Re: Re: propellor and rubber of a F1D model

From: LeRoy C Cordes <lcordes_at_juno.com>
Date: Tue, 11 Mar 2008 14:08:25 -0500

Thanks, John

LeRoy Cordes
AMA 16974
Chicago, IL
In God We Trust

On Mon, 10 Mar 2008 21:29:04 -0000 "John Barker"
<john.barker783_at_ntlworld.com> writes:
> Kevin
> I am not clear what your propeller block is like and how you are
> using it
> but I think it is easier to ignore it for the moment. You mention
> that at a
> radius of 50 mm you have a blade angle of 64 degrees. The sums are
> correct
> but is the angle calculated from an assumed pitch of 650 mm or is
> it
> measured?
>
> In any case the blade angle so close to the hub is not a good
> reference
> point. The significant factor in propeller performance is the blade
> angle
> at the radius three quarters of the way out towards the tip. I
> think with a
> modern F1D that you will find this angle is usually about 32 – 36
> degrees.
> That is the place to check your propeller and by measuring the
> angle, not by
> some guessing at what the prop block may give.
>
> It is a pity that pitch has such a hold in model stuff. (in
> propeller design
> text books it is rare to see the term mentioned.) It is imprecise
> and
> virtually impossible to measure and is usually assessed by a
> calculation
> from the blade angle, so why not just stay with the blade angle.
>
> Le Roy
> Kevin and Bill Gowen have already explained very well the benefit
> of
> specifying rubber as grams per metre but I would like to add a
> further
> point. When using rubber width to define the motor it is common for
> people
> to assume the thickness is standard at the manufacturers 0.04”.
> This is not
> the case and difference of 0.002” are not uncommon. As an example
> of the
> effect of thickness variation take the case of the 0.6g motor made
> from 1.3
> g/m rubber that I gave in an earlier post.
>
> The motor was 1.3 g/m it made up into a loop 9.1” long and took 1594
> turns.
>
> If the rubber had been 0.002” thicker then the g/m would have been
> higher
> with the following result.
>
> Motor 1.365 g/m would make a loop 8.7” long and would take 1482
> turns.
>
> A turns formula based on g/m takes care of these variations much
> more
> easily.
>
> John
>
>
>
>
> [Non-text portions of this message have been removed]
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>



LeRoy Cordes
AMA 16974
Chicago, IL
In God We Trust
Received on Wed Mar 12 2008 - 06:44:44 CET