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  #1  
Old 01-12-2012, 04:30 PM
GusBiz's Avatar
GusBiz GusBiz is offline
 
Join Date: Apr 2011
Location: Melbourne, Victoria, Australia
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Default AirScoop or smooth cowl, which is ACTUALLY Faster???

so this may fly in the face of lots of opinion here, but after much research and study on the true nature of drag I have come to realise that things are not as straight forward as first thought.

My question is;

"Is the snorkel/Airscoop under the cowl actually doing you any good performace wise?"

So most would say well of course it is you ignorant fool, can't you see the little hole where all they good air can get pushed into the engine giving you more oxidation of fuel and more Manifold presure and greater power at altitude, not to mention the speed increase it gives you while looking at it on the ramp....!!

Yes, I agree I thought that too, so looking at how this works I thought about this. I am going to assume that the manifold presure readings people are seeing (an inch or more with the scoop ) is real and I am ignoring all engine config variables.

So you have more presure, great. That supposedly gives you more air to which you burn more fuel and get more horses right?.......

......and that is where it gets ugly. I thought hold on here I am at 14k feet and I am leaning it like a demon, and putting in as small amount of fuel as I can and I am trying to put more air in there to burn more? Well I guess that will mean I am able to get more power than I would at 14k feet so instead of just 60% power I am what 63% power?

This made me think well how much more because that is relevant. Why so, because the scoop is sticking out there taking on parasitc drag so that HP increase doesn't come for free, could it be that what is happening is that we are getting an incread in engine HP at a cost of drag reduction which at top speed at max cruise is actually considerable and could be more than the engine power.

Could we be getting less power out of our engines than the cost in drag at high speeds?
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  #2  
Old 01-13-2012, 12:50 AM
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Default Hmmm

Almost 200 views of this topic and yet no response.

I think that means people are interesed in this topic but no one has any answers......
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  #3  
Old 01-13-2012, 01:02 AM
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frankh frankh is offline
 
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Location: Corvallis Oregon
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Default Well

Even if you do get more manifold pressure then the dissapointing fact is that for a fixed orifice (note engines are not fixed orifices but two Identical 360's can be assumed to be the same) the extra airflow (into the engine) only increases as a square root of the manifold pressure increase.

So lets say your crusing along at altitude..now you get 5% more manifold pressure.

your actual airflow into the engine will go up by about 2.5%.

Not all that much really.

Frank
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  #4  
Old 01-13-2012, 06:02 AM
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Flybuddy2 Flybuddy2 is offline
 
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Default

Quote:
Originally Posted by GusBiz View Post
Almost 200 views of this topic and yet no response.

I think that means people are interesed in this topic but no one has any answers......
I'm interested but not familiar with the smooth cowl. How does the induction air work?? Sounds like you're pulling it from inside the cowl based on pressure. If that's the case aren't you getting significantly warmer (even less dense) air into your carb or FI? Also, how does the induction suction impact the transit of cooling air??
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  #5  
Old 01-13-2012, 06:28 AM
David-aviator David-aviator is offline
 
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Location: Chesterfield, Missouri
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Default

The reason there is not much opinion expressed on this subject is that "smooth cowl verses air scoop" is not so much a performance issue as one concerning cosmetic appearance.

I suspect there is very little if any performance difference. The frontal area is essentially unchanged. The engine has to have air to live and there has to be an intake hole somewhere to meet this need.

I like the air scoop version because it provides best manifold pressure and maximizes cooling inlet air. The internal snorkel results in a smooth looking cowl but it reduces air for cooling. It probably does not matter much with the 180 HP engine but the 200 HP creates more heat and needs all the cooling air it can get.
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  #6  
Old 01-13-2012, 06:48 AM
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Default

The smooth IO360 cowl is wider than the O360 cowl with scoop, that feature may offset any gain from being "smooth" drag wise.

I have a very good friend whom I used to fly with often, he has a tail wheel with smooth IO360 cowl (and HC pistons), while I have a A model with snorkel and stock 360, we run neck and neck and any speed difference between us in almost imperceptable.
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  #7  
Old 01-13-2012, 08:23 AM
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Alan Carroll Alan Carroll is offline
 
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Default

Check out photos of Nemesis and a number of other very fast planes that use a cowl air scoop. Clearly the scoop doesn't have to be a speed-killer.

Less clear to me is how much MP pressure is actually gained from direct ram induction over an efficient filtered arrangement. I know that my manifold pressures with an O-360 and FAB seem to run a bit lower than what are sometimes reported for ram-air induction on IO-360s, but I'm not sure how much of that difference is loss due to the carb vs. the induction system. My race speeds seem to be similar to comparably equipped RV-8s that have snorkel induction (provided they don't also have bigger engines or extensive cowl modifications).
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  #8  
Old 01-13-2012, 08:55 AM
Aden Rich Aden Rich is offline
 
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Location: Port Angeles, Wash
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Default Get the scoopless cowl!

It just looks faster.......and that's all that counts.
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  #9  
Old 01-13-2012, 09:04 AM
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CFI1513840 CFI1513840 is offline
 
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Location: Evans, GA
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Default Smooth cowl looks better IMHO, but...

I was looking at some pretty extensive modifications to adapt the Vans snorkel to my Mattituck IO-360, so I sent the snorkel back and chose the Rod Bower ram air induction instead. I'm willing to give up a knot or two for the convenience.

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Last edited by CFI1513840 : 01-13-2012 at 09:15 AM.
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  #10  
Old 01-13-2012, 02:18 PM
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Neal@F14 Neal@F14 is offline
 
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Quote:
Originally Posted by Aden Rich View Post
It just looks faster.......and that's all that counts.
The smooth cowl is a tiny bit easier to clean the dead bug splatters off. That's probably the real extent of any performance difference
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Last edited by Neal@F14 : 01-13-2012 at 02:23 PM.
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