SJ 50 just not turning the quoted RPMs  
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SJ 50 just not turning the quoted RPMs - 8/21/2007 5:57:19 PM   
still4given



Posts: 387
Joined: 1/27/2006
From: Victorville, CA, USA
Status: offline 		Hi Bob,

I have given my SJ50 quite a bit of time to start producing the RPMs I see others quote. With a 10x6 APC the most I can get out of it on a peak needle setting is about 15.500. I broke it in on a bench first using an APC 9x7 prop and ran almost 1/2 gallon of Power Master 15% fuel through it before I ever mounted it on a plane. It will turn about 17,200 with a 9x7 and about 16,600 with a 9x8 but I can't get the 10x6 to turn anywhere near what others state they are getting. I'm running it in a Phoenix Strega which I fly in the RCPRO Warbirds class. There is a guy running a stock OS55 with an 11x6 prop and I can't pull him. From everything I've read, I should be leaving him in the dust. This is the engine that I was having trouble keeping the muffler on a while back. I've got that solved but it just seems I should be getting more out of this engine. Any ideas what might be wrong or what to look for?

Thanks, Terry

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RE: SJ 50 just not turning the quoted RPMs - 8/21/2007 6:31:16 PM   
bob27s



Posts: 4590
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		I am not 100% sure what to make of this one.....

The regular blade 9x8 and a regular blade 10x6 usually turn the same rpm.

If i recall you are flying in the high desert area...... but that should not be a big difference in rpm ... from say sea level Houston or here in Ohio (low with wet air..... or higher with dry air... tends to work out about the same)

The only thing I can think of right off the top of my head, that you could fix yourself at least ----- take the muffler off..... look in at the exhaust port ..... and make sure the liner is alinged up properly with the crankcase. If it is...... see the following. If it is not lined up, remove the head..... carefully line it up..... and reinstall the head clamp (finsh) ... torque the screws down snug, but not over-tight.

The rpms do appear below what most of us see. There may be something amiss with the engine engine, and if so it is worth sending it back to Dub. The very first step, however, is to put the engine on a solid test stand, and run both the 9x7 and 10x6 props so you can get a baseline peak rpm for your location. You would want to provide that information to Dub if you choose to send the engine in.

Bob


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RE: SJ 50 just not turning the quoted RPMs - 8/21/2007 7:40:41 PM   
still4given



Posts: 387
Joined: 1/27/2006
From: Victorville, CA, USA
Status: offline 		I will check the RPM's again when I get home tonight. I quoted those numbers from memory and they could have been from different days. I have been thinking about going to a 10x6 because it seems i'm getting pulled out of the turns and I'm thinking the 9" props are the culprits. The guys I am flying against are using 11" props on their stock engines. I remember you saying once that Dub doesn't send out an SJ50 unless it turns an APC 10x6 prop at 17,000. Well mine had never come close to that. we have a race this weekend so there is no time to send it to Dub. I may consider it afterward though. I have checked the alignment recently. I had a problem right after I bought it of the head bolts being loose and the button turned when I changed the glow plug. Since then I check the head bolts regularly as well as the sleeve alignment.

Yes I do live in the high Desert at about 3500'. It's been over 100F during the day for the last month.

Thanks for your help.

Blessings, Terry

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RE: SJ 50 just not turning the quoted RPMs - 8/21/2007 7:47:00 PM   
grant118


 

Posts: 156
Joined: 5/29/2003
From: simpsonville, SC, USA
Status: offline 		I am not an engine guy but higher elevation and higher temp will produce less hp. Plus you have to run a leaner mixture in the summer than winter time.

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RE: SJ 50 just not turning the quoted RPMs - 8/22/2007 2:28:32 AM   
MJD


 

Posts: 1492
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From: Orangeville, ON, CANADA
Status: offline
quote:

ORIGINAL: grant118

I am not an engine guy but higher elevation and higher temp will produce less hp. Plus you have to run a leaner mixture in the summer than winter time.


But the prop is spinning in less dense air so the rpm shouldn't change too dramatically I think.

Actually Bob, this point came up some time ago, a few of us were discussing the issue of lower air density and rpm with a particular prop. I do realize that turning a particular prop at say 16k at sea level requires more horsepower than turning it 16k at 3500' for example. I believe the relationship there is proportional to density squared as per the drag equation. But does horsepower decrease as the square of the density change or is it a different relationship? IOW would setting the same engine to the same relative misture setting with the elevation change result in same rpm, less, or ???

Sorry, I am not trying to hijack the thread, but I thought there was potentially some relationship between this question and the observations made here, although rrunning one engine against the other in the same atmospheric conditions should not change the merit of one engine versus the other. Just curious is all, and I know you have all the answers..

MJD

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RE: SJ 50 just not turning the quoted RPMs - 8/22/2007 3:55:56 AM   
grant118


 

Posts: 156
Joined: 5/29/2003
From: simpsonville, SC, USA
Status: offline 		I think you are right. I was thinking why full scale piston engines use super chargers for high altitude flying. Higher altitude, lower o2, less capacity to burn fuel.

high reving engines unload so much in the air and apc props act way different than master airscew props on the ground your engine may be running correctly.

