Palamedes

Palamedes

Well Known Member
So a buddy of mine were talking about my RV10 and he was asking me which propeller I was going to get etc..

He brought up a question that I didn't know the answer to which basically distilled down to;

At what point does adding more propeller blades start to negatively affect performance?


We were all over the place but the conversation was sort of along this line..

Propellers generally have from between 2 to 6 blades for the really big turbopops..
Where as a jet engine has dozens of little blades..

Why do we not see lots of blades on all planes - jets and nonjets alike? My theory was at some point adding more blades negatively affected performance but I have nothing more than wild assertions to back that up.

So why do most Vans aircraft only use a 2 or 3 blade prop? Why not a 6? or 12? etc..
 
Typically the more blades, the less the efficiency. Turbine engines use more blades to absorb horsepower where using a larger diameter is not practical.

Usually a 2 blade prop is the most efficient. Three blades usually improve climb but sacrifice a little cruise efficiency. That theory assumes everything else is equal, which is almost never the case. 3 blade props are usually quieter and smoother.

Graig Catto has perfected the 3-blade, F/P, but since you are building a -10, I'm reasonably sure that you will want to go with a C/S.

Hope this helps a little.
 
Thrust requirement and diamater limitations

Hi,

Generally, for efficiency, you would rather have a few long blades rather than many short ones.
Usually there is a practical limit on diameter, either from:

1) tip mach number (blade tip speed approaching the speed of sound, with local flow around the blade airfoil going supersonic - which starts to waste power on noise and drag

or

2) practical ground clearance limitations.

Once you have reached a practical limit on diameter, the number of blades is determined by how much power you are going to put into it, and how much thrust you want out of it. If you have too few blades, the individual blade airfoils will need to be at high angle of attack (high pitch angle) and will run out of lifting ability. If you add more blades, you can get the same thrust with lower pitch angle. But, you also have more wetted surface area over the blades creating skin friction drag that robs torque.

On an RV-10 with an IO-540, an 80" two-blade prop works fine, it is capable of absorbing 300 hp without any trouble. at 2700 RPM, the tip speed will be Mach 0.85, so that is getting up there a bit. The same prop on a Continental IO-550 turning 2850 is VERY NOISY on takeoff. Adding a third blade but keeping the diameter at 80" would be a waste of skin friction drag for no benefit.

The other alternative would be a 78" three-blade prop. Now it is better to have the extra blade to help make enough thrust with the shorter blades. It would be quieter for sure, tip Mach down to 0.835 but might have a lower top speed because of the skin friction drag on three blades instead of two (although again, since the blades are shorter, the difference may not be very significant). It becomes a trade-off of compressibility drag vs skin friction drag.
 
number of prop blades

Some performance is gained and/or lost by changing the number of blades.
Generally speaking a 3 blade will have better take of acceleration than a 2 blade. If take off distance is the only issue a 4 or perhaps even a 5 blade my work best.

For cruise performance a single blade (yes there have been single blade props certified) is theoretically the best, each added blade slows the max achievable level cruise speed for the same amount of applied power. The problem with the single blade is it is dynamically stable at only a narrow RPM range, while the 2 blade is balanced at virtually all RPM's. Fewer blades result in higher speeds for cruise but, the trade off is poorer take off acceleration and climb performance.
 
Props

Mel said:
Typically the more blades, the less the efficiency. Turbine engines use more blades to absorb horsepower where using a larger diameter is not practical.

Usually a 2 blade prop is the most efficient. Three blades usually improve climb but sacrifice a little cruise efficiency. That theory assumes everything else is equal, which is almost never the case. 3 blade props are usually quieter and smoother.

Graig Catto has perfected the 3-blade, F/P, but since you are building a -10, I'm reasonably sure that you will want to go with a C/S.

Hope this helps a little.
Click to expand...
While this may be true with a Hartzell or McCauley prop it certainly is not true with custom fixed pitch props. The record breaking Phantom Racing Biplane went fast with a three blade and faster still with a four blade, designed by the late Paul Lipps and built by Catto.
 
jrs14855 said:
The record breaking Phantom Racing Biplane went fast with a three blade and faster still with a four blade, designed by the late Paul Lipps and built by Catto.
Click to expand...

There is a lot going on here besides just the number of blades.

Short Cliff note version;

To get the HP, they run the engines at a lot higher RPM------which means that the prop diameter must be reduced to keep the tip speed down, but due to the increase in HP, and the need for the prop to absorb that HP, and the fact the prop cant be made longer, therefore the prop needs more blades.
 
Props

Paul Lipps ran the three blade on his Lancair 235 at relatively normal r/m.
The downside of the 3 and 4 blade Catto/Lipps props is the cost.
 
Mike S said:
Short Cliff note version;

To get the HP, they run the engines at a lot higher RPM------which means that the prop diameter must be reduced to keep the tip speed down, but due to the increase in HP, and the need for the prop to absorb that HP, and the fact the prop cant be made longer, therefore the prop needs more blades.
Click to expand...

That would be the CliffsNotes version of what Steve Smith said.

