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Adding additional props to a draganflyer--

BIG GUNZ 45

Heliman

Bethesda, MD usa

I am converting to a brushless system for my draganflyer--a lot more power and was wondering if I added additional props to the bottom shaft if that my help with my lift--any thoughts??

I am talking about the motors that have a shaft on each side--has anyone done this? Problems with this?

thanks

Gunz

03-09-2009 Over year old.
DKNguyen

Senior Heliman

N/A

I wouldn't add additional props to the bottom. It would increase lift but reduce efficiency. Like in a contra-rotating helicopter, two rotors sitting on each other produces much less than double the lift of a single rotor.

THe lower prop has to accelerate air that has already been acclerated by the top prop, so it has to push a lot harder to have the same effect. It's like trying trying to shove a bumper car from behind when it's already moving in your direction- you gotta hit it a lot harder to get the same effect.

I would use larger rotors or more blades if possible, or stack multiple rotors right next to each other on the same shaft (ie. 2-blade propellers 90 degrees apart to get a 4-blade propeller) that way the propellers wouldn't be sitting each other's downwash. I think it would be more efficient that way. THat's just a theory though.

03-11-2009 Over year old.
BIG GUNZ 45

Heliman

Bethesda, MD usa

THANK YOU for your input--Draganflyer has the X6 6 motors on 3 pods counter rotating --not sure how they get it to work--

stacking is an idea--I will look into that--

thanks again

gunz

03-11-2009 Over year old.
DKNguyen

Senior Heliman

N/A

Well, in the case of the X6, they pull off the counter-rotating props by having one prop for each motor. SO there are actually 2 motors per arm and 6 motors total.It just lets them use a tricopter configuration and still be able to one counter-rotating prop for every regular rotating prop to deal with the torque reaction. THey also claim it gives some redundancy, although I would think the yaw gets seriously messed up if you lost a rotor.

It's less efficient (and heavier) because they are using twice as many smaller motors rather than half as many larger motors (and larger motors are more efficient and pack more power for their weight).

Plus there's that thing where they claim it gives double the thrust...which it doesn't due to the "one rotor sitting in the downwash of the other rotor" effect I was talking about.

THey do not use one propeller for each shaft on a dual-output shaft motor like you were talking about. The only reason to stack rotors is to cancel the torque reaction. Sure you get more thrust, but more efficient ways of doing that are adding more blades or making the rotor bigger. If you did use two props on opposite ends of the same motor...well you don't cancel out the torque reaction at all. Instead you just get all the inefficiencies of one rotor sitting in the downwash of the other. THe only thing you gain is a little more lift (and nowhere near double the lift for the second rotor you just added on).
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I've been thinking about it, and it seems my entire idea was based on the assumption that multi-blade propellers are more efficient than equivelant propellers that are stacked (and possibly counter-rotating) which may be entirely false.

As the stacked propellers move closer together, their efficiency gets worse because the downwash of the top rotor interferes more and more with the lower rotor (to the point where you might even say that the bottom rotor is no longer spinning under it's own power- moving air from the top rotor is spinning the bottom rotor like a wind turbine does.). Propellers with more blades are less efficient because the rotor blades move into the wake left behind by the preceding blade. Puting two propellers so closely together might be the perfect situtation to get the absolute of both worlds- close enough to have the wake interference, but not close enough to eliminate the downwash interference...in fact close enough to get the worst possible downwash interference.

So both your idea (prop on both ends of the motor) and multi-bladed propellers is probably better than my idea of closely stacked propellers. Which idea is better between the two? Can't be sure. But don't expect to get double the lift for adding second propeller. I think the numbers say that if the propellers are spaced by half their radius you get around 20% more lift. THe farther they are, the more lift and efficient they get.
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One thing is for sure though...bigger blades is the best way to increase lift. And another thing I should add...your brushless motors have more power, yes. But adding bigger blades, more blades, or more propellers to make use of that extra power will only work if the brushless motor's extra power comes in the form of extra torque. If it is in the form of extra speed...well just stick with your current rotors and spin them faster (like more blades, or stacked propellers, you will get more lift and less efficiency).

03-11-2009 Over year old.
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Adding additional props to a draganflyer--

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