Forward strut attachments

Today was very productive!  I started with the right forward strut attachments, completing the bend and the through-hole on the aft plate.  Then I slid everything half-outside the garage and worked on the left to get it up to the same spot.

As with all the 1/4" attach points, I keep them as 3/16" holes until everything is held in place together and can be match-drilled to the full 1/4" diameter.  This might be overkill since it needs to be a sloppy 1/4" for a slip-fit pin insertion, but it feels better to do it this way.  The final match-drill is so much more satisfying knowing there is no slop.

The Goat 1, 2, and 3 all use an AN42b eye-bolt for the cabane attachment to the strut.  Since my cabanes were already built, they were too long to do this without remaking the aft cabane tube.  Instead, I'm trying to redesign the junction.

This is the right strut attach point with a first look at the new cabane attachment.  It's not exactly done yet.  For starters, the connection is just a tab that the strut pin goes through, which could slip off the end of the quick-pin if the bungees broke.  Much better to have two tabs so the pin has to fall out all on its own.  I'm still thinking...

Here is the tab on the cabane.  Like I said, I'm not totally sold on this alone.  I'm thinking about a bent tab similar to the Goat-1 compression ribs (G1W13) using the two obvious bolts.  Then again, I might space apart the two tabs on the struts and slide this next to the fuselage carry-through tube to be inside the strut tabs and therefore captured.  We'll see.

I did shorten the main cabane tube by about a half-inch.  That was a definite committal moment.  But you know, reworking this joint is maybe a good way to show how forgiving the "boltalicious" Goat construction is.  Need to make a change after it's all built?  Just unbolt, make a new whatever, and bolt it back in.  Pretty awesome.  No welding or re-riveting needed.

One consequence of attaching the cabanes along with the strut quick-pin is they are pinched in closer to the center-line than on the original Goat designs.  This means the cabanes pinch and interfere with the seat-back, which is now too wide.  I could lean the seat forward a bit (which is what I did for now), or narrow the upper seat tube.

Finally, here is how I spent most of the day, with Goat hanging half-way in the garage and half-way out.  The forward struts are simply taped on the leading edge as a mock-up, but it was enough to make some measurements and sort out the lengths needed for setting the dihedral angle.

Oh also, the forward sweep cables really do make a big difference in holding the leading edge in place ... right at head-height in this orientation :-)

Starting the struts

Had to make a few additional brackets for attaching the wing struts.  Break out the measuring tools, hacksaw, file, and clamps plus some elbow-grease for good measure.

I wanted to start with the forward strut at the fuselage first so I could jig the whole thing outside and measure the exact correct length for the strut.  And I also wanted to cut two of everything to take care of both left and right at the same time. 

But first, I need to sort out converting the Goat-4 carry-though to a Goat-3 style.  For the strut attach points themselves, I'm planning to follow the Goat-3 drawings.  It's a change to the cabanes too... that's the fun one. I'm not sure what I'm going to do for that one yet.

Beautiful spring day for a Goat

I'm sitting outside on the porch looking over an amazing sight: the first time Dan's Goat has been assembled with the wings on.

Now I started down the path of a cable-braced wing and will be switching to using wing struts, so use your imagination to remove many of the temporary lines.  However, this is the first time I have the big-picture view of sitting in the pilot's seat and looking down the length of the wings above me.  Such a cool sight.

During assembly, there were a few lessons learned already.  Handling the wings is a challenge at this stage, with a lot of flexing and wobbling to move them into position.  I know with covering on they'll get more torsionally rigid, so there's no cause for concern, just noting an 18ft by 4ft panel is big to move around.  The LE and TE pins aren't too tough to align and install.

Next is the fuselage attachment.  This involves awkwardly standing the fuselage on the tail, pinning the LE tube, then moving the fuse into place for pinning the cabanes.  Now on the strutted version this will be different, but no doubt it's going to be a magic step.  I also definitely need a piece of carpet or something to toss on the ground and keep the tip of the fuselage from poking into the dirt.

Rotating the fuselage and wings up is going to be a lot easier with rigid struts.  Using some 150lb test cord I had laying around, I rigged up the kingpost and flying wires.  It definitely took pulling a lot more tension in the upper lines than I initially thought to keep the wing with positive dihedral.  There is a lot of stretch in my thin lines.  But once the wing and fuselage are triangulated together, they rotate up as a unit just fine by pulling on the nose to rotate everything up onto the wheel.

Since it's so nice outside, I had to attach the tail as well.  This too is a little awkward as the fuselage attach point is leaned a little so the vertical tail assembly needs to be rotated as well.  Attach the lower pin first, then the upper pin, not too tough.  The horizontal stabilizer is a piece of cake from there.  Then connecting all the control cables is quick and painless.  Viola, Goat!

