Replies to This Discussion

Permalink Reply by Haldir on August 23, 2010 at 11:08am

Hexa/Octacopter:
+ more payload
+ one/two motors can fail without a fatal crash
- more expensive
- shorter flight time

Special disadvantage for the coaxial copters: more dangerous for fingers because of the props below the frame

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Permalink Reply by Martin Seven on August 23, 2010 at 11:23am

>there's not really much interest in determining which is the best design (deppending on the use) on the net.

So not true :o) there's constant research in this field going on all over the net (usually known as "flamewars"). Here's my pick:

General rules
More engines means more power! and more lift. That means more batteries. That means more time in the air. Brushless electrics like to run slow (lower RPMs), so bigger means more efficient. Also, if efficiency is your goal, coaxial = evil.

The breakdown
Tricopters: cheap, easy to build, least stable, not as robust (tail servo and mechanics), low lifting power and flight time (because the motors have to run faster to hold it all in the air). No engine out capability.
Quadcopters: mechanically simpler than tris. While they weigh almost the same they have about 1/3 more lift, they are usually more stable (no servo issues) and are capable of staying airborne for a little while longer (they can either lift larger batteries or fly more economically because the weight is spread across 4 motors and not just 3). Still no engine out capability. If it fails, it goes down.
Hexacopters: All the good things that quads have, plus more power and more lifting capability. As a bonus they add limited engine out capability - a hexacopter can lose any single engine and still land (it will lose yaw control though), and if it loses one or both engines on the neutral torque bar it could even continue flying unaffected. Downside is that they are larger and a little pricier, especially if you're running high-grade motors like AXI.
Octocopters and heavier: All the good things from hexacopters, plus true engine out ability. Loses any single one and still flies fine. This is what you fly if you need horsepower and reliability in one package. This is what you strap that $1300 Canon 7D under :)) Even more expensive though. Also heavy craft are really power hungry and unless you have some serious chargers at hand they require a lot of work on the ground before a flight can be made (charging say 5 packs for 25 minutes in the air).

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Permalink Reply by Andy Gongora on August 23, 2010 at 2:31pm

Quote Haldir > Hexa/Octacopter: shorter flight time
Quote Martin Seven > More engines means more power! and more lift. That means more batteries. That means more time in the air.

So now I'm getting a bit confused. @ Haldir: did you mean with the same batteries??
Speaking about batteries. Is the relation wattts/wieght constant? Or how does it evolve?


> >there's not really much interest in determining which is the best design (deppending on the use) on the net.
So not true :o) there's constant research in this field going on all over the net (usually known as "flamewars"). Here's my pick:

Martin, your breakdown is the first post I've read on the net which is so clear telling the differences. I've been searching for 2 days because I was sure there had to be a site where they are compared. This breakdown should be added to the FAQ :P


So, more motors means better in all aspects but costs increase.
How does an increaed number of rotors affect velocity and maneuverability??

Thanks to both of you =)

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Permalink Reply by Martin Seven on August 23, 2010 at 10:32pm

The energy density (watt-hours per kg) is dependent on the technology used. Right now the best (or almost the best) is lithium-polymer, or lipo, so that's why we're all using it. It gets better with time as the technology progresses.

Re more motors and velocity/maneuverability - that depends. It depends on the weight-to-lift ratio. If your machine weighs say 2 kg and each of your 4 motors gives out 625 g of thrust (4*.625 = 2.5 kg), then you have a ratio of 2.5/2 = 1.25, which is not really good. However, if your quad weighs 2 kg and each motor maxes out at 1 kg of thrust (4 kg total), then you're looking at a lift-to-weight ratio of 4:1, which means plenty of power and speed for acrobatics.

If you have more motors you have to consider the battery ratings: a 5000 mAh 20C battery has a max current of 5*20 = 100 A. If you have eight motors and each can draw up to 17 A (136 A total), then you either need to use a stronger battery (or more of them in parallel) or limit the engines in software using a current monitor.

Re flight time - a hexa will be heavier and therefore draw more amps to stay in the air. It will however be more efficient, i.e. it will use less watts to lift one kg, so it could afford to lug a heavier battery and in effect stay in the air longer. However with the same battery as a quad it would fly a shorter amount of time.

