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[Motorman]

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[frank williams]

3 blade Brian Eather Blue flat bottom 13 x 4.2 (4.2-4.4)

[Randy Cuberly]

Thanks Frank. Do you think some of that prop is used to tame that big engine? I'm wondering if an electric set up would use 12" diameter.


MM

Sure, but likely with different results!   

Randy Cuberly 

[Brett Buck]

Thanks Frank. Do you think some of that prop is used to tame that big engine? I'm wondering if an electric set up would use 12" diameter.

   The prop is the size it is as a compromise between vertical performance (larger) and cornering performance (smaller). The diameter is on the small side for a 75 as far as engine performance goes, and definitely permits the engine  to run faster than it would otherwise, and therefore smoother.

   The rest of the engine is set up very conservatively, with a relatively small venturi - I forget, but it might be as little as a #12 AWG drill with a 1/16" spigot, with a choke area less than a stock 20FP. I know it was that way when David won the World Championship. Of course, it will suck much more air and therefore much more fuel through that small venturi, like 8 ounces of 10%.

    I emphasize - this prop is *small* for a 75. The engine would have no problem with a 15-4 3-blade of the same design, and I have run the 13-4 Eather on my relatively dinky 61, and it wasn't strained by that at all. I have heard of people running *16-6* 2-blades on Jett 76s and the PA is stronger than that. It's just loafing.

   But there's no way in Hades that you want to try to run those sorts of props on a 630 square inch, 4-lb airplane that we used to fly with 11-6's and ST46s. The corner quality noticeably degenerates with even 1/2" more diameter. This is far more important than using more of the engine capability, don't worry too much about getting to the top of the circle.

     Brett

[Randy Cuberly]

So a 13" prop corners without too much wobble but a 12" prop doesn't really pull overhead like you want. Interesting, I did not know that. I guess the same holds true for electric. I wish I knew that before I made my 12" prop maker lol. Oh well I know a guy that welds aluminum, I can make extensions. Thanks Brett.

MM

I don't think it reducing "wobble" in the corners that is really the concern.  It's simply that the added gyroscopic effect of the larger prop tends to make the corners bigger and requires more force to turn.  The precession has a potential of wobble effect on the turn .  Of course the larger the prop the more effect of the wobble potential also.  That can be trimmed out to some extent but the extra required force is more difficult to deal with!

Randy Cuberly
PS:  Brett may have a different opinion (He usually does and I'm not going to argue with him!)
 

[Brett Buck]

So a 13" prop corners without too much wobble but a 12" prop doesn't really pull overhead like you want. Interesting, I did not know that. I guess the same holds true for electric. I wish I knew that before I made my 12" prop maker lol. Oh well I know a guy that welds aluminum, I can make extensions. Thanks Brett.

     Hmmm, I didn't say anything about 12" props, since we aren't using those. But the engine response and pitch makes more difference than the diameter. One of the most successful systems in stunt history pulled a 700 square inch, 64 ounce airplane around well enough to win 5 NATs and a World Championship with an 11.3" prop.  I won the 2006 NATs doing 5 flights in a row with about a 11.75" prop with fuzzy tips, and came in third at the 2000 NATs with about an 11 1/4" prop, again, on a substantially larger airplane of about the same weight.

     I wouldn't worry too much about vertical performance, it's a PA75 for goodness sake. You can stick almost anything on there and it's going to work better than anything ever used in stunt before about 1985. David is trading off a bunch of things to *optimize* the system, not just trying to get it to do patterns.

     If want to use a 12" prop, go right ahead, but you certainly don't need a 75 to do that, it's like a shaft run.  I flew A LOT of flights with  40VF and a 12-3.3 prop. 12" 3-blade, maybe a PA51, and it will be absolutely idling around.

 I cannot overemphasize that David's 75 system is intended to basically operate at a near-idle in a deep 4-stroke on the extremely light load of a 13-4 3-blade. First time I flew it, it took off, and I yelled out asking if it was broken, because it was running SO deep in a 4-stroke. He's pumped it up a little bit since then but it still, when right, sounds like couldn't possibly be putting out enough power. The idea is to get it so far away from a 2-stroke that there is almost no chance that it breaks more on outsides than insides -since it never breaks at all.

    And it's not about "wobble" in the corners, it's just that larger diameter slows the corner, requiring other trim changes that destabilize it in other parts of the flight.

  However, like I said recently in another case, it would be highly advisable to not get too innovative until you have a working baseline system, and use parts that others have managed to get working.

      Brett