Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Internal exhaust Ducting... ??

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Old 09-21-2003, 11:49 AM
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RPZ
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Default Internal exhaust Ducting... ??

FINALLY made it to the fuse portion of the build on my Patriot... Spent a LOT of time on the wings and tail feathers to ensure they were clean, strong and hinges set up perfectly... No I am at the point where I get to decide how to mount the engine.

The plan is to mount a Jett FIRE 60L... note that this is a rear exhaust motor. Originally, I was thinking of ducting over the FW and the tank into the canopy area and out over the turtle deck - a lot of mods and not too clean looking. Then I was thinking along the belly-pan with the engine inverted - except the pipe will sort of be in the way of the nose retract and possibly mounting the wing may be difficult. Now I am thinking of a conventionally mounted engine, moving the tank elsewhere and running the pipe through the fuse and exhausting in the tail cone (after all, that's what it's for right??

I was wondering if anyone had any thouhts on this - obviously, I do not mind deviating from the plans, and this bird is targeted for 170mph... streamlining is verrrrrrrrrrry important to me on this one. any ideas are welcome. I am not looking for any real aerbatic capability, just all-out speed. If the tank(s) were moved back over the wing saddle, would this be too far away from the carb? what about a one-way check valve on the return feed from the pipe to ensure positive pressure on the tank? What about a bladder tank?

If I run the exhaust internally, how much airflow will I need to keep things cool enough in there? Maybe those fake intakes could be made real or something... a scoop just behind the cowl, or something??

Questions, questions, questions...

Thanks to all who chip in...

Cheers,

Randy
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Old 09-21-2003, 12:21 PM
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Razor-RCU
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Default RE: Internal exhaust Ducting... ??

Only thing I can think of use a header tank (2 oz.) if you are gonna put the tank back by the wing- My tank is 1/2 in front 1/2 in back of where the wing dowels enetr the fuse... I use that above mentioned header tank and I have had no problems...

170MPH?
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Old 09-21-2003, 01:15 PM
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schmleff
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Default RE: Internal exhaust Ducting... ??

I might be wrong but it is going to take more than a streamlined pipe installation and gobs of power to make it go that fast. I would think that you will need a thinner airfoil too.

I would make a duct tube out of glass, line it with aluminum tape (to reflect radiant heat) and duct a fairly small amount of air through it. Again, I am not speaking from expierence, but I would think that you would run into the highest duct temps just after engine shutdown.

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Old 09-21-2003, 01:54 PM
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Default RE: Internal exhaust Ducting... ??

go have an ask on the Pylon threads, these are the guys in the know. My Magnum has the pipe in the canopy, the cable tie melts but no problems.
have you considered a side option? you could hide the head and pipe in a "scale" side intake. also makes the perfect streamlined duct
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Old 09-22-2003, 03:52 PM
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Default RE: Internal exhaust Ducting... ??

You can put your tank as far away from the engine as you want as long as you use a perry pump.
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Old 09-22-2003, 06:58 PM
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Default RE: Internal exhaust Ducting... ??

Make a duct that follows the shape of your pipe, and make it about 1/2" larger in diameter as the pipe. That way you have 1/4" of distance pipe to duct, which is plenty. Don't make the duct too large because then you'll need a large intake at the front to get enough airflow... smaller is better when it comes to this sort of thing.
You don't need space, you need airflow...
The pylon guys only have 2 to 3 millimeters of space, and nothing melts.
The portion aft of the pipe, that goes to the tail of the plane: make it slightly diverging. Meaning: if you have a duct diameter of 2" where the pipe ends, then make it 2.5" at the rear of the plane. This way you get a kind of suction that improves airflow again, and it also compensates for drag and temperature losses in the aft portion of the duct...

Try to streamline the airflow inside the engine cowl. Again you don't need a lot of space and turbulence, you need a smooth airflow through the cooling ribs and into the pipe duct.

This sort of thing becomes very important when going extremely fast... but I don't think it will make all that much on your Patriot. Just make a good pipe duct to keep things cool.
If you want to go all the way on the "keep it cool" thing you could make a sandwich duct: that is glass on the outside, balsa in the middle and alu foil on the inside. But it shouldn't be necessary if you get a fast enough airflow.

Don't use a check valve in your tank pressure line... you'll get way too much tank pressure on a piped engine.
You could consider a Cline regulator setup, or a Perry pump... but you said aerobatics aren't important, so a header tank like Razor suggested should be fine.
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Old 09-22-2003, 09:45 PM
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Default RE: Internal exhaust Ducting... ??

As far as ducting goes, you could also let the end of the duct extend 3 or 4 inches past the acutal exit of the exhaust. By doing this, you would create an augmentor tube that would aid cooling. By doing this, you can use a smaller inlet area for your engine and pipe cooling. This is common on 1:1 aircraft.

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Old 09-23-2003, 09:32 PM
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Default RE: Internal exhaust Ducting... ??

Thanks so far - this stuff really helps. Dub Jett has been telling me a few things about his engines - specifically the 60. It is possible I could go with a different engine. I have a little bit of research to do - so stay tuned once a decision is made on the engine.

Cheers!

Randy
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Old 09-25-2003, 08:36 AM
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Default RE: Internal exhaust Ducting... ??

I am also going to run my pipe through the Fuse. Most of the suggestions thus far are spot on. If you're serious about going fast I would forget the Jett60L. It is basically ported to be a 3D engine and any well tuned Rossi or YS 60 will outurn it.
I am putting a Jett BSE FIRE 90 in mine that has had the timing changed (Got another 1200 RPM from "as shipped" from Dub. Turns an uncut APC 10X8 at 18300 RPM static.
A Rossi 81 or new style small case 90 Ducted Fan Engines would also be good choices.
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