Iteration 5: FPV Weight#

from unyt import km, m, mm, inch, g, kg, hr, minute, s, degree, radian, volt
import numpy as np
from matplotlib import pyplot as plt
from math import pi as π
%matplotlib inline
rpm = minute**-1
ρ = 1.225*kg/m**3 # air density
μinf = 1.81e-5*kg/(m*s) # viscosity of air

Goals:

  • Use more accurate weight estimate with FPV system

Initial Design#

b = 3050*mm # wingspan
c = 230*mm # chord
S = b*c # wing area
AR = b**2/S
AR
unyt_quantity(13.26086957, '(dimensionless)')

Weight Estimate#

Weight estimate decreased from 3047 to 3000 (included real FPV system weight, more realistic telem weight, increased payload from 500g to 700g).

W = 3000*g # weight

Part

Volume

Fraction solid

Weight

Area

Count

Weight total

Wing

6.42188e+06

0.3

257.793

2

515.585

Fuselage

54465.1

1

7.08046

1

7.08046

Tail boom

353429

1

197.92

1

197.92

Wing spar

120000

1

63.4921

2

126.984

Horizontal stabilizer

140000

1

15.08

1

15.08

Vertical stabilizer

37500

1

4.03929

1

4.03929

Stabilizer mount

51465.8

1

6.69056

1

6.69056

Battery

604.8

1

604.8

ESC

70

1

70

Motor

300

1

300

Prop

87

1

87

Pixhawk

38

1

38

GPS

30

1

30

Telem

40

1

40

Servo

35

4

140

GoPro

74

FPV OSD

3

1

3

FPV VTX

6

1

6

Power distribution

30

Payload

700

Total

2996.18

Airfoil Selection#

V = 30*km/hr # cruise speed
CL = (2*W)/(ρ*V**2*S)
print(f"CL: {CL.to_value('s**2/m')}")
CL: 0.1005425691302893
(V**2)
unyt_quantity(900, 'km**2/hr**2')
Re = (ρ*V*c)/μinf
round(Re.to_value(), -3)
130000.0
α0 = -3.75*degree
Clα = 1.05
e = 0.8
CLα = Clα/(1+(Clα/(π*e*AR)))
CL_unitless = CL.to_value('s**2/m')
α = α0+((CL_unitless/CLα)*radian)
α
unyt_quantity(1.90919406, 'degree')
CLmax = 1.4*s**2/m
Vstall = np.sqrt((2*W)/(ρ*S*CLmax))
print(f"Cruise speed: {V.to('m/s'):.2f}, Stall speed: {Vstall:.2f}")
Cruise speed: 8.33 m/s, Stall speed: 2.23 m/s

Wing Loading and Thrust to Weight Ratio#

WCL = W/(S**(3/2))
print(f"Wing loading: {WCL.to('kg/m**3'):.2f}")
Wing loading: 5.11 kg/m**3
TtoW = 1.1
T = TtoW*W
print(f"Required thrust: {T.to('kg'):.2f}")
Required thrust: 3.30 kg

Propulsion System#

The propulsion system selection is identical to iteration 4 (16x10/500Kv/6S), because the weight has only been slightly adjusted.

FPV System#

I’ll be using Holybro’s Micro OSD V2 + Atlatl HV micro. The former uses 5V power from the telemetry port and the latter can run off of the 6S pack.

Hobbyking currently has an excellent offer (€26.09) on the Quanum Cyclops Diversity DVR FPV Goggle, which as the name suggests are a single-screen model with build-in DVR. They are 5.8GHz 40CH, like the Atlatl HV micro VTX.

Servos#

By using Radio Control Info’s Servo Torque Calculator I calculated that I need servos with at least 5kg of torque for the ailerons.

I decided to go with four of the HXT12K Metal Gear Servo 25T 11kg / 0.16sec / 55g (two in the wing for the aelerons and two in the fuselage for the elevator + rudder). They produce ample torque for all control surfaces.

I did consider the Turnigy TGY-813 Slim Wing DS/MG Servo 25T 9.0kg / 0.09sec / 30g, which are quite a bit lighter and of course more low profile. However, the HXT12K’s fit in the wing anyway, and my wrong weight estimate of 140g vs the actual 220g will not make that big of a difference. There were not that many other options outside of these two categories, at least not with 3+kg torque.

Radio#

I went with the incredibly simple Turnigy TGY-i4X Mode 2 AFHDS/AFHDS 2A Switchable 4CH Transmitter/Receiver . It has no special qualifications except for having 4 channels and being the cheapest option. On the receiving end I’ll be using the FS-A8S 2.4Ghz 8CH Mini Receiver with PPM i-BUS SBUS Output for SBus output.

I was considering using a completely digital system (using the 2-way telemetry connection for control) as I do plan on flying autonomously for the most part, but decided against it for safety/redundancy reasons.

