Zenith Aircraft Builders and Flyers
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Added Head Room For An Old CH 701
(Last Revision May 20, 2012)
The added roof space on the new CH 750 at SNF 2012 attracted great attention.
What about CH 701 community?
There are at least two members of this group that have built/modified their CH 701 with added head room, as well as a smoother air flow over the roof at high angle of attack. Both members I know have already enjoyed flying their planes. Mine is unfinished, untested and untried. I am showing old photos together with newer ones. With wing folding kit installed, showing the horizontal yellow tube, I could not just follow the Beanie Mod of Joe Spencer, and so... I did it my way. Comments are welcome.
Of course, to do this mod., the wing roots need to be rebuilt with new Rear Root Ribs and new Wing Root Top Skins (that you make them yourself. I live too faraway from most of you to offer a kit. Freight charge can be uncomfortable. I can advise one on one if needed). You will need a 3' x 4' sheet (1.5' by 4' for each wing) of wing skin, and 3' by 4' sheet of wing rib material (1.5' by 4' per wing, with excess material) at the start. Kansas Airparts, for one, can cut 3 ft. length off their rolled aluminum sheets for you.
Roof modification without welding
Living and flying with the long aluminum diagonal on roof is an easy choice for most. No need to take the cabin tube frame off the fuselage for cutting and welding, or risk the trouble of welding it in position. WIth wings off or firmly supported, cutting the aluminum tube at 120 mm from each corner starts the transformation. More on this tube later.
Forget the acrylic bubble windshield. A 1/8" thick one-piece windshield and roof polycarbonate sheet doesn't crack as easily, while not as expensive. It won't restrict the roof curve as does the bubble windshield. The new windshield now continues to go over the roof in a very smooth curve up to 90 mm above the original roof. An inverted L angle of 20mm X 20mm at 7 millimeters above the front cabin tube helps the windshield to sit snugly in its place on the first try.
I did not quite place the Left Roof Side Rib on the exact center of the left roof side tube, and had to relocate the rivets.
The rough sketch below explains how I shaped the Roof Side Rib. The inverted L angle cut short 3" at each end helps the windshield to curve easily to its left and right edges without heat forming required. My coordinates for the curve over roof side tubes that you may use to start your cardboard templates are:
0-0, 100-57, 200-85, 300-90, 400-78, 500-55, 600-32, 700-6, 720-0
You may adjust the first and the last points as you see fit.
Note 1. If the inverted L sits too low for the rivet length, you will not be able to install the rivet properly.
Note 2. If you want to use the bubble windshield since you have invested big money, you can lower the height of the curved up roof to keep a smooth curve from the middle of the bubble windshield to the roof, or you can still keep the 90 mm high curved roof for a higher efficiency and live with a small rolling at the joint of windshield and roof.
The Wing Root Ribs have to match the curved up roof, but just a little bigger to allow Wing Root Top Skins to overlap the roof sides. Picture below shows the left wing root with wet wing tank unfinished. The hole in Rear Rib 1 is for fuel sender off a pick up truck. You can guess it right that making wet wing slows me down.
Back to the cut-off tube on the middle of the roof.
Press the middle of the cut-off tube against a narrow work bench or the corner of a low table, then hand bend it so that the middle gets 70mm off a straight line. I did mine at 45mm since I planned to have a narrow rib on top of this tube at the highest point. 100mm length of curved steel tubes are inserted at both ends. The curved steel tubes mentioned are steel conduit that are a little bigger than the aluminum diagonal cross tube. I flattened them a little to fit the aluminum tube inside for riveting 6 of A-4 on top and bottom while filling the void between the tubes with epoxy steel filler the best I could for a firmed joint.
Roof support along the diagonal cross tube. The curved steel connector 100mm long (a comfortable length for my hand) at the cockpit front left corner will be used as the pilot's handhold. Same length of connector is used at the rear right end.
Showing two roof supports, with the twisted Z angle in the middle.
With polycarbonate roof installed. Looking from left to right.
Close-up of the twisted Z angle of .025" 6061T6 joining the diagonal cross tube and the middle roof thin rib, looking from left to right.
From right to left. Another leg of X on this 2mm roof is not necessary unless flight testing proofs otherwise. The curved roof feels firm to the touch as is.
My inverted L angle over the cabin front tube. AN-3 bolt in the middle holds the wing-fold cross tube (painted yellow in picture).
On Acrylic and Polycarbonate
Acrylic has been flying fast and high with jet planes for so long but I prefer polycarbonate to acrylic sheet for my small plane. Properties of polycarbonate can be found on the internet for those interested.
Roof modification with welding
I take off the three aluminum tubes on roof frame, then weld in the steel side tubes and short diagonals at the two front corners. After seeing it done at CH 750 Forum (similar to CanZac made cabin tube frame for CH 750), later I will add a lower cross tube at the 'lower edge' of instrument panel, and another one joining the rear ends of roof side tubes, with a short diagonal, between the side tube and rear tube, at each end.
My diagonals needed to be cut and extended a little. Continuous welding (became too hot and) caused a pull inwards of the side tube rear ends.
