Michael T.
  • Durham Region Ontario
  • Canada
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Michael T.'s Discussions

Rotax gearbox newbie

Started Aug 10, 2020 0 Replies

hi everyonecomplete newbie to the Rotax world ... I know thier motors are bulletproof...but was curious about the gearingim quite familiar with torsional vibrations and was wondering how Rotax deals…Continue

VIP or Viking In-Flight Propeller System

Started this discussion. Last reply by Jan Eggenfellner Jul 24, 2020. 1 Reply

I was watching your most recent videoCan this system fully feather the prop?…Continue

Can you modify the banana shaped flap slot?

Started this discussion. Last reply by Daniel Niendorff Jul 13, 2020. 3 Replies

hi guys I'm building a CH640 so I don't know much about the high wing members of the familyIs the flap slot open on the fuselage side....on all models?.... I was curious if something can be done to…Continue

Wanted to buy anything CH-640 or Alarus related

Started this discussion. Last reply by Geoffrey Alexander Oct 1, 2020. 3 Replies

I already have plans...if you have anything Alarus CH-2000 or CH-640 related..please get in contact...be interested in talking to you about buying it..ThanksMichaelContinue


Michael T.'s Page

Latest Activity

Michael T. posted a discussion

Rotax gearbox newbie

hi everyonecomplete newbie to the Rotax world ... I know thier motors are bulletproof...but was curious about the gearingim quite familiar with torsional vibrations and was wondering how Rotax deals with themfew questions:why did rotax stop selling the c gear box... was the rubber donut system not work as well as the newer clutch systemi was trying to figure out the new gear boxes on the more powerful rotax engine..  they are using a clutchis this how they are dealing with the torsion issue and…See More
Aug 10, 2020
Michael T. posted a discussion

VIP or Viking In-Flight Propeller System

I was watching your most recent videoCan this system fully feather the prop?https://www.youtube.com/watch?v=_Lnq4sb-BDQ&t=64sThanksMichaelSee More
Jul 23, 2020
Michael T. posted a discussion

Can you modify the banana shaped flap slot?

hi guys I'm building a CH640 so I don't know much about the high wing members of the familyIs the flap slot open on the fuselage side....on all models?.... I was curious if something can be done to close up or modify the flap slot...I thought it might get drafty in the winter..I saw a you tube video of a plane that had removed it by making the flap shaft internal to the wing...is that a common modification?if you guys could educate me on this item...much appreciated...MichaelSee More
Jul 12, 2020
Michael T. posted a discussion

Flying without a cowling

Hi everyonereally informative forum...I learning a lot going through all the posts..Ive noticed on Vikings videos that flight testing has been done with no cowl and other members mention having flown with no cowl...i was was wondering what the fuel penalty is for not having a cowl ondoes the plane handle much differently if you get caught in the rain, is there any hazard to the engine  was thinking if flying with no cowl is no big deal(water cooled engines).....you could get into the air sooner…See More
Jun 27, 2020
Michael T. posted a discussion

Is there a tips and tricks section on the forum?

was wondering if maybe I missed itdoes this site have e tips and hints section?...See More
Jun 24, 2020

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Ch-640 Items

Posted on July 6, 2018 at 9:30pm 0 Comments

Posting some pictures of recent work done on the plane

Rudder ribs and formers for the wings, flaps, ailerons and elevator

Comment Wall (25 comments)

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At 11:02pm on November 5, 2018, Mark Burrows said…

Hey Michael,

Thanks! I do still have it. I had to put it on hold for a bit, but will jump back in here shortly. I should be posting more in the near future.

At 2:14pm on November 2, 2017, Scott Stewart said…

Hi Michael,

I do have a materials list for the CH640.  I had to develop the list myself from the plans, because it did not exist when I started in 2008.  Also, I developed my own part layout drawing for the same reason.

I'm happy to share what I have.  Please contact me directly at scott_stewart@keysight.com.


At 7:42am on November 2, 2017, Greg Harris said…

Since I haven't seen the CH640 plans, it's tough to know what kind of brake you'll need. Based on my experience with the STOL CH750, I'd strongly advise against building a Dave brake. He built a 601 with it, and it probably worked okay for that. I broke my Dave brake twice trying to form the upper firewall stiffener, which is .063" and 893mm long. I used a metal piano hinge from Home depot, and the rivet failed the first time. I repaired it with solid rivets and the second failure occurred when the piano hinge metal just stretched away. Even if you use a quality stainless steel hinge from the aircraft materials suppliers, I don't believe it will hold. Other builders have had similar experiences. I built mine 8 feet long, and that wasn't long enough to do my tail section spars (each of the 3 tail spares was almost 9 feet long). I have access to a 10' industrial brake with a modified shoe, so that it bends at a 1/8" (3.2mm) radius. I repaired and modified my Dave brake several times before I could do most of the plane, but it's still not the right brake for the job. I can't bent the thicker fuselage channels and the wing trailing edge skins.

