The construction methods of the new CH-750, are a horse of a different color

I wrote a small article a little while back, that has just been published in the latest edition of the " Zenith Newsletter ". Since not all of the Zenith Aero members have as yet signed up for this excellent Newsletter, I though it might also be of interest to post the article on " Aero " so that all of you could read it. Although the original article was reduced because of space issues, the following is the complete article, as it was submitted.
Different assembly methods

Congratulations Mr. Heintz, the 750 really is a horse of a different color.

I was well into the process of scratch building my 701, when I first started hearing about the new 750. The only way in which I was able to obtain any information, was by visiting their webpage multiple times a day. In doing so, I was finally able to link up to a hidden page, that turned out to be an order sheet for the new CH-750 model. This happened in the week preceding the 2008 show in Oshkosh, and I then realized that the reason they had not mentioned anything about the 750, was because they were keeping all the " Hoop-La " for the shows opening day.

Since I was able to open the file and print out the sheet, I immediately filled it out and made sure that my credit card was in shape. Then, on the shows opening day, at exactly 9:01 in the AM, I called Shirley at the factory, and placed what was then to become, the very first retail order for a set of plans for the 750. I would have loved to order it in kit form, but having just recently reached the rip young age of 65, and living on a disability pension, the cost of a kit was not one of my options.

Since I did not have access to any other details regarding the aircraft, and from the sketchy details I could get my hands on, it looked at the time as though the 750 could fill a notch that had not been available, or possible with the 701. It had a much wider cockpit, increased gross weight, better visibility then the 701, and especially a much broader choice when it came time to choose a power plant. Over all, it looked to me as being a 701 on steroids with a little extra room. How wrong could I have been.

Now, in order to explain to you what motivated me to write all this, let me start by getting into the nitty gritty. I am in the process of building the tail feathers for my 701 ( for a photo, check out the " Rudder Album " ) and during that time I couldn't help but follow along the 750 plans, to see what differences there were in the material, and the assembly process. In the beginning what I noticed was that most of the parts have been beefed up, and in some cases, dramatically improved as far as the fabrication and assembly process goes. What stood out the most in my mind, and is the reason I took the time to write this, is how easy Mr. Heintz has made it to assemble the kit, or for that matter, scratch build the aircraft.

For starters, let me explain to you a few of the changes, and things that stood out.
In the 701 model, the trailing edges of all the flying surfaces, are joined together at the rear using a large quantity of soft rivets. This in itself, dictates the use of multiple parts, not to mention additional tools, along with a substantial investment of time, and energy in order to accomplish the operations. In the case of the 750, rather then force us to go through the same riveting process, Mr. Heintz has re-designed the trailing edges of every flying surface, so that you only need to bend the aluminum sheet over onto itself, and it automatically becomes the trailing edge of every part. This applies to all flying surfaces, including the slats, flaperons, stabilizer, elevator, even the main wings themselves. However since we can't always improve on a good thing, in the case of the rudder, they are almost identical with only the thickness of the rear spar, and the lower horn being different. By looking at them once they have been assembled, you cannot tell them apart.

We now come to the assemble process, and because of time and space, I will only comment on the stabilizer, and the elevator. As most of you builders are aware of, the frame of the stabilizer on the 701 consists of a full length rear spar, a shortened front spar, and two full end ribs, that are initially only attached to the structure with a few small rivets at the outer edge of the rear spar. Thse end ribs, are an one area where in the very least, I would have liked to see the use of A5 rivets. The reason is because these two thin rib's are all that hold the outer elevator hinges to the stabilizer, and I believe a little extra rivet strength in this area would certainly not have been over kill.

I have no engineering background, but in my humble opinion, the structure that was chosen for the 750, although similar in design, has a much better chance of being a strong, rigid platform to which the outer skin will be attached. The other parts are quite similar, however the 750 has a front spar that is the complete length of the stabilizer, and once the frame is all together, there is quite a difference in rigidity.

However the 750's strong point, is not in the frame itself, but rather the skinning, or covering of the frame. For a better reference, we will start with the designers recommended process for the 701. In this case, the factory ( plans, instruction manuals, etc, ) recommend that the builder start by attaching the skin to the round lower surface of the stabilizer, and once clecoed in place, turn the stabilizer over so that the skin can be pulled back far enough over the frame, to reach the rear spar. For those of you who have not as yet gone through that process, I guarantee that it is not something that is particularly easy to accomplish.
The reason is because of the initial bend that is given to the skin. The skin will easily follow the curved lower stabilizer section, but it is extremely difficult to try and have the skin lay down flat , especially following the bend in the first few inches of the top leading edge. Builders use different methods, including boards, 2x4's , and what ever they can lay their hands on, but the fact is, the skin just doesn't want to lay flat in this area. I have seen more then one stabilizer, damaged beyond use, because of the extra pressure that was put on the straps to try and get the skin to lay flat.

Now comes the 750 skinning. Here the designer did a complete about face, and completely reversed the skin installation process. The newly recommend procedure, is that the builder, first attach the skin to the top , flat section of the stabilizer, and that using the strap-ratchet procedure, wrap the skin around the curved lower section of the stab. Well guess what, in going about it in this way, it becomes so easy to get the skin in it's proper place, one could almost do it without the use ( although I don't recommend it ) of any straps what so ever. My thinking is that when it is done in this fashion, the process is made much easier, because the difficult top, flat area is tightly attached to the frame, before any bending pressure is applied to the aluminum sheet. I feel obliged to apologize to those of you who find it difficult to follow my some times ill chosen words here, but I promise you that you will only have to attempt it once, to appreciate the difference in the difficulty level this will have, when installing your skin.

