Contents
Aspect Ratio
ASPECT RATIO ....
...is all about shape. In particular we will take a look at the overall
shape of a sail or underwater board. Aspect ratio, let's call it AR,
has always been a concern to aircraft designers all the way back to the
Wright brothers who did a lot more than just fly the first airplane.
They were also pretty good scientists, I think, and in the Air Force
Museum I recall seeing a wind tunnel built by the Wrights, very state
of the art back then.
Anyway, AR is defined as AR=bxb/S where "b" is the span of wing in
aircraft terms and "S" is the total area of the wing. Look at this:
What this is supposed to show is two airplanes from a top view. They
have the same wing area and if they had the same weight and wing cross
section they should have the same stall speed and so forth. But they
would fly totally different from each other. The plane with the long
narrow wing might be a modern sail plane. High powered jets might have
a short wide wing. So what's the deal with the shape?
I GOTTA TELL YOU...
...that aerodynamics seems at times to be like witchcraft, with
correction factors piled on high to theories that never seem to explain
everything. Luckily a hundred years of research has left us with many
choices of correction factors. Hey! One day I'm sitting at the missile
factory and the boss is agonizing over having to add little studs to
his missile so it will fit a new launcher. "They will ruin the range
with extra parasitic drag, everybody knows that," says the aero
department. "But we gotta have them," says the boss. "We'll build it
with studs and fly it and see what happens," he says. The test goes
very well indeed, the studded missile outflying the nonstudded. "The
studs acted like vortex generators and energized the boundary layer
thus reducing drag, everybody knows that," says the same aero
department. Well, they have lots of majic wands in aero but they know
their aspect ratio effects. To start take a look at this:
This is an airplane viewed head on. The thing flies because the air
pressure acting on the underside of the wing is higher than that on the
top. But in real life the air near the tip will try to escape from the
bottom to the top by flowing around the tip, being pushed by the
difference in pressure there. It swirls from bottom to top as the plane
flies forward making a vortex. You can see them under certain
conditions as they cause swirling clouds to appear at the wingtip.
Now, this doesn't happen in wind tunnel testing of wing sections
because the wing model usually goes across the tunnel so that air can't
swirl around the end. That testing is supposed to be independent of
"3D" effects so the wing section test data is reallty for a
hypothetical wing of infinite span, no tip losses. So adjustments are
made to calculate the effect of real life wingspan. The effect is I
think mainly in increased drag and I don't recall ever seeing any
wingspan related adjustment to maximum lift.
Great efforts have been made to reduce the swirls such as twisted wing
tips, endplated wingtips and lately elaborate winglets on the tips.
But clearly a long narrow wing has less wingtip than a short wide wing.
NOW TO THE BOAT...
When a sailboat is close hauled, that is sailing close to the wind,
both the sail and the boat's underwater board are acting as airfoils,
the sail through the air and the board through the water. Here is an
end view....
The wind pressure on the sail causes a vortex at the top and bottom of
the sail, just like an airplane's wing held vertically. I've never
checked for them in real life but I suppose you could with tufts or
ribbons that will align to the air flow there. Or maybe a good excuse
to start smoking cigarettes could be made in saying you hope to view
the vortex at the bottom of the sail. Or if you are say an Englishman
you might sail in fog a lot and actually see the vortex.
The sideforce on the underwater board is a reaction to the sideforce
made by the sail in a close hauled sailboat. The board also acts like
an airfoil and actually flies through the water (which is about 1000
times as dense as air). One difference here is that the board has no
gap to the hull at the top. There can be no vortex there. Same with a
rudder or leeboard which extends past the water's surface. So it has
half of the usual vortex effect.
Hey! If you have a leeboard boat you can sometimes see the vortex at
the end of the board! I've done it with Piccup Pram. Raise the board a
bit to get the tip closer to the surface and effectively reducing the
aspect ratio. Get the boat close hauled in such a way that you can peer
over the side at the tip of the board. There it is! Looks like a white
rope twisting off the end of the board.
Now, you ask, what if the sail also extended to the deck? Would that
not eliminate one vortex. Yes and some sails have been made to do that.
