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School Project - Help

Old 09-20-2013, 10:49 AM
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Petkov
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Default School Project - Help

Hi,

I am currently in an University competition representing my school and my team is tasked with building an RC plane which will be designed to undergo several challenging missions. Most of us are total newbies when it comes to RC planes and I am sure all of you guys are really experienced so I came here to look for some help.

As a mechanical engineering student, I know a decent amount about the construction of the plane and have help build a few over the years. My main issue currently is with the propulsion, and this is where I figured you guys could help me out. I need to figure out what kind of batteries, motor, esc and fuse to use.

Fuse:
I am limited to use a 15 amp fuse but this can be a slow blow type. In fact I am trying to get as much over that 15 limit to give me an edge over the competition. Do you guys have any recommendation for a 15 amp rated fuse that can get 17, 18, or even 19 amp averages over 5 minutes without blowing? (yes, competition rules require a fuse because it is used as a quick way to make sure everyone is following the power requirements without taking the plane apart and for safety reasons. The fuse is the last thing to add before takeoff and first thing to remove after landing.)

Batteries:
They only need to last 4-5 minutes. This includes take off, flight, and landing. But my main constraint is 15 amp current. Unfortunately, I am limited to NiHM and cannot use LIPO ones :/ Do you guys have any advice on how I can determine what batteries would be best for the application?

Motor:
I am expecting to buy something between 300 and 500 watts just based on my power constraint. I am allowed to spend up to $200 for the motor. Once I decide on the batteries and motor I will look at props. For my 2 missions in the competition I will need a prop to bring my plane up to full speed and then carry a heavy payload. For the speed mission I am guessing something around a 12x12 will do the trick and for the payload mission, I was going to try a few in the 13x10 range. I could use any advice you guys have for me on how to select the right motor/prop combination.

Speed controller:
In this category I really don’t know where to begin, I was just going to follow the recommendations of a hobby story sales person.

I am very lost on the whole electrical aspect of this project and I appreciate any help you guys can give me.

(official rules: http://www.aiaadbf.org/2014_files/2014_rules_31Oct.html)
(I have a similar thread on RC Discuss going: http://www.rcdiscuss.com/showthread....-use-some-help)
(supplementary RC Universe threads: http://www.rcuniverse.com/forum/elec...challenge.html and http://www.rcuniverse.com/forum/elec...my-set-up.html)

Updates:
I received a huge help from the forums and have a strong understanding of the electrical system. Now my team has to order the parts and optimize through testing once everything arrives. For the first test flights we are "cheating" by using lipo batteries until we have the propulsion system is optimized.

The team has built the first prototype. The plane is very crude and heavy because we used old parts and cheap material such as cardboard and duct tape. We can get away with this design because the final plane is expected to carry a heavy payload. Future prototypes (we should have 1-2 more by February) will be much better designed.

Team had first test flight. We discovered the plane had some longitudinal stability issues.

Team had second test flight. Longitudinal stability issues resolved but we discovered lateral stability issues.

Current task: fix lateral stability and get more test flights in.

Last edited by Petkov; 11-18-2013 at 05:40 PM. Reason: updating!
Old 09-20-2013, 01:25 PM
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jester_s1
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I have never heard of putting a fuse into an RC power system. There are lots of negatives, and frankly no positives. Does your professor actually know anything about RC planes?

As for your questions, I have one- do you have a voltage limit? If not, you can go pretty big on your voltage with a low Kv motor and still stay under 15 amps. That 500 watt idea would require 33 volts from your battery to happen for example. If there's not a voltage limit, I'd say this is a very poorly designed assignment. If you are allowed to go for ridiculous voltages, that obviously will impact your battery and motor choice. If your prof has his head on straight, there should actually just be a watt limit on the project's power system. That will be easy to test with a cheap watt meter and would allow you all lots of leeway on how to design your power system to work well with your airframe. The speed controller is just a gas pedal. Once you pick your motor, you just choose one that can handle the power system you are using.

