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Author Topic: R O Jett 40  (Read 2842 times)

Offline Bootlegger

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R O Jett 40
« on: April 06, 2014, 10:42:33 AM »

 Guy's I have a R O Jett 40 rear exhaust in a model and I want to run a 4" pitch prop, but want to try a 12" dia .

 Is this too much dia, or not.  If so what do you suggest??

 Any suggestions will be appreciated.
 I will be running 5% - 10% nitro with only synthetic oil.   Thanks
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Offline Brett Buck

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Re: R O Jett 40
« Reply #1 on: April 06, 2014, 10:57:51 AM »
Quote from: Bootlegger on April 06, 2014, 10:42:33 AM
Guy's I have a R O Jett 40 rear exhaust in a model and I want to run a 4" pitch prop, but want to try a 12" dia .

 Is this too much dia, or not.  If so what do you suggest??=

   2-blade?  Probably too big, but one way to find out. A Bolly is a lot less load than most so that is what I would suggest to start  but I have run a 12-4 Rev-up on a 40VF (for a pretty long time) and it was OK. You aren't going to hurt it. 11000 at launch, 3.8", maybe 18.25" pipe to start with?

     One of the mistakes people still make is trying to run too much diameter, because 30 years ago that was a good plan. It's not necessarily a good plan now. You don't need to get use diameter to get efficient power transfer when you have nearly unlimited power available.

    Brett
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Online Dave_Trible

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Re: R O Jett 40
« Reply #2 on: April 06, 2014, 11:50:15 AM »
Brett has a good plan to try.  I have three of these and used them a bunch until I 'went big'.
They are wonderful engines but you can sure put too much airplane and likely too much prop on them.  To me now I think it's the most perfect classic airplane motor there is if you choose to use a modern engine.  About 60 ounces is tops,  620 sq. in.  I always ran mine on an APC 11-4 and launched at 10000-10200 RPM on most my ships.  Never really tried anything larger in props.  I think it would drop the R's with not much to gain but worth a try.

Dave

I got one out and measured the pipe length I was using.  17 5/8".
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Offline Bootlegger

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Re: R O Jett 40
« Reply #3 on: April 06, 2014, 12:27:22 PM »

  Guy's thanks for the reply's, after going back and reading MY post I failed to mention the fact that I will be running a rear muffler.

  And after reading the posts I started thinking about reducing the dia from 12" to 11" by cutting the tips by 1/2" each.
 Hopefully this will clear up my OOOOP'S...
  Again thanks...
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Offline Brett Buck

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Re: R O Jett 40
« Reply #4 on: April 06, 2014, 01:12:47 PM »
Quote from: Bootlegger on April 06, 2014, 12:27:22 PM
  Guy's thanks for the reply's, after going back and reading MY post I failed to mention the fact that I will be running a rear muffler.

  And after reading the posts I started thinking about reducing the dia from 12" to 11" by cutting the tips by 1/2" each.
 Hopefully this will clear up my OOOOP'S...
  Again thanks...
   
    I wouldn't necessarily start out cutting the prop, its pretty hard to put it back on later.

    With no pipe, I would be tempted to suggest starting with something like an 11-5 or 11-5.5 or so.

    Brett
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Offline Randy Cuberly

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Re: R O Jett 40
« Reply #5 on: April 06, 2014, 01:23:21 PM »
I hve 3 of these little engines, one side exhaust and 2 rear exhaust.
I agree with everything Dave and Brett have said but would add that an APC 11.5-4 is about the largest prop I've been able to run and keep the engine in it's "happy range" which seems to be about 10,000 to 11,500 RPM.  The 11.5-4 demands about 9,600 to 9,800 launch rpm to make a reasonable speed and any lower seems to fall off the power curve!

Mine seem happiest with an 11-4 Bolly!  or a 10.5-4.25 Phelps style on a smallish airplane like a Nobler etc.

Randy Cuberly
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Offline Allan Perret

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Re: R O Jett 40
« Reply #6 on: April 09, 2014, 06:32:35 AM »
Quote from: Brett Buck on April 06, 2014, 01:12:47 PM
   
    I wouldn't necessarily start out cutting the prop, its pretty hard to put it back on later.

    With no pipe, I would be tempted to suggest starting with something like an 11-5 or 11-5.5 or so.

    Brett
With 5 ~ 5.5 pitch the rpm would have to be much lower than the 11K with a 3.8" pitch, right ?    What is it about using the same motor with a pipe vs a muffler that requires it to operate at such different rpm ?   Is it just the fact that the higher pitch represents more load to the engine, or is it something else ?
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Offline Steve Helmick

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Re: R O Jett 40
« Reply #7 on: April 11, 2014, 08:57:32 PM »
I almost hate to suggest it (yet again) but don't overlook the 11 x 4.5 Thunder Tiger Cyclone and APC 10.5 x 4.5.

I don't think there is anything chisled in stone about having to run more pitch with a muffler, or even that you can run more pitch with a muffler. Also, tho I have been running a recarved APC 12.25 x 3.75 (at 11.5" dia.), I would avoid it if possible.  It is definitely not just chopped down to 11.5". Try every other possible option first. y1 Steve
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Online Dave_Trible

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Re: R O Jett 40
« Reply #8 on: April 11, 2014, 09:43:47 PM »
You surely can go up in pitch and down some in RPM.  The last few years that's what many 'pipe' guys have done.  Early pipe ships used engines really designed for racing more or less and the pipe was a governor keeping the higher RPM engines in a desired range and controlling speed.  To get the best from these turbines the pitch was very low and let er' rip.  Later more purpose-designed engines with milder timing developed good power at lower R's and made the engines last much better.  I know I cooked a few good engines until I learned to add pitch and drop the revs some.  Putting .61-.65's in .46 size airplanes and .75-.76 in .46-.60 size airplanes meant there was gobs of power without going nuclear in RPM.  The ROJett will do about anything but you can double the life if you go 4+ on pitch and try not going over 10500 or so.  My .61's operate around 9000-9200 and the .76 about 8700.  Everything has to do also with pipe adjustments as well but that too seems easier at lower speeds.  If you don't think you are getting enough 'go' then maybe the airplane is too much for the engine (or you are getting to be a pretty good flyer)

Dave
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Offline Bill Little

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Re: R O Jett 40
« Reply #9 on: April 19, 2014, 04:59:50 AM »
Hi Dave,

An interesting story on my Geo Juno (Saturn modified to Juno looks) and the engine progression.  It started life (in 1997) at 54 oz. RTF with an OS .40VF and a common pipe/prop set up (Paul Walker).  It really flew good.  Next I picked up a PA .51, set the pipe like Randy said, used the prop he said and it flew even better.  The power increase was noticeable!  Then a PA. .61 fell through a hole in the roof........ called Randy again.  The power and engine control was even more noticeable!  Of course I missed a couple PA .65 and .75, DARN IT!

Now, this model would have flown fine with a good ST G.51 up front if I had decided to go that route, but man were my eyes opened!  I started to understand a little better what P.J. was talking about by using a .51 (or bigger??) in his Nobler.

If you have a OS .40LA, shelf it and replace it with a .46LA run box stock in a wet 2 cycle using a 4-4.5 pitch prop.  It is much more consistent, stronger and lighter. 9200-10,000 rpm.  Get the engine running right and pitch the prop to get airspeed.  Forget lap times until the plane is flying right THEN take some lap times.

BIG Bear
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Offline bob whitney

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Re: R O Jett 40
« Reply #10 on: April 19, 2014, 10:46:47 AM »
 i have a Trivin 12.5x4.5 as was used by Todd Lee and a REV-UP 12x4.5 both in carbon 
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