Cheezits

Hello, i would just like to thank everyone in advance for all the information and suggestions. I am currently a senior in college for mechanical engineering. One of the year long classes i am currently taking is called Senior Design. In this class, every group is required to choose or come up with a project that will be monitored, critiqued, and entered into national competitions by the end of this school year. For my project, my team will be designing and constructing a micro sized RC airplane that can lift as large a payload that it can possibly carry.

Our current design goal is to have a plane with a 34 inch wingspan and weigh approximately 5 lbs. But of course the plane will be carrying a payload of a few pounds and up.

My current job is to analyze the power requirements and create an effective power system for our plane. So far i have narrowed it down to brushless motor and lithium-polymer technology as opposed to a nitro-methane engine. However i need some advice and suggestions for a few aspects of this power system. I am leaning heavily towards a single outrunner motor since it will be less expensive and much less complicated for mounting purpose (Part of the competition is being able to easily assemble/disassemble the plane). However i am curious as to what the benefits of having multiple motors is. Can someone suggest to me how i would know what size motor i would need and how many LiPo cells i should be using? Would i need a heatsink? I know that these motors tend to get very hot on my RC cars, but thats even only on full throttle once in a while. I'm assuming for RC airplanes you're almost constantly on full throttle. Also what are some good companies for Brushless motors, ESCs, and Lipo Batteries? Thank you!

Gray Beard

My Feedback: (-1)

At the very bottom of the forums list is the watt forum for electric powered planes. Maybe there are some other engineers there that have some information for you about electric motors and batteries.

ChuckW

Heat shouldn't be an issue unless you use a prop that the motor can't pull well or you enclose the motor so it can get no air. Since you probably aren't concerned with aesthetics, you could just have the motor mounted to the front of the airplane completely exposed and not have to worry about heat at all.

Battery selection will be based on the motor requirements and how you want it to perform. I would follow the motor manufacturers recommendations.

I personally wouldn't bother with a twin motor airplane, especially if this is your first desing. It is just more to figure out, more parts, more to go wrong.

You can get some good information on motor, speed controller and battery selection here: http://e-fliterc.com/

Or here: http://www.electrifly.com/config/

A good place for low cost motors, speed controllers an batteries can be found here: http://hobbycity.com/hobbycity/store/uh_index.asp Not a lot of information on the site though, just low cost stuff.

I am definitely no aerospace engineer but one thing that is obvious with aircraft capable of high payloads is that they all seem to have large, high-lift wings.

Have you or anyone you're working with flown an RC plane before? You may want to find a local club in your area to get some advice and learn a little about building & flying.

Good luck and have fun.

ChuckW

Also, 5 pounds is pretty porky for a 34" wingspan airplane. with an 8" chord that would come out to a wing loading of 42 ounces per square foot. That would be pretty much impossible to fly with a plane that small and it definitely would not carry any payload at all.


You can build a 60" plane out of balsa and light plywood that would be in the 5 pound range complete with flight gear and engine.

Your airplane should probably be in the 2 pound range I'm guessing when completed with a 34" wingspan. That would be just over 16 ounces per square foot for wing loading (assuming an 8" chord). This would be much easier to fly and could tolerate a small payload. If you could build it even lighter, that would be even better.

Generally, the larger an airplane, the more it can tolerate added weight. Small airplanes don't like weight at all.

aerowoof

I was going to say the same thing you need to design lighter than a 5 pound plane with a 34" span.Heck I have a 36" champion Eindecker with a dragonfly .049 engine ,3 mini servos,4 ch rx and 225 mah pack that weighs 16 ounces fueled.if you are limited to 34 " span you may want to consider a bi plane.the ww2 grumman duck for example.the b-24 used a davis airfoil.it was a high lift variety.

Gray Beard

My Feedback: (-1)

If you have to design your own plane you should look at some of the planes designed for high load lifting, planes like the Telemaster, King and DHC-2 Beaver then you can get a better idea of what it takes and borrow from this type of plane..

