I don't understand how a 13B can flow 277.77 cu ft/min at 6000rpm, while a 20B flows 275 cu ft. min.(Al's numbers), unless the 275 cu ft figure is for 4000rpm instead of 6000 rpm.  Using Ed's formula, I get 416.67 cu ft/min at 6000rpm, or 31.66 lbs/minute.  Which number is correct?
 
Mark S.

At 12:23 PM 2/1/2004 -0800, you wrote:

 

Subject: [FlyRotary] Re: air flow

 

 

 Here is a formula   80 CID * RPM/1728.  Example for 6000 rpm,  80*6000

rpm/1728 = 277.77 Cubic Feet of air per minute.  A cubic foot of air (at sea

level standard day) is approx 0.076 lbm/cubic foot   so 277.77*0.076 = 21.11

lbm/min air mass at 6000 rpm .  Just change the rpm to what ever value you

want.  Also note the weight of air of course decreases with altitude so

277.77 CFM provides 21.11 lbm/minute at sea level but less at altitude.

 

Note this assumes 100 % Volumetric efficiency.

 

Ed Anderson

 

Oh; boy, now you ve done it.  My beautiful intake scoop is already installed, and I apparently designed it based on the wrong number.

 

You re numbers look right to me.  I ll have see if I can figure out what I did.

 

Thanks.

 

Al

 

Your Welcome, Al

 

    Perhaps you used the correct number for the design and just had a brain fade when you quoted the number for CFM - Hopefully?

 

Ed Anderson

 

Ed:

Well, it seems that I did use my number (275 cfm) and then computed a scoop inlet area of 3.9 sq. in. based on an airspeed of 100 mph (steep climb).  When I fabricated it, I made a little larger so it would be less restrictive for low speed.  Yesterday I made a more accurate measurement of the opening, and found it to be 5 ¼ sq in.  Assuming everything is ideal, this would be about right for zero ram pressure at about 130 mph, which is close to my typical climb speed.  At typical cruise of 200 mph at say 8000 ft, I should have same ram pressure for a little better cruise efficiency.  So all that is fine.

 

There is always the trade off of a scoop that is not restrictive on the takeoff roll, and one that gives you some ram air advantage without much drag at high speed.  I am considering a small, weighted trapdoor in the bottom of the intake duct inside the cowl that will open and allow more air in when the pressure in the duct is less than ambient, and closes when the pressure is more than ambient.

 

Al

 

 

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