cyclops2

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larrysogla

Sooooooooooooo........it is really simple to make a dual axial flow impeller just by substituting a longer shaft in the motor so that the shaft extends front to back and be able to mount an impeller both in the front and in the back. All we need are two identical impellers. Then make the stator blades to straighten the airflow in-between the impellers with credit card plastic and VOILA!!!!!!!!!!!! we have a dual axial flow impeller EDF. In conclusion.............since it is so easy to make with ordinary everyday available materials, somebody has already done it and I am guessing now............efficiency did not improve. Must be the extra drag of the rear impeller. The real simple solution to increasing efficiency is to increase the diameter of the impeller. It is just so intriguing because we know that the current EDF's consume so much watts compared to the prop electrics for the same amount of thrust. Somebody has to find the physics solution(other than increasing diameter of the impeller) to this battery draining EDF's that we have now. Again, this was a very interesting discussion, thanks.
larrysogla

sheik480

Alright, I just have to ask this to: Why are more blades less efficient?

larrysogla

sheik480,
I learned new stuff in this discussion........such as True Helical Pitch..............so I am expecting to continue to do so with multi-blade props. Of course a lot more folks here know more than I do................but I would hazard a guess that multi-blade props increase drag with every additonal blade.
larrysogla

sheik480

That could be so, but a more blades could be turned proportionately slower and net an exponential drag decrease for the same amount of thrust.

BMatthews

Also it's not just about putting a ring on the tips of the blade. The "ring" needs to form a duct of sufficient chord that it smoothens the airflow both before and after the prop or rotor disc. This is the exact factor that an early test found with ducted fan units used without a duct. Most of them were long enough BEHIND the rotor but required a venturi like inlet addition to smoothen the airflow entering the duct.

BMatthews

You got me. Because the chord is fixed such a strip of material will not generate a truly helical pitch. What is needed is a stack of narrow "popsicle" sticks with a pin run down through a hole in the middle of the stack. When fanned out to a "prop like" shape it would produce a proper helical pitch other than for the stair steps between layers.

HighPlains

My Feedback: (1)

Still no.

A prop maker (Chris Machin) once said: "a prop is not a screw going through a block of cheese."

The angle of your example would be the same when you are 1 inch from the hub or 3 inches from the hub.

The formula for the angle is:

Angle = Arctan [ pitch/ (distance from hub x 2 x 3.14159)]

For a 10-6 prop:

@5" radius 10.8 degrees (tip)
@4" radius 13.4 degrees
@3" radius 17.7 degrees
@2" radius 25.5 degrees
@1" radius 43.7 degrees (hub)

These angles give a THP for a prop, but a pitch curve like this is never used in practical props. Most props are washed out at the hub, because it works better on the airplane and if the prop is made from wood requires wood blanks that are thinner (cost). Many sport props are also washed out at the tips, though racing props are not.

BMatthews

Highplains, we're now describing the same thing. My fanout of popsicle sticks will form a shape determined by the sketch below. And your equation describes what the sketch is showing graphically.

I'm also the first to agree 100% that no one uses such a thing other than some of the very early indoor models that used built up props that resembled the rotor on a dutch windmill.

HighPlains

My Feedback: (1)

My mistake. You are correct and I can see it with your sketch.

larrysogla

Sooooooooooooooo..............it means that the founders of the prop factories really had to sink their teeth into the mathematical formulas and the real life R&D(flying those model airplanes and probably a thrust indicator machine to mount the props). No wonder I am just the average Joe Public......'cause just thinking about the complex effort involved already makes my head spin. My hats off to these engine, prop and Radio founders/creators. They are gifted..........no matter what you say. Folks, thanks for the information you share......they are all new to me. Learn something new everyday.......they say.
larrysogla

B.L.E.

Both ducted fans and open props make thrust the same way, thrust is the forward reaction of accelerating air backwards. A fan typically has a small diameter and so it makes a given amount of thrust by accelerating a small mass of air to a very high velocity, that takes a lot of energy.
An open prop, on the other hand, typically has a large diameter and so it pumps a lot of air but accelerates it to a relatively low velocity, it gives the air a lot of momentum but not much energy.

Momentum = mass X velocity
Kinetic energy = mass X velocity squared / 2

A bowling ball launched by a bowler has a lot of momentum but only a modest amount of kinetic energy. When it strikes bowling pins, the pins go flying. If you built a shoulder mounted catapault to launch a bowling ball using bungee cords or something similar, you would find it has a lot of kick.

On the other hand, a .22 bullet has a lot of energy but very little momentum, when it hits a bowling pin, the pin might barely tip over if you hit it just right. However since the bullet has little momentum, the rifle that launched it barely kicks.

Shooting a small amount of air backwards at a high velocity results in little kick but takes a lot of energy, it doesn't mean the fan is inefficient, if each joule (watt-second) of energy input results in air being accelerated to a kinetic energy of 1 joule, then the fan is 100 percent efficient. It's just that accelerating air backwards to a velocity of 300 mph in order to deliver thrust to an aircraft going forward at 80 mph results in the lion's share of the energy being used to accelerate air backwards.

Think of it as the equivalent of an electrical impedance mismatch.

RAPPTOR

My Feedback: (41)

i put a bellmouth in front of a 5 inch fan once. still had full intake ducts.best setup i ever had.. is this becauese i sped up the air to the fan?? i feel this helps. not slowing it down as it reaches the rotor..

pimmnz

Nothing to do with changing the velocity of the inlet air, a properly designed bellmouth merely ensures that the full face of the fan receives clean, unturbulated air, with the increase in propulsive efficiency you have noted.
Evan, WB #12.

RAPPTOR

My Feedback: (41)

ah ha.. thanks for clearing that up..hnow about the divider, that is caused by the joining of the left and right intake duct..?? keep it short, or as close to rotor as possible?? thanks again..

BMatthews

The sooner your duct joins into one single duct the better. But with that in mind you want the transition to blend smoothly. Too abrupt and it'll cause tubulence at the joint that restricts airflow and that is truly the kiss of death to the thrust output. The fan can't accellerate the air if it's having to also suck it into the blades. This is why so many ducted fan models resort to cheater holes in the belly. At our size of models and especially with the smaller EDF models the wall turbulence makes the ducts hellishly inefficient and restrictive at moving the large volumes of air required to avoid a low pressure area at the mouth of the fan.

RAPPTOR

My Feedback: (41)

thanks for info.i made the stadard looking lazY {S} duct. put suction on the rotor end. put string into the duct. "it went right down the center".

Lnewqban

RAPPTOR,

The string will always point to the center, where the fan blades cannot do much useful work.

The flow restriction problem is happening on the internal surface of the duct and at the mouth, where there is a drastic change of direction and acceleration of the air.

Check these pictures out:

http://www.bloodhoundssc.com/car/intake_design.cfm

RAPPTOR

My Feedback: (41)

looks like turbulace started , were the divider ended.also area rule was not mantained..show me the perfect intake ducts. two.one on each fuse side..