Riveted ribs

Riveted together the inner ribs with the angle rib brackets

Checked the bearing on the crankshaft by placing it into the casing together with the aft bearing. Sonex is correct, it fits without "sticking".

Wing ribs

Deburred and primed all the inner wing ribs and brackets and spacers.

Received message from Sonex that the outer wing rib is fixed by trimming the thick reinforcement plate in the aft spar. Also received message from Aeroconversions that the "sticky" bearing is OK because it will not run that far aft. I have to put it in the casing and see for myself. They also said the conrods were OK...

Wing fold reinforcements

Continued on the ribs, and finished the wing fold reinforcements.

Then I run into a small problem (I hope). Something is too large or positioned wrong, but what? Hopefully Sonex has the answer tomorrow. With the pre-made parts with pre-made holes, things should be in the correct position, but that is not always the case, since the pre-made holes are often wrong. Time will show.

inner wing rib brackets

Drilled all the holes for the inner wing ribs.

Propeller moment of inertia

In my quest for investigating the press fit hub, the propeller moment of inertia was a needed parameter. The only practical way of finding the inertia is by doing an experiment with the propeller as torsional pendulum. This is explained by EAA here, and by Hepperle here.

I don't have a Rotax diagram, so I did it as described by Hepperle. My dimensions were:
m = 2.5 kg (measured with a bathroom scale, not very accurate), 2e = 6 inch, a = 126 cm. I measured 5 full periods, and they were 38 s on average (did it 4 times). The period time, T, became 7.6 s. The propeller moment of inertia is:

Ip = m*g*T²*e²/(4*pi*a) = 0.165 kgm²

The bathroom scale could be rather inaccurate for this low weight, but good enough for the purpose.

Rear wing spar, mid wing, polysiloxane paint, engine

Riveted and bolted on the rear wing spar

Then started fluting and deburring and straightening up the ribs for the mid wing section.

I also fetched the polysiloxane test I have had laying on the veranda. It has been there two winters and one summer now, almost 1 1/2 year. There are some superficial signs of corrosion on the non-painted part, but nothing worth mentioning. The polysiloxane painted part looks as if it was painted yesterday.

I can only conclude that the 6061 will last for ever, and the polysiloxane paint will last even longer. Too bad it is impossible to achieve a nice finish.

Sonex found a new rod that weighs 1.5 gram more than mine, leaving me some degree of trimming in case of variations in the scales. They sent it right away, free of charge. Excellent service from Sonex I have to admit. But, this has given me some second thoughts about the engine and the choices done on it.  

Major setback on the engine

When torquing the bolts for the connecting rod I felt one bolt became a tiny bit "mushy". Wishful thinking kept me from examining it right away. But the next day I did, and as expected the bolt had shredded the threads.

I contacted Sonex, and they told me to weigh the rod, and they should see if they could find a new one within 1 gram. If they can't find one, I have to get a new set of 4. They would offer me a set for the price of 3 single, hmmm.

Looking a bit closer at this, the rod is modified by Sonex. A rather large part is filed off, probably to prevent interference with other parts in the casing due to the larger stroke. The modification has resulted in a much shorter thread length than originally. The thread length is only about 4.5 mm.

Enough thread length is a difficult subject, depending on lots of factors and the application. Common engineering rule of thumb for steel, is a thread length of 0.8 to 1.5 times the diameter. The reason for this is, in case of overload, the bolt shall break before the threads. Threads in a piece of machinery are very difficult to "replace" once they are shredded, but a bolt is easy to replace.

A nut has typically a thread length of 80% of the diameter (larger for small sizes and shorter for larger sizes), and the reason behind this length is to be sure there are enough threads so the bolt breaks before the threads. You don't want the threads to be the critical part of a design.  More than this length does not improve the thread strength further. A nut and bolt usually have equal material properties. Female threads are stronger than male threads, due to larger diameter and more material for each thread.

The diameter of the threads on the bolt is 9.5 mm (seems to be 3/8 inch bolts instead of some metric dimension for some reason) . This means a minimum thread length is about 7.6 mm if a nut is used. But, since it is not a nut, it could very well be be 9.5 to 14.3 mm according to normal engineering practice. Originally the thread length seems to be about 8-10 mm, but this is difficult to say exact, because I don't know exactly how much material Sonex has trimmed off.

I did a more exact calculation for this bolt. The bolt main diameter (D) is 9.525 mm. I measured a pitch (p) of 1.077 mm/thread.

The tensile stress area for the bolt is:
At = pi/4 * (D - 0.938194*p)² = 56.94 mm²
The thread shear area is:
Ass = 0.5 * pi * dp * Le
where dp is the pitch circle diameter of the thread, dp = D - 0.64952*p and Le is the thread engagement length.

To be sure the bolt breaks before the threads shred, Ass must be twice that of At, which means that:
Le = 2 * At/Ass = 2 * At / (0.5 * pi * dp) = 8.21 mm.

Simply spoken, the correct minimum length for the thread engagement for this bolt is 8.21 mm, or at least very close to this number. With a length of only 4.5 mm, there is absolutely no margin for error of any kind, and the threads have to be 100% exact. This is not the case here, and the original length of these threads are 8-10 mm before Sonex decided to modify them. The original thread engagement length is there for a reason.


I'm not sure what to do about this. I don't think the bolts and threads themselves are too weak. They are probably made for a much higher RPM than 3500. But the torque used to fasten them seems to be too much for their own good, with this way too small engagement length. The threads in all the other bolts could also be weakened, and this is not good for fatigue, not good at all.

Main spar

Had some accident with the engine, so I continue with the aircraft riveting/bolting on the main spar.

Camshaft and cranckshaft

I already had purchased 2 sorts of Loctite, and I thought that was more than enough. Then I found out I would need the "strong red" Loctite.

Also purchased a large 20-220 Nm torque wrench at Biltema. My small wrench for AN bolts is not nearly large enough for the torques used in the engine.

Mounted the gear wheel on the camshaft with "strong red" Loctite and torqued to the right spec. Then cleaned the piston rods and bearings and mounted the bearings on the crankshaft. I had the star type bolts. These shall not be locked with "strong red" Loctite, but greased with "moly lube" and torqued much higher, in 4 steps.

"Moly lube", "white litium grease" ? what's wrong with giving some real specs? I used my assembly grease instead. My other book about VW engines also specify "white litium grease", but say that assembly grease is better. Moly lube and white litium grease can be all kinds of things, and I don't know if it even will dissolve properly in engine oil. Assembly grease is made for this kind of work, and will dissolve 100% in engine oil.

Measured the side to side play and it was well within limits. All the rods could be moved easily and freely by hand with the exact same "resistance" from the assembly grease, so I guess they should be OK.

Lower aft fuselage

Deburred and primed the lower aft fuselage skin. Then primed the skin where the gear will be seated, so I can mount the gear and forget about it.

I have to do some thinking before mounting the main spar and fitting the tail surfaces. I will need to reorganize the shop somehow to get enough space, but not sure exactly how. Maybe just build the engine in the mean time. Bought some engine assembly grease and gasket stuff. The engine manual say "white lithium grease", but "white lithium grease" can be all kinds of things, so better to get some real assembly grease instead.