More or less. Advantages of both.

[DucktileMedia]

It has been said that single rotor helis are more efficient due to the fact they have only 1 motor and ESC. It has also been mentioned by someone that their quad was way more stable than their octo. The reason to have more motors is usually redundancy and payload. But what if at the end of the day it was better to use fewer motors and esc's that were just the best you can buy with the most output possible? Surely a motor exists that can exert twice the force as two smaller comparable motors. fewer things is often less to go wrong, right? I kind of like the Y with 3 plettenbergs that could easily lift anything you can throw on it. Even if you had to swing a 24" prop, wouldnt that slower head rate be less vibrations? Or does that work the other way around?

Yes, a bunch of questions all wrapped up in one run on sentence. but I would very curious to know the tribe's thoughts on this matter.

 

[Bartman]

yuri,
i plan to try a hybrid. with my +8 configuration I plan to have the cardinal points contribute to lift and control pitch and roll like a quad while the corner four motors do lifting chores only.
i'm thinking maybe the combination of three motors per side speeding up while three on the other side slow down for every little control input may be contributing to poor AV quality. it's just an idea. not necessarily saying an 8-rotor is inherently poor for AV just looking to play with different ideas.
bart

 

[matwelli]

theres a limit on low motor count systems, as you need a fast response rate (acceleration/deceleration) in order to properly controll, and the more mass, the harder it is

I havent heard of many systems over 14" props

 

[DucktileMedia]

After posting this I thought of that factor as well. It would make sense. Oh well. then the question is, would a very reliable quad with monster motors be any different than an octo with the same lift capacity? I think a quad could still obtain enough lift using small enough high pitch props. I really dont know much about these things from experience. I am just thinking intuitively.

 

[matwelli]

I like Quads, but they are a compromise.

When Yawing it essentially is balancing on two opposite motors.

You can whip up some motor combos that will give you about 4kg lift, out of a 160 gram motor, so would have a 16kg capable quad

more rotors= less speed diferential between them to achieve a desired movement = smother control - hence why the hex's and 8s are more popular for stability, but are also affected by windy conditions

The Y6 and X8 seem to be the best all-rounders, albiet with a 10-12% efficiency loss due to the coaxial motors

thats my take on it anyway

 

[DucktileMedia]

Why is there that much efficiency loss with Coaxials? Is it that much better in a octo configuration?

 

[matwelli]

bottom prop is in dirty air - efficiency suffers

 

[Gary Mortimer]

There is also the height factor creeping in. You end up with tall platforms.

 

[jes1111]

Biggest MR I've seen is a quad - 17" or 19" props, can't remember - all rotating the same way - yaw control with rudder vanes under two motors.

View attachment 704

Yaw control seems to be the limiting issue with many variations/sizes/weights.

 

[RTRyder]

Biggest MR I've seen is a quad - 17" or 19" props, can't remember - all rotating the same way - yaw control with rudder vanes under two motors.

Yaw control seems to be the limiting issue with many variations/sizes/weights.

Now that's what I would call a Rube Goldberg quad! Complexity for the sake of making it complex, with moving rudders, the metal framework, four batteries, and all the other stuff on there that thing must be HEAVY but probably steady in a wind. I know from my heli experience that I'd rather be flying a 12 pound gasser than a Trex 450 on a windy day, but there comes a point where things get so big and unwieldy that its just not fun anymore.

My theory is simple and proven works, multi rotors today are where automobiles were probably 70 years ago in terms of development and we've got a long way to go before the ideal setup is finalized.

Ken

 

[Bartman]

if you've got an idea, build it and see what happens. i'd guess flat plates directly below the props aren't exactly tops in terms of limiting airframe vibration but maybe there's something else to be learned there.
i think when the evolutionary process really kicks in we'll see things diverge wildly from what we're considering Standard layouts today. Denny's projects are evidence of that.
jmho

 

[Macsgrafs]

4 x ducted fans or how about 4 x trex 450 heads, 1 common motor driving via torque tubes & then you can control the pitch!

 

[warthox]

It has been said that single rotor helis are more efficient due to the fact they have only 1 motor and ESC.

i think thats totally wrong. the efficiency is much worse due the high count of moving parts and the friction. every bearing or moving part costs u power.

 

[DucktileMedia]

That would be hard to argue with, agreed. But in respects to the efficiency loss from using multiple motors as to one motor was more my question.

 

[jes1111]

To lift a craft off the ground you need to "do work" - in this case, the "work" is "electrical work", i.e. Watts. A percentage is always a percentage - 20% power "loss" on one big motor is the same as 20% power loss on eight smaller ones. Nevertheless, I see what you're getting at, but I think the "efficient" part of a helicopter (in this context) is the large swept are of the rotors - lift produced is proportional to the disc area. A helicopter has a surprising low "disc loading" (ratio of disc size to AUW), therefore requiring less power to generate the necessary lift than a multirotor (which has a surprising high disc loading) .

(Sorry - not enough coffee today - hope you can follow my garble)

 

[jes1111]

4 x ducted fans

... like this one (with vectored thrust)

 

[DennyR]

Mat I ran a test on my motor dyno some while ago and coax. systems were way down on efficiency for a heavy lift machine. As I have never seen a reliable redundancy system for motor/ESC failure it comes down to a quad being the best option for efficiency. Better to buy quality stuff that wont break down. I was quite staggered at how much efficiency is lost by having the props above the arms. My last design was about 30% more efficient in terms of battery time due to a revised arm design. The low thrust line is amazing at lift-off. Hardly touching the throttle it is hovering in ground effect 2 feet off the ground it is drawing only 20 amps total. Thats not bad for a 4.5 kilo model carrying 2x5000 ma batteries and a camera system. It would be much less efficient out of the ground effect. 14x4.7 prop came out on top but I now have some three bladed home brew carbon ones that are a little better. 13x3.5 swept back paddles.
Having the center of aerodynamic lift on a plain that is close to the center of mass also helps efficiency. The taller your model the more pendulum effect and the harder the motors have to work to stay level after a lateral movement. I suppose one could call it the inertia multiplier syndrome. Sounds cool anyway.

 

[DennyR]

Ducted fans are a completely wrong solution due to the high losses incurred inside the duct. A modern jet engine looses about 80% of it's theoretical efficiency through internal air friction. Ducted fans only work to create a high pressure ratio.

That is not to say that a upstream bell mouth will hurt your lift efficiency. I't's performance gain is about the same as the weight incurred to create it.. The prop must be significantly smaller than it's bore diameter, as it needs to use the bernoulli effect to induce lower pressure along the top surface of the bell mouth and the coanda effect to increase mass flow. Nice safety feature though!!!

Upward propulsion is derived by accelerating a mass of air downwards. A viscous coupling drag is incurred and that movement of air by nature will produce an equal and opposite regenerative flow upwards. this cycle of movement will happen due to the atmospheric mass distribution of high pressure flowing towards low pressure. An open frame therefore does not capitalise on this effect.

 

[DucktileMedia]

http://www.zipzepp.com/2X-R.html

 

[Droider]

That looks far to easy! ;-)