Rigging jury struts

I'm installing the jury struts and have a question on alignment.

My question is: Are the jury struts supposed to be longitudinally aligned with the slip stream?

I understand the struts should be vertical and perpendicular to the cabin floor. I also get it that the lift struts should be straight and the jury strut brackets should be in line with the strut tubing and centered on the spars.

When I get all the points lined up, the jury struts are angled outward by maybe one inch. If I align the jury struts fore and aft, the front bracket is short of reaching the forward spar by 3/8 inch.

I plan on making longer brackets, I just want to know if I'm missing something. Is this typical of the Kitfox?

John Pitkin
Greenville, TX

Hi John,

I do not know if the situation you describe is typical or not. I can tell you my experience - very similar to what you have observed.

For the record - my kit was built by the previous company.

1) When everything is secured - the lift strut tubes have to be absolutely straight - running a tight fishline or string down the lift strut will provide a reference.

2) The Jury struts are intended to be perpendicular to the spar tubes. What I don't know is if it makes any difference if they are angled a bit to be sure the lift strut tubes are straight.

This may be the Norwegian way of doing this (I can say that since my name ends in ...son); but, I elected to cut off the upper 1 1/2 " off the ends of the jury struts - weld on 4130 tubes of the same dimensions that were a little longer - trim the length, flatten the ends and drill new holes. This way the jury struts were perpendicular to the spar tubes and the lift strut tubes were absolutely straight.

Sincerely,

Dave S.

KF 7 Trigear
912ULS Warp
St Paul, MN

As I understand it, jury struts' purpose is to prevent bending/buckling of the main strut during negative G conditions. Thus the jury strut should be perpendicular to the spar or it's tendency would be to buckle too under high negative G loads. When all was said/done with my Model 2 kit, I also had a little angle to the jury struts when installed per the manual. The angle was < 1.5 deg though and the simple clamp style fittings that attach the jury to the main were easily adjusted. This puts the jury maybe an inch or so further out the span of the main but preserves it's perpendicular orientation.

Chris

It sounds like this is a minor alignment problem. I’ll just make a longer set of brackets at the front where the jury strut connects to the forward spar.


This will accomplish two things:


I’ll get the jury strut alignment into the slipstream which probably won’t make a bit of difference in drag but makes me think I did a better job; and,
The forward attach bolt will not be buried halfway in the fabric covering.


One thing I have to say is how impressed I am with the kit. I got this far without a major snag in construction and this is only a minor detail. Sure, I had a couple of oops along the way like cutting a trailing edge piece on the wrong side of the line. But, so far, I haven’t run into anything that couldn’t be corrected. Great kit!
JP

Well, it turns out I don’t need longer jury strut brackets.



I made a short phone call to John McBean at Kitfox LLC to ask about fitting the jury struts. He was very helpful in telling me my situation was not normal. We agreed I needed to look for the cause. While not able to diagnose over the phone he did give me some tips on where to look. I have to say the Kitfox customer support is terrific.


Per the assembly manual, I used a taut string to ensure the forward lift strut was not sagging; but the jury strut was still low by 1/4 inch. Shifting the front of the jury strut outboard to raise it up put the fore and aft alignment off by one inch. Not ideal in my book.


With some clues from my conversation with John, I checked the rear lift strut and found it had a warp in the upward direction. That was totally unexpected. I had assumed it would be sagging from gravity.
I gave a quick tweak to the rear lift strut to remove the upward warp and everything came into place.


While some may find this episode a bit boring, I pass this information on as a reminder. When you get stuck, keep asking questions. Try different sources. Keep searching. And feedback, no matter how small, is a big help.


And lastly… if something doesn’t seem quite right, it probably isn’t.
Net result here is: The problem is resolved and I get to move forward.

John Pitkin
Greenville, TX

Good job John. An engineer's perspective for sure and while I can't always claim I'm as persistent about such problems and sometimes compromise where I shouldn't, reading someone elses success stories helps to keep me motivated.

