A question of props 2 blade VS 3

A 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.

In general changing from 2 blades to 3 does decrease diameter and therefore tip speeds for lower Mach numbers.

Hope this helps, Curtis

There are a lot of variables in prop design.  First, static thrust really isn't a good predictor of in flight performance, except perhaps very low speed aircraft like PPGs.

In general, larger diameter is better.  It's more efficient to accelerate a lot of air a little bit than a little bit of air a lot. 

3 blades tend to be less efficient than two for the above reason, and also because the blades are closer together as they pass, so they're more affected by the disturbed air from the preceding blade.

A prop has to be sized to absorb all the power the engine can produce.  If the diameter is limited (by airframe or ground clearance, or tip Mach number) and two blades still can't absorb the power (i.e. you overspeed or have so much pitch the blades stall at climb speed), then adding a third blade, or more, is done.  But it's rare that a smaller 3 blade will give you as much thrust as an optimized 2 blade (unless the 2 blade is limited by diameter).

I'm actually in this same dilemma at the moment... my Kolb has (and is limited to) a 50" prop, currently 2 blades with a 35HP Cuyuna and 2:1 redrive.  I have a 447 (40HP) with 2.58:1 redrive, and I'm weighing a 3 blade 50" prop vs a 2 blade and regearing the 447 for 2.04:1.

Thank you For your comments

For my dilemma.

I had some input by the props Tech. he assured me I was within spec. to use the 68 inch, And for my problem I should look elsewhere.

Aside from his suggestions. I intend to re drive it by a slight change to the engine pulley  . In spite of maybe  having resolved the initial power shortage. but I feel I'm at the bottom of this props potential match up.  There seems to be a lot of controversy  of the HP  produced by the 440 Kawi.

The  current ratio is actually 2.72 to 1 polley V drive

The drive pulley is 2.2 in dia. A 2 inch will make it 3 to 1 on the suggestion of another in my circle that would be  a maximum.

My plane is at the max weight of any U/L  being a seaplane  gross weight will be around 530# I need thrust  as high as possible optimized over speed with in the safe margins . I'm not interested in fuel economy as most want efficient cruse.

This usually throws of my discussions with other people influencing my choices.

I'm sure being a late comer to all that's been done already  Most of my problems is getting to the optimized  setup. Because there are so many variables. I have not reached out to anyone that has done what I am about to do . Setting up a seaplane as an U/L has different needs and  just getting enough power and staying in the U/L is tough.

Regards Norm

Do not cut the blade length at this time. I have seen others who have done that prior to first flight and found in the flight testing that they wished they still had the longer blades. You may well find that as the longer blade unloads in flight that the pitch is just right. Static rpm needs to be several hundred rpm below max power in order to stay below redline or peak power on the climb out. 

You can always cut the diameter down later but you can not add it back. You do not need max performance for initial testing.

Grant Smith

Dana's points above are right on the mark.  I have flown a variety of 2-blade and 3-blade props on my 447 powered trike.  The 2-blade versions mostly gave more thrust at takeoff  (You could feel it) but at cruise the difference was less obvious.  There is a lot of difference between different brands.  You can't compare apples to oranges.  I have a 57 inch 3-blade that gets  better fuel economy than either of my 60 or 62 inch 2-blade props due to it's airfoil but both of the 2-blades take off quicker and climb a little better. With a ground adjustable prop a slight pitch change can make a big difference.  I love 3 blades because to me they are smoother.

RE Dana's remark:..."3 blades tend to be less efficient than two for the above reason, and also because the blades are closer together as they pass, so they're more affected by the disturbed air from the preceding blade.

I'm not an engineer but have been told emphatically this explanation above is partially incorrect.  Even for an airplane that is tied down it is said the cone of air moving thru the prop arc is moving at a speed that precludes any wake effect from the preceding blade.

One reason 3 -blades are less efficient ( at a given rpm) than 2 blades is due to drag. Not induced drag, but parasitic or profile drag.  If they are of equal diameter, 3 blades have 5o% more leading edge that must be pushed thru the air.

--dan grunloh

Actually, Dana is correct.

A propeller blade operates in a perpetual downwash of the preceding blade, just like the airplane tail is in the downwash of the wing in front.

Note that there is a difference between downwash and wake. 

Downwash is simply a changing of the direction of flow. If you visualize the airflow over a wing, the air ahead of the wing takes an upward turn in "anticipation" of meeting the wing---and then aft of the wing it turns downward, in reaction to the lift produced by the wing. So first comes upwash ahead, then downwash behind and the transition point is right about the quarter chord point, which is the aerodynamic center of an airfoil (generally).

