6 Bladed Props

[Trikenut]

Hi
I was just wondering why some people go for 5 & 6 bladed props?
Does it help in any way
I personally don't like the look but statistics...

[Morph]

The more blades you add to a prop the less efficient they become. However on the other hand they become progressively smoother.

2 best but rough,
3, smoother, minimally weaker but still lekker

[John Boucher]

The most efficient prop is a single blade but due to inertia and balance not practical. The blades have to follow in the wake of the other thus becoming less effective. Would not like to set up the pitch on each blade.... would take an age!

There is obviously a practical and simplistic reason why 5/6 blades are used on turbines eg Hercules etc.

A well-designed propeller typically has an efficiency of around 80% when operating in the best regime.[1] Changes to a propeller's efficiency are produced by a number of factors, notably adjustments to the helix angle(θ), the angle between the resultant relative velocity and the blade rotation direction, and to blade pitch (where θ = Φ + α) . Very small pitch and helix angles give a good performance against resistance but provide little thrust, while larger angles have the opposite effect. The best helix angle is when the blade is acting as a wing producing much more lift than drag.

A further consideration is the number and the shape of the blades used. Increasing the aspect ratio of the blades reduces drag but the amount of thrust produced depends on blade area, so using high aspect blades can lead to the need for a propeller diameter which is unusable. A further balance is that using a smaller number of blades reduces interference effects between the blades, but to have sufficient blade area to transmit the available power within a set diameter means a compromise is needed. Increasing the number of blades also decreases the amount of work each blade is required to perform, limiting the local Mach number - a significant performance limit on propellers. Wikipedia

[John Boucher]

Sorry... didn't answer ther question!

Answer : They look cool when standing still and the okes using them want to be different!

[priester]

[quote="Bad Nav"]The most efficient prop is a single blade but due to inertia and balance not practical.

The silent motor glider comes with a single blade prop.

[John Boucher]

Hi Priester

Tell us more....!

I popped onto a Silent website and it seems it uses a 2 blade fold up prop driven by an electric motor?

This one doesn't use any blades at all..... check it out!

http://www.popularmechanics.com/science ... 03066.html

[Griffin]

They have a counter weight to balance them.



http://en.wikipedia.org/wiki/Single-blade_propeller

[John Boucher]

Okay..... so why do WE use 2 / 3 / 4 / 5 / 6 blade props?

[Wargames]

Okay..... so why do WE use 2 / 3 / 4 / 5 / 6 blade props?

I have just done a few calculations, help if im wrong.

Normally with 2 blade - we use a b-box with 2.58 ratio. So at engine revs of 6600 we get prop revs of 2558 if ratio is 2.58. Times 2 we get 5116 blades through the air at top of flange.

Normally with 3 blades we use a c/e -box with 3.47 ratio. So at 6600 revs the prop revs will be 1902. Times 3 we get 5706 blades.

To convert this to a single blade, you can almost run it without a gearbox, and by doing that you will lose a lot of torque on the prop.

By doing this calculation, there is not much difference between a 2 and 3 blade prop if the correct gearbox is used.

Any opinions??

[John Boucher]

Where is the clay man?

MORPHEUS ... come out hither and wave your magic brush old wise one!

(By the way Morph, congrats on the Tree Tousand post.... I see now you have gone yonder by 17!)

[justin.schoeman]

Okay..... so why do WE use 2 / 3 / 4 / 5 / 6 blade props?

To 'put the power down on the road' so to speak.

Generally, the maximum prop diameter is determined by the airframe (prop must be short enough that it won't touch the ground, or the airframe).

Given this maximum diameter, you can work out the maximum RPM (you don't want the prop tip to go beyond M0..

So, now you have a limited size prop running at a limited speed, but everybody knows that you can never have enough power. So how do you go about adding more power? Can't spin the prop faster, or its efficiency will degrade rapidly when it goes trans sonic. Can't make the prop bigger without chopping off your tail boom. Only option left is to add more blades .

Two blades are only marginally worse than one blade, but are generally the most convenient length.

Justin

[extra300s]

Nee Dirk, dit klink 100 vir my. Goeie verduideliking.

[Morph]

I have just done a few calculations, help if im wrong.

Normally with 2 blade - we use a b-box with 2.58 ratio. So at engine revs of 6600 we get prop revs of 2558 if ratio is 2.58. Times 2 we get 5116 blades through the air at top of flange.

