aerodynamic advantages of bumpy leading edges

[flieslikeabeagle]

There's a fascinating item in the August 2004 issue of Scientific American magazine on the aerodynamic advantages of bumpy leading edges. The short version: biology professor Frank E. Fish noticed in the 1980's that humpback whales have large, evenly spaced bumps along the leading edges of their flippers. After two decades of experimentation (!!), including the wind-tunnel testing of plastic facsimiles of humpback flippers with and without the characteristic bumps, the conclusion is that the bumps confer significant aerodynamic advantages. To wit: the flippers with bumpy leading edges generate as much as 32 percent less drag, and as much as 8 percent more lift, than similarly sized flippers with a conventional smooth leading edge. Further, the fins with the bumpy leading edges resist stall, withstanding "40 percent steeper" angles of attack than the smooth fins.

Apparently these results come from the creation of counterrotating vortices at either side of each leading-edge bump. The vortices "inject momentum into the fluid flow, which keeps the flow attached to the upper surface rather than allowing it to separate".

Bumpy-winged RC planes, anybody? Heck, a third less drag is one heck of an improvement!

-Flieslikeabeagle

[vintage1]

google turbulators.

also water is denser than air..

[Majortomski]

Actually it's not all that new, the biologist was just surprise that the Whale had the benefit naturally.

These "turbulators" trip the boundary layer and make most airfoils behave like they're laminar. Look at the leading edge of any Learjet and you'll find little machined diamonds attached near the leading edges.

Odd that our "newer" laminar flow airfoils loose their efficiency when bugs and such stick to the leading edge. I recall the story of a little Quickie. The guy flew his test flights off before he'd finished the paint job, and the plane flew great. After painting a few stripes on the wing and fuse the airplane was almost unflyable because the edge from the paint stripe was destroying the laminar flow on the previously clean wing.

[olmod]

and now you know why golf balls have dimples :D

[flieslikeabeagle]

Anyone know why the whale's tubercles are so much larger in proportion to the dimensions of its fin? Is this just a Reynolds' number scaling effect? And how do Reynolds numbers for a swimming humpback whale compare with a model aircraft?
I know auto companies have for years towed small-scale models of automobiles through water tanks at low speed as a way to replicate the effects of (more expensive) high speed wind-tunnel testing.

-Flieslikeabeagle

[vintage1]

Thast no whale, thats a top secret Al Qaeda submarine....equipped with listening devices in its phased array fin..:D

[Purdue Aero Man]

dunno about the whole al qaeda thing, but the bumps do serve the tactical purpose of housing the whale's numerous 20mm cannons used for under-water combat.

My memory of the quickie-type airplane accident was that it had a laminar airfoil and then while trying to land while raining, it crashed and killed the pilot. I could be wrong though.

The bumps being spaced that far apart for vortex generators is probably the correct idea, but I can't quite figure out the effect of them that far apart. I've never heard much about Re's affect on vortex generator location and size. Interesting idea though. Good thread topic.

[Salto]

I think it's instructive to seperate two different effects here. The issues of bugs and rain effecting "laminar flow" airfoils, and the dimples on golf balls, etc. are small scale effects which are uniform right across the span (or around the ball's circumference). They are related to tripping laminar flow into turbulent flow, which in the case of the golf ball, energises the boundary layer and helps delay flow detachment. Momentum is transferred from the free stream down to the lower levels of the boundary layer through uniform, small scale effects, that are confined to within the boundary layer itself.

But I think the whale fin effect is something completely different. Here, there appears to be non-uniformity of flow as you move out along the span - the "counterrotating vortices" mentioned above. This is a completely different effect on a larger scale. Looking at the scale of the bumps, I suspect these vortices reach right up beyond the boundary layer, well into the free stream.

Both mechanisms achieve the same thing - bringing energy down into the boundary layer - but they do it quite differently and on different scales.

BTW, I've seen those whale bumps and ridges on surfboard and sailboard fins, with anecdotal claims of delayed stall and higher lift.

There might be another effect from the bumps and ridges. That of forcing the flow into a chord wise direction, which would have the effect of reducing the span wise component of the flow near the tips, thereby increasing the amount of lift generated there, as well as reducing the tip vortex (induced) drag.

Graham.

[Purdue Aero Man]

Why doesn't somebody just ask the whales?

[Peter Khor]

fwiw discussed over at Electric Talk back in May:

http://www.rcgroups.com/forums/showthread.php?t=229830&highlight=whale

No follow-up though, anyone care to try it on a wing?

[Sparky Paul]

Those things are called "vortilons" on full-scales.