You'll definitely want to look through your entire set of plans and determine what the longest bends are in specific thicknesses.  Around 1000mm of .063 will give you a lot of trouble on a Dave brake. If your spars/spar caps (likely in .040) require long bends, you'll need access to a brake that can handle that. Dan Hill has an excellent video series on building a HomebuiltHELP brake (free plans on their site). I'm assuming the CH640 has a lot of similarly formed components as the CH750. If so, you will have a hard time building a single brake that can do everything. The HomebuiltHELP brake (especially as Dan built it), will do probably 99% of the plane, minus the longest bends. 

The Dave brake is easy to build, but if you're going to scratch-build an airplane, building the HomebuiltHELP brake is an excellent project to start with. When my project is done (I only have a few skins and my cabin interior structures to form), I'm scrapping my entire brake. If/when I start building the CH640, I'll build up a brake similar to the HomebuiltHELP brake, but I'll also have a hydraulic press brake. In another thread, one of the builders  said the longest bends in the CH640 are the spar caps in .040, and they're almost 11 feet long (3218mm)! I am almost certain you will not be able to bend those on a Dave brake. Further, the Dave brake has no adjustable set-back, so you can run into trouble with different thicknesses and bending acute angles with it. If you have access to other brakes regularly, then a Dave brake might work for you. I would check out Dan's brake build series before you commit to a brake, though.

Sorry that was so long, but it's a complicated issue, and I'm planning on doing a video that addresses all of this. I get a lot a brake questions, so a brake video would seem prudent.

At 1:54pm on October 30, 2017, Scott Stewart said…

Hi Michael,

When I bought the plans, Zenair included a CD with lots of pictures. They also include a very detailed rudder assembly manual which describes the rudder skin drilling.  Did you get this CD?

The idea is that you should lay out the hole pattern on the rudder skin and drill undersize (3/32") rivet holes in the skin. The rudder skeleton should only have centerlines drawn on the spar and rib flanges. You then lay the skin around the skeleton and line up the pieces so you can see the drawn centerlines through the rivet holes. Once everything is aligned, you match drill the rivet holes into the spar and rib flanges, and then drill all holes to full size.

You use this same technique throughout the build: layout and drill undersize holes in the skins, align to center of ribs/spar, final drill assembly.

I made matching right and left blocks for my wing ribs. Every place I needed a forming block, I made two in order to firmly secure the aluminum. For those pieces, like ribs, where I needed to make left and right, I put fluting notches into both blocks. In single pieces, like the firewall, I put fluting notches into just one.


At 2:41pm on October 26, 2017, Scott Stewart said…

Hi Michael,

You will want to make as smooth of a curve through all the points as looks good.  This may require you to offset from the points giving by +/- 1 mm.  The smoothness of the curve is more important than the actual measurements.  The lift performance will not change if the measurements are +/- 2 mm, but the drag will increase if the curve is not smooth.

You will notice on the wing ribs and the curved bulkheads that you will need a little artistry to achieve a good curve, using the numbers provided as only a guide.

-- Scott Stewart

At 2:20pm on October 25, 2017, Scott Stewart said…

Hi Michael,

Do not use the "~305" dimension they call out on your drawings.  350 +/- 20 is what you want.  I used 350 and this is how it looks completed:

Notice the position of the lightening hole relative to the rivet holes on either side.  If you used 305mm, you would overlap the rivet holes.


At 2:07pm on October 25, 2017, Scott Stewart said…

Hi Michael,

It looks like Zenair revised the drawing.  On mine they showed "~405". I crossed that out and wrote 350.

For the tight Z-bend, you need to cut two matching pieces of wood to use as the forms.  For example, get two 2x4 pieces that are one foot long.  On each one, cut a 3/8-inch step down the full length into half of the long dimension.  The steps on each piece match together.  You then use a vise to smash the aluminum between the two wood form block.  The aluminum will bend around the wood step.

