Received-SPF: none
 receiver=logan.com; client-ip=216.240.18.37; envelope-from=echristley@att.net
Message-ID: <4CB328ED.4000301@att.net>
Date: Mon, 11 Oct 2010 11:10:37 -0400
From: Ernest Christley <echristley@att.net>
Reply-To: echristley@att.net
User-Agent: Thunderbird 2.0.0.24 (X11/20100623)
MIME-Version: 1.0
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Prop Chord vs RPM
References: <list-4498981@logan.com>
In-Reply-To: <list-4498981@logan.com>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: quoted-printable

Al Gietzen wrote:
> A prop is a wing that flies in a circle.  Right?  The most efficient
>=20
> wings (the ones that put on gliders), are very long and thin to get a
>=20
> high aspect ratio.  Right?  Given this premise, it only stands to reaso=
n
>=20
> that the best prop would be the longest one that would leave some groun=
d
>=20
> clearance, and then absorbs any remaining Hp by increasing the chord.
>=20
> =20
>=20
> That's my story, and I'm sticking to it (unless I'm wrong).
>=20
> =20
>=20
> Nice try, Ernest; but the term =91gross over-simplification=92 comes to=
 mind=20
> J. =20

No doubt.  But we're talking rule of thumb as a guide to choosing a prop =
for a light plane.  Unless we're building our=20
own, oversimplification can be useful, even necessary to the selection pr=
ocess.  You first oversimplify to get a basic=20
idea of the appropriate area to be looking in, and then iteratively add c=
omplication to narrow the selections down to one.

> With this wing; every increment is traveling at a different speed,=20
> has a different AOA; and it=92s always making a sharp turn.=20

Nearly all of the methods I've seen for dealing with this complication in=
volve dividing the wing into sections and=20
calculating incidence, AoA, lift, etc, for each.  The technique is just a=
n approximation, but the approximation become=20
very accurate as the number of sections approaches 8. It isn't an extreme=
ly DIFFICULT thing to do (after completing a=20
refresher course in trigonometry), but it is an extremely TEDIOUS process=
=2E  Thank God for computers and spreadsheets.

>Not to mention=20
> various turbulence factors, material stress, etc; but then I know you=20
> knew all that.
>=20

Granted.  I supposed it would be possible to get a prop so long on a tall=
 taildragger that the blades would simply bend=20
forward instead of pushing air back.  For most of us, though, ground clea=
rance will be the first limitation, followed=20
closely by tip speed.

Another big factor is going to be what is behind or in front of the prop.=
  I think Tracy demonstrated that pretty=20
conclusively with the prop he was using that was designed for a pusher on=
 his RV.  The air is much slower at the center=20
section of the prop on a pusher, and the prop designer had taken that int=
o account.  When put onto a tractor=20
configuration, the center of his prop was seeing a much different AoA tha=
n what it was designed for.  It is easily seen=20
if the conditions are known and just the sections near the hub are analyz=
ed.

But then we're not at the first stages of the selection process anymore. =
 My story is that for our category of=20
airplanes, length is the primary determinant of efficiency and should be =
the first criteria to use in selecting a blade.