Cheezits

Ahh! Thank you for all this information! This will definitely help! As for the weight of the plane, i made an error... we are actually aiming for the plane to have a total weight of 5lbs with the payload. Because we want the motor and battery combination to be as light as possible, we are going to use a power system that can just adequately propel the airplane. We only need to lift the payload for a very short time since the plane just has to complete a basic circuit with a couple turns. So we don't need a power system that's so strong the plane can do aerobatics hehe. By the way, because this i the micro class we are limited to a certain sized box and a 34" wing span is as large as we can get.

Also i was aiming for about 80 Watts/lb, so that came out to 40.4 Amps if i use a 3 cell LiPo (10V with load). So for the 40.4 Amps, i basically would just need to find a motor with a Max Continuous Current Rating of 40A+ correct? Because the Max Continuous Current Rating is the max amount of Amps that the motor will draw, as in if the battery can only provide 30A, thats how much it'll draw, and if a battery can provide 50A, the motor will only draw 40A? Thanks!

ChuckW

Even if the 5 pounds includes the payload, I'd think you'll have a very difficult time flying it on that 34" wing. I could be very wrong of course; my experience and knowledge in this are is fairly basic. Maybe if you used a fairly wide chord and thick airfoil? You might want to visit the aerodynamic forum here. Some of the guys there know a heck of a lot on the subject.

A lot of the better motors will list their watt rating.

TFF

Build it as a biplane or a "pizza box" to get the wing area. I am not an electric expert, but max continuous of the motor is its safe rating. You can put a smaller prop on it and it will draw more amps and burn up. The motors dont self regulate; you have to match the whole system.

bkdavy

My Feedback: (1)

Building as a Pizza Box flyer with a 34 inch square with a total weight of 5 lbs gets you into the 10 oz/sq ft wing loading area, which should be very managable. Keeping the wing loading that light can help you use as smaller power system. But control will be harder, as this style has very little inherent stability.

ChuckW

Pizza box or flying stop sign type of plane... didn't even think about those.

Cheezits

Hmm, so a smaller than recommended prop will cause the motor to use more current? I'm worried about this since we will probably be using a smaller prop than recommended since it produces enough thrust. My current setup are these electronics: The thrust we calculated with Profili program came out to about 2.8lbs of Thrust which is just what we need. Also, we only need about 2-3 minutes of runtime. RPM came out to about 12000 RPM and 297W of power.

http://www3.towerhobbies.com/cgi-bin...?&I=LXTRE2&P=0 (Servo)
http://www3.towerhobbies.com/cgi-bin...&I=LXLWU0&P=ML (Motor)
http://www3.towerhobbies.com/cgi-bin...ksy5&search=Go (Speed Controller)
http://www3.towerhobbies.com/cgi-bin...vte7&search=Go (Receiver Battery)
http://www.promatchracing.com/prodde...hp?prod=3S3400 (Main Battery)
http://www.amainhobbies.com/product_...tem-TX-RX-Only (Radio & Receiver)

firehopper

from what I've seen.. smaller prop = less current use, less chance of burning up a motor, bigger prop = more current.

On a aeolian motor Ihad, 3 cell lipo, Iwas getting 60amps on a 10x6, and on a 9x6 Iwould get 41 amps, 9x4 prop drew 35amps.

Cheezits

Sorry i haven't responded in a while everyone, so currently i am still concerned about the power system. I'm not sure if what i listed above is sufficient, or overkill. The motor's specs are 30A max constant draw. How do i know when it will draw that much? It depends on prop size, weight, and drag correct?

