Bill Freeman wrote:

I agree with Vance.  To get the HP absorbed the

narrow props

were overpitched.  I thought it interesting that

the spreadsheet

showed this effect.  The problem with going

narrower in chord

is that you need to run very high blade angles to

absorb the

power and the blade works badly at lower speeds..

As the

chord is moved into a more appropriate range, the

required

CsubL to absorb the HP drops,  therefore the

blade angle drops

and the drag drops, increasing the efficiency.

Remember

these are all at the same HP.  The narrow chord

prop would

be a very bad prop over the range, but appears to

be a good

prop if you look at the top end (design poin).  It

actually is

essentially equal at top end.  This is why I was

looking at this

data.

I thought this process was reasonable, and that

this is what the

spreadsheet was showing.  I am very open to anyone

pointing

out where I may have made and error or bad

assumption.

The spreadsheet is very much in the development

stage and

these were not intended originally for public

release,

just to show Paul and Perry how things are

progressing.

The basic calculation method is from Dommasch

"Airplane

Aerodynamics".  I have embelished the CsubL and

CsubD

with functions that accurately reflect the local

alpha, plus a

estimate on the Mach corrections above 0.6 M.

Dommash

was doing it by hand and used essentially fixed

CsubL and

CsubD without any Mach effects corrections.

Bill
Long EZ builder & pilot
mechanical engineer

The narrow chord and high pitch sounds like a rational
explanation. Many of the "classic" prop design
approaches concentrate on the "design point" and use a
high lift coeff at that point - penalizing climb and
static as you showed. In my program width is set using
about 0.3 CsubL and gradiated over radius in search of a
constant power per square foot of disc area over most
of the blade. Off design cases are iterated with strip
element techniques to converge with momentum based on
velocity thru the disc. Vance

 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org