JonathanMilbank
Joined: 14 Apr 2012
Posts: 388
Location: Aberdeen area
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 Posted: Sat Feb 04, 2017 5:15 pm Post subject: Cooling drag question for Bud |
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Bud,
I'm in the process of converting my Classic to XS firewall forward and keeping the Rotax 912S with Airmaster CS prop. You recently wrote an interesting treatise on various causes of drag and I was particularly intrigued by your explanation of cooling drag.
Now I better appreciate the need to seal the cooling duct effectively and how important the bubble of pressure at the cowling intake is to encourage unneeded air to slip past the outside of the cowling. This raises the notion in my mind of encouraging some back pressure.
The pictures I've seen of other XS installations show the radiator extending across the duct and hanging down more than halfway. Behind that hangs the oil cooler, often extending a bit lower than the radiator, but never all the way down to the cowling.
The question I ask is whether there would be a drag-reducing improvement by reducing the outflow of air from the tunnel with say metal plates partially blocking some of the gap, or perhaps this would simply cause oil and water temperatures to rise.
I've also heard that the duct exit at the rear of the bottom cowling is the cause of a low pressure thereby increasing drag, so some outflow of air from the duct could help to reduce this effect.
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