Quickie JETT on FAI fuel?
10-24-2004, 03:21 PM
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Quickie JETT on FAI fuel?
Hello Bob,
Happy Halloween!
Now to the point. As you may or may not know, we do not use nitro in the Quickie event over here. So when I test my QJ engine on the recommended props, I see about 500 rpm less than "guaranteed". Those 15% of nitro you guys use over there could easily explain the performance drop, so the engine is fine IMO.
On an APC 8.75x9W the engine just makes it to the 19000 rpm mark... absolute peak setting... not too bad I guess.
But since the engine is optimised for 15% fuel, is there anything I can do to get a little more out of it on FAI fuel? Or do I just have to "live with it" and run a little less prop? The heaviest prop the other guys over here use on their Nelsons is the 8.75x8.75, but most of the times even less... so I guess they're in the same boat.
I'm just tempted to lower the head some... but will it get me anywhere? (besides at Jett Engineering, shopping for parts)
Happy Halloween!
Now to the point. As you may or may not know, we do not use nitro in the Quickie event over here. So when I test my QJ engine on the recommended props, I see about 500 rpm less than "guaranteed". Those 15% of nitro you guys use over there could easily explain the performance drop, so the engine is fine IMO.
On an APC 8.75x9W the engine just makes it to the 19000 rpm mark... absolute peak setting... not too bad I guess.
But since the engine is optimised for 15% fuel, is there anything I can do to get a little more out of it on FAI fuel? Or do I just have to "live with it" and run a little less prop? The heaviest prop the other guys over here use on their Nelsons is the 8.75x8.75, but most of the times even less... so I guess they're in the same boat.
I'm just tempted to lower the head some... but will it get me anywhere? (besides at Jett Engineering, shopping for parts
)
10-25-2004, 10:29 AM
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RE: Quickie JETT on FAI fuel?
Hey there......
Thanks for writing......
With the quickie jett, you should have no direct issues running the FAI fuel. Sounds like you are getting good performance, but you are correct..... at 19K peak on the 9W, that is 500-800 rpm below what the engine typically turns on 15% fuel.
My guess is the guys with the nelson engines see the same rpm drop. If that is the case, I'm surprised they are running those props.
The key here is to keep the RPM up. The engine runs best when you can get a ground peak in the mid-high 19K's, and a launch RPM somewhere over 19K....... around 19.2K to 19.4K. Only one real way to get there.....
Drop a prop size. Experiment a bit with the above numbers in mind.
The 9W is about as large as you would want to go based on what you described, but may be almost too big. You may notice that it is hard to set the needle... the engine is trying to trickle down off the pipe anything below 18.8K rpm which may give you a false peak.
Tip here - as an experiment..... stick with the 9W...however... take a spare 9W, clip 7 or 8 mm off of each tip. That prop should easily exceed 19K. Use that to set the needle. Find peak rpm, back down 500 rpm. Remember the needle position. Then reinstall the full-blade prop. When racing, turn the needle out 1/2 turn..start the engine....... wait 10 seconds......turn it back to where it was.....then throttle back a bit or have your launcher/caller hold the nose of the plane down, until its time to launch (or the engine will seriously overheat). Go to full throttle just before the flag drops. As long as the engine can unload over 19.5K in the air, it should run great [8D]
Although the others are using the 8.8x8.75, try running an 8.8x8.5 if you can get ahold of one. Its a very quick prop, and the engine should turn it up into the RPM range we are talking about here. One of my other favorites on the 'early edition' QJ-40 was the APC 8.75x8.25 - its an amazing prop. Another option is the 8.75x9NN.
Do you know what shim stack us under the head right now ??
Let me check with the boss. Might be worth dropping the head a bit. This will help it unload in the air, but it will not do a great deal for ground RPM. It might get you a couple of hundred rpm to get over that 19K "hump", in which case it may be useful. Prop selection is still most important... let the muffler do its job.
Bob
Thanks for writing......
With the quickie jett, you should have no direct issues running the FAI fuel. Sounds like you are getting good performance, but you are correct..... at 19K peak on the 9W, that is 500-800 rpm below what the engine typically turns on 15% fuel.
