Mark Drela - October 21, 2000

For those wanting to make their own pod mold,
you might consider turning the basic shape on
a metal-turning lathe.  That's how I did mine,
and it was far easier, faster, and more accurate
than whittling the shape by hand with templates.

It is the lathe analog of the "template-free"
sanding method described on the CRRC Apogee page.
But the lathe tool bit is used instead of the 
sanding block, so it's much easier.  There are
no curves to hand sand, just simple-curve wing deck
which is best done with a belt sander.

This is the procedure:

1) The wood block is two halves lightly spot-glued
together with Ambroid, with the chuck rod sandwiched
between them and sticking out the back.

2) The block is turned to a cylinder the same size
as the outermost diameter of the pod (1.72").

3) The pod is then shaped by machining off cones
of material.  The cone angles and positions are
set up to be tangent to the pod shape surface. 

4) The remaining slight corners are then sanded off
by eye and the whole thing is finish-sanded on the lathe.

It takes less than 2 hrs of work up to this point,
starting with raw lumber.

5) Separate the mold halves by wicking a lot of acetone 
into the joint, and putting the pod into a Ziploc bag
with some more acetone squirted in to give it time to
dissolve the Ambroid.  Go for lunch and the halves 
should fall apart when you return.

6) Bandsaw in the wing deck shape into each half,
and sand to final shape with the edge of belt sander,
if available.

7) The A-Lite pod shape also has the upper half over the
hatch area tilted by about 0.25" at the wing leading edge.
I did this by putting in a horizontal wedge of wood
at the z=0 waterline.  The wedge's point is about 1.375"
back from the nose (at the spinner base of the electric version),
and the wedge's 0.25" base is about 6" back from the nose.
To free the part of the plug above the wedge, I made the
crosscuts with a razor saw, and the rip cuts with an X-acto
with the point broken off a bit. 

8) The halves are taped together where possible.  
The vertical walls just under the wing are built up with 
a microbaloon/epoxy paste, with vertical shiny tape-covered balsa
planks pressed against the sides to set the vertical wall shape.

9) The extra wood and microballoon build-up is sanded to 
the final shape of the wing deck.  Round off all corners 
slightly for a better layup.

10) The halves are very lighly tacked together near the edge 
for putting on the finish.  I used four thin West 105/205 coats,
waiting for jelling between coats.

11) The plug is split again, and the halves are RTV'd down on
Plexiglass for the mold build-up.  I added a half-rod shape on each
half for the tailboom channel.  On the A-Lite this is angled down 
by 1 degree, but with the flying stab it really doesn't have to be.

For the record, below are the coordinates of the basic pod shape,
in inches.  Plot like any airfoil.


Allegro fuselage
    14.731470    0.250000
    14.297490    0.307644
    14.108469    0.332751
    13.735859    0.380746
    13.300978    0.434408
    12.847121    0.488696
    12.392019    0.541616
    11.940777    0.591091
    11.493261    0.635403
    11.046713    0.673862
    10.598965    0.706708
    10.148935    0.734529
     9.696363    0.758036
     9.241480    0.777884
     8.784513    0.794665
     8.325661    0.808931
     7.865219    0.821179
     7.403608    0.831837
     6.941461    0.841169
     6.479687    0.849176
     6.019036    0.855586
     5.559897    0.859943
     5.102450    0.861730
     4.646736    0.860427
     4.192564    0.855531
     3.739969    0.846677
     3.290033    0.833479
     2.845440    0.815217
     2.411125    0.790438
     1.993749    0.756547
     1.600530    0.710201
     1.237052    0.647624
     0.911963    0.570529
     0.646387    0.488966
     0.448524    0.411913
     0.306388    0.341494
     0.203808    0.275422
     0.128245    0.213878
     0.071414    0.157861
     0.030875    0.104593
     0.007390    0.051469
     0.000114    0.000059
     0.007522   -0.051371
     0.031158   -0.104494
     0.071851   -0.157752
     0.128837   -0.213779
     0.204604   -0.275330
     0.307473   -0.341386
     0.450017   -0.411781
     0.648427   -0.488797
     0.914596   -0.570247
     1.240131   -0.647123
     1.603922   -0.709416
     1.997410   -0.755493
     2.414993   -0.789141
     2.849457   -0.813704
     3.294152   -0.831772
     3.744161   -0.844794
     4.196815   -0.853487
     4.651041   -0.858237
     5.106813   -0.859408
     5.564314   -0.857506
     6.023505   -0.853049
     6.484202   -0.846555
     6.946014   -0.838477
     7.408201   -0.829088
     7.869849   -0.818386
     8.330330   -0.806105
     8.789223   -0.791819
     9.246225   -0.775028
     9.701146   -0.755181
    10.153762   -0.731689
    10.603830   -0.703892
    11.051622   -0.671084
    11.498213   -0.632673
    11.945779   -0.588422
    12.397074   -0.539022
    12.852226   -0.486191
    13.306129   -0.432004
    13.741055   -0.378449
    14.113694   -0.330555
    14.302728   -0.305503
    14.731470   -0.250000