In need of HELP: system sizing

[Erik Sink]

Hello, I am a college student working on a senior capstone project, designing and building a UAV. I am interested in using an electric propulsion system, but have practically no experience at all with RC electric motors. I've played around with ECalc and MotoCalc for a couple days now and am thoroughly confused! Our plane does not need to have aerobatic maneuvering qualities, it needs to perform somewhat between a trainer and a sailplane. The target flight-time is 30 minutes, pretty much the whole time will be cruising except for take-off and landing.

Here are some of our plane characteristics:
Gross Weight: ~20 lbs
Design cruise speed: 50 mph (73.3 fps)
Thrust Required at Cruise: ~2.78 lbs
Wing Area: 6.67 ft^2 = 960.5 in^2
Wing Span: 6.055 ft = 72.66 in
AR= 5.5
CD0=0.055

I was looking at the following motors:
Hacker C50XL
AXI 5330/18
Astro 120
LMT 2280 (not sure on the # of windings)

We are not set on a prop size, but I'm expecting it to be between 12-15 in. diameter. It is in a pusher configuration. The smaller diameter, the better, muti-blade props can achieve this, correct?

Any help would be GREATLY appreciated! I would definitely prefer to get a good prediction before spending all of this money, thank you!

-Erik Sink

[Andrew McGregor]

It would help a lot to get an airframe empty weight... that is, everything but batteries, ESC and motor.

[Ron]

first thing you have to do if you want this to be successful on a reasonable budget is get the weight down. at the moment you have a brick
960 sq. in wing area....320 ounces weight...equals the glide angle of a
concrete picnic table.......If you can get the wing loading down into a reasonable area, then your chances of success are greater. Aim for as close to 10 ounces to the square foot as possible...I know you likely won't make it, but this would be a good target for wing loading. Use as large a prop as possible, and keep the RPM down under 4800 assuming you are going to be using about a 15" prop...to figure out max speed for a larger prop, figure out the tip velocity on the 15" prop at 4800...don't exceed this tip speed the amount of thrust per rev does down drastically once you go over this .
Also make sure the tail moment isn't too short..you don't want this thing to be twitchy. Don't use a symmetrical airfoil...use a semi symmetrical one like an Eppler 195, or a Selig 7055....no more than a 12% airfoil max.
I know this stuff is likely something you already know, but it may be of some help to be reminded. one more thing....don't get carried away with using exotic materials to keep weight down...it is very difficult to build something lighter/stronger than a properly designed and executed balsa model.

[eflightray]

Eric

There is a very good UAV forum here on RCGroups, link - UAV Unmanned Aerial Vehicles (http://www.rcgroups.com/uav-unmanned-aerial-vehicles-238/)

Also a very good aerial photography forum, link - Aerial Photography (http://www.rcgroups.com/aerial-photography-128/) if this is more inline with what you hope to achieve.

[mariuslj]

Erik

Silly questions: What are the design criteria? Is it possible to use a model that's proven to work rather than re-inventing the wheel?

[LaxLife]

Ron's advice is right on. To the practical question of which motor to choose, I've used both the Hacker and the Axi 5330 and I'd suggest you mount the big 5330 for a 20lb craft. A 20" prop and a 10s lipo power source should provide the minimal 80 watts/pound and that will get this project into the air. The motor/esc/batteries will weigh about 4 pounds. If that makes your craft 24 pounds I'd say you have a problem. If that just gets you to 20 pounds it is manageable although Ron's comments on wing loading are right on. What is your ESC choice (electronic speed controller)? go with a Castle 85 HV (high volt), a JetSpin99 or a Hacker90 ESC. Some of the no name knock offs are said to be pretty good (www.unitedhobbies.com) Use a safety shunt!
http://www.atlantahobby.com/shopexd.asp?id=6606
Build this into your powertrain between the ESC and the batteries so you have positive cut off when you're not expecting the prop to spin. And setup your transmitters failsafe settings so that throttle goes to zero (not full as most default settings have it) when the transmitter to receiver connection is lost. This might happen in flight but is more likely to happen on the ground when somebody turns off the transmitter inadvertantly. When that happens the plane's rx will default to failsafe mode and the throttle will open to full. In a gas/glow plane this is ok because the engine won't start by itself. In an electric this protection doesn't exist and the prop will got to full speed immediately and chew up everything in its wake, and it will taxi/fly uncontrolled. This is bad as the prop on your project should have the power of a small lawn mower. Would you play around with an overturned lawn mower with the blade spinning? Be careful. Read these forums for help and ask lots of questions. What school are you attending?