Wake happens only when an airfoil is stalled. It is simply very turbulent air that does not flow in streamlines, but is churning all over, behind the stalled airfoil (or any unstreamlined object; a cube in a flow of air will produce a large wake for example). The problem is that a wing or prop blade flying through such a wake will experience much higher drag. 

This is not true of flying through a downwash, although there is a small aerodynamic penalty here too. The wing or prop blade flying in downwash will need a higher angle of attack to make the same lift. Think of it as flying "uphill," because the airstream is coming down at an angle that the wing must "climb." The extra alpha (angle of attack) needed means the lift vector will be tilted back a bit, which means an additional drag component.

So a prop blade will always be in downwash, but it will only be in a wake if the prop is stalled. Stalling of props usually is only seen with fixed pitch props with high pitch, which would be used on an airplane with a high top speed, say 200 mph or so. In that case, the blade can stall when the airplane forward speed is slow or standing still---because of the high angle that the blades are set. That is why fast planes need in-flight adjustable props, so their low-speed performance does not suffer.

As for the question of 2 blade or 3, the two blade will generally perform better, partly due to less downwash as Dana said. But there is more to it, such as as blade diameter, etc. 

Now about the specific question, I'm assuming the engine is about 45 hp so a good prop should be making about 250 lb of thrust. A good diameter would be about 66 inches, for an all round prop. This would be for a plane designed for about 75 mph top speed.

Gordon

The engine  for my plane is less Hp

It is infact only 35 + enhance by carburation and exhaust supposedly able to produce near 40 hp.

According to my suppliers it was able to use this prop  having more to do with the props construction  being thinner. The IVO is composite ground adjustable.And is in the left hand pusher configuration. I suspect it is still at the limits end of this engine  and need some compromise to use . Such as increasing the redrive ratio to 3-1

There are good explanations here as to why  2 is better than 3

Is there any reason for adjusting to one or the other  based  mainly on the specific needs of the airplanes use. Such as needing to over come the water environment . and  or carry heavier loads into the air and trading speed for low seed load carrying . If the wing can carry a specific load at low speed why does the prop have to be configured for speed . Like plowing snow I put it in low gear I can move a bigger load or over come the friction I just can cruise the highway with the others going along slower doesn't bother me.

I still don't know how to ask this question to get  where I want but this is what I am thinking

Proper engine loading is very important to the 2 stroke users.You only need to watch the video [ Is your 2 stroke engine failing].  Available here somewhere sorry i don't remember . Then you will understand why I think its more important than how fast  you fly.

scenario #1 big 2 blade drive change slower prop speed getting to the engine power curve and using  more pitch , compensate  for the reduced load and load the engine in that manner.While not exceiding the pitch to a stall. VS using  the ratio at a higher prop speed to create engine loading which is where its at but not reaching the power curve for the engine.

Second scenario is to use the 3rd blade and a smaller dia this increase the engine load by the drag . but increases the thrust  to move the heavier weight My understanding is the 3 blade produces more thrust  even if it has more drag. if the smaller 3 blade can never improve the equation

I was trying to understand how to get the heavier load into the air from viscus regime and not  optimize for cruse I am not needing to go anywhere fast or efficient or do cross country flights. I also think it relates to heavier pilots needing to over come if speed is not paramount . the engine loading always is.

I just seem to always get the optimized answers instead of a special needs one.

Regards  to all  Norm

Norman, the reason I am asking is that I have written a prop program that lets you quickly find the best prop for your airplane. Works from ultralights to 300 mph airplanes. All you have to enter is airplane power, prop rpm and top speed. 

What is the expected top speed of your airplane? 65 mph I think is the limit for UL, but I think many can go faster. I put in 40 hp and a prop rpm of 2400 rpm and got a best choice for prop for high static thrust of 69 inches, 2 blade. My program also gives pitch and actual blade geometry so you can cut your own prop from wood if you like or have someone make it for you. This includes blade angle at each of 20 blade stations, blade chord, thickness, etc.

Anyway, with a seaplane it is better to have more thrust because the friction is higher than rolling wheels. So you want as much engine power and you want a "climb" type of prop, instead of "cruise."

If you want to contact me directly I can send you a copy of the program and play with it yourself. You only need a spreadsheet to run it, such as the free open office suite you can download. I am at goarnaut(at)yahoo.com