A new design

A few months ago I started on a new design for Liverpool Hope University. We're looking at small solar winged aircraft as a platform for research and interesting student projects. Its a subject that's interested me for many years, and I figured it was time to put my design and building skills to the test.

The concept is to build a small (3-4m wingspan) aircraft that can stay aloft for many days. Larger solar aircraft have been built and flown, for example Solar Impulse and Zephyr. The challenges of miniaturizing everything for a much smaller aircraft set my mind racing. Working with one of our PhD students we made a software model of the aircraft and showed that the idea is credible on paper.

We've broken the project down into a series of phases of increasing complexity. This first phase is to develop construction techniques for an extremely lightweight and low power aircraft. Over the summer break I've started the deep design of our first prototype so the project is truly underway!

Just to whet everyone's appetite, this is the CAD drawing for the first version of the left wing. Spars are carbon fibre, leading and trailing edges are balsa. The inboard 3 ribs are 3D printed for strength, and the rest will be 3mm Depron for lightness. Its beginning to take shape on the mag table. Once assembled I'll start to investigate ways to simplicate and add lightness to the design.

Dr Rocket
Interesting!
I suppose my first observation is "many days at a time". This could raise a potential "line of sight" issue if it is to be flown in the UK without special CAA permission.

Just a thought but does a high aspect ratio wing give the best area to weight ratio for a solar powered plane?
It is possible a more 'modest' wing design could end up providing a better compromise between structural and aerodynamic efficiency.

We've already been in contact with the CAA and both parties think that the best approach is to set up a TDA for trials. This could give up to 90 days window for test flights. Initially we want to develop and prove all the strands of the technologies, so we're probably not going to get to the overnight stage for about 2-3 years. Who knows what CAP 722 will look like by then! We have a fallback position at an overseas university should UK regulations become unworkable.

The wing is one of the key challenges. The upper surface is to be covered in solar cells (we have a source of very lightweight flexible cells) so it will have to be strong enough to support them and their cabling. Energy management is the biggest challenge so we see automated soaring as one energy saving measure, hence the glider-like profile.

For this proof-of-construction-concept phase I'm going for a simple 13% Clark Y as its easy to build. We want to understand construction issues such as where we can save weight, which materials work and which don't, how much flex we can expect (solar cell fatigue over a long flight is a concern). The next version will probably be one of the Eppler profiles with some tapering and a bigger root chord.

Eric wrote: Without intending to throw cold water on your ideas, why are you trying to re-invent the wheel?
A solar-powered UAV has already been built and flown, a couple of years ago, as a flying test-bed for an airborne radio relay system.
It was basically a huge flying wing, with multiple motors, but proved so fragile, to minimize weight, that it couldn't be flown in anything except a flat calm, or the clear air turbulence broke it!
If I recall, it was intended to drift around in the upper atmosphere, way above present aircraft, using battery at night, and solar during the day.
After a few trials, everything went very quiet.
www.unifly.aero/news/solar-powered-drone...fly-for-up-to-a-year

Very helpful I'm sure....

As one of the unenlightened can someone please explain the advantages of using an Eppler profile as opposed to the Clark Y ?

A truly fascinating project DR,
I'm a complete electrickery numpty but I'd love more details, energy management etc.

Hi Eric. No cold water there! The Aerovironment/NASA projects like Helios contributed a lot to solar aircraft development, mainly through NASA's open approach to publishing data. Since then the much publicised Solar Impact has flown around the world, and Zephyr has shattered its own endurance record with a 25 day flight. I was involved in Zephyr before 'retiring' to part-time academia. Another academic institution (ETH Zurich) has completed a 4-day flight with a 5.6m solar aircraft. Aeromodellers have managed flights of a few hours using a mix of solar and battery, and some of these are on You Tube.

As a research project we want to see how small a functioning aircraft we can build. There's loads of interesting tech to explore there. I'm currently on the prowl for a new PhD student to explore some of our ideas. For the undergrads and masters students there's a shedload of interesting projects, especially for the robotics and electronics students.

Regarding Eppler: its a matter of getting a low speed & Reynolds number, high lift/drag ratio aerofoil. The Eppler series are particularly well suited to that flight profile.

So that's you told Eric!
Truly fascinating DR, thank for that, I will be following with interest.

Hi Paul. At a purely electronics level there's not much new in energy management for a solar aircraft. Solar panels feed a max power point tracker (project - MPPT needs to be miniaturized) and voltage converter to provide core power. Excess power is used to charge batteries, which release power overnight.

Algorithmically its a whole new game! Kinetic and potential energy are negligible, so the strategy for managing electrical energy is key to success. Airspeed has to be slow to keep propulsive power low, especially overnight so the aircraft can fly to well beyond dawn on stored energy. Decisions to climb (high power) need to be made when there is excess solar power, so climbing at night is unlikely except in dire emergencies.

Research and simulations suggest that flying the aircraft is the easy bit. Most of the operational effort revolves around planning the generation, storage and consumption of energy.

Thanks, it'll take me some time to digest that, wonderful project and I wish you every success.

I would agree entirely that minimising the power to fly is an key factor for this sort of project.
Power to fly is a function of a plane's total weight and its 'gliding' sink rate.

My own experiments in duration power flying suggest this is not necessarily achieved with typical high aspect ratio glider wings which are intended to achieve the best lift to drag ratio but have a structural weight penalty rather than obtain the minimum power to maintain level flight.

Your team could be in for quite a bit of experimentation.
I will follow with interest.