Maybe Tracy will post again with more info, but my interpretation is that the inlet dia of his inlet 'pipe' remained the same, but instead of the simple 'pipe' pointing forward, he made a fat radius lip to fair the *outside* of the 'pipe' into the outer cowl shape. If mine doesn't cool with the stock James inlet rings (which are relatively sharp lipped), I'll just fair them out to the biggest radius I can manage in the space available. Frontal area won't change, so no loss there. He mentioned cooling improvements in climb; that seems likely to be caused by the inlet being basically an airfoil, and the sharp edge trips to turbulent flow at the inlet, while the 'fat' rounded lip acts like a fatter airfoil at high angle of attack in climb.

I gave up on 'optimum' while I was making my radiator & oil cooler diffusers. It just wasn't reasonable to expect optimum shapes given the space and fabrication constraints. I'll be elated with good enough.

Marc, 

The discussion isn't about the shape of the inlet; it's the shape of the 'lip' around the inlet. (Examples: stock RV cowls have really fat lips; the Sam James (round inlet) aftermarket cowls have sharper, thinner lips.) It's widely accepted that a circle is the optimum inlet because it has the best area to surface ratio, but hardly any of us have round heat exchangers, so going from round to rectangular makes it harder to maintain the perfect expansion rate in the diffuser (duct) and keep air flowing all the way to the corners of the HE.

As always, I could be wrong.

Charlie

On Sat, Jul 3, 2021 at 1:44 PM Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:

Thanks Charlie. Got a pretty good grip on that.

I was referring to Tracy's post on 4/28/2011:
"Finally got around to finishing my cooling inlets. (pictures attached). Up until now they were simply round pipes sticking out of the cowl. The pipes are still there but they have properly shaped bellmouths on them.The shape and contours were derived from a NASA contractor report (NASA_CR3485) that you can find via Google. Lots of math & formulas in it but I just copied the best performing inlet picture of the contour. Apparently there is an optimum radius for the inner and outer lip of the inlet. There was no change to the inlet diameters of 5.25" on water cooler and 4.75" on oil cooler."

So, which inlet picture?
And where do you see the optimum radius?

I'm beginning to think that Tracy want's to keep it a secret! First the wrong report number, then ... :)

Finn

On 7/3/2021 9:49 AM, Charlie England ceengland7@gmail.com wrote:

Hi Finn,

I was hoping someone with better memory/understanding would jump in, but I'll take a swing at 'general principles'. 

If memory serves, the guys who have applied the paper to hardware recently have said something like this:

The sharper edged lips are the most efficient shape where the flow is 'perfect' into the diffuser, and there's no spillage over the outside of the lip. But in our typical situation we need lots of flow at low airspeed (climb profile; high power) and less flow (relative to freestream) at high airspeed (cruise). If  airspeed is high enough that some of the air in front of the inlet can't get in and must divert around the lip, the sharp edge of the lip will cause turbulence and drag on the outside of the airframe (at high airspeed, where it hurts the most).  So, we can't truly optimize the lip. If a cowl flap will be used to increase cooling flow at low airspeed and reduce it at high airspeed, that means there will be significant spillage around the lip at high airspeed. So while the fatter lip is less efficient in the ideal flow situation, it ends up being better in the real world, because we must be able to cool at low speed and we want minimum drag at high speed. 

There's also the 'internal vs external diffusion' issue. I think Bernie Kerr was the 1st person that talked to me about that. Internal (K&W duct, for instance) is theoretically more efficient, and could use a sharp edged lip, but it's really difficult for us 'measure with a micrometer; cut with an ax' builders to get perfect. So the safer thing is to go big on the inlet to ensure plenty of flow, and then throttle the outlet to match actual flow to conditions, with a cowl flap. Since we'll have a lot of spillage around the inlet with the flap closed, that means a fat lip on the inlet to minimize drag when there's lots of spillage.

Or I could have an advanced case of oldtimer's, and I'm remembering it wrong.

I hope someone will correct me if that's the case.

Charlie

On Wed, Jun 30, 2021 at 10:18 PM Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:

Thanks. I guess I kept missing it because I don't understand what I'm looking at.
Can anyone help me understand the following and show me the optimum inside/outside radius ratio (which I assume is one of the A-10 to A-40 contours):


Finn

On 6/30/2021 9:45 PM, Charlie England ceengland7@gmail.com wrote:

Seems likely; I don't recall seeing a CR3485 referenced anywhere, either.

On 6/30/2021 4:42 PM, Finn Lassen finn.lassen@verizon.net wrote:

I tried in vain to find NASA_CR3485 (well, I found a NASA contractors report on animal studies).

I wonder of he meant 3405?


Anyone found a better match?

Finn

On 4/28/2011 9:07 AM, Tracy wrote:

Finally got around to finishing my cooling inlets. (pictures attached)� Up until now they were simply round pipes sticking out of the cowl.�� The pipes are still there but they have properly shaped bellmouths on them.�� The shape and contours were derived from a NASA contractor report (NASA_CR3485) that you can find via Google.� Lots of math & formulas in it but I just copied the best performing inlet picture of the contour.�� Apparently there is an optimum radius for the inner and outer lip of the inlet.�� There was no change to the inlet diameters of 5.25" on water cooler and 4.75" on oil cooler.

The simple pipes performed adequately in level flight at moderate cruise settings even on hot days but oil temps would quickly hit redline at high power level flight and in climb.�

The significant change with the new inlet shape is that they appear to capture off-axis air flow� (like in climb and swirling flow� induced by prop at high power)� MUCH better than the simple pipes. �� First flight test was on a 94 deg. F day and I could not get the oil temp above 200 degrees in a max power climb. �� They may have gone higher if the air temperature remained constant but at 3500 fpm the rapidly decreasing OAT kept the temps well under redline (210 deg F).

I have an air pressure instrument reading the pressure in front of the oil cooler and was amazed at the pressure recovered from the prop wash.� At 130 MPH the pressure would almost double when the throttle was advanced to WOT. � That did not happen nearly as much with the simple pipes.��

These inlets ROCK!

Tracy Crook

Virus-free. www.avast.com