2 propeller blades os opposed to 3 blades

[Ultra long hauler]

Hi,

my question is simple……….certain LSA have 2 propeller blades, while all planes at my club happen to have 3 blades.

What determines this please?
Is it the engine make that is most important, or the airframe design as well?

I know that in theory a 1 blade propeller is the most efficient propeller as the blade encounters "clean air" at all time, but vibration issues make this an option that´s better left on the design table.

But what about 2 as opposed to 3? And why not 4, while we´re at it--> like some larger aircraft carry?

Cheers,

###Ultra Long Hauler###

[WILCO.XMG]

from my limited knowledge its all about how much "lift" which acts as thrust the propeller can output.

You are correct in saying that 1 blade would be more efficient, however 4 blades are quite inefficient as the airflow they encounter is quite turbulent. Thus reducing the "lift" the prop creates.

The angle of twist in the prop is probably the most important feature, not number of blades, im quite sure vari pitch props act different however im not sure to what extent.

Like i said im no expert but there is my $0.02. Please feel free to offer criticisms,

[soay]

A 2-blade prop needs to have longer blades to give the same thrust as a 3-blader, so its tips run faster through the air, making more noise. This also reduces ground clearance, making a prop strike more likely. However, it is simpler to make and lighter, which can be critical for nose heavy aircraft.

[Cusco]

When our 2-blade Arrow has a severe prop dink and needed a new prop we fitted a three blade prop as if surprisingly was massively cheaper that a two blade prop.

The performance is no different and If I'm honest, despite static and dynamic balancing, the vibration is a bit worse with a three blade.

True the ground clearance is better and I think from the ground it's a teensy bit quieter.

However, with a three blade prop the aeroplane looks seriously fab..............

[Lister Noble]

Interesting stuff,submarines have multi-bladed props,which I imagine is to reduce noise,and turbines have many blades but in a confined tube.
There must be a lot of tech theory some where to show which is most efficient,quietest etc.:)

[moreflaps]

It is not true that a 3+ blade prop is automatically less efficient than a 2 blade. It is much more complex than this. Power loading per disc area has a huge influence as does tip mach number. Just look at the prop fans that are present on some modern planes to see what having even more blades can do for efficiency... If you include noise production as an important 'efficiency' factor, more blades are clearly superior! In fact, test showed that the efficiency difference between a 3 blade and a 2 blade could be very small if designed properly (see Fig 44, http://aerade.cranfield.ac.uk/ara/1945/naca-tn-947.pdf)

All this is discussed in a excellent text:
Airplane aerodynamics and performance By Jan Roskam, Chuan-Tau Edward Lan

Cheers

[moreflaps]

Hi Lister
The mutiblade props used on modern subs is mainly due to the need to absorb engine power without cavitation. Cavitation leads to a huge drop in efficiency as well as noise -as you noted.

Cheers

[Jan Olieslagers]

I remember a four-blade prop on a Rotax 80hp and it was a lot less noisy than the usual 2 and 3 blades.
While we're on the subject: it is surprising how much noise comes from the prop, I always felt the engine was the main source of noise - until I saw/heard an electrically powered trike demonstrated.

[Trolltuner]

As has been pointed out, three bladed vs. two bladed props are generally less noisey because their swept diameter may be reduced. This means that their tip speed for any given RPM, and thus their ultimate helical tip mach number (I just love using that phrase :)) is lower. Many GA piston types have numbers between 0.7 to 0.9 mach at takeoff. The closer to Mach 1 at the tips, the louder it gets. Ever hear a Grumman Duck at takeoff? With tips at, near, or past Mach 1 the noise is deafening. Not to pick on Grumman, but the twin engined Albatross isn't much better. :O

Having stated this, the perceived noise level or PNL may be quite different amongst two to multi-bladed props because the latter tend to shift the frequency spectrum of the noise higher. Human annoyance at different levels and frequencies of noise vary quite a bit. Personally, I long for the drone of a DC-6 or Connie so common in years gone bye. :)

Cheers,

Tom

[Zulu Alpha]

3 bladed props generally have smaller blades than the equivalent 2 bladed.

This means less force on the hub, therefore the hub can be lighter. So, more weight total in the blades and less weight in the hub. Hence the possible increase in vibration.

