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Therapy Project

That makes sense Charlie. Automotive ignition systems have improved tremendously over the years to a point where they are virtually fool proof. I don't know what is used in race cars, however I do believe that the P-mags are the closest ignition system available which has the attributes of both the automotive and self powering of the magneto. The P-mags only need a small outside voltage input for operating at low RPMs.
 
Question about front gear fittings

I've been whirling around in circles making and remaking gear fittings. I finally have some put together.

But a gnawing question keeps coming back to me: I keep reading about welds cracking on a washer at the cabane attach point on PA 18s. Here is what my drawing says to do:

front gear fitting washer side.jpg

Two questions:

1. The bolt hole size is 1/4 inch. Seems like I've seen it larger for heavy duty gear installations.
2. Is the weld-on washer (red arrow) going to present a problem? I've been contemplating using a doubler that crosses both holes.

The plans have a gross wt of 2200. I might be second guessing things, but as designed it seems kind of light.

Vic
 

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I'll let others speak to the size of the 1/4" bolt. I agree it seems to be too small. Perhaps for a lower gross weight?

The reason for the AN970-4 washer is to increase the bearing strength of the fitting in order to prevent the bolt from pulling out the end of the fitting. The increased thickness increases the bearing strength. The drawing indicates that the washer is welded completely around to the fitting except along the edge. This concentrates the loads on the weld thus when over loaded it will break along the weld.

You could increase the thickness of the fitting to equal that of the original fitting plus the thickness of the washer, leaving off the washer. This will give the same strength without the stress concentration at the weld.

Lacking the availability of the thicker materiel just make two fittings, edge welding them together. Blend the edge along the cross tube to distribute the weld loads.

If I were to use a washer as shown in the drawing, I would edge weld it to the base fitting leaving off the weld extending from the 10 o'clock to 5 o'clock location with the exception of placing a tack at about the 2 o'clock location. Or one at 12 & another at 3 o'clock.
 
The 1/4" bolt is for the 1500 lb gross weight. That is upped to a 5/16" bolt for the 1750 and 2000 lb gross weight. Airframes makes a thicker fitting where they doubled up the material thickness between the cabane fitting bolt hole and gear fitting bolt hole. All depends on you mission and amount of abuse you intend to inflict.
 
...


Lacking the availability of the thicker materiel just make two fittings, edge welding them together. Blend the edge along the cross tube to distribute the weld loads.


That's exactly what I was contemplating. I appreciate what you said about weld stresses. I seem to remember repeated references to the joint breaking at the weld of the washer.


The 1/4" bolt is for the 1500 lb gross weight. That is upped to a 5/16" bolt for the 1750 and 2000 lb gross weight. Airframes makes a thicker fitting where they doubled up the material thickness between the cabane fitting bolt hole and gear fitting bolt hole. All depends on you mission and amount of abuse you intend to inflict.


Thanks, that is what was causing the gnawing in my head. I must have been thinking of the beef-ups for the gross weight increases.


It seems like a good idea now to fabricate them with 5/16 in mind and the doubler.


Of course, my full intent is to baby this project, operating only from golf-course-style grass runways with obstructionless approaches, etc., but... I can't help but notice the gravel river beds on the nearby rivers, or the notable airstrips just across the WA/ID border from me. I'd like the structure to be up to the task.
 
Increased thickness would be best. But doubled is better than single. Think about load path - with severe loading the bolt bends some (probably elastically) , primarily loading the inner layers of the fitting. That transfers to the outer layers of the fitting via the welds or deformation of the holes. Given your 2300 lb gross weight, I'd suggest going to the trouble of making the fittings out of heavier material if that isn't too big a pain in the rear.

Also, maybe consider longer "fingers" where the gear fittings weld onto the fuselage tubes, to distribute the load.

Opinion - - -
 
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Increased thickness would be best. But doubled is better than single. Think about load path - with severe loading the bolt bends some (probably elastically) , primarily loading the inner layers of the fitting. That transfers to the outer layers of the fitting via the welds or deformation of the holes. Given your 2300 lb gross weight, I'd suggest going to the trouble of making the fittings out of heavier material if that isn't too big a pain in the rear.

Also, maybe consider longer "fingers" where the gear fittings weld onto the fuselage tubes, to distribute the load.

Opinion - - -

Right now the fitting is spec'ed from .090 4130 sheet. The only bend is the 90 degree bend at the top to reach the cross tube. I see Wicks has .190 sheet available, but it's pricey! I suppose the bend could be made with a suitable radius.

I took a look at Airframes' heavy duty gear fitting. I see it has doubled pieces, and I noticed the internal washer is only edge welded on the outside. I assume that is to prevent the cracking I've heard about.

https://www.airframesalaska.com/Front-Heavy-Duty-Gear-Fitting-p/af21272-4.htm
 
Yup, as Sky mentioned that weld on the outboard (away from centerline of the fuselage) edge of the washer could present a stress riser. And absence of the weld could present a crevice corrosion opportunity.
 
