mole mite wing design and construction5) richard mole... · a lifetime’s interest •ppl from raf...

Mole MITE Wing Design and construction

23

4

2 5%da t um

Mole "MITE' G.A.

2 5% da t um

10

,7°

92

8

2 16 2

10

00

4 9 8

316

08 00

16

13

64

4

34

3

4 39 0

58

6

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

23

4

2 5%da t um

Mole "MITE' G.A.

2 5% da t um

10

,7°

92

8

2 16 2

10

00

4 9 8

316

0

8 00

16

13

64

4

34

3

4 39 0

58

6

MITE Flight envelope, design speeds

Vs1 VG VF VA VC Vne VD

Knots 43 62 66 89 90 113 126

Kph

(MC-30)

80

(81)

115 121

(114)

165

(169)

167

(180)

210

(225)

233

(250)

Limit flight envelope at mauw

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Knots

n

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

Luciole

Jodel

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

Pind= [W/b]2* K/ (550 * ½ ρ π V)

MC-30

Luciole

MITE

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

1.5mm GL1 central lower skin

Beech 600*5.5*20 profiled as shown

4,5

°

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

1.5mm GL1 central lower skin

Beech 600*5.5*20 profiled as shown

4,5

°

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