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Oops. Didn't read your question well enough, Al

. But, even assuming your 9.5" of STATIC pressure (which is of course what you are reading) is 100% of the free air stream dynamic potential at the entrance of the inlet, you can not recover more than 84% of whatever free air dynamic potential that 9.5" might represent inside the duct.

if the free air velocity (160) converts to 12"H20 and you had a streamline duct inlet actual had that coming in then theoretically you could get approx 12 * .84 = 10.8" inside the duct. Since you measured 3.25" static in front of the core, that would indicate a significant lack of pressure recovery inside your duct (what ever the reason). There are several reasons this might be happening.

1. The air flow and velocity is considerably reduced from what you are expecting (too small opening/exit - which I don't believe to be the case)

2. The boundary layer is a significant part of your duct total air flow and as a consequence its lesser velocity has less dynamic pressure potential.

3. A significant part of your duct flow is chaotic with eddies which does not provide recoverable pressure - or it is much reduced. (The boundary layer could be contributing to this)

4. Some combination of the above.

Right, now I would suspect that the boundary layer could be the culprit in that it can contribute to 2 and 3 above. But, as you know, this is speculation on my part. Just throwing out some thoughts that might give you some ideas.

Best Regards

----- Original Message -----

From: Al Gietzen

To: Rotary motors in aircraft

Sent: Saturday, July 14, 2007 12:37 PM

Subject: [FlyRotary] Re: FW: Oil cooler air flow

Hi Al,

If a "full-strength" Streamline duct were tested under the conditions of

9.5"H20 at the entranced to the inlet then at the widest part of the duct

you should theoretically measure 9.5*.84 = 7.98.

Ed;

Perhaps you could enlighten me on this. Where does the .84 come from? Is this static pressure? I thought that as the air was expanded and slowed, dynamic pressure was converted to static pressure and the value increased. You’ve obviously studied this more than I.

Regarding your second sketch, since you do have slower moving boundary layer air moving next to the skin of the fuselage (and duct), the vane would probably help it turn around the corner. However, you are still ingesting slower moving air with less dynamic pressure to recover from it. It is my opinion (no experience or hard data) that moving your inlet fuselage side of your inlet opening approx 1 1/2 - 2"away from the fuselage would make an improvement.

You are very right about that. Actually in the location that it is, even an inch or less away from the surface would make a big difference. Keep in mind that the scoop is about 23” wide with about 1 ¼” opening. How do you get BL diversion with that configuration? Of course this configuration began the way it did because another Velocity builder had put his standard aircraft oil cooler (for a Lyc) in the same location, and said it worked great without a scoop – just the differential pressure above and below the wing was enough. Go figure.

As I as looking over this diagram more carefully (Winginstallation.jpg), it became apparent that it main point was to show one duct installation with the inlet stand off (bottom one) and the other without inlet stand off using a vane to assist the airflow. So, one could draw the conclusion that you have a choice? either use inlet stand-off OR using a vane.

FWIW

Where did that diagram come from? Very interesting. I think that my scoop opening is large enough, and the BL thin enough, that if I can get effective diffusion in the duct it should work just fine.

Thanks for your input on this.

Al

Thanks,

Al