To bring some discussion off of twitter.
Frist, Dave, good on actually getting the airframe in. :-) I can't wait to see what you do with it.
Second, lets discuss N-rotor versus penny farthing or even coaxial helicopters. I'm just going to call them quad rotors instead of N-rotors for now. The only reason for more rotors is for more power, and if you've got good software, you might even see redundancy.
Here's how I see it. Quad rotors were made because, more or less, they could. The concept was tried at the start of the helicopters development, and discarded. It was even revisited again in the 60's and discarded. (
http://en.wikipedia.org/wiki/Piasecki_PA-97 )
I am going to start with what I see as advantages with Quad Rotors. First, quad rotors have only four moving parts. They usually have brushless so your prop is directly attached to the outrunner frame. They have no pivot points. They are pretty simple. A frame could be as simple as two paint stirrers.
I see simple as good. Very good. But simple to control, I think trumps that. We'll get back to that
Now quadrotors are completely unflyable. At least by people. They are utterly dependent on a multi axis gyro, and a fast acting microprocessor to stay in flight. The "stability" comes entirely from the fact you've got a microprocessor or four going nutty trying to keep the thing airbore. In todays electronic world, it's not a crippling thing. But you also are completely out of luck if anything goes wrong. I know of a couple people who have issues with the outrunner style motors losing sync. When a brushless motor unstarts, the CPU doesn't know about it. Then the helicopter crashes.
Okey, so we know we have mostly reliable electronics. And sensored motors fixes the unstart issues. Woo! What about the airframe itself, and power-train choices. To talk about that we'll need to discuss the comparison between the aircraft.
A quadrotor has four, high speed propellers. This is not a very efficient method of moving air. You've got tiny little wings, moving very quickly. Each propeller tip making it's own little wake and wasting energy. Shorter propellers have more losses due to vorticies, and I can't imagine the interaction between the propellers does it any good either. The "thump thump thump" you hear from a real helicopter is the interaction between the rotor wash and the tail rotor wash!
With a quad rotor, you're blowing on the frame with every propeller. And you've got the frame up close behind the rotor, which doesn't help the aerodynamics of the blade itself. Also the motors are fairly large, causing large dead space at the center of each rotor. (it's the least effective part of the prop, but it's worth mentioning) You also need to consider the downwash of the whole plane is going to interact with the chassis in the middle. As you can see with the dyson air multipliers, the effects of airflow aren't very well constrained to the path of the prop. You've got the huge, unfaired, chassis in the way of your downwash.
So that's a problem that can be taken care of with horsepower. Horsepower means bigger motors, bigger batteries, bigger speed controls. Big horsepower means fast response too. ;-) So there are some upsides to flying with the "big stick" method.
A penny farthing (read: conventional) helicopter, while still a brutal machine, is much less a "club the air into submission" vehicle. Now, the rotor head of a helicopter is pretty complex. At minimum you need hinges on the blades, and a way of controlling pitch and roll. You'll also want collective. (provided you want the full aerobatic capabilities of a quadrotor...) Which is a third servo.
Now, here's a video from six years ago:
That was state of the art then. That heli would carry anything you'd ask it to. And there are lots of electric models that are just as capable now. The advantage i see with using servos, is that they self recover, and require little power to operate. While a quadrotor needs lots of surge power available to make it's rotor speed adjustments. Servos also tend to fail in a progressive manner. They start to glitch long before they completely fail. The heli will still fly with glitching servos! (this is experience talking *grins*)
So the helicopter can be flown by a human, with human response times. And can fly in amazing directions, I'll even venture to say it's agile enough to keep up with those silly princeton quadrotors. Because they're somewhat stable they also make decent camera platforms. Even without computers handling everything.
Conventional helicopters are also available in a wide range of sizes to accommodate almost any payload. Way back when, it was common for film crews to have a man on call with a .90 size heli so they could fly cameras into unusual places. If you watch old tv shows on special effects, you'll see someone with a r/c heli.
http://www.diyphotography.net/introduction-to-radio-controlled-helicopter-aerial-photography here's a fairly recent article on the subject. Evidently an eflite 400 is capable of carrying a significant camera aloft. (I own an eflite MCX, a Novus FP, and some larger 280 sized chinese heli)
Conventional helicopters also only have two rotors to mess about with. The main rotor has long blades with high aspect ratios providing smaller tip vorticies. You also have only one big motor to power, which means a savings in speed control weight. On most medium to large helicopters the tail rotor is powered off the main rotor, meaning that the tail rotor cant' lose control so long as the gearbox stays intact.
The bodies on conventional helicopters are narrow, and usually have a fairing, providing an aerodynamic advantage over quadrotors. The tails are frequently smaller than the support struts on a quadrotor, and are tubular, so should have less drag as well. This means you can carry a greater payload, and use a smaller powerplant to do it. Also it means a given battery will take you further.
I suppose I could have taken this rant down to "well in the 80's they did it with conventional helis, the quadrotors are a new thing" and been done with it. :-)
I will say, quadrotors are neat.
Now, for flying down a canyon, I think a single rotor has a big advantage. You only need one frame to protect the rotor from the canyon walls, and typically the rotor blades are hinged, so brushing against something may not be a critical failure. (I do that with my helis all the time...)
I can't wait to see how your project turns out dave!
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