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Home arrow Engineering arrow Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
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Weight and Center of Gravity Control

Once the preliminary geometry has been defined in 3D CAD and a set of suitable analyses has been carried out, the total airframe weight and the position of the longitudinal center of gravity (LCoG) should be reassessed. These estimates will be made from CAD-based mass and centroid predictions plus the known weights and locations of the various bought-in components that will be used. It is standard practice at this stage to maintain these estimates in tabular form, usually as a spreadsheet. Control of the LCoG is, of course, vital to establish pitch stability of the aircraft. We always weigh our aircraft after building and before the first flight to establish the final maximum take-off weight (MTOW) and LCoG. We typically do this with sets of calibrated scales placed under the wheels. It is important when calculating LCoG values that the aircraft is horizontal and the contact points with the scales are in known locations.

Weight Control

Given that an adequate structural definition has been established, it should be possible to estimate the weight of the aircraft with a good deal of precision; we typically work to the nearest gram. To do this, we try and avoid relying on manufacturer-stated weights for components; rather we prefer to weigh all the parts we intend to use in-house and add these to our weights build-up. If such weights are not available, some form of scaling will have to be used, see Tables 11.4-11.6. Table 15.1 shows a typical weight analysis for one of our aircraft, in this case the ducted wing unmanned air vehicle (UAV) already seen in Figure 4.22. Figure 15.1 shows this aircraft being weighed after final assembly. The component weights are all established by weighing the items to be fitted to the aircraft, while those of the selective laser sintering (SLS) and foam parts are taken from the CAD definition using a relative density of 0.95, which is based on weighing previously manufactured SLS nylon parts. Note that as the design progresses, further detailing of the CAD models for the SLS parts will rapidly intensify. This will, of course, modify the weights of these parts, but if a simple constant wall thickness, of say 2 mm, has been assumed for the initial structural model, the shift to internal stiffening of

Small Unmanned Fixed-wing Aircraft Design: A Practical Approach, First Edition. Andrew J. Keane, Andras Sobester and James P. Scanlan.

©2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.

Category

Part

No. off

Material

Weight each (g)

Total

weight (g)

LCoG

(mm)

Moment (g mm)

Spars

Main spar CG31.3/28.5 1 400 mm X 31 mm OD

2

CFRP

306

612

0

0

Tail booms CG21.8/19.0 950 mm X 22 mm OD

2

CFRP

147

294

-536.67

-157 781

Rudder posts and hinge pins CG10.0/08.0 490 mm X 10 mm OD

4

CFRP

26

104

-1 157.5

-120 380

Aileron hinge pins 843 mm X 7.5 mm OD

2

CFRP

20

40

-196.88

-7 875

Elevator hinge pin CGI 6.7/14.0 1 070 mm X 16 mm OD

1

CFRP

102

102

-1 070

-109 140

Main threaded rods and nuts

3

steel

55

165

177.5

29 288

Foams

Main wings

2

Foam

261

574

-42.7

-24 518

Ailerons

2

Foam

34

75

-217.04

-16 235

Inner wings

2

Foam

20

44

-61.5

-2 706

Rudder fins

2

Foam

100

220

-1 124.61

-247 414

Rudder flaps

2

Foam

22

48

-1 124.61

-54 431

Elevators

2

Foam

135

297

-1 125.67

-334 324

SLS

360 mm main fuselage with wing supports

1

Nylon

795

795

-83

-66 176

Nylon

250 mm fuselage with hatch

1

Nylon

454

454

496

225 397

Conical real' fuselage

1

Nylon

235

235

-326

-76 598

Front lower fuselage

1

Nylon

275

275

672

184 704

Engine cowling

1

Nylon

34

34

737

25 045

140 mm Fuselage section

2

Nylon

264

528

231

121 957

Port duct

1

Nylon

1 020

1 020

-80.86

-82 477

Port wing tip

1

Nylon

120

120

-95.94

-11 513

Stbd duct

1

Nylon

1 020

1 020

-80.86

-82 477

Stbd wing tip

1

Nylon

120

120

-95.94

-11 513

Port tail connector

1

Nylon

160

160

-1 131.76

-181 082

Port outer elevator end

1

Nylon

38

38

-1 123.24

-42 683

Port inner elevator end

1

Nylon

40

40

-1 199.01

-47 960

Port rudder cap

1

Nylon

38

38

-1 127.49

-42 845

Stbd tail connector

1

Nylon

160

160

-1 131.76

-181 082

Stbd outer elevator end

1

Nylon

38

38

-1 123.24

-42 683

Stbd inner elevator end

1

Nylon

40

40

-1 199.01

-47 960

Stbd rudder cap

1

Nylon

38

38

-1 127.49

-42 845

Servo mounting plates (wing)

