Therapy Project

That could be helpful. I took a drawing of the floats and did some basic trig to set the angle, but double checking doesn’t hurt.

BTW, it was from one of your comments on a different thread that made me look for the measurements.


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jimboflying said:

I made aluminum pulleys and pressed in rollerblade ball bearings from VXB.com. They seem to work well.

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Same. 608 bearings work great:

Bottom Channels

This was sort of tedious and the drawings were little help. The drawings showed "channel" under the fuselage in the front cockpit area. It also said, "radius all channels" with little arrows. No reference as to how much. I'm guessing that when the plans were drafted this sort of question was part of the common knowledge base for your typical scratch builder.

BTW, no dimensions were given for the channel, either. But based on what I could discern from a photo on the Wag Aero website, it looks like standard 3/8 channel.

I'll mention something that has bothered me about the Wag Aero plans from the start. In most places, the plans follow what I call the "propagation of error" method. That is, they show measurements from some other joint or intersection instead of from a datum reference.

No big deal if you mark out your own references--extra work and double checking, though. When I did that, sometimes the dimensions did not add up, so you end up choosing what looks right.

So the channel locations followed the same format, except some measurements were left out. I got the feeling that it was more of an eyeball the thing and make it work sort of process.

Oh, and the fuselage profile showing the channels did not match the dimensions for the fuselage layout. I spent a few hours looking at various pictures of 2+2 Sportsmen and saw that there were subtle variations with how the boot cowl was shaped. I decided to try to avoid the bulge I saw on some of them, double checking for clearance of the elevator arm on the torque tube.

Once I got into it, though, it was not so hard. I used battens and stiff paper to make sure that there was a conical projection from the firewall template, and did the same for the middle set of arches. Tedious, but I have it all tacked.





I did not have a metal shrinker or formers, so I used the needle-nosed plier approach to bend the curves. Once it was all tacked and braced, they seemed plenty rigid.




For kicks I unrolled some aluminum flashing to see sort of how it looked.



The plans, as far as I can tell, are silent on how to fasten the lower cowling. I think nutplates? I saw someone mention a clever way to seat them with epoxy instead of rivets. One picture showed them pop-riveted....More things to figure out.

Screws into the channel has been done as well

CharlieN said:

Screws into the channel has been done as well

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Of course, LOL! I'm showing how little experience I have at these things.

Just a few months ago I put back in a bunch of screws in my Commonwealth. You'd think that would have told me something.

My opinion they would tell you to weld on tabs and use nut plates of some form that will remain durable long after we no longer care.
If time and cost were a concern the PK screws are the way.

PKs are a horrible way to go. The screws chew up the channel and wallow out the holes until the channel cracks and breaks. I've seen many aircraft with nutplates riveted into the channel but that means three holes for each screw instead of one. Some have said that they used adhesive to hold the nutplates in. My trick for these is to use 1/4" square nuts for #6 screws. These will just fit inside of the channel. Glue them into the channel with pro-seal so that you can dig them out if needed. Then you can attach the boot cowl or panels with #6 machines crews with nylon washers.

Web

Nut plates during the rebuild and use the original sheet metal hole as the middle


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wireweinie said:

My trick for these is to use 1/4" square nuts for #6 screws. These will just fit inside of the channel. Glue them into the channel with pro-seal so that you can dig them out if needed. Then you can attach the boot cowl or panels with #6 machines crews with nylon washers.

Web

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Thanks for that! I'd been kicking around some sort of homemade nut plate idea--this is right what I was looking for. I even have a little box full of those nuts and I always wondered if I'd ever use them.

Pressing on, kind of slow, but I wanted to pop up to show I'm not totally derailed. Lots of life things accumulate and slow down progress. Plus, my four herniated discs in my back decided to flare up again, so I stagger around the shop and sit a lot and think. Or so I say....

But I did have to ponder things because I am working on the bird cage and stringers. The plans are pretty vague, at least to me. As I mentioned before, the plans showing channel work don't match the profile of the tube structure. I decided to quit trying to figure it out and go for flow.

