acgeorge

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What are the pros and cons regarding four blade props? Why are they so scarce? I would think anyone molding nylon 3-blade props, could do 4-blade. With the growing facination with scale warebirds, there should be a market. I would love to put one on my P-51 .60, if for nothing else, esthetics. Are there any sources, that won't charge more for the prop than my palne cost?

Andy

bubbagates

My Feedback: (32)

Andy,

I'm gonna take a stab at this but these will be my opinions and are not based on anything else other than maybe some hearsay

\I think one of the reasons you do not see allot of three blade props on 60 size planes is the diameter that is required is small enough to not make them very efficient in the air or so I have heard. I believe they look good and I have tried them and also noticed they are quieter. BUT, I did notice that they tend to increase the "P" factor slightly as now you have more air flowing over the rudder on the left side. Actually not more but more constant airflow. Torque probably also plays a slight factor in this when you get into the bigger diameters.

Now I agree a Mustang screams to have a 4 blade prop for the look but again in a smaller size effiencey all come into play. I'm sure the "P" factor and torque issues also increase.

I do know this, on the full scale Mustangs with the Merlin engines and big 4 blade props, the pilot has to be standing on right rudder if he intends to slam the throttle to full as "P" factor and torque will want to roll the plane to the left very hard. I've heard stories from Mustang pilots not hitting the rudder hard enough after quickly applying full throttle and the plane all but rolling over on it's back

britbrat

For model airplanes there very few pro's, except appearance. 3-blade, & even worse, 4-blade props, have extremely limited applications on models.

They are heavier & more costly than equivalent 2-blades. They are also less efficient -- primarily due to the necessity for reducing diameter, if the same blade pitch is to be retained, in order to keep the engine load within an acceptable range. Additionally, they have more blade-frontal-area and more wetted surface than an equivalent 2-blade, resulting in greater parasitic power losses.

Bubba has it mostly right about there being more left yaw resulting from the multi-blade set-up, although he is a bit off-base with the P-factor thingy. The greater airflow over the left side of the vertical tail is caused by the spiral slipstream from the prop. This pushes the tail to the right.

P-factor is something else altogether. It only occurs when the plane is at a positive angle of attack, as when a tail dragger is sitting with its tail-wheel on the ground. This causes the descending blades (right side) to be at a greater angle of attack than the ascending blades (left side) -- & results in greater thrust being developed on the right side of the aircraft -- thus pushing the nose to the left. When the tail rises & the AOA essentially reaches zero, the P-factor dissappears.


Where multi-blade props are usefull, is in their ability to transmit more power for a given diameter (disregarding lower efficiency). If a model has a prop clearance problem that would otherwise dictate a less powerfull engine because of limited prop size, a multi-blade prop can be the answer -- thus permitting a stronger engine to be used. However, the efficiency losses don't go away.

acgeorge

My Feedback: (3)

Wow! Thanks for the reply

Andy

dr_wogz

My Feedback: (3)

I'll add my bit too..

making a 4 blade mold takes up a big area. The same area can be devoted to a few 2 bladers.

Now, let's also talk about balancing a 4 blader! 2 is realtively easy. 3 gets tricky. 4.... well, you get the idea.

When you increase the blade number, you typically drop the dia by an inch. Some also suggest dropping 1 in pitch as well. So, a typical 14x7 for a .90, becomes a 12x7 (or 12x5) and that can look kinda dopey on a plane!

And, our little engines turn (spin) real fast! Much faster than what we're trying to copy / model. And this churning of the air, with more an more blades kinda renders them useless, as the next blade is biting into the slip stream / turbulence from the previous blade. So, it is not effective, as it is not biting 'fresh clean' air.

Now, these new 'RCV Engines' apparently have a 2:1 reduction built in, so the spin is more 'realistic' (slow) and can probably use a 4 blader more effectively / effeciently. Hence why they use a larger dia prop with a bitgger pitch for their size.

there was a time (apparently, so I'm told) that you had one blade props. (They had a counter weight on the oppocite side) these apparently were quite effecient.

As with much in engineering, it is a series of comprimises & trade offs. Ideally, all planes should have a single blade, to ensure clean non-turbulent air for maximum performace. 2 is nice, as they counter balance each other (and look right). But the size & power of some engines late in the war (since we're talking warbirds) forced the designers to either increase the prop dia to an ungodly diameter (requiring planes to sit on super long gear) or just increase the overall number of blades to maintain the same prop diameter.