We are very lucky to have guys like Steve and my Kousin Kevin from Kanada ;)
 
Paul's "Elippse" props certainly looked different, but Mr. Catto (who actually built the prop to Paul's design) is of the opinion that Paul's claims of superior efficiency may have been over-stated (i.e. perhaps real, but if so only marginal). The sharp corners on Paul's design were additionally areas of greater vulnerability, so for most of us would not have been "better" as an every day prop.

I looked into buying Paul's old 235 from the current buyer as of a year ago. For some unexplained reason someone had removed most of the paint and the aircraft had not been flown for several years (which gave me concerns about interior engine rust etc.) so I passed.
 
Theodorsen genius...

Palamedes said:
So a buddy of mine were talking about my RV10 and he was asking me which propeller I was going to get etc..

He brought up a question that I didn't know the answer to which basically distilled down to;

At what point does adding more propeller blades start to negatively affect performance?


We were all over the place but the conversation was sort of along this line..

Propellers generally have from between 2 to 6 blades for the really big turbopops..
Where as a jet engine has dozens of little blades..

Why do we not see lots of blades on all planes - jets and nonjets alike? My theory was at some point adding more blades negatively affected performance but I have nothing more than wild assertions to back that up.

So why do most Vans aircraft only use a 2 or 3 blade prop? Why not a 6? or 12? etc..
Click to expand...

Many years ago in a galaxy not so far away an aeronautical engineer perfected aerodynamic theory to a level not since equaled. His name? Theodore Theodorsen. His analysis of aerodynamics was so groundbreaking his theories are still considered a standard by which others are measured.
His propeller theory has been studied by many great prop makers including Craig Catto, Bernie and Margie Warnke and Gary Hertzler. It's an old yet surprisingly timeless NASA document worth your time to peruse and answers your question quite well albeit in Engineering terms.
Needless to say his conclusions show that like anything scientific, it depends.

My own experience testing eleven different propellers (including the first Catto RV prop) on my RV's and HR2 show that 2 blades are slightly faster at top end than three. Three blades are smoother, quieter and climb better. For the RV10 I have flown the MT3 and Hartzell BA for extended periods and my previous comment holds true.
Higher HP airplanes (P-51, C-130J, Turbines etc) are another story altogether. :)

For Theodorsen's propeller Theory, read the document...
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091989.pdf

V/R
Smokey
 
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Prandtl too!

The original theory goes back to Prandtl. Theodorsen made some interesting additional contribution, but mostly put the whole theory in usable form with graphs of the optimum circulation distribution as a function of blade number.
Andy Bauer made a nice contribution by accounting for the non-uniform inflow velocity caused by the flow disturbance from the fuselage.

smokyray said:
Many years ago in a galaxy not so far away an aeronautical engineer perfected aerodynamic theory to a level not since equaled. His name? Theodore Theodorsen. His analysis of aerodynamics was so groundbreaking his theories are still considered a standard by which others are measured.
His propeller theory has been studied by many great prop makers including Craig Catto, Bernie and Margie Warnke and Gary Hertzler. It's an old yet surprisingly timeless NASA document worth your time to peruse and answers your question quite well albeit in Engineering terms.
Needless to say his conclusions show that like anything scientific, it depends.

My own experience testing eleven different propellers (including the first Catto RV prop) on my RV's and HR2 show that 2 blades are slightly faster at top end than three. Three blades are smoother, quieter and climb better. For the RV10 I have flown the MT3 and Hartzell BA for extended periods and my previous comment holds true.
Higher HP airplanes (P-51, C-130J, Turbines etc) are another story altogether. :)

For Theodorsen's propeller Theory, read the document...
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091989.pdf

V/R
Smokey
Click to expand...
 
I think a direct answer to your question is the solidity ratio (how much area of the rotor the blades takes up as a percentage of total "disk" area; the swept area of the blades) directly affects parasitic drag; the higher the solidity ratio the higher the skin friction drag. A lot of effort goes into determining the highest lift performance with the lowest parasitic drag. At a basic level, airfoil and rotor solidity are parameters that strongly influence the design method.

A simple statement of overall in plane H force is:

CH = 0.25*sigma*CDo*mu+CHi

where, sigma is solidity ratio of the rotor (blade area/disk area). The other terms should be apparent.
 
Theory vs reality...

scsmith said:
The original theory goes back to Prandtl. Theodorsen made some interesting additional contribution, but mostly put the whole theory in usable form with graphs of the optimum circulation distribution as a function of blade number.
Andy Bauer made a nice contribution by accounting for the non-uniform inflow velocity caused by the flow disturbance from the fuselage.
Click to expand...

Thanks for that Steve, I've been enjoying the fruits of smart guys like you labors for many, many years.

V/R
Smokey

PS:I'm sure Prandtl and Theo would appreciate my absolute favorite engineering marvel, the F-16. Even while flying it I marveled at it's design capabilities and despite being 40+ years old it soldiers on as a frontline fighter combating ISIS as we speak. A testament to AE's like yourself.
Of course, The first RV3 Kit appeared the same year...:)
 
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One more thing, hub size is important. You want the smallest hub you can get away with, but also strong enough at the rotor root that it wont part from the aircraft. More blades means a bigger hub, reducing the lifting area and adding weight.
 
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