Building the MachNone

Q: What is a Sky Pup? 

A:   It is a FAR Part 103 legal ultralight which means that it can legally be flown without a pilot's license. Of course, you would be crazy to fly without instruction which is available from instructors associated with EAA (Experimental Aircraft Assocation), the USUA (U.S. Ultralight Association) or the ASC (Aero Sports Connection).

It is not a kit. You buy the plans and then construct every part (except hardware) yourself. It was designed to be constructed out of readily available materials: wood (mostly Douglas Fir and some hardwood), styrofoam (Blue Dow insulation board), fabric (dacron or polyester), sundry pieces of aircraft grade steel, aluminum and hardware, and epoxy glue. All the construction is done with ordinary handtools (table saw, drill press, maybe a bandsaw being the most exotic). It is flown with a lightweight 15-20 hp engine.  It has 31 foot wingspan, 16 foot long, weighs 200 to 220 lbs if built carefully and will carry a gross load of 400 lbs and have an initial climb rate of  approximately 400-450 fpm. Please note: heavier pilots will want to consider a diet or a different aircraft

It is the simplist kind of ultralight utilizing 2 axis control (elevator and rudder, no ailerons) for simplicity of design and construction as well as lightness.  

Q:  Why build a Sky Pup?  

A:  The short answer is: you want to fly? The longer answer: You want to fly cheap. The full answer: You want to fly, cheap but safe. The Sky Pup, although an older design, is one that is fully engineered. The previous prototype to the Pup, the Bluelight Special, was designed by a team of Cessna engineers (off-duty!) to explore alternate methods of construction. One of the engineers, Steve Wood, continued the process in the Sky Pup, a design he released to the public through Sport Flight Engineering. 

The plans are a model for scratch build aircraft. 

"The only bad thing about the Skypup blueprints is that all the other prints you ever see will seem lousy and ruin your attitude. The Skypup plans are really really outstanding."-- Scott 

"Plans are available from Lafe Wood (Sky Pup designer's son) at: SKY PUP 6790 Reeder Mesa Road Whitewater, CO  81527 Plans are $70 within the USA and $75 for foreign orders (shipping included). 

"Lafe still has a number of the original color brochures available.  They are $5 within the USA and $6 for foreign (shipping included). Payment for either of the above can be made by check or money order. 

"Secondary Source: http://vulaorg.vws0104.fast.net/blueprint_pricelist.htm." --Edwin  

Q:  How much  will It cost? 

A:  "First of all this is one of the cheapest airplanes you can build, even if everything is store bought new! It all depends on where you start, if you have a complete wood shop, or access to one like a high school or college wood working classes, you can build this thing dirt-cheap. You can cut and mill your own wood instead of buying pre-sized wood from an aircraft supply source. I bought my wood (Doug Fir) through a local cabinetmaker who ordered it in special. I had him get me 2 ea. 2" x 8" x 18 ft planks for around $80 ea. I was able to cut all my wood including spars from these 2 boards. (plan carefully) I am fortunate to have access to a wonderful wood shop (Dads) and also am employed by a very large hydraulic manufacturing company where I have access to every machine in the shop. Needless to say, what little metal work there is, didn't cost me anything. I bought all my foam from a local concrete company for very little, they buy it in bulk and were very good about not screwing me on the price. The most expensive things you will buy are the engine, propeller and the AN hardware. Please for your own safety and possible future owners, NEVER substitute the crap you get at the local hardware store for aircraft hardware, Nuff said!. The plans have a complete list of everything you will need, so you'll be able to get a pretty good idea of what the cost will be for your particular situation. As far as the engine goes, there are a lot of different engines out there, some OK and some not so OK! I have a friend that's flying a 215 cuyuna and is very pleased with it. He bought his new back in the mid eighties, flew it for a short time and then stored it for 10 years. Now he flies it pretty regularly with out fault. Try to stay away from those high time engines if you can, remember that it's your heiny sitting in the seat. Believe me, those off field landings are NO FUN when the prop quits turning! Done it once due to a planetary re-drive failure! I would prefer it didn't happen again! Hope all this helps!" -- Curt.   

