I've nearly finished building a T67 Firefly, semi scale RCplane but theground clearance for the most suitable prop is much too small.
My idea is to use a three bladed prop!
Has anybody used a three bladed prop in similar circumstances and if so was there sufficient power.
I have a 4 stroke RCV 58 CD engine.
Thanks.
</p>

Something like an 11x5 or 10x6 3 blade is fine.

As a general rule if you are using a 11x6 2 blade and want to go to a 3 blade you either drop one in pitch, to 11x5, or 1 in diameter to a 10x6. A 3 blade is scientifically less efficient but it is hard to see the difference.

These are as small in diameter as I would go. If there is still a ground clearance issue in these sizes try changing the wheels. It may be a decent idea to go with slightly bigger wheels anyway.

THanks Phoenixangle,
sounds like good advice and probably the only selection of options I could use. The plane would look silly with longer wheel hangers (another idea )  but I could get away with a slightly larger wheel.
Cheers, quantumsquid.

Hi!
Those props are too small!!!!
You have to use larger ones if you don't want to over rew your engine.
3-blade 11x7 or 12x6 are more suitable.

A point to be brought up is your engine itself. The rule of thumb when going from a two blade to a three is one size down in length and one up in pitch. The big however is your engine itself. The RCV engines in this country are almost never seen. I have only built one plane that I talked the owner into using the RCV, the one that looks like a honey comb. That was years ago and the plane has never been flown. The engines are gear driven and swing a big prop, you may want to go into the glow engine forum and ask about props for that engine and see what people that are really using it have done. The normal rules of prop selection may not be correct for the RCV. I know they have two different models but that's about it, we just don't see a lot of them in use and they are a bit different then any other engine. When I had the one I installed in the plane, A Hell Cat, I read the instructions and just the break in for the engine was different then anything I have seen before. Better to be safe then sorry, go ask some users of the engine and be safe. They really are cool but I still haven't seen one in use.

Hi!
I think this RCV engine has no gear reduction! It has 1:1 out put. All other RCV engines have a reduction gear.

The props sizes I posted are within the range suggested in the engine manufacturers manual for a 2 blade. In the manual for the engine itself they suggest an 11x6 or 12x6 apc type 2 blade prop.

I would trust the manufacturer knowing what is ok for their engine. From the manual I would not post them without first researching what the manufacturer suggested themselves for the engine.....

Thanks guys,
created a bit of interest with this problem.  I have the manual and bought the 12/6 prop after reading a test report on various prop sizes, but my problem is that if I fit a 12/6 the blade will be grass cutting at  zero ground clearance.
The RCV58 CD has a gear driven shaft and is 1:1 and will take conventional size props. It has no poppet valves or springs.

I think a 10/6 will still not give me enough ground clearance and so I thought a smaller diameter 3 bladed would do the job.
At the moment I haven't seen any small diameter 3 bladed props for sale but I spotted "sports props".. 2 bladed 7 inch diameter with a big 7 or 8 pitch. This type of blade has got me thinking again.. I wonder, could I use one of these ??

You can use any prop that keeps the engine within it's working RPM range. The RCV I installed was the SP model and I have no real idea of what prop sizes would work on it. The instructions tell you what props the designers found to work within that rpm range so they give you an idea or starting point. Most normal four strokes are run well below there top RPM range to keep the valves from floating, that's why you see bigger props on them. I have a box of different sizes of props and I test all my engines to make sure I don't over rev them then I test to see what prop size I, the pilot likes on the plane/engine combo. A 7 or 8 pitch isn't very big. Some of my engines in a pattern plane are calling for 12 to 14 pitch in a shorter prop. That's why I said to go into the glow engine forum and talk to people that are really using these engines. You may over rev your engine with that small of a prop. If you contact RCV directly and tell them your problem they may have a much better answer for you. I'm sure there are forums in the UK where the RCV is used a lot more that could be of more help. I don't even see RCV advertised very often here in the states these days. 8 or 10 years ago they were advertising a lot here but the engine never caught on. The reason I talked the customer into the RCV SP was he was going completely scale and didn't want the engine seen or holes in his cowl. The SP was the only engine I knew about that would do what he wanted. The plane came out so well the fellow has never flown it so I never got to see the engine even run.
Do a search yourself in the glow engine forum, this is just a general Q&A forum.

