From: Charles.K.Scott@dartmouth.edu (Charles K. Scott)
Newsgroups: rec.aviation
Subject: Building a Christavia, #12 plans problems?
Date: 13 Aug 92 14:48:49 GMT
Organization: Dartmouth College, Hanover, NH


 Building a Christavia - Plans comment

As many of you who have been following my postings about my efforts to
construct a Christavia know, I'm a first time airplane builder.  Many
of the rules concerning aircraft construction I've learned as I
progressed, many others remain to be learned.  I realise that there are
many do's and don'ts some of which are just common accepted practise
and others, hard and fast rules you can't deviate from.  I do a
considerable amount of reading on the subject and talk freely with any
and all who might be of help.  While I'm a first timer at airplane
construction, I have some background in mechanics, I worked as a dealer
auto mechanic for eight years, focusing mostly on Subaru's and Mazda's.
 Add in several years as a carpenter and you arrive at the level of
skill I apply to the project.  While I'm familiar with reading
blueprints and manuals, I had not studied a construction plan for an
airplane before and I'd like to address this posting to my beginners
impressions of the plans for the Christavia as received from Ron
Mason,the designer.

At first I was pleased with what appeared to be a very complete set of
plans.  Everything I felt I would need appeared to be there.  We are
talking about a plans built airplane, not a kit.  All the drawings for
fittings and brackets are full sized which made transposing them to the
material very convenient.  However, I noticed as I began to think and
plan for the construction of the many brackets for wing and strut
attachment weren't as precise as I had first thought.  For instance,
the drawing for the strap bracket that will attach the wing to the
fuselage had the centerline drawn slightly off center.  Not much, but
Ron specified a 2 degree angle of incidence that would naturally occur
if the bolt bracket for the fuselage was welded in place properly and
the brackets themselves were drilled and installed on the wing
properly.  I didn't like it knowing the center line was off a little so
I didn't use the drawings to transpose the dimensions to the steel
material.  Instead, using the radiuses and dimensions from the drawing,
I made templates out of aluminum flashing starting with the centerline.
 This insured that the template would be dimensionally accurate (as
long as I was accurate in my work and fashioned them that way).

Then, while laying out for the measurements of the centerlines of the
compression struts along the spars, I noticed that one drawing of the
wing layout which was at the top of the page, didn't match exactly the
measurements of the drawing of the spar directly below (they were
supposed to).  They were off exactly 1/2.  So then I was in a quandry,
which one was right?  When I called Ron about it he didn't seem
concerned and suggested I just defer to the top drawing.  Well OK, I
could do that, and that's in fact what I did do but I was unsettled to
find that error.  

The next little problem reared up when ordering the hardware to bolt
things together.  Several pages listing hardware and materials was
provided for both wings and fuselage.  The instructions implied the
lists were complete, and Aircraft Spruce provides a complete kit of
hardware spec'd to the Christavia and I didn't see and significant
deviations in types or quantity.  Careful scrutiny of the list,
however,  revealed some discrepancies.  For instance, the plans called
for each bolt used to thread through the spar into the compression
strut to be of the drilled head variety so that it could be safety
wired.  OK, there are six compression struts per wing which means a
minimum of at least 12 drilled head bolts per wing or a total of 24 for
both wings alone.  Yet the total number of drilled head bolts listed
for the entire airplane was well under 24, and several of them weren't
even the correct dimensions and must therefore be intended for use
elswhere.  Confused, I called Ron again.  "Oh?" He said, "I was the
first person to notice that error in the plans."  At least 12 to 20
other Christavia IV's are under construction and I'm the first to
notice?  "Just drill the bolt yourself," he finished in a slightly
exasperated tone.  Well OK, I can do that too.  It takes a special
drill attachment which I don't have yet but it's not a big deal.  But
as I hung up the phone I realized that making plans for an airplane is
a complex job and it must be stupendously difficult to get every detail
(like calling out every single bolt) right first time.  Also, I have
since learned that some producers of kits take the time to build a
prototype, make plans from the prototype and then attempt to build an
airplane from the plans to see how the plans work.  This inevitably
results in sweeping changes in the plans and construction manual as
problems are encountered.  Ron built only one prototype, and has
recently sold it when he was made an offer he couldn't refuse.

