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Thread: Air Intake Options

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    Registered User berridos's Avatar
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    Air Intake Options

    Hi everybody

    I wonder which of these air intake design options you prefer.
    Both are fast planes. The first one allows variable air inlet size.
    I plan to channel the cooling air and the exhaust fumes thru a tunnel between the pilots and let it exit behind the pilot at the belly where the fuse start contracting.Air Intake Options-sharkinlet.jpgAir Intake Options-ar-6primerfrontcua.jpg

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    Registered User bmcj's Avatar
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    Re: Air Intake Options

    Quote Originally Posted by berridos View Post
    I plan to channel the cooling air and the exhaust fumes thru a tunnel between the pilots and let it exit behind the pilot at the belly where the fuse start contracting.
    Just make sure there is no leakage into the cockpit cabin.

  3. #3
    Registered User berridos's Avatar
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    Re: Air Intake Options

    I thought about keeping the exhaust tube inside the channel and cooling the whole channel by the air that passed thru the water cooler. Both flows exit together at the rear.
    The channel would be built out of high temp epoxy or a ceramic composite matrix with basalt fibers for example, and a proper heat shield around the exhaust pipe.
    Maybe a 2 yards exhaust pipe is too heavy...

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    Moderator autoreply's Avatar
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    Re: Air Intake Options

    Static cooling (ground) can be your major problem. Motorcycles do it all the time though, without any airflow, some good (carbon!) isolation does wonders. Even running them completely shielded from airflow the carbon ones stay surprisingly cool when you run it into the red. The further from the prop axis your inlets are, the more static airflow through your engine.
    Aude somniare

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    Registered User berridos's Avatar
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    Re: Air Intake Options

    So according to your tip you would favor a NACA duct mounted in the lowest possible position of the fuse with a variable opening intake door and building the tunnel between the pilots out of carbon to avoid heating up the cabin and melting the channel.
    Do you have any reasoning for why carbon acts in such an insulating manner? Isnt the matrix responsible mainly for heat conductance?
    By the way i need to study which sandwich cores are heat resistant and arent heat conductive. Initially I belief nomex honeycomb would be the best option.
    Last edited by berridos; November 18th, 2011 at 03:56 AM.

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    Registered User wsimpso1's Avatar
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    Re: Air Intake Options

    In F1 racing, cooling drag reduction is a huge part of doing well. These folks tend to use the small inlets that face directly forward and are on stagnation points. Submerged ducts (NACA and the like) just do not do very well for feeding primary cooling systems, where big pressure drops and large flows are needed. Everyplace I have seen them used for primary cooling, they end up with a raised lip to make them work. Go with the picture on the right.

    Billski

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    Registered User Jan Carlsson's Avatar
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    Re: Air Intake Options

    I second what Billski say, on the pic to the PORT SIDE, the NACA intake is at a area that might have a low pressure or suction, from the rounding, might be a little better at climb, I guess that is the reason for the inlet flap. further back on that cowling is probably higher pressure at the bottom, specially at high alpha.

    The 101 rules, say, inlet at a low point, in a clean, high pressure area, outlet at a higher level then inlet, warm air like to rise, at a low pressure area, and or where you have distubed air anyway. below the prop center line at the side or just under spinner you have high pressure, especially during climb, when you need the most cooling.

    On top of cowling just after spinner at the side of, if a boxer engine, you have suction, just as you have just after the curvature shape of radial cowling, (as the one above) Also at the side far forward on the cowling you can have good suction. (as usual it depends on the devil that have his fingers in the details)
    There are planes with NACA inlets that work, but they have to be placed in a high pressure area, like the inlet just below LE of wing, those that have made NACA inlets on Vari-Long-eze planes, where the inlet is at the rear of fuselage in a negative angle, and probably in a suction area, it have not worked good, a pitot style is probably better then.

    Jan
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    Last edited by Jan Carlsson; November 18th, 2011 at 01:20 PM. Reason: We see things from different side of the screen! :-)
    Jan.