With some answers on the voltage question, expected flying weight of the plane, whether or not brushless is allowed, and seriously, what is your professor thinking requiring a fuse to be used, we can give you more help.
Old 09-21-2013, 10:09 AM
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Lone Star Charles
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You will probably get a better response if you will give us all of the design parameters. You mention power constraint, but then tell us that it's between 300 and 500 watts?

Also, although we do enjoy this type of challenge, we don't usually do homework assignments.
Old 09-22-2013, 04:42 AM
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jester_s1
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There was a project like this posted last year where the parameters really made no sense at all. It was for a prop driven land vehicle, but the teacher really didn't know what he or she was doing so it was going to just be a horsepower contest where the student who spent the most money would win. Teachers really would do well to get on RCU themselves and ask a couple of questions about RC and the power systems we use before making these assignments. I would love to design a project for students to do, and I think I could make several that would be very educational and fun without being expensive or complicated.
Old 09-23-2013, 04:52 AM
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Petkov
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Thank you for taking interest.

There really are not many restraints besides the 15 amp restriction and 1.5lbs of HiMH battery limit. Although I understand that most RC systems do not use a fuse due to added resistance and I am sure other negatives, the only purpose of the fuse in my competition is make sure everyone follows the parameter. The only loophole is that I could use a slow blow fuse.

The two missions we have to perform are: 1) see how many laps the pilot can make around a preset course in a 4 minute limit 2) once the plane is loaded with 6x6x6in 1lb blocks how fast it can compete 3 laps. For the first mission the score is based on the number of laps, for the second it is based on number of blocks and time. the total score is then divided by the weight of the empty plane, so lighter the plane the higher the score.

For the motor I only mentioned 300-500 watts as a guess. It is not a requirement. I would like to follow the "sports plane" type of model (6:1 or 5:1 aspect ratio for the wings) with a high wing and slight dihedral and foward taper.



Last edited by Petkov; 09-23-2013 at 05:42 AM.
Old 09-23-2013, 05:24 AM
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1.5 pounds of batteries? Your teacher really doesn't know anything about RC. Seriously, you should suggest that the teacher talk with someone who flies RC planes to get some perspective on the project. I did a little bit of math for you, and found that using sub-c 3000mah cells a 1.5 pound battery pack would provide 68 volts. Even going up to 5000mah packs comes out to 46 volts, which is still way more than a typical RC system has. Even allowing that the solder tabs and such are going to cost you some weight and therefore won't allow you to run as many cells, you're still talking about a power system that is not appropriate for RC gear.
And the fuse idea is just foolish. A wattmeter will accomplish the same goal of keeping the contest fair without the negative of having a fuse blow and causing the planes to crash. Fuses are for keeping things from burning up in the event of a failure when you'd rather have the equipment shut down than be damaged. No RC pilot in his right mind would put one in his main power system. If the parameters are correct that you've given us, there won't be a single plane in the class that will make it even once around those pylons if they even get off the ground at all. The amp limit is too low, the electrical system engineering is flawed, and the battery parameters are all wrong, not to mention that it sounds like you are also supposed to design a plane to work with this power system that may or may not be a viable flying machine itself.

It may be an exercise in futility, but I'd be willing to talk with your teacher about designing a project that will be appropriate for beginners, practical, and give a decent chance for success. Feel free to share my user name here and have him get in touch.
Old 09-23-2013, 06:02 AM
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Petkov
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I think the battery limit is just to make sure we don't build anything too crazy. As for the fuse, I do agree with you that it is unnecessary but thats is how they set up the competition and no complaining from our team will do any good.

