Two Blades Vs Three Blades

[Trav737-400]

Hi everyone, I just have a quick question. What is the difference between an aircraft that has a two-bladed prop for example a C172 and an aircraft that has a three-bladed prop like a C182?. Does the three bladed prop generate greater thrust or is it just a cosmetic thing?

Thanks.

[jarjar]

Simply put, YES.

thrust produced by a propellor depends on the acceleration the prop imparts on the air Formulae something like this:

T= M x (V1-V) M= MASS OF AIR V1= Air after acceleartion by prop
V= Air before acceleration by prop

Three blades impart a larger V1 than two bladed props hence producing more thrust.

There are many websites around that can explain indepth the theory of propellors, the NASA website could be a start, if not just punch "propellors"into your search engine and browse the results.

Hope I could Help

[SeldomFixit]

The HP of the engine must also be considered. The blades must be able to absorb that power and this can influence the type of installation.

[rmcdonal]

By adding a third blade you can reduce the size of the other 2, this can give you a greater ground clearance, also it keeps the prop speed down.

[Whiskey226]

The number of blades you put on the thing depends on a few factors, including engine HP, physical restraints etc. Generally the fewer blades you can get away with the better, as pushing all that metal through the air at high speed saps power - it is another airfoil, after all.

[jandakotpilot]

I Have flown a 182RG with both a 3 and 2 blade prop. The noise with the 2 blade was greater so was the performance.
Another Arrow that I used to fly had a 2 blade on it and when its time came up they put a 3 blade on it and it became a snail.

[OZBUSDRIVER]

3 look better. However, the howl of those squared off tips on a C182 on takeoff do sound the business:ok:

[NAMPS]

You do get more thrust (depending on engine HP), although there is also a weight penalty.

[Trav737-400]

Thanks guys, that clears it all up for me. I am now more the wiser!:8

[flyby_kiwi]

And believe it or not a one bladed prop is the most efficent although obviously unpractical. Not sure if its ever been applied to an a/c - maybe there are pics out there????

[Ultralights]

A longer 2-blade prop would give more thrust than any smaller
diameter multi-blade prop.
In fact, years ago, you could get a SINGLE blade prop (with a
counterweight opposite the one blade) for some of the low
powered Aeronca's etc. The advantage was better thrust from the
larger diameter, when the engine was not strong enough to turn a
2-bladed prop of the same diametre.
In all the old warbirds, prop diam was limited by ground clearance.
The Spitfire started with 2 blades, and as the power increased, the
only way to absorb it was with more blades, 3, then 4, and finally 5 !!
Not as efficient as a bigger 2-blade, but it was diameter restricted.

On CriCri, the ideal props would be 2-blade, about 36" (1 metre)
on engines with an HONEST 18-20 hp. HOWEVER,
that would create problems of a longer, heavier engine strut,
Vmcg, Vmca, bigger controls, more weight, more fuel, bigger
wing to lift it, ad infinitum. You end up with more expense
and no real gain in performance.

So a 3-blade prop is to absorb the power of a motor bigger

[Noted & Disregarded]

I believe they use one bladed props with a counter weight on model aircraft that are designed specifically for racing. And I also believe it is true that the more blades you put on the less effective or efficient the propeller becomes but as previous posts have stated it just isn't practical to put 2 big long blades on powerful aircraft .

Noted

[djpil]

The biggest factor in propeller efficiency is diameter - the larger the diameter the higher the efficiency, the higher the thrust. Everything else being equal, a 3 blade prop is less efficient than a 2 blade prop.
You need enough blade area to absorb the power - more power needs more blade area - get that from increased blade chord, increased diameter or extra blades. Others have mentioned most of the considerations. A 3 blade prop is also smoother.
I guess that you can get details of the above with a good googling session.

i.e. I disagree with jarjar. This question needs more than just simple actuator disc theory.

PS - I have seen a pic of a single blade prop on one aeroplane.

[Wizofoz]

I still think the major consideration is noise. In Europe, because noise is such an issue, you see a lot of aircraft retro-fitted with FOUR bladers purely for noise reasons.

[Woomera]

And to further confuse the issue, go read about the Unducted Fan (http://home.swipnet.se/~w-65189/turbine_engines/types_of_turbine_engines.htm), in effect, a super efficient multi blade prop!

Woomera

[onthedials]

Performance is just part of the story. A smooth running engine brings additional satisfaction and probably lower costs over the life of the aeroplane associated with gyros, avionics and such.

A new 1997 Cessna 182S was fitted in the factory with the 2-blade McCauley B2D34C235/90DKB-8. The 2-blade propeller worked well enough. There was a little vibration, but much less than than is common in other singles. Interestingly the vibration seemed to increase at the lower end of the green tach band, so the RPMs were kept up around 2300 rpm in the cruise and little more was thought about it.

A bit more than a year later a 1998 182S which had the 3-blade McCauley propeller installed was utilised on another task. The owner thought it was a different aeroplane. The noise was different, the vibration levels were lower and any performance difference was imperceptible. This 3-blade also had a significantly smaller diameter (79 vs 82 inches) and on dirt airstrips that was an added advantage. The negative was the weight: the 3-blade added 16.6 pounds right on the front of the aeroplane, the last place anyone wants to add weight to a Cessna 182.

