first 750 assembly processes

With the 701 wing gone from the shop and into storage, the bench is now clear for my 750 tail group and I can get to building my airplane using the skills and knowledge I've learned.  This blog post will try and capture several days of shop work over the past week - it's been busy and to the uninitiated observer looks like it's moving really fast, but there's a bunch of work that goes into this that pictures will never capture.

The factory tail spars come predrilled, with all the rivet spacing laid out - this saves a bunch of time on layout, but you still need to consider order of operations and keeping everything square.  Slight imperfections in scratch built parts means measuring everything closely.

​And here it is.... my 750's first drilled hole (back drill from rear spar hole to doubler) and cleco.  This is the basic procedure and first step of everything to be assembled to come.

Each doubler is done the same way, A3 hole in the spar as guide, A3 drill through doubler and clecoed.  I pleased my doublers fit as nice as they do.

With everything together, drill again, this time upsizing to A4 and again to A5 where needed and cleco.  It's quite repetitive but this ensures nice clean and centred holes.  The other thing we did was line both edges of the table with angle iron clamped to the edge.  2x2 HSS tubes laid perpendicular across the table provide a level reference point (flat table) to work with.

With the spars upright and referenced flat, I started to trial fit the rear and nose ribs.  These to are back-drilled from the spar to the rib flange, then up drilled to A4 and clecoed.  A quick measurement confirms my scratch built ribs are correctly sized and by default the spars are spaced correctly.  Excellent!

Spars and inside ribs are drilled and clecoed in place, so next are the tip ribs.  The key here is make sure they are square to the spars and the spars are square to the rest of the assembly.  The front end of the tip rib is connected to the front spar (left side in picture) by a fabricated 025 "L" bracket.  It takes a bit of ingenuity to make it all square and drill it, but not overly difficult - and certainly a TON easier that repairing the completely un-square 701!

To help square everything up before final drilling of the tip ribs, we added some uprights to the bench (there were eventually 4 of these, one at each spar end front and back).

The use of standard "L" around various parts of the airframe allows for small adjustments to square everything up as well.  Kinda explains why I need so many of these (see this post). 

Here is where the tiip rib attaches to the stabilizer spar.  Drilling this takes some thinking about which order to do it first, but the uprights hold everything in place while the drilling is completed.  Another check of measurements first, then final drilling to A4 size clecos.

​With everything squared up, the horizontal stabilizer rear bracket assembly is prepped for installation.  This 063 thick aluminum plate is fun to bend, but again if you think it through it isn't that difficult.  The tabs are bent inwards to 98 dregrees from flat.  This is an important measurement as these tabs are where the horizontal tail attach to the fuselage.  I used a protractor and drew out an example angle on  paper to confirm both tabs were correct before test fitting it on the rear spar.

Clamp the bracket in place and back drill from the pre-drilled spar holes.... all good.... right.... wait a minute... damn...did I just put this in upside down?!?!

This is where paying close attention to the plans was important.  A traditional horizontal tail looks like a small wing - flat on bottom and curved on top.  The Zenith STOL aircraft have inverted tail airfoils, where the flat side faces up.  There are a number of reasons for this that I won't get into here, but what makes this confusing is that it goes against common thinking.  Multiply that by the fact the plans for the ribs and forms are drawn with the ribs flat side down, it'e easy to get confused which way is up!  I'm also building the horizontal stab flat side down to take advantage of the flat table to get everything square.

So, what saved my bacon?  The predrilled factory holes!  The fact that they are symmetrical and the bracket is centred on the spar horizontally means I just had to invert it before up drilling to A4 and eventually A5 holes.  What could have been a nightmare  was avoided, but I'll be much more careful next time!

With the bracket turned up to the correct orientation and DOUBLE CHECKED AGAIN before drilling, I worked on right sizing the holes and clecoing things in place.  Another check and everything is now where it needs to be.

Next to go on is the centre elevator hing bracket, one of three attach points between the stabilizer and the elevator.  The attach point is at the tip of the triangle and will be drilled later when we match up the elevator.

Supporting the hinge bracket is a support gusset, which was chaleging to bend correctly - again the plans give a somewhat confusing view, but a little figuring, fitting, and adjustments finally got things where they needed to be.  It's getting crowded in this area with all the clecos!

Like the ribs, clecos can be moved inside the spar or to the other side of whatever it is attached to - clecos don't seem to care and that's a good thing :)

Making notes on the parts as you go through assembly helps to remind you in the future.  Here you can see I wrote a note that an "AN-bolt", not a rivet goes in this spot - a reminder that the hole is drilled to a specific size that is slightly larger than an A5 rivet.  Before anyone comments that my grammar sucks, the term "AN" stands for a particular type of fitting (in this case a bolt) used to connect flexible hoses and rigid metal structures. It is a US military-derived specification that dates back to World War II and stems from a joint standard agreed upon by the Army and Navy, hence AN.

The final parts to be added to the horizontal stabilizer being measure/confirmed and laid out for bending - accuracy here is critical to make sure all three attach points for the elevator and the two from attachment for the stabilizer to fuselage are correct.  This will make rigging the plane easier at final assembly and a straight plane ALWAYS flies better!

Left/right attach brackets on the let and left/right horiontal stab/elevator tip hinge attachments on the right

I flipped the stabilizer over, making it easier to attach these brackets.  Alignment bother vertically with the proper amount protruding above the spar and horizontally in relation to the centreline of the spar is critical for tail alignment.  Looking at this picture is decieving, the brackets are perfectlyperpendicular to the spar - I know because I measured it 5 times and confirmed orientation 5 times.... no second chances here.

Next I measured up the stab/elevator hinge attachment brackets for rivet hole spacing and drilled them out.  I thought I might drill them as a stack, but I was concerned about alignment so I did the individually.  The plans are very clear here on where they attach to the rear spar and they installed accurately without issue.  So nice working with newly made, undrilled parts!

So,with everything drilled correctly, together with clecos and measured as correct, it all comes apart for final deburring, cleaning and corrosion protection.  Now was a good time to lable anything that might get confused when it goes back together for riveting.

Well, that's a lot of work done, approximately 14 hours in total.  I'm updating my build time tracker on the right of the page as I go.

Next up, prime everything for corrosion protection.  In hindsight, I think I'll wait to prime/corrosion protect parts until all the drilling/deburring is done - the drill and debur swarf tends to get stuck to anywhere the primer is already applied (it's somewhat sticky even after curing) which makes cleaning more time consuming when it doesn't have to be.  After that reassembly of the entire horizontal stabilizer for final riveting - my first rivets on the 750 will be so satisfying :)

Thanks for reading, more to come!