How good is Eppler no. 836?
Johannes Schoon
centerboard for my dingy. For a long period of time, I was almost
certain I would use the NACA 0012 section.
This week however, I happened to take a closer look at the Eppler
section E836. I found it in a red book (I think you know which one I'm
talking about. I merely Xeroxed the relevant pages and forgot to write
down the title.) It seems to be the right thing for me judging by the
short characterization of it:
"Hydrofoil E 836
This symmetrical hydrofoil represents a compromise between
cavitation and boundary-layer requirements. It may be adequate fore
vertical surfaces like the centerboards of small sailboats ore the
skegs of windsurfing boards. It is relatively thin, which is
sometimes required in the rules for contests."
It looked good after comparing the Cd(Cl = 0) values at various
Reynolds numbers with those of the NACA 0012.
Now comes the tricky part of this story, the part that makes me raise
my voice and ask for your advice.
In that red book (the title still eludes me), there were diagrams for
the NACA 0012 too. There were, however, no diagrams for the NACA 0012
with a smooth surface. Because of this it is impossible to compare his
NACA 0012 results with measurements published elsewhere. Also, it is
impossible to compare the NACA 0012 with the E 836 section. Any of
these tests would have been interesting.
How far off in its predictions is the Eppler code?
What reputation have his foils?
Johannes Schoeoen
dept. of Naval Architecture and Ocean Engineering
Chalmers University of Technology
Gothenburg, Sweden
dr...@mit.edu
sch...@na.chalmers.se writes:
|> One of these days, I'll finally get my act together and make a new
|> centerboard for my dingy. For a long period of time, I was almost
|> certain I would use the NACA 0012 section.
|>
|> This week however, I happened to take a closer look at the Eppler
|> section E836. I found it in a red book (I think you know which one I'm
|> talking about. I merely Xeroxed the relevant pages and forgot to write
|> down the title.) It seems to be the right thing for me judging by the
|> short characterization of it:
|>
|> "Hydrofoil E 836
|>
|> This symmetrical hydrofoil represents a compromise between
|> cavitation and boundary-layer requirements. It may be adequate fore
|> vertical surfaces like the centerboards of small sailboats ore the
|> skegs of windsurfing boards. It is relatively thin, which is
|> sometimes required in the rules for contests."
|>
|> It looked good after comparing the Cd(Cl = 0) values at various
|> Reynolds numbers with those of the NACA 0012.
I've never analyzed the E 836, but here are some things to consider:
First of all, cavitation is a non-issue on your dinghy, unless you
plan to sail it at 30 or 40 knots!
You should also look at Clmax, not just Cd(Cl=0). The most demanding
condition for a sailboat keel occurs just after a tack, when the speed
is the lowest and the sail fills up again. Here the boat wants to slip
sideways, and you need a high Cl from the keel to quickly regain speed.
In a tacking duel, this may be more important than a low Cd(Cl=0).
A suitable airfoil "goodness parameter" is Clmax/Cd(Cl=0).
For what you need from a dinghy keel, the NACA 0010 or 0012 sections are
hard to beat. At your Reynolds numbers (I'm guessing 1/2 million), they
have about 80% laminar flow at Cl=0, and they also have much higher Clmax
than traditional "laminar" sections. If the E 836 is better, it won't
be by much.
A keel has serious problems with separation of the thick hull boundary
layer at the keel root. It's probably a good idea to locally reduce
the % thickness there -- maybe down to 0008, by suddenly flaring the
chord while keeping the absolute thickness the same.
Mark Drela First Law of Aviation:
MIT Aero & Astro "Takeoff is optional, landing is compulsory"
Martin Schoon AR/RH
|> In article <950202235...@nasp.dfrf.nasa.gov>,
|> sch...@na.chalmers.se writes:
|> |>
|> |> This symmetrical hydrofoil represents a compromise between
|> |> cavitation and boundary-layer requirements. It may be adequate fore
|> |> vertical surfaces like the centerboards of small sailboats ore the
|> |> skegs of windsurfing boards. It is relatively thin, which is
|> |> sometimes required in the rules for contests."
|> First of all, cavitation is a non-issue on your dinghy, unless you
|> plan to sail it at 30 or 40 knots!