First, I used some clear fir for bottom stringers. Had to scarf them.



Welded more tabs and discovered that 102F in the shop is not so bad after our 118F heat wave.



Side stringers (just a mock-up--I'll use better wood eventually)



Then I cobbled together a channel-bender because everyone else has one. It is homely and clunky, but worked pretty well for the channels above the cockpit.


You put it into a vise and fine tune the bend angle with a large set of channel-locks. Then push/pull the channel through. It doesn't take much to make too much of a bend. This shot was using scrap:



Looking at the photo I see the washer could bind, but I didn't notice when I used it. Some improvement can be made there. And the tube/bushing on the right really ought to be another roller, but I was out of stock. It did the job.



For some reason I don't have pictures, but I set the curve using wing ribs aligned on the wing attachment points and then tried to account for plexiglass width. Now I just need to add cross members and additional bracing.

I'm at a decision point whether to go with a swept-back curve like the PA-18A design or to go as designed. I put up the arches as designed, but I sort of like the ag profile better. More things to sit and think about.

I try to do everything that's practical to stay away from PK or any sheet metal screw...... I even tap my spars for 4-40 rib mounting screws so as to not use the PKs. I found these "weld nuts" years ago and have been using them ever since. Inexpensive alternative to use in secondary structures.............. You can spot weld them in, form flanges or trim and bond like in Piper Channel that we all use. The square nuts DO work well too in the Piper Channel and have used them a lot before finding the weld nuts. I think I have them in #4 thru #10. They have them in several shapes too like for corners, but they are a little thicker like the ones on the left of the box.

Good to hear from you again Vic

Fun with pulleys

I'm never going to make it as a content provider. I'm too slow these days.

But I've been working on where to attach pulleys for control cables.

Others may have no issues with this, but I have to confess the Wag Aero plans were very sparse on details for pulley location. Basically a line drawing showing the rough location of cables. No specs on pulley size, although they do provide pulley bracket details.

But I just couldn't get the pulley brackets, as designed, to line up well with the center of the torque tube.



Note, the pulleys shown are too small, I changed that. I also tried flipping the brackets the other way, but that resulted in interference with upper frame tubes.

While I was messing with this, I researched pulleys and aircraft cable. That was a mistake (not really, just an eye opener).

1/8 inch, 7X19, cable, of course, is specified. As I read more, I discovered that the minimum recommended pulley size, for aircraft application, for this cable is 5 inches! I ran across lots of charts and ponderous references supporting that.

Well, that threw me a bit. Plus, some folks recommend using coated cable if you go with aluminum pulleys.

Hmm. That didn't seem right, especially in the world of cub-like aircraft.

So I decided to look at my Commonwealth to see what they did. 2" aluminum pulley at the wing root and from the torque tube. Best I could tell, the cub drawings have 1 3/4" pulleys. I decided to make some 2" pulleys and press on.





I'll clean them up and bore them for ball bearings sometime later.

Finally, after trial, error, fiddling with various temporary brackets to hold in place, I think I have located where they go. By the way, the plans call for a fairlead near the door, but that bend is more than 13 degrees. I put a pulley there too.





For me, the critical thing was to have the cable aligned in planes for each pulley, that is, making sure the cable enters and leaves each pulley square to the axle. It took a lot of fiddling and tongue holding, but I think the other side will go quicker. They are tacked in place for now, waiting for final welding once I finish the other wing.

RVBottomly said:

1/8 inch, 7X19, cable, of course, is specified. As I read more, I discovered that the minimum recommended pulley size, for aircraft application, for this cable is 5 inches! I ran across lots of charts and ponderous references supporting that.
View attachment 57589

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This one "looks" to be a bit small. High load near the control stick turning nearly 90 degrees. The cable may tend to fray/break individual wires due to bending. Larger pully diameters minimize the concentration of the bending loads on each individual wire.