[raptor22]

Something I saw a few years ago that was interesting concerning about the new "fastskin" fabric on the full-body competition swimsuits made by speedo. They placed bumpy strips every little bit on the smooth fabic. The water tunnel tests showed that the strips energized the boundary layer locally, and the higher energy portions of the boundary layer would twist around the more laminar areas creating a vortex.

When the boundary layer would normally seperate, the vortexes *reportedly* twisted away from a swimmers body and continued on. Supposedly, these vortexes effected the water around them as they spun away and stopped the water from creading that tubulent eddy-type flow that causes so much drag.

They reported that their suits tested "far better" than those that were perfectly smooth.

--Alex

[50+AirYears]

In a book by a German test pilot from WW2, there was a statement by him that they were rarely able to get captured P-51 mustangs to acheive the performance that we dumb Americans got. Found out after the war that the laminar flow wings had to be kept scrupolously clean. Small amounts of dirt hurt the airflow.

[AndyKunz]

Why doesn't somebody just ask the whales?

Or better yet, the Designer. He knows!

Andy

[mwraight]

Hey -- Purdue Aero Man -- I went to Purdue too!

It seems to me that this would work well for the application it's designed for -- Whales! That is probably the extent of it. Remembering of course that whales actually flip their flippers and move them around -- also that whales are working their way through water at relatively low speeds. Whales are also bigger than our models, so it may not work on a smaller scale. There are a hundred considerations here.

I did build a model and cover it with bed sheet fabric -- and it developed little bumps like that on the leading edge and all the way down the top and bottom of the wing. It flew, but then again, most things will fly on the slope! A friend of mine also built a glider with a serrated coroplast leading edge. No word from him yet how it flew.

I would tend to lean towards the observation that we're at the pinnacle of aerodynamics and there isn't going to be any major breakthrough discovies that greatly increase efficiency. There is no perpetual motion -- you always have to remember that. There are always trade offs - and lifts trade off is drag. You just can't get away from it, try as you might. We're at a point where there may be minor improvements here or there, but don't expect anything magical.

[vintage1]

I am not so sure. We have followed one model - laminar flow =- as far as we can.

But teher are others - insect flight for example - that actually rely in turv=bulent flow.

We haven't had the computing power before to anaylyse it properly, but now we may have.

So I wouldn't be surprised if some novel humps don't start appearing here and there.

[flieslikeabeagle]

mwraight wrote:

I would tend to lean towards the observation that we're at the pinnacle of aerodynamics and there isn't going to be any major breakthrough discovies that greatly increase efficiency.

Oh oh. Those words have been spoken before about other areas of science - and in every case, huge revolutionary discoveries followed. Not long before Einstein's theory of special relativity, the Bohr atom, the Heisenberg uncertainty principle, and all of quantum mechanics arrived, physics teachers were telling their students that the only thing left was filling out more decimal places in all the know scientific constants.

Not to mention, there are examples of hugely superior aerodynamics right in front of our faces. When we can build an aircraft that has a wing-span of a few millimetres, a weight of a few milligrams, that can do a loop in a couple of body-lengths, and then land upside down on the ceiling, I'll think we're getting pretty good. Any housefly can do all of that :D

Back to the bumps: all those factors (whale size, whale speed, water vs air) are taken care of by the Reynolds number. As previously posted on this thread, the Re values for the whale are around the same as a large bird or RC model, and there is therefore a good likelihood that what works for the whale would work for some birds and/or RC planes.

I suspect that just the "starved horse" effect of cloth covering on wooden rips will not effectively duplicate the whale tubercles. A few million years of evolution often succed in iterating to a very optimal solution...just look at the human eye, so sensitive it can respond to a single photon, with a diffraction-limited, almost perfect lens that lets you see clearly from distances of a few inches to light-years.

-Flieslikeabeagle

[mwraight]

I agree that there have been revolutionary discoveries in many areas when people felt they could go no farther -- but we're not talking about relativity, Einstein, Heisenberg or quantum physics here. There are trillions of discoveries left to be made, but there are areas that have been pushed to their very limits - aerodynamics is one of those. It's like the internal combustion engine. We can make improvements here and there, but we're reaching the pinnacle of efficiency - a point where not much more can be done because of the limits of what we're dealing with. The internal combustion engine will always have a limit - nothing can be 100% efficient - nothing can be anywhere close. So, you see, if we were talking about something like theories of physics or medicine or other areas I would whole-heartedly agree. But a wing is a device, like an engine, that can only get so efficient. We've pushed that efficiency right up to the cusp. I believe that there are improvements out there, and people are going to make awesome discoveries, but it isn't going to be anything revolutionary. Nobody is going to discover a wing that doesn't create drag or one that is 90% efficient

Another point I'd like to make is that a whale doesn't fly - and it doesn't need to develop lift. Those fins are not designed for flight. However, I would encourage someone to experiment - you never know, those bumps might work magnificiently. I'm just saying that you should expect a miracle in aerodynamics.