My guess is the guys with the nelson engines see the same rpm drop. If that is the case, I'm surprised they are running those props.
The key here is to keep the RPM up. The engine runs best when you can get a ground peak in the mid-high 19K's, and a launch RPM somewhere over 19K....... around 19.2K to 19.4K. Only one real way to get there.....
Drop a prop size. Experiment a bit with the above numbers in mind.
The 9W is about as large as you would want to go based on what you described, but may be almost too big. You may notice that it is hard to set the needle... the engine is trying to trickle down off the pipe anything below 18.8K rpm which may give you a false peak.
Tip here - as an experiment..... stick with the 9W...however... take a spare 9W, clip 7 or 8 mm off of each tip. That prop should easily exceed 19K. Use that to set the needle. Find peak rpm, back down 500 rpm. Remember the needle position. Then reinstall the full-blade prop. When racing, turn the needle out 1/2 turn..start the engine....... wait 10 seconds......turn it back to where it was.....then throttle back a bit or have your launcher/caller hold the nose of the plane down, until its time to launch (or the engine will seriously overheat). Go to full throttle just before the flag drops. As long as the engine can unload over 19.5K in the air, it should run great [8D]
Although the others are using the 8.8x8.75, try running an 8.8x8.5 if you can get ahold of one. Its a very quick prop, and the engine should turn it up into the RPM range we are talking about here. One of my other favorites on the 'early edition' QJ-40 was the APC 8.75x8.25 - its an amazing prop. Another option is the 8.75x9NN.
Do you know what shim stack us under the head right now ??
Let me check with the boss. Might be worth dropping the head a bit. This will help it unload in the air, but it will not do a great deal for ground RPM. It might get you a couple of hundred rpm to get over that 19K "hump", in which case it may be useful. Prop selection is still most important... let the muffler do its job.
Bob
10-25-2004, 11:44 AM
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RE: Quickie JETT on FAI fuel?
Thanks Bob,
I kinda expected that answer...
I guess I'm just the tinkering type.
My engine came with one 0.005" aluminium (?) shim and two 0.002" brass shims. Well, I measured them in metric sizes of course, and those are the closest values after converting... they were actually 0.004921245" and 0.001968498" respectively.
So according to the Quickie JETT .40 Technical Bulletin I'm already at the lower limit...?
Are those the only shim sizes available?
I kinda expected that answer...
I guess I'm just the tinkering type.
My engine came with one 0.005" aluminium (?) shim and two 0.002" brass shims. Well, I measured them in metric sizes of course, and those are the closest values after converting... they were actually 0.004921245" and 0.001968498" respectively.
So according to the Quickie JETT .40 Technical Bulletin I'm already at the lower limit...?
Are those the only shim sizes available?
10-25-2004, 01:01 PM
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RE: Quickie JETT on FAI fuel?
I checked on a few things
More recently we have been running the head set between .008 and .012. Your shims add up to .009" (.25mm). You may be running on the high end.
Leave it there. At least that is my initial suggestion.
Another .001 - .002 or so is not going to do much. Considering we use to run these at .010-.015 at one time, its set about right.
If you want to tinker, pull one of the .002 shims. I would not suggest more than that though unless you accurately measure the deck height first to see where the head really is.
The key is the RPM is the muffler and timing. Regardless of what you do to the engine, you still have to hit those rpm numbers.
There are not too many other little miracles.
Let me know how you make out
More recently we have been running the head set between .008 and .012. Your shims add up to .009" (.25mm). You may be running on the high end.
Leave it there. At least that is my initial suggestion.
Another .001 - .002 or so is not going to do much. Considering we use to run these at .010-.015 at one time, its set about right.
If you want to tinker, pull one of the .002 shims. I would not suggest more than that though unless you accurately measure the deck height first to see where the head really is.
The key is the RPM is the muffler and timing. Regardless of what you do to the engine, you still have to hit those rpm numbers.
There are not too many other little miracles.
Let me know how you make out
