# UL3

### Help Support HomeBuiltAirplanes.com:

#### PTAirco

##### Well-Known Member
I really need to think of a better name for this thing....

Single seat, all metal, partial fabric on wings.
45-65 hp, two stroke power (likely MZ201).
High wing, taildragger, enclosed fuselage.

Wing area - 100sq.ft
Empty weight -440 lbs (inc. ballistic chute)
Gross weight - 800lbs

16 gall. fuel, lots of baggage space.

For pictures, see my albums.

Wings are 95% done, tanks installed (welded al.)
Front fuselage structurally complete, currently working on installing controls.

Tapered steel landing gear legs on order from Laing Machining.

Last edited:

#### PTAirco

Getting to the fiddly parts - controls. Stick, torque tube, link tubes, belcrank and idler arms are done and in place temporarily. Checked the actual throws I am getting and everything seems fine. Not too much friction in there yet either, though the nylon bearing blocks are less than ideal in my opinion. $80 worth of rodends so far. Rudder and brakes are different story; despite lots of CAD drawings and measuring and eyeballing, the rudder cables foul the other controls in a couple of places. I wanted both brake and rudder cables to run through a central tunnel, but will need to move them to the fuselage sides. Cut up the already welded pedal and brake assembly and shuffled everything around and re-welded. Had to improve the geometry at the same time - it's a little cramped in the front of the fusleage; size 14 shoes max!. I am using separate heel brake pedals set inboard from the rudder pedals. I think toe brakes are fine but they put a lot of strain on the rudder hinge and I am using a piano hinge for that - I can see that wearing rapidly if one stands on the brakes a lot. Even considered going without differential brakes and using a brake lever on the stick. But I am sure there will come a gusty crosswind day when full rudder won't stop me from from a groundloop and I'll curse myself for taking the easy way out... #### PTAirco ##### Well-Known Member Received landing gear legs from Laing Machining -$550 plus shipping, but worth it. They fit my socket tube perfectly without any honing.

Machined axle sockets to size, welded them and then re-machined due to the (expected) distortion. Machined axles.

Then came the fun part: the gear legs are held in the socket tube at their upper end by two 3/8" bolts. There was no way to rig this whole thing up under my drill press so they had to be drilled by hand. Lined everything up, clamped it and started by doing small pilot holes and working up in drill sizes. I used all those odd bit sizes that normally never see any use. It was HELL drilling through the hardened (220.000psi) gear legs with a hand drill. **** near burnt out my drill too. Had to stop and re-sharpen drill bits every 1/8" of an inch or so. If the point angle wasn't just right or the pressure or the drill speed, nothing would happen except screeching, jamming, over heating etc. Spent two days of my spare time doing this. By the end I figured out the right combination and it worked ok; moderate pressure, moderate speed and lubricant and a very flat point angle seemed to do it. I was on the verge of going out and buying a cobalt drill bit, but it got done.

After much headscratching I settled on a little toe-in for the axle sockets. I know some people advocate toe-out on a taildragger, but I just can't see the physics behind that making any sense. The amount? 1.458 smidgins. No idea what that is in degrees, it looks about right. Having separate sockets, I could adjust it if absolutely necessary.

#### Attachments

• 63.3 KB Views: 386
• 47.5 KB Views: 447
• 56.9 KB Views: 327

#### PTAirco

##### Well-Known Member
Wings are done! Finally. Tanks installed, torque tubes, inspection panels.

#### Attachments

• 65.8 KB Views: 583

#### PTAirco

##### Well-Known Member
Received my streamline struts from Carlson aircraft - very nice. I was originally going to have struts with no jury struts but the weight difference is considerable (about 8 lbs), so I'll have to fit them. They have a variety of streamlin shapes at Carlson, all 6061 aluminium extrusions. the nice feature is a flat area inside desinged to accept square bar or tube that makes designing end fittings much easier, increases the metal thickness just where you have the attachment bolts and I also (marginally) increases the section modulus of the strut. Alround, I am very happy I found them. I was resigned to using round tube and then making some kind of fairing for it.

On their website they have some strange strength figures for these srtuts, which made no sense to me at first. He quotes the ultimate strength of the struts in "psi", whereas he really means "lbs" in tension and compares them with equivalent round struts of equal strength - in tension.

He does not give equivalent figures for compression strength which is really the only thing we're interested in, but you can work that out fairly easily yourself.

Hung up a wing and after lots of fiddling with water filled tubing as a level, plumb lines and help from Mr Pythagoras, got it squared up and the rear root attachment drilled in the right place, which determines pretty much everything about the wing and it came out ok. Right wing is next.

You can see in the picture why I left off the rear fusleage - there'd be no way to rig the wings inside my shop.