I think large headsails of racing boats in particular can be made this
way. But they are deck sweepers and clearly are a big bother on the
usual day sailer. I remember reading somewhere that the gap needed to
prevent the vortex is quite small, say one tenth of the sail's width or
less. The sail on my Bolger Birdwatcher might approach this. You can
sort of get away with this on a Birdwatcher because you are always
supposed to be in the cabin. But even here the decksweeper is a very
dangerous thing. So a decksweeper sail is a feature I try to avoid.
DOWNWASH....
So far I'd be tempted to say "So What!" The short wide wing has bigger
tip vortices than the long narrow wing. But the aero guys point out
that the swirl produces a "downwash" over the entire wing, not a good
thing at all since the wing is supposed to be pushing us up. The aero
guys simply add the downwash to the real wind like this:
So what we have here is that the pilot sees he is flying at say 10
degrees to his flight path. But the wing tip vortices are swirling the
wind on the wings downward by say 5 degrees. So he is really flying at
5 degrees to the wind the wing is experiencing. That might not be a
huge factor to a pilot who can correct by changing the aircraft's pitch
and get the lift proper simply by finding the pitch that gives level
flight. He might find that the real limitation here is that on final
approach to the runway he is pitched so far upward that he can't see
the runway! I suppose the Concorde airliners were the best example
here. Short wide wings are great for supersonic flight, like say the
Space Shuttle, but they produce a downflow such that the required angle
of attack on landing resulted in the design of a folding nose to give
pilot vision over the bow.
Now, according to the airfoil section data his wing might stall at say
15 degrees angle of attack. The plane might appear to have a flight
path of 30 degrees in very slow flight but the airflow local to the
wings is actually below the stall angle once you correct it for
downwash. I am guessing at all these numbers but I hope you see how it
works.
Let's contrast this with the sailplane with the wing aspect ratio of
25. His wing tips are tiny and so are his wing tip vortices and so is
the resulting downflow on his wings. He is approaching 2 dimensional
flow like the section tests in the wind tunnel.
Of course the glider pilot's idea of "level flight" is different from
the Concorde's pilot. He is slowly gliding downward but the angle can
be quite low, maybe 50 to 1 in a super glider (I'm guessing). I am
going to guess again and say maybe his "cruising" speed is twice his
landing speed, unlike the Concord which might cruise at ten times its
landing speed. So the glider may not see downwash as a huge part of its
life. He flies and lands flat. The downwash issue has been designed out
when the long long high aspect ratio wing was designed in.
WHAT! MORE NUMBERS......
When the pilot tilts his plane upwards to correct for downwash a nasty
thing happens. See below...
Same drawing as before but note that by adding pitch to account for the
downwash the wing's force also tilts backward and creates an added drag
component "induced" by the downwash. It is proportional to the
aircraft's lift and to the downwash.
The aero expression for calculating the induced drag coefficient (to be
added to the basic parasitic drag coefficient to calculate total drag)
is fairly straight forward. Essentially it is just (Cl x Cl)/(3.14 x
AR) where Cl is the lift coefficient and AR is the aspect ratio. And
right off the bat you can see that in our very first example the short
winged plane with AR=4 always has about six times as much induced drag
as the long winged plane with AR=25.
Induced drag is at its worst when the lift coefficient is high such as
when landing or high G maneuvers. It can become very high for short
wing airplanes. It can overwhelm other types of drag at times. A low
speed glide angle might be quite steep in the downward direction. Or if
thrown into a high G turn the short winged pylon racing airplane will
slow down a lot and lose his lead to the racer with the long narrow
wing. But the Concord cruising at mach 3 is at a very low value of Cl
and induced drag is of no concern then.
I suspect the close hauled sailor is always operating at a high value
of Cl or at least at the best lift/drag ratio his sail can produce. And
for the close hauled sailor that extra induced angle of attack comes
right out of his sail's ability to point into the wind. We'll take a
closer look at that next time.
Contents
Jewelbox Junior
SAILBOAT, 15' X 5', 400 POUNDS EMPTY
JEWELBOX JUNIOR
This is Jewelbox Junior, a 15' version of the original 19' Jewelbox
built a while back by Karl James in Texas. That Jewelbox went all
over the US and parts of Canada and I understand was sold a few years
ago to someone in Florida and replaced by a larger sharpie that Karl
had designed. Here is a photo of the original Jewelbox.