    A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away. Antoine de Saint-Exupery
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    Registered User bmcj's Avatar
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    Re: Air Intake Options

    Quote Originally Posted by wsimpso1 View Post
    Go with the picture on the right.
    Quote Originally Posted by Jan Carlsson View Post
    I second what Billski say, on the pic to the left
    Just a word for clarity... referring to the picture on the "left" or "right" can be a bit ambiguous. Depending on how one sizes their forum window, the pictures can move around on the screen. These two photos swap sides, depending on my window size.

    Quote Originally Posted by berridos View Post
    ...with a variable opening intake door.
    Now a question... are the NACA ducts typically adjusted with a flap that sticks out into the airflow, or can they be regulated by reducing the opening with a restrictor vane on the inside?

    Bruce

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    Registered User berridos's Avatar
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    Re: Air Intake Options

    This one has a variable inlet. Open at taxiing and climb and closed at cruise.

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    Registered User djschwartz's Avatar
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    Re: Air Intake Options

    Years ago I read a discussion by the designer of the NACA duct. It was never intended to provide maximum pressure recovery. Its intent was to allow air to be drawn in from the slipstream with minimal disturbance of the flow regardless of whether or not air is flowing through it. As such it is most effective for applications such as the intake for a ventilation system or a fan powered forced air cooling system. Efficient engine cooling requires maximum pressure recovery. To achieve maximum pressure recovery you need to avoid including a well developed boundary layer in the cooling air flow. There are basically two ways to achieve this. The most common is to draw the cooling air in at what would naturally be a stagnation point at the leading edge of the aerodynamic shape, in other words, at the very front of the aircraft. The second is to use a scoop that is set off from the main structure by a distance just a bit larger than the expected thickness of the boundary layer at that point. That is the method used by the P-51.

    You may also want to reconsider your cooling outlet position. In this case, best cooling efficiency is achieved when the cooling air is exhausted into a naturally occurring low pressure region. If your fuselage shape is properly designed, the point at which it necks down behind the wing and cockpit is actually a point at which pressure is rising back towards ambient. That's why these shapes are call "pressure recovery" designs. Just as with the airfoil of the wing the point of minimum pressure on the fuselage will be somewhere near the point of maximum thickness; though, the complex shape of a fuselage makes it a bit harder to predict exactly where without some wind tunnel testing or 3D flow simulation.

    The biggest reductions in cooling drag come from paying attention to what happens to the air inside the cowling. Having proper diverging inlets and converging outlets, making sure that all the air drawn in to the cowl is forced to flow around the engine itself in a manner most effective to cool it, etc.

  11. #11
    Registered User Dan Thomas's Avatar
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    Re: Air Intake Options

    Quote Originally Posted by djschwartz View Post
    Years ago I read a discussion by the designer of the NACA duct. It was never intended to provide maximum pressure recovery. Its intent was to allow air to be drawn in from the slipstream with minimal disturbance of the flow regardless of whether or not air is flowing through it. As such it is most effective for applications such as the intake for a ventilation system or a fan powered forced air cooling system.
    And that's what I understood, too. I made a NACA duct for the cabin heat system on the Glastar and it provided no pressure at all unless I slipped the airplane to get some ram pressure into the duct. I ended up with a lip on the aft edge to scoop air into it. I've seen others modified that way on homebuilts.

    The NACA duct works well for induction air. The engine can draw what it needs without fighting suction nor experiencing pulsations off the prop blades. Such pulsation upsets fuel flow and mixture distribution in the carb and can cause a rough engine.

    I have experienced this with the old 182s and many 172s when running them up with the cowl off; the cowl usually deflects the airflow so that it slides past the air intake and pulsation doesn't affect the carb; take the cowl off, and the intake gets battered by prop blast, and the engine will be somewhat rough in the runup. Put the cowl back on and it's smooth. The best ram recovery you can get at typical small-airplane speeds is so tiny as to be practically worthless: 1"Hg at 160 MPH, and one-quarter of that at 80 MPH. You older guys will remember the "Ram Intake" setups found on 1970s muscle cars; all marketing hype, no actual gain at all. Those intake scoops were a hair above the car's hood surface, where the boundary layer and turbulence would slow the air anyway.