My last posting dealt with the situation discovered with the fuel bay
area and the difficulty in removing the fuel tank if needed once the
wing is covered.  No provision had been made for this possible
eventuality without slicing off the painstakingly applied fabric. 
Maybe Ron didn't worry about this.  Perhaps he felt that the fabric
should be finished in such a way that slicing it off and patching in a
new section is the proper way to repair a fuel tank problem (one netter
has confirmed that this is what he had to do, and if Dope is used, the
patching isn't obtrusive), I don't know.  He certainly didn't mind my
modifying the fuel bay area in the manner my knowledgeable friend
suggested to avoid cutting fabric.  Ron seemed to say, "sure, whatever"
(although he really didn't, it just seemed that way).  He did make sure
I was going to leave in the diagonal brace which makes fuel tank
construction difficult in that the brace runs right through the tank. 
OK, so I can do that too.  But part of my last conversation with Ron
was revealing in that he related that he had to deliberately over
design the airplane for two reasons.  1. He couldn't predict the
building skills of the average homebuilder, which vary widely and
therefore made the specs more robust than necessary (the spar and rib
specs exceed those of the Waco UPF-7 which is certified to 9 G's
positive and 5 G's negative) yet the airplane is designed for the light
airplane transport category which requires only 4 G's positive and
around 1.5 negative (I think).  2. Ron conceived the airplane for
missionary work.  He saw it flying out of short jungle strips heavily
overloaded.  He set the max. gross at 2100 lbs, but told me he felt
that would be routinely exceeded during normal operations by the
missionaries.  THAT'S why he put in the diagonal bracing.  Looking at
plans for some of the older and very similar designs such as
Taylorcraft, Piper and Aeronca show no diagonal brace through the gas
tank.  Ron put it there because he saw the airplane being subjected to
gusts and turbulence while heavily overloaded and flying low and wanted
to be positive the wings would stay on.

I have yet one more example of the need for flexible thinking with the
Christavia plans.  The design calls for the fuel tank to be removed
from the bottom.  In order to make the tank removable, the plans show
two removable 3/8" square tubes installed underneath the tank bolted to
the spars fore and aft.  They bolt to the spars with 2  - 3/16"bolts at
each end and are a continuation of a false spar that has not internal
bracing, just the wooden cap strip on top and the square metal tube on
the bottom.  Just on the other sides of the fuel bay cap stips, are
complete noses and tails of the ribs around which sheet aluminum will
be wrapped.  The drawings show the brackets for the square tubing lined
up directly with those wooden pieces, but in reality, that can't
possibly happen because the tail and nose pieces would be on top of the
bolts, and you wouldn't be able to remove them.  The wood could be cut
out to allow clearance for the heads, but it just didn't feel right.  I
thought I would have to offset the square tubing to gain bolt clearance
which might look a little awkward, but would certainly work.  However I
wondered if I should make this deviation from the plans.  Yet another
call to Ron confirmed my conjecture.  'Just offset them," he said, "you
can't run the bolts through the wood."  OK, I'm getting the hang of
this now but again, that isn't what the plans called for, I had to call
Ron to find out.  This is beginning to make me very cautious.  

I don't mind fudging plans now and again if I know what I'm doing and
have experience, I used to do that all the time as an auto mechanic,
but I went to school for that and I knew what I was doing.  As I've
said, this is my first airplane and I'm reluctant to make construction
deviations on my own.  I've been calling Ron when I felt it necessary,
but I'm getting the distinct impression he feels the calls aren't
really necessary, that I could have figured things out for myself. 
Maybe that's true, perhaps others attempting to build this airplane see
these problems and just brush them aside and build it like they know it
should be built.  But I feel I'm probably pretty average as a first
time homebuilder, and as such, I'm probably feeling the same way anyone
else would.   I wonder if other builders could speak to the
completeness and accuracy of the plans they are working with? 

Corky (slightly daunted now but still moving ahead) Scott