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stevron

2 or 3 blade prop what is the performance difference

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Posted (edited)

Let’s hear from the most informed amongst us. Does a 3 blade prop perform better than a 2 blade , I am looking for a little better climb rate and yes I can loose weight and that will help. Let’s hear some facts that would encourage a poor and struggling pilot spend the hard earned to the purchase a Performance improver. I am happy with all facets of the planes performance except take off performance. Which I think is asthmatic to say the least.

Edited by Guest

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Hi Stevron - no offence mate but this topic has been "don to death" many times - the simple answers are:

 

In general, for low powered piston engine applications, a two blade is better than a three (or more). In recent times the fashion has been to fit 3 + blades however there is rarely convincing evidence that there is a performance improvement commensurate with the additional cost and weight.

 

Knowing what sort of aircraft you fly may assist the debate.

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For what it's worth, I think a 2 blade is great for fairly aerodynamic airframes (especially with IFA props) and 3 blade more suited to stol types where you hit that wall at about 90/100 kts and it takes a massive amount of power (and fuel consumption)to get any real increase over that.

 

Most aircraft come with ground adjustable blades these days and it's very easy to add or take a half degree and test it.

You need to post some performance figures and airframe/engine type. WOT climb, WOT flat and level etc

It's no good changing props if you're not getting the most out of what you've got now....

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The main reason to go to more blades is tip speed and sometimes ground clearance which doesn't apply to our types really. If you have a slow speed draggy plane where it needs more thrust, the geared engine helps and so does more blade area. Matching the prop to the plane is needed. Nev

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Two for go, three for show is a bit of a cliche that has been done to death but has some merit. Three blades have more blade area and with a geared engine like the 912 on a draggy airframe climb performance will probably be better. The more blades though the more drag so top end performance is likely to be better with 2 blades. In fact single blade props are the most efficient having the lowest drag but balance can be problematic. Many props are now ground adjustable as mentioned above so experimentation is easy. I opted for a 2 blade Bolly Bos5 as the original 2 blade non adjustable wood prop did not perform well. It took 3 adjustments to get it to what suits me. I get 1500 fpm climb one up at 70-75 knots & 1100-1200 fpm at 80 knots. WOT 3300rpm S&L is around 140knots. Cruise at 2800 rpm about 110 knots & cruise at 2950 rpm 125-130 knots. Comfort & fuel consumption play a big part too so I usually cruise at 2800 rpm at lower altitudes at around 17-18 lph. Fuel consumption climbs dramatically for those extra knots and at 2950-3000 rpm consumption is 26-28 lph.

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...............................................................................................................

Comfort & fuel consumption play a big part too so I usually cruise at 2800 rpm at lower altitudes at around 17-18 lph. Fuel consumption climbs dramatically for those extra knots and at 2950-3000 rpm consumption is 26-28 lph.

 

I am always fascinated fuel consumption against performance;

 

As I have stated before I have my two blade Fiti ground adjustable, set for advantage climb (so not best cruise).

 

My fuel flow indicator seems to be "all over the shop" these days however I always log my actual fuel used (as manually measured back in to the tank) against engine hours. I consistently have a fuel burn of 13L/h average, with just me on board and about 13.5-14 L/h for two up.

 

I trip plan at 14 L/h to be conservative.

 

On a trip I usually cruise between 4800 - 5200 rpm for indicated air speeds of 100-110 knots. On occasion I will open the throttle to 5400 rpm, for an indicated airspeed of 120 knots and if my dodgy fuel flow is to be believed 17-18 L/h

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2 blade vs 3 blade is comparing apples with pineapples unless you clearly specify what props are being compared. I changed a wooden 2-blade (GT) for a carbonfibre 3-blade (Bolly) and both my acceleration, climb and cruise have improved slightly (most consistent 1-2kts more speed at the same rpm and fuel burn). The carbonfibre blades are thinner and narrower than the wooden blades, off-setting the 3rd blade area and cross section. If you simply add a 3rd blade of the same thickness and planform, the result will be a less efficient prop.

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If you consider sailplanes, the longer wings give better performance. The same is true for propellors, so as facthunter says, tip speed and ground clearance would be the reasons to consider a 3 blade prop, other things being equal.

Have a look at the props on the man-powered planes to see what is efficient: big, slow, 2 blade props.

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I ran both on same engine and airframe and 2 blade better at everything

Could argue three blade gave more climb performance but im sure 2 blade could be pitched to match if that was the way you wanted performance - i did not

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Metal and later carbon etc which have finer sections are better than wood for efficiency. Where I've compared wood and metal on the same airframe the wood is noticeably less "noisy" (rumbly) through the airframe and nicer to fly behind.. It feels easier on the engine (and probably is). . Thin chord blade (toothpick) are ok on fast less draggy planes but slow ones or higher flyers need more blade area. 2 blade (laminated) wood is the easiest thing to make

The DC 3's used in PNG were equipped with what was known as" paddle blade" props for higher altitude aerodromes and climbing over the big rocks in the clouds..

The prop needs the correct (helix) twist in the blades and to be pitched for the engine and aeroplane with a bit of bias towards your take off performance when you use a short strip all (or most) the time, or operate heavy. Nev

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Dynamic prop balanceing removes mch of the rumbly feeling on more rigid CF props

Three blade can be good for more blade if you want to increase ground clearance

Timber from Jabiru used to move around a lot, tracking, depending on the weather, reckon this was masked by timbers inherent vibration damping and flex.

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