Howdy,
My name is Justin and I wanted to explore the idea of using different motors, other then your standard sizes, in helicopters for fpv and surveying. My thought is that in general traditional helis are intended to be serious 3D machines and are very over powered if your just looking to fly around. So would using a different motor with say a shorter can work? I haven't really looked around at options on the net but I think a smaller more compact motor my increase efficiency and therefore flight times in our helicopters.
This idea came from looking at threads on here, helifreak, and rc groups where they are using asymmetrical blades with a very reduced head speed, about 1900rpm. If the head speed is reduced considerably why do we need big powerful motors?
The bird I intend to build is a "480" size streched 450 running on 6s with 350mm spin blades and a direct drive tail. This will not be a 3D machine, nor do I want to build it so it is capable of 3D, just strictly fpv and perhaps some 3D mapping in the future.
Replies
The problem with the asymmetric blades from Spin blades is that the shorter ones have a very wide chord. Aspect ratios in the from 8-9. The asym blades in 800mm lengths have an AR=12.3 but they spin counter clockwise. What kind of modifications would be needed to spin my blades counter clockwise, swap wires and flip a one-way bearing? Swash?
In symmetrical blades they are wider than comparable Align blades (except for 800mm).
I I think I asked century for a semi-symmetrical 800mm blade that spins clockwise and they offered to cut a blade down for me but the chord was pretty wide 70mm or more.
I'm still looking.
Ray, I just discovered this a few days ago. Check out the Spinblade Scale 4-blade sets. They are asymmetric (still not sure if this is good or bad), but are much narrower than the 2-blade sets. So you buy a set of 4, and get 2 2-blade sets of high-aspect ratio. :)
I haven't tried yet, but it's an option. There's some concern about root shear on heavy helis. Basically, can the narrow blades take it. I bet it will be fine, but be careful.
To run CCW blades, I don't think would take much work. Yes, get the motor spinning backwards, and then flip the OWB. Also look at which way the tail blade spins, this is fixable too. I think the swash should be OK, worst case the phasing will be a little out, but you can fix that easily with the H_PHANG parameter.
Hi Justin,
I've also been thinking about this motor power set up idea as well. My set up at the moment is a 1800kv motor for 6s, but running on 3s with 2000 head speed. I'm thinking about a 1000kv motor running at 6s, but it seems the 2212 1000kv motors for quadcopter are not capable, it will probably overheat. 28xx might worth a shot.
There's a lot of merit to this discussion. I think that it would definitely be possible to fly with a smaller motor, and gain some efficiency. I have thought of it myself but didn't try it yet. There's a bit of an issue getting pinions to fit the smaller motor, that would be the biggest thing.
It would also help to have some logic in the flight controller that would limit the climb rate to something manageable for the motor, because even though we don't fly using anywhere near full power most of the time, you will see occaisional peaks sometimes.
However, the real gains in flight time are not in the motor, but rather in the rotor system. You've mentioned rotor speed, and that's a great place to start. But also the exist rotors on the market are far from ideal. And my latest testing and thinking seems to show that the Asymmetric blades are not all they're cracked up to be. They are capable of creating very high lift at reduced speed, but I do not think they are actually efficient. What we need is actually a much narrower, symmetric blade.
How about these from spinblade? http://spinblades.com/en/products/scale-4-blade-system/4-bladesyste...
They're the company's "scale" blades and look to be a lot narrower then the usual asymmetrical sets, and they are about the same price as buying two sets.
As far as pinions to fit the smaller motors I was considering turning down some appropriate shafts for the motors, or even rewinding motors. I've been trying to use that ecalc to estimate power systems, with out much success, that would fit in the standard 450 frame. Then I saw the "vwinrc" 450 pro frame on another thread with the top mounted motor, and think that frame might be the ticket to more options because there is quite a bit of extra space to move a motor around. At this rate my heli is goingito be a real Frankenstein by the time its flying.
Yes, actually, I was thinking of those as well. It might be worth a shot.
But there are other problems with these blades:
They have a nasty ridge on the leading edge. I believe this is from the manufacturing process. This likely destroys any chance of laminar flow, which would cause a lot of drag.
The cambered profile will also likely have a sharp Cl/Cd profile. What this means is that, while the peak Lift/Drag of the blade may be better than a common NACA 0012 profile symmetric blade, that only occurs at a narrow range of Angle-of-Attack. At all other AoA, the NACA 0012 will be better. And with helicopters, very little of the length of the blade will actually be operating at the perfect AoA.
It's an extremely complex situation. At the end of the day, assuming you're not planning on making your own blades, the only thing you can do it buy different candidates, and test them. The narrow asymmetric blades are worth a shot.