It appears that the research summarized in that article has concluded that irregular leading edges produce more lift before stalling and have significantly lower drag than typical straight and smooth leading edges.
While I am not advocating that we should all go out and start cutting irregular leading edges on our keels or rudders, I still thought it was an interesting finding. I will see if I can get my hands on the paper and see what it has to say (Reporters generally do not give enough of the important details that we are able to draw difinitive conclusions from their articles).
As I recall, the old Hawker Hunter had a dog-tooth or saw-tooth on its wing leading edge to do the same thing. Look forward to your further report. I would guess that they’ve gotta be testing in a borderline laminar regime, so the turbulator effect of the scallops give net positive gains…
Yes, that is my guess as well. The mechanism they talk about is the vorticity streaming off the leading endge which is energizing the flow. By that I assume they mean boundary layer re-energization which would certainly postpone laminar seperation in the borderline laminar regime. If I had to guess at a reynolds number for a humpback whale flipper, I might come up with a number of like 100,000 - 200,000 so that is certainly right in the ballpark.
Another possible mechanism would be enforced vortex shedding which would break up the tip vortex and thereby reduce the induced drag. If you look at the flipper model that the guy in the article is holding up, all the ridges are out near the tip of the flipper which may indicate that they are interacting in some way with the tip vortex. This would be similar in nature to winglets in their role in reducing induced drag.
Either way, I’m most interested in seeing if the drag reduction gains are across the lift spectrum or only in certain lift points (high lift for example). You certainly would not want to pay a drag penalty at low lift just to gain a drag reduction at high lift when you spend most of your time at lower lift.
It sounds like there will be some follow-on studies as well. So the definitive data on this may not be published for quite some time.
One thing I have learned over the years is that the Creator is not only a master craftsman, but He is really quite brilliant. He has rarely created features with no purpose. It is a shame that we humans take so long to figure out why He has done certain things. The most successful designs in almost any field are usually inspired by His work. I would not be surprised to see the same happen here…
Guys, if I’ve got this incorrect I apoligise.
It seems that whale fin rudders are a new thing to you?
They have been used for a few years now.
Whale fin rudders have already been proven to be far more efficient through the water than standard shaped ones.
It is not the planform shape that is new here, but the scallops on the leading edge. Humpback whale flippers have some pronounced scallops on the leading edges. take a look at the picture from that article to see what we are talking about:
Lester is right that some aircraft have used scalloped leading edges on wings (both intentionally and in the case of fabric stretched over frames, unintentionally). But this is a much more pronounced scalloping effect than I have ever seen on an aircraft wing. I have never seen scallops like this on a yacht rudder.
[:-magnify]
The shape of that fin is very similar to the rudder on my SeaWind. Hmmm maybe I need to get out the file and add a little character to the leading edge.[:-viking]
If this shakes out to be something to try. Would the keel benefit from this?
What I appreciate most about this research is that it reminds us of the complex nature of the dynamic system of a boat and sails going through the water and air. Seems like just when we think we’ve got things figured out, another factor that hadn’t been considered comes into play.
Its why yacht design is still an art and not 100% a science.
Wil
I didn’t think that you wouldn’t have seen, heard about whale fin rudders. That looks interesting. My opinion is it is taking things a little to far. It might work properly through the air, but I doubt it will work on sailing boats.
Just my opinion, I have been known to be wrong several times before.
Keep in mind that mother nature designed those fins to work in water, not air. But as long as the various scaling parameters are held constant (Reynolds number, etc) what works in water will generally also work in air.
Roy,
I agree that there are many many factors that we have yet to find the best solutions to. But I think the biggest thing that keeps yacht design fluid (pun intended) is that it is always an act of comprimise. In full size boats, you often need to trade off interior volume and ammenities versus speed. Safety also comes into play. In model boats interior volume and safety are not as big of concerns, but cost can be. and an optimal design for light wind might be a dog in heavier breeze such that you need to comprimise the design relative to the conditions you expect to see. and above all, there are the rules (rating or otherwise) that limit what you can do. Designers are constantly looking for “rule beater” designs which leads to more tradeoffs which might hurt the overall speed, but will help the rating such that the boat will finish higher on corrected time (which is why I prefer one design classes over open classes…)
Tom,
Don’t get your file out yet. This is a relatively new set of data and you really need to see what it says before you run off trying to apply it… But it may apply to the keel as well as it does to the rudder…
What I meant is that for application to sailing boats the “grooves” on the leading edge may cause there to be to much disturbed water flowing over the rudder. This is not good for sailing boats, but whales don’t seem to turn to quickly.
I’m now thinking that disturbed air for planes won’t be all that good either. I know nothing about planes so can’t comment further.
HI,
I was under the impression that the Leading Edge mods on the Hunter were to improve the airflow at supersonic speeds ACROSS the wing surface.
Later aircraft had Wing Fences fitted to the wings for much the same reasons, the Fences jutted out from the leading edge and guided the air flow to the rear of the wing.
Copying things from supersonic aircraft to put onto model boats, is pushing things a bit too far I think.
John.
scalloped_flipper.jpg