[raptor22]

I agree that there have been revolutionary discoveries in many areas when people felt they could go no farther -- but we're not talking about relativity, Einstein, Heisenberg or quantum physics here. There are trillions of discoveries left to be made, but there are areas that have been pushed to their very limits - aerodynamics is one of those. It's like the internal combustion engine. We can make improvements here and there, but we're reaching the pinnacle of efficiency - a point where not much more can be done because of the limits of what we're dealing with. The internal combustion engine will always have a limit - nothing can be 100% efficient - nothing can be anywhere close. So, you see, if we were talking about something like theories of physics or medicine or other areas I would whole-heartedly agree. But a wing is a device, like an engine, that can only get so efficient. We've pushed that efficiency right up to the cusp. I believe that there are improvements out there, and people are going to make awesome discoveries, but it isn't going to be anything revolutionary. Nobody is going to discover a wing that doesn't create drag or one that is 90% efficient

Another point I'd like to make is that a whale doesn't fly - and it doesn't need to develop lift. Those fins are not designed for flight. However, I would encourage someone to experiment - you never know, those bumps might work magnificiently. I'm just saying that you should expect a miracle in aerodynamics.


People say we have reached the pinnacle of effeciency of powerplants. This may be true that we can utilize the most energy we will ever draw from the source of power, but there will always be a way to pack more power for that utilization into a smaller lighter package; thereby still increasing performance.

The same lies true with aerodynamic effeciency; while we may be at the pinnacle of effeciency (which I don't think we are at all), we can extend that effeciency to larger Re ranges, higher max Cl's, or wider drag buckets. There will always be progress.

I would like to remind you that in the 1920's there were several to aerodynamicists with what seemed very convincing "proof" that aerodynamics could advance no further. They were wrong, although the math they offered "proved" their theories.

I'd like to also add that, while a whale doesn't fly, it's flippers do produce liflt while it is deflacting them as a control surface. During this, they experience all of the same phenomenae of our own airfoils. Therefore, they CAN be used as an example.

--Alex

[mwraight]

People say we have reached the pinnacle of effeciency of powerplants. This may be true that we can utilize the most energy we will ever draw from the source of power, but there will always be a way to pack more power for that utilization into a smaller lighter package; thereby still increasing performance.

You've taken my statement and modified it in an attempt to suit your argument. If we can utilize the most energy from a source of power then it has reached the highest in efficiency it will ever reach, regardless of the size of the package. You're trying to argue two separate things. For example, if we were to be able to extract a maximum of 50% of the energy in gasoline and turn it into motion (the remaining 50% going to heat and other losses), it wouldn't matter what size that engine was - it is at the pinnacle of it's efficiency. Now if you make the engine lighter you're dealing with something else entirely - the efficiency in which you build that machine and use materials. But if 50% of power is the maximum you can achieve - that is the limit. A .40 size engine will never be capable of delivering the power of a 10 cylinder Cummins turbo diesel.

The same lies true with aerodynamic effeciency; while we may be at the pinnacle of effeciency (which I don't think we are at all), we can extend that effeciency to larger Re ranges, higher max Cl's, or wider drag buckets. There will always be progress.


I full heartedly believe that we will increase efficiency, but I don't think you're getting my point - it is highly unlikely that there will be any huge/major/miraculous advances. There are always little things we can do to tweak efficiency and increase performance - but no discovery in aerodynamics is going to allow us to lift a 100,000 pound load with a 1 foot wingspan at 100 mph airspeed, or fly a 747 around the world on 16 ounces of fuel. We've got lots of tweaking but nothing huge remaining.

Furthermore this discussion keeps on drifting towards things like making things smaller - like creating tiny, insect like flying machines. While I also agree this is a field we know virtually nothing about - this has nothing to do with my observation. This thread started on discussing the merits of bumpy leading edges as it relates to making model aircraft more efficient. The flight of an insect and a model airplane are not relevant comparisons.

I don't want anyone to think that I'm a nay sayer or trying to prove the Earth is flat. I believe that there are so many wild and wonderful things to be discovered that we'll never be able to comprehend them all. But you have to temper that with realistic expectations and knowledge that there are limits to everything. We can all dream up wonderful things, but many of those things will never come true. Every mechanical and physical thing has the laws of physics to obey - gravity, friction, etc. There are limitations to everything in the universe. I simply propose that we are nearing those limitations in aerodynamics. This is not to say there will be no improvements - just no fairy tales or fantasies.

