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Multi-Rotors Discuss your quad copters and other multi engined choppers here.

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Old 05-15-2014, 01:25 AM   #1
zoomy
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Default KK2 Quad unstable after prop size change

I have a Turnigy HAL frame and components I put together from HobbyKing. I followed a youtube recipe for the parts, and it flew impressively well from the word go. However, after numerous crashes, I ran out of spare props. I wanted colored props for orientation reasons, and all I could find were sets in 10x4.5 size, whereas my originals were 8x6.

Excited after mounting my new props, I test flew and I found the machine goes into an oscillation right away until I lose control. With the old props, it hovered in a very nice stable fashion. I noticed if I accelerate vertically, it become stable during the acceleration phase, but then when held to hover, it begins oscillating again.

Is it possible to reprogram the KK2 controller to compensate for the larger props I wonder? I can't find anything in the manual that describes a procedure for prop size. Thanks.
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Old 05-15-2014, 02:41 AM   #2
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Spacing between props is probably the issue. You have cut the spacing by 2 inches. That allows the props' tip vortices to interact more.

It may or may not be able to work when flying at speed as that would change the interaction.

I think you need to either find a set of the 8X6 props (or close replacements) or space the motors further apart.
Another potential fix is ring shrouds around the props. 50-50 on that working.

I don't think you'll fix it with programming.

Careful spray painting and rebalancing cures the color issue.
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Old 05-15-2014, 02:50 AM   #3
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It appears that you've "bumped" the prop sizes up a bit to much.......prop size (like most RC craft) is dictated more by the motors/ESC/batteries ability (spec's) to deliver enough power to the rotors........I seriously doubt that your KK2 controller has any compensation or the ability to recognize prop size.....and you are taking a chance of frying one (or more) of the ESC's if you continue to run the nearly 1.5 greater size props than recommended by the manufacture..... (you did not state the size and weight of your multi rotor or number of motors your'e running).....!

Distance between rotors can cause a problem on smaller/light weight 4 rotor craft......but, anyone who has actually been enaged in multi rotor flight/repair/build....will notice that the spacing for hex, six and eight blade craft.....varies considerably due to length/width of the frame/motor apertures, blade length, motor shaft height/weight and control board protocol.....all strictly determined by the "engineering" design provided by each manufacture.

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Old 05-15-2014, 03:03 AM   #4
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He traded off pitch for dia... probably not too bad of a load increase (if any) on the motors & ESCs at hover power. The lower pitch, larger dia props would be more efficient for hover. They'll be turning considerably lower RPM to get it off the ground.

It might even be lifting off at a lower current demand.
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Old 05-15-2014, 03:08 AM   #5
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I would just reduce the gain (P) on the gyro (roll and pitch) and if you are using self level turn the gain down on that also. The gain setting is causing the wobble.

Turn the gain down, take off and land if it still wobbles. Repeat. The thrust difference between the props changes the needed gain. (Also the disc size, moment of inertia,and other physics stuff)

A good rule of thumb is for your multi to hover at half throttle. too much prop will have it climbing strong at half throttle.

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Old 05-15-2014, 03:13 AM   #6
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That's a smart start Flubber......although I have seen an increase in blade diameter cause some issues with full rotation and yaw on quads smaller than 450 size.......and the blades must be perfectly balanced all around....the slightest deviation will cause instability!

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Old 05-15-2014, 05:36 AM   #7
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Originally Posted by Flubber View Post
I would just reduce the gain (P) on the gyro (roll and pitch) and if you are using self level turn the gain down on that also. The gain setting is causing the wobble.
+1 Usually some tweaking of these variables is required to get your multi rotor flying properly. I'd be surprised if you couldn't tune out the oscillation.

I'd also be concerned that your motors might be pulling too many amps with the bigger props. Power goes up to the power of four of the linear diameter increase (meaning that going from 8" to 10" doubles power required), so even though you have reduced pitch the new props will still most likely be putting a lot more load on the motors. You should put a wattmeter on one of the motors and check amps are within ESC and motor limits when at full throttle.

As noted above, balance is also important, and vibration might effect the gyros in the controller.
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