|
|
<... Half of somethin' will always be greater than nothin' ...>
True as far as it goes, but even NACAs are more than nothin'. A lot of the serious speed merchants in the EZ community are going with downdraft instead of NACA and getting good results. As I said in an earlier post, there's more to it than just frontal area or inlet efficiency. I would surmise that a good downdraft setup buys you back more drag than it costs by enabling you to do important things with the lower cowl and get more/cleaner air to the prop disc.
At least the heavy hitters seem to believe so ... Jim S.
Ernest Christley wrote:
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*)
|
|