where does the model roll?

[Thomas C. Weedon]

There is no such thing as perfection, just "close enough". My design rolls
with no noticeably wobble (unless I've been drinking) so I don't worry about
the rest. The wing is 1" below the thrust line and the stab is 1/2" above.
Like we used to say in the service, "close enough for government work"
Tom W.

-----Original Message-----
From: discussion-request at nsrca.org
[mailto:discussion-request at nsrca.org]On Behalf Of george kennie
Sent: Friday, January 10, 2003 4:25 PM
To: discussion at nsrca.org
Subject: Re: where does the model roll?


Boy, this stuff can really get you started doing a lot of head scratchin'. I
think Oddino has the right slant on this one.It seems to me that it all
comes
down to the location of the vertical C.G.  Any displacement of the wing
centerline from the location of the vertical C.G. should logically cause the
wing center line to travel concentrically around the point of vertical C.G.
placement. This would mean that the wing, while rolling, would execute a
very
small barrel roll around the C.G. point. If it were possible to get the
location
of the vertical C.G dead-on on the centerline of the wing, and also dead-on
on
the pitch axis C.G., then you would be the lucky owner of a perfectly
axially
rolling aircraft. However, as John points out in the case of the Hideout
with
the tank in the canopy, the problem becomes more complex due to the fact
that as
the tank empties the vertical C.G. is under constant change which means that
the
rolling axis will be under constant differential adjustment.
This is a great brain-teaser and has really made me consider a set of
parameters
that I had never really thought about before.It seems to me that when you
consider the layout of our contemporary models with the head of the engine
hanging down below the datum line,the pipe on the bottom of the bird, fixed
gear
with wheel pants sticking down 12 or more inches below center that the tank
in
the canopy would probably be great at the beginning of the flight. Sounds to
me
like good reason to maybe get the wing centerline somewhat below the datum
line
so as to attempt to align the wing center line with the vertical C.G. as
much as
possible.
Maybe the next big fad we'll see is guys putting two 8 oz. tanks fore & aft
of
the wing tube,............hmmmmm...........
Georgie

John Ferrell wrote:

> IMHO:
> The airplane that rolls around the thrust line presents better for our
> purposes.
> The location of the neutral point, cg and center of lateral area appear to
> play a part as well.
> You have a measure of control with the shape and positions of the
ailerons.
> You can usually influence the roll axis with aileron differential.
>
> I obviously don't really understand the subject because I cannot quantify
> it!
>
> One reason the Snaky airplanes like the Prophecy were such a success is
that
> it about impossible to get the neutral point and the CG too far from the
> thrust line or the wing center line.
>
> I never saw a flyer that was happy with a Hydeout that had a big tank
> stuffed in the canopy area.
> It is another condition to consider with the wide bodies that are so
popular
> now.
>
> John Ferrell
> 6241 Phillippi Rd
> Julian NC 27283
> Phone: (336)685-9606
> Dixie Competition Products
> NSRCA 479 AMA 4190  W8CCW
> "My Competition is Not My Enemy"
>
> ----- Original Message -----
> From: "Koenig, Tom" <Tom.Koenig at actewagl.com.au>
> To: "'Discussion'" <discussion at nsrca.org>
> Sent: Wednesday, January 08, 2003 6:31 PM
> Subject: where does the model roll?
>
> > Hi Guys,
> >
> > I've just been asked this question.........it never occurred to me to
> really
> > think about this, now I'm open to opinions, facts and anyone that has
the
> > cold hard knowledge.
> >
> > I have always assumed that a model rolls around the datum line ie: the
> > crankshaft center line (not taking down and side thrust into
> consideration)
> >
> > Now a mate of mine swears black and blue that in actual fact the model
> rolls
> > around the wing center line ie: it is the wing that rolls and all else
> > scribes an arc around this.
> >
> > Can anyone can set us straight......please?
> >
> > Thanks
> >
> > Tom
> >
>
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