I guess (hope) that what Eppler really was aiming at in the cited text
was 1avoiding ventilation (air being sucked down from the surface).
|> You should also look at Clmax, not just Cd(Cl=0). The most demanding
|> condition for a sailboat keel occurs just after a tack, when the speed
|> is the lowest and the sail fills up again. Here the boat wants to slip
|> sideways, and you need a high Cl from the keel to quickly regain speed.
From my own experience (and Johannes') I know that this is very real.
My own boat is a cat and it is 5.5m wide and there is only a rather
open net between the hulls so one has a pretty good view of the
daggerboards. If you make a badly performed tack and come out of it at
low speed and full load you can actually see the eddies from the
daggerboards and how they 'disappear' as the boat accelerates and the
flow becomes more 'normal'.
<snip>
|> For what you need from a dinghy keel, the NACA 0010 or 0012 sections are
|> hard to beat. At your Reynolds numbers (I'm guessing 1/2 million), they
|> have about 80% laminar flow at Cl=0, and they also have much higher Clmax
<snip>
The engineering challenge here is really the wide Re-span the
daggerboards (and rudders) must cope with. Take for instance my cat
again (back of the envelope figures) At top speed to windward Re~2.5
million and Cl~0.3-0.4 (very much back of envelope!). Coming out of a
tack (half-badly performed) Re~1 million and Cl~1. Then we might have
this light weather race when the average Re~0.6 or maneuvering at low
speed (starting line or harbour). Top-speed reaching gives me Re~4
million...
|> A keel has serious problems with separation of the thick hull boundary
|> layer at the keel root. It's probably a good idea to locally reduce
|> the % thickness there -- maybe down to 0008, by suddenly flaring the
|> chord while keeping the absolute thickness the same.
Not a very practical thing to do to a retractable foil such as a
daggerboard and glancing in Hoerner's books I get the impression that
it is absolute root thickness rather than relative root thickness that
is the culprit.
Hm, I have tried to come up with a first law of sailing but so far I
have had no success.
[Moderator's note: The first law of sailing is that if you have to
ask how much it costs, you can't afford it. This is also the first
law of aviation, I'm sorry to say. MFS]
============================================================================
Martin Schoon <era...@eras70.ericsson.se>
"Problems worthy of attack
prove their worth by hitting back"
Piet Hein
============================================================================
Craig Wall
>First of all, cavitation is a non-issue on your dinghy, unless you
>plan to sail it at 30 or 40 knots!
>
>You should also look at Clmax, not just Cd(Cl=0). The most demanding
>condition for a sailboat keel occurs just after a tack, when the speed
>is the lowest and the sail fills up again. Here the boat wants to slip
>sideways, and you need a high Cl from the keel to quickly regain speed.
>In a tacking duel, this may be more important than a low Cd(Cl=0).
>A suitable airfoil "goodness parameter" is Clmax/Cd(Cl=0).
>
>For what you need from a dinghy keel, the NACA 0010 or 0012 sections are
>hard to beat. At your Reynolds numbers (I'm guessing 1/2 million), they
>have about 80% laminar flow at Cl=0, and they also have much higher Clmax
>than traditional "laminar" sections. If the E 836 is better, it won't
>be by much.
>
>A keel has serious problems with separation of the thick hull boundary
>layer at the keel root. It's probably a good idea to locally reduce
>the % thickness there -- maybe down to 0008, by suddenly flaring the
>chord while keeping the absolute thickness the same.
Mark, it's been my experience with dinghies (and I'm talking about
the lower performance types, like the Optimist Pram) that flat plates
work better in many cases, the reason being that for a decent profile
to obtain significant lift it requires a substantial angle of attack,
especially if you hope to reduce the absolute size ot the surface.
The problem with this is that the hull is now mis-aligned with the flow
to a significant extent. Unless you wish to incorporate variable
incidence in the mounting of the keel, so you can adjust the AOA independent
of the hull, you may wind up with more drag overall. Sure, the *keel*
is clean, but the hull is now dirty.
What you might want to look at is how much AOA is required for the keel
to do it's job. For flat plates it's often much less than for a foil
section. (This was brought home to me vividly when I put a foil
section on the rudder of my pram- it suddenly required much large
tiller deflections to steer the boat, and was noticeably draggier
than before I streamlined the "slab" section I originally had.)
Craig