I'm reminded of the pulley next to the rudder pedals on a Taylorcraft. They are very small, the cable turns nearly 90 degrees and they wear out quickly.

skywagon8a said:

This one "looks" to be a bit small. High load near the control stick turning nearly 90 degrees. The cable may tend to fray/break individual wires due to bending. Larger pully diameters minimize the concentration of the bending loads on each individual wire.

I'm reminded of the pulley next to the rudder pedals on a Taylorcraft. They are very small, the cable turns nearly 90 degrees and they wear out quickly.

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I was thinking the same thing. Piper has an 80421 pulley, which is 1 3/4" phenolic. The Backcountry build manual shows the same size in aluminum. Mine is 2".

I could probably fit a 3" pulley there.

I would pay attention to pulley size and amount of wrap. More bend or heavier load I run larger diameter pulleys.

Pulley Progress and Static Stress observations and worries

I've built more pulleys and inserted ball bearings in them. That took a long time on my mini-lathe, but it was fun learning how to custom-make cutting tools and trying to be precise. FWIW, I overshot on boring one pulley and ended up putting in a larger bearing. That's what happens when you lose count of your thousandths.



Meanwhile, I've been breaking things in the name of learning. I decided I should quit pondering wing stress with my flap modification and do something about it. I had to go to basics. Beam theory, statics, and then material stress analysis. Etc.

I read a lot and found myself wishing I had gone on beyond the 2nd year in engineering school instead of switching to Soil Science and Music. But my fossilized brain started to catch on.

First, I decided to use the standard rectangular beam formula on various pieces of douglas fir I had laying around. I calculated the Moment of Inertia of each piece, stuck it in a big vice, and used a fish scale to bend them until breaking. Then I calculated the bending moments and bending stresses to compare them to the table in ANC-18. Pretty cool. Every piece I tried broke right around the modulus of rupture listed, plus or minus 10%. The pieces ranged from 1/4 x 1/2 up to 3/4 X 3.5. I did this to see if I understood how the formulas worked.

Interestingly, I tried it also on some 1/4 X 1/4 spruce scraps I had laying around. Both had some leftover epoxy on them, so that is a confounding variable. They actually broke at nearly 2X the listed fiber-stress limit in the table.

So, with that under my belt, I thought, "OK, let's look at these wood spars."

Ugh.... Front spar is .75 x 6.25. Rear spar is .75 x 4.5. I ran numbers different ways--tried point source loading, equal distribution loading, assumed center of lift at 20% chord, then 30%. Ran an Excel spreadsheet. Etc.

WagAero lists this 2+2 as having a 2200 pound gross weight. When I use that figure (one-half per wing), the numbers don't work unless I go with a 2.7G load and no factor of safety. It's supposed to be 3.8G with FOS of 1.5.

Scratching my head here, but all along I suppressed suspicions. The wing looks like a J3 wood wing, except the N-brace offers some rigidity. The numbers work out fine for a Gross Weight of around 1450.

To be clear, I was doing a very basic static load analysis--something I should have learned how to do long ago, but here I am.

Also, I'm starting to run numbers on aluminum spars and the larger ones from Carlson look encouraging. Still, I'm wondering if I'm falling into "fake engineer trying to analyze things" syndrome.

For the engineering minded, the basic problem is that at 3.8 G and 1.5 FOS, the bending moment at the lift strut is around 70000 inch-pounds. Using a moment of inertia for the 3/4 x 6 1/4 wood spar, [(a)h^3/12], I get 15.26. Then using the bending formula Bs=M Y/I, I get a result of between 10,600 to over 14,000 psi depending on where I put center of lift.

Spruce's fiber stress limit is listed at 6200. So, depending on angle of attack, etc., I think I'm over 2 times ultimate load at a 2200 gross weight.

Hmmm........

Not making me feel confident.

