Conference vmszoo::rc

812.0. "SCRATCH BUILT CESSNA 172" by CLOSUS::TAVARES (John -- Stay low, keep moving) Tue Dec 27 1988 12:02

I started my next project, a scratch built scale Cessna 172 over
this last weekend.  I selected this plane just after the Master's
Qualifier in June when someone in the office brought in pictures
of their newly-painted 172.  This is one very nice plane, it is
white with metallic blue trim and gold striping.  It should make
a fine subject. 

The owner is an accomplished pilot who frequently goes on long
cross countries, so the plane has just about every antenna and
geegaw hanging off it that it can.  He is also very cooperative,
even to the point of offering the combination to the hanger so I
could take measurements, etc.  I declined on that at this time,
but will probably take him up on it later.   He also loaned me
the IPB for the plane, from which I copied some details of
internal structure.

I decided to make an ongoing note for this project because,
despite the fact that I'm a duffer pilot, I'm a fairly
experienced scratch builder.  Perhaps this note will stimulate
discussion of this subject, but if nothing else, it'll be fun to
talk to myself and keep a journal of the project.  The plane is
far above my head in terms of flying ablility required, but I
plan to take anywhere from 18 months to 2 years to do it, and I
should be ready by then.   Of course, I'll be doing other things
in th meantime; there's a Sagitta 600 coming in the mail, and I
need to finish the Tutor (already 18 months and counting).

After I decided to build it, I took the month of July while on
vacation to verify that this is indeed the model I want to build,
and to look for sources of documentation to save me the time and
trouble of doing my own drawings.  I even planned out my model
building and flying activities for the next 5 years!

I found an article from 1968 on building the 182 Skylane, nearly
identical in outline, and sent off for the Royal 172 plans.  When
the plans came, I went out to the plane and took a roll of film.
I set the plans on the floor and did a close check against the
plane.  Turns out that the Royal plans are for a late '60s plane,
whereas the one I want is '76 vintage. The main things different
are a droopy wingtip on my model (actually drawn in erroneously
on the Royal plans), some simplification of the tip of the fin on
the Royal plans, easily fixed, and a different shape on the wheel
pants. 

The model will be at 1.5-inches/foot, giving a wingspan of about
54 inches.  I will be using a Fox .36 in the plane.  Actually
this was partly responsible for the long range plans for my
activities.  I know darn good and well that a small model like
this will not be treated seriously in competition, so I decided
that I will build 2 planes.  The first one will be at the Royal
plans size, the second will be at 2-inches/foot.  I will use the
first one to make my mistakes and to get my feet wet in sport
scale, the second, well, look out Al!

This decision also impacted the way I will build. I normally make
tracings of the parts on vellum and glue the vellum directly on
the wood to make the first part, then use the first part to make
all additional parts.  This time I'll glue the tracings onto gift
box cardboard (from xmas; I went through the pile looking for the
boxes, and for that clear plastic they use on boxes to give a window
to the child's toy inside -- great windshield material).  Now,
when I build the second one, I'll enlarge the templates on a
copier by 25% and make the new templates.  Between the existing
plans, and a few lines on the back of some 18" computer paper
(great stuff!), I should be able to build the larger plane with
little additional problems. 

As a side note, this is something that seems to discourage folks
from scratch building, the thought of all the drawing that must
be done.  Actually, given a good plan at 8 1/2 x 11, it takes
relatively few full-sized lines on paper to build the actual
model.  Most of the info on a plan is construction notes.

The Mystery of the Aft Section Taper kept me going for a while.
Both the Royal plans and the ones for the 182 show no taper on
the sides of the aft section, from the rear window to the tail.
I went out to the plane and measured with a plumb bob about
1-inch per foot taper.   This meant I needed to re-plan the
entire aft section.  I don't know why the two sources have no
taper; its easy enough to put in, and the 182 article is a 
serious model, my current theory is that the early planes had no
taper.

I must've spent 15  or 20 hours just going over the plans, checking
and planning for all the gotchas, and mainly trying to find ways
to save weight, not a terribly hard thing to do with Royal!  I
decided yesterday morning to build the frame by doing the
doublers first, getting them into perfect alignment, then gluing
3/32 sheeting on top of this frame to form the actual outer skin
shape of the plane (trust me, this is the slick way to do this,
based on lots and lots of research and thinking). 

The Royal plans specify a 3 1/2 to 4 pound finished weight, much
too much for this model.  I'm shooting for 3 to 3 1/2 pounds,
meaning that I'll be watching the wood I put into it very
carefully.  This will result in an under 20 oz/sq ft wing
loading, which is acceptable for me.

There are two difficult areas, the back window area, which must
be built up of solid balsa and carved to shape, and the fairings
around the tail, which Royal shows as being carved from solid.  I
will do this, but I will also hollow it out as much as I can to
save weight. Royal shows a solid empenage (sp?), but I will build
it up using the 182 plans as a guide. 

Also, I'll use 3/32 inch ply versus 1/8 ply for the aft formers,
and for some non-load bearing formers in the front section.  Come
to think of it, I could probably get away with 1/16 ply for these
formers.  Gonna check that out.

The wings are pretty straightforward, Royal shows them planked,
I'll use Coverite and only plank the forward D-section.  It is
TBD on the flaps -- I don't think they'll be worth the trouble
(they're Fowler type) for this small model.  

So, at this point I've got the doubler templates drawn up, and
have re-drawn the first tapered former, all on Clearprint vellum.
I'll be finishing this up and making the templates in the next
week or so.  Luckly, the tapering process does not change the
fairing of the stringers, so I don't need to do a top/bottom
view. 

