N1EDM

My Feedback: (2)

I've run 3-blade props and didn't notice much change in HP. I don't know where I'd get a 4-blade prop that I could fit to the .125. I've never flown a 4-blade, either... All I know is what I've read. The 4-blade prop would be more scale, for certain. There's a good question to pop out to this forum.... Has anyone flown the P-51 with a 4-blade prop??? And, how does it compare to a 2-blade prop???

I won't turn my nose up at the idea, that's for certain. Doesn't t his take a 4-inch spinner???

Bob

stevegauth30

I know from the few tests that I've done, a 3B produced less thrust than a 2B. Does that mean 4 would make less? That's one area where I'm not so up to speed. Need to research it. I'd love to fly my warbirds with a 4B , just never thought it was practical.

Zor

stevegauth30,

It depends on the actual props you were using and we do not know that.
Also we do not either have figures of thrust you have measured and the rpms produced with whatever engine or motor you had in your tests.

A prop is a device to convert power available from the engine ( or motor ) into thrust; a forward pulling or pushing force. The value of a prop is in its efficiency in doing that. I believe that a four blade prop can be more efficient than a two blade prop. We, as modelers, are limited to the commercially available choices.

Other thoughts are that there is many other factors that makes a modeler decide on its choice of available props on the market. One of the main ones is 'scale appearance'. That factor is often prevalent against prop efficiency ( amount of thrust developped ). Most models are over powered anyway.

I certainly like to see a prop that is close to scale particularly on a warbird.

Have your choice and enjoy.

Zor

saramos

Two problems with 4 blades. First, just like with 3 blades, you have to reduce diameter . You end up with such a small prop that it looks cartoonish and not too good on the performance. To get the diameter up, you have to decrease RPM. Now your engine is not working efficiently, and is overloaded. The solution is to have a high torque, lower RPM power source. The best solution is to go with a geared motor. For this size plane, there's not a lot of choices. The only real IC choice for this size plane is the RCV SP engine. This is an unusual 4 stroke engine that has it's prop shaft mounted to the cylinder, which rotates around the piston which drives the crank shaft which is mounted vertically at the back of the engine. The crank shaft turns the cylinder/prop shaft via a ring gear with a 2:1 gear reduction. The result is a 90 4 stroke that can turn the APC 15.5" 4 blade prop. Not quite scale diameter, but close enough that it doesn't look ridiculous. A couple of things to note with this engine. First, with this setup, the plane will have a lot of torque, but at the expense of a lower top end. Second, they run hot, and it's real tempting to fully conceal them in the cowl because they fit the shape of inline engine models pretty well. Solutions are big cooling holes, or experiment with active cooling. Finally, be prepared to draw a crowd at the field because of it's unique sound, look, design and ability to turn what many will at first think is a static display prop. If you want to turn a 2 blade, it'll be 18" diameter. The other choice is to see if you can find the right electrical motor/gearbox/Kv/battery combo. I bought an e-flite 110 with the intent of trying it on the TF P-51B, but the project was put on the back burner quite a long time ago now. Here's some shots of my TF Spit with the RCV. Note the use of a ducted fan for cooling and the APC 15.5" prop.

Scott

raptureboy

Thanks Saramos for a more detailed explanation, I was trying to be a little more direct. The P-47, F6F Hellcat, and the Corsair, as well as 51's and spits all had 4 blade props in order to to harness the horsepower available efficiently. If they had 2 or 3 blades they would of needed a blade so long it would of come close to striking the ground. They had lots of torque like the RCV so they could swing them and maintain optimum RPM. I agree the 4 blade looks very nice but for most 4 and 2 strokes they are too much to pull in a diameter that is scale. I use a 15x8 3 blade on my P-40 with a DLE 20 and it looks a little small but it sounds more quiet than the 2 blade 17x6 prop I had. Nice looking Spit by the way.

Zor

Hi saramos,

I separated the two choices you mentioned above in their own paragraphs.

Your Spifire sure looks gorgeous. You are a super-builder.

I do not agree with your conclusions and you are not backing up many of your statements.