This won't be too much for scratch builders. Hopefully, this mod will give enough elevator control on power off approach from flare to touch down.
Champ
Comment
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Comment by Ron D Leclerc on April 27, 2012 at 10:14am
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G'Day champ...
Are you using metric thickness for the PC? I checked for what is available here and I can get 1/8"(3.3mm), 0.093"(2.4mm) or 0,08"(2.0mm). I would use the 1/8" for both thr roof and windshield... I was also thinking of a butt joint! The thiness PC is 0.06"(1.6mm)... I would use for the doors!
They use 1/8" for the 750 roof and there is no spec for the 701 roof!!!
Ron
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Comment by Chumphol Sirinavin on April 27, 2012 at 1:01am
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My second thought is butt joint will be better without the bulging of the overlapped edges and you don't have to look for rivets with long grip. I will try 3 butt joints with 2 mm. scrap pieces, using hot glue, epoxy and dichloromethane.
A friend suggested (rather late for me) that 3 mm is much more stronger for windshield.
Champ
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Hey champ...
Just looking at your beany mod... and what do you do with the joint between the windshield and new roof, is that a butt joint or is the windshield overlaping the new roof/ How thick is the new roof... I checked out my drawings and Zenith did NOT indicate a material thickness. Just the door(0.04) and window(1/8")!
Ron
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For CH 701 roof mod without the diagonal tube, see my comment on Sebastien's blog New CH 750 Features.. My first CH 701 roof will remain with the bent up diagonal tube as shown above.
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Comment by Paul Saccani on April 22, 2012 at 10:08pm
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With regard to an air balance tube between the wings. This strikes me as a useless exercise, which under some cases (like unbalanced flight), could cause loss of fuel via flow from a high side tank to a full low side tank and overflow.
Varying fuel flow from tank to tank would have much more signicant elements (fuel level, hose routing, unbalanced flight, vapour locks) than ram air pressure differences, so I would suggest that it would just be weight put to no good purpose.
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Hi Paul,
Please see my comments on Sebastien's blog New STOL CH 750 features introduced at Sun'n Fun and look at my pictures there for my cabin roof frame mod.
Actually, I used 2mm polycarbonate and I do not think the curved up roof will ever cave in. Now, If you use 3mm, you will have a very strong roof with no tubes or angles to obstruct your view through the roof, like the 3rd edition 750.
Best,
Champ
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Paul,
My internet connection is rather erratic today. I could not attach pictures for you. I will have to try with a direct email later.
Dave,
Just imagine that each of your wing tank has two outlets, one at the front and one at the rear. one of them or both will keep fuel supply to the engine, the front one at nose low attitude and the rear at nose high attitude. If you have fuel hoses from both wings connected at a common low point, one of the wing tanks if not both will keep fuel flowing. For unbalanced fuel between the two wings, a T-connector at each wing root before going through a valve will give you choice to use fuel from which wing or from both. The balance tube you mention might work, but I need some time to think it over for all flight attitudes possible.
Best,
Champ
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Comment by david shrader on April 22, 2012 at 3:01am
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I love it champ. I keep thinking there has to be a way to incorporate a balance tube for tank venting now that the wing profile is carried thru over the cabin. Looking at your pics it appears the beanie mod. almost allows enough room (height) to keep a balance tube from being a low spot. I know your tanks are wet but if they were standard could a tank vent balance line make it straight across from L/H tank to R/H tank without having to drop down at the cabin? I've read that fuel flow rate is different between wings (fuel imbalance) because there is a slight difference in ram air pressure between L/H & R/H tanks, the easiest solution would be to install a balance tube between them if there isn't a low spot where fuel could block the line.
Dave
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Yep, plenty smart. I was thinking of just drilling new hole where the windscreen contacted to the tube, but it is much smarter to spread the stress with an L angle. Perhaps a 2 or 3 cm strip of 20 thou between the windshield and tube, at a tangent to the contact point might serve as well for stress relief, for less weight?
When I do a major overhaul next, I think I will be making a new 7F18-1SP (cabin frame) and deleting 7F12-2 (cross tube). On 7F18-1SP, I would delete the diagonal tube and entry handles and extend the 4130 upper tubes to 250, with a diagonal tube of 3/4" 4130 from 200 to 200 from the corner of the upper tubes and the centre tube of the cabin frame on each side. Much like the 3rd edition 750, needless to say..
I have a stupid question - on the wing, the ribs do not act in isolation, so the upwards force on the skin does not tend to pull them towards the sking. On this one, the cabin root ribs seem to have a fair bending moment on their attachement to the upper tubes of the cabin. Is the curve of the 3mm PC over the ribs going to provide enough strength to stop those ribs bending inwards? Polycarbonate is a good deal more flexible than aluminium, but on the other hand, that is quite a thickness. Please forgive the curiosity.
Regards,
Paul.
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G'Day Paul,
If without this inverted L angle, the upper corners of the windshield will be more stressed with a smaller radius of turn.
Have a nice day.
Champ
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