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Design and Construction of the Mole Mite

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Design and Construction of the Mole Mite Mole MITE Wing Design and construction 800 2162 25% datum 1613 928 10,7° 234 3160 343 644 586 498 1000 Mole "MITE' G.A. 25% datum 4390 A lifetime’s interest • PPL from RAF scholarship while at school • School to Vickers Armstrongs Aircraft shop floor • UAS APO rank and BSc Aeronautical Engineering • 2 hrs flying Tiger Moths was a good investment • A career in Business Schools – but no flying ! • Family & domestic commitments took over • I desperately wanted to fly again before it was too late Almost flying - at last En route Riga Northern Norway in the light of the midnight sun 800 2162 25% datum 1613 928 10,7° 234 3160 343 644 586 498 1000 Mole "MITE' G.A. 25% datum 4390 Vs1 VG VF VA VC Vne VD Knots 43 62 66 89 90 113 126 Kph 80 115 121 165 167 210 233 (MC-30) (81) (114) (169) (180) (225) (250) Limit flight envelope at mauw 4.50 4.00 3.50 3.00 2.50 2.00 1.50 n 1.00 0.50 0.00 -0.50 -1.00 -1.50 -2.00 -2.50 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Knots MITE Flight envelope, design speeds Francis Donaldson in G-LUCL with builder Richard Teversen looking on e-Go prototype G-EFUN late 2013 W lb mauw climbing at R ft/min requires W*R/33,000 hp MC-30 Jodel Luciole D18 Climb rate ft/min 800 770 Mauw Kg 200 499 Power required hp 11 26 Engine shp hp 22 85 Propeller ɳ .8 .8 Power available hp 18 68 Power for climb % available 60% 40% Power for level flight % available 40% 60% Two main conclusions • A decent climb rate on low power depends even more than usual upon containing the level-flight power requirement. • An engine problem that allows continued running at say half the power gives a Luciole pilot less of an issue to deal with ! Containing the level flight power required at climbing airspeed A low-power aircraft benefits disproportionately from • A flapped wing that permits a smaller wing area and lower wing profile drag when clean. • A low extra-to-wing drag (no open cockpits, draggy struts are to be avoided, undercarriage must be well faired etc). • A long span to contain the power absorbed by wing induced drag 2 Pind= [W/b] * K/ (550 * ½ ρ π V) MC-30 MITE Luciole Weight W lb 441 463 lb Span b ft 22.6 20.74 K K 1.08 1.08 Speed V fps 90 90 Pind 2.2 2.9 Rate of climb ft/min 800 ? 800-0.7*33,000/463 = 750 Aspect ratio 10.3 7.9 Structural elements of the wing • The narrow box spar reacts normal loads only Flanges are sized by beam theory for local BM. The web is sized for the local Shear Force. Web to flange overlap determines bond stress. • Wing skin reacts chordal loads & torque around the s.c. Acquires direct stress from spar flange flexure. • Ribs maintain the wing section. Central ribs react crew inertial load on seat & walkway • The TE False Spar has negligible bending strength Carries the control surface hinge loads. Carries shear stress, as it’s part of the torsion cell. • Centre Section Rear Spar reacts seat & walkway loads. Flanges of the narrow box spar • Flanges are laminated from 2 lengths of pultruded carbon rectangular strip, 20mm wide and 8mm deep • These bought-in pultrusions are produced under controlled conditions. • Randomly drawn coupons are tested for consistency of material properties as part of the acceptance procedure for material stock. Testing coupons of pultruded carbon for the spar flange Advantages of Carbon pultrusions • Pultruded fibres are much straighter and more densely packed than a hand lay-up and thus have superior physical properties. • In particular, the properties are consistent within a manufacturing batch (if of aeronautical quality!) • They can be tapered, laminated and bent rather like wooden members. Simple restraining jig when tapering pultrusions Mechanical grinding completed Laminated carbon flanges • Each flange consists of 2 pultrusions laminated together. • Top and bottom flanges are symmetrical. • The flanges are laminated while bent around a centre-section former. • The width of the former is the same as the fuselage and subtends an angle of 9°. • The spar emerges from the fuselage sides at 4.5° dihedral. The spar at the centre section 4,5° Beech 600*5.5*20 profiled as shown 1.5mm GL1 central lower skin Laminating the top flange inside the spar mould Protection from surface damage • It is essential to avoid surface damage. • Barely Visible Surface Damage (BVSD) can be avoided by encasing pultrusion within a ply shell. • Internal ply & Beech