flyrotary@lancaironline.net Mailing List Archive

True, it has yet to be tested on a hot day, nor have I tried any sustained high power climbs. I do get 100+ degree delta Ts on my RV-4 because it was specifically setup to use the absolute minimum air volume to cool (thick rads etc). That is one reason why I had to install cooling plenums on my coils & alternator. It is very hot under the cowl and I fried several D581 coils without the plenum. Haven't gotten good delta T data on the -8 yet.

The prop blast may be helping a lot on the -8. I used the Sam James 'Holey Cowl" with the round inlets set well outboard of the spinner. Not sure what it means, but Harmon Rockets are cooling 260 - 300 HP IO - 540s with inlets smaller than mine. 5" is the dimension after I enlarged the openings. Sam James supplies 3.625 - 4.25" inlet inserts for use with the 200 HP Lycs. I do see significant pressure in the oil cooler diffuser when running the engine up prior to starting the takeoff roll so I assume the rad is getting similar benefit. My Catto prop was built with a lot of attention to proper pitch near the hub as well.

One other observation from yesterday's tests. When my oil cooling was improved, the water temps went down as well, even though airflow probably went down through the rad when I improved airflow through the oil cooler. There is apparently a lot of internal coupling of the water & oil temperatures inside the engine.

Supposed to be warmer tomorrow. Will do some more flight tests.

Tracy

On Wed, Dec 23, 2009 at 11:20 AM, Al Gietzen <ALVentures@cox.net> wrote:

Tracy wrote:

My 5" round inlet for the radiator looks ridiculously small compared to yours but so far it is cooling the 20B OK.

Now that sort of boggles my mind as it seems to violate the laws of physics. Let’s just take a modest climb power of, say; 225 hp. At that power, the energy going into the coolant is about 6000 Btu/min. In order to remove that amount of heat, at a typical air temp increase of 75 degrees; takes about 4000 cfm air flow. A 5” dia inlet is 0.14 sq feet, meaning an average inlet velocity about 29,000 ft/min, or 330 mph. Even at 100 air temp increase (unlikely on a 90F day) it’s 250 mph. And I’m guessing your climb speed is half that. Similar math suggests you’d be limited to a steady state (cruise) power of about 50%.

Of course, being a pusher driver, I think of inlet air speeds in terms of the speed of the airplane. So does the fact that the inlet is behind prop give a much higher effective inlet velocity? I’ve been thinking that the turbulence in the prop wash would negate a good portion of the extra mean air velocity because of reduced inlet effectiveness.

It will be interesting to know how it works out on a hot day.

Great that you have your 20B in the air and working well.

Merry Christmas and Happy New Year to all.

Al G