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Yup, as Sky mentioned that weld on the outboard (away from centerline of the fuselage) edge of the washer could present a stress riser. And absence of the weld could present a crevice corrosion opportunity.

the original washers are only partially welded(edge welded, and its at the end of that weld where they crack)
 
the original washers are only partially welded(edge welded, and its at the end of that weld where they crack)

Thanks for that clarification. I read some threads on the dog-bone doubler to deal with that. Do you think a double-thickness fitting would do the same thing?

I suppose while I'm at it, should I make it more triangular and add a safety chain hole, too?
 
Gear fittings

10.5 hours over the past two weeks. That's what it took to layout the gear fittings and finally get them parallel and square.

I tried various jigs and finally ended up lightly tacking, tapping with a hammer, grinding the tack away and shifting, measuring in three dimensions, tacking again.

Despite having wooden and, later, a steel jig holding my all-thread exactly parallel, it would still end up off after tacking.

But it finally happened and I'm fairly happy with the alignment.

If I were to do it again, I'd run the 1/2 bushing tube between both front and back fittings to keep those aligned. Then it would only be two dimensions of jigging. But, alas, I didn't have enough material.

10.27 layout.jpg

gear oblique.JPG

gear parallel.JPG

There were diversions, however:

10.27 fishing.jpg

Now I need to order some more tubing for the Vee cabane and gear assemblies. I have plenty of material to keep doing other things, though, so plugging along continues.

I looked at my build log and tallied hours to date: 221 hours over 10 months. I think I need to step it up a little if I want to get to Johnson Creek next year.
 

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Another random update and some musings

I'm waiting for my order of additional materials for the landing gear to arrive--should be here tomorrow. I spent time tweaking and checking the fuselage alignment and finishing off missed weld spots, etc. I was pleased with what I had. I measured from a tight string marking the horizontal reference line to all sorts of locations, and the measurements were very close to what they should be. Eyeballing the line straight from tailpost to the firewall showed it to be at the center of every station.

So I welded the x brace in the tail and, without really noticing it until later, I ended up with lower longerons bowed in.

Picture shows the fuselage upside down:

20181125_142251881872914.jpg


So now I have more to do at the tail end... and it got me wondering if the x-brace really strengthens things. I'm sure it does, but it seems odd to have it tying into the middle of the braces like that. I'm half tempted to weld up shapes with 1/16 rod and try twisting them with a light torque wrench to see the failure modes.

But I'm also looking ahead and starting to calculate wing materials. So many questions are popping up; and so many rabbit-trails beckon. I end up musing a whole bunch.

I'm still trying to pin down flap forces in various configurations.

Lately I've been flying a rented 180hp C-172 with a constant speed prop. It is quite a performer. I've taken to studying its flaps carefully. It has the manual style that go up to 40 degrees. Pretty effective, it seems to me. I'm wondering if something like that would be nearly as effective as the Keller flaps. But the forces involved, it looks like a lot of twisting leverage?

So that leads me to ponder the little rear spar of the Piper design. I don't have a PA 14 drawing, but the Wag Aero metal wing kit uses the same spar dimensions as the Northland drawings. I wonder why not use one with a wider web and trim the rib capstrips to fit? Or is it necessary? I've been drawing various ideas for compression struts that would double as ribs at the flap hangars.

I don't even want to bring up my idea for a combination AOA indicator and low airspeed indicator that looks suspiciously like a trolling speed log.

Or variable pressure inflatable bushwheels that you could pump up or down from the comfort of the cockpit.

I think I have it bad, but at least these diversions generally occur at night before bed, instead of when I'm working on the project.
 
I'm half tempted to weld up shapes with 1/16 rod and try twisting them with a light torque wrench to see the failure modes.
Please do. I've had a hard time trying to quantify that tail bracing. X-bracing and boxing. Mine is boxed, but I can't explain why it's a good idea.
 
701c039280570d3f095aa61e8aec5aa9.jpg


It keeps the longerons from bending , and the diagonals, and keeps the two points where the jackscrew attaches from bending and being put in a bind

bff181805ea6596e6e54668717ad5fd8.jpg


After the longerons and diagonals were replaced I put the horizontals on and found the rear attach was off quite a bit, causing the jackscrew to not turn. I can’t find a better picture but the rear attach was off quite a bit.

I can tell you exactly where I bent the tail, landed on a narrow road I mean runway, turned around and the baby bushwheel hit a rock or hole. That’s about the time the trim stopped working.


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Cabane Vee

Finally got back to the shop. The past two weeks have been a disaster in my other life--I ended up getting assigned cases from another public defender who, for various reasons, has now been found unable to represent clients. My planned-for quiet fall and winter has turned into a perpetual state of triage.

But that is why there is a therapy project. I was pleased to make a little progress on the cabane vee.

I cut from .090 4130 sheet the four shapes to be doubled into two parts.

20181205_1847021305116883.jpg

Edge welded them:

02 edge weld.jpg

And then drilled the two pieces at once to keep the holes lined up.