All this being said, I most certainly do not want to imply that you should all go out and start building a 750 ( although that would really be cool :o) but to rather look seriously into the new construction methods, and techniques that have been implemented for the 750, and when applicable, use these methods to facilitate the building process on your own 701 project. Personally, I have incorporated most of the changes in the build process for my own 701, and it has made the assembly much more user friendly then was originally thought it could be. By the way, you do not need to have access to the 750 plans in order to compare the construction methods, since the 750 photo manuals are being published as they are completed, on the main Zenith " builders " website.

One last part that I would like to address is the elevator, and this will certainly be of interest to those of you who have, or who will, eventually need to skin their elevator. Everyone is aware of the fragility of the bare aluminum sheets in the .016 thickness especially when one is alone to handle it. It is easy to bend, and " smiley's" seem to appear from nowhere and immediately attach themselves to any large sheet that has been moved around indiscriminately. In the case of the elevator, the sheet is almost eight feet wide by three feet in developed length, so if you blow it, there goes another 4' by 12' sheet of pricey aluminum.
In order to install it correctly, one must first bend it the proper distance, not to mention the right angle, and then with the care that is given to a new born child, wrestle, as best as one can, the flimsy sheet around the shaky elevator frame, all the time, being very very careful so as not to be attacked by a smiley. I guarantee that you will be more then a little grateful to have some extra hands around the table when the time comes to install the skin.

Next enter the case of the 750 stabilizer. Although it is slightly bigger, and incorporates a beautiful trim tab as part of it's structure, the skinning can be done by the builder himself, and with out so much as a dent or blemish, of any kind. Why you ask, well it has become relatively easy, since the skin is in four different sections, making it much easier to handle, not to mention that the two largest sections are now of the thicker .020" aluminum. By having the skin in four different sections, when the time comes to cover the frame, each individual piece can be precisely adjusted, one at a time, and it makes for an extremely smooth, blemish free surface.

IMPORTANT, please read carefully

To those of you who might be interested in covering their 701 stabilizer using the 750 procedure described above, there is one change you will need to make in order to insure the skin is bent in it's proper place. At the upper right hand corner of page 7-S-O, you will find a small box indicating the H & X values necessary to correctly bend the aluminum sheet for the various parts. In order to get the exact, very precise bend for the stabilizer skin, you will need to change the X value, from 19m, to 60m.

If done in this way, and skinned "a la" 750 method, you have my personal guarantee that the aluminum skin on the front nose ribs, will fit so tight, and wrap around so very close, that it will be impossible to insert a knife blade between the skin and the rib itself, even before the parts are riveted together.

During the installation of the skin, I have also devised a small trick to keep the nose ribs from protruding through the skin, marring the leading edge, and making those small unsightly bumps that are always seen after skinning. I take a 2 x 3 " piece of .016" square aluminum, and before covering, tape it across the front of the nose rib's. When done in this way, the additional surface area keeps the dents from forming, and gives you a beautiful clean, leading edge.

As I mentioned previously, I am certainly not implying that everyone should go out and start building a 750, but rather look seriously into the construction methods that have been put in place by Mr. Heintz for this new model, and when ever possible, use the changes that might be beneficial in the construction of your own 701.

Sorry for having gone on for so long, and thank you for having taken to read through my little story.

Happy building to all of you, and don't forget that getting in some quality time with the family, is also part of the building process.

Robert Pelland
Trois Rivières, Québec,
the 701 & 750 Scratch

robert.pelland@cgocable.ca

www.autoretro.info

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Hi Robert,
I like you purchased the 701 plans before the 750 came out. I am considering buying the 750 plans and building it because my engine choice is a corvair, which will work in a 701 but it is marginal. One question; what is the longest bend in a 750? I just completed an 8 ft brake and would love to think that I wouldn't have to rebuild it.

Thanks,
Scott Sutton
Hello Scott,
So far the longest bend I have had to make is the rear spar in the main wing. Unlike the 701 It is a one piece spar, but because of the length, it is built in two parts, and joined together near the area where the rear strut attaches to the wing. My I ask who brake did you build, was it Mac's http://www.macsmachine.com/, or Dave's http://www.ch601.org/tools/bendbrake/brakeplans.pdf.

I also built a brake from plans , http://www.ch601.org/tools/brake_plans.htm, but ended up purchasing a professionel brake from " Brown & Borg.

As for your choice in aircraft, you would be far better served with the 750, rather then the 701. Everything connected to the 750 aircraft , is a big improvement over the 701.

If there is anything else I can help you with, please get in touch with me at my personal e-mail address at the following. robert.pelland@cgocable.ca

Good luck with your build
Robert
Hi Robert,
I like so many other guys built my brake into my work table. It works like a conventional brake in that it has center rotation and 2 hinge pins that the bending leaf rotates around. This is a little unusual for a home built brake that is 8ft long. Most guys use piano hinges or multiple hinges to accomplish the rigidity needed for the brake and sacrifice center rotation. This is obviously not a problem, I just wanted to see if I could make it operate like a conventional brake. The upper leaf (pinching leaf) is located to give a 1/8" radius and is spring loaded so that in normal position material can be inserted. I am very satisfied with the brake, it produces a very straight part and I can make bends over 90. However, If I were to build it again, I would make it free standing and make it out of all steel. Incorporating it into the work table was much harder than expected, I have allot of steel bracing under the table to prevent flexing.

SS

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