No, seriously, I have been waiting for someone to ask just such a question so we could walk through the math. There's a caveat, though, that theory will only take you so far. If you've set out to design a new aircraft, you'd best have the budget for some failed parts and the time to experiment.
First of all, I assume you don't want to build your own propellers. Since 30 lbs is rather large, you'd want to pick some larger props. There is a compromise here, because the larger the diameter of the props, the more efficient it will be (which translates to longer flight durations). Unfortunately, larger props have more rotational inertia, which translates directly into more sluggish control responses. As a starting point, we could chose 24"
I am also assuming that you say 30 lbs of thrust because that will be the gross weight target of the aircraft. Let's add at least 30% to that for control headroom and round it all off to 40 lbs total just for convenience. That means each 24" prop must produce 10 lbs of thrust.
Using momentum theory as a starting point, that means a disk area of 3.14 square feet. The formula for induced velocity is the square root of (thrust in pounds divided by (2 times the density of air times the disk area)) or sqrt (10/2 * 0.00238 * 3.14). The answer is 25.87 feet/second ^-1. The ideal power at the rotor disk is the original thrust (10) times the induced velocity or 258.7 pound feet/sec ^-1. Simply divide this number by 550 to get an ideal power of 0.47 horsepower. To convert to watts, 746 * 0.47 = 351 watts. But this is the ideal power, assuming the propeller is 100% efficient, which of course, no propeller is.
Based on testing, we know that model airplane propellers are somewhere between 25 and 50 percent efficient. Assuming worst case (and allowing for some margin), then your motor must be capable of 351/.25 or 1,404 watts at the shaft. Since it can be assumed that a brushless DC motor with controller is about 80% efficient, you'll need to supply 1,755 watts of power from your battery. Under load, a lithium polymer pack usually sustains about 3.4 volts per cell, so that's a 6 cell pack and 86 amps approximately. You would do well to upsize the ESC and motor a bit, because VTOL aircraft stay at nearly full throttle for much longer durations than airplanes do.
Now you have some shopping to do. :-)
The tricky part here, of course, is the KV rating on the motor. Ideally, the full maximum battery voltage times the KV rating should not exceed the maximum recommended RPM of the propeller. So, 6 X 4.2 X KV /=/ > RPM Max. Also, besides obviously needing a pair each of matched regular and counter-rotating props, you need to find them with a relatively low pitch, lest the static thrust conditions (no inflow) exceed the propeller section stall angle. Perhaps something along the lines of a 24 X 8 through 24 X 12 ought to do the trick. You might want to get a static thrust RPM recommendation from the prop manufacturer and back into your motor KV spec that way.
Keep us posted!
Holy cow! :)
Spot on - 30lbs GVW. At first I was thinking exactly the same: something in the range of 20"x8", 200W/lbs. So I went around and found a sweet motor/prop/bat calculator which seems to be very accurate. To check its accuracy, I took this guy bench test as reference - 19"x8" with 17lbs of thrust @ 7500rpm.
The problem is that I can not find such propeller with cw/ccw config! The best scenario today is a 13"x6.5" with a 16.000rpm maximum. It turns out from the calculator with a 5.5lbs of thrust @ 7.000rpm and a whopping 8lbs @ 10.000rpm - a too good to be true figure for me. Still just right on, no headroom.
I would feel a little bit more confortable investing in a slightly bigger prop/motor configuration.
Yep, you're pioneering uncharted territory, at least from a commercial product standpoint. However, those German guys with the flying pilates ball got their props from somewhere...
Precise! Would love to know where they got their props from...
there probably Zoar props.
sir, how to we calculate this data?
i'm trying to do a conceptual design of multicopter... amazed looking at this calculation :)
please tell me what to refer..
There's a difference between knowledge and understanding.
Calculators are good if you want knowledge. For most people, "just give me the answer" works.
However, if you're actually designing, you should understand what you're doing. Assuming you're familiar with basic physics, this should be a good primer:
If you have further questions, ask away.
thank you so much! :)
this is my mini-project that i'm working on..
anyway, my goal is to design this stuff with full understanding... i'll be following the discussions...
thank you again.
Another direction is to increase the number of propellers. Why not go with an Octo or even with 16 props? Less prop momentum issues, more reliabililty etc.
As Brad explains in his early posts here, if you go with 8 or 16 props, it will be difficult to achieve a proper disk loading while maintaining the wing span reasonable. Despite this, I dont see any why not... ;)
Regarding big, I have some practical numbers for a 24" setup I did on giant scale R/C airplane.
Setup: Turnigy 80-100 180kv motor, Jeti Spin 200A ESC, 12S Lipo and APC-E 24x12 propeller.
Result: 6200W, 37lbs static thrust at 6500rpm.