3 blades have less gyroscopic effect so put less load on the crankshaft during aerobatics.

Also at overhaul, 3 blades have to be overhauled instead of 2 so slightly more expense

[sunday driver]

Ok - a stupid question coming up ...
If fewer blades means more efficiency, why do intelligent people do this ...
A400M Countdown #2 - A PROGRESS REPORT FROM AIRBUS MILITARY (http://www.a400m-countdown.com/index.php?v=2&spage=9)

[WILCO.XMG]

Efficiency doesnt always equal performance. And with any military vechicle performance is more important than efficiency, otherwise you wouldnt be spending millions of dollars on rockets to fire into tumbucktu.

But with the A400 doesnt it have that clockwise on port and anti-clockwise on starboard rotation.

Not sure if it makes a difference to prop performance?

[IFMU]

When our 2-blade Arrow has a severe prop dink and needed a new prop we fitted a three blade prop as if surprisingly was massively cheaper that a two blade prop..............
My observations on our Arrow are about the same as all yours. Our CG is a little further forward than I would like though, I think the 2 blade must have been better.

[moreflaps]

Hi SD

no sure if your Q was sarcasm, but the key is that >2 blades can be more efficient than 2 bladed props in (especially) high power applications.

Cheers

[IO540]

3B props look better :)

They also tend to have bigger vibration problems, not made any easier by Hartzell delivering them well out of balance. Dynamic balancing is a must in most cases.

I don't think there is a real performance improvement. I believe the most efficient prop has just one blade, but of course that would have some interesting balance issues too ;)

The more blades, the more power the prop can absorb for a given RPM. So more powerful planes have more blades because that is the only way to absorb say 1000HP-10000HP without the blade tips exceeding mach1.

Alternatively, more blades allow the prop to rotate slower which makes less noise.

[sunday driver]

Flaps
Well there was more than a hint of genuine curiosity, since I don't have any useful knowledge of propellor technology (or anything else).
Having thought about it I'm guessing they need all those blades to soak up the eight-plus megawatts provided by each turbine. If the mission is a military matter, efficiency is not so pressing. And, yes, apparently adjacent engines do rotate in opposite directions.
SD

[Ultranomad]

I believe the most efficient prop has just one blade, but of course that would have some interesting balance issues too
AFAIK, single-blade props were once quite popular in control line models, and there was at least one production model of single-blade prop for manned aircraft, made by Everel Propeller Corp.:

http://www.notplanejane.com/images/AeroMatic/Everel/EVEREL-Prop-Corp-E88-1_sm.jpg

On the other hand, concerning the fan-like 6-8-blade props, all these models are fairly recent and have been optimised using CFD techniques. Allegedly, their efficiency is quite good.

[moreflaps]

Anton
What a great picture and exam question for 1st year engineers! That prop cannot be dynamically balanced as the second moment of each 'blade' is obviously different. I wonder how much it shook the engine -poor bearings and crankshaft! I wonder if the designer got sacked...

Cheers!

[Deeday]

Single-blade propellers are still used in some motor gliders with retractable prop/engine assembly, because of their reduced dimensions.
Crucially though, the blade and counterweight are hinged to the hub in the same way as a two-blade see-saw helicopter rotor, which produces a lot less vibration than the Everel prop.

[moreflaps]

Hi Dday
Yes, you've got to allow the CofG's to align (not possible with out allowing some flexibility) but the different intertia could lead to a phase error in dynamic balance and mass related vibration. That is why one needs to match the blade mass _and_ radial CofG position in each blade (as is done routinely in helicopter blades). BUT that still does not remove the 2nd moment of planoform area, which means that for a single blade prop the center of thrust is rotating around the shaft trying to bend the boss from the motor... I'd worry about resonance too making it all even worse, I could imagine with a hinged blade counterbalanced with a shorter larger weight that the whole lot could get really ugly at some RPM -excited by the rotating thrust vector.

Cheers

[Ultra long hauler]

Thanks for the answers so far………

So if I were to buy an Ultralight / LSA today with a box standard Rotax or Jabiru engine of roughly 80HP, how many propeller blades would you recommend, at the end of the day?
That is assuming I have enough ground clearance for either 2 or 3 blades………


###Ultra Long Hauler###