The complete set of rules is on: http://www.aiaadbf.org/2014_files/20...es_draft5.html if you guys are interested. There is a lot of crazy stuff they expect us to do.
Old 09-23-2013, 08:12 AM
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jester_s1
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I read the contest rules. Lockheed/Martin sponsors a similar contest this is only a weightlifting event, but they specify an OS .65 AX as the power system with any prop they teams wants. Their electric category is one motor and battery combination too so that teams can focus on their airframe design instead of worrying about their power systems. To give you a useful answer, I can tell you that you'll want a low Kv motor running with a lot of voltage to turn a big shallow pitch prop. You'll need a lot of wing to meet the STOL requirement, and you'll probably do best in the taxi contest with a tail dragger with the tail wheel fairly high so that the nose isn't sitting up too much. Use the biggest wheels you can to handle those bumps, and consider buying the DuBro pneumatic tires since they are so much softer. If the data is available to you, look at previous designs that did well in similar contests to get an idea of what works and what doesn't.

I've worked the Lockheed contest a couple of times at my club field and have noticed a trend of students designing planes that are maxed out for straight line flight efficiency (some really elegant designs) but fail the practical test of turning well or being able to handle gusty wind. Watching novice pilots with their finely built but so horribly unforgiving airframe designs take them home in a plastic bag can be a hoot, especially when they walk around so cocky at the beginning. So my point is that the most technologically advanced design with the best airfoils for lift and the least draggy tails and such may not be a good move for you if you don't have an experienced pilot to fly the plane. Add unpredictable wind conditions to the mix, and going with a fairly easy flying plane has a lot going for it.
Old 09-23-2013, 08:49 AM
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Lone Star Charles
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Outstanding! I just read the rules - what a great contest. Sounds like a lot of fun as well as a trip to the aviation mecca of Wichita.

The rules seem pretty challenging, yet simple and straightforward. As you develop your concept aircraft, you will probably find this site most beneficial if you describe your design and ask for input regarding pros and cons for whatever element you are working on - whether it is powerplant(s), fuselage, wing layout and design, etc. The first design that comes to my mind is kinda like a Short Skyvan, not the prettiest nor most efficient, but will probably fly with anything you can close the door on. It also would stand a good chance of clearing the 2X4 on the taxi tests.

Check out some of the Electric forums here for advice on power systems. Multi-engine may be a good way to go here since it sounds as if the fuse(s) requirement is on a 'per powerplant' basis (I could be wrong here).

In any event, it's sure to generate some interest here. There is a wealth of experience on this site. Don't be discouraged if you get some "that'll never fly" comments. Hyperbole seems to run rampant in these forums sometimes.

Best of luck. Oh, and by the way, what school do you represent?
Old 09-23-2013, 08:52 AM
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Petkov
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Thanks for the response I will definitely put your advice to good use. I might actually look over the Lockheed/Martin contest to get some ideas as well. I did look at past reports of this competition but the amp restriction went down from 20 to 15 and the teams do not do a very good job justifying how they came up with the combinations. It would have been very convent otherwise.

We have already decided on the tail dragger and big wheels so its big plus to get a confirmation from someone with experience. The high tail wheel, is something we did not think of, and I believe you bring up a good point. I usually aim for practical designs when designing anything so I will make sure I push my team not to do any crazy and not try to "reinvent the wheel". Hopefully with the dihedral and high wing, the plane will be stable enough and our pilot is decent so I hope we would not be needing any plastic bags after test flights.
Old 09-23-2013, 09:07 AM
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Petkov
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Charles, I will keep you guys updated on progress with the plane. Looking at past reports, most winning teams do not use multi-engines so I am a bit skeptical or trying it out but I will look into it. Thanks for the plane reference, I will check it out at this point i was looking at planes similar to the c-27J spartan so hopefully its something like that. I am new to this whole forum thing, any advice to which specific thread I should post once I am ready?

I represent The University of Massachusetts - Lowell

Thanks!

Last edited by Petkov; 10-01-2013 at 08:24 AM.
Old 09-23-2013, 04:58 PM
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The fuse is not a big deal if you go for a higher voltage pack.