I have flown the same model with the same engine just switching from two blades prop to a three blades prop without any problems and without any evident difference in behavior of the model. Let us face it _ _ _ all the prop is doing is converting the engine power into thrust and the throttle controls the rpm ( thrust ).

Not much choice in finding four bladed props that look reasonable scale. It appears you did very well.
As I discussed earlier ( can be in another thread ) I do not agree that an IC engine can be overloaded; it either overrevs when not sufficiently loaded by a prop too small or revs down when the prop is too big in diameter or pitch. I will gladly stand corrected on that if anyone offers a logical explanation.

No doubt there is some reasons why these RCV SP engines are not more popular.

I do not see any reasons to use multi-blades props that are too small. It is easy enough to use a slightly larger diameter prop of the desired pitch and trim it down then balance it properly.

Zor

N1EDM

My Feedback: (2)

One theory that I heard of for why 4-blade props aren't as efficient is that the prop blade is still not clear of the disturbed air from the retreating blade, i.e., it's in cavitated air because of the high RPM of our engines. In real life, with a full-sized warbird turning 2000 RPM (give or take) the plane has the opportunity to move forward enough to get clear of the cavitated air from the retreating blade.

I don't know how true or valid that is, I just read it somewhere. That's my story, and I'm sticking to it....

Bob

Zor

Playing with props is ( can be ) part of the hobby.
I have cut prop tips before to reduce their diameter and then of course balance them.

I wonder how many might remember the Martin 404 airliner. It had three blade props that were quite wide ( compared to most ) and the blade width ( LE to TE of the blades' airfoil ) kept going wider as the tips were approached and then the blade tips were square.

I flew from Atlanta to Miami one day in the cockpit with the crew. The captain was one of my students many years before and that was before all the security now being applied. In those days the captain was REALLY the onboard commander.

Those engines were reving at 1800 RPM in cruise. I nearly felt I could follow the revolutions and very low prop noise level in the cockpit. I do not know the noise in the main cabin; I was never there in flight.

Prop experimentaiton is a fascinating part of the hobby..

Zor

Zor

Hello Bob,

That theory has been around for a long time and I have also read it many times.
I personally do not have any faith in it. When visualizing the airflow associated with a working prop in flight ( forward speed ) I see the air mass being accelerated backward in a rotating vortex. Visualizing the following blade ( 90 degrees behind in a four blade ) I think that following blade sees the incoming air from ahead in a not disturbed condition.

I do not intend to criticize your wording of "cavitated air" but I do not believe the air accelerated by the preceeding blade is cavitating. If it was then that blade would be in a constant stalled condition and I do not think that is reality.

I believe and visualize the air flow as being smooth and I associate this with the variable pitch of each which is diminishing from blade root to blade tip as the blade airfoil keep travelling faster as the radius of rotation keeps going up.

So I have my own uderstnding and visualization of what is going on.and like you I am sticking to it LOL .

Zor

Quikturn

My Feedback: (12)

Engines have an operating range where they operate best at and to maintain the longevity of the engine. Operate outside those ranges, you won't get the best power and/or can cause damage to the engine. If you put such a large prop on a given engine to where it can't get into an rpm range where it develops it's power that would be referred to as overloading.

FlyerInOKC

My Feedback: (6)

I think with propeller efficiency its a matter of mass, the more blades the more mass the prop has and that is what affects performance. It's a matter of balancing the overall mass of the prop to the engine's ability to create thrust without overloading. Just my 2 cents.

Zor

I understand your description of under what conditions we can use the word 'overloaded' ( overloading ).
I consider that if an engine is rated by the manufacturer to operate within a specific RPM range there would be some RPM outside of that range that could damage the engine.

I find it odd ( for example ) that my manual for the Saito 125a shows on page 16 Propeller Selection a note that reads ,,,,,

NOTE: Observe operating rpm ranges as excessive rpm can result in damage to the engine.

Nowhere in the manual is that rpm range shown ( written ).

I read the whole manual three times (again).

It says nothing about lower RPM due to oversize propellers. The propellers recommended for various usage (sport, scale, aerobatics) for the 125a is from 15 x 6 to 17 x 6 .