[mwraight]

People say we have reached the pinnacle of effeciency of powerplants. This may be true that we can utilize the most energy we will ever draw from the source of power, but there will always be a way to pack more power for that utilization into a smaller lighter package; thereby still increasing performance.

You've taken my statement and modified it in an attempt to suit your argument. If we can utilize the most energy from a source of power then it has reached the highest in efficiency it will ever reach, regardless of the size of the package. You're trying to argue two separate things. For example, if we were to be able to extract a maximum of 50% of the energy in gasoline and turn it into motion (the remaining 50% going to heat and other losses), it wouldn't matter what size that engine was - it is at the pinnacle of it's efficiency. Now if you make the engine lighter you're dealing with something else entirely - the efficiency in which you build that machine and use materials. But if 50% of power is the maximum you can achieve - that is the limit. A .40 size engine will never be capable of delivering the power of a 10 cylinder Cummins turbo diesel.

The same lies true with aerodynamic effeciency; while we may be at the pinnacle of effeciency (which I don't think we are at all), we can extend that effeciency to larger Re ranges, higher max Cl's, or wider drag buckets. There will always be progress.


I full heartedly believe that we will increase efficiency, but I don't think you're getting my point - it is highly unlikely that there will be any huge/major/miraculous advances. There are always little things we can do to tweak efficiency and increase performance - but no discovery in aerodynamics is going to allow us to lift a 100,000 pound load with a 1 foot wingspan at 100 mph airspeed, or fly a 747 around the world on 16 ounces of fuel. We've got lots of tweaking but nothing huge remaining.

Furthermore this discussion keeps on drifting towards things like making things smaller - like creating tiny, insect like flying machines. While I also agree this is a field we know virtually nothing about - this has nothing to do with my observation. This thread started on discussing the merits of bumpy leading edges as it relates to making model aircraft more efficient. The flight of an insect and a model airplane are not relevant comparisons.

I don't want anyone to think that I'm a nay sayer or trying to prove the Earth is flat. I believe that there are so many wild and wonderful things to be discovered that we'll never be able to comprehend them all. But you have to temper that with realistic expectations and knowledge that there are limits to everything. We can all dream up wonderful things, but many of those things will never come true. Every mechanical and physical thing has the laws of physics to obey - gravity, friction, etc. There are limitations to everything in the universe. I simply propose that we are nearing those limitations in aerodynamics. This is not to say there will be no improvements - just no fairy tales or fantasies. We won't be making gold out of lead.

[raptor22]

I am saying that, even if we do not increase max effeciency through this, we may delay the stall to increase max effeciency over a larger Cl range. Therefore it could make a HUGE difference in the flyability of an airplane, as we sould be able to stretch thinner lower drag foils over larger amounts of flying conditions that would require a draggier airfoil to delay stall or laminar seperation bubbles.

--Alex

[raptor22]

As far as engine power goes, my point was that there will be other methods of storing energy more compactly; ie better buels, etc. Like Lipos and brushless on electrics; a better power source. Then, with the power compacted into a smaller tank, throwing more energy for a given amount of fuel, the engine power would increase as well for a given size.

That would pack more power into a smaller and lighter package, and there are nearly infinite amounts of fuels to discover. Maybe we'll find a better way to produce biodiesel and other organic fuels that are even better w/ energy density. Who knows?

--Alex

[flieslikeabeagle]

mwraight wrote:

It's like the internal combustion engine. We can make improvements here and there, but we're reaching the pinnacle of efficiency


I took thermodynamics classes in college in the 80's. All current internal combustion engines are a *long* way below the theoretical efficiency predicted by thermodynamics. No mystery, energy is lost to friction between rings and cylinders, as heat in the exhaust, and as turbulence in the intake and exhaust charge, not to mention a host of other things.

IC engines have now had a century of development, and it may be that development is slowing, but gasoline engines still are around 30% efficiency - that's a lot of wasted energy!

Anyway, this is drifting off topic, and I do not wish to get into a long argument on whether various bits of technology are at their peak of development or not, so let's agree to disagree. You say potato, I say pot-ah-to...

The bumpy whale flippers have been demonstrated to resist stall, generate higher lift at high angles of attack, and generate lower drag at those high angles, than smooth wings. The flippers do indeed generate lift, which, combined with their ability to articulate and thus alter the direction of the lift force, enables the whale to manoeuvre unusually tightly for such a large animal. And the Re numbers for the whale are in the same range as some large birds and some model aircraft. These are all facts that have been demonstrated at this point.

I would be very surprised if none of these demonstrated benefits of the bumpy leading edges had any application to RC planes, of some size and speed range, in some flight regime, for some types of flying.

-Flieslikeabeagle