laminates protect the inner pultrusion face from fretting damage by soldiers. • A ply laminate bonded to the external face can be chamfered, if necessary, to match the section. • These additional materials erode the comparative advantage over Spruce only a little. • The encased MITE spar 24mm wide and 6.32m long weighs 6.4 kg The spar at the centre section 4,5° Beech 600*5.5*20 profiled as shown 1.5mm GL1 central lower skin Pultruded carbon Vs Spruce I • VLA ACJ 572(b) permits stresses in carbon up to 40 daN/mm2 or some 58,015 psi Fcp= 3,530 psi for Spruce, some 16 times lower. • Because Fcu= 4,700 psi for Spruce, limit load stress cannot exceed 3,133 psi. The comparison of allowable stress levels increases in favour of carbon to 58,015/3,133= 18 • Composite structures are subject to a composite factor of 1.25 provided the coupon tests demonstrate adequate consistency . • The comparison in allowable compression stress in favour of carbon becomes 18.5/1.25= 15 Pultruded carbon Vs Spruce II • A Spruce compression flange is about 15% of the spar depth and the ‘Form Factor’ is approximately FFu= 0.5+0.5*0.15= 0.58 • FFu*Fbu= 0.58*9,400= 5,452 psi. Comparison in allowable limit stress falls from 14.8 to 14.8*4,700/5,452= 13 • Density of carbon pultrusions is 1.5 g/cm3 some 3.5 times more than Spruce. • Specific stress advantage is 12.8/3.5= 3.6 • Carbon pultrusions make a very much more compact spar flange at about 30% of the weight. Ready to close up the spar Encased spar released from the mould Two last considerations • Bending 8mm deep pultrusions before laminating them together to form the 16mm flange at the centre-section ‘locks-in’ their local bending stress. • This reduces the remaining allowable stress. • Reducing the cross-sectional area of the flange to match the local BM may not be feasible. • Flange area decreases more or less according to a square law (rectangular wing). • But the Normal Shear Force will decrease linearly to a first approximation. • The depth of the MITE flange that preserves acceptable bond stresses to the web becomes the design criterion around mid-span. The spar test rig The proof of the pudding • The spar weighs about 6.4 kg. • It supports 722 kg on each wing distributed over 3.16m according to the Schrenk distribution. • The 1,422 kg total is almost one and a half metric tons (3,183 lb). • Load/spar wt ratio = 1,422/6.4= 220 • Photo shows case D4.2 at n= 1.25*1.5*4.2= 7.875g • Repeated loading gives identical deflections. • All the elements of structure remain within the elastic region even at fully factored ultimate load. The n= 0 ‘torsion with drag’ test A4.2 at ultimate combined loads Building the flight wing 3.1m*1.26m*1.2mm ply with 34mm id Celebrating the starboard skin safely applied Plain wing at the stall Angular stall margin, 802.5mm constant chord,b=6.32m 16 15 14 13 12 11 10 9 8 degrees 7 6 5 4 3 2 1 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 wing stn clean stall 1.5*Vstall Section & Wing lift curves Constant c=802.5,b=6.32,43014.5 at R=1 mil,alpha0=-2.3 Section CLmax=1.532 at alpha=14.25, a=0.100 Wing CLmax=1.400 at alpha=16.8, K=1.097, a=0.077 1.6 1.16 1.5 1.4 1.14 1.3 1.2 1.12 1.1 1.0 1.10 0.9 0.8 1.08 K 0.7 0.6 1.06 0.5 0.4 1.04 0.3 0.2 1.02 0.1 0.0 1.00 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Section CL Wing CL, 0 degs washout Induced drag K Flapped section & wing lift curves Rectangular wing b=6.32,c=802.5mm, 43014.5, Re 1*10^6 Flap section clmax=2.485 @ 10.4 degs, a=.083,alpha0=-15.8 Flapped wing Clmax=2.000 @ 15.3 degs, a=.072,alpha0=-12.1 2.5 1.25 2.4 1.24 2.3 1.23 2.2 1.22 2.1 1.21 2.0 1.20 1.9 1.19 1.8 1.18 1.7 1.17 1.6 1.16 1.5 1.15 1.4 1.14 CL 1.3 1.13 K 1.2 1.12 1.1 1.11 1.0 1.10 0.9 1.09 0.8 1.08 0.7 1.07 0.6 1.06 0.5 1.05 0.4 1.04 0.3 1.03 0.2 1.02 0.1 1.01 0.0 1.00 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 alpha Section cl Wing CL clean section cl Induced drag K Flapped wing at the stall Angular stall margin with 25% flaps, c=802.5mm, b=6.32m 20 19 18 17 16 15 14 13 12 11 degs 10 9 8 7 6 5 4 3 2 1 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 flapped stall at 1.5*V flapped stall Reality checks before design • What it means to build an aeroplane • Design for production • Design for maintenance • Fuel costs • Engine costs • Hangarage costs • Qualities that can make light aircraft fun and safe: Good low speed handling Better than 500 ft/min climb Good cockpit environment Excellent field of view Simple fuel systems Design for survivability .
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