I wonder is there a 440?

Quoted from PUFF

I wonder is there a 440?

Yes there is. same bottom end and top end. Just bored out and bigger piston.

Cool. I just checked the other thread for the site. There's even a 48 HP 440-B... and even bigger 440-C....  NICE

Engine Mounting

NICE! might save a bit of weight by making some aluminum blocks to substitute for the stacked washers...

Quoted from PUFF

NICE! might save a bit of weight by making some aluminum blocks to substitute for the stacked washers...

That's a good idea. It is surprising how much those little washer add up to.

There might be a problem with the washers. The engine puls the plane so the force on the bolts through the washers is perpendicular to their axis. The washers don't provide any support in that direction so the holds will flex and the bottom of the bolts will apply a large side moment on the intermediate wood plate. The design was not build to resist that moment and I am concern that the bolts holding the engine will brake that plate therefore your engine will depart the plane.
This is particularly dangerous since the damage is unobservable as it will start from the bottom of the intermediate plate and when reaching the top will be hidden by the washers.

Let's say the thrust is 200 lbs.  The force on each bolt is 50 lbs.  The bearing stress on a bolt hole is the force divided by the projected area of the hole so for a M10 bolt in a 3/8" thick plate the stress is 337 psi.  The maximum bearing of birch ply parallel to the face grain is 3350 psi.

@Bob Daly, I beg to differ.
The stack of washers allows lateral movement so the bolt can pivot around the nut below the intermediary plate. The moment of that pivot is F_thrust*A, where A is the stack height. This moment is then applied at the bottom of the plate by the washer below the plate. If the washer radius is B then the upward force is F_thrust*A/B and that might be quire high at that point. I don't have all the data to calculate it but I would be concerned.

If the stack of washers is replaced by a solid spacer, then your calculation is correct.

The washers work similar to the solid spacer in that they maintain the parallel faces of the plate and engine mount.  This is important because the bending moment put on the bolt is shared by the bolt holes in each.  The bolt bends twice.  The bolt can be treated like a cantilever beam.  Then the total bending moment on the bolt is 50 inch-lbs if the washer stack is 1" high.  The moment at the mounting plate is then 25 inch-lbs.  This moment must be reacted by the plate hole. If the plate is 3/8" thick, the force is 66.67 lbs.  Then the stress is 450 psi, still well below the 3350 psi allowable stress.  

1. I don't agree that the force is equally divided by the 4 bolts since there will be only a couple of them sharing the load in worst case scenario. The other 2 will start bearing load only after the wood board will give in a bit.
2. In the failure mode, the force is perpendicular to the wood board, not parallel.
3. The washers are stamped not milled. Therefore, they allow a bit of vertical movement, meaning that the bolt only needs to flex a bit, forming a single arc, not two.

This is a very interesting discussion.

My major point is that the failure mode is different than the design mode. It might be that it's still within the bearing tolerance of the wood plate. The correct design would be to eliminate the spacer or as an alternative to have a solid spacer.

Another point is that the stack of stamped washers will vibrate and lower the clamping force of the bolt. That bolt needs to be secured to prevent it from unscrewing.

Having said all that, this is a very fine build thread! Great of fine workmanship. I don't want to trash the thread with this discussion anymore.

Isn’t it easier if the builder make a square piece of plywood that fits under the engine block? I believe a plywood would be even lighter than all those washers.

Assembled and trial fit the tuned exhaust. 17 inches right on the money from the exhaust port to the tuned pipe.

My Dad used to call this "chicken shit welding" cause it looks like chicken shit. But it will work.

I cant weld good enough to get paid for it but I can weld good enough that I don't have to pay someone to weld it for me!