2

Nylon

14

28

-60

-1 680

Servo mounting covers (wing)

2

Nylon

11

22

-60

-1 320

Servo mounting plates (rudder)

2

Nylon

9

18

-1 125

-20 250

Servo mounting covers (rudder)

2

Nylon

11

22

-1 125

-24 750

Servos

Ailerons Futaba S3470SV

2

58

116

-60

-6 960

Rudders Futaba S3470SV

2

58

116

-1 125

-130 500

Engine Futaba S3470SV

1

58

58

642.5

37 265

Nose wheel Savox SC-1268

1

67

67

642.5

43 048

Elevators MKS HBF380 X8

2

79

158

-1 125

-177 750

Farge control horns inch screws

4

8

32

-592.5

-18 960

(continued)

Category

Part

No. off

Material

Weight each (g)

Total

weight (g)

LCoG

(mm)

Moment (g mm)

Large control horns support pads

4

1

4

-592.5

-2 370

Engine and motors

OS GF30 plus exhaust, ignition, propeller, spinner & fuel line

1

1 517

1 517

752.5

1 141 543

DuBro 8 oz. fuel tank plus stopper, breather & filler lines and clunk

1

110

110

192.5

21 175

Stainless SLS engine mount

1

Steel

130

130

697

90 667

Hacker A50-12S V3 motors plus mounting nuts

2

345

690

-200.53

-138 366

Two Jeti Advance 70 Pro SB speed controllers plus main harness

1

359

359

160

57 440

Master Airscrew propellers E-MA1470T 14x7 three-bladed (tractor)

1

76

76

-240.53

-18 280

Master Airscrew propellers E-MA1470TP 14x7 three-bladed (pusher)

1

76

76

-240.53

-18 280

Avionics

Futaba R6014 HS Receiver + ribon cable + two leads

1

35

35

282.5

9 888

SkyCircuits SC2 autopilot with GPS and 2.4 GHz aerial and lead

1

414

414

407.5

168 705

Pitot tube and connecting hose

1

brass

40

40

0

0

Overlander LiPo FP30 6S 22.2V 5 000 mAh 30C main motor battery

1

704

704

582.5

410 080

Spektrum LiFe 2S 6.6V 4 000 mAh avionics battery

1

243

243

524.5

127 454

Nano-Tech LiFe 30C 2 100 mAh 2S avionics battery

1

108

108

524.5

56 646

Double pole single throw 10 A switch plus local wiring harness

1

70

70

407.5

28 525

LED voltage indicator strips

2

4

8

500

4 000

2-6S LED balance voltage indicator

1

4

4

500

2 000

Baseboard (main)

1

Plywood

36

36

496

17 714

Baseboard (receiver)

1

Plywood

19

19

300

5 700

Baseboard (tank)

1

Plywood

19

19

160

3 040

Baseboard (speed control)

2

Plywood

19

38

0

0

Wiring in wings and tail booms

2

150

300

-400

-120 000

Servo linkages

6

7

42

0

0

Misc. cable ties and screws

1

50

50

0

0

Under-carriage

Nose wheel and leg

1

79

79

642.5

50 758

Nose wheel upper steering column inch collets, springs, and cap screws

1

steel

30

30

642.5

19 062

Steering arm bore 6 swg/5.0mm plus springs

2

3

6

642.5

3 855

Main suspension, wheels, and axles

1

0

0

Totals

13 572

13

170 791

Channel wing aircraft being weighed after final assembly

Figure 15.1 Channel wing aircraft being weighed after final assembly.

a thinner structure will reduce the weight of the SLS parts while leaving the centers of mass broadly unchanged. Then the impact of structural detailing will generally not adversely impact on either the overall aircraft weight or its LCoG position.

If at this stage the aircraft is significantly too heavy, some form of weight control exercise can be entered into (in our experience it is very rare for an aircraft ever to be too light). This can be very difficult to achieve, but typical measures could be as follows:

  • • Reduction in structural element sizes (typically by making elements thinner, especially where finite element analysis (FEA) reveals that stresses are low), although it is difficult to make substantial savings in this way without considerable design effort;
  • • Reducing battery sizes;
  • • Lightening wing covers, either by reducing foam thicknesses or by adopting less robust claddings;
  • • reducing the size of servos by accepting lower torque or by using a higher power-to-weight ratio and probably much more expensive items;
  • • fitting smaller diameter wheels.

Hopefully, the weight budget will not have been too greatly exceeded, but it is in the nature of all vehicle designs to increase in weight during design as the final build is approached, largely because extra items keep getting added to the build specification, either because they were simply not allowed for at the start or because higher specification items are selected or mission creep has set in. For this reason, it can be wise to add a design contingency at the outset of 5%, but this can, of course, become a self-fulfilling prophecy of weight growth.

 
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