Sky Pup General Information
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 ofapproximately
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 Sky Pup 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, CO81527 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.
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: Did I hear your right, this plane is made of styrofoam?
A: Yes indeed! In order to minimize weight and increase strength the Sky Pup uses
a method of spar construction where the styrofoam essentially defines the shape,
but it is bonded to two wooden spars. Think of an I-beam, the styrofoam is the web,
the vertical component of the beam while the wooden spars are the top cross pieces
that take the actual load of the beam.
One needs to exercise caution in using styrofoam. White styrofoam that appears to
be constructed with thousands of plastic beads is totally unacceptable. What your
are looking for is higher density styrofoam at 2 lb.per cubic foot. Foams come in
many colors, but you will not go wrong with blue Dow insulation board. Owens Corning
pink foam MAY be a close second, but blue Dow board is the foam of choice.
Q: Can I readily customize the plane?
A: Any deviation from the plans must be VERY CAREFULLY considered. Are you and engineer?
Can you be sure that your alteration will not weaken or increase the weight of the
plane. The gross weight of the Sky Pup is only 400 pounds. Assuming you build it
light (200 lbs) that only leaves 200 lbs for pilot and fuel.
"Probably the most considered alteration to the Pup is adding ailerons. It's been
tried and found wanting! As one modifier comments "The airplane flies "ok" but I
am disappointed in the way the ailerons respond. I wish I would have installed spoilers
instead." --Pupbuilder2. [Ed. Note: Some of the same problems noted below would also
apply to spoilers.]
“Adding ailerons would require a major redesign of the wing which would add enough
weight as most of them are made[ 230 lbs without ailerons] put it out of the ultralight
category. The benefits of changing from dihedral & rudder control to aileron control
can be questioned. Without ailerons, it forces the pilot to lift the low wing with
the rudder. Proper [safe] technique. Using ailerons[ an inheritly natural action]
for this just above stall speed will result in a stall and possibly a crash. So,
as designed, the skypup is safer than it would be if modified to carry ailerons.
It also depends on the pilot. Never wager that a competent pilot in a two axis p[ane
can not handle crosswinds. Ultralights [legal ones] are not really suited to fly
in windy conditions. A Cessna does that better. And a F86 does it better than a cessna.
It is all relative. Expect to use different airplanes for different tasks." --Dennis
Specific Construction Questions
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
14151 Raket 120 Aero ES
15421Hirth 240
15281Hirth f36 (including redrive, exhaust, carb!)
28351Hirth f33 (42 lbs. with redrive)
15382OMC opposed [direct drive]
16422hith291
16141Solo 180
17301Solo210
18?381Rotax 247
18 ?481Rotax 247 fan cooled
19461Sachs 294
20481Kohler 292
20421Cuyuna 215 fan cooled (The Sky Pup was designed around this engine)
20-25172Limbach 275 (twin opposed)
21381Yamaha 292
21341Zenoah 250
22511Zenoah G25 B-1 (complete engine, cd ignition)
2134Zenoah 250
2138Yamaha 292
21?472Kawasaki 250
22322JPX 425
22?481Rotax 299
25251JPX d 320 (35 lbs. with redrive, exhaust, elec. start)
25442Tohatsu 295
25431Chotia 430 [direct drive]
25372Hirth 400[opposed]
25?481CCW 312[fan cooled]
25492KFM
2532JPX 505
2537Hirth 400
2642Rotax 277 (55 lbs with redrive & carb)
2742OMC
3244Kawasaki 340(note thatthis weight is for a cdi ignition engine)
37-4044Kawasaki 440(points ignition engines weigh 3 lbs more)
25-4041Kawasaki 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.
Information on Flying the Sky Pup
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?
A: Flying the Skypup in light to moderate crosswinds is not a problem. You simply
set a crab angle to keep your groundtrack following straight down the runway path.
You hold the crab through the entire final approach, adjusting the angle to keep
your groundtrack lined up. Just before you touch down you smoothly take the crab
out so that the airframe is aligned with the runway. If you do this too soon, you
will drift sideways and consequently put heavy sideloads on the wheels and landing
gear. However,if you time it well there won't be a problem. The crosswind becomes
problematic when you slow down and your rudder looses effectiveness. You will have
a tendency to weathervane into the crosswind. You can land in stronger crosswinds
than you can taxi!
BTW this technique works best on grass strips, pavement is harder.
You don't want to be flying in heavy crosswinds, Dan Grunloh describes his experience
in windy conditions: "Headwinds were 18 mph and I landed in a crosswind. The landing
was almost entirely on one wheel and the pup turned 90 degrees to the runway before
coming to a stop."
