I have a lengthy design document for the selection and design of the copter system being used in my development. Some select sections have been posted here for documentation purposes and the document will be included with my project. 

(link to lengthier design document)

Project requirements

• The project shall be completed within the budget.
• The system should be able to resolve a 5cm by 5 cm logo.
• The system shall be able to resolve 20 by 20cm objects.
• The system shall fail in a safe manner that will not damage either the system itself nor pose any risk to the user, other people or objects in the vicinity of operation.
• The system shall be capable of imaging an area of 100m by 100m.
• The system should be able to image an area of 400m by 400m.
• The system should be capable of flying at a variable height with a minimum of 5m.
• The system should have minimum cost of upkeep.
• The system should require minimum training to operate.
• The system should be capable of operating outdoors.
• The system shall be capable of operating in light adverse conditions (light breeze).
• The system shall carry a 100g payload.
• The system should have enough battery to image the area in as few trips as possible.
• The system should have enough payload to support future sensor additions.
• The system shall carry a camera, onboard stabilisation, some means of connecting to a computer and be able to store the images.
• The budget shall include the mechanical parts of the craft, control electronics and batteries
• The budget should include radio receiver and camera.
• The system shall include the electronics required to allow autonomous recharging

Requirements due to legality

• The system shall have power monitoring.
• The system shall have a means of transmitting telemetry data in order to inform the operator of the systems operational status.
• The system shall have a means of failsafe.
• Radio transmission power and frequency shall comply with Australian regulations for all transmitters used.
• The system shall failsafe systems like return to base / flight termination.
• The system shall have a means to terminate the current operation and allow user control.
• The system shall be considered a small uav under casa regulations (under 100kg gross weight).
• The system shall not be commercial applications.
• The system shall not operate near structures, people or an aerodrome.
• The system shall not operate at a height higher than 400m.

Notes on copter design

The selection of components for the H550 and Q450 designs required many tradeoffs between price, payload, flight time and availability of components from as few distributors as possible in order to minimise money spent on shipping. Some justifications and reasons for the selections of components were

Propeller selection:

Larger Propellers

Higher thrust/payload capacity

lower flight time

Less responsive

Heavier weight

May burn out motors if they are too large

Limited by frame size

Smaller propellers

Lower thrust/payload capacity

Higher flight time

More responsive

Lighter weight

Useable on high kv motors

Battery selection:

Larger batteries

Higher cost

Higher flight time (with diminishing returns)

Less responsive

Heavier weight

Higher more battery cells

Higher thrust /payload capacity

Less flight time

Smaller batteries

Lower cost

Shorter flight time

More responsive

Lighter weight

Fewer battery cells

lower thrust /payload capacity

more flight time

Motor selection:

Higher KV

Higher thrust/payload capacity

lower flight time

More responsive

Limits propeller size

Higher power

Higher thrust /payload capacity

Less flight time

Able to turn larger propellers

Lower KV

lower thrust/payload capacity

Higher flight time

less responsive

can have larger propellers

Lower power

lower thrust /payload capacity

more flight time

limited in propeller size

There is a delicate balancing act between those three main copter components as seen above so there are a few rules of thumb that have been developed to try and guide designers

Try not to exceed a ratio of more than 0.5*weight to thrust to remain responsive for flight or more than 0.8*weight to thrust to be able to maintain stable flight. Lower weight to thrust ratios result in a more responsive system allowing acrobatics and advanced maneuvers. Hence aerial photography systems usually have a weight to thrust ratio of 0.5-0.8 since the system does not need to be responsive for flight and payloads are normally heavy due to gimbals cameras etc

For long flight times try to have a battery weight to copter weight ratio of 1gram copter and payload for every 2grams of battery you are carrying or even more battery. This is because the motors on copter systems require a large amount of energy to run and deplete batteries very quickly.

For long flight times pancake style motors with low KV, high power and large propellers ,but they are limited in the payload they are capable of carrying and these systems are more expensive as well

Other craft design considered

-Hexacopter, Octocopter and n-copter

Decreased flight time and increased cost compared to quad

Higher lifting capacity for additional future upgrades such as sensors or camera gimbals

Depending on the design there may be more stability in wind

This project does not require large payload capacities

Sometimes able to land safely after losing a motor or propeller mid flight

-Fixed wing

Can have much higher flight times ,but it comes with the trade off of maneuverability

Unable to hover to take still photos

No vtol

-Blimp

Can have large carrying capacity, but needs larger envelope. Can have huge flight time and hover

No off the shelf systems so everything must be custom

Considering the design limitations of the above systems I chose a build a hexacopter system to allow for the overhead of the experimental payload (which could be reduced in weight once sufficiently prototyped).