OwenDuffy.net 


SK450 quadcopter (10/11/2013 build)

*** DRAFT ***

Introduction

This article is a collection of build notes for my SK450 quadcopter project.

Key elements of the configuration are:

The mass of the fully populated quad is ~1100g.

Frame

The size is 437mm diagonal between motor shafts (not 450mm as the type suggests) and it can swing props up to about 11", though gear in the middle may limit propellers to something less. This project uses 10" propellers and clearance to the battery is very tight.

Fig 1:

Fig 1 shows the built quadcopter.

The spaced between the lowest and middle deck is too small to accommodate a flight computer or battery... it is basically useless. The lower deck is too close to the ground to accommodate a battery, and in any event, slinging a battery under the frame has significant disadvantages.

The upper deck was raised with longer standoffs so that the flight computer would fit underneath, and the batter secured to the top. The battery is in the propeller plane, and great care must be take to prevent propeller strike. In this case, 10" propellers were used to provide additional clearance, but it is still very tight.

Fig 2:

 

Fig 2 zooms in on the motor mount. The black FRP disk is screwed to the underside of the motor, and then the disk is attached to the polyamide leg by three cap screws from the top. Once the black disk is fixed to the leg, the motor screws are basically inaccessible from under the frame, the disk and motor must be removed from the frame to tighten or remove the motor screws. The PCD of the three screws is 35mm, and allowing clearance for the head of the screws, will not accommodate a motor body more than about 30mm diameter (essentially 28xx series motors are the maximum). The whole thing seems rather flimsy, but it has not failed with a small number of flights however great care has been taken in balancing the propellers to minimise vibration. The hole in the centre of the motor black disk was 8mm diameter, and had to be ground out to 12mm to accommodate the bearing/shaft projection from the motors used.

So, for a nominal 450mm frame, it is quite restricted in propeller size and motor diameter.

The 3mm carbon rods at front and back stress the frame and twist it out of alignment.

The motors tilt inwards, as much as 4° on one motor, but at least 2.5°. 

When the spares on hand are exhausted, they will not be replaced, they were about twice the price of the frames used on the X450 quadcopter and not nearly as good. I strongly recommend against this frame.

Design review

Drive system

Fig 3:

 

Fig 3 shows the characteristics similar to the the HK 1045 SF propeller. The green dots are for an operating point of 950g thrust at WOT for each propeller, and the indicated motor power is 114W at is 7,006RPM.

Expected hover is at around 4,500RPM for around 360g thrust per propeller at about 4A current for each motor. Endurance should be about 10min on a 3000mAh battery.

Implementation

The following are notes on the construction. Some issues were noted during initial build, some a little later.

Fig 4:

Fig 4 shows the FC mounted with self adhesive foam (for vibration isolation) to a small square of acrylic which is again foam mounted to the frame.

Propellers

The quad uses nominally 10"x4.5" two bladed slow fly propellers. These are available from a range of sources, and initially the cheap ABS plastic ones have been used.

Propeller mounting

The motors included propeller mounts with collets to suit the 4mm plain shafts, and M6 threaded extension for the propeller. Propellers needed to be reamed out to M6 to suit the adapters.

Power distribution

Power distribution is a Hobbyking distribution board with XT60 connector for the battery and four way 3.5mm connectors for the ESCs. As received from Hobbyking, input connector had a dry joint and disconnected during flight leading to a crash that broke two frame legs.

MultiWii 328P Flight Controller w/FTDI & DSM2 Port

Features include:

The flight controller board came with no documentation, no schematic and inadequate information as to the settings in the configuration config.h to build the appropriate flash module.

Table 1: Receiver connections
Channel Use FC pin Tx Comment
1 Throttle 1 Thr  
2 Roll 2 Ail  
3 Pitch 3 Ele  
4 Yaw 4 Rud  
5 Aux1 5 THR  
6 Mode D12 3POS  
7 Discovery beeper NC AIL  
8 Spare NC GEA Vibration capture

Table 1 shows the receiver connections.

