# 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

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.