Black Magic and twin diesel build

[John Payne]

I have now assembled the left had wing and I have made some changes to the inner structure to help with the strength.  The first change I have made is to include cap strips which will help the torsional stiffness of the wing (thanks for the help Malcolm).  The second major change was the inclusion of shear webs between the two main spars.  I used 1/16 sheet for this, with the grain vertical, since I was using balsa spars and I was worried that the engine weight and radio assistance would add extra strain to the structure. I will also replaced the original wing bracing for plywood dihedral braces front and rear of the main spars. As it will be a one piece wing, the design to allow the wings to be broken down for transport wasn't relevant.

 

I also took some time over deciphering the plan as far as the tip detail was concerned.  There was no detail for the shape of the tip, aside from the plan view.  After much deliberation and searching on the web I settled on keeping the lower spar flat to the end of the wing and angling the upper spar down to it over the last couple of rib bays. The washout was kept as per plan at 3/16.

 

The first step was to assemble the trailing edge and end sheeting to produce a complete assembly that was secured to the building board.  Next the lower front sheeting was pinned down and the lower spar glued down on to it.  The sheeting was positioned under the end sheeting and the spar cut to length to stop at the sheeting and no glue was applied here at this point.  The detail photo below shows this.  Next the lower cap strips and central sheeting were glued in position between the trailing edge and front lower sheeting sheeting.  The ribs were then glued over the spar and onto the cap strips and central sheeting.  The front of the ribs were not glued at this point.  This assembly was all flat on the building board and allowed to thoroughly dry. At this point the washout packing was positioned and finally the upper spar was glued in position across the ribs with the end being shaped to come down onto the lower spar at the wing end.   The end rib was glued in using the template on the plan to get the right angle.

 

This photo shows the assembly to this point.

 

This photo shows the wing tip detail with the end sheeting now glued in position.

 

The next stage in the construction was to fit the shear webs and glue up the front sheeting to the ribs.  My preferred way to do this is to put the glue in place for the ribs, working the glue well under the ribs and then, using other sheets under the lower sheeting, wedge the sheeting up under the ribs by using progressively thicker sheeting to keep the sheeting touching correctly.  Sounds more complicated that in is and it takes no time to do, but it does fully support the sheeting while it is drying. Teh end sheeting was also glued to the lower sheeting and this time.

 

I then fitted the leading edge strip of 3/16 square balsa, securing it to the end sheeting, and added to the end sheeting to build it up to allow a decent gluing area for the top sheet.  Once in place I carefully sanded the end sheet infil to allow the top sheeting to sit down properly. At this point the upper sheeting is applied and pinned down and again the whole assemble allow to thoroughly dry. 

 

The next photo shows this assembly stage with LOTS of pins to hold it down.  You will notice that the lower spar is still securely pinned down to the building board.  Oh, and make sure that you remove all pins from under the upper sheeting before you glue it on.  You have been warned!

 

Nearly finished now.  The upper cap strips are now glued in place and to help this I was given a good tip (thanks again Malcolm) to shape the 1/16 by 1/4 strips before fitting.  Once you have cut them to just over length, carefully draw the strip between your thumb and the back of the scalpel handle (make sure that you do not create any red balsa, you have been warned again!) and it nicely bends the strip to shape.  Do not do it too hard or you will stretch or even break the balsa.  I lost a couple this way, but practice makes it easy. Once the strip is shaped it is a simple matter to cut it to length and glue it in place.  It will not tend to spring back up off the rib. The upper piece of central sheeting was also glued in at this point.

 

The final pieces to go in are what I take to be anti-warp struts whitch go from the upper spar to the trailing edge. Tricky operation this, as the strut goes across two rib bays and has to be cut and glued to the middle rib.  Take your time here, but I think it adds a lot of strength. The final photo shows the end result before removal from the board and final sanding. 

 

 

Quote

[John Payne]

I think the post was a little long and missed off the last photo.  Here it is.

 

 

The next steps are to build the right hand wing, join the wings and add the dihedral braces.  I'll go into that next time.

[John Payne]

Just a quick update on the build.

 

The right hand wing is now built, and as it is a mirror image of the other wing there are no new pictures to put in the log.  One thing to mention is that the plan only contains one half of the wing drawn on it and so I couldn't build the right hand wing directly on the plan as I had done with the left hand wing.