JB Jr is also narrower than the original boat, the bottom now planked
with just two sheets of 1/2" plywood. Perhaps a good comparison of
the two boats would be that Jewelbox needs 16 sheets of plywood and
JB Jr needs 9. In particular I hope that JB Jr could be towed behind
a small car. Two protoypes of JB Jr were completed last fall. One by
Vern Stevens in Idaho and the other by Erwin Roux in Pennsylvania.
Vern had a chance to take this photo before winter hit:
And Erwin sent quite a few great photos taken on a beautiful autumn
day.
Here you see that JB Jr has that Birdwatcher cabin. The idea behind
the Birdwatcher cabin, invented by Phil Bolger in his Birdwatcher
design, is that the crew sits low inside the cabin looking out
through watertight windows.
The crew weight thus acts like ballast. The boat becomes more stable
with extra crew where a normal raised deck boat becomes less stable.
I did some paper studies of the self-righting abilities of Junior.
With it lighter bottom planking, Junior is bound to be less in that
department than Jewelbox, which Karl says has righted from having its
windows submerged. By my studies Junior should self right from up to
65 degrees of roll. Beyond that and she would roll another 15 degrees
and become stable on her side. She won't flood due to her Birdwatcher
cabin. If you couldn't rock the boat back upright you would have to
exit, right the boat from the water by stepping on the leeboard, and
climb back in. And you would need a reliable step to do that in any
high sided boat. These are just paper studies. I would expect my IMB
design to behave the same way. Larger heavier designs, like Jewelbox
and Scram, should self-right from a full knockdown. More weight on
the bottom, especially another layer of plywood there, would be a
good investment if you could tow the extra weight.
I've shown Junior with a sharpie sprit rig, although you could
substitute a lug sail as Stevens did.
JB Jr plans are $35. Simple nail and glue construction with no jigs
or lofting.
Contents
Prototype News
Some of you may know that in addition to the one buck catalog
which now contains 20 "done" boats, I offer another catalog of 20
unbuilt prototypes. The buck catalog has on its last page a list
and brief description of the boats currently in the Catalog of
Prototypes. That catalog also contains some articles that I wrote
for Messing About In Boats and Boatbuilder magazines. The Catalog
of Prototypes costs $3. The both together amount to 50 pages for
$4, an offer you may have seen in Woodenboat ads. Payment must be
in US funds. The banks here won't accept anything else. (I've got
a little stash of foreign currency that I can admire but not
spend.) I'm way too small for credit cards.
I think David Hahn's Out West Picara is the winner of the Picara
race. Shown here on its first sail except there was no wind.
Hopefully more later. (Not sure if a polytarp sail is suitable
for a boat this heavy.
Here is a Musicbox2 I heard about through the grapevine.
This is Ted Arkey's Jukebox2 down in Sydney. Shown with the
"ketchooner" rig, featuring his own polytarp sails, that is shown
on the plans. Should have a sailing report soon.
And the Vole in New York is Garth Batista's of
www.breakawaybooks.com, printer of my book and Max's book and
many other fine sports books. Boat is done, shown here off Cape
Cod with mothership Cormorant in background, Garth's girls are
one year older. Beautiful job! I think Garth is using a small lug
rig for sail, not the sharpie sprit sail shown on the plans, so I
will continue to carry the design as a prototype boat.
And the Leinweber's make another prototype! This one by Sandra,
an Imresboat shown here on its first outing. They are taking it
on a "cruise" so more about it later.
And a new Down Under Blobster, now rightside up for final finish.
Looks like another beautiful job....
A view of the Caroline prototype showing a lot of the inside,
crew on fore deck. Beautiful color:
I gotta tell you that on the Caroline bilge panels I made an
error in layout and they are about 1" too narrow in places on the
prototype plans. I have them corrected but it always pays, even
with a proven design, to cut those oversized and check for fit
before final cutting.
Contents
AN INDEX OF PAST ISSUES
BACK ISSUES LISTED
BY DATE
SOME LINKS
Mother of All Boat
Links
Cheap
Pages
Duckworks
Magazine
The Boatbuilding
Community
Kilburn's Power
Skiff
Bruce Builds
Roar
Dave
Carnell
Rich builds
AF2
JB Builds
AF4
JB Builds
Sportdory
Hullforms Download (archived copy)
Plyboats Demo Download (archived
copy)
Brokeboats (archived copy)
Brian builds Roar2 (archived copy)
Herb builds AF3 (archived copy)
Herb builds RB42 (archived copy)
Barry Builds
Toto
Table of
Contents
