Stuka Stunt Main Forum · 18 of 18 known posts recovered
Any formulas or guidelines for relative sizes of three bladed and two bladed props? That is, if you are happy with the performance of your plane and engine using a two bladed 10-6, what size three bladed prop would you get?
Assume you are perfectly propped now, and are looking for identical performance from a three blade and want to switch just for appearance purposes, what would you choose for any size two bladed prop as the right size three blade?
Larry F
The big problem is we don't have a lot of equal choices here. If you are running a 10 x 6 BY&O two blader, there is no 9 x 6 BY&O three blader. And just as switching from a 10 x 6 BY&O to a Zinger to an APC will give you a different load and different performance with your engine, so switching to a different brand or style of 3 blader will, also, give a different level of performance.
Therefore, take this as a guide only. As even with 2 blade props, we need to experiment with 3 bladers to see what works and what doesn't. The carbon fiber props are a little easier to work with in this respect as the blade shapes are basically similar and we can easily repitch them when needed. Otherwisse there just aren't a lot of choices out there.
Leonard Neumann
Don't forget the Tornado Magnum series. They have a 9-6 and 10-6 3 blade, alonng with similarly shaped 2-blades. They also have 10-4 3 blade that works occasionally on the stock 40FP, along with corresponding 2-blades. We even ran it on Ted's PA51.
These are essentially the same as the old silver polyester props, but are glass-filled nylon and plenty stiff enough. They cannot be repitched, but if you get one that's otherwise right, they work well. Plus they hand-balance them at the factory. The other nice thing is that they are cheap - dirt cheap! The 10-4 2-blade (ideal for the stock 25FP/Twister) is somthing like $1.25.
Brett
You have mentioned these before.... can you help with where to get them? Locally, asking for Toranado props yields the rubbery white critter (a pusher is available, if you really want to punish a new guy!).
A web site or something would be appreciated, unless you want to get into the mail order business, at which point I'll send you a box of $$$!
Thanks
Peabody
The three blade 9-6 is $3.70 and the 10-4 and 10-6 three blade props are $4.09.
JZ fiberglass three blade props are available in 9-6 and 10-6 sizes for $4.29.
In all of these it is going to be an experiment because they are not equivilent to something else we may be using, and you cannot readily change the pitch as you can with a carbon fiber or wood prop.
It would be nice if someone like APC, who molds their props anyway, would come out with a series of three blade props using the same blade shapes. Demand is always the problem. If there are just a few of us wanting them, it isn't going to happen.
Leonard Neumann
You might be surprised. The 11.5-4, the 11.5-4 wide, and the 12.25-3.75 were designed and produced at David F. and Bill Werewage's request. The 11.5-4 works good on the 40VF (I used one at the 94 NATS), and the 12.25-3.75 was designed for the 46VF.
The setup they have out there lends itself well to one-offs and low-volume production. He just programs his CAD machine, puts some aluminum in the machine, and walks away for an hour or so. Later, he takes the finished molds, and puts them in the injection moding machine and goes to bed. The next morning, he has 2 dozen props waiting for him.
The real problem is that we could never decide what the next prop should be.
Brett
Try the big R/C mail order house, Sheldon's:
http://www.btown.com/sheldons/
Go to Aircraft, select Grish Brothers, and order away. The prices are about 10% higher than at their retail store. Examples:
MAGNUM 9 X 4 PROPELLOR.................. 1.53
MAGNUM 9 X 6 PROPELLOR.................. 1.53
MAGNUM 10 X 4 PROPELLOR................. 1.71
MAGNUM 10 X 6 PROPELLOR................. 1.71
MAGNUM 11 X 4 PROPELLOR................. 2.07
MAGNUM 11 X 6 PROPELLOR................. 2.07
MAGNUM 12 X 4 PROPELLOR................. 2.57
MAGNUM 12 X 5 PROPELLOR................. 2.57
MAGNUM 12 X 6 PRPELLOR.................. 2.57
MAGNUM TRI-BLADE 9 X 6 PROP............. 3.21
MAGNUM TRI-BLADE 10 X 4 PROP............ 3.41
MAGNUM TRI-BLADE 10 X 6 PROP............ 3.41
Hard to complain for hand-balanced props, really.
Brett
You are right - nicely priced. Thanks for posting the web site.
You made the comment that the 10-4 three blade SOMETIMES works with the Max FP40.
If you don't mind, can you further elaborate on "Why sometimes?"
Regards - Bob Kruger
I'm not sure exactly why, but some individual engines can be made to work with 4" of pitch, and others can't. About 50% of the time, it just runs away, indicating the need for a 10-2.5 3-blade. Note I am talking about box-stock motors, in the unfortunately labeled "40-sized" models like the Twister. Modified motors are a different story.
I hate to even mention running 40FP's stock, since I just don't think the chances of success are high enough to suggest it to people. The stock 35 is much more likely to work, in my experience, and 10-4 3-blade will work there too. If you want to run a 40FP, the chances of success will be higher with a modified engine, like the SSW, Big Art, or Randy. But then the 10-4 doesn't work too well.
Brett
Oh, yes, earlier engines came equipped with larger venturis, also. This is something else that could add to the run away problem. And few people know what size venturi they have on their engine. They just run what comes in the box.
Leonard Neumann
Leonard Neumann
That is surprising. I know that other OS engines (VF in particular) are like clones of one another - all essentially identical, and run the same. They also cost 3 times a much, so maybe that's it.
But it would certainly explain the problem. The only thing I saw in common was that *none* of the early model iron-liner FP's (pre-ABN) that I saw would run very well at all. The ABN version seemed to to me much more reliable.
Brett
Leonard Neumann
My crude math model shows 9.5% to 7.1% reduction in diameter for a three blade compared to a two blade, depending on the original diameter. Shorter props require a higher percentage reduction, and the percentage of reduction is non-linear (according to the crude model).
Reason I posted this was I assumed there was general consensus as to the relationship, and I hate to do math on problems others have already solved.
Yeah, Leonard, we don't want to fool with the pitch if we want the same lap speed. Never trust those RC guys! It is an effective disk area problem, which gets right ugly real quick mathematically.
Practical bottom line is start with something like a shy 9% and trim tips until performance (lap times or RPMs) is the same for both props. If three blade RPMs are too high, add some length to the prop <g>.
I'm happy. Let's not add to this thread unless you have something major.
Larry F
I haven't tried to repitch these, but I have a question: "How much heat did you have to apply, and how sure are you that you did not hurt the integrity of the prop?" A carbon fiber prop has the carbon tow going lengthwise through the blade to hold it together. If heated hot enough that the tow actually moves inside the blade and resets in its new position, it usually will hold the new pitch. If heated with the hot water method as some will do, the carbon fiber prop will spring back to its original shape in a hot car or on the hot surface of the flying field.
This would be my concern with the Master Airscrew props as well. With no tow going through the blade, they could lose their strength by heating and twisting and they could spring back in the heat of the (car) (asphalt surface). CHOOSE ONE
Leonard Neumann
That's where the Master Airscrew props usually break anyway. Since these are molded with heat, they may be OK if you don't overheat them in the repitching process, or work them back and forth too much. I would hate to see the prop shed a blade while the engine is running. It could be lethal.
Leonard Neumann