#### Attachments

• 77.8 KB Views: 439
• 95 KB Views: 586
• 52.6 KB Views: 409

#### PTAirco

##### Well-Known Member
Engine arrived!

Finally signed a big check and bought an engine.
I had eventually settled on a 65 Hirth, rebuilt with a used gearbox. A little heavier than I liked for my design, but it had outgrown my original choice of the MZ201 of 45 hp. I ended up with a MZ202, over 60 hp with toothed belt reduction drive, pull start, twin Bing carbs. I may go for an electric starter eventually.

As you can see in the picture, it is light! The engine was never used or installed, but sat in its crate for a while and I got it at a reasonable price. Still the most expensive thing I bought for a long, long time.

Need to make a cooling shroud for it eventually but that will be a little while.

#### Attachments

• 73.4 KB Views: 667
• 87.3 KB Views: 647

#### PTAirco

##### Well-Known Member
Finished engine mount and have the engine sitting up there to see about fitting cooling baffles and how to cowl the whole thing. The trouble with two-strokes of course is that huge exhaust. It sticks out maybe two inches past the fuselage sides, so I think it will actually go outside the cowl on that side. The baffling will take air from the left side of the cowl, and exit on the right. The carbs also stick way out and the air filter will have to be very flat and go into a bulge on the left side. And the plugs need a bump in the top of the cowl. Starting to look like a ME109 soon.

Also my prop is marginally off-center - the tensioning mechanism on the belt drive alters the center very slightly, not enough to worry about, but there won't be any precisely aligned spinners on this one.

#### Attachments

• 83.4 KB Views: 525
• 64.6 KB Views: 335

#### PTAirco

##### Well-Known Member
More progress on the cowling:

made a plug by wrapping the engine in Saran wrap and aluminium mesh and then squirted foam-in-a-can all over it. carved to shape and then used joint compound to smooth it all. I laid up the cowl directly on this, no mold, so there was plenty of sanding and filling to do. In fact it never ceased to end. So far the most tedious part of the project. I am really bad at working with glass and resin....

The baffling was also complex; trying to seal the awkwardly shaped front end of the engine to the cowl and getting the air to flow through the fins of the cylinders and nowhere else. The exhaust manifold will run outside the cowl and re-enter the bottom where the muffler is. Still working on an air filter set-up.

Firewall is covered with a fireproof material which weighs very little; I don't have a steel firewall for weight reasons. The material will undoubtedly do the job, but being white and fragile ends up looking a little scruffy after a while.

Hooking up and routing wires, throttle cables and pull start. After months of trying to communicate with Compact Radial Engines, the people that make the MZ engine, I have to report some lousy customer service. I asked over and over again, what parts I need for an electric start system and price, without much success. At this stage I will just go with the pull start.

#### Attachments

• 90.5 KB Views: 592
• 76.5 KB Views: 479

#### PTAirco

##### Well-Known Member
Very little progress all year - paying off my Aeronca project and too much flying. But. Rear fuselage is coming together. Fin mounted, tailplane mounts almost done.

Finally figured out a solution to my skin stiffening scheme: there is not a lot of internal structure in the fuselage; basically a box held together with angle longerons and uprights at intervals. I propose to use external stringers, but until now haven't come up with a simple method of fabricating them. ideally I'd have the skin corrugated in strategic places, but that is outside the realm of practicality.

I have come up with a simple method of forming "pointed-hat-section" stringers. Strips of sheet about 2.75" wide and as long as practical are simply placed over a male die, formed simply by laying down 3/4" angle iron on a flat surface. The female die is a 1" angle placed over the strip and whacked down hard with a mallet all along the length. Result is a nicely formed "witches hat" section. Pictures show the prototype. These will form external "corrugations" about 6" apart, running lengthwise along the fuselage.

This leaves the open ends to be dealt with, primarily a cosmetic thing. I could have made up glass fiber endcaps, but I seem to enjoy doing things the hard way. Experimentation yielded a sort of deep drawing die, with a the male part cut and filed from square bar. The wood top plate will be replaced with a metal one, now that I am getting good results. It's amazing how much difference subtle changes in technique make to this process. Annealing is essential - see the crack in attempt No1. Polishing the dies and blank with furniture polish helped enormously; everything slides much easier without wrinkling.
I need about 40 of these, so mass production makes the effort worthwhile. You can see the gradual improvements in the many attempts.

One can argue the merits of external vs internal stiffeners - certainly these cause drag, but it makes the structure very simple to assemble and you don't need to notch bulkheads and formers; both uprights (inside) and stiffeners (outside) can be continuous. But mainly I like eccentric engineering and always liked corrugated airplanes!

#### Attachments

• 68.8 KB Views: 273
• 64.8 KB Views: 246
• 68.3 KB Views: 238
• 76.9 KB Views: 221
• 59.2 KB Views: 309