Back Order

Two of the spar tubes were on back-order at Aircraft Spruce, so I'm still patiently waiting for them to come in before A.S. ships the rest of the order.  I've been told by the end of the month.

Update 1/26/12: Spruce called today and should be shipping the tubing order out tomorrow.

Some work in AVL

MIT published as freeware the panel method CFD code called Athena Vortex Lattice for analyzing the geometry of aircraft.  I input the Goat4 geometry into the program to do a little investigating.  I used a 300lb takeoff weight, a 29% root chord CG location (the wheel location), an 11.7% ClarkY smoothed airfoil (from UIUC's database), and tried hard to match the wing/tail angles to Sandlin's drawings.

 

I also plotted this in AVL Editor, a tool by Cloud Cap Technologies.  Their tool shows control surfaces a bit better.

 

The incorrect tip taper isn't a huge issue considering the tips aren't loaded heavily and quite frankly there is enough other guesswork right now the tips are of the least concern.  I haven't added cabanes or the nose or a pilot either.

As a start, I'm using 30 mph (44 ft/s) as a cruise condition and assuming sea level atmospheric properties.  AVL gives the neutral point at 40.6% root chord for an 11.7% static margin and predicts a 0.2 deg (down) elevator to trim.  Speeding up to 45 mph (66 ft/s) requires 4.1 deg (down) to trim and slowing down to 25 mph (36.7 ft/s) requires -1.1 deg (up) to trim.  Based on watching the in-flight Goat videos, I'm seeing more up trim for typical cruise, so this CG location may be a bit far aft.  Sandlin recommended balancing nose-heavier than this 29% location anyway,

Moving the CG position up two inches to 15.4 in aft of the LE (25.7% root chord) and re-running the 30 mph case yields -2.1 deg (up) elevator, 45 mph case yields 3.1 deg (down) elevator, and 25 mph case yields -4.4 deg (up).  That's just for trim.  There is some +-25 deg or more of total elevator travel for maneuvering.

I don't have anything good right now to compare these numbers to, but this is a starting point.  More number crunching later...

Wing order is in!

I finally hit submit on the wing material order!  Should be here in a week or two.  Holy muffins, it's going to be a big pile of tubes...

I am going with similar mods described by the Yando Goat builder: G3 wing struts & attachments, G4 fuselage/tail/cabanes, up one size on the spar and flying strut tubes (0.049 spars, 0.058 struts), and fingers crossed I can keep it all under 155lb :-)

I'm still not 100% sold on the simple flat-bottomed airfoil compared to something spiffy like the G3's attempt at an FX63-120.  But you know what, I have the power to run the numbers and I just haven't done it yet.  Regardless, I'm keeping the wing area of G4.

I do feel better about the structure after seeing someone else doing load testing.  I don't have more details right now ... reprinted from here:

  

Posts may become more regular as I actually have material (and more financial incentive) to continue on.