Front spar to me seems small, might just be me and too dang long since I have run the numbers on wood wings. Way back I built Cub type wings, I built the spars full height, as in skin to skin, The ribs attached to the faces rather than sliding over. That added height makes a big difference. Mine was also thicker, 7/8 or 1" or such. I think I started with 5/4 material.
How much of a plywood doubler do you have at the struts?

Pulleys, I drill & ream the holes for the bearings.

CharlieN said:

How much of a plywood doubler do you have at the struts?

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3/32 birch ply both sides. I didn't factor that in, but I think it would only help around 17% max.

Pulleys, I drill & ream the holes for the bearings.

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Right. I was going to do that but didn't have the right size reamer. So the slow, hard way!

Charlie, I plugged numbers into my spreadsheet along your lines. If I run that the spar width out to 1.25 wide and 6.75 high, I'd be close.

No way I'll find a piece of spruce like that these days. Laminating could work.

The height makes the biggest change. Laminating caps provides alternate grain reducing the chance of uncontrolled fracture. As you see in your numbers the serious bending loads are all in the region of the front strut. The root is primarily a compression load as a result of the lift strut angle forcing inwards, root lift for the most part is counteracted from the outboard portion of the wing's lift forcing downward at the root.

CharlieN said:

The height makes the biggest change. Laminating caps provides alternate grain reducing the chance of uncontrolled fracture. As you see in your numbers the serious bending loads are all in the region of the front strut. The root is primarily a compression load as a result of the lift strut angle forcing inwards, root lift for the most part is counteracted from the outboard portion of the wing's lift forcing downward at the root.

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Right. I did a free body diagram and the biggest bending moment is at the lift strut. The bending moment inboard of the strut is less than half.

Just to see if my approach was correct, I did the same analysis on my Commonwealth Skyranger. The wing is similar, uses spruce spars, etc. Gross weight is 1450.

Lo and behold, at 3.8 G and 1.5 factor of safety, the stress at the lift strut point was 6206 psi. ANC 18 list spruce fiber stress limit at 6200. If you account for the rounded wing and move center of lift aft a little, the figure is well under the limit.

So those engineers did things by the book.

I can't be the first person wondering about these things. Quite a few have been built. Marty57 said he had an engineer look at his and called it overbuilt. I'm not seeing how.

Sounds like you're approaching this in a reasonable way - I wonder if there are planes out there with a span-wise wing loading similar to your -14 and that have wood spars you could compare to. Perhaps another approach would be to find (or compute) the moment of inertia of metal spars for similarly loaded wings and see how they compare. I have a set of 2300 gross Dakota Cub wings here for our Exp -12 build I could give you some dimensions from if that would be helpful. I'm pretty sure they are 6061 aluminum, but don't know the heat treat condition. I bet a call to Dakota could answer that. Since they are certified wings, I bet they could also tell you the max fiber stress they're designed for.

Thanks, Gordon. Maybe an Interstate or old Scout.

I would like to see the specs on the Dakota Cub wing. That would help me understand what can be done at various points to address stresses.

I started running numbers on the original Super Cub and came up with some surprises too. Not as extreme, but the Northland drawings seem to suggest gross weights over 1500 are dicey....I'm pretty sure I need to rethink my moment of inertia calculations on those spars.

So many new questions! LOL. I don't yet know how to analyze what a doubler might do. I do see from my moment diagram that the doubler on my wing doesn't really extend far enough if my bending moments are correct. Back to reading ANC 18 a little more thoroughly.

It may be I'm chasing my tail after finding a new toy. But something about these wood wings just didn't seem right. Too bad I didn't decide I should learn wing analysis before building the first one!

For the doublers, just use superposition - assuming the doublers are well bonded to the primary structure.

Please shoot me an email - I know I had yours at one time, but somehow now I don't. gordon.misch@gmail.com

Resolved, sort of....

Wing spar concerns turned out to be overblown. I corresponded with folks who know what they are doing in wing design. They pointed out that my assumptions were conservative in the extreme.

First, I found out you don't use factor of safety when you are checking the proportional bending limit (aka fiber stress limit). So I should have used 3.8 G instead of 5.7 G with that figure.