 

There are several solutions to this. I could have traced the wing onto greaseproof paper and build on the reverse of that.  I could have applied parafin to the plan to make it transparent enough to see through (an old method but a smelly one!).  My simple solution was to put white paper onto the building board, turn the plan over and work with what I could see through the plan.  It worked because of the quality of the print allowing it to show through.  Had it not worked I would have drawn out the position of the ribs, spars and trailing edge and worked from that, using the left wing as a pattern.

 

Now to clean up the wings and fit the dihedral braces.

[ken anderson.]

john - it's looking good-give me a call when it's completed-i'll call down and collect it--thank's.............

 

     ken anderson...a needy cause....'s...

[Simon Chaddock]

John

What a nice piece of carpentry.

 

Anti warp struts? Interesting.

 

With a fully sheeted leading edge (and sheeting between the spars?) you have in effect a big torque resistant box and coupled with the strong capped ribs I would have thought the structure would have been stiff enough but short of actually testing its torsional stiffness with and without the struts its only an opinion!

 

Cant wait to see the twin completed and did you say you are thinking of a 4?

[John Payne]

Hi Ken, that one made me smile. I guess the likelyhood of it happening is linked to how well it flies...........

 

Thanks for the interest.

[John Payne]

Hi Simon, thanks for the note.

 

There is a lot of structure in there, but as it will be taking an engine bigger in size and power than the original I thought it would be better to err on the safe side.  Still, for all that wood it is still light.

 

As for the engine, it is coming along now and the crankshaft is finished.  I have a couple of pictures and will put them up soon. Yes we are thiniking of a 4 cylinder, flat configuration.  It is planned to be modular so that the main components are not changed too much and will basically be two twin cylinders bolted together. Once we are on to that one we plan to create a build log for it.  Not quite sure which forum to use for it but I can ask for guidance from Timbo on that one.

[Malcolm Fisher]

John,

 

Please keep us all informed of progress and certainly let us know about the four cylinder version when you get on to it.

 

I, for one, would be very interested in having one in the, probably unlikely, event that you go into mass production...

 

Malcolm

[John Payne]

I certainly will Malcolm.  I have persuaded my brother, the engineer of the two of us (!), to start adding to this build log on matters concerning the twin and as I said in the post above we are then intending to create a new one for the flat 4.  We thought we might get some interest in the engines and it is nice to see that we have.

 

Thanks for the interest.

[John Payne]

On with the assembly of the wings.

 

The first job was to roughly plane and sand the wings to shape and make sure that they are the same profile.  This was done using a razor plane and a Permagrit sanding block.  I stopped at getting the wings to roughly near shape as I still had to handle them and the possibility of minor damage meant that the final sanding to shape could be left until the complete wing was ready for the covering stage.  To get them together I glued them up and held them in place using 5 minute epoxy and held it until it had achieved a good enough set to hold them in place without my help.  I then left the assembly to completely cure. The final result is shown below after the wings were joined.

 

Next I had to fit the dihedral braces to the front and rear of the main spar.  The original plan shows a two piece wing with tubes and dowels holding them together for flight.  I had to modify this for RC use and putting a 1/16 ply dihedral brace at the front and rear of the main spar looks as though it will be strong enough. I took the braces out to the third rib. To get them into place I cut out the lower sheeting to give me enough room to clamp the glued in brace to the spars.  I did it this way round to make sure the braces were a good fit to the final dihedral, rather than juggle with the central rib angle to suit the braces (don't forget these braces are my addition and not detailed on the plan). To cut the slots I used two hacksaw blades together which gave me a slot equal to the width of the plywood used.  Once the slot was cut into the relevant ribs, it was a quick clean up to ensure a good glue joint and the rear brace was glued in and left to fully dry.  The photo below shows these operations, with the blades cutting the front slot after the fitting of the rear brace.

 

The wing was now solid enough for the fitting of the front dihedral brace, using a similar cut and fit exercise.  Doing it this way round I believe that no undue stresses are introduced into the wing in this area and the braces were slid into place and secured, rather than trying to get the wing together around the dihedral braces in one assemble operation.  Just a personal choice really.  The photo below shows the securing of the front brace.

 

To finish the wing I simply glued back in pieces of sheeting to replace those removed to allow access and a final sanding will make the cut and shut exercise look OK.

[John Payne]

For those people interested in the engine, I have included below a couple of photos of the crankshaft, which is now virtually finished and can report that the liners are now done with the exception of the transfer and exhaust ports.

 

From memory, only the prop driver taper has to be finished on the crankshaft and this will be done at the same time as the prop driver itself.  The sharp eyed will notice the rougher apperance of the diameter where the taper will be.  The extension on the front of the crankshaft was to lead in the die used to cut the thread on the crankshaft and will be removed later.

