Exhaust within subcowl

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MolsonB

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I'm just in the process of building a teardrop subcowl for under the plane. My oil & coolant rad outlets will be dumped into the subcowl, so they can be streamlined and spit out the back. Along with it, the exhaust will flow through there.

My idea is to add louvers when the exhaust pipe enters the subcowl, so it pulls the air in the subcowl (oil & coolant, plus any air that leaks into the main engine cowl) and thrusts the exhaust. I found a company that just sells the perforated louvers without a muffler shell around it. Of course the subcowl and bottom of plane will be protected with Fiberfrax and SS plates.

Can the exhaust pipe (2.5") end inside the subcowl, or should I extend a pipe (3") outside the airplane for a few feet ?
subcowl.jpg
 

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don january

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The way I see it you could try it ending in the sub cowl and watch for a problem during run time and if there is a heating problem or the like you can take steps to cure it. IMO
 

Toobuilder

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Do a search for cooling augmentor tubes. Plenty of airplanes (most notably the T-34) use such a scheme.

As for using what appears to be a muffler/resonator, don't. I did a very similar thing and it won't survive prolonged 1400+ degree temps. In my case the internal core distorted, cracked and broke in just a few flights. This was a heavy guage, high quality automotive race muffler. No bueno.
 

Marc Bourget

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John Thorp designed augmentor exhausts for many aircraft, and, surprisingly, a Porche 911 that allowed the owner to dispense with the cooling fan and save 20-30 HP loss.

It will be hard (from what I understand of your approach) to "harness" the power of the exhaust pulse. You'll gain a better insight if you go to the EAA Sport Aviation archives and read his second article on cooling engines. It will be in the 1961-63 timeframe.

FWIW
 

TFF

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If the P-47 is any indication, you can run an exhaust a log way. The longer it is, the more chance you will loose horsepower. Much harder to retain the horsepower in a long exhaust than short. And extra weight too.
 

pictsidhe

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Your scheme will do nothing for muffling. It may even make it noisier. The stainless duct will do as an exhaust pipe, but will get moderately hot, so needs to be kept away from heat sensitive stuff.
 

MolsonB

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The muffler/resonator was a picture of the internal perforated louvers only. I wouldn't have the outer casing on, there is a company that just sells the inside pipe. The subcowl would act as the outer casing so it sucks in the residual air in the main cowl. The exhaust pipe is 2.5", while the perforated louvers would be 3". Making the louvers not in direct path of heat.

The rotary is very high pitch supersonic exhaust. From my understanding, by allowing slower moving air getting injected into the exhaust, it helps spin and collide with faster exhaust to cancel out sine waves. Also creating a vacuum pump.

Idea is to mimic the Aero Exhaust, which is well known for reducing Rotary exhaust decibels.
https://youtu.be/iDTTOa77efs?t=26
 

pictsidhe

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I'd suggest a mild steel ground experiment before trying some nice stainless. Muffling is a black art, I suspect your scheme will act more like a megaphone. Exhaust driven ejector pumps are known for being loud. The explantion of how the Aero exhaust works is plain wrong.
 
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Toobuilder

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If you intend to use the sub cowl as the "outer casing", I'd bet it will in fact amplify the sound.

Are you trying to reduce cockpit sound or for those on the ground? If the former, I've found a simple turndown on the exhaust and a great set of ANR headsets clamped on my head to be the lightest, simplest and most effective.
 

Himat

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I'm just in the process of building a teardrop subcowl for under the plane. My oil & coolant rad outlets will be dumped into the subcowl, so they can be streamlined and spit out the back. Along with it, the exhaust will flow through there.

My idea is to add louvers when the exhaust pipe enters the subcowl, so it pulls the air in the subcowl (oil & coolant, plus any air that leaks into the main engine cowl) and thrusts the exhaust. I found a company that just sells the perforated louvers without a muffler shell around it. Of course the subcowl and bottom of plane will be protected with Fiberfrax and SS plates.