What else can I say?  It's coming together.  This feels like I've crossed the half-way point.  Structurally, I have the struts, their respective attach points, and the jury struts remaining.  Then it's all secondary structure for the wing: lots of ribs, wing tips, control surfaces, hinging, control rigging, and d-tube sheeting.

Oh I also weighed the current structure while assembled: 62.8 + 6.8 + 17.2 = 86.8lb  This doesn't represent any particular reference state, but does give me a warm-fuzzy that there is 60-something pounds to stay in the Part 104 glider class.  The struts aren't light and there is a lot of aluminum in the ribs and control surfaces to go.  Then covering / paint.  Fingers crossed guys.

Getting back into it?

So I finally finished off the kitchen renovation project, which entailed dismantling the old kitchen down to the studs, removing a wall, and moving all the appliances, not to mention new tile floors, new cabinets, new countertop, and new backsplash.  It's been a busy year.  And that only compliments the other house projects that have been keeping me busy. 

Yeah...

The funny thing about putting this project down for months is that I don't remember where I stopped.  Even with all the photos and descriptions here, it's a bit of a mystery what my next few moves were supposed to be.  The left wing panel was further along, so I finished match-drilling holes for the two ribs and riveted in the two compression ribs.  While in the area, I also bolted in several tangs and used rope tied with a bailing knot to pull taught the internal bracing.  This did add some triangulation for support, but it's still pretty flopp
y and will be so until the struts hold the spars in place.

This reminds me where I stopped.  Getting the right wing up to this same point should be routine now that I have the left wing assembled.  But the next big question is how to attach the struts and, importantly, how to align the spars & fuselage to cut the struts to length.  I started making a GOAT-4, but liked the looks and improved aerodynamics of the strutted GOAT-3.  This leaves the strut attach point half-way between the two currently.  The cabanes attach to the fuselage carry-through and there is nowhere to tab through for the struts.  Yet.  This will be my pondering point for the time it takes to get the right wing up to par.

The garage isn't long enough for both wings, any my driveway slopes down.  May be fun aligning the wings for measuring the required strut length.

Wings Over Wasatch

Had a day off of work while on travel in Utah and made an appointment to get a hang glider lesson with Steve P. from wingsoverwasatch.com.  Only have a few photos, but am sharing what I have.  Steve made a GoPro video shot from the left wing.  High value from the morning's instruction!

I'm still in one piece after a whole morning of drills, short hops, and a few longer hops.  Max altitude was in the neighborhood of 20 feet.  Turning by weight shift is such an unnatural feeling and I got caught twisting instead of shifting early on, resulting in my only crash landing.  Glad I was wearing a helmet because my head whacked the keel tube as one wing tip caught on the flare and pulled my nose over and into the dirt.  No damage other than ego. Not having direct yaw control was weird, making crosswind flying awkward and left me wanting for a rudder to help move the nose around.  It was a lot like my last hand launch glider contest when my rudder pushrod broke and I had to tape the rudder at neutral.  Maybe that's why it was harder than I expected to hold a straight ground track line in the occasional crosswind.  At any rate, the cross breezes gave the opportunity to try some gentle turns.  If my shoulders weren't painfully turning black and blue from holding up the wing, I would have kept working up the hill.  I launched from at least twice as high as the dunes of Kitty Hawk if that helps build a mental image.  The South Side hill is several times taller than the dunes and a whole lot less soft :-)

Many thanks to Steve P. at wingsoverwasatch.com.  If you're ever in the Salt Lake City area with a free morning, give these folks a call and enjoy their excellent training and gentle encouragement.

All in all, very awesome to get back in the air and definitely an incentive to pull Goat back off the back-burner (at least back as a side-project to the kitchen, hehe).  Thanks for sharing with me!

The promised video:

More ANSYS runs

Using the 75/25 load split between LE and TE, and with the addition of the forward sweep cable, I re-ran the 6g loading case on my Goat wing.  Not altogether different, as you might expect.  The bowing shape is still roughly the same.  The maximum stress is roughly the same.  At first glance, the results are pretty equivalent.

 Maximum stress is still in the leading edge tube between the jury strut attachment and the wing root pin joint.  There is still some bowing of the main struts upward as the jury struts are pulled on by the main spar.  
Do note that these deflections are listed as approximately 0.5 inch.  With such a low-fidelity FEA, I'm expecting somewhere between 3-12 inches of actual deflection during the load test, but the point is to say that the shape shown in the analysis is scaled to be visible.  ANSYS has a true-load deflection option and it shows essentially no discernible deflection.  Beware of length scaling.