No, but I understand where you are coming from. In order to compare you have to use the same engine, same gearbox ratio. A 912ULS only has one gearbox ratio any way

The prop is a wing. The wing has lift, drag, angle of attack and wing size(surface area) like a normal wing. The lift produces thrust to drive the plane forward. The drag loads the motor limiting your max rpm. To get more thrust, increase the size of the wing, or increase the angle of attack, or both. However everytime you do this you increase the load on the motor, reducing the max rpm (this also pushes your EGT temps up). You can increase or decrease surface area by either making the prop shorter/longer or thinner/wider

Then you have the max length allowed by the airframe, ground clearance etc. So assume a 72" 2 blade(1.828m) , would have an average width of 100mm the area of the blades would be 0.18m squared(.09m sq each blade). Now assume amount the pitch to give you a maximum rpm of 6600. This would give you a nice cruise RPM of about 5800

By adding a third blade with the same length, width and pitch would increase the area of the blades to 0.27 m Sq, the drag to the motor would increase by 1/3 and reduce the max rpm produced drastically (can't give you an exact but probably like 6100 or lower). To add a third blade, and to maintain the 6600 and cruise rpm you have to do one(or a combination) of three things. Reduce the angle of attack of all the blades by say 1/3, or replace all the blades with thinner blades of 66mm wide instead of 100mm or reduce the length of the blades to 1.8m (70.9")

You can't really narrow the blades down too much because there is a strength integrity you have to worry about so mostly you have length or pitch (angle of attack) to play with.

Most guys will look at shortening the blades slightly, i.e. the Bushbaby 2-blade is 72", 3-blade is 69" with the same thickness blades

The advantage of the more blades is sound and smoothness. More blades = reduced pitch or surface area = reduced noise, and a more balanced prop.

The big negative is the moving prop disturbs the air, creating a wake of turbulance. I single blade allows one whole revolution for the wake to clear before the blade comes around again. A 2 blade 180deg, 3 blade 120deg, 6 blade 60deg. The more blades, the less time to clear and the result is a reduced efficiency.

[Morph]

Normally with 2 blade - we use a b-box with 2.58 ratio. So at engine revs of 6600 we get prop revs of 2558 if ratio is 2.58.
Normally with 3 blades we use a c/e -box with 3.47 ratio. So at 6600 revs the prop revs will be 1902.

Remember that the prop tip speed is not allowed to exceed to sound barrier. This speed is determined by the distance the blade tip moves in one revolution X revs per second. A 72 " (1.829m diameter)blade will travel 5.7460 meters in one revolution. It would require 3445 rpm to break the sound barrier. A shorter prop travels a shorter distance, i.e. 62" (1.574m diameter) will travel 4.947m per revolution. Here 4002 rpm will break the sound barrier

Let's limit these props to max 80% of speed of sound, the 72" would need a rpm of 2756 and the 62" would be 3201rpm. Now for a motor spinning at 6600 rpm your ratio of 6600/2756 = 2.39, i.e. 2.39:1 for the 72" and 6600/3201 = 2.06:1 for the 62"

If we put a 72" prop on the lower ratio box, we would get a tip speed of 3201rpm/60=53.35 rp second X 5.7460 m/s = 306m/s which is very close to the sound barrier.

So typically your higher ratio boxes, require more engine revs per prop rev. This means the props swing slower allowing the use of a longer prop. The moment you shorten the prop you can increase the speed, by changing the ratio. However swinging a big heavy prop on a lower ratio box takes the tip speed closer to the speed of sound and to trouble.

A higher ratio box gives you a slower prop speed. Slower prop speeds allow you to swing longer props.

In my previous post I said you would need to shorten the blades or reduce the blade pitch of a 3blade to get the same thrust as a two blade without overloading the motor and with the same ratio box. By changing your ratio from 2.58 to 3.47 would allow you to increase the length of the 3-blade again back to 72" which is probably why they recommend it.

End of lecture, class dismissed

[Wargames]

Hi Morph,

Thanks for the lecture. You have just it on the head.

Would it be fair to say, to answer trikenut's question in post one, that you can put on a 3 blade prop with c-box(3.47) and a 2 blade prop on a b-box(2.58) and almost get the same result in terms of time to clear the wake of previous blade?? In essence, if you play around with different ratios and different number of blades, you can maximize your efficiency??

Point number 2: Lets take a rotax 503 with an even 50hp. Transfer that power through a c-box(3.47) and the power on the prop is 173.5hp. Through a b-box(2.58) the power on the prop is 129hp. Is there a benefit for the gain in power on a c-box??