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Thread: Understanding low drag engine intake / cowling design

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    Understanding low drag engine intake / cowling design

    Can anyone lay some widsom on me, or point me to some online literature, which explains the theory behind good cowling design for low drag air intake for the engine and low drag cooling / air extraction through the engine and out the exhaust tunnel?

    I need to design a filtered air intake for a fuel injected Lycoming 540, and I can't find any examples or explaination of what's good and what's bad design.

    I understand the basics like creating a baffled plenum for passing air over the engine, its the details of low drag vs. high drag air intakes / filter locations etc. which are baffling me!!
    Flying Bearhawk kit no.125

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    Registered User Vision_2012's Avatar
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    Re: Understanding low drag engine intake / cowling design

    Try 'Firewall Forward', Toni Bingelis.
    I think it was exhaust s/b 3x input. He also shows some neat auto-opening doors.
    For bafflement, try eyeballing Bonanza or any other production downdraft spamcan with same engine.
    Alan Laudani
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    TFF
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    Re: Understanding low drag engine intake / cowling design

    Look at RVs. There is the option to have the intake inside the left hand air inlet. looks good. Cirrus has the top induction Continental and picks up the air at the same place. Yahoo Tailwind group has some old articles by John Thorp on cowl design. I know the old style Mooney and Bellanca cowls with the openings below the spinner centerline have lots of drag and dont cool any better than the smaller above the spinner center.

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    Re: Understanding low drag engine intake / cowling design

    Quote Originally Posted by Vision_2012 View Post
    Try 'Firewall Forward', Toni Bingelis.
    I think it was exhaust s/b 3x input. He also shows some neat auto-opening doors.
    For bafflement, try eyeballing Bonanza or any other production downdraft spamcan with same engine.
    There is a lot of reasons to do an updraft as opposed to a downdraft for natural convection to clear the cowl when static. Peter Garrison did a bunch of experimenting on this front on his Melmoth.
    Jay K.

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    Re: Understanding low drag engine intake / cowling design

    One good design is that used on the Glasair III. This has actually been discussed here in several forms before - you might wan to use the search function and see if you can find the particular threads.

    The most important thing to understand is that the inlet only "lets" air in. No matter how big you make it or how close you move the inlet to the prop, it cannot take any more than the engine's fins can pass. That number in turn is a function of the fin passage area and pressure in the volume downstream of the engine. As such, the cowl's cooling exhaust size and geometry tends to be more important that the inlets. The Glasiar is a great example - the inlets are very small but the exhaust is designed so the low pressure field it creates literarily draws the cooling air out.
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    Re: Understanding low drag engine intake / cowling design

    Quote Originally Posted by TFF View Post
    Look at RVs. There is the option to have the intake inside the left hand air inlet. looks good. .
    That is a tidy solution, I noticed it on Mr. Staton's fuel injected Bearhawk.
    I was wondering whether this design competes with the engine cooling airflow, or whether there is just too much ram air to make a difference? Based on Orion's comment I guess it makes very little difference?

    Would that arrangement typically be more or less efficient than simply putting the air filter in the middle of the nosebowl (like a Cessna, discounting the different shape of the nose)?

    Thanks for the suggestions or updraft cooling, etc, but I dont have that kind of freeboard for design changes unfortunately.
    Flying Bearhawk kit no.125

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    Re: Understanding low drag engine intake / cowling design

    Quote Originally Posted by Jay Kempf View Post
    There is a lot of reasons to do an updraft as opposed to a downdraft for natural convection to clear the cowl when static. Peter Garrison did a bunch of experimenting on this front on his Melmoth.
    Wouldn't an upwards outlet also have much better cooling in climb (lower cp during high aoa flight)? Having the exhaust streaks over your windshields sounds a bit less comforting though...
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    Re: Understanding low drag engine intake / cowling design

    Some thoughts...

    Bearhawk is not the fastest plane on the block, so I was wondering if your question was about sufficient cooling or lowest drag cooling?

    Slowing the air down is important. Fast moving air through the fins just adds drag without adding cooling ability.

    The advantage of updraft cooling (convection) seems intuitive, but if it is better, why don't more planes use it?

    Getting the air to "wrap" around the cylinders is desirable for more even cooling, rather than just cooling the area between the cylinders.

    Updraft cooling means that if you blow something in the engine and start spraying oil, it will likely end up on the windshield unless you can route your outlets to the side or bottom.

    ... just some thoughts, and probably worth what you paid for them.

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    Re: Understanding low drag engine intake / cowling design

    There's lots of info in the NACA reports from the 30's on. They're available on the NASA Tech Server.