    Dan

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    Moderator autoreply's Avatar
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    Re: Air Intake Options

    Quote Originally Posted by Dan Thomas View Post
    You older guys will remember the "Ram Intake" setups found on 1970s muscle cars; all marketing hype, no actual gain at all. Those intake scoops were a hair above the car's hood surface, where the boundary layer and turbulence would slow the air anyway.
    Motorcycle manufacturers still make those kind of claims. 10% more power @ 120 mph due to "ram-air". I'm still being laughed at by owners who don't believe that it's total nonsense.

    But indeed. NACA-inlets are useful when you sometimes need that airflow and are worried about drag when not in use (or ingesting stones, pressure fluctuations).
    Aude somniare

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    Registered User Toobuilder's Avatar
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    Re: Air Intake Options

    Quote Originally Posted by Dan Thomas View Post
    And that's what I understood, too. I made a NACA duct for the cabin heat system on the Glastar and it provided no pressure at all unless I slipped the airplane to get some ram pressure into the duct. I ended up with a lip on the aft edge to scoop air into it. I've seen others modified that way on homebuilts...
    I have a NACA duct on each door of the Hiperbipe and they blow a fantastic amount of air. These are 6 feet behind the prop and still capture the propwash at idle on the ground. These are perfectly flush to the surface, with the "lip" being only a slight radius inside the OML of the skin. These small ducts are almost too effective, and are often mostly shuttered closed. "Exact" shape is a requirement for these to work well. It's a case where "close enough"... isn't.

    Quote Originally Posted by Dan Thomas View Post
    ...The best ram recovery you can get at typical small-airplane speeds is so tiny as to be practically worthless: 1"Hg at 160 MPH, and one-quarter of that at 80 MPH...

    There are plenty of people who would do backflips for an additional 1 inch of MP.


    Quote Originally Posted by Dan Thomas View Post
    ...You older guys will remember the "Ram Intake" setups found on 1970s muscle cars; all marketing hype, no actual gain at all. Those intake scoops were a hair above the car's hood surface, where the boundary layer and turbulence would slow the air anyway...
    Yes and no. While the claims of increased manifold pressure "ram air" were marketing hype, the fact that these systems allowed "cold" air to the engine was a legitimate and verified performance advantage in many cases (Some were more effective than others).

  14. #14
    Moderator autoreply's Avatar
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    Re: Air Intake Options

    Quote Originally Posted by Toobuilder View Post
    I have a NACA duct on each door of the Hiperbipe and they blow a fantastic amount of air. These are 6 feet behind the prop and still capture the propwash at idle on the ground. These are perfectly flush to the surface, with the "lip" being only a slight radius inside the OML of the skin. These small ducts are almost too effective, and are often mostly shuttered closed. "exact" design is a requirement for these to work well. It's a case where "close enough"... isn't.
    Suction (from the tail) can allow huge amount of air to "blow" (actually suck) into the cockpit.

    Without a pressure difference you won't get airflow, so you either blow in air with ram air, or you "suck" it out with an outlet. Preferably both for minimum drag at maximum airflow power.
    Aude somniare

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    Registered User Toobuilder's Avatar
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    Re: Air Intake Options

    Quote Originally Posted by autoreply View Post
    Suction (from the tail) can allow huge amount of air to "blow" (actually suck) into the cockpit.

    Without a pressure difference you won't get airflow, so you either blow in air with ram air, or you "suck" it out with an outlet. Preferably both for minimum drag at maximum airflow power.
    This is true, of course, since my ears don't pop when I crack the vent open. Rag and tube airplanes are not known for being exactly "airtight". For something tighter (like a Glassair), a well placed outlet would probably be a lot more effective for cockpit venatalation than trying to "fix" a NACA duct with a lip outside the OML.

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