BOM#

Also available as iteration_5-bom-and-receipt.

Part

Name

Store

Link

Price pp

#

Price total

Flight controller

Pixhawk 2.4.8

second hand

https://www.2dehands.be/a/hobby-en-vrije-tijd/modelbouw-radiografisch-helikopters-en-quadcopters/m1658504570-pixhawk-2-4-8-px4-32bit-apm-flight-controller-with-power-mod.html?previousPage=lr

40

1

40

Servo

HXT12K Metal Gear Servo 25T 11kg / 0.16sec / 55g

HobbyKing

https://hobbyking.com/en_us/hxt-10kg-servo-metal-gear-10kg-0-16sec-55g.html

8.88

4

35.52

Control horns

Control Horns 16x20mm (10pcs)

HobbyKing

https://hobbyking.com/en_us/control-horns-16x20mm-10pcs.html

1.22

1

1.22

Linkage holders

Nylon Arm 2.8 x 6 x 6mm (10pcs)

HobbyKing

https://hobbyking.com/en_us/nylon-arm-2-8-x-6-x-6mm-10pcs.html

0.48

1

0.48

Motor

PROPDRIVE v2 4258 500KV Brushless Outrunner Motor

HobbyKing

https://hobbyking.com/en_us/propdrive-v2-4258-500kv-brushless-outrunner-motor.html

46.1

1

46.1

Prop

TGS Precision Sport Propeller 17x10

HobbyKing

https://hobbyking.com/en_us/tgs-precision-sport-propeller-17x10-1pc.html

6.14

3

18.42

ESC

YEP 80A (2~6S) SBEC

HobbyKing

https://hobbyking.com/en_us/yep-80a-2-6s-sbec-brushless-speed-controller.html

43.49

1

43.49

ESC programmer

YEP ESC Programming Card

HobbyKing

https://hobbyking.com/en_us/yep-esc-programming-card.html

5.92

1

5.92

Battery

Turnigy nano-tech 4000mAh 6S 35~70C

HobbyKing

https://hobbyking.com/en_us/turnigy-nano-tech-4000mah-6s-35-70c-lipo-pack-w-xt-90.html

63.39

1

63.39

Radio transmitter

Turnigy TGY-i4X Mode 2 AFHDS/AFHDS 2A Switchable 4CH Transmitter/Receiver

HobbyKing

https://hobbyking.com/en_us/turnigy-tgy-i4x-mode-2-afhds-afhds-2a-switchable-4ch-transmitter-receiver.html

31.49

1

31.49

Radio receiver

FS-A8S 2.4Ghz 8CH Mini Receiver with PPM i-BUS SBUS Output

HobbyKing

https://hobbyking.com/en_us/fs-a8s-2-4g-8ch-mini-receiver-with-ppm-i-bus-sbus-output.html

7.59

1

7.59

FPV goggles

Quanum Cyclops Diversity DVR FPV Goggle

HobbyKing

https://hobbyking.com/en_us/quanum-cyclops-diversity-fpv-goggle.html

26.09

1

26.09

FPV Cam

CADDX Baby Ratel Starlight 1.8mm 1200TVL 14x14mm 4.6g FPV Camera w/160° FOV (Yellow)

HobbyKing

https://hobbyking.com/en_us/caddx-baby-ratel-starlight-1-8mm-1200tvl-14x14mm-4-6g-fpv-camera-w-160-fov-yellow.html

25.51

1

25.51

FPV OSD

Micro OSD V2

Holybro

https://shop.holybro.com/micro-osd-v2_p1207.html

13.17

1

13.17

FPV VTX

Atlatl HV micro

Holybro

https://shop.holybro.com/atlatl-hv-micro_p1186.html

17.29

1

17.29

Telemetry

Transceiver Telemetry Radio V3

Holybro

https://shop.holybro.com/transceiver-telemetry-radio-v3_p1103.html

37.05

1

37.05

GPS

Pixhawk4 2nd GPS Module

Holybro

https://shop.holybro.com/pixhawk4-2nd-gps-module_p1145.html

34.58

1

34.58

Charger

Turnigy P606 LiPoly/LiFe AC/DC Charger (EU Plug)

HobbyKing

https://hobbyking.com/en_us/pd606-charger-eu-plug.html

43.15

1

43.15

Total

490.46

Receipt#

Name

Link

Price w/o fees

Paid

Fees (shipping/conversion/handling/service…)

Second hand: Flight controller

https://www.2dehands.be/a/hobby-en-vrije-tijd/modelbouw-radiografisch-helikopters-en-quadcopters/m1658504570-pixhawk-2-4-8-px4-32bit-apm-flight-controller-with-power-mod.html?previousPage=lr

40

40

0

HobbyKing

https://hobbyking.com/

348.37

397.59

49.22

Holybro

https://shop.holybro.com/

102.09

147.84

45.75

Total

585.43

94.97