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Originally Posted by osxuser
I think you might be better off putting the smile inlet more flush on the cowl, right below the spinner if your going for that. To me, and i'm no expert, that doesn't look as efficient a design as a regular round ram inlet.
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The location near the spinner is absolutely the worst place to get induction air. Look at the shape of the prop blade near the spinner? It is a blunt club beating the air to death. The spinner is not help either with airflow any which way but straight. The further out towards the prop tips the better. Many a flight test has been done, see the book Speed with Economy by Kent Paser.
This is one of the biggest myths around, that you can get huge boost from your intake scoop design. It is a urban legend and claims of boost are like claims of high top speeds. However it is harder to get away with an exaggeration the top speed of a RV because most people know how fast RV's go, which are fairly consistent. Any gain over ambient air pressure at RV speeds is possible but not common.
As far as intake shape, there is no magic and the
claims of tremendous ram rise are questionable .
Often pilots report large MAP increase mostly because their alternate source is so restrictive. It is more you are dropping MAP by going to alternate than reading your MAP increase from RAM pressure.
In actual fact you would be good just to get ambient pressure with no losses. The standard pressure @ 8,000' std day is 22.23 in-hg. Most Lycomings RV's have a hard time getting 21 inches much less +22 in-hg plus at 8,000' @ say 190 mph.
From my recall most RV's claim MAP of 21-22"-hg at 8,000', wide open throttle. I have seen claims of, 22.5", 23", 23.5 or even 24". Keep in mind the standard pressure is 22.23"-hg.
24" at 8,000' is over 1.75"-hg raise. That is more total RAM pressure than is available, which is only about 1.2"-hg at 190mph. This claim of cruise MAP (8,000' WOT) was from an O-320 powered RV. I would have to question the accuracy of their MAP gage or actual true ambient pressure (corrected for temp) or density altitude, unless he was going 385 mph, which I doubt. I can believe about 0.3"-hg with a very fast RV. However given the casual method most take MAP measurements, any claim of RAM rise has to be suspect.
For any one getting more than a true honest 22.0" or 22.5"-hg they have a great intake, very effective system with no boost but with no loss, which is real good. Boost is a myth unless you are going 250-350 mph or have a special designed intake very close to the prop (not near the hub).
If you want boost you need a turbo or super charger.
The best way to measure RAM pressure in the intake is put a pressure probe in the intake. Using a separate airspeed gage hooked to a reliable static source, you could read airspeed and convert to pressure or ram or MAP pressure. The problem is with small errors in static source when measuring small a pressure (which it will be negative to small). For accurate flight test data, static cones are pulled behind the aircraft to get accurate static pressure.
If you could recover even 1/4 of the total RAM pressure at 190mph it would be about 1/3 in-hg. Again that is a pretty good trick and has been done with special installations that place the intake snorkel real close to the prop (out from the hub), at angles (to the right 10 degrees I recall) to take advantage of the true combined relative airflow of the prop (thrust) with the fwd velocity of the aircraft. After all that the gains where small. With the stock Vans installation and the Smiley intake you are lucky not get a loss, which is way better than most that have losses.
When you start going well over 200mph, 250-350mph you can get more RAM just because there is more available. However the idea of massive boost (.5" to 1"-hg or more) is a little dubious for a RV. Van's airbox is about as good as it gets with a filter.
The snorkel extended from the cowl to with in 1/4" to 3/8? of the prop blade, positioned and angled properly looks weird and thus is not popular. Plus the gain is small, not to mention may be a little more drag. You see these induction tubes on Reno formula racers, but for every day flying the stock position and shape it very good.
If you like the
-face scoop for looks (like a P-51) that is fine but doubt the claims of great gains in RAM. It may be better than a round intake in that the wide horizontal mouth may take advantage of the fact the relative air under the cowl is not perfectly straight. However if the area is greater, the drag will be greater, negating the advantage from (small) RAM rise of MAP.
Cheers George
Smile cowl inlet
CJ
Myth Buster? That is not a good location
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