|Mar 03, 2011, 11:33 AM|
Please help me determine size and prop of Flying Wing
Hello world !
Gear available = 7.4 LiPo, 50-70 Watt brushless slow-fly, 8060, 8037, 8050
and such props. This is a slow fly combination.
Mission = put together a flying wing made of Blue Cor.
 What weight and wingspan flying wing do you think I should put together, at that setup ? Im not interested in crazy climb speed, just a smooth flight.
 And generally, how the heck does one determine what plane to build, GIVEN the power plant ? E.g. suppose I wanted a Cessna, is that possible ?
PS: I recently posted about a Hurricane build. The plane parts are ready but I put them aside because Im not ready yet to build it correctly. I need to get familiar with some power/speed experiments first. Im sorry to those that responded that Im not building it, but I hope this is temporary !
|Mar 03, 2011, 12:58 PM|
For general slow flight 50 watts / lb is fine, for sportier flying 75-100 w/lb, and for 3d or powerful aerobatic performance 125-150w /lb is at least required (so you can point it up and climb vertically ).
I can only estimate at what you have there but given a 50 watt motor at peak power, I would say for satisfactory flying keep your weight below 8-10 oz TOTAL (including battery). That means you'll most likely be using a 2s-3s 400-600 mah lipo around 1-2 oz at most, with the plane and motor/electronics taking up the rest of the weight < 7-8 oz.
If you make a flying wing, that should be very easy to do, as all the weight is the wing itself and no fuselage or tail etc., to weigh it down with. 30-40" wing span should be doable.
I am going by the fact that I have also made a well flying delta wing plane, around 28", using a 20 gram 2200 kv motor and 3s 600mah lipo, 5" prop, with AUW around 8-9 Oz. The motor put out about 50 watts as well. While the plane is no 3D machine, it will fly and putt around on a calm day just fine, and even perform loops and lazy rolls.
|Mar 03, 2011, 01:47 PM|
That's good news ! I have a 2 cell 850 mAh battery (50 gr ~= 1.5 oz) and an AUW (estimated) of 250 gr (=8.8 oz). Wingspan is 80 cm ~= 32". Well at least in theory Im good
Thanks for the valuable power/weight ratios!
|Mar 03, 2011, 07:17 PM|
My calculations say I am pulling 8amps for 12 minutes constant power and is on 3s so about 100w and I am getting a 54" span 40oz wing to climb well perform lazy rolls and fast loops from higher altitudes dropping in.. so that's under 50w per lb but I think the wing loading is a very important factor and Im@ 5oz per sq ft. So will fly in place in 7mph winds pretty much. With full throttle takeoff@ 180w its airborne in 5ft distance.. so prob would takeoff at half throttle but that's just a theory and is yet to be proven..
Your prop will be determined by your power system, your motor, the voltage, the esc all play a factor, a simple prop calculator will help, but at those light sub 16oz weights an 8x4 I def think will work if your motor can turn it.
I also think it is worth saying that there are more factors involved in flight than power plant, and while that is obvious, I'm just trying to relate the fact that lift is another huge part of it besides the power input.. It's not too hard to strap a motor to a piece of foam with a 1:1 thrust ratio and get it airborne, regardless of piloting skills it will prob takeoff and at least fly for a short time... It's much harder to take a 150hp engine and design a plane that will weigh 2500lbs or more and have a 1/2 thrust ratio and still have great performance... It takes a little more effort in the design stage but is totally doable.
So the lift part; I used a very long chord length in relation to wingspan for big wing area because the angle of attack doesn't have to be very steep nor do you need much speed to generate tons of lift, which means it is friendly for a low power drive system.. Think in terms of vertical performance for an extreme case, the closer and closer you bring your angle of attack to vertical, the more and more power it will require to remain airborne and with larger wing area you have a slower sink rate, or a higher tendency to "float" on top of the cushion of air you're flying on. Also, a shorter wingspan with a longer chord will have an inherent strength advantage, meaning you can build lighter if you want. Also you can have a higher flying weight as it will take less airspeed to generate the needed lift to keep you airborne when you have low wing loading, which I think is almost the #1 factor of flying, I mean you could certainly design a wing that would float along at 3mph with a 30w power system if you wanted to, and it would be based upon your total wing loading, a light loading means you can almost float in place at a walking pace, that has nothing to do with your total weight alone, but is a relation of your total weight AND wing area, so while there are certainly rules of thumb that are based on vertical performance, the basic physics of it should make enough sense that you could build a very large wing if you wanted and fly it with a tiny power system which would be called I guess a "glider", which is what i think they basically do. With a fully built up airfoil you will also have a large speed range, from crawling slow to pretty fast also, so you can always do your aerobatics in a dive from high altitude or whatever floats your boat.. Just saying you don't have to follow every suggestion you hear to reach the desired outcome if you understand the basic concept.. I think this is easily proven with the fact that real planes use rather long runways to build speed slowly in a lot of general cases, as they just don't have the power to jump right off the ground. They're budget constraints are big $$, ours are "do I get the $12 motor or the $18 motor" sorta thing...
All that means a slow flying wing that will climb easily with not much power and with a good airfoil it'll be efficient, and wings are great at aerobatics too, so sort of the best of both worlds with not much power. I guess eliminating the fuselage is a big factor on the drag part too and you end up with a much more efficient design with a wing.
My 54" wing thread will kind of explain the way I built mine if it's of any interest to you.
|Mar 10, 2011, 02:32 PM|
Sorry for the delay to reply.
Great thread, fpv and stuff !
I have already built mine, its made of Depron and BlueCore and the wing is flat. Unfort. the wind is very strong so no maiden yet.
Well my problem is that I had, as you said, a cheap power system lying around and wanted to build some airplane to utitlize this system. I know this is the opposite way to build things but heck this little motor lying there haunts me ever since I crashed a small foamie back in 2006. HAD to be used in something
I have a 8x6 prop which revs at 5000 at that voltage (at least this is what the motor manual says) so this gives about 28-29 mph, which should be enough I suppose.
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