 

I have also persuaded Frank to join in with the forum to explain, better than I can(!), the progress of the engine manufacture and pick up any queries anyone may have about how he is machining the components.

[Frank Payne]

Hi all,

My name is Frank and I have the dubious honour of being John's brother.  We have been threatening to do this type of joint build for ages and so far he is way in front with his part of the deal! Please let me know if there is any particular issue I can expand on. The idea is to update as we go along and eventually show a video of the Twin running(and then flying?). Like most engineers, I love to talk about the techniques used, and also like most engineers, if I do not know the answer to a question, I will gladly make something up! I look forward to hearing any thoughts you may have.

 

Regards

 

Frank

[Former Member]

[This posting has been removed]

[Malcolm Fisher]

John

 

I missed the post about the wing joining.

 

 What you did is pretty much what was done on the Flair r/c version I built as this also has a one piece wing. I can't remember just how far the dihedral braces went or even if they were inserted on one wing and the other slid into them after the first part  had the glue set. 

 I use slower setting epoxy for this job as it gives more time to set things up accurately and IMO gives a stronger result than the rapid cure version. I also give the ply a light sanding to make sure that there's no relase agent left after its manufacture.

Malcolm

[Brian Parker]

Frank, welcome.
 I, and I'm sure others, would be interested in details of the method used for the crankshaft manufacture.

 Also some information on the equipment at your disposal would be interesting and would, I suspect, make us all jealous.

Thanks for sharing the build with us.

[Tim Mackey]

Hi Frank and welcome aboard.... look forward to seeing more of your lovely machining work.

I always puzzle about how you turn those journals on a twin crankshaft.

Edited By Timbo - Moderator on 20/07/2009 21:09:31

[Klippy]

Hi Frank, love your work! I'm still waiting for my drawings of the twin! I intend to make one myself. Never think for one moment that nobody's watching, there's lots of 'lurkers'! me included. Timbo, I imagine the crankshaft requires the use of a 4 jaw chuck to spin it off centre. Could be wrong of course.

[John Payne]

Hi Malcolm.  You didn't miss much.

 

I used quick set epoxy for a couple of reasons.  The main one was that as I was physicaly holding them in alignment while it set, teh whole thing took a lot less time and I deidn't get too much cramp in the fingers!  I do it this way as I have had wings move in the past when I thought I had got them secured well during gluing, only to find they have moved out of alignment. Not easy to correct!

 

The second reason was I figured that as a butt joint the glue would be strong enough to hold the wings together while I secured the braces in place.  After that , I'm not sure that the joint itself comes under that much stress so a stronger epoxy wouldn't add that much. I am also going to add a narrow strip of nylon around the centre joint to provide bonding between thesheeting on the wings and this should add a lot more strength for a very small weight addition and provide support at the rear of the joint. At the moment the wing stands at 5 1/2 ounces.

 

As you suggested I did lightly sand the plywood to get rid of any release agent on the wood and make for a better bond.

 

Thanks for the note. 

[Frank Payne]

Thanks for the welcome everyone it is much appreciated.

Brian,  The manufacture is not that difficult just takes a fair amount of light cuts. I will write out a more comprehensive explanation but basically  drill three centre holes in line in both ends and use a carrier to drive the workpiece between centres. Sorry if that sounds a bit technical but it is not meant to be. Once I get a few pictures together it will make sense.

Timbo, I have promised to explain in more detail, so I guess you will be reminding me if I start dragging my feet!

Tony, you are not wrong, you can certainly off set the workpiece in the 4 jaw but lining up the second journal exactly opposite the first can be a mite tricky. Using the 4 jaw for a single throw is a more rigid and straightforward method. So straightforward in fact that even my brother can do it!

I am lucky enough to have a Myford Super 7 and a clapped out Bridgeport with a few extra pieces of kit with it. I am one of the happy band of machinists who like to share information with anyone who takes the trouble to ask, so please ask anything about the techniques used on any part. I am currently making the con rods with their 10 BA screws, next week, by way of a relaxation, I am going to redecorate my hall through the letter box.

Remember as my first Foreman said to me "Blimey Frank If you cannot make it size can you at least make it shiny"

Best Regards Frank

 

[Frank Payne]

Thanks for the welcome everyone it is much appreciated.

Brian,  The manufacture is not that difficult just takes a fair amount of light cuts. I will write out a more comprehensive explanation but basically  drill three centre holes in line in both ends and use a carrier to drive the workpiece between centres. Sorry if that sounds a bit technical but it is not meant to be. Once I get a few pictures together it will make sense.

