two vs four blades

[S55]

This is to add more gas on the fire of various disputes about multi vs. two blade props.
My geared motors were doing fine with 9x7 props, but I wanted a 7 inch diameter and, if possible, without sacrificing static thrust and pitch speed. I started to play with various props. First thought was try a larger higher pitched one and cut it down to 7 inch. I quickly abandoned them: too much needed power for poor results. I then tried to mount two props of 7 inch at 90 degrees to make a 4 blade prop. The best results I got came from using the 7x? props from the Sky Runner. The pitch is unknown. The same props come from Megatech (???), but also unmarked.
In few words I get the same static thrust and more pitch speed for a little less input power.

9x7 prop, 2.8/1 geared 280 motor, 158 g thrust, 32 Watts input, 3600 RPM, 21 mph pitch speed

2x(7x?) prop, 5.3/1 geared 300 motor, 158 g thrust, 27 Watts input, 3950 RPM, 22.5 mph pitch speed

I hope this may help others or fuel other tests.
S55

[S55]

Two blade chart

[S55]

Four blade chart

[S55]

I forgot to ask this: does anyone know how to speed up the guessing? Say I want to switch from two blade D1xP1 to four blade D2xP2 keeping roughly the same static thrust, pitch speed and electrical power. Gear ratio may be changed if needed. I agree there may be some testing needed, but how do you move in the right direction quicker?

Thanks,
S55

[Lancair]

motocalc (www.motocalc.com)

[HarryC]

Originally posted by S55

2.8/1 geared 280 motor,

5.3/1 geared 300 motor,

Sorry, but your conclusion about the 2 or 4 blade prop is not valid because you have changed other parameters. It could be the change in gearbox or motor that is causing the change, you have chosen to ignore that and ascribe the effects entirely to the change in prop because that is what you wanted to see.

There is no dispute in science about the efficiency of the number of blades on a prop, the only dispute is amongst modellers who don't know the science.

H

[Dick Huang]

Originally posted by S55
I forgot to ask this: does anyone know how to speed up the guessing? Say I want to switch from two blade D1xP1 to four blade D2xP2 keeping roughly the same static thrust, pitch speed and electrical power. Gear ratio may be changed if needed. I agree there may be some testing needed, but how do you move in the right direction quicker?

Thanks,
S55

S55
For propellers of the same power consumption:
D2=D1*[B1/B2]^0.2 where D is the prop Diameter and B is the number of blades.
For example switching from a two blade to a three balde propeller requires a new diameter of D3=[2/3]^0.2=D2*0.922.
BTW I agree with Harry C about the validity of your conclusions; it shows you can change motors and gear ratios to get the answers you are looking for between two and four blade props.
Also how did you get pitch speed of your 7" four blade prop if you did not know the pitch?

Dick Huang:D

[jrb]

Don’t have a RoT (Rule of Thumb) for your question specifically; but with respect to power absorbed by a multi (N) blade prop as compared to an equivalent two (2) blader with the same pitch (Dick, thanks posted this before I finished my typing):

DiaN=D2*(2/N)^.2 >> D4 = D2 * 0.871 >> 10” 4 blader loads up like an 11.5” 2 blader of the same pitch.

Agree that most modelers subscribe to many myths about props; based on experience with high revving glow/internal combustion engines.

No comparison to using an electric motor and gearbox; specifically if the gearbox ratio can be varied such that input power is maintained. Doing this nullifies the myths.

Even some well known aero authorities will argue/disagree about multi blade performance versus less blades.

A special case that is often cited is that of 1/2A Control Line Speed where a single blade is used. They’re used here because these models must ROG! They need thrust (Diameter) to Take-Off; the reduced prop diameter on a two blader reduces the Area (prop disk) enough as to cause ROG problems. It should be noted that the 1/2A engine rev up at over 20,000rpm and that the counter weighted single bladed prop is precisely indexed to the crankshaft.

These days though (maybe this is what HarryC meant) more blades are used for Increased efficiency and Less noise – Mother Nature typically doesn’t allow things to be less efficient and quieter!

The following photo shows measured data and MotoCalc predictions for 3 props: 4 blader, 2 blader of same D & P, and an Equivalent 2 blader

I’m quite happy with the performance of the 4 blader I use on my Mustang (40 ARF coversion) as can be seen in my gallery ( http://rcgroups.com/gallery/showgallery.php?cat=998&stype=2&si=jrb ); couldn’t turn an larger prop and a smaller 2 blader just doesn’t look right.

Here are several searches that lists other threads about the same topic:
http://www.rcgroups.com/forums/search.php?action=showresults&searchid=1529759&sortby=lastpost&sortorder=descending

http://www.rcgroups.com/forums/search.php?action=showresults&searchid=1529768&sortby=lastpost&sortorder=descending

[S55]

Lancair,
After reading a thread where Sparky Paul measured numbers that were far from what MotoCalc predicted, I did not even bother to look at it.


HarryC,
No matter what I change, if it is to my advantage, I will use it. When working with two blade props only, it is common practice to change props, motors, gears, cells to improve performance. Why is this not OK when changing the number of blades?

Dick,
That prop looked a bit more pitched that a 7x6 GWS and I thought it cannot be 7x8, therefore I assumed it must be 7x7.