<font size="4">Thanks Gray Beard, I'm beginning to get to grips with prop choice now with the valuable information received from yourself and others from this forum and speaking to fellow fliers over here in the UK.  (Thanks for letting me know about the other sections on this site).  The one thing everybody is saying is that I wont get enough thrust from a 7inch blade, regardless of the pitch.
Following some general guidelines dropping down from the recommended 12/6 it would be 11/7 to 10/8 with a 10inch still in recommended range for the RCV58CD.
Although the ground clearance will be minimal (about an inch) experienced pilots say that's all you'll need so I'll start with the 10/8 and see what happens!!
I've e-mailed Alan at Weston UK, the RCV suppliers, a number of times about prop choice and not yet had a reply.
 
</font>

Hi!
As your engine has no reduction drive, and is more or less like an ordinary poppet valve four stroke, you need a either a 10x8, 10x9, 11x6, 11x7,12x5 or 12x6 3-blade prop.Best props on the market are RAM, Graupner Cam-prop and APC. But of those only Graupner and APC makes 3-blade props.

Thanks Jaka,
I'll buy a few 10inch props with  different pitches. 11 inch with two or three blades would be to large and leave me with no ground clearance.
Will also look for 10inch 3 bladed.

This plane has 1/2 inch ground clearence with the prop. That's nothing new with a lot of pattern type of planes. The trick is to three point the landings and not lift the tail too much on take off. This prop has been on the plane sense I got it set up. It gets flown pretty much every week 5 or 6 times a day. To get all that ground clearence I made up new gear legs onn inch taller and added larger tires. The prop is a 15X8 on a YS 1.20 and is about as small as I could find that would keep the engine in it's working rpm range. A bigger pitch on the 15 didn't fly the plane the way I liked. If you are using the engine of your choice then sometimes you have to make mods to the plane to use it. Graupner does make some nice 3 blades, I think, I bought a few years ago and still have them. Try picking up the phone and calling RCV, I have found a phone call much better then email. It's hard to tune out the ringing so someone will pick up. Put a prop on your plane then lift the tail to see the angle your plane will have to be in to nip the prop. If your landing or taking off at that angle you have a bigger problem then you think.

It's reassuring to see your plane that has only 1/2inch ground clearance. My model has similar characteristics to your plane's undercarraige setup. So with confidence!! I will fit a 10/8 while still looking for a suitable 3 bladed.
Still some work to do on my Slingsby T67 FIrefly but I'll post a few photos when it's nearer to test flying.
RInging Weston is a good idea, only a National call for me over here in the UK. 

I have unlimited calling for state side calls so I discovered a long time ago the phone is your friend. I don't deal with anyone that hasn't a phone number listed and I have been know to phone them just to make sure there is someone there to pick it up. I have discovered a lot of places have people answering the phone and English is not there first language, sometimes they think they are speaking English and they speak it better then anyone else in the store/office, if I can't understand them then I don't order from them. Other places just don't bother answering, another place I bypass.Just by ground testing with a tach you will know if the prop is going to keep the engine in it's RPM range. Then flight testing will tell you if you are going to like the way it fly's your plane. I have a big box of props and spend a complete day sometimes looking for the prop that works for me. Someone else may not like the way my plane fly's with that prop, who cares, it's my plane. On that pattern plane I tested a lot of props but even with longer gear legs a 15 inch was as big as I could use, I did try a Graupner 14X7 three blade, it flew the plane but lacked any vertical pull, the plane just does everything better with the APC two blade. The engine is a YS 1.20 FZ, it is under it's Max. RPM range and works very well. If I went to bigger tires I could go up in size a little. I fly off of a flat dry lake bed or asphalt field. The LB is flat but not smooth. Once in a while I hit a clump of dirt with the prop but so far so good.

Hi!
A 10x8 is way way too small!! You must use at least a 11x9,11x10 if you go the 2-blade route or a 10x8,10x9 if you use a 3-blade (which will give bad performance).