Hello:

     How does one determine the prop load of a 3 blade prop as compared to a 2 blade prop? I would like to try some 3 blade props but I have no idea as to which props to try.  For example, what is the equivalent 3 blade prop for a 10x6 2 blade prop etc.?

     In the past, I used the formula for a 2 blade prop as 2x diameter plus the pitch to determine then prop load.  How would I compensate for the third blade? Would I just add the pitch of the additional blade?

    Any suggestions?             

                                                                                                          Tia,

                                                                                                          Frank McCune

        Hi Chris:

        Thanks for the information.  This may help me to determine what sizeps prop I will use.  I am looking for a bit of more ground clearance for the prop.

        The only prop that I have used is a 10x6 three blade prop on an OS FP .40 converted to Diesel via a DDD head.  Yes, it looks cool on my Zero fighter.

                                                                                                                   Be well,

                                                                                                                   Frank mcCune

                                                        
                      

    Hello:

     How does one determine the prop load of a 3 blade prop as compared to a 2 blade prop? I would like to try some 3 blade props but I have no idea as to which props to try.  For example, what is the equivalent 3 blade prop for a 10x6 2 blade prop etc.?

     In the past, I used the formula for a 2 blade prop as 2x diameter plus the pitch to determine then prop load.  How would I compensate for the third blade? Would I just add the pitch of the additional blade?

    Any suggestions?      

       I am not sure what you mean by "prop load" in this context, and if you mean "how hard it is to turn" then the pitch part looks backwards. All else being equal, it's harder to turn a 5" pitch prop than it is a 6" pitch prop, because you have to turn the 5"  much faster to get the same airspeed, and/because it is less efficient. The 6" is harder to turn on the ground, but not in the air.

    Additionally, the usual concept for a 3-blade is to get more blade area without the detrimental effects of diameter, so in a sense the idea is to increase the load.

      A very general rule of thumb it ~10% less diameter for a 3-blade VS a 2 blade of a similar type. But you really do just have to experiment, because there is *far* more going on with props than the loading of the engine.

      Brett

      Hello Brett et.al.:

      What I was mainly concerned about was placing a too large of a load on the engine in order to gain a bit of ground clearance.  A friend of mine runs a 12x6 three blade prop on his OS .40 LA.  This combination appears to work well for him but I do not think that the OS .40 LA would be stressed enough to endure this heavy load.

Stressed enough???  As in parts breaking off or something?

That's not how IC engines work.  Put too big of a prop on it and it'll run slower than it's "happy speed", and, if the venturi is big, probably get hot from a combination of overwork and inadequate cooling.  But you can to some extent change an engine's "happy speed" by venturi size changes and what fuel you use -- I suspect that I could get a 40LA to run cool and happy all day on a 12x6 three-blade; it'd just be putting out as much power as a typical modern 15, at a really low RPM and with poor speed regulation.

If you want the same rpm and engine load this is reasonable, but increasing pitch will increase theoretical max speed, which may be a problem.  Keeping pitch and decreasing only diameter may cause rpm to increase, due to less load, which will also increase theoretical max speed...

Catch 22

Phil

also affected by the camber of the blades , under camber, and "philips entry"
the only way to really know is to bolt it on and see what happens

Without Frank replying I am going to assume that "stressed" really should be "strong" (enough).

Running a diesel for stunt is often counter intuitive - think more like a fourstroke, they will pull a high gear at lower rpm and then have self imposed limits that prevent overspeeding.

     Hello Chris, Tim and others:

     When I said stressed I meant that the parts of the OS Glow engine may not be strong enough for Diesel Operation.  In the early 70's I was running ST .15 Diesels and was having a great time with 7x6, 8x4 and 9x6 props.  The Brits liked 8x6 props for combat .15 Diesel engines but I never tried one as I thought that the
     ST Diesel engines were not strong enough for turning large props. The moving parts were about the same as the Glow engines.  There may have been some heavier rods and cranks for the Diesel but I would have to look through my parts to confirm this.  The crank shafts for the G20/.15 Glow and the G20/.15D were different as the Diesel required a much smaller shaft port.  I was advised to go to 9x6 and 10x6 props on the Diesels.  I did not do this but I have broken crankshafts, con rods, crank pins , pistons and have blown the jugs, cylinders, off the crankcases on Russian Diesels. I even had a great running Fox .35 Stunt puke a crankshaft in Foxberg on a 9x6 prop.  Overstressed?.  It was running on 10% nitro.

       I think that an engine designed to be run as a Diesel has more robust parts in it compared to a glow engine.  Yes, I realize that one can adjust the compression on a Diesel to match the load.  This is not a mystery as all has to do is listen to the sound of the engine.

      Enough for now! I do not want to get into fuel burn characteristics mixtures etc. glow vs. Diesel. Stoichiometric ?

                                                                                      The best to all,

                                                                                      Frank McCune