So in my initial research I think I understand some basics insofar as prop size: larger props may be less noisy as they require less rotational speed to achieve the thrust necessary, however they may lead to a less stable craft as their higher inertia makes speed adjustments less snappy.Would it be possible to build out an 8 propeller copter, 4 blades being much larger and responsible for the majority of the usual thrust load, and 4 being much smaller and responsible for the majority of the balance and stability requirements. The idea would be that since the smaller blades would not need to generate large amounts of thrust, they would not need to spin up to the higher RPMs, but their lower inertia would still allow them to bring more stability to the platform.Is this possible? Would it require crazy mods to firmware? Could it achieve the design goal of a much quieter but still stable platform?
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A further issue with large props is gyroscopic precession, which will resist deflection. So adding your four extra smaller props will improve the maneuverability, but not as much as you might expect. Still worth experimenting with though, go for it.
There was actually a guy in India who had a large central gas engined coaxial set of rotor blades providing primary lift for the copter with 4 separate electric rotors providing roll pitch and yaw.
It actually worked, but I wouldn't have wanted to be anywhere around it.
Large rotors are much more efficient than smaller rotors and that is why they are desireable.
The more expensive large props are made with a balsa or foam or honeycomb core and Carbon Fiber covering so they are actually quite light and capable of responding quickly.
One of the problem's with quadcopters with large diameter propellers is that the propellers can turn so slowly that prop blade position can actually provide a pitch or roll offset to the copter.
And since the blades turn asynchronously you can get a periodic craft pitch imbalance caused by complementary and unfortunate alignment of blade pairs.
Usually this problem is worse if the copter is comparatively light, the heavier it gets this problem tends to dissipate as the blades are spinning faster to keep it in a hover.
However, it is probably optimal to simply design and tune the whole airframe for the intended weight as a quadcopter rather than to introduce further design complexity and inefficiency with small blade maneuvering rotors.
Could it be made to work, sure, would it provide a benefit versus other means, probably not.
If the 4 big props are all attempting to do the same thing (that is spin at the same speed all the time), then there really isn't any reason to have four of them. You could have a coaxial counter-rotating pair of really huge props/blades, with four blades around them for maneuvering. Of course, at that point you are getting close to figuring out a helicopter might be more efficient than a quad. You could also have two main rotors and 4 maneuvering props, without the need to make them coaxial. Two would be relatively easy to power via a gas engine, if you wanted to go for crazy flight times. The quad maneuvering portion would mean your main blades could be fixed, which I suppose decreases complexity a bit over a heli in terms of moving parts, but not by much.
You could always set up four large props to spin at a constant rate to lift maybe 80% of the weight all the time. Then you could use the smaller ones connected to an autopilot to do the maneuvering, provide stability, etc. I think the end result would be insignificantly different from several medium sized props at the right speed. Good props properly sized Amd balanced are really pretty quiet.
so you are just trying to cut down on noise and you want larger props...The noise from a prop is due to the pitch of the prop slapping the air. To cut down on vibrations from the larger props? Balance them....
From what I understand is that small props are able to spin faster and therefore provide faster response to what you input. Think acrobatic platforms...
A larger prop spinning slower gives you more thrust. Think aerial photography platforms carrying heavy payloads. These/we people spend time balancing our large props and motors to insure there are no erroneous vibrations in the platform.
I did a little research and could find no other person who has tried this! So my thought process would be to make octo with four arms longer than the other four. Placing the smaller props on the longest arms so they come into play upon immediate tilt of the platform and the larger four that are closer to the platform provide the lifting thrust.
I don't think this will need an adjustment to the firmware only different size motors. So if you went this route you would need to plug in the four speed controllers of the same size props (with props off) and calibrate those then do the same to the other four. This should calibrate all your ESCs to work.
Cool idea but not needed to do what you want. much quieter = less pitch / stability = bigger props then again you are talking about eight props spinning so how much noise do you really think you are going to quite?
Replies
A further issue with large props is gyroscopic precession, which will resist deflection. So adding your four extra smaller props will improve the maneuverability, but not as much as you might expect. Still worth experimenting with though, go for it.
There was actually a guy in India who had a large central gas engined coaxial set of rotor blades providing primary lift for the copter with 4 separate electric rotors providing roll pitch and yaw.
It actually worked, but I wouldn't have wanted to be anywhere around it.
Large rotors are much more efficient than smaller rotors and that is why they are desireable.
The more expensive large props are made with a balsa or foam or honeycomb core and Carbon Fiber covering so they are actually quite light and capable of responding quickly.
One of the problem's with quadcopters with large diameter propellers is that the propellers can turn so slowly that prop blade position can actually provide a pitch or roll offset to the copter.
And since the blades turn asynchronously you can get a periodic craft pitch imbalance caused by complementary and unfortunate alignment of blade pairs.
Usually this problem is worse if the copter is comparatively light, the heavier it gets this problem tends to dissipate as the blades are spinning faster to keep it in a hover.
However, it is probably optimal to simply design and tune the whole airframe for the intended weight as a quadcopter rather than to introduce further design complexity and inefficiency with small blade maneuvering rotors.
Could it be made to work, sure, would it provide a benefit versus other means, probably not.
Best Regards,
Gary
If the 4 big props are all attempting to do the same thing (that is spin at the same speed all the time), then there really isn't any reason to have four of them. You could have a coaxial counter-rotating pair of really huge props/blades, with four blades around them for maneuvering. Of course, at that point you are getting close to figuring out a helicopter might be more efficient than a quad. You could also have two main rotors and 4 maneuvering props, without the need to make them coaxial. Two would be relatively easy to power via a gas engine, if you wanted to go for crazy flight times. The quad maneuvering portion would mean your main blades could be fixed, which I suppose decreases complexity a bit over a heli in terms of moving parts, but not by much.
http://diydrones.com/forum/topics/h8-uav-370-development-settings-s...
Have a development that is attempting to do this, doing a modification of the AdruCopter code to captilize on the differnt motors/props
so you are just trying to cut down on noise and you want larger props...The noise from a prop is due to the pitch of the prop slapping the air. To cut down on vibrations from the larger props? Balance them....
From what I understand is that small props are able to spin faster and therefore provide faster response to what you input. Think acrobatic platforms...
A larger prop spinning slower gives you more thrust. Think aerial photography platforms carrying heavy payloads. These/we people spend time balancing our large props and motors to insure there are no erroneous vibrations in the platform.
I did a little research and could find no other person who has tried this! So my thought process would be to make octo with four arms longer than the other four. Placing the smaller props on the longest arms so they come into play upon immediate tilt of the platform and the larger four that are closer to the platform provide the lifting thrust.
I don't think this will need an adjustment to the firmware only different size motors. So if you went this route you would need to plug in the four speed controllers of the same size props (with props off) and calibrate those then do the same to the other four. This should calibrate all your ESCs to work.
Cool idea but not needed to do what you want. much quieter = less pitch / stability = bigger props then again you are talking about eight props spinning so how much noise do you really think you are going to quite?