JRB,
Thanks, I have seen you are a multi blade supporter.

S55

[vintage1]

If all things were equal a multi blade prop at a given pitch would be precisely identical to a larger two blader at the same pitch and RPM. However things are not identical and prop blades at high RPM running in the wake of the preceding blade can suffer as a result.

If you look at fullk size, where a lot of work has been done, you find most lighty aircaft use 2 bladers, most geared turboprops 4 bladers, and radials tended to use 3-4 blades.

A very few warplanes used 5 bladers, and thats it. No one uses 6 blader that I am aware of.

what this suggests to me is that its not a cut and dried issue. It looks like a compromise depeniding on efficiency, thrust, RPM and size available, with fans being too ineffcient, and single bladers too hard to balance, or too big.

[HarryC]

Originally posted by S55

HarryC,
No matter what I change, if it is to my advantage, I will use it. When working with two blade props only, it is common practice to change props, motors, gears, cells to improve performance. Why is this not OK when changing the number of blades?


Yes you should change other factors to suit the prop. But what you did was state that all the improvements, or lack of less thrust, could be credited to the 4 blades versus 2 blades. You can't draw that conclusion unless everything else was exactly the same. Clearly it wasn't, differences between the 2 systems you tested could have been due to using a quite different motor. They could have been due to you using the different gear ratio, maybe a different box altogether with different friction characteristics as well. Nothing that you did allows you to draw any conclusion solely about the number of blades.

H

[jrb]

There’s the old one thrashes the other argument. Doesn’t look like it in this photo; nor would it be done quietly:

[jrb]

I fly my 40 ARFs at a lot less rpm then do the glow guys, thanks to the gearbox. Allows me to use larger more efficient props.

How many blades?
The Lockheed Martin C-130 is the US Air Force principal tactical cargo and personnel transport aircraft, and the C-130J Hercules is the latest model, featuring a glass cockpit, digital avionics and a new propulsion system with a six-bladed propeller.
http://www.airforce-technology.com/projects/hercules/


US Navy C-2 & E-2C:

http://www.hamiltonsundstrandcorp.com/hsc/proddesc_display/0,3809,CLI1_DIV22_ETI2937_PRD80,00.html

Here’s a picture of an ungeared multi blade solution:

[HarryC]

Originally posted by vintage1
If all things were equal a multi blade prop at a given pitch would be precisely identical to a larger two blader at the same pitch and RPM. However things are not identical and prop blades at high RPM running in the wake of the preceding blade can suffer as a result.


There is more to it than that. Blades are just wings and like all wings they have a spanwise distribution of lift, they lose lift (thrust) at the tips, which in turn are generating drag from tip vortices which just uses engine power for no good. The more tips that you have the less lift or thrust you get for the same power. You may have noticed that gliders are not biplanes, there is just far too much lift lost and drag created to make it viable.

But props suffer a double whammy, they lose lift at the roots as well as at the tips. The spanwise distribution of lift is not spread from tip to tip like a glider wing, it goes from root to tip. Only the central section of each blade is converting power into thrust, the root and tip are wasting power as drag.

So the more blades you have, the more roots and tips you have thus the more power you waste.

There are 2 main reasons why some aircraft go for more than 2 blades. The first is the physical size of a 2 blade prop can not be accommodated - think how large a 2 blade prop would be on a Corsair, and how much more gull wing it would need for ground clearance. Try a 2 blade on a turbo-prop that has several times the power of the WW2 fighters and the thing would be impossible to fit. Although the multi-blade prop is less efficient, the extra engine power being fitted is far more than the prop loss so it is worth doing. The other main reason is that if you fit a larger prop, the tip speeds go up in proportion to the diameter and will quickly hit the transonic and supersonic flows, which is baaad news! Keeping the diameter lower and absorbing power with more blades allows you to keep the tips below the sonic flow speeds.

H

[Sail 'n Soar]

HarryC's last post is a good summary cutting through much of the chaff. A really good place to see optimum design approach put to practice is with man powered aircraft, where optimum propeller efficiency is critical when you are extremely power limited. There two-bladed props are king.

Of course I'm surprised that nobody has raised the topic of even greater efficiencies with single-bladed props. In fact, single-bladed counter balanced blades have been used effectively in some model aircraft applications. In that case, there are relatively few applications where the greater propulsive efficiency offsets the mechancal and structural static and dynamic defficiencies of such a design.

[markdrela]

So the more blades you have, the more roots and tips you have thus the more power you waste.


No, this is backwards. When going from 2 to 4 blades you have twice as many tip vortices, but each vortex is half as strong. There is actually a slight net decrease in the wasted energy (induced loss). This is analogous to the situation with a wing -- a biplane has less induced drag than a monoplane of the same span and same total lift.

In any case, this benefit of increasing the number of blades is tiny relative to the large penalty of decreasing the diameter. So to address the original question...
There is no way you can retain the same static thrust (at the same power!) by adding more blades. Disk area is all-important.

[markdrela]

Originally posted by Sail 'n Soar
A really good place to see optimum design approach put to practice is with man powered aircraft, where optimum propeller efficiency is critical when you are extremely power limited.