    An intake can be oversized with little or no penalty - if it's designed properly. Orion's comments on the baffles were spot on but lacking detail. That detail can be found in the Tech Server files.

    The difficulty with the Bearhawk is the amount of high powered flight below 100 MPH (on take-off) The engine mfgr's basically "spec" 5.0 to 5.5" H2O Delta P across the baffles for cooling. You get that dynamic pressure at slightly over 100 MPH. "Getting it right" at speeds from 50-100 MPH is the crux issue. John Thorp's Nov & Dec 1963 articles in Sport Aviation cover the basics and low speed cooling issues rather thoroughly. I knew him for 15+ years and had numerous conversations on cooling, but he writes very condensed and I still learn something new each time I re-read these articles.

    1:3 intake to exit ratio is grossly over estimated. The heating of the cooling air argues (Boyle's Law) for an increase of about 10%, not 300%. Too much exit increases turbulance/drag and allows any benefit from slipstreaming to be wasted. The oversized exit simply allows recovery to ambient, reducing the Delta P.

    IIRC, 5" H2O is the equivalent to the drag on a cigarette. The effects at this level are subtle and these subtleties are generally ignored by builders.

    Sufficient and lowest drag cooling are interdependent - and worth exploring - but it takes effort and focus to achieve.

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    Re: Understanding low drag engine intake / cowling design

    Quote Originally Posted by Battson View Post
    That is a tidy solution, I noticed it on Mr. Staton's fuel injected Bearhawk...
    I fly an RV with the filter in the left inlet and while it looks good, there is a substantial performance penalty. The pitot/snorkle style induction will produce a significant increase in MP over this setup. I'm working on a new induction setup now.

    On another note, you seem to be asking two questions in your initial post- are you interested in engine induction, or engine cooling?

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    Re: Understanding low drag engine intake / cowling design

    Both!


    Can you please show me what you mean by snorkle / pitot type setup?
    Flying Bearhawk kit no.125

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    Re: Understanding low drag engine intake / cowling design

    Quote Originally Posted by Marc Bourget View Post
    The difficulty with the Bearhawk is the amount of high powered flight below 100 MPH (on take-off) The engine mfgr's basically "spec" 5.0 to 5.5" H2O Delta P across the baffles for cooling. You get that dynamic pressure at slightly over 100 MPH. "Getting it right" at speeds from 50-100 MPH is the crux issue. John Thorp's Nov & Dec 1963 articles in Sport Aviation cover the basics and low speed cooling issues rather thoroughly. I knew him for 15+ years and had numerous conversations on cooling, but he writes very condensed and I still learn something new each time I re-read these articles.
    For those looking for a link to this article:
    http://xa.yimg.com/kq/groups/3854470...ame/nov+63.pdf
    http://xa.yimg.com/kq/groups/3854470...ame/dec+63.pdf

    Thanks for the information Mark
    Last edited by Battson; May 28th, 2012 at 06:23 PM. Reason: Second half of article
    Flying Bearhawk kit no.125

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    Re: Understanding low drag engine intake / cowling design

    Quote Originally Posted by autoreply View Post
    Wouldn't an upwards outlet also have much better cooling in climb (lower cp during high aoa flight)? Having the exhaust streaks over your windshields sounds a bit less comforting though...
    Just cooling exhaust upward exit not exhaust outlet. Peter Garrison did a bunch of CFD and then field testing of his single inlet below prop and double exit above the prop after the high pressure nose of the cowling. He had thermostatic low drag cowl flaps as well. The entire package had pretty significant cooling balance performance and definitely provided a benefit in climb and at all high HP demand regimes.

    I suppose if something went bad in the cowl it could bleed out some fluids to the screen but he had that anticipated too with the air travelling in and then rear then up through the cylinder baffling then forward to the outlet. So any entrained fluid would basically be cyclone separated out backwards to the firewall and drain out to the belly. Very clever thinking through the whole design.
    Jay K.

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    Re: Understanding low drag engine intake / cowling design

    The other potential issue that John Thorp cites with updraft cooling is the increased pressure of the cooling airflow on the windscreen, which by his calculations is significant for a typical light aircraft. For those reasons he discounted it for single engines, but suggests there is no excuse for not using it with twins. So for me, its not at option in any respect.

    Those articles are good stuff.
    Flying Bearhawk kit no.125

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    Re: Understanding low drag engine intake / cowling design

    Quote Originally Posted by Battson View Post
    ...Can you please show me what you mean by snorkle / pitot type setup?
    Kind of an extreme example, but this is a good picture.

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