The F4U Corsair has that really cool 'inverted gull wing', not because it's cool (which it is), but to try and get the fuse high enough to spin its big prop while keeping the landing gear as short as possible for carrier landings / abuse!

They do sell 'static' props for the scale birds, for 'static judging'..

Once the [proper] prop is spining, it's a blurrrrrr!!

dr_wogz

My Feedback: (3)

wow, 3 replies in about an hour of each other!

I guess my bit is kinda redundant now!!

multiflyer

Andy,

More blades are less efficient as discussed above, but offer 2 alternative features. Also as mentioned above, the more blades you have, the smaller the diameter can be. This is useful if you have a short landing gear. I put a more powerful engine one of my planes and started breaking props on less than perfect landings. so I run 3 blades to get back to my original diameter. The other bigger reason for multiple blades not mentioned above is noise reduction. Noise is becoming an ever larger concern in this hobby. Smaller diameter gives slower tip velocity and significant noise reduction.

Multiflyer

ballgunner

My Feedback: (1)

drwogz - there was a single blade prop in use about 1946 or 47. It was mounted on a Piper J3 and indeed was much more efficient than even a two blade. The single blade had a counter weight to balance it. The trouble soon proved to be that the uneven thrust wore out the main bearings in a very short time. The cost of replacing the bearings soon outweighed the efficiency of the sigle blade.

dr_wogz

My Feedback: (3)

Thanks Ball gunner for that tid bit.

I can fully understand the worn bearings issue!

I did not know it was truely tried in full scale.

rc-sport

My Feedback: (12)

I have a SKybolt with a Saito 120 and had a APC 15-6 prop on it. The plane was really fast with this prop even at 1/2 throttle, I switch to a Graupner 14-7 3 blade prop to calm it down. I swear it's just as fast as before but now it pulls vertically like a tractor. The 3 blade really added to the look of the plane. If you want a 3 blade without losing performance try a Graupner.

JohnBuckner

My Feedback: (1)

Single blade props in years past were fairly common especially in high performance applications such as controlline speed due to the efficiency. Here's and early publications suggestions for ways to do so.

John

britbrat

Sounds like it was underpropped with the 2-blade. Try your Saito with a 15-7 2-blade, or a 14-8 2-blade.

sscherin

My Feedback: (1)

Anyone rember Byron's Pow'R Prop system?
It was a gear reduction setup for Quadra motors to turn 4 bladed props.
Iron Bay still sells them
http://www.ironbaymodelcompany.com/


As for Single Blade setups most of the CL speed guys at the 89 Nats were running singles.

Sport_Pilot

And now they don't, because the drag of the counterweight was dead drag with no thrust. The loss in efficiency is very small, hardly noticable, in full scale you would have to do carefull measurement of distance and fuel to notice the differance. What we are after is performance and often a muli blade prop with lower pitch and the same diameter will give much better thrust, or a smaller diameter with the same pitch will give a bit more thurst at the same effective speed.

The real problem is selection, to have a multiblade prop that will replace all of the two blade prop's you would have to have the same if not more sizes and pitch's. Instead there are less. Invaribly someone does the rule of thumb selection of 1" less pitch, or 1" less diameter, and it is not really the right size for that plane and engine. They assume that multi blade propes are worse because of less efficency and broadcast that over the net.

britbrat

Since when? They're still doing it. The counterweight is burried in the spinner on the better set-ups.

britbrat

Interesting comments.,

The multi with the same diameter & lower pitch will indeed develop more thrust --- if more power (RPM) is available. Similarly the multi with smaller diameter & the same pitch will transmit more thrust --- but only if more power is available.

If the same powerplant is used, the multi will transmit less power & have lower overall efficiency in either case.


Where this gets confusing to some modellers is the situation where they change to a multi-blade prop & their speed goes up or their climb goes up. In those cases, they were under-propped with the 2-blade.