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Permalink Reply by Haldir on August 23, 2010 at 11:11pm

Yes, my less flight time is based on the assumptions of same/similar batteries.

Keep in mind that there are some legal limits to the weight of such a copter in certain countries. Some countries have limits of 1kg (like flying without insurance, flying in urban areas etc.), 5kg is another limit.

As Martin said, if your goal is aerial photography you'll will certainly look at hexa/octocopters, for just-for-fun flights in urban areas you might want to look at an ultra-light quadcopter.

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Permalink Reply by James Pistorino on August 23, 2010 at 5:18pm

Help me understand why you say that a single motor out on a quadcopter means that it is going down. Do you mean down in an uncontrolled way, or just down as opposed to sustained flight?

On motor failure, why couldn't you just shut down the opposite motor so that the torque is cancelled out? That would leave you with two opposed counter-rotating props, which, depending on their size, etc. should allow for either controlled descent or sustained flight.

Thanks

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Permalink Reply by Alex Zarenin on August 23, 2010 at 6:30pm

Quads (as far as I understand) have two motors on one cross-beam rotating in one direction while two other motors on other cross-beam counter-rotate. This way you can compensate for yaw without changing the overall lift by increasing lift on both motors on one cross-beam and similarly reducing lift on the other two motors.

If two motors on one cross-beam counter-rotate it is still possible to compensate for yaw rotation, but it will also affect either roll or pitch (whatever is going along this cross-beam). While it is theoretically possible to stabilize quad with motors on each cross-beam counter-rotating, it will make control algorithm much more complicated versus the case when counter-rotating is provided by motors on different cross-beams.

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Permalink Reply by Martin Seven on August 23, 2010 at 10:36pm

Each beam (two opposite arms) are co-rotating, meaning their torque adds up, not cancels out.

If one motor fails, the MCU would have to kill the motor opposite to the failing one, otherwise it would flip over. So now the quad has only two good motors and no way to stabilize itself. If the CG was below the prop plane then it could stabilize itself, to a certain degree, naturally, however the torque of the two running engines would spin it quickly out of control. So yeah, in an uncontrolled way.

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Permalink Reply by Peter Seddon on August 24, 2010 at 3:31am

Ok here is some real data from a Tricopter and Quad just as they get flying (there will be some ground effect help as I couldn't lift off and clear the ground:

Tri:
Ready to fly weight 825g
Lift off power 135W (at 10.1A from 3S battery)
42cms motor centres, CF2822 motors swinging 8 x 4.5 props

So 45W per motor and 163W/kg to fly
Using a 2300mAH battery flight time would be of the order of 13minutes

Quad:
Ready to Fly Weight 1050g
Lift off power 93W (at 7.5A from 3S battery)
28cms motor centres, Xtra 2829/13 motors swinging 10 x 4.5 props

So 23W per motor and 88W/Kg
Using a 2300mAH battery flight time would be of the order of 18minutes

I started an Okto and it is currently running as a Quad but not stable enough to take some actual measurements. It weighs in at 1.3Kg as a quad and should weigh 1.75Kg as an Okto. It is fitted with 10 x 4.7 props and KDA20-22L motors. Anyone care to tell me the power it will consume and current on a 3S battery.

Are these measurements comparable - they have to be treated with caution as the motors are different and the props are different so the thrust per amp will not be the same?

Peter

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ModeratorPermalink Reply by Gary Mortimer on August 24, 2010 at 3:46am

You've forgotten the best one, the V like the Falcon 8 from Astec

Advantage

You can look UP

For commercial work quads are so yesterday ;-) The charging point is valid, we only use one battery at a time in the Falcon but still have a charger that charges four at a time as you always need to have some handy for in case. The jobs not over till you've got the shot.

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Permalink Reply by Andy Gongora on August 24, 2010 at 4:28am

You've forgotten the best one, the V like the Falcon 8 from Astec

I'm not sure if you mean this one

Yes, you are right. Maybe someone is looking for the diferences between radial and lineal airframe desings. I'll add this one to the main post aswell. (Is it called lineal? :S )

I'd really like that in the end, putting in one single post all the information, to create between all users a general guide about the different airframes. (Although I now it's difficult to generalize when everything depends on the used components)

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ModeratorPermalink Reply by Gary Mortimer on August 24, 2010 at 5:05am

Yes thats the chap, there is also the T out there as well.

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