|
Aligning the tail boom raised a couple of unexpected challenges.
It was no problem to run a central string guide along the ground, set by Plumb lines hung from the centre of the front frame tube and engine frame tube . But hanging a Plumb on a loop of string around the end of the tail boom as recommended rang some alarm bells. Try it yourself - you can make the plumb hang almost anywhere - up to two inches off centre.
Here’s my solution. Lay a 12 inch rule on the floor with the centralised string on the six inch mark. Then plumb down from each side of the tail boom in turn. When the boom is correctly aligned you’ll have an equal measurement each side of the six inch mark on the rule.
The manual tells you to orientate the first tail boom bulkhead vertical, then the fourth - like they’re separate? I figured the fourth was the most important. I clamped the digital protractor to a short piece of aluminium angle and measured the vertical from two 1/8 inch drills inserted into the holes in the bulkhead. It’s tight - but possible.
 |
At this stage the two pilot holes for the horizontal fin should be horizontal and square to the tail boom centre line. ‘Fraid not. First I opened the holes to 10 mm (or 3/8 in) so I could get a straight piece of 10 mm bar (or 3/8 in) through and put masking tape on the area. I then sat a spirit level horizontally on some Plasticene above the holes. Measuring down from the spirit level to the bar will tell you how far out it is horizontally and simply holding the square on the side of the tail boom along the bar will show any front-to back misalignment. Mine was about 5 mm (3 /16 inch each way). Don’t forget that the hole is not supposed to be exactly on the horizontal centre line
Double checking with the Digi Level
Once you know how far out of alignment your piece of bar is you can calculate and draw an offset 3/4 inch square box around the 10 mm hole and enlarge it with a file to correct the misalignment. (The red dot shows the position of the original hole.)
You can double check the front to back deviation by measuring from the square frame tubes. This is another recommended method of aligning the tail boom - but it only works if the horizontal fin holes are correct. I’m no aerodynamicist (can’t even spell it!!) so I’m not sure how critical this is - if at all, but I figure better right than wrong.
I used Epoxy Resin for the taped joints of the aluminium horizontal fins and the GRP side wings. Polyester resin doesn’t adhere well to aluminium.
This picture is a little confusing but it shows the horizontal fin’s diamond shaped mounting plate and its reflection in the polished aluminium of the tail boom. I’ve tweaked the plate to best match the curved shape of the boom skin but short of re-making the welded joint it’s impossible to obtain an exact flush fit. You can see the gap between plate and boom. Something’s gotta give when the bolts are fully tightened and it ain’t gonna be the steel!!
Enter the trusty packing tape mould release. I mixed a thimble full of Epoxy Putty, warmed the mounting tube slightly to help keep the putty soft during assembly.
The putty was rolled into a 3 inch x 1/4 inch diameter sausage and pressed around the inside face of the mounting plate. The mounting bolts were tightened just enough for the plate to make contact with the boom, squeezing out the putty.
Left overnight to cure, the fins are easily removed, the bolts cleaned up and the putty trimmed. Perfect fit.
The vertical fin mounting bracket gets the same treatment.
I’ve read about potential problems with the standard Vertical Fin mountings, partly due to the forward two bolts being tightened on the hollow tube section inside the fin. I decided to reinforce the tube in this area with a plug of Epoxy Resin. I mixed half a cup of resin thickened with Mica powder to the consistency of thick whipped cream. I made an injection bag from a small polythene zip-lock bag and a nozzle from a standard mastic cartridge. The resin mixture was spooned into the bag and injected into the tube through the drilled holes by squeezing the bag - just like icing a cake. The thick consistency of the mixture ensured it filled the tube and stayed in place until it cured. I then redrilled the holes.
I
Something else that niggled me. On my ship there is no possible tail boom mounting position that does not result in one rivet head ending up beneath the flats on the mounting tubes, holding them off the tail boom skin by the thickness of the rivet head. Tightening the securing bolts will, to my thinking, do two things. 1 Slightly bend the flattened end of the tube as bolt tension closes the gap and 2 induce unnecessary bending (or breaking) tension in the bulkhead casting ears around the rivet hole It’s a small issue and probably wouldn’t matter too much in reality. But I pointed this out to my CAA inspector and I suggested replacement of these rivets for countersunk ones. He agreed.
I made an 80 cm long drill bit for drilling the second bulkhead at the aft end of the ballast weight tube support frame.
I drilled the end of a piece of 10 mm stainless rod and silver-soldered a drill in the hole. Easy peasy.
With all the holes drilled and preparation done I decided to paint the Tail Boom at this early stage in an attempt to minimise the continual Assembly/Disassembly process. I bolted a piece of studding onto a 3 mm aluminium plate which I fixed across the open end of the boom using the ballast weight frame holes and one of the pre-drilled 1/8 in jigging holes. Both ends were supported on step ladders so the boom could be easily rotated without falling off!! We once painted a friends 289 Cobra in a beautiful Lexus Silver with a Ferrari Red stripe. It looked stunning so I chose this as my colour scheme.
Here’s Matthew laying out the Vinyl stencils for the mandatory tail boom markings. The registration lettering should be around 6 inches tall and clearly legible but the ‘DANGER’ and arrow markings must be the specific size required by the CAA
When it came to refitting the tail boom I didn’t want to damage the paint as I slid it into the four mounting arms. I wrapped the arms with a thin piece of card and smeared a little Petroleum Jelly on the card. The tail boom slid into position a treat and the card was pulled out. No damage.
This picture shows my chromed ballast weight mount bolted into place on the tail boom. The curvature of both the mount and the support frame inside doesn’t match the curvature of the tail boom. Simply bolting the sandwich together could distort the tail boom skin so I masked the inside and outside surfaces of the skin with the good old packing tape mould release (clear tape this time) and bolted the components together with a thin bed of filler on each mating face. The excess squeezed out leaving a perfectly mating set of three components.
I used red filler on the frame and white on the mount but it doesn’t really matter. The thickness of filler varied between zero and 1.5 mm. Not much, but worth getting right.
I used a Laser Level to align the Vertical Fin. You can buy these now for less than £10. Amazing. I machined a packing block to support the rearward end of the level parallel to the centreline of the fin
And here’s the laser dot at the seatback position - almost at the required 15 1/2 inch dimension to the pilots side of the main rotor shaft.
The Vertical fin’s rear mounting design has been a concern for some builders and it’s clear to see why. The mounting spacer is simply a short piece of 3/4 inch drilled aluminium bar that clamps between the rear fin and the Tail Boom skin with one 3/16 bolt. A sensible solution is to fabricating a similar saddle-type bracket to the forward one but that’s a change that we would not be allowed to make in UK. So to improve on the standard method I moulded some epoxy ‘Metal Set’ putty around and under the end of the spacer (on packing tape, of course). It now fits accurately to the contour of the tail boom skin and presents a larger contact area to form a more secure and stable fixing.
|
