Mystery model

[Jmk89]

I did some more work on the latest build today while waiting for the balsa to turn up, which it has now done, so this was an efficient use of time.

Today's task was to make the prop blank.  Here's a photo.


The wood is paulownia.  Since I had a block 300mm long to hand, I made the 400mm length by splicing 2 pieces 300mm x 38mm x 32mm together with about a 87mm long splice.  Then I remembered to wait for the glue to dry and to then drill the hole in the post drill before shaping the blank.  On my calculations this should give a 400mm pitch (i.e. P/D of 1.0).

[Jmk89]

Here's a start on the fuselage.  It's a bit different in that all sides are 0.8mm sheet and the "spacers" or "bulkheads" are glued to the sides first.

Here are a couple of pictures:



To hold them down while the glue sets (I like using aliphatic) without putting holes in the spacers or the sides, I made some little plastic holders with the 3D printer which are held in place by pins and which just push down enough on the spacers to make a good contact with the balsa side.

Here's a photo of a couple of those little gizmos:

[Jmk89]

Some more progress - wings and fins built and ready for sanding

[Jmk89]

I attach a picture of the Mystery model's tailplane plan.  I can't work out what the plan means.  Is it:
1.    sheet on bottom with a little triangle of balsa on top?
2.    sheet on top with extra triangle on top?
3.    sheets on bottom and top and extra triangle on top?
4.    something else?

[Jmk89]

5.   little triangle let into the top sheeting (change of grain direction)?

[dputt7]

Sorry Jeremy, sat here for half an hour trying to make sense of it.  The plan definitely shows sheeting on the top and I think the extra piece is laid over the top, Is that the L.E. could it be to reinforce the L.E. Joint.

[pb_guy]

Is it held on by rubber bands? If so, it might be for strength.

[TheLurker]

The plan definitely shows sheeting on the top and I think the extra piece is laid over the top...

Yup.

All I can think of is that it's to provide a 1/16" thick panel for not much extra tail weight to give a rigid/robust mounting point for the fin. Perhaps also allowing the fitting of the fin with the LE offset to port or starboard.

[Jmk89]

Thanks for the replies, chaps.

The tailplane is held on by rubber bands, according to the article.  But I'm not going to use bands but will use a wire and tube hinge to hold the TP on and for the DT.

I think that the first time I saw this was a picture in a 1950's AeroModeller showing a fitting made by Jim Fullarton - I couldn't find it in the excel index, but it's a bit like Dave Hipperson's hinge for Senator but the tail tips rather than the fuselage.

The sheeting can't be a fin thing - it has twin fins stuck on the TP tips!

[Jmk89]

Today I carved the prop for this model. 

Here are some of the tools I used - Sloyd carving knife, staight carving knife, razor saw and Liogier (French woodworking rasps).


The carving knives are from woodcarving times and are good and rigid in the blade and hold an edge much better than "hobby" knives. 

The French rasps are hand-stitched (the process of making the teeth is called stitching) and are much more effective than the machine stitched rasps that are usually available.  The main reason is that they are slightly out of perfect line and size so that they don't create tracks in the wood which is what happens when each tooth goes along the same line.  So the finish strangely is much better.


I have two of them - a bigger and a finer each in tapered half-round shape.

The other tool that I find very useful when making the saw cuts is a carver's vice - the jaws swivel to hold the workpiece which makes it much easier to hold the prop still while sawing it.



The eagle-eyes will notice that the prop is balsa not paulownia.  When I was carving the paulownia blank shown earlier in this thread, I wasn't using my woodcarving knoves and  trying to hold it still using a "hobby" knife, I snapped that blank and did it so comprehensively that the prop couldn't even be rescued by gluing it back together.

[Prosper]

Hmmm - the Carver's vice and the French rasps - never heard of these. I feel they ought to be in my toolkit. Thanks.

Stephen.

[Piecost]

Jeremy,

Thanks for the propeller carving information. More details of the process would be appreciated.

Tim

[OZPAF]

That's interesting Jeremy - an insight into wood carving and associated tools. That vice does sound interesting.

Thanks and more info on your prop carving would be appreciated.

John

[Jmk89]

Thanks for all the very kind comments. I am by no means an expert at prop carving, but I have done it a few times and I find it easier every time I do it. The first couple of attempts were frankly pretty bad, but one learns quickly. If doing this the first time, buy enough material to make three or four props. It is almost certain that you will have to throw away the first and one or other of them will be broken as you go along.

