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JETT engineering .46 FIRE - Fuel (max nitro?)

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Old 02-23-2015, 07:13 PM
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Default JETT engineering .46 FIRE - Fuel (max nitro?)

Good evening

As the title says engine in question is a jett engineering .46 FIRE with a full length tuned pipe.

My questions is what fuel mix would provide the most power . This engine will be used in a competition so engine life isn't top priority. I know Dubb suggest 15% nitro and says more power will come from more nitro. How far does this hold true?

Has anyone run 30+% nitro in these engines? Did you shim the head?
What glow plug did you use?

Prop will probably be an 11x4 maybe 12x4.

Thanks
Steven
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Old 02-23-2015, 09:05 PM
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If you know what you are doing and have lots of spare matched P/L sets and rods, you can do like the Top Fuel drag racers do...use mostly all nitro, a dash of methanol and replace pistons / rods after every round.
You can also just use 0 - 5% nitro with a high compression head and a specially sized pipe and get outstanding performance that way.
Shimming the head and prop selection are your "tools" for controlling the heat and ignition timing when tuning. Controlling the heat generated during the run is assisted by adequate oil content and perfect mixture control. The best advice in general is to use the least amount of compression, nitro and propellor that gets the job done and makes you happy.
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Old 02-24-2015, 07:47 AM
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I don't understand the need for every last bit of power running a 4" pitch prop. That being said, call Dub and ask him to set the engine up for high nitro. He built the thing, he can tune it for you. You will have to send him the engine, pipe, and prop you plan on using. The compression will have to be lowered to run more than 15%. You can guess how much or have him do it.
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Old 02-25-2015, 07:47 PM
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What event limits you to .46 ci displacement, and where a 4" pitch prop is suitable?
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Old 02-25-2015, 07:58 PM
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Hey guys sorry for the late reply.

This infact is NOT for a fast airplane I posted here because when i think JETT I think fast .
The airplane is actually a heavy lift competition gross aircraft weight will be around 30 lbs. Max engine displacement is .46

This subforum probably has the most experience with this engine sorry if posting here offended anyone.
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Old 02-25-2015, 08:11 PM
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In that case, what I said earlier still stands. Call Dub and talk to him. He is a really nice guy, and can tell you how to get where you need to go.
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Old 02-25-2015, 08:18 PM
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Nobody offended. Something didn't ring quite right until you explained the application, now it makes perfect sense! There's some neat stuff happening in those competitions.

what pylonracr said.. talk to Jett.

I wonder if Dub will suggest spinning the crap out of a 10-4?
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Old 02-25-2015, 08:30 PM
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I was kind of thinking that myself, how fast can you turn a 4" pitch prop before you start wasting power making lots of noise.
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Old 02-25-2015, 09:51 PM
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I doubt any .46 could max out the usefulness of a 11 x 4 or 12 x 4.
I would be surprised to see any .46 out perform my Morris Hobbies Rossi in this department.
That engine was designed for 3D.
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Old 02-26-2015, 06:38 AM
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These would probably satisfy Jett's "RPM RPM" mandate..
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Old 02-26-2015, 07:27 AM
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We used to run 80% nitro with 5% castor and 15% Klotz for record attempts. Some guys were using propylene oxide mixed in. Really 50% is lots, and I doubt there would be any further gain. I watched Propworn doing the Heavy Lift thing a while ago. It was quite amazing. He was using a .61 as to the rules. It was honking pretty good. Saying 30 lbs to fly is one thing, but carrying the plane from the field to the pits makes it seem impossible to fly, or even taxi.
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Old 02-26-2015, 11:32 AM
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The doctrine must be clean clean clean so the engine can pull it at enough speed to sustain flight. What sort of level flight speeds?
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Old 02-26-2015, 11:39 AM
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It wasn't too fast. The wing had a lip under the trailing edge, maybe 3/8" down. I guess it lifted better. It was pretty light before the weights of course. I think the rules were to do a circuit and land. Maybe the same university rules as the OP?
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Old 02-26-2015, 12:51 PM
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if buying new from dub ask him to set it up for that prop assuming you will run an APC prop tune the pipe to run the engine around 18,000 on the ground with what ever prop will do this. then find the prop in the 11" range like a 11x3 or 11x4 so that it tachs 17,000 on the ground and in the air should unload just past the 18,000. Dub can raise the timing some and get these numbers.

