AncientCityFlyer

I am assembling an ESM C-47 and have a couple of questions in regard to the engines I'm thinking of using and coresponding props. The recommended engine is a .91 4 stroke. I've read some of the threads on the RCV .91 CD and I'm getting mixed feelings on that engine. I like the size as it will fit inside the cowling w/ very little eng exposed and still allow for proper cooling. I'm just not sure if the engine will be
adequate . I want to keep it as scale as possible which limits me to a 3 blade prop of no more than 13" in dia. RCV recommends props with a higher than normal pitch. A good 3 blade would seem to be a 13 x 10, but it seems no one makes one. Graupner makes a 12.5" x 7" 3 blade but I'm thinking I'll be over reving the engines. The flying weight is supposed to be around 21-22lbs. Any ideas or suggestions most appreciated.

Thanks, Mike

earlwb

The RCV website engine info suggests a two blade prop of 13x6, 14x6, 14x7 sizes. so a 13x8 three blade is a reasonable choice then.
Master airscrew happens to make a 13x8 three blade. http://www3.towerhobbies.com/cgi-bin...&I=LXAA39&P=ML
The RCV engine does not have poppet valves so overreving it is not a big issue, you just wind up outside of its power band then.
http://www.rcvengines.com/rcv91cd.htm

AncientCityFlyer

earlwd, thanks for the input. My only issue is I've used Master Airscrew props before and they tend flex more than I like, however if that's my only option I'll give it a try. Mike

earlwb

I would fly using a two blade prop and switch over to the three blade "scale" prop for static display purposes.
I really haven't had any luck getting three blade props to work all that well on anything, including the little 1/2a engines.

cahusker

My Feedback: (50)

I have this engine and am flying a Graupner 12.5x7 three blade prop on it. It is on a BH T-28 60 size plane and flies it well. These engines are good sport engines but are not the most powerful of this size engine. The nice thing is they fit under the cowl which is why I chose it. Be sure to baffle the cowl air to keep the engine cool as they quit rather abruptly if they get too hot, from my experience with them. They also have a unique sound that is different from the standard 91 four stroke.

airraptor

My Feedback: (66)

earl hit the nail on the head.

RUN the two blade for flight and static get any 3 blade you want. I see no reason to fly with a scale looking prop unless that is part of your flight for scale competition where you have the judges watch you during start and you have a onboard starting system. other wise the judges cant tell you have a three blade prop in flight............

mike109

G'day

I used to have an RCV CD 91. It was a good engine. I put it in a large 60 size trainer to give it some air time and it really hauled the big trainer around extremely well. I am pretty sure I had a Master Airscrew 14 x 6 on it. It was an amazing sounding engine in the air. It sounded like a big four stroke motor cycle

I initially ran it on a "four stroke fuel" which had only synthetic oil in it and it was not very happy with this so I went to my usual 5% castor, 15% synthetic, 10% nitro, 70% methanol mixture and it was much happier.

I did not go ahead with the plane I bought it for and so a friend has it now and is going to put it in a 60 size warbird eventually.

Hobbsy

My Feedback: (102)

Imperial Metric
Manufacture Die Cast
Engine Type 4-stroke - Glow ignition
Displacement 0.91 cu inch 15 cc
Max Power (approx.) 1.5 bhp 1.12 kw
Weight (exc. silencer) 25.2 oz 715 g
Length 4.72 inch 120 mm
Height - centre line axis 3.23 inch 82 mm
Cowling Radius R85
Propeller shaft diameter 5/16" UNF
Practical RPM range @ prop 2,200 - 11,000 rpm

Example prop sizes (2-blade) 13x6, 14x6, 14x7
Recommended Fuel 10% Nitro / 15% Oil including max 6% Castor

A Graupner 12.5x7 or 12x8 3 blade would be a good choice and look much nicer than any others. The 14x7 would be a bit much unless you're talking about the .91 SP. The one pictured is a 14x7 which I use on my K&B 1.00 Diesel.

buzzard bait

ACF...on the high pitch recommendation I think you might be confusing your 91 CD with the 90 SP. They are completely different because the 90 SP has a 2:1 gear ratio, so it turns 5000 rpm where a normal four stroke turns 10,000. That's the reason for the higher pitch recommendation. But it applies ONLY to the SP series (60, 90, 120). So if your engine is a 91 CD, then it doesn't need any higher pitch than any other four stroke.

Going with a lot of pitch might not be a good idea on your plane/engine combination. I'm guessing you might want good low speed thrust for safe takeoff, but I don't know the plane.

There is a big controversy on another forum about the efficiency, or lack thereof, of three bladed props vs two bladed props. Passions run high on this subject! You will find lots of people insisting their 3 bladed props do just fine or better than 2 bladed.

I think it comes down to the match between motor, prop and airframe. If it's a good match, people like the prop, whether it has two blades or three.

Jim

Hobbsy

My Feedback: (102)

Jim is correct, the old wives tale about 3 blades being less effecient went out the window long ago. Consider that the prop is constantly moving forward into new air there is no disturbed air for the next blade.

buzzard bait

Well, not to be argumentative, but just as there are losses at wing tips, so must there be losses at blade tips.

A three bladed prop has three tips where draggy vortices are generated, just like a biplane has four wing tips generating vortices. So there's a theory.

But in practice, it seems any good prop is about 85% efficient and there isn't a great deal of room for improvement. People get convinced one brand, or type, whatever, is better than another, but it's really because the prop load happened to match the engine and airframe better than the one they think is inferior.

So I think as a practical matter, if you want to fly with a three bladed prop, the thing to do is get two or three and experiment with which one does best with your plane and your engines.

Jim

blw

My Feedback: (3)

Passions run high about this subject for some reason. However, there is no reputable source that I know of that shows a 3 bladed prop as draggy or less efficient.

I'm not sure about the tips being that big of a factor when the plane is moving forward. Yes, there are vortices but they are gone in an instant. They aren't ingested back into the prop.

Actually, depending on your definition of efficient, but model props can hit between 90-95% efficiency. One thing to consider is that the higher volume of air being pulled into the prop disk is generally a higher efficiency.

The 3 bladed and 4 bladed props will generally be smoother running. You will have better braking action with them over a 2 bladed prop which is good on landing and downlines.

earlwb

I think the real problem with the three blade props is getting a good selection to try and choose from. Usually we are very limited as to which prop to use with three blade versions. But with the two blade props, we have a rich variety of different props to choose from and to use. Plus some brands tend to be not as good as some others too. so it is easier to have good success finding a two blade prop that works well with a particular engine and airplane. But when you do find a three blade prop that works with your plane and engine, you have a good combination, it can work really well.

buzzard bait

I think that's exactly right.

AncientCityFlyer

Buzz bait. I agree with you. Got looking at the numbers and the high pitch props would not be the way to go. I'm also flying a Platius Porter (104" W.S.) with a Saito 1.80 and swinging a Graupner 16 x 8, 3-blade and it performs great. Like most folks there is a lot to debate about 2-blade vs. 3-blade props. I think I'll go with the Graupner 12.5 x 7 3-blade and see how it performs.

Thanks to everyone for input. Mike (ACF)

blw

My Feedback: (3)

Those are supposed to be good props.

Hobbsy

My Feedback: (102)

Her you can get just about any diamter and or pitch you want. These props are high quality and they get them to you quickly even when they have to make it.

http://www.zingerpropeller.com/3_bladed_Propeller.htm