January/February/March 2015
Project location - Independence, OR USA.
This
Mitchell Wing B-10 was partially completed by Bill Jackson back in
the early 80's. He obtained plans at the time and fabricated
the center D-cell assembly, the wing tip D-cell assemblies, both
elevons and all of the ribs from materials he purchased (this was not
a kit). I moved the pieces from his garage in Beaverton, OR to
the EAA shop in Independence, OR on January 13, 2015. The next
day, I started in on finishing the ribs which amounts to first,
pulling all those staples out.
The plans Bill Jackson used were an earlier set. The plans I
purchased recently in November 2014 from U.S. Pacific you can find
on-line. The plans run $250 and may not be available in the future
since Don Mitchell passed away in 1991 giving Richard Avalon rights
to his designs. Richard also recently passed away and his wife is
now trying to fill orders with what plan sets are left. The plans I
have include various revisions to the individual sheets as listed
here:
Sheet Number Rev date
2-7-82
6-28-82
8-25-80
2-7-82
6-17-81
8-17-81
2-26-81
6-23-80
8-16-80
4-6-82
7-30-82, 6-10-82,
2-11-82
2-26-81
6-17-81
6-23-80
One key revision included the stabilator counter balance weights and symetrical rudder profiles. These were described as a required modifications after in-flight problems became evident.
My main incentive for
tackling this project is to obtain a relatively inexpensive, high
performance rigid wing hang glider that offers a reclined hang cage
flying position vs the standard prone position common to most hang
glider pilots these days and also one that is feasibly transported to
the launches available in the NW, many of which are mountain sites
with roads that will not allow one to tow a trailer to launch. To my
knowledge there are no foot launchable gliders with this type of
pilot position that are available for a reasonable cost for the
average pilot. If one has the resources and “reasonable cost” is
not a concern then there is the Swift which is currently being
manufactured in Europe however, even this glider (which is really
expensive) cannot be taken to many launch sites I intend to fly with
the Mitchell Wing. My plan to transport the finished wing at freeway speeds is to use
an 18 foot flatbed trailer (I just happened to have one). I may decide I need to fabricate a heavy "bag" to protect the wing from rain and wind buffeting. Once I get
to the flying sites, I plan to construct a car top rack that will
accommodate the folded wing at slower road speeds to get it up to
launch.... Similar to what I did with the Easy Riser (see my other
blog, easyriser1).
I went onto Amazon and
purchased a book written by Larry Collier, copy right 1982 titled
“Building & Flying The Mitchell Wing. Nearly half of this book
is devoted to Don Mitchell's P-38 ultralight and motorized versions
of the B-10 and U-2. About a third of the book may have some useful
material on the build process of the basic wing but I'll need to get
involved in deciphering the plans to know for sure. Another book I
have that does provide some very good discussions on learning to fly
the B-10 from a conventional hang glider pilot's perspective and also
other aspects of dealing with the wing in both transporting it and
landing it. This book is available currently through Amazon and
titled “In Search Of World Records” written by George
Worthington, copy right 1980.
Finishing
the ribs
Pulled
all the staples out of the ribs Bill had built and then cut out all
the excess gusset material. Discovered the ribs weren't quite done.
I will need to build two more #6 ribs (there are four required).
Sanded completed ribs in prep for several coats of polyurethane.
Modifications
review
After
spending time reviewing the a) plan set sheets, b) Collier's book and
the c) the plan set directions "booklet" (came with the
plan sheet set), I discovered that Bill Jackson made some
modifications to the original design that I cannot feasibly change
including the substitution of a solid aluminum center bow vs the
plans which used a laminated wood bow. He also angled the tips
of the main leading edge D-cell to be parallel to the chord line.
His plan was to mount the ribs parallel to the chord line vs at
right angles to the D-cell web per the original plans. I will
need to build a jig to hold the ribs and carefully sand in this angle
along the face truss member (most will be 1/4x1/4 spruce) sticks so
that they will mate properly to the D-cell web on the back of the
D-cell. Another mod was the elimination of the 1 inch vent holes in
the D-cell web between each foam nose rib. Bill was concerned about
weakening what he felt to already be a very weak part of the wing
structure. His solution to achieving an air vent path to each sealed
cell was to punch a small hole through the foam ribs. Drilling a
hole through the web after skinning the D-cell will likely result in
significant tearing of the plywood web so I plan to implement Bill's
idea of venting through the foam vs the web. Another modification
was the D-cell center section. The plans have this as a 3 foot
straight section orthogonal to the chord line. It's not clear yet if
this will allow the rib assembly to go according to the plan or not
but I think it will. Bill wanted to reduce the drag profile of the
wing hinge plates that are epoxied onto the joint area at the end rib
(#6) position. There are two in the plans set about an inch and a
half apart. Prior to unfolding or folding the tip sections, a pin or
bolt is inserted through a hole drilled through these plates to serve
as a hinge bracket. On the right side, the rear plate has the hinge
tab profile removed (or Bill just installed it with a straight piece
of steel plate with no tab). I will likely epoxy a tab onto this
plate to match the front tab to reduce the possibility of having the
folding or unfolding operation put a torque on the single plate
weakening that epoxy joint. A fairing for these tabs could easily be
fabricated and applied prior to flight if I get that concerned about
drag.
First
material list order
I
decided to put together an order list for materials to fabricate both
rudders, all metal parts and the two remaining ribs. This will
put me in a position to begin attaching the ribs to the leading edge
D-cell.
Order
placed with Aircraft Spruce and received the end of February. Spent
more time studying the plan sheets regarding the mixer box assembly
for the control stick, rudder and stabilator interconnection.
Template
construction & Rib build
Traced
out metal parts onto drafting mylar and cut them out. Also
fabricated the three rudder rib templates. Researched the EA 9410
epoxy referred to as the primary adhesive throughout the plans.
Hysol EA-9430 is the “new” , “improved” replacement for this
old epoxy. This adhesive is mixed most easily by weight which requires access to a scale cabable of 0.1 g resolution. I bought a small, digital Ohaus for ~$135 which works well for this task. Transferring the tar-like component (referred from here on as "part A") into a mix cup first and getting an accurate measurement to calculate the water-like component ("part B") worked best. I used a medical syringe (cut the needle off with a pair of dikes). The material was compatible and it worked nicely to meter in to a tenth of a gram. I cut a template of the #6 rib out of mylar from a scrapped hang glider leading edge insert. I traced this out onto a 3/4 inch plywood panel and used 1 1/2 inch finishing nails to form the 1/4 inch cap spars to the profile. Made a small block with a groove to hold the nails perpendicular to the work surface so as not to twist the cap spars.....just doing this freehand is not good enough and you'll end up with the cap strips twisted out of plane very easily. I followed the instructions and mixed about 12 g of part A then did the 100:23 ratio calculation to get the weight of part B which gave me about enough to glue up one #6 rib (truss sticks, gussets and trailing edge block) with about a third left over. At 70 F (the can said use 75F) the epoxy set up in about an hour at which point it got hard to spread. This only got to be a problem when I started the second rib, glued one side of it up and then went over and glued all the gussets onto the first rib, opposite side. I got it done but the epoxy was fairly stiff at the end. I used a desk-top office stapler for everything except the TE block which was 1/4 inch ply. I used a heavy duty staple gun for that and still had trouble getting staples into the ply....very hard material. I did not have a minature hobby table saw and ended up using an Exacto razor saw to cut all the truss spars. I also did not have a bench sander to aid in doing the final fitting so it took me quite a while to get a good fit on all the pieces. Pencil, ruler and scissors to do all the gussets.