Q: What kind of engine can I use in my Sky Pup? 

A: There are two major factors to consider when shopping for a Sky Pup engine. First is power, the Sky Pup manual specifies that an engine between 15 and 20 hp is sufficient to fly the Pup. Less than that, the performance may be insufficient and if you were able to take off may find yourself persistently flying near stall. Not a fun scenario. An engine much over 20 hp will give snappy performance, but may drive the plane faster than the Vne (speed not to exceed) of 69 mph. One might possibly find himself flying without major portions of the airframe. Not good. The second factor is weight. "Steve Wood told me that the weight allowance for the complete engine is about 62 lbs including prop, redrive and muffler. . . ." --Dennis 

"Possible Sky Pup Engines 

"Power  Weight  Cylinders  Name

14             15         1              Raket 120 Aero ES 

15            42          1               Hirth 240 

15             28         1               Hirth f36 (including redrive, exhaust, carb!) 

28            35          1               Hirth f33 (42 lbs. with redrive) 

15            38          2               OMC opposed [direct drive] 

16            42          2               hith  291  

16            14          1               Solo 180 

17            30         1               Solo  210 

18?          38          1               Rotax 247 

18 ?         48          1               Rotax 247 fan cooled  

19            46          1               Sachs 294 

20            48          1               Kohler 292 

20            42          1               Cuyuna 215 fan cooled (The Sky Pup was designed around this engine) 

20-25      17          2                Limbach 275 (twin opposed) 

21            38          1                Yamaha 292 

21            34          1                Zenoah 250 

22             51         1                Zenoah G25 B-1 (complete engine, cd ignition) 

21            34                             Zenoah 250 

21            38                             Yamaha 292 

21?          47          2                Kawasaki 250 

22            32          2                JPX 425 

22?          48          1                Rotax 299 

25             25         1               JPX d 320 (35 lbs. with redrive, exhaust, elec. start) 

25            44          2                Tohatsu 295 

25            43          1                Chotia 430 [direct drive] 

25            37          2                Hirth 400  [opposed] 

25?          48          1                CCW 312   [fan cooled] 

25             49         2                 KFM  

25            32                             JPX 505 

25            37                             Hirth 400 

26            42                             Rotax 277 (55 lbs with redrive & carb) 

27            42                             OMC 

32            44                             Kawasaki 340    (note that  this weight is for a cdi ignition engine) 

37-40      44                             Kawasaki 440    (points ignition engines weigh 3 lbs more) 

25-40       41                            Kawasaki 440L     --Dennis 

Note: Engines beyond 25 horses (except the lighweight JPX) will probably have to be run direct-drive to lessen weight and de-rate the power to fly adequately in the Pup. In any rate, the prop/engine/carb/exhaust/redrive combination will have to be kept underneath the approximately 62 pound limit in order that the weight and balance of the Pup will be in an acceptable range. 

Q: What kind of glue should I use? 

A: Want to start a debate? Ask about glues. To keep it simple use epoxy,  the structural epoxy that mixes 1 to 1 in volume. T-88 is a good choice. Use plastic gloves, some people develop a skin sensitivity to epoxy. There are some non-sensitizing epoxies out there.  In situations where you are gluing wood to styrofoam, mix your epoxy with microballoons to make a kind of past. This will make the epoxy cover more, be lighter, without compromising strength in the foam/wood bond.

In wood-to-wood bonds care must be exercised as this is usually a critical structural joint. Glue joints are dependent upon the glue seeping into the cellular structure of the wood so take care in these areas. Clean surfaces allow the glue to bond better. Factory surfaces often have a waxy coating (a lubricant) that prevents the epoxy from easily penetrating the surface. Surfaces that are cut by sharp edges like a plane cleanly cut off the cells of wood allowing the epoxy to flow in easily. Sanded, ragged surfaces clog the microstucture and do not let the epoxy flow in well. To glue your prepared surfaces apply epoxy to BOTH surfaces. Let them set a bit, add more if the pieces start to look dry. Mate your pieces carefully applying only enough pressure to insure the pieces don't slip. Too much pressure will squeeze out the epoxy (some squeeze out is good) and what is left may be wicked into the wood leaving the bond-line dry. This is a weak joint.