10 x sub c cells 1.5 lbs 150 watts
23 x AA cells 1.5 lbs that will give you about 400 watts.
28 2/3 AF cells again 1.5 lbs 500 watts

I was involved in something similar years ago using an IC engine as the standard power plant and max weight carried as the criteria.

Your mission profiles are quite challenging.
Old 09-23-2013, 07:24 PM
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jester_s1
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You might start by looking at the planes than have been designed and successfully flown in rough "bush" conditions. The DeHaviland Beaver and Piper Super Cub come to mind immediately. Both have high aspect ratio wings with flaps, big taildragger wheels, and tall chunky fuselages. They also are both squirrely on the ground handling because of the short fuselages and small tails. Have a look at the landing gear designs of those planes because they are proved to be tough enough to handle the bounciest places.

I watched a lot of planes crash at the Lockheed contests. What was fairly typical was an inexperienced pilot takes off, fears being near the ground, so he points the nose about 30 degrees up and flies the plane into a full stall. The planes with the airfoils that the books say give the best lift at slower airspeeds would do a snappy stall at that point and head for Earth. Some pilots recovered, while others didn't. The more moderate designs survived that terrible takeoff and then had the wind to handle. Maybe 1 out of every 6 planes had aerodynamics that made them somewhat manageable in the wind. That's the kind of thing that a pilot thinks about, not an engineering student. If they managed to correct in the wind turbulence they then had to land. Overflaring was by far the most common error there which usually results in a nose in. This is where not going too hot on your airfoil really pays off because gentle stall characteristics make for gentle landings. Again, Cubs and Beavers.
Old 09-24-2013, 04:41 AM
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500 watts is only 2/3 HP which is less than a sport 40 size engine. There is no way you will drive a 12x12 or 13x10 prop with that. You will have a hard time with a 9x7.
And before you repeat that 300-500 watts was your estimate remember that at 15 Amps you will need 33 volts (22 cells) to get 500 Watts. As has been said you instructor really needs to talk to someone in RC familar with electric planes or helicopters.
Old 09-24-2013, 04:57 AM
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Petkov
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duncker, I was thinking of using 28 of the Elite 1500 2/3 cells. they seem to have very good discharge rates compared to the 2000 and 2100 AAs. The winning teams used those last year so I think it would be a safe bet. What configuration do you think will give me the least amount of resistance?

jester, thanks for the model suggestions, will definitely look into those. I tend to use very practical designs, here is the plane from last year: http://www.youtube.com/watch?v=rY-3xUZWBM0. We could get only about 2 lbs of thrust compared to 6-7lbs that other teams were getting. We couldn't take off in the 20x20 square unless there was a moderate headwind.

Last edited by Petkov; 10-01-2013 at 08:24 AM.
Old 09-24-2013, 05:12 AM
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jester_s1
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The takeoffs in your video are exactly what I was talking about - gun and jerk. It almost cost you the plane in the second takeoff when it stalled about 8 feet off the ground. Look at how the big commercial jets and military bombers take off to see how a heavily loaded plane should take to the air. It does require more pilot skill, but you'll be amazed at how much more weight you can carry when you learn to takeoff properly. It will also allow you to design a plane that flies at a lower airspeed, which lets you use a bigger prop to overcome the drag that all that extra weight is going to create.

I'll make one other suggestion. I see lots of planes with a boom fuselage like yours, but there isn't any good reason to do it that way. It probably creates more drag than a smooth shape would, and you can probably save some weight with a balsa box back there that has some width to it.
Old 09-24-2013, 05:43 AM
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Petkov
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great advice! As for the take off, we really needed the pilot to lift it off the ground asap and we still barely made it before reaching the edge of the takeoff box. The pilot was not part of the team last year, we had to borrow him from the airfield, but he said he had over 20 years of experience.

