Conceptual Design of a Solar Powered UAV

[sligo565]

Hi Fellas,

I have been stuck a few days trying to figure out my Range and Endurance equations.

I am using the ones provided in Anderson: Introduction to Flight.

As I am working with batteries and not Fuel,

I was just wondering if anyone could tell me if i can just make the Specific Fuel Consumption and the fuel weight constant (cancel down to one)

[Accu157]

Because you will be working with batteries, you probably will need to use something else. I'm not aero engineer, and I would agree with your wonderings. However, I wonder if you even need to use those.

[sligo565]

Probably not but only god knows that now!!!!

[DiveBombDave]

Fuel weight in electric IS constant, makes endurance calculations much easier because the AUW is not constantly changing during flight.

Find (or estimate) the number of watts consumed while at your cruising airspeed. 5 watts/lb is for the most efficient setups around. 10 watts/lb is more reasonable for the average builder, if all the right choices are made.

Multiply your battery capacity (in amp hours) by it's nominal voltage to get watt-hours. As example, a 3s lipo of 2100mah cells would be 11.1v times 2.1amp hours for 23.31 watt hours. Divide this by your plane's consumption in watts, a 3 lb plane at 10 watts/lb would need 30 watts, so 23.31 watt hours divided by 30 watts is 0.777 hours, or over 46 minutes.

I use motocalc to estimate durations in "idea" planes... once the plane is built, it's as simple as time a flight and wattmeter the recharge, that's the proof in the pudding.


Have you seen the SoLong project? Solar sailplane?


Dave

[sligo565]

Hey Dave,

Just one last question,

First of all heres the website where i found some pretty niffty Lithium Ion batteries:

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

I was talking to one of my buddies and he gave me the following advice which im not sure is true or not seeing that hes a bit of a know it all!

Each cell can produce 201Wh/kg * 0.045kg(weight of one battery) = 9.3465Wh. You have 51 cells so the total energy available is 476.6715Wh. At 2.242kW draw (Power Required) the battery will last for 476.6715/2242 = 0.21 hours.

Now im a bit sceptical of these numbers as it seems too harsh to be true!!

[DiveBombDave]

PM sent - math is good. 2242watts drawn from a 476watthour source will last for 0.21 hours. This is a short duration but a ton of power - 2242 watts would carry a 44lb plane on a respectable climb. At 10 watts per pound, my personal "loiter" standard power consumption, that power would loiter a 224 lb plane!

Dave

edit -

Just got in from flying my Sig Kadet EP42, and I ran it's numbers through to see how many watt per pound it consumed on a normal, boring (just enough throttle to maintain altitude) flight.

Power system is a lesser efficient chinese clone type motor, 2215 sized with a KV of 1100. Clone type 15A ESC. Thunder Power 3s 2100mah. APC slofly 8x6. Draws 12a or 130w at full throttle, so 2.1ah / 12a = .175hr, or 10.5 minutes of full thottle. (You could instead look at power instead of current, and say 2.1ah X 11.1v = 23.31watt hours, and then / 130 watts for the same .175 hr abouts).

To figure what my loitering consumption is, I flew for 30 minutes exactly and consumed 1050mah out of my 2100mah pack (how convenient). So I know I used half the pack in half and hour, tells me it'd loiter for an hour (note that you really shoot for a 75% discharge of your pack, so I'd safely fly 45 minutes in this example). You can assume then that it used on average 2.1 amps or 23 watts during that flight.

Divide by the models weight of 1.48lbs, and you can see it was using 15.5 watts per pound on average to hold altitude.

And this is with a clone motor and ESC... if I had an Axi-Jeti (or other high eff brand motor esc), and took off my landing gear for their drag, I could get that value down into the 10-12watts per pound range.

Sooooo.... long story short, I haven't looked at your airframe numbers from the other thread, but you should be able to design it to hold altitude with those same Watts per Pound numbers above, and then you can easily calculate duration based on the size of your battery.