Happy gluing!  

Q:  I don't know anything about props and redrives? Where do I start? 

A:  What you need here is a bit of voodoo! Not really, but unless you are lucky or a lot smarter than I am, this could very well be the most complex part of building the Skypup. I've been running the Cuyuna 215, gone through two different redrives and 4 separate props three of which I've carved myself. Every combination is different and sometimes difficult to get to work effectively. As a starting point, with a 20 horsepower engine you want a reduction ratio of between 2.4:1 to 2.5:1 if you are using a wooden prop, try a 58 inch diameter prop with a 24 inch pitch (commonly referred to as 58x24).  If your reduction drive is belt driven, you need a right turning prop. If it is gear driven, then a left turning prop. You will save yourself a lot of hassle if you use a ground adjustable prop so you can adjust the load more effectively. I designed and made my own props and got lucky that they worked with some trimming. 

"There are basic things we need to know about buying redrives: 

It is not just an accident that gear redrives do not usually come in an even reduction ratio. eg 2:1 this is to avoid destructive resonance in the drive system. [I do not know how destructive it is.]  With belt redrives there is a lot of bending of the belt and a thick belt can not handle the rapid flexing of our 6000 rpm motors. The thickest belt you should use is a 3VX belt. The 5VX belt has an operating limit [without looking it up, I'm guessing here] a couple thousand rpm lower. The shape of V belts gives lots of friction area so they do not have to be wide, however as they are small there will have to be several of them to have the required power handling capacity. As multiple belts will be run, it is critical theat they all be the same length. Manufacturing tolerances are not tight enough for even tension so when a set of belts is used, they are MATCHED belts specially selected to be the same length.  Flat belts, being thin, work ok at high speeds, but they have to be tight to not slip. So for increased friction, multiple grooves are used on some belts. They mate into ridges in the pullies.

These redrive belts must be quite tight so as not to slip. These are commonly used in automotive application where they can be bent back and forth with impunity. The belts used in cars are narrow and do nt transit a lot of power. Wide belts transmit more power but as the tightness is a force in pounds per incremental width of the belt, the total force is even greater in a wide belt."

Cogbelts are toothed flat belts. The tension must be high enough on them so that the cogs can not climb the " pulley" teeth. Even reduction ratios and their resonances should be avoided with cogbelts. They run MUCH more quietly than a roller chain and their longevity is better also." --dennis 

 There are two interdependent factors you must strive for: first you must match the load of the prop to the engine and second, you must provide sufficient force of air past the prop to drive it forward. 

Let's talk about load. Load is affected by the diameter of the prop and the pitch of the prop. If the diameter of the prop is too great, it will take more horsepower than you have to spin the prop fast enough to drive the plane forward. the same is true if you have too great a pitch on the prop (the angle it presents to the air.) Some props can be trimmed to reduce the diameter and the load. And ground adjustable props can adjust the load by reducing the pitch.  

You can tell if your prop is loaded properly by the RPM that the engine reaches at full throttle. Each engine will be different, but your manual should tell you at what RPM the engine should turn while on the ground (static rpm). If the engine will not produce the necessary RPM's then you have to do something to reduce the load. You can do one of three things: 

1) Change your reduction ratio. I currently have a 2:1 reduction ratio. It's crappy for such a small engine. Don't use one! It's like driving your car in high gear all the time. If at all possible get a 2.4:1 to 2.5:1 ratio. My original redrive was 2.37:1 and it worked like a charm. 

2) Change your pitch. If you have a ground adjustable prop, by reducing the pitch you can reduce the load. But the catch is that if your combination of ratio and diameter is not right, it is possible to have the load correct as measured by sufficient RPM's but the blades of the prop may become so finely pitched that the prop doesn't get enough "bite" of the air to give you a decent cruise. I know, that's where I am now. 