P.S. In rebuilding the Six Million Dollar Eaglet, I did a
practice paint job of the 172 -- and it turned out lousy, but I
learned what not to do.  The first thing is that I'll not be
using dope on the model, I'm switching to acrylic lacquer from
now on!  Also, I wish to announce that my beloved Nitrate dope
has fallen into disfavor...

I've built the frame and started planking it early this week.

Here's one for anybody using Royal plans to scratch build --
beware!  The outline is nearly perfect, but the drafting
of the formers is very poorly done.  The formers are given upside
down or sideways, with no indication, and in one case, the former
was totally wrong.

I opted to make the aft formers from 1/16 ply, and this seems OK
now.

I started out the frame construction by fitting the main former,
F5 in.  Then I put in the firewall and the one former between the
nose and F5.  When I did this, I found that F5 was a poor fit, so
I ripped it out and re-made it.  This one was my fault, because I
had drawn F5 with straight sides, and it needed some taper.  Then
I added the aft formers.  Only one required rework.  It was too
short to properly fair in with the top surface.  It think this
was because it was drawn upside down and when I put the taper in
I carried the mistake to the pattern.  The one former that was
totally wrong was the last one: it was given the same size as the
previous one, which I found out when I brought the sides
together.  Of course, I re-made it to the right size.

Monday I started planking.  The first part is the two lower rounded
corner planks running from about the rear window to the tail.  I
CA'd them down along one edge and wet the outside with warm
water, then pulled them down and let them dry with rubber bands.
Then I CA'd them along the other edge.

Lessons learned so far:

1) "Not good enough" is the most frequently used expression in
the scale modeller's vocabulary.
2)CYA is wonderful for spot tacking pieces into place as the whole
is brought into alignment.  This one is a lesson because I've not
used it previously in my smaller models.
3) Rubber bands are also wonderful -- I've used them far more in
this project than ever before. 

Finally, let me close with a plug for my favorite tool -- a piece
of board with sandpaper glued to one side.  I've been using this
for a couple of years now, but this project has brought home the
value of it in getting things perfectly straight.  I cut some 3/4
inch thick pine 3/4 x 1/8 strips.  Then I glued two pieces at
right angles:

                 ----
                 |  |
                 |  |
                 |  |
             -------------
             |           |
             -------------

and used #80 sandpaper.  A few passes with this on a plywood or
balsa edge and its straight.  Actually I've got a bunch of these
using all the way from #50 to #220 for different purposes, but
the one above is the handiest.

Got a question:  How much is "allowed" to be seen through the
cabin windows of a scale model?

The reason for this is that I'm cleaning up the rear window area
now; I'd say its the most difficult thing I've ever done on a
model.  Anyway, once this section is sealed up, which includes
the cabin from the aft windows to the bottom of the rear window,
it will require major surgery, like removing the rear window to
get into.  I'd like to put my servos back there -- I think I can
gimmick up a mounting that will allow me to remove them.  But the
hitch is that when you look in through the window you'll see
them.  Do judges take off points for this -- they'll only be
visible in the top view.

For the front part of the cabin, I'll put in a removable
instrument panel/cockpit floor that will extend from the
instrument panel aft to the front of the rear windows.  This
floor will run about 1/2 inch below the side windows, and will
contain the instrument panel, the figures, and the seats.  I'll
have the radio, batteries, and throttle servo beneath this false
floor.  I could continue this piece into the rear window area if
needed, but would rather not because of added complexity.

You sure have to think ahead with these buggers.

    John,
    
    Judges are not allowed to deduct outline points for concessions
    made in order to install model engine, radio, etc.  However, you
    may lose craftsmanship points if servos are visible at the 15' judging
    distance, particularly if, in the judge's opinion, they _could_
    have been hidden.
    
    Unless you expect to have a potentially very competitive model,
    mount them for convenience.    

      |
      | |      00	 Adios,      Al
    |_|_|      ( >o
      |    Z__(O_\_	(The Desert Rat)

    
    
    I guess this is a s good a place as any to this in. Mod if you want to
    move it feel free to do so.
    
    I'ld like to ask if anyone has seen, built, or have any comment on the 
    Royal Cessna 172. It looks like a real clean plane but as always there
    are things you don't see untill you build it. I'm basiclly looking for
    a good scale/trike landing gear/.40 plane and this one fits the bill.
    I this one because it has a scale interior cockpit kit. Please give me
    feed back on this plane and or one that has the same features.
    
    thanks
    Bruce 

Alas, the project sits abandoned.  I became discouraged when I
realized that despite all the work it would take to finish the
job, it would still not be a serious, or even semi-serious
competitive plane because of its size.

All my earlier remarks on the model, including the ones on the
drawings, are still valid.  The Royal drawings are not very good,
and represent a mixture of several different 172 models.  The one
shown in the drawings is of about '67 vintage, though it sports
the later curved wingtips, for instance.

Be forewarned, it is a very complex model in the way its
engineered.  You should think about placement of the servos and
other internal components almost as soon as the fuse is framed.
Also, the construction of the back window is about as tricky as
they come.

You noted that it requires a ..40 engine.  While its rated up to
a .40, I think you'll have a problem fitting and flying with
anything bigger than a .30.  Keep in mind that Royal wants to
sell kits.


Personally, after seeing and working with the drawings, and
noting the prices of Royal kits, I wouldn't even think of buying
one.  

In fairness to Royal, I must admit that I'm not as motivated a
scale builder as I'd like to be.  The Robin has not been touched
in weeks, and when I get back to it, I dread the metalwork I need
to complete for the landing gear.  Though buying a set of wheels
for it last week really dressed it up and made me start facing up
to working on it again.  

Check out the PICA 182, it looks like a real winner.