Analysis and Full Scale Testing of

the Joby S4 Propulsion System

1 Alex Stoll / 3 August 2015

2

3

S4 vs. R44

4

S4 R44

Disk loading 9.4 lb/ft2 2.9 lb/ft2

Rotor diameter 9.5 ft each 33 ft

Max gross weight 4,000 lb 2,500 lb

Max payload 840 lb 828 lb

Nominal hover performance (estimated)

Shaft power 560 hp 245 hp

Power loading 7.1 lb/hp 10 lb/hp

Rotor tip speed 370 ft/s 705 ft/s

Cruise performance (estimated)

Cruise speed 200 mph 135 mph

Shaft power 113 hp 190 hp

Max range at cruise speed 200 mi 300 mi

Range at max payload 181 mi 175 mi

5

$0

$1

$2

$3

$4

$5

$6

$7

$8

$9

30 50 70 90 110 130 150 170 190

Fare

pri

ce p

er

mile

fo

r a 2

00-m

i trip

Block speed (mph)

Robinson R44

Joby S4

Uber X (LA)

Uber X (NY)

Motor-out trim

6

0% 150%

0% 150%

150% 150%

7

1

1.1

1.2

1.3

1.4

1.5

6 7 8 9 10 11 12

Mo

tor-

out

trim

thru

st incr

ease

Number of propellers

8

9

𝑏 ≥ 2𝑅𝑁

𝐴𝑅 ≥2𝑅𝑁 2

𝑆

𝐴𝑅 ≥ 𝑁24𝐴

𝜋𝑆

𝐴𝑅 ≥4

𝜋𝑁2

𝑊/𝑆

𝑇/𝐴

10

Radius

(disk loading)

Power

Tip speed

Solidity

Nacelle weight

and drag

Battery weight

Max torque

Controller weight

Motor weight

Noise

Bird strike

Downwash

11

12 2,850 N (640 lb) thrust at 15 m (50 ft)

“Noise Detectability Prediction Method for Low Tip Speed Propellers,” Barry, F.W., and Magliozzi, B., 1971

Theory

Experiment

13

Hover

3,100 N (700 lb)

Transition

45 m/s (87 kt)

1,000 N (230 lb)

14

Radius

Solidity

Hover CL

Inner twist

Outer twist

Blade

element

code

Transition RPM

Hover power

Transition power

Airfoils

15

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0.5

1

r/R

c/R/

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

-10

0

10

20

30

r/R

Tw

ist

(deg

rees)

Constrained to

facilitate folding

Inner twist Outer twist

16

17