LunaSea

if you are limited to a 34" span for a total 5 lb weight, consider a biplane and use an undercambered airfoil with a relatively wide chord for the wing. The UC airfoil will generate MUCH more lift at the low speeds that you are likely to acheive w/ the electric power and weight considerations. In any event, it will likely be prone to wicked stalls, so keep the flying speed as high as you can manage.

iron eagel

I would suggest for a five pound airframe you would want at least 750 watts of power for it alone.
Add 100-125 watts/pound for any additional internal payload 150/lb if the payload is external.
Typically for that a good high power ESC which is already heat sinked should work fine as long as you provide airflow over it.
Use an outrunner type of brushless motor and a good li-pol battery. I would recommend a higher voltage setup on the order of six li-pol cells.
You can vary the prop size and pitch to keep the current/wattage within acceptable limits.
Edit to add:
Your 30 amp motor at 11.4 will give you only about 342 watts of power. Even if you exceed the recommended continuous rating of the motor by a max of 20% that gives you a max of 410 watts.

iron eagel

this should help
http://www.castlecreations.com/suppo...alculator.html
A good source for motors also
http://www.bphobbies.com/view.asp?id=V819427

jimmyjames213

My Feedback: (2)

i have that motor, im running a 10x7 and can get around 10000 rpm out of it on 11.1v. if you need a exact reading i can get it for you, it has been a while since i tached it so i may be wrong
however im betting it will have a very hard time flying (if it could at all) 5 lb+ on a 34 in wing [X(]

bkdavy

My Feedback: (1)

Go to http://www.electrifly.com and use their system to pick the brushless motor you want. They have a calculator that you put in your desired final weight, prop diameter, and then select the performance type (use Trainer) and it will provide you with a recommended motor, ESC, and battery for you system. For example, I chose 5 lbs weight, Trainer, and a 9 inch prop, and it recommended

Motor Recommendations
Best Combination
Performance Trainer
Motor 35-36-1500 / GPMG4625
Gear Ratio N/A
Gear Drive Not Required
Propeller 9x6 in., Electric
Battery 3200 mAh, 11.1v

ESC SS-60 / GPMM1850
ESC Motor Adapter GPMM3123
Motor Mount Medium / GPMP1255
Current Draw 47 amps
Approximate Speed 70 mph
Approximate Thrust 46 oz.
Weight of Power System 13.9 oz.
Approximate Flight Time at Full Throttle 4 minutes
Watts per Pound 104.4

Brad

mclina

Just as a point of reference, the attached picture is a 36" wingspan plane that I built for snow flying last winter. It was a little over 3 pounds, and flew pretty well. During the maiden flight, I found it tail heavy and strapped a pair of pliers to the nose for balance. At that point it was almost 4 pounds, and still flew pretty well. I do not think that it would have gotten off the ground at 5 pounds. That's a lot of weight for a 36" wingspan. The biplane or pizza box idea is a good one.

Here is an electric setup that I have on another plane, which is about a 3 1/2 pound plane that is very aerobatic. This system would fly a 5 pound plane easily, and it is pretty cheap.


http://www.hobbycity.com/hobbycity/s...idProduct=3887

TR 35-42B 1250kv Brushless Outrunner (Eq: AXi 2820


http://www.hobbycity.com/hobbycity/s...idProduct=2165

TURNIGY Plush 40amp Speed Controller

http://www.hobbycity.com/hobbycity/s...idProduct=7363

Rhino 2150mAh 3S1P 20C Lipoly Pack



Good luck, and let us know how it turns out.

Cheezits

Holy cow! This is gold! Thank you everyone! I'll get back to you all as we progress.

Jetdesign

My Feedback: (8)

I think you should be able to get the plane below 5lbs. Since you are not building something meant to last you can get creative with material selection. You could also go for a stick design and get a cheap carbon fiber tube from hobbycity.com for your air frame.

Here is a thread that has tons of information for determining electric power systems. Some of it is pretty basic, but sometimes that's just what we need:
http://www.rcuniverse.com/forum/m_7100376/tm.htm

Here is a link to the slow-stick. I just figured it's a good example to show how simple an airframe can be. You can concentrate your energy (and weight) on the wings and payload system. Note the picture next to the specs that shows the power system:
http://www.*********.org/gws_slow-stick.htm#