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Lendo

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Sockmonkey, I had a similar design in mind but my centre tube was open to outside airflow, cooling the exhaust and temps, while helping with exhaust extraction, however I never got to test it. Put a tapered funnel in the front and no holes in the tube and you have my design.
George
 

Sockmonkey

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Sockmonkey, I had a similar design in mind but my centre tube was open to outside airflow, cooling the exhaust and temps, while helping with exhaust extraction, however I never got to test it. Put a tapered funnel in the front and no holes in the tube and you have my design.
George
Great minds think alike.
I think what you're describing is a pulse jet thrust augmenter shroud.
See that short little bit of extra tube? The hot exhaust draws air into it and produces a significant amount of extra thrust. It's been found that augmenters like that work better with pulsing flow than a steady one, so using them on your exhaust system nets you a few more pounds of thrust basically for free.
 

lr27

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If it sounds like a pulse jet there will be a cost. For instance, a few dB's from your hearing, urine stained upholstery and big fines for excessive noise. Or maybe loss of the field you're using. ;-p

I have heard a pulse jet model airplane, but I wasn't close enough to see it. I was as close as I wanted to get.
 

Sockmonkey

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If it sounds like a pulse jet there will be a cost. For instance, a few dB's from your hearing, urine stained upholstery and big fines for excessive noise. Or maybe loss of the field you're using. ;-p

I have heard a pulse jet model airplane, but I wasn't close enough to see it. I was as close as I wanted to get.
It wouldn't be pulse jet loud fortunately. You can actually muffle it.
 

Lendo

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Right, Augmentation is the right terminology, some use the exhaust flow to draw hot air from the engine bay as well. I considered it for the Rotary Engine, the exhaust is hot and loud, however my welder stuffed it up so I left it with him. I think if you combine both ideas you may be onto something, but I assume you need a bigger outer Tube and no holes in the inner tube, as that would be fresh air fed by an open tapered funnel, if running externally, which is what I was considering. Like some have said - anything sticking out means Drag.
 

Sockmonkey

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Right, Augmentation is the right terminology, some use the exhaust flow to draw hot air from the engine bay as well. I considered it for the Rotary Engine, the exhaust is hot and loud, however my welder stuffed it up so I left it with him. I think if you combine both ideas you may be onto something, but I assume you need a bigger outer Tube and no holes in the inner tube, as that would be fresh air fed by an open tapered funnel, if running externally, which is what I was considering. Like some have said - anything sticking out means Drag.
The augmenter could be added post-muffling as long as the muffler doesn't slow the flow too much, and as it doesn't depend on resonance to function the whole thing could be inside the fuselage with a small inlet tube to admit the aumentation air.
 

rv7charlie

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Guys, I think you're confusing (conflating) two completely different concepts.

The 'Meredith effect' involves taking cooling air *that is required to cool the engine* flowing it through the heat exchanger(s) and with 'perfect' duct design, use the heat added to the air by the heat exchanger to expand the air and accelerate the flow (using the exit duct). For the Meredith effect to be real, the thrust from the heated air has to be greater than the drag of ingesting the air and forcing it through the heat exchangers. According to engineers, it's very difficult to actually achieve net thrust, and well nigh impossible at the speeds we fly and the quantity of heat our typical engines add to the air. I know that there are people who say they've measured exit velocity higher than the a/c's airspeed, but that doesn't necessarily mean *net* thrust unless the actual cooling drag has been quantified as well, and it is lower than the measured *thrust* (not just speed of the air).

Exhaust augmentation is a *totally* different concept. It uses venturi effect to suck more air through the cooling system, and is almost always used to improve cooling at low airspeeds; typically in climb profiles. Don't ever forget that any air going through the cooling system is a drag negative. At low airspeeds (climb profile), the advantages of improved cooling outweigh the drag penalty. But if the augmentation is allowed to operate at cruise and at altitude, where speeds are higher and HP is lower (obviously, with lower heat load), then far more air than is needed is being moved through the cooling system, resulting in a significant unnecessary drag increase, and the *potential* of overcooling the engine for most efficient operation.

(Also worth remembering is that exhaust augmentation almost always *adds* noise.)

Now, there is a technique for achieving additional thrust *directly* from the exhaust. Kent Paser (Speed with Economy) did detailed testing on his Mustang II using various nozzles on the exhaust of his Lyc 320. IIRC, some nozzles created noticeable thrust increases *at altitude*, but little-to-negative benefit down low. BTW, the book is well worth owning if drag reduction interests you.

Or, I could be wrong about everything....
;-)

Charlie
 

Sockmonkey

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They're similar in that external air is taken into an enclosure, heated, and allowed to exit through a flared nozzle. A Merideth duct is essentially a low-speed ramjet that uses waste heat from the engine to expand the air instead of burning fuel in it directly. Using engine exhaust as your heat source to expand the incoming cold air is a little easier than doing a conventional Merideth duct, as you don't have to tune the geometry to pick up heat from a structure. The hot exhaust readily mixes with the incoming cold air.
 

MolsonB

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Well after 30 hours, the inside pipe started to cavitate on itself. It's 2.75" 321ss 0.050" thick. I drilled about twice the area of 2.75" of holes, hoping to reduce back pressure. Sadly I didn't really get a chance to measure.

What would be a good next step, get thicker gauge and drill more holes ?
 

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Patrickh99

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Titanium? Ace Race Parts dot com is one site that sells it.


Well after 30 hours, the inside pipe started to cavitate on itself. It's 2.75" 321ss 0.050" thick. I drilled about twice the area of 2.75" of holes, hoping to reduce back pressure. Sadly I didn't really get a chance to measure.

What would be a good next step, get thicker gauge and drill more holes ?
 

Ried

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Dec 30, 2010
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Downers Grove, IL / USA
Looks like well scorched metal. Like old burner cans from a turbocharger test stand. Not cavitation that I've ever seen.

Either titanium or inconel metal to deal with the heat.
 

MolsonB

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Based on my limited experience / knowledge, it looks like having the internal perforated pipe causes too much back pressure for a rotary engine at those RPMs, which decreases HP/increases EGTs. Although effective at reducing noise, costs speed.

So I've come across another design where they didn't even have the internal perforated pipe. Basically the exhaust shoots into the large tangential can that has internal vanes welded at an angle on the inside surface opposite the exit from the headers to help break up the pulses and direct the exhaust toward the exit.

Then I'd come up with a secondary muffler design underneath the plane.
 
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Lendo

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MolsonB. I had a design idea similar with a cool tube going through the centre of the outer tube from the front to just past the back. The idea was to cool the exhaust and help with extraction. I had it made up out of SS but never tested it. I thought it important to cool the Rotary exhaust/
George
 
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