To answer a few other questions:

• I am too far into the build to change the main strut attachment location to the wing spars.  So, what you see in the FEA is what I have to work with.  Potentially I can add additional sleeve length or inner sleeves to the jury strut location or also change where the jury strut attaches to the main strut.
• I am not using Schrenk's approximation or an elliptical load distribution.  Instead, this loading comes from the AVL model, which is likely better than Schrenk for arbitrary wing planforms.  See a previous post to learn more about the AVL work.
• I have so much margin left in the wing structure near the strut attachment that I'm not going to run the asymmetric aileron loading case.  The FEA in my mind was more concerned with showing how much margin to yield or ultimate load that I had, rather than doing the detail-work checking for aeroelastic effects.

Finally, I'm thinking it's worth moving onward with the construction.  Additional sleeving can be added at the jury strut attachment location without invasive surgery, so it doesn't hurt me to continue on.  There is a kitchen renovation in my near-term future (July), so the time is ticking away before another major delay!  Ack!

I should probably add that I'm narrowing in on a decision to load-test to 4G's.  According to the FEA, this should be well below where I'd anticipate any yielding and is more than I plan on pulling (no loops, heh).  A cohort at work has even suggested it will be difficult to pull that many G's in the airchair, considering the high drag count and low-performance airfoil.  Self-limiting is good in this case :-)

Detail work on structures analysis

Among a bunch of great questions/suggestions/feedback, I took a look at the fore/aft distribution of forces on the two spar tubes.  The previous analysis ran a simple 50/50 split of the wing total forces to get things started.  What is better?  AVL can help provide the answer.

Under a 7g (using this now instead of 6g) load case at CL of 1.5 (assumed CLmax), the element forces can be plotted (seen above).  This gives the distribution of CP along the airfoil chordwise and showing the distribution of force.  For the root, here is the element force distribution:

 Strip # 25     # Chordwise = 10   First Vortex =241
    Xle =   3.16670    Ave. Chord   =    5.0000   Incidence  =    1.7500 deg
    Yle =  -0.59947    Strip Width  =   1.20000   Strip Area =    5.999976
    Zle =   4.03135    Strip Dihed. =   -2.9939

    cl  =   1.56703       cd  =   0.73643      cdv =   0.67992
    cn  =   1.66386       ca  =   0.47903      cnc =   7.78287    wake dnwsh =   0.10109
    cmLE=  -0.55324    cm c/4 =  -0.13727

    I        X           Y           Z           DX        Slope        dCp
  241     3.19462    -0.59918     4.03134     0.17282     0.28424     5.05604
  242     3.41428    -0.59918     4.03134     0.37261     0.14534     2.82108
  243     3.83407    -0.59918     4.03134     0.54628     0.08312     2.09844
  244     4.41670    -0.59918     4.03134     0.67141     0.03037     1.77600
  245     5.11040    -0.59918     4.03134     0.73689    -0.01091     1.54034
  246     5.85353    -0.59918     4.03134     0.73689    -0.04508     1.32883
  247     6.58005    -0.59918     4.03134     0.67141    -0.06874     1.10233
  248     7.22542    -0.59918     4.03134     0.54628    -0.08517     0.86729
  249     7.73230    -0.59918     4.03134     0.37261    -0.09597     0.62162
  250     8.05563    -0.59918     4.03134     0.17282    -0.10036     0.35427

The X position does not start at zero because I use the nose of the glider as the origin location.  An X position of 3.1667 is the LE at the root if you were curious.  Plotting the X position vs the dCp value gives the distribution of pressure along the airfoil chord.

Now some simple statics can give the force distribution between the LE and TE.  Assuming each CP acts like a force on a beam, the component of load carried by the LE and TE can be found by multiplying the force by a ratio of distance to the beam length.  The beam looks like this:

Component of force F acting at a distance xa from simple support A is given by: F_a = F * xb / (xa + xb).  Carrying out this algebra using the CP distribution from AVL along the root chord gives 74% on the LE and 26% on the TE.  Note that different airfoils and loading conditions will have different CP distributions, so don't use this approximation for any other purposes.  Also, this force split does not account for a tilting of the lift vector due to pulling angle of attack, but I will be considering this split sufficient for a static loading case.  This is certainly more representative than 50/50...

Note from the AVL plot that the root airfoils are working harder than the tip airfoils, with regard to a CP peak at the LE vs more evenly distributed along the chord.  Using the force distribution at the root (75/25) should provide a conservative estimate of the LE spar loading out toward the tips.  I may also run 100% on the LE just to see what that looks like.