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RE: SJ 50 just not turning the quoted RPMs - 8/22/2007 4:38:00 PM   
bob27s



Posts: 4590
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		yeah... HP drops faster than drag with altitude... if I recall properly. Not quite sure...... where are my textbooks???

in any case...... the engine he has here is a bunch down on RPM...... not just a few hundred. Not sure exactly whats going on. Probably deserves a look.


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RE: SJ 50 just not turning the quoted RPMs - 8/22/2007 4:41:10 PM   
still4given



Posts: 387
Joined: 1/27/2006
From: Victorville, CA, USA
Status: offline 		Thanks guys,

I will send it in after the race this weekend. I have heard too many good things about the SJ50 to give up on it.
Hopefully Dub can figure out why it is not turning what it should.

Blessings, Terry

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RE: SJ 50 just not turning the quoted RPMs - 8/24/2007 4:51:13 AM   
still4given



Posts: 387
Joined: 1/27/2006
From: Victorville, CA, USA
Status: offline 		Hi Bob,

I got a chance to run some tests tonight. I didn't pull it from my Strega and I used a Tru-turn 2 1/4 spinner to help reduce the drag off of the front of the fuse. Here are the numbers I got. All of the props are APC and I peaked each prop.

8x10=16,400
9x7=17,200
9x8=16,400
9x9=16,100
9x10=14,200
9.5x8.5N=16,400
10x6=15,800

This was on Power Master 15%. The outside temperature is about 72F.

Based on these numbers, which prop would you think would give the best results on a two pylon course with the pylons 700' apart?


Blessings, Terry

< Message edited by still4given -- 8/24/2007 2:29:48 PM >

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RE: SJ 50 just not turning the quoted RPMs - 8/24/2007 2:19:03 PM   
bob27s



Posts: 4590
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		based on the numbers.....

the 9x7 or the 9.5x8.5N

but in any case... those are low....
after the race this weekend, be sure to send it in..... something is not right.


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Jett Engineering - Engine Mfg Support Forum Host

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RE: SJ 50 just not turning the quoted RPMs - 8/24/2007 4:19:31 PM   
still4given



Posts: 387
Joined: 1/27/2006
From: Victorville, CA, USA
Status: offline 		Man, I have a couple YS45's. I think I'll stick one of those on a test stand and see what they will turn. I may be swapping out an engine tonight. Struggling to run against guys who are flying $100 engines is not what I bought a Jett for. They are supposed to struggling to keep up with me.

Blessings, Terry

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RE: SJ 50 just not turning the quoted RPMs - 9/15/2007 3:47:19 PM   
still4given



Posts: 387
Joined: 1/27/2006
From: Victorville, CA, USA
Status: offline 		I did OK at the race. Went lean half way through the first lap and had to land immediately so I scratched the first heat. Won the second heat out right and lead the third heat until the last lap when it went lean again and I had to finish the race at half throttle. The other guys pasted me but they had cuts so I won that heat too. Ended up third over all.

I have switched out the engine for one of my YS45's. It's turning a 9x9 at 16,700 which seems faster on this airframe.

I've had three lean runs since I've owned this engine. When I say lean run I mean I heard the engine lose RPM and I backed it off and landed it. Tha tlast race is the only time I ever continued to fly after it leaned out and I only kept it running just fast enough to fly. Do you suppose I've damaged the cylinder/piston? If so, can't I just order a new set from Dub rather than sending it to him? Seems he's a very busy guy. Is there any special lapping that he needs to do or can I just swap out the parts myself?

Thanks, Terry

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Speak kindly. Leave the rest to God

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RE: SJ 50 just not turning the quoted RPMs - 9/15/2007 4:48:24 PM   
bob27s



Posts: 4590
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		I would highly recommend sending the engine back... as noted above.
There is always a possiblilty that the piston/liner is not the problem, or not all of the problem.

I believe we had previoiusly decided there was something not quite right with the engine. It is not performing properly.
Would have been best to have avoided racing it and getting the lean runs again.

If you set the engine 600-700 rpm down from peak rpm at launch (for racing), and it still went lean - either the fuel system is really goofed up, or the engine got hot for some other reason.


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Jett Engineering - Engine Mfg Support Forum Host

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RE: SJ 50 just not turning the quoted RPMs - 9/15/2007 7:19:30 PM   
still4given



Posts: 387
Joined: 1/27/2006
From: Victorville, CA, USA
Status: offline 		Hi Bob,

First of all, the engine would fall of the pipe if I tried setting it 600-700 off peak. About the lowest I could set it was 400 off peak or it would go rich on take-off and never get on the pipe. Maybe 400 off peak is too lean but it would rev up pretty good with the pinch test at that setting. I was thinking about it after I posted the above and I forgot that when I first got the engine the muffler kept coming loose. I'm sure it went lean every time that happened.

I guess I'll just send it in. Hopefully Dub can fix it without it costing me too much. I'm starting to lose interest.

Sorry for bugging you so much with all of this.

Blessings, Terry

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Live simply. Love generously. Care deeply.
Speak kindly. Leave the rest to God

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RE: SJ 50 just not turning the quoted RPMs - 9/16/2007 4:16:52 PM   
bob27s



Posts: 4590
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline 		You should be ok... we just have to see what is wrong.


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Jett Engineering - Engine Mfg Support Forum Host

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