So I suppose it can be claimed that a prop 17 x 8 would begin to overload the engine. I do not feel that the engne would be damaged not being able to reach its design maximum RPM if that information can be found somewhere.

I wish to thank you for replying to my posting on this matter.

Zor

Zor

FlyerinOKC,

Your valuable time to respond is certainly worth some of my time to comment.

I cannot see how the propeller mass has to do with the matter of overloading an engine.
Props mass have to be balanced; no doubt about that.

The thrust is provided by the prop. The engine has no ability to provide thrust. The engine is providing torque to force the prop into rotation. As the prop is rotating and accelarating the air in the direction opposite the airplane trajectory that thrust force is exerted on the engine crankshaft and bearings. While the prop blades are rotating there is drag on the blades and that requires torque from the engine.

As we know this torque existing at a specifc RPM is a result of the power an engine can provide.
Power is RPM multipled by torque and result in producing the rate at which work is being done which is moving the airplane through the air resistance.

I imagine that we do not all understand what is going on in the same manner.

I agree that we can use the expression "overloaded engine" if we operate it beyond ( outside ) the manufacturer's specifications when those specs are available to us. My present knowledge is that two strokes engines have a range of 2.000 to about 15,000 rpm while four stroke engines are limited to about 10,000 rpm due to valves performance.

Wishng you best in the hobby.

Zor

acdii

Mass is part of it, the other is that instead of two blades pulling air, you have double the amount of air being pulled, which is where the loading comes from. Its like a 2WD car and 4WD car. With 2WD @ a given HP, you can smoke the tires, but @ the same HP with 4WD, they would be very hard to smoke, as you just doubled the load on the engine.

BTW early Corsairs had 3 bladed props, in fact just about all in theater Corsairs in WWII had 2 bladed props, it wasn't until near the end of the war when the 4 bladed props were added as the engine HP was increased. I forget which models started using the 4 bladed props. The ones Pappy flew had 3 blades.

I have a Hacker geared motor that runs off 6S batteries that would easily spin a scale 4 blade prop, the only drawback is the batteries to power it are huge and may not fit the plane. The Hacker is for a Funtana 90S that I will never fly, and it spins a 22" prop.

acdii

Two things here. First is Mass. The heavier the object, the more power is required to move it. Take a two pound weight on a string and twirl it around, see how much energy you need to exert to spin it, now take a 4 pound weight and spin it with the same amount of energy. You will find the larger weight goes much slower.

Second is the amount of air being pushed by each blade. The lower the pitch, the less air being pushed. A 17x6 prop will move less air than a 17x8 prop at a given RPM. To reach that given RPM though requires more energy for the higher pitch since more force is required by the higher pitch to move that air. The more diameter, the lower the pitch is needed to turn it at the engines rated ability. The more blades pushing air, the smaller the diameter/lower the pitch is required for that engine to turn it.

Just the opposite is true, the more power you have, the larger diameter and pitch is required so you don't overspeed the engine and have it blow itself apart. Finding the right balance of pitch and diameter for a plane is key based on what kind of performance you are looking for.

The advantage of added mass of a larger prop is the flywheel effect, the engine runs smoother, and will idle better due to that moving mass, but takes more energy to increase the speed of that mass, and will use more fuel to do so.

N1EDM

My Feedback: (2)

Zor, I still believe in my definition about the cavitated air, so we can agree to disagree. My position is that a 2-blade, and even a 3-blade prop has the opportunity to move beyond the air disturbed by the retreating blade. A 4-blade prop may not have as much of an opportunity to do that.

The only way to find out would be to do a wind-tunnel test with smoke and a high speed camera, I think :-)

Bob

stevegauth30

That being said, back to mustangs.

Zor

Agreed stevegauth30,

I hope to make this post my last one on this matter. I obviously learned my physic in a different school and pursued my knowledge in a different way.

Again, the engine does not create thrust; the propeller does that while the engine produce the torque that rotates the propeller.

A heavier prop ( more mass ) that has the same diameter and airfoil would produce the same thrust at the same RPM. It would change speed to a higher PRM or lower RPM a bit slower due to its mass.

A four blade prop does not double the amount of air being pulled. The air is not being pulled anyway; it is being accelerated in the direction opposite the airplane speed.

