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  • September 21, 2017, 10:04:37 AM

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Author Topic: Alternate RO-Jett 61 setup  (Read 608 times)

Online Brett Buck

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Alternate RO-Jett 61 setup
« on: August 27, 2017, 07:13:30 PM »
OK, I finally decided to try a new system, primarily to try to improve the hot weather/dead air performance, where I think electric, with the heavy line tension, seems to have an advantage. I have a few flying sessions on this system and today, at 102 degrees, it was incredibly good. The primary difference is the prop

  STOCK RO-Jett 61 BSE "Brett version"
  STOCK head clearance
  Stock "long" venturi, drilled with #5 AWG drill, and SIG eyelet spigot with about .030 projection
  Eather 13-4 3-blade flat back (blue dye) cut to 12.5 and pitched to 3.75 helical.
   Billy/Randy 51-61 Slimline pipe, 17 5/8 to first baffle
   Powermaster 10% "Air" or Powermaster 10% RO-Jett or equivalent. 6.3 ounces at 102 degrees and sea level (use 15% to start in the Midwest/Southeast)
   Launch revs = 9600-9700
   
    Engine should operate in deep 4-stroke, and just touch about 50/50 4-2 in the tops of the square 8. The break is as smooth as you could possibly imagine, as good as a 46VF.

   This setup required a further-aft CG to make up the turn lost by the wider-blade prop, and the line tension is substantially heavier than the standard system.  It works incredibly well in dead air and hot conditions. It will work pretty well in dead and cool conditions. I do not know if it will work as good as the previous baseline in heavy wind (where the previous system was nearly miraculous, basically, I was hoping for 20 mph most of the time).

If you want to switch, use the same setup with the 12.5 3.75 (helical) with the same settings, I tried it and the one difference (pipe length 3/16 longer) isn't different enough.  My next test is to pull it out to 17 3/4, which probably is too long for this prop but might work even better with the cut-down 13-4. The test after that is to try it at 12.25 instead of 12.5 to improve the cornering, or at least try to get it back to the same CG. David has some buzzed-down 13-4s that I am going to trade with him, since there's no point to cutting up props if you don't have to.

   Note also that it will also spin the unmodified 13-4 3-blade with no problems in terms of the engine, but my airplane didn't like it very much, and neither did my right wrist. This is the same prop that David uses on the 75, at 4.2". Obviously the 61 doesn't have the power of the 75 so it runs it leaner, but still in a 4-stroke in level flight.

    Brett


Offline Steve Fitton

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Re: Alternate RO-Jett 61 setup
« Reply #1 on: August 28, 2017, 08:28:43 PM »
That is almost the *identical* setup I run in my good PA-61, and the description of the run quality (and wind performance) is near identical too.  Did you change pipes, or been running the same randy/billy pipe?  When I changed from the "quiet"pipe to the old slimline it seemed to make an improvement in the run.
Steve

Online Allan Perret

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Re: Alternate RO-Jett 61 setup
« Reply #2 on: September 13, 2017, 06:54:37 AM »
OK, I finally decided to try a new system, primarily to try to improve the hot weather/dead air performance, where I think electric, with the heavy line tension, seems to have an advantage.
    Brett
So why do electrics have more line tension ?
Allan Perret
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Slidell, Louisiana

Online Brett Buck

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Re: Alternate RO-Jett 61 setup
« Reply #3 on: September 13, 2017, 01:16:58 PM »
So why do electrics have more line tension ?

  I don't know.

   Plenty of people have proposed a large number of "magical thinking" reasons for this sort of thing over the years (it was a real effect even on ST46s  - one would feel weak, the next one strong, even at the same prop/rpm/lap time and just flying around level) but all them are easily dismissed.

   It's something, it's a real effect, but I don't know what it is.

    Brett

Offline RandySmith

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Re: Alternate RO-Jett 61 setup
« Reply #4 on: September 13, 2017, 01:59:21 PM »
So why do electrics have more line tension ?

Flying a given stuntship  nose  heavy, will normally make for more line tension

Randy

Offline Steve Helmick

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Re: Alternate RO-Jett 61 setup
« Reply #5 on: September 13, 2017, 09:14:49 PM »
Flying a given stuntship  nose  heavy, will normally make for more line tension

Randy

Even if the LO's are moved forward the same amount as the CG?   ??? Steve

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Online Lauri Malila

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Re: Alternate RO-Jett 61 setup
« Reply #6 on: September 14, 2017, 01:34:11 PM »
Can it be simply more contant torque/power? Electric props get impulse several times per revolution, IC only once. That should mean that E-props works at better Re-area bigger portion of time.
That effect becomes more clear with lighter props due to lessened flywheel effect.
That reminds me of my all time best Discovery Retro .60. It was stronger than anything else in my big pile of Retro's and ran really nice in all kinds of conditions. But it did not last very long.
The secret was one faulty ball bearing in Yuriy's piston grinding machine that instead of making pistons round, made them something like 16-angled.

L

Offline RandySmith

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Re: Alternate RO-Jett 61 setup
« Reply #7 on: September 14, 2017, 03:16:42 PM »
Even if the LO's are moved forward the same amount as the CG?   ??? Steve

Yes even  when moving  LO to match  CG, maybe cause of more weight and centrifugal force acting on the extra nose weight

Randy

Online Brett Buck

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Re: Alternate RO-Jett 61 setup
« Reply #8 on: September 14, 2017, 07:42:59 PM »
Can it be simply more contant torque/power? Electric props get impulse several times per revolution, IC only once. That should mean that E-props works at better Re-area bigger portion of time.

        That's possible and that was as far as I got the last time I spent any time on the topic, but it's not compelling. It *sort* explains the same thing happening with 4-strokes, because on the "dead" stroke, you don't compress the dead charge, which keeps the rpm from dropping like it does with a 4-stroking 2-stroke, it just sort of steadily goes down. Whether that is sufficient to explain the effect, or why it would matter to the line tension as long as the airplane doesn't lose consequential speed between strokes, makes it a weak argument. It also doesn't explain why various 2-strokes of the same variety and otherwise identical don't all feel the same - just like your Retros.

It is a very real effect and we had the same thing over and over, but I am still not sure what causes it. In fact, the entire idea from above counts on the same effect, but by changing the prop rather than the engine. *Nothing* else in the system is changed, just the prop (same pitch and slightly less in-flight RPM, although its substantially slower on the ground), and then, just the blade area, the airfoil and shape is about the same aside from the blade width.

      Brett

 


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