I appreciate the criticism. I believe its the best way to learn when talking to someone with experience. As for explaining why I did what I did, is that the propulsion guy said we will get x thrust so wing guy said we need a wing x feet with a cord of x and stability guy said we needed the tail to be x by x and x feet from the trailing edge of the wing and my structures team ordered a CF beam that was way too strong then they all went away for winter break and didnt comeback to the team until a week before competition. I was left with building everything myself so i made the decision to go as light as possible. I used 1/6in lightply ribs and the lightest balsa i could find for connecting everything together. It came out as a big box but it was still the lightest airplane at the competition. Do you have any blueprints for RC aircraft that you suggest I base the design for this year after? Those would be a HUGE HELP to understand what you mean.
Old 09-24-2013, 05:44 AM
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Since the takeoff's and landings are so critical, you may wish to use a non-student pilot (which seems to be allowed for in the rules). In the past few years our club has worked with university students in similar competitions. We have provided an expert pilot, safety inspections of the airframe at various stages of construction, and pointers to forum discussions on some of the design issues. It might be worth your while to contact a local club or two and see if they will give you a few minutes of meeting time to outline the competition, and ask for expert pilot assistance.
Old 09-24-2013, 06:46 AM
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You might look into a good 600-class brushed motor with a 2.5:1 to 4:1 gearbox. I run just that with a 12V Multiplex 600 BB Turbo on 10 Reedy 3000 matched C-cell Nimhs that weigh 1.5 lbs. It turns an APC 11x7E on 2.5:1 reduction, and a 13x6 with 4:1 in an old timer with polyhedral wing of 52".
There's nothing wrong with brushless, as long as you get the efficiency.
Old 09-24-2013, 08:29 AM
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I don't have blueprints, but just look at Cubs and Beavers to see the fuselage shapes. A few RC designs that are known for being very easy to fly include the Great Planes Ugly Stick, Sig 4 Star, and Hangar 9 Pulse. The wing designs are tail surface to wing ratios make those planes forgiving and good in the wind and still very maneuverable. They aren't the best weight carrying or STOL designs on the market, but if you can understand what makes those planes pilot friendly there will probably be some lessons you can apply to your student design.

On the takeoff, what I meant was the nearly 40 degree climb out angle. I understand you are required to break ground by a certain distance, but unless there is a 20 foot fence that you have to clear shortly thereafter there is no need to climb out so steeply. A smooth and shallow climb out lets the plane gain back the airspeed it lost when you rotated to get off the ground so that you don't run the risk of stalling the plane. So on that flight, a better takeoff would have been to break ground where he did, and then hold the plane a couple of feet off the ground for a second or two before climbing so that the plane has a chance to accelerate.
Old 09-24-2013, 11:23 AM
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Lone Star Charles
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I think that with the rules allowing an experienced pilot, you will be able to concentrate more on design variables and not worry so much about learning to taxi and fly.

With that in mind, I would suggest that you post your project in the aerodynamics forum on this site http://www.rcuniverse.com/forum/aerodynamics-76/ There are a lot of guys that frequent that forum with some real knowledge about aircraft design and I know that this project will pique their interest.

Since your powerplant seems to be the system with the most restrictions, I would also suggest that you post your project on these sites:
http://www.wattflyer.com/forums/index.php?
http://www.rcuniverse.com/forum/elec...-universe-227/
Pick a forum that is in line with your questions and has a lot of posts in it. It will get the most visibility.

I don't know much about this and it is not free, but I have heard that http://www.motocalc.com/ is a great help for calculating electric motor parameters.

I looked into converting to electric flight once and found this thread very helpful: http://www.rcuniverse.com/forum/elec...ic-flight.html aeajr, a moderator on that forum is a very helpful guy and probably would be good to know.

Again, good luck with this.
Old 09-24-2013, 06:01 PM
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You'll learn a lot on Wattflyer. One really neat thing about electrics though is that you can make really big changes just by installing a different prop. So there might be 5 different props that will keep you under that 15 amp limit, that will all fly the plane differently.
Old 09-25-2013, 03:49 PM
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Thanks everyone!