Chris

John

If you want to drive to Mesquite you could look at my Plane, to get your answer about the jury struts.

Thanks for the offer Paul,

John McBean answered my questions on how the jury strut it is supposed to align and I found the error was with the rear strut having a slight upward warp. It was easily corrected. All is well and the construction continues at a rapid pace.

The fun meter is pegged.

JP

just a thought ,..but are the struts backwards? meaning left is on the right and visa versa?

Not possible on the SS Model 7. The struts have an upward angled fish mouth on the fuselage end. They are left and right specific and only go on one way.

As I stated, the problem was an upward warp of the strut.

The warp was probably caused by welding heat when fabricated. It was easily corrected once it was discovered.

John Pitkn
Greenville, TX

I’ve been working on resolving an issue with the left jury strut installation and like John have a warped rear lift strut. However, the tube is warped forward (and a bit of a sag) towards the front lift strut tube about 3/16 to ¼” which, makes it impossible to correctly install an align the jury strut. I have tried to tweak the rear lift strut tube straight by supporting the ends and deflecting the tube more than 3” in the opposite direction with no detectable results in straightening the tube. It looks like when the tubes where welded together at the fuselage attach point that the angle was not correct (to small) resulting in the tube being pulled into place to weld the spacer at the wing attach point on the opposite end resulting in the warp. Trying to apply more force in the center of the tube does not seem like it will yield the desired straight tube at this point to me. Any ideas on how to proceed with this would be appreciated. Thanks, Greg

Does anyone have tips & tricks related to the placement and alignment of the Jury Struts. I have a series 5 kit from Skystar that was initially created in 1994. I have purchased the S7 jury strut attach brackets for the top of the struts that attach to the spar. I also have installed the landing gear so when I put the wings on the plane to install the butt ribs and determine where the door stops rising upward, I planned to install the jury struts at this time. According to my documentation the floor of the fuselage needs to be level, I assume both fore/aft and side/side. And the jury struts are to be perpendicular to the floor of the plane. The instructions about the plumb line are a little confusing to me. So, I'm asking if there are any tricks/techniques to installing the struts that you would be willing to share?

I had my fuselage leveled, front to back and side to side. I did this with a digital level. Wing was attached, lift strut was attached also. I had my wing supported just outside of the lift strut. I ran a string down the lift strut to make sure it was straight (you do not want sag in it). I think the measurement was somewhere around 53" outboard of the fuse for jury strut placement (this may be specific to my SS7). That gets you close though to begin the preliminary attachment. I then took the hardware/clamps and jury struts and loosely fitted them onto the lift struts (over tape to protect the powder coat of the lift strut). I then placed the prefab brackets on the top of the jury struts with the hardware, did not tighten though. They were curved to the radius of the spar. I used 3" hose clamps to clamp them to the spar, again loosely.

The entire setup to this point was tight enough that I could shift it up or down the the spar/lift strut if that makes sense.

The idea from there is to run a plumb line over the top of the spar and let it hang right next to the jury strut. If the fuse floor is level, this plumb line would be perfectly vertical. You can then optically make sure the jury strut is parallel to it. That is what is needed. The 53" measurement may vary slightly which is ok.

Once it is aligned, tighten everything up. The clamps should be in parallel with the jury strut, if that makes sense. The bracket coming off of the spar should be parallel with the jury strut, not at a weird angle. You might have to loosen/adjust/retighten a couple of times to achieve this.

Drill the spar for the bracket, etc.

Hope this helps.

Thank you. Did you drill the attachment holes in the top attach brackets before trying to fit the jury strut/brackets to the lift strut brackets? The ones I received do not have a hole drilled to which you would attach the jury strut and it appears to be something that you would determine where to drill it in the fitting process. Same for the bottom brackets, no hole so, do you drill these holes prior to putting them on the lift strut?

Yes I drilled mine ahead of time. There were dimensions of where they should be, and those brackets are of a size such that you would not want to deviate from the dimensions.

Once it all bolts up loosely it can move around enough to get the geometry right.