Chapter 15
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.

Table 15.1 Typical weight and LCoG control table (LCoG is mm forward of the main spar)

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 31 mm OD	2	CFRP	306	612	0	0
	Tail booms CG21.8/19.0 950 mm 22 mm OD	2	CFRP	147	294
	Rudder posts and hinge pins CG10.0/08.0 490 mm 10 mm OD	4	CFRP	26	104
	Aileron hinge pins 843 mm 7.5 mm OD	2	CFRP	20	40
	Elevator hinge pin CG16.7/14.0 1 070 mm 16 mm OD	1	CFRP	102	102
	Main threaded rods and nuts	3	steel	55	165	177.5
Foams	Main wings	2	Foam	261	574
	Ailerons	2	Foam	34	75
	Inner wings	2	Foam	20	44
	Rudder fins	2	Foam	100	220
	Rudder flaps	2	Foam	22	48
	Elevators	2	Foam	135	297
SLS	360 mm main fuselage with wing supports	1	Nylon	795	795
Nylon	250 mm fuselage with hatch	1	Nylon	454	454	496	225 397
	Conical rear fuselage	1	Nylon	235	235
	Front lower fuselage	1	Nylon	275	275	672
	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
	Port wing tip	1	Nylon	120	120
	Stbd duct	1	Nylon	1 020	1 020
	Stbd wing tip	1	Nylon	120	120
	Port tail connector	1	Nylon	160	160
	Port outer elevator end	1	Nylon	38	38
	Port inner elevator end	1	Nylon	40	40
	Port rudder cap	1	Nylon	38	38
	Stbd tail connector	1	Nylon	160	160
	Stbd outer elevator end	1	Nylon	38	38
	Stbd inner elevator end	1	Nylon	40	40
	Stbd rudder cap	1	Nylon	38	38
	Servo mounting plates (wing)	2	Nylon	14	28
	Servo mounting covers (wing)	2	Nylon	11	22
	Servo mounting plates (rudder)	2	Nylon	9	18
	Servo mounting covers (rudder)	2	Nylon	11	22
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 HBL380 X8	2		79	158	1 125	177 750
	Large control horns incl. screws	4		8	32	592.5	18 960
	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 incl. collets, springs, and cap screws	1	steel	30	30	642.5	19 062
	Steering arm bore 6 swg/5.0 mm plus springs	2		3	6	642.5	3 855
	Main suspension, wheels, and axles	1			0		0
Totals					13 572	13	170 791

15.1 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 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.

Figure 15.1 Channel wing aircraft being weighed after final assembly.

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.

15.2 Longitudinal Center of Gravity Control

Table 15.1 also shows the longitudinal center of gravity (LCoG) computation. To do this, the LCoG values of all the SLS nylon and foam parts are calculated by the CAD Program, while those for the bought-in components are established by their locations in the design drawings, assuming that the individual centers of gravity lie at the center of each component. The analysis shows that the estimated LCoG is slightly forward of the centerline of the main spar (which lies at the quarter chord point). This is, of course, for an aircraft without gasoline in the main tank, which itself lies forward of the quarter chord point, so the aircraft will have positive trim stability even when all fuel is used.

Should the LCoG not be as required at this stage, consideration must be given to changes to the overall geometry of the airframe or the positioning of heavy internal components like the batteries. When designing with SLS nylon fuselage elements, we find it a relatively simple matter to adjust the LCoG by simply changing the lengths of one or more fuselage elements; then provided there are some relatively heavy elements in the nose of the aircraft, the LCoG can be adjusted as required. In the case of the aircraft tabulated, the batteries and main gasoline engine all lie well forward, so rather small adjustments in fuselage length gave good control of the LCoG without the need for major redesign, though extending the fuselage does, of course, increase the weight.