This is not the reason why 2-blade props are most efficient on HPA's and on most model aircraft.

The real reason is because the blade chords on a 2-bladed prop have to be about twice as big as the blade chords on a 4-bladed prop of the same diameter and rpm. Twice the chord means twice the Reynolds number, which means a considerably smaller profile drag loss. For this reason a one-bladed prop is aerodynamically the best on many small models which are especially sensitive to Reynolds number. But 1-bladed props cause headaches with balance, so they are not seen very often. A 2-blader is the next best thing then.

On large airplanes the blade Reynolds number doesn't matter too much, so here the ideal blade number is set by other considerations, mainly getting practical blade chords.

[HarryC]

Originally posted by markdrela
but each vortex is half as strong. There is actually a slight net decrease in the wasted energy (induced loss).

Where on earth do you get that from?

H

[markdrela]

Originally posted by HarryC
Where on earth do you get that from?


From Betz-Prandtl's blade/vortex theory for propellers, developed circa 1930 if I recall. There have been subsequent refinements by Theodorsen (ca 1950), and Larrabee (ca 1980). All are consistent with this regard.

[Sail 'n Soar]

markdrela

I believe we are in violent agreement.

The real reason is because the blade chords on a 2-bladed prop have to be about twice as big as the blade chords on a 4-bladed prop of the same diameter and rpm.

Who said anything about the same diameter? My point was focused on your statement in the post right after mine:

In any case, this benefit of increasing the number of blades is tiny relative to the large penalty of decreasing the diameter. ... Disk area is all-important.

i.e., better to have a larger diameter 2-bladed prop than a smaller diameter similar cord/RE n-bladed prop - an MVdV thing.

But 1-bladed props cause headaches with balance, so they are not seen very often. A 2-blader is the next best thing then.

As I said,

In that case, there are relatively few applications where the greater propulsive efficiency offsets the mechancal and structural static and dynamic defficiencies of such a design.

Counter balance is one aspect. In addition to being heavier even when balanced, the one-bladed prop is going to creat a cyclic aero load, which is going to give other structural response problems problems to light weight structures.

[HarryC]

Originally posted by markdrela
From Betz-Prandtl's blade/vortex theory for propellers, developed circa 1930 if I recall. There have been subsequent refinements by Theodorsen (ca 1950), and Larrabee (ca 1980). All are consistent with this regard.

Are you able to elucidate on this? I don’t have any of their papers! IIRC power absorption is related to the 4th power of diameter, indicating that changing from 2 to 4 blades would see a reduction in diameter to approx 85% of the 2 blade diameter. If aspect ratio is maintained then chord is also 85% of the 2 blade indicating a reduced vortex, assuming that the same rpm is achieved and allowing for a lower tip speed due to the reduced diameter it would indicate that the sum of the tip vortices size and energy is still well above the 2 blade prop. If you add in the spanwise distribution of lift along the blade then the more blades you have the greater the area that has reduced lift as a fraction of the sum of the blade spans. What is the propeller theory maths that shows that increasing the number of blades does not affect the thrust (ignoring diameter efficiency)?

H

[Dave Hederich]

I thought it might be interesting to completely remove the motor and gearing factors from the equation and look at the efficiencies of blade count on wind turbines. Perhaps those with engineering degrees can comment on the validity of studying devices that convert wind into energy rather than vice versa. It turns out that most modern wind turbines, for a variety of reasons, have three blades, but that some also have two blades or even one. Blade counts higher than three are typically reserved for wind-driven pumps only.

I found the following to be the most interesting of what I was able to read in the limited time I had available:

As a blade rotates, it moves into the space occupied by a previous blade. The limit to speed of rotation is that this space should not contain air strongly perturbed by that previous blade; therefore, fast-turning rotors should have few blades. However, having more blades increases torque on the rotor shaft.

The general rule for the optimum number of blades on a rotor depends on its function, and can be outlined as follows: electricity generation requires high speed at low torque, so the rotor has few blades; water pumping (and historic milling) requires large torque at low speed, so this rotor has many blades.

The minimum number of blades is one, which is possible with a dense counterweight; however the rotor motion is very uneven because the wind speed is higher with the blade up than it is with the blade down. Having two blades is common, but motion is still not steady, and the visual impact can be slightly disturbing. A three-bladed rotor has a steady motion, is quieter and is visually the most acceptable. However blades are expensive, so the fewer of them there are, the cheaper the turbine. It is suggested that three-bladed turbines will become the norm on land, but two-bladed machines may become common for offshore wind farms.

http://www.jxj.com/magsandj/rew/2003_03/wind_turbines.html

And this:

For each blade on a wind generator's turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbine.

To reduce the precessive stresses, modern turbines have three blades, only one of which is in a maximum stress position (vertical) at a time. The major historic design defect is to have an even number of blades, so that two blades are vertical at the same time. Two-bladed turbines have the highest cyclic stresses.

When there are four or more blades, the blades of a high-speed, high efficiency turbine start stalling in the disturbed air from the previous blade.

