Exhaust flow

[BrickPilot]

If introducing turbulent exhaust flow into the slipstream creates drag, would making it non-turbulent eliminate this drag?

I was thinking about how they make the leaping water fountains. Basically, they remove the turbulence from the water flow by passing it through a tightly packed set of tubes the size of drinking straws. When the water exits, it is a laminar column and stays together as it shoots through the air.

I was wondering whether or not something like this has ever been tried at the end of the exhaust tailpipe.

[Bob Axsom]

There are a lot of obvious differences but the only way to know is to try it. I recently cut off the downward bend of my exhaust pipes and the speed increased. It seems to me that if you get this to work you would would have to point the tip of the exhaust pipes in the opposite direction of the flight path - not down in other words. Good luck.

Bob Axsom

[gmcjetpilot]

There where some great threads on exhaust recently, try the search.

Most of the turbulence is where the exhaust mixes with the free air stream. Your flow tubes may have some benefit but probably some draw backs as well. It kind of always works that way. All the small tubes would add a restriction, more back pressure, especially where they start internally, in side the pipe.

There are books written about exhaust design and megaphones and megaphones with revers cones do all kind of wonderful things. Also at high altitude a "nozzle" or necked down exit that accelerates the exhaust flow can add "jet thrust". THE DOWN SIDE, like everything there is no free lunch, so when you play with the exhaust and gain one place you lose another, like a fixed nozzle would cause back pressure at sea level and high power, thus a loss of HP. Megaphones are LOUD! and they would be draggy out in the breeze.

Can I be honest? Leave it alone and keep the exposed pipe and angle and/or down turn to a minimum to avoid external drag. The closer the exhaust exits parallel with the free airstream the better.

If you want to get into it i suggest the book "Speed with Economy" by Kent Paser, and "Scientific desgin of intakes and exhaust systems". Also got to Cafe Foundation Org. Check out their research papers. They have three or so great articles on exhaust desgin and in-flight analysis with pretty cool pressure probes.

[RV8RIVETER]

Sounds like a great idea... at first. If memory serves those "straws" are extremely small and very tightly packed indeed. That would create alot of backpressure to get the exhaust through them. Just the dramatic increase in wall surface area alone would really slow down the exhaust gas. Then let us say it is possible to get the flow completely uniform, you will still induce turbulance when the two air streams meet, because of the large temperature difference of the gasses. I think most of the drag in this area of the cowl is due to the speed difference of the engine cooling exit air and the airstream and you would probably get more payback experimenting with mods to that.

But on the other hand, you can always make a small experimental fixture to test your idea and see what happens for yourself.