This is particularly prevalent with 4-stroke engines, wherein their power flattens sharply above a particular RPM & the engine is unwilling to wind up any higher. A modeller may be dissatisfied with the performance of his plane & put a multi-blade (usually 3B) on it & voila -- it takes off like a rocket. The engine spins the multi at the same RPM as the 2B & he immediately concludes that the #B is outperforming the 2B. He is correct --- except that the engine was incapable of spinning the undersized 2B at an adequate speed. Had he installed a "stiffer" 2B prop, he would have seen the plane's performance substantially improve -- with the engine still spinning at essentially the same RPM.

BTW, the rule of thumb for props fitting the 40 - 75 engine class: to convert to a 3B from a 2B, go down 1" in diameter for the same pitch --- or keep the diameter the same & lower the pitch 2". This is only a rule of thumb, but it is a good starting plan.

acgeorge

My Feedback: (3)

I would like to use a 3-blade prop on my .60 size P-51. What would be an appropriate prop if I up the engine size to .90? I'm thinking 12x6 ro 12x8 or 13x6.

What do you think? Is a .90 too large for a .60-.75 recomended engine size plane?

Thanks

britbrat

A 90 is not too large, particularly if it is a 4 stroke.

It is somewhat difficult to accurately pre-judge prop sizing, because it depends on the airframe, on the brand & model of engine, the brand & style of prop, & of course whether it is a 2 stroke or 4-stroke. If yours is an average ABC BB 2-stroke, a 13-6 3B should work OK, or even a 14-4 or 14-5 -- depending on ------ whatever. You will have to try it & see. The way to limit your costs during the experimental phase is to determine the best-fit 2-blade, then try a 3B based on that.

Sport_Pilot

<i>Since when? They're still doing it. The counterweight is burried in the spinner on the better set-ups. </i>

Find one and post it here. I haven't seen this. In fact the larger prop diameter for the single blade is sometimes a problem, less thrust at speed.

Sport_Pilot

At low speeds yes. But at high speeds you will get more thrust from a smaller column of air with a higher velocity than a larger column of air at a lower speed. That is why you will find more and more blades on the large Reno racers. Sure it's less efficient, but going fast is always less efficient than going slow!

britbrat

You get less thrust from a column of air at higher speed than you do from a larger diameter column at lower speed (assuming the same power is being applied to move the air columns).

What you get from the higher speed column is greater Delta V. You also get greater shearing losses & less usefull power transferred.

The Reno racers use multi-blade props because of the need to transfer very large engine outputs within the available diametral clearance of the propeller. They are indeed less efficient than they would be with fewer blades of greater diameter -- however, there just isn't the clearance space available to go that way.

Sport_Pilot

You get less thrust from a column of air at higher speed than you do from a larger diameter column at lower speed (assuming the same power is being applied to move the air columns).

What you say only works on the ground and at low speeds. If you are going 400 MPH and the prop stream is going 430 MPH you are not getting much thust, the only way you can go faster with the same power is to put more blades on the prop and move a smaller column of air at a higher speed.

Look at this photo of Rare Bear.

http://image.rcuniverse.com/forum/up...12/Bz77687.jpg

Scads of prop clearance here, if what you said was true then they would have used every amout of area possible and used that excess clearance!

britbrat

Sport_Pilot, Thrust isn't speed. You are confusing thrust with delta-V -- they aren't the same. Thrust does not determine speed, discharge velocity (delta-V) does that --- regardless of how fast you are attempting to fly, or how high.

For example, the turbo-fan engines on commercial jets make much more thrust than the turbo-jets & low-bypass turbo-fans in fighters, but their discharge velocity is much lower, & hence so is top speed. Even if you put a 50,000 lb thrust turbo fan on a small airframe it wouldn't go particularly fast. However, if you put a an afterburning turbo-jet, or afterburning turbo-fan, with exactly the same 50,000 lbs total thrust, the airframe would go much faster. Afterburners are used to increase both thrust & delta-V in jet engines. An afterburner does two things -- it increases total thrust (power), to be sure, but more importantly it greatly increases the velocity of the exhaust stream.

Lets take two similarly sized jets as examples -- the Harrier & the F-16. Both generate essentially the same total thrust, but the afterburning F-16 has a discharge velocity more than twice the speed of the exhaust discharge in the Harrier. Despite having as much thrust as the F-16, and despite actually being slightly smaller & lighter, the Harrier remains firmly subsonic, while the F-16 can hit Mach 2.