There are a number of vibrations that decrease in peak intensity as the number of blades increases. Some of the vibrations, besides wearing out the machine, are also audible. However, fewer, larger blades operate at a higher Reynolds number and are therefore more efficient. Also, the cost of the turbine increases with the number of blades, so the optimum number of blades turns out to be three.

http://en.wikipedia.org/wiki/Wind_turbine

For those who have more time to search, simply copy and paste the following into the Google search engine:

"wind turbines" "optimum number of blades"

[Sail 'n Soar]

Perhaps those with engineering degrees can comment on the validity of studying devices that convert wind into energy rather than vice versa. It turns out that most modern wind turbines, for a variety of reasons, have three blades, but that some also have two blades or even one. Blade counts higher than three are typically reserved for wind-driven pumps only.


From an engineering standpoint the same thing goes whether ou are speaking propulsion or energy extraction. Wind turbines I have seen have two blades. This includes individual wind turbines across Germany and the large wind turbine farms across California.

I don't believe I have ever seen anything other than two-bladed wind turbines in actual practice. To your question, I believe the same thing applies whether you are speaking propulsion or energy extraction - as long as you don't exceed the capability of the individual blade.

[markdrela]

Originally posted by HarryC
What is the propeller theory maths that shows that increasing the number of blades does not affect the thrust (ignoring diameter efficiency)?


I never said that increasing the number of blades doesn't increase thrust or power. Of course it does (assuming rpm is the same).

I said that it will have very little effect on efficiency, provided the thrust or power are kept the same (via reduced blade chords). The original question was for the case where power was being held fixed, so that's what I was working with.

[jrb]

Its always amazing how theoretical these discussions turn when most of the work with props has been empirical!

Lets just for the moment use MotoCalc as an unbiased scientifically based analysis; the attached is data for my Mustang. Its shows that the 4 blader has more static thrust and a lot better climb rate than either the equivalent power (ep) or equivalent pitch & diameter (pd) two bladers. Though the pd has lower climb rate its traded for a slightly higher max flight speed; which on is important to you? The ep is not a good choice at all.

Climb rate is more important to me; as this will get the plane off the ground and out of trouble better than having a few mph more max speed. For another’s mission profile speed may be more important.

I suggested using MC just for the moment above as I find its prediction of “pitch speed” (specifically static) to be grossly inaccurate; and subsequently I question any calculations using pitch speed, i.e. flight speed.

We all in the end fly with the prop that gives us the best flight performance based on empirical analysis – how it flies for us!


Someone’s interest or use of a multi blade prop shouldn’t just be chastised or thrown our of hand based on the empirical analysis of others.

S55, have you made an empirical/flight analysis?

[S55]

JRB,

To make that analysis I need some experience to begin with and I do not have it. I could probably judge that more thrust is needed, for example, but where to go from there? There are many things that cannot be changed without affecting others and I cannot cut and try by flying the model. Too much time and often the differences are hard to notice and you cannot really tell if you went the right direction. Experimenting in the garage is much quicker and if you know what your variables are, you can easily see that the 4.2A reading is lower than the 4.4A you got before, for example. It took less than a weekend to play with more than a dozen various propellers and three different motors, and this included the usual weekend chores plus two trips at the hobby store, because the first time I forgot my wallet home.

So anyway, I would need to fly it first. And even before that, I need to build it. My only constraints were: wingspan of 40 inch max and battery power of 60 Watts max. Like I said my motors looked OK with 9x7 props, but the plane look with such big props did not look right to me. This is why I was looking for some smaller props and I thought 4 blades may help me get there. And they did.

S55

[Dave Hederich]

>> Like I said my motors looked OK with 9x7 props, but the plane look with such big props did not look right to me. This is why I was looking for some smaller props and I thought 4 blades may help me get there. And they did.

After reading many hundreds of messages here on this subject over the last few years, I've found that "appearance" is the primary reason why people want to use 3- and 4-blade props on their R/C aircraft in place of 2-blade props. The second most common reason is ground clearance.

A few people believe they can get better overall performance with more prop blades. Of all those I've read, JRB has gone the furthest toward making a case for better performance with more blades. But I still haven't seen anyone convinced to change their position on this, so I guess it would require more compelling evidence to accomplish this.

That's why I brought up the subject of blade count on wind turbines. By taking the motor, gearing and aircraft out of the equation, we get closer to determining the efficiencies of the prop itself without all the variables. The fact that fewer blades appear to be more efficient on a wind tubine and a greater number of blades is preferable for a wind pump suggests that there might be performance-related issues that favor one over the other in various aircraft applications, as well.

There is evidence of this in the aviation industry, as well, where the most modern tuboprop aircraft (Saab 2000, Fokker 50, C-130J) use props with more than 4 blades. Just what all this means in terms of relative performance for scale model e-flight appears to be open to debate.

[jrb]

Here’s a thread that might indicate another reason for using a multi blade prop:
http://www.rcgroups.com/forums/showthread.php?s=&threadid=152273

This link shows some interesting info; similar to Dave’s wind turbine info:
http://www.rcgroups.com/forums/showthread.php?threadid=145863

Not sure what’s more compelling evidence than what’s being done ; especially in an empirical world! -- “There is evidence of this in the aviation industry, as well, where the most modern turboprop aircraft (Saab 2000, Fokker 50, C-130J) use props with more than 4 blades. Just what all this means in terms of relative performance for scale model e-flight appears to be open to debate.”

