Endurance of Batteries and cruise Wattage

[sligo565]

For my final year project/thesis I have to carry out an in-depth conceptual design study of a high-altitude (65,000ft), solar powered unmanned air vehicle. The project includes a comprehensive survey of previously conducted research and emerging technologies that will impact future projects (e.g. light-weight solar panels, batteries). The primary design parameters (i.e. size, weight, cruise speed, endurance, powerplant type, materials) also have to be determined.

The main objective of the report is to try and find out a way of a 24hr+ flight

Uploaded is the back of napkin sketch I have of my UAV design. I would greatly appreciate any help you can offer with finding such things as Battery Endurance etc. Below are calculations on how far i have gotten with the project:

Reynolds Number: 129,788
Wing Loading: 13.82 kg/m^2
Power Loading: 35 Kg/Kw
Aspect Ratio: 19.24
Taper Ratio: 0.45
MAC: 0.836
MGC: 1.196
Airfoil: E423
Cl Max: 2.4 at AoA of 14 degrees

I have selected a 3kW motor for my propulsion, I know this doesn’t exist but I prepared a graph of Weight vs. Power of different Electric motors and found that a 3kW motor would roughly weigh 900g (0.9Kg). Seeing that I have 4 motors, this means I will have a power output of 12kW. But in cruise one of these motors will be switched off meaning that i will have a max power of 9kW at cruise

For my batteries i have selected SAFT VL 41M Lithium-ion cells. I have uploaded the details of this battery which can be found on the link below. As my solar panels will only be producing 4.5kW of power I will need my batteries to produce 7.5kW meaning i will need 51 cells.

Finally for my solar cells I have selected Sanyo HIP-215NHE5 panels with a power output of 215W. I will hope to have 21 panels of these cells meaning a Max Power output of 4.5kW

Where I have become stuck is trying to figure out the Endurance of my batteries and also how to figure out what wattage I will need for cruise so that the UAV doesn’t fall out of the sky!!! Will I need max power all the way through the flight?

If anyone could guide me in this step of the airfoil and other steps after this it would be greatly appreciated!

[danstrider]

I would appreciate getting a copy of your final report once it's done!

Nice link by the way on the Li-ion cells. Those are some MEGA capacities!

Dan

[CDN-uavpilot]

I would think the principles contained in Dan Raymer's book on aircraft design would be useful for determining the battery capacity needed. Note that Raymer uses gas engines in his examples. The fuel capacity will need to be translated to a Ah capacity.

see link http://www.aircraftdesign.com/

[Krogoth]

You'll need to set up a big set of simultaneous equations if you want to find the optimum solution for infinitely sustained flight. I did this recently when I was trying to design much the same thing, only it was a surveillance blimp that could fly non-stop for weeks, in theory.

Basically, you'll need to try and figure out what constraints there are on your system. For example, you'll have a Wh/kg for your batteries, a W/m^2 and W/kg for your solar cells, a thrust per velocity per watt for your motor/prop combo etc. All of these affect each other - if your plane weighs more, it'll need to generate more lift, which means it needs more power and bigger wings, which means you get more drag, which means you need more power and therefore more weight etc. - and you're back to where you started.

What I did was to come up with a basic design (like you have) and then try and source the best available components I could for a reasonable price. For example, is the Saft the best energy density you can find for batteries? I've found one that gives 201Wh/kg for $US13 per 9.36Wh:

http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2763

Sure, it's OEM, but it's cheaper and has 1/3 more capacity per unit weight... For solar cells, check out the ones that MIT KID and co used on their solar powered UAV elsewhere in this forum - for memory they offered 39W/m^2 with 66 W/kg, compared to the 15 of the Sanyo solar cell you mentioned.

Remember, when you're working out the endurance of your UAV you need to make sure you get your battery capacity and solar capacity correct - assume, for example, you get 8 hours of sun a day. Therefore, given a constant power draw, you will need to generate 3 times that power draw during the day, and have enough battery capacity for 2 times that at night (for the other 16 hours). 8 hours is a generous estimate, too.

I don't know enough about aerodynamics to give any comments on building a plane to fit your requirements - i'm sure it can be done, the question is at what scale - a $5k backyard creation, or $50k 100m wingspan giant? (The first blimp that popped out of my equations was 200 metres long and 40 metres wide... better component selection shrank that down to 12m and 1.5m respectively).

Hope that helps,

Will.

[sligo565]

Thanks, this is just a conceptual design....the price of everything isnt really an issue, which is brilliant!

[matttay]

There are folks that have taken data loggers aloft in their airplanes, so you should search the forums for some plots or users. I'm sure you'll find a lot of users happy to share. Graph several points, a trend will emerge, fit a line, extrapolate, etc

Roughly, a plane that has 50W/pound is known as a docile flyer, and a plane that has 200W/pound can do just about anything you want (unlimited vertical)

Assuming the 50W/pound is required for takeoff and climb (full throttle), then it feels about right that cruise would be around half throttle, or perhaps 30 or 35W/pound, probably cruising somewhere around 35 or 40 MPH.

You could also get motocalc and start to get a feel for a larger system simulation.