An Interesting Science Fair Experiment

[Earl Boebert]

The International Science and Engineering Fair was in town this week, and one exhibit caught my eye:

The student built two wings, one with a straight leading edge and one with sinusoidal bumps (as in a whale flipper) and tested them in a wind tunnel. The whale flipper had 14.8 % lower drag in angles of attack from 4 to 18 degrees and an average of 20.9 % greater lift at all angles except 16 deg. Most interesting to me was that the stall characteristics (decrease in lift) was much gentler for the whale flipper.

Hmm. How long before we see "whale flipper" fins, rudders, and keel wings?

Cheers,

Earl

[Guillermo]

Hi Earl,
See: http://boatdesign.net/forums/showthr...=whale+flipper

[Eric Sponberg]

Already been done, but the public in general is slow to catch on. About 20 years ago, David Pedrick, a well known fellow naval architect in Newport, RI, developed what he called the "Whale Tale" keel, which closely resembled a whale's tale.

On rudders particularly, you may see some rudders that have a straight leading edge, rounding at the tip rather sharply, and then rounded at the trailing edge, a type that has been quite common. This sort of orientation is backwards. If that rudder were turned around so that the curved edge was forward and the straight edge aft, it would be more effective--more lift, less drag. You can see an appropriate shape on my rudder for Bagatelle, picture below.

I learned to sail on a 1965 Sunfish, which I still own. Sunfish owners may know that the older style daggerboards had the curved tip with a tighter radius forward, gentler radius aft. See figure attached. Following the reasoning of the whale's tail, the daggerboard would be more effective if it were flipped leading edge for trailing edge.

Finally, as in the orientation of the Sunfish daggerboard and Bagatelle's rudder, we find that windmill blades and airplane propellers, there is less drag and more lift if the tip edges are swept aft. In airplanes these are called scimitar propellers. See photo.

Eric

[frosh]

In boats, aircraft, wind turbines and probably ather applications, this is a HUGE increment in knowledge. I am just blown away by the significance of this, and much more so that I have only just become aware of it this moment, after a lifetime of being passionate about aerodynamics and hydrodynamics. More power to the person who elucidated this knowledge and then proved it scientifically. I pay homage.
Sam Frosh

[Tim B]

So....

We all know that sharp corners at the trailing edge give cleaner vortex detachment and better induced drag (read Hoerner etc.).

Somebody explain to me WHY this scalloped planform performs better. Let's see some flow visualisation. What dependancy does it have on Reynolds number? Is it playing with the boundary layer?

To be honest I'm starting to think that a single or double-taper foil with a parabolic or Kuchaman tip is about as good as any.

Saying it's better just because it gives the answers you wanted is dangereous territory in my opinion. It needs to be backed up by some good theory.

Tim B.

[Guest625101138]

I design high efficiency props. These typically operate below 2 degrees angle of attack.

So how did the sinusoidal leading edge work below 4 degrees?

[Raggi_Thor]

Originally Posted by Earl Boebert

...one with sinusoidal bumps (as in a whale flipper)...

You mean the leading edge looks like a repeating sine curve in side view?
One reason for not usng that planform would be production costs.

[Raggi_Thor]

Wait a moment, that shape would in effect make the foil larger wouldn't it?
So you have to compare with a larger "normal" foil?

[Earl Boebert]

The shape was that shown in Guillermo's link above; a row of "bumps" on the LE, diminishing toward the edge. The pictures in the linked thread show it clearly. The displays were not "audience friendly;" they consisted of storyboards set back behind a barrier and the typefaces were typically too small to read from the aisles (obviously, the judges could get close).

Cheers,

Earl

[kach22i]

Originally Posted by Guillermo

Hi Earl,
See: http://boatdesign.net/forums/showthr...=whale+flipper

Yep, see the articles and pictures in that thread, that guy really knows his stuff.

This old article from Hamilton is interesting, post WWII. Don't click if you are on "Dial-Up", it's a big page.

http://www.combatreform2.com/killerbees2.htm


As for winglets on props, see this:
http://www.tampabayaerosport.com/propellers.html



As a general rule: 1) curved blades are more quiet,
2) winglets (like close tollerence ducts) control tip vortex,
and 3) Riblets keep air on the desired surface preventing it from wandering off.

Turbine Blades
http://cirrusaviationinc.com/jt8d.htm

[Tim B]

The problems encountered (thinking back to the last generation of Spitfires) wasn't that they couldn't achieve high speeds, it was being under control at the time. That was where the all-moving tail came in. That though has little to do with the origenal subject.

From what I have heard, riblets have not shown a conclusive benefit.

Tim B.