3) Change your diameter. If you have a fixed pitch prop (wooden) then this is your only alternative. My original prop was a 58x26. I couldn't easily change the pitch so I started trimming the blades. I ended up with a 52 inch prop before the RPMs were high enough to make the engine happy. That worked well enough. However, be aware that propellers are much less efficient when they get to the short end. I once flew with a 48" prop and it didn't work very well. 

All of these factors interact with one another. Change one, change the other. Whatever you do, make SMALL changes and make sure your prop is balanced. 

Let me stress the importance of getting the RPM right once more. You might think it will be easier on the engine to let it spin more slowly at max throttle. Don't, the engine is overloaded and chances are that you are courting a heat problem (Your CHT (cylinder head temperature will likely be high) and the engine is more likely to wear out. Also watch for the opposite problem. Your engine has a maximum safe RPM range. If you allow your prop to spin faster than that,  you are risking catastrophic failure. Probably your EGT (exhaust gas temperature) will be high as well.  Also, be aware that the prop unloads when you begin moving. You can expect the prop to spin about 300 rpm faster while flying than it did while you were stationary on the ground at full throttle..

Q: What kind of styrofoam should I use? White, blue, pink, green?

A:  "Styrofoam is the Dow trademark for their blue board 'extruded' polystyrene, sometimes referred to generically as EPS.

"But this too can be confusing because polystyrene comes in both expanded and extruded types, both of which are sometimes referred to as EPS. The expanded type EPS is what you see in foam coffee cups and electronics packaging, while the extruded type is that fine textured blue or pink board which is not made of expanded beads, but which you
can find next to the expanded material in big-box hardware stores.

"Technically, legally, styrofoam cups are not styrofoam cups because Dow owns the trademark and has not released it for use in coffee cups. Generically, in common usage, we all call polystyrene foam 'styrofoam'. Wicks' use of 'styrofoam' and 'polystyrene' foam as two different products, I am guessing, is simply delineating between Dow blue board
and, perhaps, Owens/Corning pink board, which is not legally called 'styrofoam', but which, although many builders are critical of it, is 'extruded' polystyrene similar to Dow's Styrofoam.

"Seems the d'ruthers dictate that Dow's product is best, yet, while some would even suggest using one of the higher compression strength versions of Dow's blue board, the plane was designed around the standard 25psi compression rated 'extruded' polystyrene whether it be Dow's 'Square Edge' 'Styrofoam' or Owens/Corning's 'Foamular 250' available at Home Depot.

"There is currently, and has been in the past, discussion about the shear strength of these foams; and someone even floated a 75psi rating for Dow's product even though Dow does not publish such a rating, and wisely shies from promoting their product for anything but use in the construction industry. Shear strength is a more important quality for
the designer's consideration than compression strength, as the wing spar foam web is placed under longitudinal shear forces more than anything else. If my engineering background serves me correctly, technically, shear strength is a derivative of compression and tension strength qualities of a material.

"As for the dimensions of the polystyrene required for building the Skypup, most people settle for what they can get, then epoxy laminate it together for the dimensions they require before then hot-wire cutting it to shape and finished size. The epoxy in the laminating joints cuts just as easily with the hot-wire or router or saws as does the foam itself.

"For me, I would be satisfied with standard Dow 'Square Edge' blue board, and would have no problem with using Owens/Corning 'Foamular 250' pink board; but would reject the weaker 'Foamular 150'. I personally think, like many here, that going to Dow's stronger comm'l product, 'Highload 40', 40psi compression, could create some other issues in the Skypup.

"All three of these products I just mentioned have an approximate 2lbs per cu/ft density, which is specified by the Skypup designer, Steve Wood, and are interchangeable in that respect. " --Terry

Q: How hard is it to hot wire ribs and other styrofoam parts? 