Then you check with the factor of safety to see where the spar breaks in reference to the modulus of rupture (not fiber stress limit).

That made a big difference.

Then there is the practice of subtracting wing weight from the load because of inertia. Wings with fuel tanks can get pretty heavy, so my wing loading calculations were off.

Finally, you figure linear wing loading from center of wingspan, not just length of wing, because the fuselage creates lift too.

After accounted for all these things, I can say the Wag wing meets the 1944 ANC 18 standards. It's close to the limit, but within margins.

But...one quibble. ANC 18 changed in 1951. The proportional bending limit was reduced to 5300 from 6200. Apparently they changed the load test from a 3 second load to a 15 second load. I guess it makes sense.

The Wag wing as I analyzed it falls short of the 1951 standard. Not by much, and, as Gordon pointed out, doublers may make up the difference. I haven't gotten that far yet.

I've learned a lot in the past few days. One thing I want to explore is how flaps change the loading as the center of lift moves aft. It might have some implication for slow flight in areas with wind rotors, for instance.

The other thing I learned is that my Commonwealth is built like a tank. The wing analysis I did shows that it is far stronger than required under either version of ANC 18.

Plodding along

Like wading through molasses, but a little more progress.

I made bigger pulleys and tacked them into place.




Still need some alignment fine-tuning. But after all this, I now see the cables interfere with the floor. So either I lift them up with a different kind of bracket, or modify the floor. Now I see why Wag specified pulley brackets for smaller pulleys. Still, they also specified a fairlead with a 14 degree bend, which I didn't like.

While I kick that around, I fabricated the elevator bell-crank and support.










I had been putting this assembly off because (my excuse) my back was sore and stiff and I had to crawl around inside the fuselage. Well, my back was not getting any less sore, so I figured how to do it with stools and supports. Bah. I need to kick things into gear a little harder.

But the other weight is my wife's condition. It takes a lot of time to keep things together in the house. And, I keep thinking the original mission for this project is now off the table. I can barely get her into a car, let alone an airplane. I'll keep building because I like to--as the title says, it's therapy. But now I'm wondering if it ought to be more of a light fun flier instead of a camping truck.

Things to ponder as I plug along.

RVBottomly said:

But the other weight is my wife's condition. It takes a lot of time to keep things together in the house. And, I keep thinking the original mission for this project is now off the table. I can barely get her into a car, let alone an airplane. I'll keep building because I like to--as the title says, it's therapy. But now I'm wondering if it ought to be more of a light fun flier instead of a camping truck.

Things to ponder as I plug along.

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The family unit is important........ As you say "it's therapy". Finishing it IS Therapy. Not finishing it will only generate more disappointment in one's self.

If it is to become just a solo flying machine perhaps consider moving the front seat into the middle? Leave out the back seat. It can still be a fun flyer. AND you will have the satisfaction and pride of completion. Not completing at this stage would be anti-Therapy.

Thanks, Sky. You are right. I've been tending toward moving the seat to the middle. An extra wide body cub, LOL.

It can still be a truck--a lot of room behind, I could fit a Honda 90 back there if I could lift it in. Certainly, room for one of Courierguy's electrified bikes.

Pressing on.

RVBottomly said:

It can still be a truck--a lot of room behind, I could fit a Honda 90 back there if I could lift it in. Certainly, room for one of Courierguy's electrified bikes.

Pressing on.

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With those thoughts ................. Consider provisions for a removable lightweight hoist to assist in loading the Honda. There is almost always a way to complete a mission. Just turn on some brain cells, you may be surprised what may come out.

skywagon8a said:

Consider provisions for a removable lightweight hoist

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Or a simple ramp...

RedOwlAirfield said:

Or a simple ramp...

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I'm not really sure I want a Honda 90. I was just using that for reference. There are some pretty cool looking motorized scooters out there that are even smaller.

FWIW, I rolled a mountain bike into the space with a simple ramp and it fit easily.