Getting back to propellers -- the number of blades is irrelevant to discharge velocity, & hence speed -- it is only relevant to power transmission. More blades mean that more power can be transmitted for a given propeller diameter. However, there is a penalty associated with multi-blade props -- weight & drag. They obviously weigh more, but more importantly they cost more power through parasitic losses. Airframe builders only go to more blades when they can no longer transmit increased power through the existing prop due to constraints in blade diameter. The constraints can be limits on available clearance, or limits in blade tip velocity &/or mechanical strength.

In the case of your Bearcat example, it is up against propeller RPM limitations, so it is forced to use a 5-blade prop. It can't use a 4-blade of the same diameter as the 5-blade to transmit the available engine power (not enough blade area) without an unacceptable rise in RPM.

Multi-blades are no magic solution to speed. The Schnider trophy racers had an absolute requirement for speed & they used 2-blade props. The 2-blade fixed-pitch prop on the Supermarine S-6B float plane was quite sufficient to transmit well over 2,000 HP and push it to 407 mph. That is a FLOAT plane with floats, braces, struts & wires hanging out in the breeze going more than 400 mph. If they could have gone faster with a multi-blade prop it would have been fitted instantly -- except that the multi would have been slower due to lower overall efficiency.

Sport_Pilot

Sport_Pilot, you are confusing thrust with delta-V -- they aren't the same. Thrust does not determine speed, discharge velocity (delta-V) does that --- regardless of how fast you are attempting to fly, or how high.

No I am not, you cannot accelerate to a higher speed without thurst. Thrust is Delta V * Area (of blades). Leaving out a few details of course.


Getting back to propellers -- the number of blades is irrelevant to discharge velocity,& hence speed --


wrong, you cannot get the speed with the same area without increacing power. You have to decrease the area, once you decrease the area, then the extra blades help increase the thust.


In the case of your Bearcat example, it is up against theboth clearance & RPM limitations so it is forced to use a 5-blade prop. If it had adequate ground clearance to swing a larger diameter 4-blade prop of the same pitch as the smaller diameter 5-blade that you are so entranced with -- it would generate MORE thrust than with the 5-blade & would certainly accelerate faster & likely be able to go slightly faster.

I never said anything about 5 bladed props, in fact I think the Rare Bear has a 4 blade prop. It has plenty of excess prop clearance.

However, the blades could possibly fail through excessive centrifugal loads.

Centrifigal loads, you are really pulling at straws. Think sonic.

The 2-blade fixed-pitch prop on the Supermarine S-6B float plane was quite sufficient to transmit well over 2,000 HP and push it to 407 mph. That is a FLOAT plane with floats, braces, struts & wires hanging out in the breeze going more than 400 mph. If they could have gone faster with a multi-blade prop it would have been fitted instantly -- except that the multi would have been slower.

I think the supermarine was a bit before these things were worked out, I don't know how much of this they knew back then, but I don't recall 400 MPH speeds from that era, not saying you are wrong. Just that 300+ MPH would seem more likely.

When I think about it the Reno racers may not be the best example, if I recall correctly multiblade props are at a speed advantage at altitude, or more so. Both the Reno racers and the Schnider cup was at low altitude.

britbrat

You cannot accelerate without delta-V, regardless of thrust. You can inrease delta-V without increasing thrust -- which is what you do with a smaller diameter higher-pitch prop. The thrust is often actually less in those cases

Actually you are wrong --- you can increase speed with the same area by increasing pitch (delta-V) -- up to the limit imposed by the required power. Try it yourself -- put a flat pitch prop on your model & measure the speed -- change to a steeper pitch prop with the same blade area -- you will go faster, despite a possible decrease in power.

Tip Mach number is important for prop efficiency, but tips can actually go supersonic in many cases without failure, whereas centrifugal loads impose absolute mechanical limits on prop RPM.

The Supermarine S6B set a world speed record of 407 MPH -- with a 2-blade fixed-pitch propeller.

Speed is always dependent upon power, regardless of altitude. The ability to deliver power at high altitude is indeed dependent on propeller blade area. Since propeller RPM can't be increased due to tip speed & mechanical limits, & diameter has the same limitations, as well as practical limits on size, therefore more blades are the answer -- but that is not different from the low-level case.