Table 2: ESC connections
FC pin Motor Comment
D9 Right rear  
D10 Right front  
D3 Left front  
D11 Left rear  

Table 2 shows the ESC connections.

Table 3: Bluetooth connections
Bluetooth pin FC pin Comment
Vcc AX3  
Gnd AX3  
Rx AX3  
Tx J1 middle pin  

Table 3 shows the Bluetooth connections.

diff.htm is a report of the differences between the distribution config.h and that used.

 Fig 5:
 

Fig 5 shows the voltage the MultiWiiConf configuration tool screen with the quad hovering but loosely held in the hand. The purpose of this test was to evaluate the residual vibration sensed by the board.

Receiver

Fig 6:

 

Figs 6 shows the FrSKY V8FR-II HV receiver receiver.

Bluetooth

An inexpensive Hextronix Bluetooth module was connected for field tuning purposes using a smartphone app. The module was configured for name Q01 (COM44 on W09), 57,600bps.

ESC

The Afro30 ESCs were upgraded to tgy (SimonK) bootloader + tgy afro_nfet_mw.hex firmware.

Current version: 1f75da384e5c83f83916aa752819ca6eed712565 afro_nfet_mw.hex 24/03/14

For this application:

.equ MOTOR_ADVANCE = 15 ; Degrees of timing advance (0 - 30, 30 meaning no delay)
.equ TIMING_OFFSET = 0 ; Timing offset in microseconds (max 4096Ás)
.equ COMP_PWM = 1 ; During PWM off, switch high side on (unsafe on some boards!)
/^\.equ RC_CALIBRATION/c\
.equ RC_CALIBRATION=0 ;disable stick calibration
/^\.equ STOP_RC_PULS/c\
.equ STOP_RC_PULS=1140 ;Stop motor at or below this pulse length
/^\.equ FULL_RC_PULS/c\
.equ FULL_RC_PULS=1850 ;Full speed for pulses longer than this

It delivers better drive system efficiency, and faster / finer resolution control.

Motor

The motors are Turnigy D2836-8 1100Kv brushless DC motor, specifications:

Battery: 2~4 Cell /7.4~14.8V
RPM: 1100kv
Max current: 18A
No load current: 1A
Max power: 336W
Internal resistance: 0.107 ohm
Weight: 70g (including connectors)
Diameter of shaft: 4mm
Dimensions: 28x36m
Prop size: 7.4V/11x7 14.85V/7x3
Max thrust: 1130g

Battery

Zippy 3S 3000mAh 20/30C.

Voltage detector

A Turnigy TGV-Detector was used to warn of low battery. Several of these were purchased, and they are fitted with a 2.54mm shunt to go on 2mm pitch header pins resulting in a loose and intermittent / unreliable connection. More Hobbyking quality!

The TGV-Detector was fitted with a 2mm shunt, and it performs reliably though at the first audible alarm it is vital to get the quad down within 15s, it isn't much of an early warning.

Measurements

Drive system

Fig 7:
 

Fig 7 shows a test of the quad at 12.0V. There was 0.6A current drawn by the four ESCs and FC with the motor OFF, so 0.6A needs to be deducted from the figures in the graph.

Expected hovering rpm is around 4,300, avg motor current consumption was 17.5-0.6=16.9A.

A series of full stick accelerations was conducted, and current peaked at 72A, rpm peaked at 7,600.

Crash damage

Crash damage has mostly been to the propellers which seem reasonably robust... but they are breakable.

A couple of spare frames were purchased to provide spares. 

Further experiments

 

 

Links / References

Changes

Version Date Description
1.01 10/11/2013 Initial.
1.02    
1.03    
1.04    
1.05    

© Copyright: Owen Duffy 1995, 2016. All rights reserved. Disclaimer.