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