Pistons problemo!

[Pylonracer]

The piston supplier regularly supplies the re-bore firm, (Saunders in UK),

Had a few engines done at Saunders in Cadnam. I think they are in their 100th year, I live just down the road from them.

 

[graham-xrf]

Had a few engines done at Saunders in Cadnam. I think they are in their 100th year, I live just down the road from them.

Yes - the very same place! One thing I found out was that when aircraft owners need the engines overhauled, they have to take them to the places that will provide the certifications. These places make them pay the big bucks for that, but they send the engines on to Saunders to do the actual machining work. Saunders have lots of of Lycoming and Continental engine overhaul work. I found them to be super helpful.

It's just a pity I (and indeed also the piston supplier), walked into a unexpected piston identification mistake. They may yet come up with something.

 

[Pylonracer]

Yes - the very same place! One thing I found out was that when aircraft owners need the engines overhauled, they have to take them to the places that will provide the certifications. These places make them pay the big bucks for that, but they send the engines on to Saunders to do the actual machining work. Saunders have lots of of Lycoming and Continental engine overhaul work. I found them to be super helpful.

It's just a pity I (and indeed also the piston supplier), walked into a unexpected piston identification mistake. They may yet come up with something.

Oh I did not realise that they were doing Lycoming and Continentals there, that is probably what keeps them going. Good on them.

 

[RJSakowski]

Yes - as per the "Jimmy's World T-shirt. Quite a lot actually. The one in your picture was a modified design that made it safe, although all are a bit prone to problems if the wind is a bit more than "slight". It was banned in France until the design fix could be found. Henri Mignet's first design is the one I am dealing with.

View attachment 486811

It had the unfortunate characteristic that if one had an engine with that little bit more power, or otherwise one went a little faster by pushing the stick forward. it would obligingly nose down into an unstable state. If one pulled back on the stick, it would nose down further, instead of coming back up. Pull back hard enough, and you crash into the ground!.

So look at it. There are NO ailerons. The entire fin at the back is all rudder, but there are NO rudder controls for the feet. The way it banks is by secondary effect from the rudder, (worked by the stick) . At least the yaw is in the correct direction, because of the lack of ailerons.

Look at the lower "wing". Is that a (sort of) biplane, or is that what would be the tailplane elevator made huge and moved forward? Is it a "tandem wing"? The way the movement of the stick made it go up and down was by tilting the entire upper wing, hinged at the leading edge. The air from under the wing, in the slot, going over the "rear" wing, gives that "rear" wing enough lift to tip the whole plane forward, and the faster you go down, the more it tips forward.
The problem was, Henri Mignet (designer) was utterly crap at conventional plane controls coordination. He was out to make the controls "simpler", so he settled for only a stick - NO ailerons, and NO separate rudder control.
Pull back => UP.
Push Forward = DOWN
Stick left => TURN LEFT
Stick right => TURN RIGHT
Bank in a turn comes as a bonus secondary effect from the big fin at the back. The cables from the stick that, in any other airplane, would work ailerons, were connected to the rudder instead.

Modern versions
There are hundreds all over France, and some have been modified to have a real tailplane elevator at the back. They are slow, and safe, but not easy if the wind is more than about 8mph. Originally, folk built them at home from a magazine article series.
After the safety design change, here is what they used to look like..

View attachment 486815

And then, they evolved..

View attachment 486816_ _View attachment 486817

This one has a canopy..

View attachment 486818

And this one even folds up to stash in the garage!

View attachment 486819

BUT - I need to find a solution for the pistons. Whatever specialized synchronous tool cam movement lathe used for making a "oval" in a piston, and simultaneously a non-linear taper is something I do not have. I have only just started the hunt. All this, by the way, interesting as it might be, is a total distraction from the stuff I would would rather be getting into.

Turning an oval should be a piece of cake for a CNC lathe. On a less expensive note, I believe that a lathe equipped with the Rocketronics ELS is also capable f turning an oval. https://www.rocketronics.de/en/els/

 

[Pylonracer]

Turning an oval should be a piece of cake for a CNC lathe. On a less expensive note, I believe that a lathe equipped with the Rocketronics ELS is also capable f turning an oval. https://www.rocketronics.de/en/els/

I don't think the oval piston would be a piece of cake at all on a normal cnc lathe. I would have thought that the spindle would have to be synchronised with the tool and that is normally only a function available in threading operations, where the movement of the tool is controlled for threading.