A: Step 1: Making an accurate template: "For the wing ribs, I cheated. I found a program that traces wing ribs on the computer printer (I think it was "Foils 4.2" but that was a while ago). Then I photocopied printout of the pertainent parts (eg nose rib). I took the photocopy and placed it face down on the template material and ironed it down with and ordinary iron. The ink in a photocopier is heat applied and the iron melts it and transfers it to the wood.  (Alternately, you could glue the printout on the wood.) I then carefully cut out the templates. Also I found it helpful to cut out two matching templates and pin them through with nails in order to guarantee that the edge of each rib was perfectly square. With one template, it was very easy to get an undesired slight bevel because of the flexing of the hot wire"--Roger 

Step 2: Cutting the ribs. "There are two simple and quick [a full set in 15 minutes] ways to cut foam ribs:1 using a hotwire 2 using a router. As I've done both methods, here is a comparison and you choose which one you like. Begin with a wooden template in each case. for the router it should be 3/4" to an inch thick. For hot wire it can be a thin piece of paneling or formica etc. The wire is heated by high current, low voltage electricity supplied by a large battery charger, or a toy train transformer or perhaps a car battery if you include a fuse in the wire. The wire must be kept tight during cutting and nichrome seems to elongate a little less than stainless steel safety wire. The trick is to make a nice straight cut as the wire will be deformed into a curve by the drag of the styrofoam. Nice cuts are a matter of experience. Using a router with a laminate cutter inserted, place the router into a router table and then copy the countour of your pattern. It will help to have a couple of nails coming through the pattern to hold the foam in position. Absolutely no operator skill is required to get perfect ribs the first time. Best of all, they do not have to be sanded as there is no layer of melted plastic on their surface. The only difficulties are that it is loud and terribly messy. Go outside to do it or your wife will hate you forever." --Dennis 

Q: How hard is it to cover the Skypup airframe? 

A: The plans give adequate, but antiquated instructions on covering the Skypup. The original plans called for covering the airframe with "sheath lining material" which is the shear dacron bought from local fabric stores normally used to line clothing like inside of a suit. It was to be glued to the structure with with latex contact cement and then shrunk tight by application of an ordinary electric iron. Then finally sealed with clear polyurethane varnish (the color to be provided by your choice of fabrics). There are several problems with that approach: 

1. Fabric bought from a fabric store is now invariably pre-shrunk. Consequently, if you use it, it will not shrink or very little and you will have a baggy looking aircraft. So I have bought 1.8 oz dacron fabric available at Aircraft Spruce for $3.10 a yard (almost as cheap as fabric store sheath lining). 

2. Latex contact cement was originally chosen because it is waterbased and doesn't eat styrofoam like ordinary contact cement. It seems currently unavailable (at least I couldn't find any). It appears to have been replaced with "neoprene contact cement". I tried it, it stinks. So I am using 3M Fastbond 40 waterbased contact cement, an industrial grade contact cement that in one form amazingly turns from a blue to green as it dries. (available from Grainger Industrial Supply). Some folks taking the high road are using aircraft certified "Cecobond" contact cement (also available from Aircraft Spruce) however it is about $40 a quart and you will need over a gallon (3M Fastbond is about $17 a quart). 

3. Polyurethane varnish has some UV protection factor (especially "spar varnish") but I think better protection and more varied color choice is provided by using latex exterior house paint as many homebuilders have used. 

Q: "How safe is the Sky Pup? 

A: "I believe I have heard of two structural failures in the 20 years since the design was introduced but I never knew any details. I think one might have been in Florida, and maybe the other was in Canada. 

"There was also a report of a spar separation while taxiing which was due to the foolish use of a partial butt splice in the spar cap. 

"There is no reason to doubt the airworthiness of the design, if built correctly. The engineering numbers can be verified. The original spar design concept was proof tested in many different configurations. It is one of the few ultralights to be subjected to a complete finite analysis of loads in the entire airframe. 

"Quite a lot of Sky Pups (almost all) were built and flown over the design gross weight but that has never been a problem as yet. Some examples have been flown way over design gross, and way past VNE. Some have flown nearly 1000 hours. 

"Structural problems could arise in these areas. 

     1. Selection of wood. 

     2. Use of substitute foam 

     3. Changes in the design 

     4. Trouble with bonding 

"The Sky Pup is fairly forgiving to build, has large bonding areas where important and has a sufficient margin of strength in the wood and foam. However, I HAVE seen or heard of problems due to every one of the above factors. Absent any evidence of design overload, I think any problems will likely be traced to factors such as the above. 

