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Hey, I'm not against NACA - if they work and you like them and they fit you need, then go for it.
The report did not say they would not work, just that they would not work as well as other types (streamline duct for instance) if you need pressure recovery (decrease airflow velocity and increase pressure recovery from dynamic energy).
However, I don't think using a streamline duct after a Naca duct is going to avoid the "bullet", the problem is any thing that causes a pressure recovery is going to cause "back pressure" to the air flow. From what I think I understood the Naca duct is great where you keep the air velocity up (like it being sucked into a jet engine) but don't have a lot of pressure recovery. But whatever you have (radiator, streamline duct) that causes back pressure tends to adversely effect the performance of a Naca duct because it slows down the airflow. At least that is what I think I understood.
Since I have my ducts right up front and am not faced with some of the challenges of the pusher type installation - I can say I don't have a dog in this fight {:>)
Ed A
----- Original Message ----- From: "Ernest Christley" <echristley@nc.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Wednesday, October 19, 2005 2:02 PM
Subject: [FlyRotary] Re: NACA scoops
Ed Anderson wrote:
Hi Thomas, afraid not exactly an old wives tale. Here is an extract from an old NACA report that appears to take the position that submerged (NACA?) ducts are not the best suited for radiators or anything requiring much diffusion or pressure recovery.
Ed, I took my lunch break to read the 5I20 report. What FUN!!
I think we're tripping over semantics here. There is a subtle difference between "supplying air to a radiator" and "using a NACA inlet for pressure recovery." The subtlety is the root of the argument. Just because it isn't good a pressure recovery doesn't mean that it will make a bad cooling inlet. It only means that you will have to do some pressure recovery in some other way, like a streamlined duct. "We don't THINK it will work for radiators, because there isn't enough pressure recovery" IS a long way from "it can't supply air to a radiator and you shouldn't waste your time trying ***because the inventors said so***". I'm only objecting to the very last clause. The authors might fully agree that a submerged inlet is a perfect compliment to a streamlined duct. The submerged inlet collects air in a low drag manner which it does well, and the streamline duct is responsible for pressure recovery which IT does well. But we don't know that, 'cause they don't make any attempt to speak to it. I think it would be appropriate to assert that the authors would agree with the statement that sticking a radiator against the back end of a NACA inlet is only marginally better for cooling than hanging it out the bottom of the fuselage.
Having said all that, Thomas' original statement is spot-on. The report authors did indeed recommend against using the NACA inlet as a pressure-recovery device.
Jim, the report Ed and I are discussing has a section on drag. As I read it, they didn't bother with a drag study because it is so low with a submerged inlet. All the losses due to drag end up as pressure where it's needed. If those NACA guys are to be believed, even though the downdraft scoop was half the size, it will still have more drag. Half of somethin' will always be greater than nothin' 8*)
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