Timbo, I have promised to explain in more detail, so I guess you will be reminding me if I start dragging my feet!

Tony, you are not wrong, you can certainly off set the workpiece in the 4 jaw but lining up the second journal exactly opposite the first can be a mite tricky. Using the 4 jaw for a single throw is a more rigid and straightforward method. So straightforward in fact that even my brother can do it!

I am lucky enough to have a Myford Super 7 and a clapped out Bridgeport with a few extra pieces of kit with it. I am one of the happy band of machinists who like to share information with anyone who takes the trouble to ask, so please ask anything about the techniques used on any part. I am currently making the con rods with their 10 BA screws, next week, by way of a relaxation, I am going to redecorate my hall through the letter box.

Remember as my first Foreman said to me "Blimey Frank If you cannot make it size can you at least make it shiny"

Best Regards Frank

 

[John Payne]

Slow but positive progress on the airframe this week.

 

One thing that has given me something to think about has been the way that the radio guidance would be completed and I didn't want to have any more evidence of it than necessary. I finally came up with a method of mounting micro servos in the lower part of the tail fin.  This would do a few things.  I am sure that tail weight will be needed to counter the weight of the engine. I wanted to keep the installation simple. It his the controls under the tail plane. It gave me an opportunity to make the tail plane more secure as I am not convinced about the narrow fixing on top of the lower part of the fin.  The original had a banded on tail which was on top of a diamond shaped ply plate on the lower fin portion.  If it got knocked on landing it was simple to refit. The following photos show the servos positioned in the lower fin, not yet screwed into place.

 

 

The servos are 2Kg units.  As the intention is for radio guidance rather than anything aerobatic I think these will be fine. I have used "full size" square servo mounting grommets and will use a single screw to secure them. They will get a cover over them to keep the diesel residue off them and it is this cover that will help secure the tail plane to the lower fin. I haven't worked out the final details yet but it should help brace the joint.  I cannot complete this yet as I haven't finished the engine installation, which cannot be completed until the engine is finished, hence the slow progess at the moment (my excuse anyway!).

 

I have also bent up the tailwheel mounting wire from piano wire.  this will be fixed to the former shown earlier in the build log.  It just needs the holes in the former to enable the wire to be bound into place.  I will fill in around the exit point for the wire so that I can cover the fuselage properly. The photo does not show the shape of the mounting very well as it is a straight on view.

 

[John Payne]

Progress on the airframe has been slow but steady over the last couple of weeks. As I said earlier in the build log I have to wait for the engine to be finished so that I can finally complete the front of the model.  We don't know how the exhaust system or fuel tank are going to be put into the frame and rather than finish the tail end and potentially damage it when I move it around to fix the front I will leave it unattached.

 

So the progress to date is to complete the tail components to a position where they can be fixed when ready, fix in the tail wheel mounting plate, fix in the servos and fit the wing seating strips in.

 

The first photo shows the tailplane and elevators, from 1/4 sheet, with the joining wire, from piano wire, for the elevators. In the photo below you can see the slot I had to cut to get the rudder control down to the servo.

 

The secured tail wheel mounting, with wire bound on with thread and epoxied in.

 

The next photo shows the tail fin and rudder with the control wire, again from piano wire.  The rudder was locally reinforced to take the control wire.  The connection to the control rod can just be seen. It is a simple aileron connector secured by a collet.

 

The wing seating strip it there to allow the wing to sit nicely on top of the cabin area and is made up of 3/16 square strip.  The wood for the cabin header is made up of two pieces of 1/4 balsa and will be finally shaped once the front end is complete.

 

The final photos show the final assembly to this stage and is one thing that I like to do regularly as I go through a model build, just to enjoy the way it progresses to a final model.

 

 

The next bit will be to cover the wing, which I can finish, and finalise the connections to the servos and the servo cover, but I can't secure it yet.

[Simon Chaddock]

Hi John

Lovely work and there is nothing wrong with putting it together "just to see what it looks like". It certainly helps the motivation on a long build.

 

It also gives a good opportunity to spot any small errors or weaknesses before you get too far and I follow the principle that if I can't see any then it means I am not looking hard enough!

 

[John Payne]

That is true Simon, one or two little things did show up on the quick assembly. Easier to fix now than once it is covered, that is for sure!

 

Thanks for the post.

[Sheldon Holy]

This looks gorgeous! Great building there, and i like the engine. Was the engine a kit or did you machine it out yourself? I have been using those BRC servos on .40 size models, and they are just as effective as the standard servos.

 

Sheldon