"All builders should be aware that epoxy is the only approved adhesive for the Sky Pup. Some early examples were built with other wood glues. There was not a problem as such but Steve Wood concluded that the average Sky Pup builder would get more reliable results with epoxy." --Dan Grunloh.   

Q: Can a plane really fly without ailerons? 

A: Hey model planes do it all the time. The point of ailerons is to roll the wing (tilt it off of horizontal) into a bank, so that the lift of the wing now has a component that is pulling it in a non-vertical direction. So it is actually the lift of the wing that turns the craft, not the rudder or ailerons. The Sky Pup, in common with many trainer-type model aircraft, has a pronounced dihedral (the v-shaped angle of the wings with respect to horizontal). It is this angle that allows the Pup to roll in response to the input of the rudder. When the plane yaws from rudder input (twists to the side) the wing that leads forward produces more lift than the other,  thus rolling the plane and allowing a portion of the lift to turn the plane in a bank.. 

Planes that rely on dihedral/rudder for roll control tend to be self righting and stable. They alway want to return to the level position. The only real drawback to the 2-axis control method is that the pilot lacks the same level of control in cross-wind landing situations that a 3-axis would have. However, skilled Pup pilots have learned to handle moderate crosswind components and a plane as light as the Pup has no business flying in heavy winds anyway.  

Q: I heard it is hard or near impossible to land a two-axis plane in a crosswind. Is that true? 

Q: I want to trailer it to the field to fly. How hard is it to assemble? Can I do it alone?

A:   My plane is now hangared, but I used to tow it to the field (about  1/2 mile away) to fly. I made a tow bar that attaches to the tail skid and I would tow the fuse (backward) to the field on it's own wheels. Then I'd go back home to get the wings which I had on cradles on a small trailer. It generally took me 45 minutes to an hour to set up including the traveling back and forth. That is doing it all alone too. If you decide to go this route, it's not too hard to do alone. I take a wing to the centersection, fasten it loosely with the lower bolt to the wing attach bracket so it will act as a pivot. Then do the same to the other wing. Then I prop up one wing with a broom handle  (padded end) wedged in the tie down strap under the wing. Then adjust the broom handle until the top bolt holes line up and put in the top bolt. do it to the other side and then put on the nuts and leading edge attach bolts, wing gap covers, etc. Pre-flight and off you go! If you don't count the travel time, the hitching and unhitching, it takes about 25-30 minutes to do it alone, 15 with help. By the way, it helps a real bunch to have a dedicated tool kit with exactly (and only)  the tools you need to get the job done. I bough one of those cheap step-stool  toolboxes. Comes in handy when rigging the plane. It takes two sockets, socket wrench,  two wrenches, a hammer, a long 3/8 drive socket extension, pliers, safety wire and a 6" long piece of 1/4" steel rod to drive out the bolts.

Q: Where can I get the plans? 

A:   SKY PUP 

Whitewater, CO  81527

Plans are $70 within the USA and $75 for foreign orders (shipping included). Lafe still has a number of the original color brochures available.  They are $5 within the USA and $6 for foreign (shipping included).

Q: Are there any other sources of construction help beside the plans and manual?

A:   The Skypup Newsletter can now be viewed online for free at  here at this web site. 

You can also check the following web sites:

Edwin's International Sky Pup Yard-Many Sky Pup pictures and tons of general information. (Temporarily unavailable)

Roger's Sky Pup Project

and above all else:

Yahoo Sky Pup Club

      The web sites of Edwin Lelieveld: International Sky Pup Yard and Roger Ford Sky Pup Site.

      Various posts at the Skypup Club Yahoo group- Sky Pup Club by Dan Grunloh, Dennis Gwynn, DanielBK, Scott Perkins,

      Terry Graham

      The Sky Pup Construction Manual by Steven Wood

      The Sky Pup Newsletters: edited by Dan Grunloh

      Other places now forgotten :-)