Can the exhaust pipe (2.5") end inside the subcowl, or should I extend a pipe (3") outside the airplane for a few feet ?
View attachment 76446
A few points to consider:

  • As the flow exit the exhaust tube it will expand,
  • Mixing inlet air with cooling air and exhaust will heat the inlet air, expanding it.
  • The expanding air will tend to block the intake, both from cowling and louvers if the exit is to small.
  • The total exit volume will be large, requiring a large exit to not block the flow.
  • At anything less than one speed, one engine load, one altitude and outside temperature the exhaust area
  • will be sub optimal. Either restricting flow or creating excess drag.
  • Sound will exit any opening in the structure.
  • Sound transmit more or less straight through thin plates of most materials.
  • Exit of exhaust inside the cowl do make it very difficult to predict the total flow.

To keep it reasonable simple:
  • Do not have any exhaust exiting within the sub cowl.
  • Run air from the cowl separate from cooling air if cooling air is necessary.
Run air from the cowl separate from cooling air if cooling air is necessary.
 

wsimpso1

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Ejectors are amplifiers for an octave or more around the fundamental frequency of the ejector chamber. If the primary exhaust note is in that range, your engine will sound BIGGER. Understand that Wankels have a lot of energy in the higher harmonics, and that has to be tamed to make the noise tolerable. Also, EGT's are really high in Wankels.

Everitt Hatch, who had a bunch of experience with Wankels in airplanes, racecars, and road cars had some important things to say on the topic of exhaust system design for them:

304 stainless may not last long with Wankels, but Inconel does;

Piccolo pipes change the character of the exhaust note, tending to take the harshness off, but does nothing for the primary exhaust note;

The "duck call" in the exhaust port gives big exhaust noise taming for low power loss, and is better than any other muffling device in this regard. His experience was that they had to be Inconel and he had to develop them to make them durable and effective at low power loss. He used them in his aero engines (no mufflers, but a desire to bring off the harshness), in race cars that have noise limits, and in road cars.

Billski
 

pictsidhe

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Low pass filtering, diffraction and destructive interference. Which doesn't sound as cool and is way, way harder to explain. If you are selling a muffler that muffles, why bother with the more esoteric facts in your infomercial? Mufflers are very complex to analyse or predict. It's only fairly recently with modern computers that design has strayed from ye olde 'suck it and see'.

You can clone it, or you can come up with another scheme. easiest just to buy one, though... If you are going to design your own, base it on something that is known to work. you probably won't get quite the performance, but you'll be in the ballpark. New ground is going to be many orders of magnitude harder.

Can someone explain 'duck call' mufflers? My googling just tuns up a lot of plastic and wood calls to buy, nothing in pieces...
 

Jay Kempf

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Duck call muffler = f art can = boy racer, Type-R Accessory from Autozone or equiv. Basically the modern equivalent of playing card in the spokes and clothes pin :)
 

rv7charlie

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Aeroturbine is an effective muffler for the rotary, but very wrong idea for an extractor. Cooling drag would get a lot worse, due to friction through all those holes. One big hole has a lot less drag than a bunch of small ones with the same total area. Google 'eductor' and look at the drawings. The energizer (exhaust) should end inside the duct, and the duct should have a smooth taper to the exit.

Charlie

Edit: if you get it working right, it will help with low speed cooling, but will likely cost you speed at cruise because the eductor will cause excess cooling air flow at speed.
2nd edit: Rotaries are different to muffle, but cooling is cooling.
 

wsimpso1

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Duck call muffler = f art can = boy racer, Type-R Accessory from Autozone or equiv. Basically the modern equivalent of playing card in the spokes and clothes pin :)
Nope, look up MCPI. Exhaust port inserts. I do not know them, but They were part and parcel of Ev Hatch's Mazda engines back in the day, he claimed that his engines made street or race legal with less muffler and more power using them instead of skipping them.

Billski
 

wsimpso1

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Don't be silly Billski. Any street racer knows that power is directly proportional to how godawful loud that a car is ��
While just letting the noise go makes more power, I was speaking of folks with sound limits: Road vehicles in some jurisdictions, autocross, many road racing classes, some drag classes, and so on. In those situations, Ev said that exhaust port inserts made for more power at legal noise than by getting there with just a muffler.
 
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