The APC 11" props styart to howl pretty good at 17,000 and scream at 18,500. on this type of plane and weight you should see around 50-67 mph at these rpms/ lots of drag. I have ran this prop on an engine at 17,000 on the ground and recorded an inflight speed of 92 mph.
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Old 02-26-2015, 06:15 PM
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What data don't you have?
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Old 02-26-2015, 06:39 PM
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lots i dont have lol. I will be doing some more testing soon on my War Bird racers. we run 14x13N, 13.5x13.5 and 14x14 props on 40-60% nitro. Some of the guys say the 14x13N unload to around 12,000 in the air.
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Old 02-26-2015, 09:41 PM
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I would stick with 15% as you also want some reliability. My jett 50 will spin an 12. 25x 3.75 apc well over 20k. It would overstress the props after 6-7 flights and they would show white cracks. Dub wil set you up. But i would think you would want to prop up a bit and get more disc area.
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Old 02-27-2015, 04:56 AM
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Hello Steven,
After reading some of the suggestions presented, I'd like give a little reminder on the rules concerning engine displacement. Modifying the internal displacement by adding shims is not allowed. All other "external" mod are acceptable. So...bottom line is whatever condition the engine was in when you received it after purchase originally is how it will operate. You can modify fuel, carb, pipe and prop but that's pretty much it. Oh...and gearing is allowed as well. Good luck at competition.

Oliver Alvarado
SAE Aero Design Rules Committee

Note to everyone else. First up will be Aero Design East in Lakeland, Florida. The flying portion is on March 14 and 15. So if you are in the area, come on by and observe the competition. It is being held at Sun n Fun and there is no entry fee for spectators. Lots of fun and usually lots of crashes.

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Old 02-27-2015, 05:19 AM
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Default SAE Aero Rules Document

For any one interested in what Steven is competing in...here are the rules.

Oliver
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Old 02-27-2015, 07:21 AM
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how does shims change the displacement?
Also if gear is allowed then why run a sport engine? for this type of even you want the largest prop you can run and gearing would be the way to go. Find a old OS 46 Ducted fan engine or better yet run a Jett FAI practice engine on a geared set up. these engines like to run around 30,000 or more and i think they are over 3 hp.
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Old 02-27-2015, 07:39 AM
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Shims do not alter the displacement, only the compression ratio, or possibly the timing of the exhaust/bypass ports if the liner is shimmed up. The displacement is only changed with the diameter of the piston, and the offset of the crankpin/stroke. I would think shims on the head should be allowed because they could even need to be thicker if the plugs don't last, or thinner or even removed for temperature and humidity changes. Just my interpretation of the rules, I am not running it or anything, but you know, rules are made to be bent.
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Old 02-27-2015, 08:40 AM
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Airraptor and Aspeed...you are both correct in that changing the shims does NOT change the displacement volume (bore x stroke length = displacement volume) and does not include the combustion chamber volume. Shims added or remove do change the compression ratio and is considered an internal modification which is not allowed by the rules. I kind of threw it all together which was a mistake on my part and I see that now as I reread my post. Thanks for calling me out on it.

I agree that gearing an engine is the way to go but in the many years that I have been involved with the SAE Aero Design contest...few student teams have ever attempted it. The ones I have seen usually destroy themselves for some reason. Running the shows I don't get a chance to get with the teams and see what failed.

Oliver
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Old 02-27-2015, 08:44 AM
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Heck...because of the strict no internal modifications rule...a team showed up with a home built electric rotor attached to the carb to increase air input. If I remember, it did fairly well as far as engine performance. Of course the best engine/airframe design usually wins...not just by engine power alone.

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Old 02-27-2015, 11:34 AM
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I think an engine with a belt reduction drive would be ideal. run a large prop at a low rpm to gain most thrust. The strongest engine to meet these rules is a MB Profi 40. you can have these set up from MB to run on higher nitro. Just a guess here but on a belt drive and say a 20x10 prop at 6000 rpm or so should give around 18 pounds of thrust. that would fly a 30 pound plane.
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Old 02-27-2015, 12:08 PM
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I think 18 pounds of thrust will fly something alot heavier than 30 pounds!! I noticed you are in Fairfield, CA. We have another Aero Design competition in Van Nuys on April 25 and 26 at Apollo Field. We have an East and West competition due to the number of teams wanting to participate. Heck...we even have to turn teams away this year. At West, teams will be coming from Poland, Egypt, India, Mexico, Canada, UK, China and of course from the US. It is really interesting to watch. Just grab a suitcase and make a weekend of it.

http://students.sae.org/cds/aerodesign/west/
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