The attached photo shows a plane that certainly doesn’t have a ground clearance issue; but probably uses the 3 blader for improved climb.

That 1st link above suggests that a large diameter prop may effect stability; where you could transfer the same power using multi blader w/o the P effect.

[Sail 'n Soar]

The attached photo shows a plane that certainly doesn’t have a ground clearance issue; but probably uses the 3 blader for improved climb.

I believe you jump to the obvious too quickly. My interpretation of the same photo is that those large baloon tires indicate this plane is flown off unimproved runways. I can easily imagine that you would need an even smaller diameter propeller in that case to avoid nicking the prop tips in tall grass and brush, etc. Besides, raise the tail of that tail dragger and much of that ground clearance goes away.

Quite a while ago Scientific American published an excellent laymen's article on propellers, but of course I can't locate it. But if you really want to find an article on the topic that doesn't require an engineering degree to understand, attack your local central library and have at it.

In terms of the theory I was able to find this from an article on manpowered flight published in the November 1985 Scientific American.

"Any propulsive device (with the exception of a rocket" that generates thrust takes in air at flight speed and expels it to the rear at a higher speed as a jet. In the case of a propeller the jet is the slipstream: the air pushed aft of the propellere. A flapping wing pushes back an amorphous mass of air with each stroke.

In every case the jet carries kinetic energy that has been added by the propulsive device and that cannot be recovered; it is eventually dissipated as heat. As the jet velocity increases, the loss from wasted energy goes up faster than the gain in thrust. Thus efficiency dictates a device that takes in a large mass of air and adds to it only a small increment of velocity. This goal calls for a propeller that has a large diameter or for flapping wings that have a large span."

The SA article I wish I could find devotes an entire article expanding on the science behind these two short paragraphs.

"P factors", sonic tip speed limitations on large platforms, the ability to absorb more power within a diameter-limited case, lower acoustics, etc., etc., may drive you to smaller diameter props with greater numbers of blades. Ignoring the topic of viscous drag for the moment, these other factors aren't going to change the basic physics that you want to accelerate the largest mass of air practical the least amount for the highest propulsive efficiency, i.e., for the most amount of thrust from the energy expended. Maximizing disk area is the way you capture the largest air mass. And unless you have more power than you can absorb with a reasonable aspect ratio 2-bladed prop, you maximize disk area by going to the fewest practical number of blades. Considering the shortfalls already discussed relative to single blade props, the fewest practical number of blades is two.

Belaboring the point, there may be other reasons you may have to go to smaller diameter and higher blade count props, but it isn't , propulsive efficiency or thrust per watt.

[markdrela]

Originally posted by Sail 'n Soar
Quite a while ago Scientific American published an excellent laymen's article on propellers, but of course I can't locate it. But if you really want to find an article on the topic that doesn't require an engineering degree to understand, attack your local central library and have at it.


The author was Eugene Larrabee. I don't remember the date of the issue. Sometime in the early 80's or mid 80's I think.

[jrb]

My data indicates otherwise.

Also, most measurement sets don’t include actual pitch speed data as mine does; clearly indicating a major deficiency.

[Sail 'n Soar]

Originally posted by jrb
My data indicates otherwise.

Also, most measurement sets don’t include actual pitch speed data as mine does; clearly indicating a major deficiency.

?

[jrb]

Not at all surprised by your “?” SnS!

Most electric wind turbines I’ve seen are 3 bladers.

[Sail 'n Soar]

Originally posted by jrb
Not at all surprised by your “?” SnS!

Most electric wind turbines I’ve seen are 3 bladers.

I did a quick image search on Google for wind turbines and almost all were 3 bladers. I stand corrected. It's most likely faulty memory on my part.

But actually, the "?" was in reference to your statement "my data indicates otherwise" and "... measurement sets...." It isn't clear to me what data and measurement sets you are referencing.

[Dave Hederich]

Most electric wind turbines are 3-bladers for reasons not related to efficiency, as noted in my previous message. The 2-blade and 4-blade configurations have cyclic stress problems unrelated to aircraft configurations. Most electric wind turbines use the minimum number of blades that does not create cyclic stresses. In the absence of the cyclic stress factor, that would be the 2-blade design, not the 4-blade design. So I think electric wind turbine blade counts generally support the concept that fewer blades are better. On the other hand, wind pumps generally support the concept that more blades are better. This is only meaningful to the aircraft blade count question if a connection can be made.

[jrb]

SnS, seems you also missed my posting [8] that spoke to ½ CL Speed and its use of a single bladed prop as you expressed “surprise” in [15] that it had not been discussed.

My previous postings [i.e. 31] and the photo below shows and speaks to measured pitch speed (velocity); rarely is measured pitch speed included in the work of others.

The dashed red line at the top is the calculated pitch speed for 11” pitched prop using the same manner as MotoCalc (p” x rpm / 1056). The measured velocities of the props with an 11” pitch are below this line – the 4 blader though, being closest.

Also note that as predicted by the fundamental equations of propulsion speed in linear with rpm, thrust the square, and power the cube (all data in this chart are “measured”).

