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Permalink Reply by Denny Rowland on March 10, 2011 at 9:55am

I usually start testing with a simple board like the KK and then when I get the prop/motor combination sorted I start working with some modified MK boards that I have. But that is now about to change as I'm definitely going up the Ardu. route so that I have more control over the final outcome and can learn a lot more from others more knowledgeable.

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Permalink Reply by Dean Lamborn on March 10, 2011 at 10:01am

As long as you are using a standard ESC it doesn't matter what the size of the vehicle might be.

The reaction to inputs is a function of the PID tuning. Fast response is a function of how much power you have in reserve. If you are committing most of your power to lifting you don't have a lot of reserve for maneuvering.

 

As with anything that lifts you run into a weight spiral. "I need bigger batteries, and they weigh more so I need more lift so I need bigger batteries . . ." You will have to pick something that you think will be in the range that you need and then calculate total lift and then start subtracting all of the things that support that lift. Nothing but fun.

 

The 8 rotor in the second picture by Denny could be flown as a 4 rotor for maneuvering with the other 4 in a throttle only mode for lift.

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ModeratorPermalink Reply by Gary Mortimer on March 10, 2011 at 10:10am

@Denny are the motors upside down to reduce the load on the bearings?

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Permalink Reply by Denny Rowland on March 10, 2011 at 10:44am

It was done to deduce aerodynamic drag on the Arms. But the best way to mount you motors is like this. The less prop. overhang from the base mount the less the vibration.

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ModeratorPermalink Reply by Gary Mortimer on March 10, 2011 at 11:00am

Yeah I have started doing something similar for the same reason!

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Permalink Reply by William Bishop on March 10, 2011 at 10:26am

Please give recommendations and if I am way off just buttstroke me and after 50 pushups I will get up and start over.  The motor here is for "airplanes", will it work for a quadrotor or am I barking up the wrong tree?

Questions:

1. Would this motor that boasts 20+ lb of thrust/lift capability, four of them @ 80 lbs, working a 30 lb load, have a reserve sufficient for maneuvering?
2. What kind of propeller configuration would be ideal?  18 - 20 inches? Pusher, Slowflyer, three blade?

 

AXI Gold 5345/14 Outrunner Motor

31-1/2 oz. weight, 2-7/16" diameter, 3-1/8" case length, 8mm shaft diameter. Very high torque motors for sport models up to 420 oz. (20+ pounds!), sailplanes up to 780 oz. (48 pounds!), and F3A competition models.

Kv (RPM/V)	Prop	Battery	Controller	Airplane	Type
225	22x10 21x14	18-12 LiPo	200 Amp Spin Opto	Big 3D to 23lbs. Sport to 26lbs.	1.20-1.60 3D, Sport/Scale

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Permalink Reply by Denny Rowland on March 10, 2011 at 11:20am

When it comes to evaluating motor prop efficiency it is pretty much a case of trial and error as the numbers given out by some manufacturers are to be taken with a large pinch of salt. However as a guideline to what your looking for consider this. Maximum lift efficiency is about the airspeed that you will be flying at i.e. if you are flying at 120 mph you need to absorb all of your available power into the maximum dia. that will provide at least that speed. In our case that speed is almost zero so we need to absorb our available power into the largest dia prop with the least amount of pitch. i.e. not a toothpick small dia. with a high pitch, but a large dia. slowflight wide section low aspect ratio prop. with a very small pitch angle. As props start to get very large the tip losses which act with a high leverage from the hub and become a significant drag factor so it becomes a trade off. I made this simple test rig dyno. to check the lift/watt performance of various props. From that data you can see on the MK site a simple formula for calculating battery endurance against the total current drawn.

This MK 3538 could lift 2.5 kilos at max 14.8 v. power which was almost 500 watts with a 14x4.7, clearly at that level of stress it would not last very long. It could however run all day at about 13-1400 grams of lift.

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Permalink Reply by William Bishop on March 10, 2011 at 3:15pm

30 - 35 lbs total is my goal.  I can't find info on prop sizes, motor sizing, RPMs, etc for hovering and maneuvering/flying.

Hate to whine and beg but before I drop $ 5 - 7K into the project, I want to make sure that I will be successful before starting. 

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Permalink Reply by Denny Rowland on March 11, 2011 at 12:02am

The best advice I can give you is to study the lift watt sheets available on the MK site for their MK3538 motor and see how many watts you need to lift 1500 grams comfortably then multiply that figure to get roughly 10 lbs or 4.5 kgs. Seems like about 600 watts to me. I would also use a 6s battery at least.

Assuming the same efficiency you need to find a good quality motor in that range that has nearly the same Kv.

Plettenberg is a company that make the best motors out there but expensive. typically over 90% efficient. As I do not have any experience with motors of that size I can't really help you beyond that. Good luck and keep us informed of progress. You could also consider a geared drive like that used on the AR drone.

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Permalink Reply by Denny Rowland on March 11, 2011 at 12:35am

 

I hope you enjoy tinkering. When I decided to get into muli-rotor platforms, I, like many, bought an MK Oktokopter and assumed that it would work reliably. Like most other people I found that it was full of problems and did not reliably fulfill my expectations. I would go as far to say that it was not of merchantable quality.  A study of the forums will show that this a very steep ongoing learning curve. Three years down the line I am still working to try and perfect that dream and take it to the next level. What you are attempting is not going to be straight forward. Theoretically yes but in practice NO. If it was that easy, there would be dozens of them RTF at a cheap price. It's a bit like motor racing - development never stops and there's a lot of dead end roads.

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Permalink Reply by William Bishop on March 11, 2011 at 8:29am

Thank you Denny.  I am researching batteries today.  I have to go back to asscrackistan for 90 days the first week of April so when I return I will have all kinds of packages waiting for me to start the tinkering process.  I want to build an aluminum or composite frame and build vented 8S or 10S battery compartments in the X as close to the center of gravity as possible. 

Where can I learn about Motors so I can understand the relationship of Kv and wattage ratings and figure out optimal prop sizes for that motor?

 

I have seen posts recommending different size and pitch props for quadrotors + or X but have not seen any explanation of the reasoning or theory.

 

Thanks again you are a great help to this knuckledragger....

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Permalink Reply by William Bishop on March 11, 2011 at 8:35am

"I see said the blind man" 4 X 10S batteries would run you up to about 12 lbs.... 

Maybe I should turn back to a 50cc gas engine with front/rear shaft outputs into T gearboxes.....

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