A 4 wheel drive car does not need twice the power from the engine.

A heavier object does not need more power to move it. It is not the mass, it is the friction (drag for a prop) that needs more power. A feather and a piece of lead will accelerate (falling) at the same rate in vacuum (no friction).

A lower prop pitch of same diameter will move the same air area giving it less acceleration and will need less torque; less engine power.

Above is some of my knowlede and understanding. It is for you the readers to decide if I make sense or not.

Again agreed _ _ _ we should return to Mustangs.

Zor

acdii

I tried to simplify it, guess it went over. Replace air being pulled/pushed with thrust(I used a visual reference) a 4 blade prop produces at least twice the thrust as a 2 blade prop of the same diameter, which requires more HP to move it. I also did not state a 4WD car needs twice the HP, I said that a 4WD car wont spin its tires with the same HP engine a 2WD car can. Simply put, power distribution to 4 wheels takes more energy than to 2 wheels, which is also true for a 4 bladed prop. Some of what you said is what I said, only in a different way. Looks like we are on the same page for the most part.

In short, the more blades of a prop, the more thrust it produces, however on the power side, the more blades, the more mass to move, the more power required to produce that thrust, hence, a smaller diameter is needed so the blades match the power output of the motor that spins a 2 blade prop with the same amount of thrust. The P-51 needed a 4 bladed prop because the engine produced more power than a 2 or 3 blade prop could effectively use while maintaining ground clearance, so they added an additional blade to create a prop size that would use the engines power most efficiently yet provide the needed ground clearance. The problem we face is we cannot get a motor that produces enough power for a scale sized 4 bladed prop, and yet maintain scale appearance and keeping it light enough to fly. The P-51 prop also spun at a much lower RPM than what our engines idle at, the engine rarely hit 3000 RPM and the prop was geared down to a slower RPM than that.

If you go electric there are motors that will turn a scale 4 bladed prop, but the power required to turn that motor exceeds the capacity of the battery pack that will fit in the plane. Thats what I ran into with mine. I have a motor and ESC and batteries that will easily do a 1/8th scale P-51 prop, but the batteries wont fit in the plane and weigh too much, they weigh more than the DLE20 that is in it now. I have a pair of 3S 8000mah batteries for my Twin Otter, and for the Hacker I take both packs and series them to make an 6S pack. Joined together they are larger than the fuse. Any smaller than that and it limits flight time to just a few minutes.

The Byron 1/5th P-51 with the 2 stroke gas and gear reduction turns a 4 bladed prop, but much smaller than scale, more like a standoff scale look. My friend has one hanging in his den, I keep bugging him to get it tuned up and fly it. I would love to hear that one go by with that 4 bladed prop.

Zor

acdii,

My post #3093 was to resume my understndings. It was not aimed at your writings.

Some of my statements came out of other posts.

Wishing you and your loved ones a Merry Xmas.

Zor

acdii

Aye, glad that is cleared up LOL SAme to you and yours!

johncoulouris

Hello everyone,

I bought this kit from ebay but the plans of the models weren't included. I tried to contact the local dealer but he couldn't find the plans from top-flite.
If anyone is willing to sell the original plans of this bird, please contact me via pm.

thank you,
John

capriman

If someone would scan them and convert to PDF format, they could be emailed to you...however, they measure 41 inches wide by 60 inches long....most copy places can only handle 36 inches wide. Trying to build this plane from scratch from plans would be very hard. You might also search Google for "p-51 on a diet" for something a little different.

skylarkmk1

Go to the Top Flite web site and look under technical info and service. It appears that they have the plans listed in the parts listing and the manual. You may need to contact customer service to order them. It took me all of 5 minutes to find the listing for P-51D Mustang Kit - TOPA0110 (60 size) and P-51D Mustang Giant Kit - TOPA0400 and find the plans listed in the parts list.

FlyerInOKC

My Feedback: (6)

Slightly off topic here, below is a link to the newsletter of the National Museum of WWII Aviation. There is an article on the Eagle Squadrons (Americans flying for the Brits).

http://campaign.r20.constantcontact....2-d4ae5284344f