Oval pistons are normally done with some sort of cam machine.

 

[RJSakowski]

I don't think the oval piston would be a piece of cake at all on a normal cnc lathe. I would have thought that the spindle would have to be synchronised with the tool and that is normally only a function available in threading operations, where the movement of the tool is controlled for threading.

Oval pistons are normally done with some sort of cam machine.

The CNC lathes that I have seen in operation are capable of machining a crankshaft in one setup.

 

[Pylonracer]

The CNC lathes that I have seen in operation are capable of machining a crankshaft in one setup.

Ok they would do an oval then.

The ones I have seen and worked with have no hope of ovals.

 

[graham-xrf]

The CNC lathes that I have seen in operation are capable of machining a crankshaft in one setup.

I expect a CNC machine is capable of synchronously nodding the tool in and out to turn an oval, but that is modern stuff. They can make arbitrary shapes. Before CNC, production machines for making pistons have been used for decades - like back to WW2 era. They use some kind of synchronized cam motion that drives the cross slide or tool as the piston turns.

Additional to oval, the pistons have a taper to the skirt which is designed to straighten out as the piston heats up.
In this case, the clearance at the bottom of the skirt, in line with the wrist pin axis is supposed to be 0.008" (min) to 0.009" (max).
The clearance at right angles to the wrist pin axis is set at 0.003" (min) to 0.004" (max).
The "oval" nature is at the bottom of the skirt. It tapers away to proper roundness, the higher up toward the top we go. I do not know if the taper is even linear.

Among all those on the planet, there may be a domed shape piston with the standard 5/8" wrist pin, and made to fit what started out as 60.8mm (2.394" or 2 25/64). Adding the 0.020 gives bore 2.414" (2 13/32") . Then the piston would be made the tad smaller by the clearances amount. I (think) there was a Honda piston that would fit this engine, but I am still looking.

The supplier is OK that I return them, because they are unused, still in their box, and they are trying to find something that will work. Meantime, I have a couple of beautifully bored engine cylinders that can only be used as door stops!

 

[Pylonracer]

I expect a CNC machine is capable of synchronously nodding the tool in and out to turn an oval, but that is modern stuff.

It is specialised stuff, not a normal jobbing CNC lathe that is for sure.

You would definitely get the oval nice if you up ended the piston and milled the skirt and blended it into the true diameter, there will be loads of guys who can do that, just need to CAD it out and run it through a CAM program.

 

[RJSakowski]

I expect a CNC machine is capable of synchronously nodding the tool in and out to turn an oval, but that is modern stuff. They can make arbitrary shapes. Before CNC, production machines for making pistons have been used for decades - like back to WW2 era. They use some kind of synchronized cam motion that drives the cross slide or tool as the piston turns.

Additional to oval, the pistons have a taper to the skirt which is designed to straighten out as the piston heats up.
In this case, the clearance at the bottom of the skirt, in line with the wrist pin axis is supposed to be 0.008" (min) to 0.009" (max).
The clearance at right angles to the wrist pin axis is set at 0.003" (min) to 0.004" (max).
The "oval" nature is at the bottom of the skirt. It tapers away to proper roundness, the higher up toward the top we go. I do not know if the taper is even linear.

Among all those on the planet, there may be a domed shape piston with the standard 5/8" wrist pin, and made to fit what started out as 60.8mm (2.394" or 2 25/64). Adding the 0.020 gives bore 2.414" (2 13/32") . Then the piston would be made the tad smaller by the clearances amount. I (think) there was a Honda piston that would fit this engine, but I am still looking.

The supplier is OK that I return them, because they are unused, still in their box, and they are trying to find something that will work. Meantime, I have a couple of beautifully bored engine cylinders that can only be used as door stops!

I had suggested a CNC lathe because you might be able to find a jobber that has the capability. The lathe would have to have a synchronized spindle. nut it should be able to make the pistons provided you can furnish a CAD model.

Another thought would be a CNC mill with 4th axis capability. My Tormach 770, along with my 4th axis could definitely machine the exterior.. There may be some difficulty machining the interior surfaces though, depending upon the complexity. The mill might not produce as nice a finish as the lathe would but but it should be acceptable for your purposes.