction in a mould
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Mouldless methods
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Mouldless methods - shells
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Foldaplane concept for 2D curvatures
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Now for something completely different
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Which does different things
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QuikR – a 3D sports motorcycle ● Tailless flexwing configuration gives low empty weight and low parts
count
● Empty 220kg, loaded 450kg
● 50-100 mph hands off, electric trim
● Lateral trim by nosewheel steering
● Range 400 miles at 80mph
● 38mph stall, 120mph Vne
● 1200fpm climb at max AUW on Rotax 912s
● Quick fold topless wing
● Seamless mandrel drawn 6082-T6 17swg tube airframe
● Extruded 6082 aerofoil struts
● Polyester mouldings with spray gel coat
● Extruded trike basetube and main pylon
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Fittings etc designed and optimised on CATIA/ABAQUS
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Sail reinforcing aramid bands and washout rods, composite tip fins, short aerofoil struts, pullwound carbon battens
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QuikR sail production
● 18 hours precision sailmaking, +- 1mm all over
● Advanced technology sail materials include Polyant HTP square sail main body, PX10T leading edge, X05 aramid unidirectional reinforcement bands, Mylar .040” leading edge stiffener with APS system.
● In flight strain gauging carried out with extensometer
● Sails produced on mylar templates with raised tape edges, shape is transferred with a 4h pencil
● Sail is assembled with seamstick double sided tape
● Simple automatic tape folder/adhesive applicator for making batten pockets
● Sewn together with Tenara spun PTFE UV proof thread
● Continued airworthiness testing by Bettsometer ( needle with calibrated spring balance)
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Sail loft with batten tape folder, raised tables and mylar template with thick tape profiles
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The last word on the subject!
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Scrapheap biplane build and flown in 3 days with 4 people
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Sailmaking for BAE Cayley glider
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Pilcher triplane replica
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Booster carbon fibre auxiliary power unit with folding propeller and retractable undercarriage