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
Glad to hear that
it all turned out OK. I would say that 5 1/4 sq inch should do the
job, 3.9 might have been a bit on the restrictive side for a WOT 3
rotor. I agree - every decision seems to involve a compromise of some
type. Reducing any pressure loss in the duct should certainly help
more air flow into the engine. Good looking installation - coming
along well.
Cruise at 200
MPH should give you some where between 0.5 and 0.75 psi additional
inlet pressure depending on the density altitude. Any estimate
of when first engine run on the airframe?
Ed