The chart shows, as does the data that I previously posted, that a 4balder turns rpm into velocity more effectively (efficiently) than a 2 blader; and, that for the same input power the 4 blader develops more velocity and hence thrust then the two blader of the same pitch and blade form.

Beyond making more air statically, the 4 blader also adds more velocity to an approaching airflow than its comparable two bladders – this of course is real interest in flight.

Dave, I suspect that difference in blade types & numbers used for generating electricity as compared to pumping water has more to do with the required input power/rpm characteristic of the output device.

[jrb]

This photo (sorry for the poor quality) shows my free-jet test stand along with my collection of synchronous & series wound AC motors and my horizontal thrust stand. I also have a vertical thrust stand; both are calibrated before each test using a set of weights.

[Sail 'n Soar]

jrb,

Just a quick comment tonight. OK, I had forgotten that you had started this thread with your test data. In terms of measured "pitch speed", I take it you are referring to measuring the down stream slip stream, or free jet, velocity. If so, I'm not sure that's what the community refers to as pitch speed. I had always interpreted it to be more related to the calculated one in the sense that you are calculating the speed at which the prop airfoil zero lift line is nolonger at a positive angle of attack to the free stream flow. That is, the hypothetical velocity at which you nolonger get thrust, ignoring RPM change as the motor is unloaded, and only indirectly related to the slip stream velocity.

I'll give more thought to your points, but can't write more tonight - getting ready to go out of town.

[jrb]

I’m not sure to which community SnS makes reference, but here’s MotoCalc’s definition and the picture that follows is from NASA – Aerodynamic Nomenclature [ http://naca.larc.nasa.gov/reports/1921/naca-report-91/ ] (my propulsion community: USAF, AEDC, UTSI, NASA, and propulsion vendors like GE, PWA, RR, MTU, Williams, etc.):

“PSpd or Efflx (mph or m/s) - Predicted propeller pitch speed (or fan efflux velocity), in miles per hour or metres per second. This is the speed of the air as it leaves the back of the propeller (or fan), relative to the air through which the plane is moving (i.e. the excess pitch speed, beyond the speed at which the plane is flying).

Copyright (c) 2003 by Capable Computing, Inc.”


Here some interesting comments about props from a manufacturer:

http://www.hartzellprop.com/engineering/sitelink_engine_faqs.htm
If two-blade propellers are the most efficient, then why don’t all propellers have two blades?
The short answer is because efficiency doesn’t propel the airplane, thrust does. Efficiency is the ratio of the power coming out of the propeller to the power going into it. A two-blade propeller is capable of achieving a higher efficiency than a three-blade propeller and so on, but at the same time it uses less power and produces less thrust.

Will a three-blade prop make my airplane quieter inside?
Yes, in most installations. Cockpit noise comes from a variety of sources; engine, exhaust, slipstream, and the propeller. Vibrations are also perceived as noise in the cockpit. A two-blade propeller produces an inherent once-per-revolution vibration that shakes the airframe, so a three-blade propeller will be inherently smoother and therefore quieter. In a single engine airplane, the propeller blade wake will beat on the windshield producing noise. Changing from two to three blades reduces the wake intensity and also increases the frequency of the beating, which is perceived as being quieter. In a twin, the reduced diameter of the three blade propeller will result in less tip generated noise and a greater clearance between the blade tip and the fuselage. Both of these characteristics will reduce cabin noise.

M O R E B L A D E S E Q U A L S
M O R E P E R F O R M A N C E
Unfortunately, two-bladed props have some inherent disadvantages.
For starters, they’re louder. Propellers are the source of most of the noise generated by an aircraft. And two-bladers are often larger in diameter, resulting in nearly supersonic tip speeds ... and dramatically increased noise levels. Refinement of propeller airfoil design can also reduce noise levels to a more comfortable level.
Two-bladers are also more prone to vibration. Two blades create two large pulses of thrust compared to a three-blader’s three smaller, smoother pulses. Two-blades often provide less ground tip clearance which leads to more nicks and scratches on the blades. And as the size of the engine increases ... requiring more blade area to effectively absorb the increased power ... two-bladed props must become unconventionally long and heavy.

Various NASA prop reports & info

http://wright.grc.nasa.gov/airplane/propth.html

http://www-spof.gsfc.nasa.gov/stargaze/Sflight.htm

http://naca.larc.nasa.gov/reports/1920/naca-tn-16/

http://naca.larc.nasa.gov/reports/1920/naca-tn-21/

http://naca.larc.nasa.gov/reports/1924/naca-tn-201/

http://naca.larc.nasa.gov/reports/1925/naca-tn-225/

http://naca.larc.nasa.gov/reports/1926/naca-tn-246/

http://naca.larc.nasa.gov/reports/1929/naca-tn-317/

http://naca.larc.nasa.gov/reports/1950/naca-tn-2192/

http://naca.larc.nasa.gov/reports/1950/naca-tn-2184/

http://naca.larc.nasa.gov/reports/1950/naca-tn-2178/

http://naca.larc.nasa.gov/reports/1951/naca-tn-2450/

http://naca.larc.nasa.gov/reports/1958/naca-tn-4365/

http://naca.larc.nasa.gov/reports/1952/naca-tn-2776/

http://naca.larc.nasa.gov/reports/1923/naca-tn-158/

http://naca.larc.nasa.gov/reports/1921/naca-report-91/

http://naca.larc.nasa.gov/reports/1920/naca-report-79/

http://naca.larc.nasa.gov/reports/1920/naca-report-73/

http://naca.larc.nasa.gov/reports/1920/naca-report-71/

http://naca.larc.nasa.gov/reports/1920/naca-report-29/

http://naca.larc.nasa.gov/reports/1920/naca-report-31/

http://naca.larc.nasa.gov/reports/1918/naca-report-15/

http://naca.larc.nasa.gov/reports/1918/naca-report-14/

[gkamysz]

Hartzell is in the full scale business. So obviously fullscale concerns are prevalent.