I will visit a field probably this weekend and any advice you guys give me on how to fly the damn thing is useful so I can know if the pilot is doing a good job. I might get a simulator to try it out myself, those look fun in the RC shops.

Eddie, I will need to look into those batteries and do my research on motors. I have no idea how the gear boxes work as of right now or even how a brushless motor is different than normal electrical ones.

Thanks for the links Charles.I posted in the electrical forum and will post in aerodynamics when I do my research and size up my plane a little better. The aeajr article was amazing. Wish I know all that info last year. As for motorcalc, I believe my school has a student version of the software. I will have to get me hands on it.

updates:
I have managed to roughly size up my aircraft using historical data of winning team planes. I estimate the plane to have an empty weight of 2 to 2.5 lbs and loaded 4 to 5.5 lbs. Using the average power-to-weight the other teams were getting(average of about 70watts/lb), I am aiming for 350-400 watts. With the 15amp restriction I will need about 22 cells of the 1.2V batteries which is about 1.2lbs with wiring and such.

Last edited by Petkov; 09-25-2013 at 03:52 PM.
Old 09-25-2013, 04:46 PM
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Originally Posted by Petkov View Post
Thanks everyone!

I will visit a field probably this weekend and any advice you guys give me on how to fly the damn thing is useful so I can know if the pilot is doing a good job. I might get a simulator to try it out myself, those look fun in the RC shops.

Eddie, I will need to look into those batteries and do my research on motors. I have no idea how the gear boxes work as of right now or even how a brushless motor is different than normal electrical ones.

Thanks for the links Charles.I posted in the electrical forum and will post in aerodynamics when I do my research and size up my plane a little better. The aeajr article was amazing. Wish I know all that info last year. As for motorcalc, I believe my school has a student version of the software. I will have to get me hands on it.

updates:
I have managed to roughly size up my aircraft using historical data of winning team planes. I estimate the plane to have an empty weight of 2 to 2.5 lbs and loaded 4 to 5.5 lbs. Using the average power-to-weight the other teams were getting(average of about 70watts/lb), I am aiming for 350-400 watts. With the 15amp restriction I will need about 22 cells of the 1.2V batteries which is about 1.2lbs with wiring and such.
I don't see any reason to limit the cells cram as many cells as you can into that 1.5 lb pack. the more volts you have the more watts.

I would have a look at the Gossamer Albatross designed and built by a team led by Paul B. MacCready, a noted US aeronautics engineer, designer, and world soaring champion. for some ideas on wing section and wing construction you might also get brownie points for a reference to the wing design used on Oracle the AC 72.in your design notes.

High aspect ratio wing and long fuselage.

There was not much wrong with the design flown on your vid. Was it loaded on that flight. If yes you could have carried a LOT more weight. As you have a short take off run I suspect flaps might be the way to go possibly full span flaps.

Large dia wheels will help on the rough taxi. 6 inch ?

Do not build a canard though !

Go brushless and listen to the electric guys on what motor/props to buy. .
Old 09-26-2013, 03:57 AM
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The short answer on brushless vs brushed motors is that brushless motors are more power efficient. That means you get more turns of the prop for the amount of amps you consume, which is a major help in what you are doing. The only negative is that the speed controllers cost a bit more, but they will fit just fine into the budget you said you have. Gearbox motors have a built in gear reduction so that a low wattage motor can turn a bigger prop at a lower RPM. In some applications, you get better power efficiency that way since the higher Kv motor is inherently more power efficient than a low one is, and so even with the losses of the gear drive you can still be better than just going with a lower Kv motor.
And if you didn't know, Kv is how many rpm the motor will turn per volt delivered to it. A 500 kv motor fed with 10 volts would turn 5000 rpm unloaded. That's theoretically at least. Actual numbers are always a touch lower.

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