For smaller steps that needed to make later, I welded together two 1/8-inch steel bars shown here:

The aluminum gets pressed between them to form the bend like this:

At 12:11pm on October 23, 2017, Scott Stewart said…

Hi Michael,

Yes, I am still on the forum.

All in all, my plans build went fairly smoothly.  There were several parts that were dimensioned incorrectly, and I needed to remake them, but there was nothing I could have done differently about wrong dimensions on the plans.  I listed these incorrect dimensions on http://wiki.matronics.com/wiki/index.php/Complete_CH_640_Errata.  Some of the parts they call out are off, like the trim cable is the wrong length.  Also, I had to redesign the flap control a little because Zenith wanted to charge $900 for a $100 linear actuator.

As you progress through the build, keep me updated.  You will need to make or borrow several specialty tools.  For example, I made a series of hole flanging dies for the lightening holes.  You will find that many people are willing to lend out tools like this for serious builders.

Happy building,


At 1:20pm on October 18, 2017, Greg Harris said…


I haven't shown a lot of measuring and drilling in my videos. I haven't done a rudder assembly video either, because HomebuiltHelp did theirs. However, my Horizontal Stabilizer video series shows that my skins were pre-marked and drilled before drilling the skeleton underneath using the method I described. I really recommend just to get going on the rudder components and finish the rudder assembly before starting anything else (you'll need a bending brake, though). It really will teach you most of what you need to build the rest of the plane. Hope that helps!



At 1:16pm on October 18, 2017, Greg Harris said…

Okay, we are looking at the same Construction Standards book. From your explanation, you're reading the standards correctly. The small "l" does mean length. However, it sounds like you're a little overly worried about them. In practicality, if you make a 3650mm spar to 3650.5 mm, you're within tolerance. However, it's a simple matter to file off that last .5mm so you are closer to perfect. That particular graph at the top of CS#300 is strictly for shearing and cutting raw stock to length, which says nothing about finish filing or the the width of the stock or spar webs/ribs, etc. You would never want a finished part to be 8mm off over a 1m distance. In the above spar length example, that'd mean your spar could be +/- 28mm!

The other graphs are for things like angle variation tolerances, bend radius tolerances, etc. From a practical fabrication and assembly standpoint, all of those are pretty simple. If you bend an angle to 15 degrees, and it needs to be 16 degrees, you can simply adjust it closer so that it mates properly with whatever surface it needs to mate to. If it is only off by a half a degree or less, it will probably be fine to mate to other surfaces, since most of the metal is thin enough to flex into position. Remember, we're not building rockets or fighter aircraft. In reality, these structures are much easier to build and assemble than they seem at first. I went through the same apprehension too!

For the tolerances we are most worried about as scratch-builders, check out the CH 701 construction manual, which was written with scratch-builders in mind. There really weren't any pre-fabricated kits when it was introduced. Most kits back then were just raw materials kits. At the top of Page 5, look at "Working Tolerances." Tolerances are much more critical on the formed/finished parts, and the 701 standards manual basically sums that up nicely for our needs, even on the 750 and anything else. The big Construction Standards for Zenair Light Airplanes is an awesome reference, packed with great information. However, it goes much further into depth than we really need to build a any of the Zenith airplanes. The most important things are that your mating parts match each other closely (e.g. spar height = rib height, etc.), and that they are not too undersized (weaker structure), and that they are not too oversized (too heavy).

Lastly, for the rudder skin, the way I do skins is NOT how some people like to do them. Some folks like to pre-drill their ribs first and then use strap duplicators to drill the skin holes. I only do that where I absolutely must, which is not very often. My preferred method is to test assemble the undrilled skin to the cleco'd or fully assembled skeleton to make sure it fits. Then I measure the skin for the center-line of each rib flange or spar flange (or whatever else). I take very careful measurements and determine where the rivet line will be on the skin. I then pre-drill the skin with an A3 bit. After that, I mark the center-line of the flanges and other places where the rivet lines are supposed to be on the skeletal structures. I use them line as sight lines through the pre-drilled skin holes to line everything up. Once the sight lines are aligned perfectly to the skin holes, I drill and cleco as I go, usually starting at the spar rivet line. I then go back and final drill for rivets once everything is lined up and cleco'd. This is how the older kits were assembled when you had to do a lot more drilling, but skins were pre-drilled with sight holes. It's a little extra drilling to do it that way, but it works well for me to make sure everything is square and fits together properly. This method is what works best for me, but everyone has their own approach.

I haven't shown a lot of measuring an


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