How many of those benefits apply to models?

As far as efficiency goes in model aircraft, my opinion is that is is MUCH more critical to choose the correct diameter and pitch to match the airframe and airspeed, before you start worrying about the 1-2% you might get with a prop with more or fewer blades.

For models there are other concerns:

4 blade prop is more expensive
4 blade prop is over twice as likely to break in a nose over
4 blade prop is hard to find in the right sizes


Have you seen this site?
www.supercoolprops.com
Click the articles link.

Greg

[jrb]

Greg, the link isn’t working and I haven’t found the site with a search as well.

Agree that choosing the “right” prop for the way you like to fly a plane is what its about.

The 4 blader on my Mustang looks good both on the ground and in the air; performance is spectacular as well – see my videos and more pictures: http://rcgroups.com/gallery/showgallery.php?cat=998&stype=2&si=jrb

Tools like MotoCalc do a fairly good job (macroscopically/empirically) indicating that a 4 blader can provide a change in performance that maybe worth considering. They don’t however do a good job (microscopically) predicting pitch speed; static specifically – slip is not applied?

Yes Hartzel does full scale propellers (small and commuter aircraft), I worked with them on their propeller test stand; their full scale propellers operate at much lower rpm (less than 1/2) than ours (E about 6k). Glow typically operates at twice ours and hence have smaller dimensions. The increased rpm and smaller dimensions do move the aerodynamic parameters (i.e.: Re, Pn, etc.) in the right direction for scalable studies – wind tunnel models are a lot smaller than their full scale counter parts.

A friend of mine wanted to put a 4 blader on his GWS Mustang after seeing mine fly (etc.); I suggested (much like Greg) that since the plane is flown w/o landing gear the 4 blader was not a good choice because of “landing” issues. Also, there didn’t seem to be a gear ratio available that would match the prop to the plane.

Knee jerk reactions based on myth are not useful for a proper understanding.

[Dave Hederich]

http://www.supercoolprops.eftel.com/

[Sail 'n Soar]

I’m not sure to which community SnS makes reference, but here’s MotoCalc’s definition and the picture that follows is from NASA – Aerodynamic Nomenclature [ http://naca.larc.nasa.gov/reports/1921/naca-report-91/ ] (my propulsion community: USAF, AEDC, UTSI, NASA, and propulsion vendors like GE, PWA, RR, MTU, Williams, etc.):

jrb,

Relax. No need for the personal attacks. There may be a few nuggets of understanding there worth considering. And thanks for ther references. There were a few there I don't recall seeing previously. But in relation to this discussion I especially like the one in the object in you post:

"(d) Pitch speed.- The product of the mean geometric pitch by the number of revolutions of the propeller in unit time; i.e., the speed the aircraft would make if there were no slip."

The statement above is a simple definition. There were two aspects of my post I was trying to get to.

First, your charts include what you call measured pitch speed. From the above definition, pitch speed is the product of pitch and arm. Is that what your are referring to as "measured" pitch speed. If so, then I don't understand the disconnect between your measured values and MotoCalc calculated values, since your plots are measured against rpm. By looking at your curves it appeared to me that your measured pitch speed was determined by measuring the air stream velocity behind the prop. In that case you are measuring an average value for the entire flow, or what I was referring to as the slip stream speed. That velocity is very much related to thrust, but it is not pitch speed. An explanation of how you measure pitch speed would clear everyting up.

The second aspect is, which I confused by referring to the zero lift line rather than the simple geometric pitch, that at pitch speed the prop angle of attack to the air stream as measured to the geometric pitch reference is zero. Ignoring prop airfoil camber for the moment, at pitch speed, i.e., no slip speed, the propeller is basically at zero anlge of attack. That means that without camber it produces no thrust. The prop airfoil camber will increase the speed at which there is no thrust - that's why I was referring to the zero lift line- but the basic point is still correct. In other words, I was discussing more the implication of the definition than the strict literal definition.

You are right. Thrust is what we are interested in. But the rest of the story is that propeller efficiency relates directly to how much thrust is generated from a given amount of power in. Paying more attention to the efficiency column in the MotoCalc tables will help you choose the prop which gives the longer motor run for the same amount of thrust. From my own experience, I switched from an 11 x 7 to an 11 x 8.5 on an Astro 803G powered model. Looking at the numbers, the 11 x 8.5 shows higher efficiency numbers at least for my specific model. The impacts were higher top speed, more excess thrust at any given flight speed - translates to better aerobatics - and when I throttle back to cruise I'm getting about slightly longer flights.