My approach is based upon my reading of, mainly, the following:

• R. H Warring, Airscrews (in the Plan Gallery);
• R. G. Moulton, Modern Aeromodelling;
• R Coleman: Airscrews en masse (Aeromodeller 1973); and
• Andy Sefton's article in April 2026 Aeromodeller (number 1067).

I don't exactly follow any one of these. There is no single "right" way to do this. What matters is not how you do it, but getting a good propeller out of the process.

I won't go into prop design or layout of the prop blank.  For that, go to the material above and the articles by John O'Donnell in early relaunched Aeromodellers (2014 - 16).

Also, the block specifications and blank layout are often set out on the plan or in the instructions/article accompanying the plan.

So assuming you have the dimensions for your blank,

• mark the dimensions onto your block,
• glue the ply reinforcement plate on the front centre of the blank,
• drill the hole for the prop shaft (it's much easier to get it plumb when you do it at this stage!)
• mark the blank to show the areas that need to be sawn away before starting to shape the blades
• cut those areas away with a tenon saw or bandsaw if you have one
• mark the blank now on the bottom and the front of both blades - to work out the front, I find it easier to put the blank down on the bench with the ply plate up and treat the further vertical side for the right hand blade as the front and then rotate the blank 180° so the left blade is now the right blade and mark the further vertical side for that blade as front as well.  So the bottom near corner of the blank is the trailing edge of the blade and the top further corner is the leading edge. 
• the next stages are to carve away the back of each blade so that at each radius of the prop, you have a right triangle with a straight line between the leading edge and trailing edge as the hypotenuse and the top and near sides as the "square" sides).Do each stage to one blade and then the other and then move on to the next stage   The first stage is to make saw cuts with a razor saw through the back triangle starting with the right angle and cutting down to the hypotenuse line at radius points along the blade - I tend to space the radius points between 8 and 10mm - the idea is to in effect mark the hypotenuse at each radius point. 
• The second stage is to carve the sawn material away using your knife - I always find I need at least two knives per blade and sharpen them, before starting and whenever the blade feels harder to sweep through the wood.  The saw cuts make the job easier because they make it easier for chips to separate from the blank, they make it simpler  to stop the blade following the grain into areas that you want to preserve, and, if you have made them nice and straight (i.e have marked the hypotenuse accurately) they show you when you are getting close to the correct depth and make it easier to see if you are creating a straight line from LE to TE or putting a convex curve on it.
• When I think I'm about 1.5mm from the hypotenuse I stop carving and use the larger and then finer rasp and then a slot sander to finish the back down to the blank corners.

Next step is to mark the thickness down to that to which you wish to carve the front of each blade. I create guidelines by making by drawing a line on the leading edge and trailing edge 4mm back from the leading edge and 4mm up from the trailing edge. That should create a blade which is about 3.5mm thick when you carve down to those lines.

I repeat the process that I used for the back -  saw cuts across the blade to connect the guidelines, carve by knife close to the guidelines trying to keep the surface flat and to avoid  convexity, and finishing up with the rasps and the slot sander.

Having done that to both blades, draw on your preferred blade shape. I must admit to trying to use more of a Larrabee blade shape (the very useful article by John O'Donnell from AMI is now available in the Plans Gallery, I notice) rather than the more traditional "paddle" blade that is often shown on the plan. I maintain that it is as authentic as any other blade shape since "dagger blade" propellers were used in the "golden age" and everyone was experimenting with everything at that time, so some of the blades used then are likely to have been very close to the Larrabee shape.

Once you have the blade shape as close as you can get it to your desired shape, it is time to shape in the desired airfoil. I tend to use something close to the ARAD 6  airfoil that I have mentioned before (there is an article on this in the Plans Gallery with the theory and also the coordinates). One reason for this is that I have created a set of 3D printed plastic templates for upper and lower surfaces of various chords and changing airfoil would require a new set of templates. For that shaping I only use the fine rasp and sandpaper.

Some final hints:

• always carve away from yourself ;
• if you are not using a vice to hold the blank while carving, remember to hold the blank not to push it into the bench - pushing hard tends to lead to blank breakage; and
• the moment the blade of your carving knife feels at all blunt, take it to your sharpening station and put a new edge on it -carving balsa or paulownia is much easier if you are using a sharp tool; it is also much safer since you don't need to apply so much force to make the blade move through the wood.

[Jmk89]

phew!  That took longer than carving a prop!