After some discussion the 1997 182S was upgraded with a new McCauley B3D36C431/80VSA-1. A dynamic balance was performed on installation. The biggest change was the lack of vibration. The climbing seemed better with the 3-blade but it was probably a knot or so slower in the cruise. The vibrations were lower right across the range of allowable revolutions and certainly less noisy in the takeoff roll. There is no way the owner would ever go back.

Earlier-model 182s (182Q and previously) often found no benefit from 3-blade upgrades which they carried out, but that is certainly not true of this more recent experience.

Hope this is of interest.

[jarjar]

DjPil,

I agree that there is alot more to it than thrust equations, but if I went into that I could have 10 pages of theory and still be going.
I look at it like this:

Increase prop diameter- this will increase the thrust and reduce drag due to increased aspect ratio, however due to aircraft design (prop clearance) this may not be feasible.

Increase Prop Rpm- Speed of sound hence drag issues

Increase Blade Chord- This will work ,however it will reduce the aspect ratio, hence increasing induced drag off the prop tips

Incease prop Camber- Think youll find must props are at their most efficient camber already.

Increase number of Blades- As engine power output over the years has increased this has been the most viable option.(as an example this is why turboprops over the years as they get more powerful have seen an increase in the number of blades). 6 blades seem to be the limit on one hub before undesireable flow patterns emerge.

[matana22]

And from a different angle.
What about price?
If an operator decides a 3 blade would be more beneficial on a 182 what would the difference in purchase and ongoing costs be?

[onthedials]

In a new Cessna 182, there's no choice any more. Even though the type certificate showed both, the 2-blade prop finished in production very soon after the 3-blade became available and they were soon all equpped with the 3-blade prop, as they are now. That would be a result of the lower vibration and lower noise (especially in Europe) rather than any performance considerations.

The cost difference to manufacture a new single with a 3-blade rather than 2-blade prop couldn't be more than a few grand.

Upgrading an existing aeroplane is a different story and complicated by the fact that the newer props on older aircraft are generally only made available through STCs that the prop manufacturer paid for; ie the price includes the need to recover the cost of retro-certification in a relatively small market. On the other hand, for some other aircraft types, those costs have long been sunk and a new 3-blade would cost little more than overhauling an old 2-blade. At the higher-end, in the US a new design 3-blade prop for an older 182 can cost upwards of $USD16,000 plus freight and installation.

As for on-going cost, the question is complicated by what overhaul period applies and the type of hub design. New propellers do not seem to have significantly different TBOs or overhaul periods based on number of blades. The overhaul of a 3 -blade prop will cost more than a 2-blade of the same manufacturer and design.

[enicalyth]

Guys

If you can get a copy of "Practice's Notebook", long out of print, do. Mine's is 1955 and at home.

The chapter on gyroscopic forces versus number of blades explains why many WWII pilots and before that were deemed guilty of "overspeeding" when blades flew off two-bladers.

The only thing I can find in a hurry is http://naca.larc.nasa.gov/digidoc/report/tr/19/NACA-TR-19.PDF

They key phrase is (Bless you Bill) something like this:

"The aggregate gyroscopic moment transmitted to the shaft by a 2-blade propeller is variable and at all instants equals twice the moment of one blade.... for a steady running multi-blade the aggregate... is constant and at every instant equals the geometric sum of the varying moments of the individual blades".

Now, shades of the "Flight of the Phoenix" and all that where the German model aircraft designer gets his slide rule out. Look up Martin Hepperle's excellent site. Select the Clark Y family of airfoils and have fun exploring efficiency. His Java Applet is okay right on up to M0.55 ATR42/72 and Dash-8 sorts but falls over a bit when trying to sort out A400M fact from fiction.

I tried to look up Bill the other day [he is 94] and was told he is golfing this afternoon and won't be back till late because he's booked to play jazz at the "Coach".

Wow.

[magnum pi]

Imho,without getting into E=MC......2 blades=greater cruise speed,slower roc,more vibration(especially once nicked a few times) & less ground clearance. 3blades=greater roc,slower cruise speed,less vibration,better grd clearance.this applies to most lighties,eg c182,c206-210(early models),mooneys,lance & cherokee 6,arrows etc. Price was normally about the same(3-4 yrs ago)$14-20,000 au$,dep on a/c type.Meat bombers with C182s usually have 3 bladers to get up quicker.Only economical once fitted prop out of hrs tho. PI :ok:

[Spinnerhead]

2 blade = speed

3 blade = acceleration

You will find exactly the same on boats

Less blades = more speed
More blades = more acceleration or pulling power.

However at altitude where the air gets thin you need big blades and/or more of them.
Big blades are a significant drag at lower altitudes.

[185skywagon]

go to this link for thrust comparisons between various props.
here (http://www.pponk.com/HTML%20PAGES/propellers.html)

[elemenohpee]

Four bladed prop has less pull and also lower top end speed than the three blades.:( Just better ground clearance!!:D

[djpil]

OK jarjar, I'll buy that.
I have sometimes been misunderstood by trying to represent 10 pages of theory in only a few lines of text here.

enicalyth raised another important subject, especially for aerobatic aeroplanes. A composite 3 blade prop has much lower polar moment of inertia than a 2 blade metal one. A lot easier on the crankshaft flange when doing lomcevaks, flat spins and snap rolls. At least double the price though.

[185skywagon]

The gyroscopic/Pfactor force is noticably less with the 3 blade prop on my 185 than it was with the 2 blade prop of the same diameter. in my practical experience, i think that only the climb is better with the 3 blade prop.
all this is on a C185 300hp.