Before bringing this post to an end, thought you would like to know that I did an experiment with MotoCalc to compare the performance of the 11 x 7 and 11 x 8.5 2-bladed props with 4-bladed props optimized for the same operating envelope. Specifically, I set the top speed the same, set the maximum power in to 200 watts, the 803G limit, then compared the thrust with speed. Per MotoCalc, on 10 CP-1300SCR cxells a 9.5 x 6.5 4-bladed prop provided the same top speed as the 11 x 7 (where thrust = drag), had a practically identical input power vs. speed curve, and slightly higher thrust across the envelope. Quite a surprise to me at least. Things also change when I limited the 4-bladed prop options to available pitches. I'm not sure if this supports your contention or just indicates a limitation of the approximations built into MotoCalc.


If you are open to new ideas you might try a similar process to choose comparble pairs of 2- and 4-bladed props to test in your experimental setup. It promises the possibility of a more apples-to-apples comparison.

[jrb]

SnS found this towards the end of your response “4 bladed prop had a practically identical input power vs. speed curve, and slightly higher thrust across the envelope. Quite a surprise to me at least.”; hasn’t been to me.

Also “I'm not sure if this supports your contention or just indicates a limitation of the approximations built into MotoCalc”; my test data also confirms this, its real.

“If you are open to new ideas you might try a similar process” with an available 4 blader and a variable ratio gearbox; the fundamental element of my thesis.

Your 805G is no different than the glow situation or direct drive electric which limits the use of props to quite a narrow range.

The 1st data I showed was from tests made using a Model Electronic Corp’s SuperBox and changing the pinion for an appropriate ratio. This allows one to use a larger 4 blader than with a fixed ratio.

Its not the higher hp of the engines on the latest turboprops but a higher gear ratio (new technology) gearboxes that allow the use and benefits of higher blade counts.

The chart in [36] was made using AC motors with a lot more available hp than used by us

My synchronous motor allows me to run a prop at a fixed/constant rpm – whether its my 22x10 or 8x6. The motor as been calibrated such that I know the power absorbed by the prop (@ the motor shaft) based on input current. So I fully characterize a prop: power, thrust, and exit velocity. And when used in my free jet test the test props rpm doesn’t change whether static or with a high approach velocity.

I can also run the props with my series wound motors at variable speed. I use performance charts from the manufacturer to obtain the output power at the shaft/into the prop.

My free jet test stand allows me to mount my motors on either end; for example to have either a fixed approach velocity (sync up front) to a variable speed test prop or visa versus, or variable in both locations.

E Flight set ups of interest can also be mounted on the back of my free jet simulator; testing static and variable speed approach velocities while measure thrust, exit velocity, etc.


I’ve generated more data points than I can analyze or keep track of; but the data clearly indicates what shown below – a propeller adds velocity to its incoming flow – pitch speed does not limit flight speed!


Therefore, I use the largest diameter prop that will still let me ROG, and then pitch and gear for the current draw I want. I’m not concerned about pitch speed or efflux velocity, my planes (1/5 Cub or .40 Mustang) fly faster then either their measured or predicted pitch speed.


My impression SnS reading your posts is that you’re using Vp as your definition of Pitch Speed. Pitch speed has always been defined ignoring slip; just like a screw into wood.

Vp on the other hand is the velocity through the prop disk; it is defined as average of the free stream and exit velocities [Vp = .5(Ve-Vo)], as shown in the following graphic from NASA. At static conditions Vo is zero; I measure Vo in my free jet tests and Ve in both static and free jet tests.

[Sail 'n Soar]

My impression SnS reading your posts is that you’re using Vp as your definition of Pitch Speed. Pitch speed has always been defined ignoring slip; just like a screw into wood.

jrb,

I still define pitch speed as ignoring slip. It was your statement that you measured pitch speed that confused me.

The measured velocities of the props with an 11” pitch are below this line – the 4 blader though, being closest.

On the basis of this I thought that YOU were using Vp as your definition of pitch speed.

Since so much of this thread was really built around your static thrust measurements for various two- and four-bladed props I thought I would end by sharing this graph with you from Mark's Mechanical Engineers' Handbook, 6th addition, pg 11-109.

[jrb]

SnS, this information is dating us!

Mine is the 8th edition and it doesn’t have the same chart; though on page 11-101 it does state “The static thrust for a given power input increases with diameter and the number of blades”.

Marks’ is an excellent source; though I didn’t use it for my post graduate work.


All the best,


Jim

[Sail 'n Soar]

jrb,

FYI, I got the 6th edition among the rewards for a 1st place level Mass State science fair project on propulsion.

SnS

[Dick Huang]

I found this pic of a F4U-5N (Corsair) at take-off with the Mustan type prop vapor trails.

Dick Huang:)

[jbatch]

With apologies for (newbie) ignorance in aeronautical science
and for an outlook unconstrained by knowledge of wherefore
I speak...

It seems to me that propulsion systems should try to avoid the
effects of swirling and wasteful turbulence, ergo they should
accelerate a stream of air directly aft. To do that efficiently using
prop or turbine blades, wouldn't there be a need for some type
of shroud around the blades (or stators) inside the shroud?
Wouldn't more blades equate to better efficiency? Just asking.