This Q&A is now closed and replaced with a new Q&A based on the latest upgrade drawings at

To all 601XL and 650 builders and aircraft owners. If you have a specific question regarding the upgrade, post it here and I will consult with Chris Heintz so that we can give you the best possible answer. Please be specific about your questions.

When posting a question or replying to an existing comment, PLEASE add it to end of the last page.

Zenith Aircraft is upgrading its 601 XL, so a lot of questions are answered on the step-by-step Blog Posts on this main page.

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Nice work! Would you also replace the 6 round spacers 6W4-5 with 0.063 thicker ones (or shim them with 0.063 stock)?

P.S. I think you meant A5 and not A6 towards the end of your post
There is no need to replace or shim the 6W4-5 round spacers and the spreading of the spar caps is minimal.
Yes, this is a fine but not required. When doing this, make sure you remove the front wing uprights 6-B-13-1 first, which gets replaced with 6-ZU-2-4.
Craig, In reference to your question of 11-11-09

I would prefer to make new spacers .063 longer.
If you draw a .156 circle and then copy it vertically .063, you have a picture of the figure 8 hole pattern that you would see with 6W4-1 moved .063' in relation to 6B10-1 Cabin floor to accomidate the thicker 6ZU1-2. This would require a .191 hole to blend the two holes and accept an A6 rivet.
I think I will wait to see what Zenith comes up with before I make any changes.
These guys know what they are doing. Besides it's been many years since I studied Aeronautical Engineering.

My questions refer to the center spar and how it will accomodate the 'thicker' spar from the wings. I assume that the spacers within the center spar will have to be increased by 0.063 to accomodate the 0.063 wing root doubler 6-ZU-1-2. If so, then the center spar doubler 6-ZU-2-1 will also have to change dimensions a bit ((54 + 0.063 or ~55). Is this correct?

These are the only inconsistencies I see with the upgrade plans at this time. Forcing the thicker wing roots into the narrower center spar just doesn't seem rignt.

I also would be interested in the possiblility of using solid rivets rather than bolts to attach the extrusion angle 6-ZU-1-1 to the spars. I realize that bolting the extrusions to the spars is the easiest (and cheapest) solution for those who have completed wings. But one of my wings is not yet completed and rivets are MUCH cheaper than bolts. Unless the added strength of the bolts is part of the design...

And thanks for publishing the upgrade. I look forward to the final drawings.

I believe that both of your questions have been answered on the above few answers.

Since we have not released the final drawings, please do not finalize the upgrade at this time.
My 601XL is a conventional gear version. What effect does the more forward positioning of the main gear have on the planned modifications. I am particularly concerned with the addition of a diagonal Z angle forward of the center spar shown on drawing 6-ZU-3. This change will need to be different on the conventional version due to the 4130 tube steel landing gear mounts that run vertically up the fuselage sides forward of center spar.
Yes, for the tail wheel version, the Z angle needs to be moved so as to not interfere with other items such as the steel tube uprights. Also make sure that the angle does not interfere with the bottom spar bolts. The exact location of this Z angle is not critical.
The final version of the upgrade kit will not include extra ribs in the wings. We may add "L" angles to the top, between the main and rear spars. The upgrade drawings already calls for "L" angles at the bellcrank area.
I would not recommend closing anything until the final upgrade drawings are released.
Yes, Chris did design a spring system that connects to one of the elevator cables. It adds about 10 lbs to the stick load at full deflection. However, we have not finalized the flight testing of this system. I hope that we can offer this as an option in the near future. Since it will go into the fuselage, the installation will be very easy and inexpensive.

Note that with the stronger airframe (once the upgrade is done), excessive elevator input will not stress the airframe as much.

If you are going to consider adding L-angles, why not go a bit further and add true stringers like the Z-angles that CZAW put into their SportCruiser wing? It seems to me that a continuous Z-angle stringer would add significant strength to the wing for a negligible weight penalty compared to the segmented L-angles that were added to the 650 wing. Reducing oil canning is nice, but, while we are strengthening the wing, why not do it right? Stronger is better, I think. Particularly when it doesn't add much weight. You could easily tie the bell crank reinforcements into a stringer reinforcing the whole wing. Why Not? CZAW put 3 Z-angles into their SportCruiser wing.

When we did the static load tests, we did a lot of testing with the weights in the front of the spar but also near the trailing edge. This was done for twist and aileron-flap extreme loads. My concern would be that if we cut the rib as per the above photo you supplied, I am not persuaded that the ribs will be strong enough when the wing is twisted. The wing we now have is so tough that even after exceeding the Ultimate Loads for the SLSA category, there was zero permanent deformation in the wing skins, ribs or spars (same goes for the fuselage). It is typically expected to have some permanent deformation at Ultimate Load and we had none!


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