|
Thanks Ed,
I like to reduce things down to the simplest
formula, provided one doesn't lose track of what's relevant.
So I guess I should look at this as 100 %
theoretically efficient - whereas we may be looking at only 80 % efficiency ( in
real terms), Do you agree with 80%?
However your saying that the lift may give
more speed ( MPH, in real terms) - So are you suggesting that these two
(variables) may balance out?
Or should we use a fudge factor, and that could be
what?
I'm not looking for a finite answer, just
close enough to be able to access the experts opinion, and we all know what an
expert is - " A drip under pressure".
George (down under)
Yes, George. If we assumed no slip in the prop
then theoretically the formula will give you the pitch of the prop you would
need to screw through the air and achieve your MPH at your RPM input.
This is based simply on the screw principal. I am certainly not a prop
expert, but I also believe that since the prop is a spinning wing
(airfoil), that there is a lift component in the direction of flight
which may result in more speed than the formula could account for.
Your second equation can be reduced further from
(Inches/Minute)/(RPM) = (Inches/Minute)/(Revs/Minute) =
inches/Rev
Ed A
----- Original Message -----
Sent: Wednesday, December 21, 2005 7:08
PM
Subject: [FlyRotary] Re: Props
Don't ask me George. I'm not an engineer or good with
mathematics. Ed A will know better.
Buly
On Dec 21, 2005, at 6:20 PM, george lendich wrote:
Thanks
Buly,
So, Pitch" = MPH
x12 x 5280
RPM x 60
I figure 12 (inches in a foot)
and 5280 ( feet in a mile) and 60 ( minute in an
hour).
So the top line is converted
into inches per minute and the bottom line RPM.
So, Pitch " = inches
per minute
RPM
Does that look right to
you ?
George ( down
under)
Thanks George. my mistake was that i did not convert it to MPH.
I was also given the formula for cruise speed:
Pitch" x RPM x
60 = MPH
12 x 5280
Buly
On Dec 21, 2005, at 4:39 PM, george lendich wrote:
Buly,
Looking at the
formula given by members on this list, 2 out of 3 were
spot
on with Hoverhawk
and the third was close.
Examples:
1. Prop Dia x
3.1416x RPM/ 720 - Correct!
2. Prop RPM x Dia x
.00436 - Correct ( 3.14/ 720 =.00436)
3. 3.14 x 64 {given
prop dia} ( 5500/2.17 {given RPM})= 508346/707 -
slightly incorrect
( as compared to the other 3 examples, which include
Hoverhawk)
i.e 3.14/ 707 =
.004809 whereas all the others are 3.14/ 720 = .00436.
Now that I have
that sorted - can someone tell me how to calculate pitch,
easily!!
George ( down
under)
Thanks Perry.
This makes me feel much better. Looks like the formulas
given to me were
not good.
Buly
On Dec 19, 2005,
at 2:00 PM, Perry Casson wrote:
I think your
math is off a bit - Here's a online calculator
http://
www.hoverhawk.com/propspd.html
I think 6900
puts a 72" prop close to mach 0.9 but
I've never ran
mine above
6500
Perry
Casson
--
Buly
--
--
|