04 drill.jpg

I am really enjoying my RF30 clone mill/drill. I have been doing just that, drilling and milling. It seems very steady, even with pretty big cuts. I do take my time, though, and keep the speed fairly slow for 4130 steel.

I even roughed out curves on the brackets freehand. Those of a certain age will remember Etch-A-Sketch. It was great training for free-handing a curve on a mill.

04 etchasketch.png

Parts after drilling:

05 shaped.jpg

Then came the Vee:

06 vee layout.jpg

07 tacked Vee.jpg

Then the brackets tacked on:

08 tacked bracket.jpg

10 second bracket closer.jpg

Finally there was trimming, drilling and aligning.

11 aligning vee.jpg

I missed a few photos where I milled the ends of the vee to accept bushings, and then tacked those in. I'm pretty pleased with the fit. Still more welding and drilling, but I see the steps leading to having my landing gear fabricated.

As I said in my wings thread, I'm working on supplies for that phase. But I've got plenty to keep me occupied into early next year, at least. Next sub-project will probably be the control torque tube assembly. I'm planning on a simplified version with one stick between the two seats, much like Paul Claus's new 4 place cub. I'm investigating aluminum versus 4130 weldment, and am open to any warnings or ideas.

Pressing on,
Vic
 

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Rear04.jpg
it helps for twisting between the tail spring bolt forward to the first set of diagonals.

I know that's the intent, and I'd sure like to see some tests to see how effective it is.
Gordon,
Consider the lever arm of the tail wheel spring against the two attach points with a side load. Then look at the column loads on the lower longeron between the tail wire attach point and the next bay forward. The new added X​ brace cuts that column load in half.
 
Pete, is that how they fail - column buckling? I thought it was in torsion. I haven't personally observed the problem. My IA had me box those tubes rather than x-brace them. If it's a column failure the boxing would be superior, I think, due to support of each tube in two axes. Thanks - -
 
for what its worth. when i built my plane i called atlee and asked him about x or box brace or none at all, he said box, so i never questioned it and went with that. he mentioned having the top tube a little farther back to clear something, but he made his clamp in one with the top tube down some. just wondering if this x and box thing is a clearance issue more than anything. but he did say to do it.
 
Gordon, I was only going by Tom's pictures in #175 where there are two tubes with a column buckle. His X brace reinforcement cuts the column length of four tubes by half. By welding the X in the center there is additional stiffening. The damage is caused by torsion on the entire aft end of the fuselage but when you narrow your focus to one tube at a time you can see the column failure.
 
Gordon, I was only going by Tom's pictures in #175 where there are two tubes with a column buckle. His X brace reinforcement cuts the column length of four tubes by half. By welding the X in the center there is additional stiffening. The damage is caused by torsion on the entire aft end of the fuselage but when you narrow your focus to one tube at a time you can see the column failure.


When you have a big tail wheel (baby bushwheel) and swing the tail around, hit something solid, the longeron bends (green line) it’s un supported from the factory. Later model cubs and a’s had the smaller tube in there (red line)

I rebuilt my cub from landing on a road and turning around and not watching the tail wheel and hitting something. I added the red tube and x brace and now I look out the back windows when turning.
Only make that mistake once
8653dd893966e9342297ec153664ab39.jpg



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The diagonal tube Tom shows in red was standard on the A models and late model 150 how Super Cubs I have worked on. Seen way too many Super Cubs with that aft section of longerons bent. That is a long piece of 3/4" x. 035" wall tubing to be unsupported. Straightened several on covered airplanes with a hammer and a 2x4. Sometimes it worked good, sometimes ok.
 
When you have a big tail wheel (baby bushwheel) and swing the tail around, hit something solid, the longeron bends (green line) it’s un supported from the factory. Later model cubs and a’s had the smaller tube in there (red line)

I rebuilt my cub from landing on a road and turning around and not watching the tail wheel and hitting something. I added the red tube and x brace and now I look out the back windows when turning.
Only make that mistake once
8653dd893966e9342297ec153664ab39.jpg



696898bc8643cf629cde18fce7c53483.jpg






Sent from my iPhone using Tapatalk

That red tube makes sense to me. You said it was smaller. What size did you use?
 
for what its worth. when i built my plane i called atlee and asked him about x or box brace or none at all, he said box, so i never questioned it and went with that. he mentioned having the top tube a little farther back to clear something, but he made his clamp in one with the top tube down some. just wondering if this x and box thing is a clearance issue more than anything. but he did say to do it.

yes, box is all i use, the tube farther back/lower is to clear the bottom elevator cable
 
The diagonal tube Tom shows in red was standard on the A models and late model 150 how Super Cubs I have worked on. Seen way too many Super Cubs with that aft section of longerons bent. That is a long piece of 3/4" x. 035" wall tubing to be unsupported. Straightened several on covered airplanes with a hammer and a 2x4. Sometimes it worked good, sometimes ok.

Worked for me to straighten, but the jack screw was in a bind, ended up having to replace the rear attachment


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