As far as the stack of washers are concerned... They are per the plans. And as we all know, the Minimax is a well proven design. There have been hundreds if not thousands of them built all over the world. Some of them are 30 plus years old and still airworthy and flying today with bigger engines than mine. Please correct me if I'm wrong but I haven't heard of any Minimaxs crashing due to engine separation. But I don't want to be the first so... While we are on the subject, has anyone ever attached a safety cable to the engine in case of engine mount failure?

Tuned pipe mount.  Gonna have to cut that exhaust pipe off and move it about a 1/4 way round and weld a patch over the old hole.

Great work, toliver!

I hate to be the bearer of bad news, but I think you will have some problems with your exhaust once you've done a few hours.

The problem is that the first flexi joint you have is not until 14" down the manifold. This means there is a long lever arm, and subsequently high load, on that pipe joint, particularly where the downpipe joins the manifold.

I strongly recommend that you use a second ball joint positioned between the start of the bent pipe & the outlet of the manifold, shortening the downpippe to maintain the tuned length.

A bit of hassle to do now, but will save you a lot of tears in the future. Our engines jump around a lot, and I don't believe it will be long before that pipe cracks, with subsequent power loss.

Bruce

What Bruce said.

Quoted from beragoobruce

Great work, toliver!

I strongly recommend that you use a second ball joint positioned between the start of the bent pipe & the outlet of the manifold, shortening the downpippe to maintain the tuned length.

Bruce

I'll have to take another picture to show it but the y-exhaust manifold is as short as it can get. The 90 degree downpipe is also as short as it can get without making it less than 90 degrees. which just won't do. I appreciate your input, but I really don't see where adding another flex joint where you pointed out is going to make any difference anyway.

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Quoted from toliver66

I'll have to take another picture to show it but the y-exhaust manifold is as short as it can get. The 90 degree downpipe is also as short as it can get without making it less than 90 degrees. which just won't do. I appreciate your input, but I really don't see where adding another flex joint where you pointed out is going to make any difference anyway.

Toliver.

You are doing a really great job on your conversion. Your detailed photos will probably help other builders too. I do however agree with Bruce regarding the extra exhaust joints. Rotax did exactly that on their two-stroke engines many years ago. This was du to breakage of the exhaust systems. There is a link to their service bulletin here: https://legacy.rotaxowner.com/si_tb_info/serviceinfo/11ul87.pdf

Petter

Quoted from Petter Strand

Toliver.

You are doing a really great job on your conversion. Your detailed photos will probably help other builders too. I do however agree with Bruce regarding the extra exhaust joints. Rotax did exactly that on their two-stroke engines many years ago. This was du to breakage of the exhaust systems. There is a link to their service bulletin here: https://legacy.rotaxowner.com/si_tb_info/serviceinfo/11ul87.pdf

Petter

Thanks for the compliments.

I can't say I fully disagree, but if I add in another ball joint it will put the tuned pipe a further 3.5 inches out from the fuselage side and also raise it 3.5 inches. Increasing the length off the y-header will also increase the lever-arm (seems counterproductive to me), also raising the tuned pipe 3.5 inches will put the tuned pipe mount into the engine mount shelf/bulkhead area of the fuselage making the spring mount imposible to access.

Azusa aluminum rims vs Azusalite nylon rims. About a 1 lb difference saving 2 lbs overall. Im not sure it was worth the expense since I already had the aluminum rims. Oh well, gives me something to sell or trade at the next fly-in.

Weight of the brake drum is 11.1 oz. Almost a full pound. I'm going to replace these with disk brakes. We will see if there is any weight savings to be had there since the disc rotors are just 3.3 oz.

Varnishing the fuselage

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The plastic rims can break. I've broken 3 and replaced each plastic one with aluminum.

Quoted from Greg Doe

The plastic rims can break. I've broken 3 and replaced each plastic one with aluminum.

Yeah, I've heard that they may crack on a hard landing on the team Minimax facebook page as well. And then on the other hand I've heard that they are virtually indestructible. So not sure which ones I will go with at this point.

I started covering the fuselage today. Got the bottom done

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