Item 1224

DESIGN: Dragonfly ~ Rotor - Blade - 0012 - Leading Edge Weight Calculation

Drawing:

The 25% of chord is at the start of zone #6.

Moment of Solid Spruce Airfoil About 25% of Chord:

 

Zone:

Area: *

Moment Arm:

Area x Arm:

 

1

.3

-1.125"

-0.3375 

 

2

1.4

-0.875"

-1.225

 

3

1.9

-0.625"

-1.1875

 

4

2.1

-0.375"

-0.7875

 

5

2.2

-0.125"

-0.275

 

6

2.2

0.125"

0.275

 

7

2.2

0.375"

0.825

 

8

2.2

0.625"

1.375

 

9

2.1

0.875"

1.8375

 

10

2.0

1.125"

2.25

 

11

1.9

1.375"

2.6125

 

12

1.8

1.625"

2.925

 

13

1.6

1.875"

3.0

 

14

1.4

2.125"

2.975

 

15

1.2

2.375"

2.85

 

16

1.0

2.625"

2.625

 

17

.8

2.875"

2.3

 

18

.5

3.125"

1.5625

 

19

.2

3.375"

0.675

 

20

.1

3.625"

0.3625

 

Total:

29.1 *

  

24.6375

 

  

1.8188 in2

 

* Area is quantity of 1/4" squares. This includes the skin.

Assuming that the weight of dried spruce is 28 lb/ft3 (see; DESIGN: Dragonfly ~ Rotor - Blade - 0012 - Wood - 4.75" - Core) then the moment per running foot of span is 28 / (48 x 48) = 0.01215 x 24.6375 = 0.2994 lbf-in

Weight of Materials for Leading Edge: (from http://www.simetric.co.uk/si_metals.htm)
 

Iron

6800 - 7800 kg/m3

2.9484 - 3.3816 lb/(in2 x ft)
 

Brass

8200 kg/m3

3.5544 lb/(in2 x ft)
 

Lead

11340 kg/m3

4.9164 lb/(in2 x ft)
 

Tungsten

19600 kg/m3

8.4972 lb/(in2 x ft)

 

Tungsten Calculations:

If the weight is circular and centered on the start of zone 2 line then the moment arm is 1.0".

Tungsten; 0.2994 / 8.4972 = 0.0352 in2 = 0.212" diameter. This will just fit, (see drawing) with a 0.035 clearance for fiberglass skin. The fiberglass glass skin will hold it in place. This skin will increase the positive moment unless it stop at 50% of chord.

The tungsten rod will have a 0.0353 in2 cross section and will weigh 0.0353 x 8.4972 = 0.30 lb/ft. for a total weight of (78"/12") x 0.30 = 1.95 lbs.

Lead Calculations:

If the weight is circular and centered on the start of zone 2 line then the moment arm is 0.75".

Lead = 0.34" diameter. This will just fit, (see drawing) with a 0.039 clearance for fiberglass skin. The fiberglass glass skin will hold it in place. This skin will increase the positive moment unless it stop at 50% of chord.

The lead rod will have a 0.0939 in2 cross section and will weigh 0.0939 x 4.9164 = 0.466 lb/ft. for a total weight of (78"/12") x 0.466 = 3.03 lbs.

Note: If the lead is shaped to suit the leading edge the moment will be longer and the weight can be less. Probably about 2.25 lbs. This looks like the simplest way to go.

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Last Revised: December 5, 2003