Articulating Foil Tail

[Erwan]

Hi Viking,

If your project is similar to " Morphing rudder blade", I can give you a few infos.

Your morphing section is not fundementally different from an articulated rudder system with a fixed leading edge and an articulated trailing edge.

More than 10 years ago, there was a debate within the 60' racing trimaran about this issue.

The conclusion was that the articulated system could be used with sharper leading edge section, creating less section drag.

Sotoff already mentionned it:

The sharp leading edge will allow full pressure recovery along the foil reducing the profile drag.

IMHO That is the point.

Regards

EK

 

[viking north]

EK thanks for that info, I did a search for that research on the "Morphing Rudder Blade" but came up dead end. I'd be very interested to read about it, do you have any info on how I migh locate the document --

 

[Erwan]

Viking,

Unfortunatly, I didn't have any workpaper addressing this issue, it is just an idea I have on my backburn. If somedays I ll try to do someting, it will be for A-Cat application.

Furthermore a few year ago, the America's cup TNZ (monohull) has found that when the rudder blade has its maximum thickness vertically aligned with the transom stern , the global wake is less draggy.

So for a Cat, I just imagine to combine the sharp leading edge advantage with the above-mentionned Kiwi research and have fence in the transom for the front part of the blade made of monolithic carbon fiber and behind the morphing part of the section.

Reliable camber mecanisms still to be discovered, there is maximum of 10mm inside clearance for the mecanism, so if this idea is promizing, it seems that it will remain promizing for a while.

Regards

EK

 

[sottorf]

Over at

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/what-significance-wing-thickness-45383-6.html

Starting on page 5, is a string of posts where I describe foil sections I am considering for wing. However, the results can also be applicable under the water to some extent (stay away from cavitation speeds). On page 6 post 85 is pretty much a summary.

It shows that a thin pointy flat surface rear half behind a big fat blunt forward portion can be very low drag. If you use the rear half as a big flap, you can get very good L/D values. If you make it a "double flap" arrangement you can also get very respectable lift values.

As shown on some of my earlier posts in the referenced thread, I was looking at blunt rear edges for a while early on. I was getting pretty good lift and decent L/D. Eventually I recognized that they just do not get down to the really low drag values and you really do want the low drag number for the low lift conditions that are a reality at times.

FYI, I probably like XFLR5 more but must admit that http://www.mh-aerotools.de/airfoils/javafoil.htm was a lot easier to just jump in and start using. His web page has decent "just getting started" info and Javafoil can be run as a web application with no downloading. I recommend giving it a try.

Scanning the thread and post 85 as you suggested, you investigate the effect of maximum thickness on L/D but nowhere in that thread to I see any reference to thickening/blunting of the trailing edge and what effect that has. It is not a problem you can solve using XFLR5, which is panel method with boundary layer theory based on Marc Drela's XFOIL. XFOIL can handle limited separation but requires a sharp trailing edge to fulfill the Kutta condition at the trailing edge. You would need to resort to a RANS or LES solver to calculate the aero/hydrodynamics of a profile with blunt trailing edge.

 

[DCockey]

Adding a fairing to a blunt trailing edge will theoretically reduce the drag IF the rubber fairing that is added is flush with the surface of the profile. The sharp leading edge will allow full pressure recovery along the foil reducing the profile drag. Secondly the sharp trailing edge will lead to a smooth separation of flow from the trailing edge without the generation of a so-called "vortex street."

'''''''.

My guess is sorttorf meant sharp trailing edge, not sharp leading edge, in the second sentence.

 

[sottorf]

My guess is sorttorf meant sharp trailing edge, not sharp leading edge, in the second sentence.

yes correct.

 

[viking north]

Yes thats also the way I read it. As one grows older it becomes normal to switch opposites --turning negatives into positives becomes a full time job for the brain to fool the body into continuing on

 

[viking north]

Ok Erwin no problem. It too was an idea that popped into my head as a result of Marchaj's reference to dolphins and their ability to slightly change body shape to reduce drag and thus attain great speeds. Thus the idea (and i'm sure not origional)of using a flexiable material that could promote good flow where it is often lacking but needed most. That being the lagging or tail section of a foil shaped keel or skeg due to a number of reasons. Anyhow an idea that might be worth playing with.

 

[PAR]

If you want to be effective, the "changeable shape" portion of the foil might take the forward to middle of the section, from slab sided to rounded, likely moving the "bucket" to a more favorable value. Maybe pneumatic bladders, to get the chord moved aft a bit, possibly a couple of bladders on each side, so low and higher speed sections could be addressed. Considering you max speed on your best day, will be in the 7 to 8 MPH range, maybe a wee bit over the top as far as engineering.

 

[JSL]

Some comments:
Having a fixed rubber trailing edge would be better than flexible. If flexible and of any length, the boat motion could create thrust (google - gause fin) and this could compromise the steering effect. Some years ago I met a fellow who put such a contraption (2 horizontal fins about 1' wide x 2' long) on the end of a dagger board on a 16' canoe. Rocking the canoe, we could get about 1/2 a knot. The big plus though was the tremendous improvement in dynamic stability (roll damping) - two of us could swap ends while standing.
I have used a variance of the schilling rudder on several boats and they work very well. One owner (85' motor yacht) said it almost made the stern thruster redundant. The minor loss is that most steering only goes +- 35 deg. and this rudder is good to +-45 deg. so you don't get the full benefit.

 

[viking north]

PAR, Have you not suspected yet that this build is Canada's challenge entry for the Lipton Jug

 

[viking north]

JSL, I was more or less toying with it's possibilities on low aspect fins or skegs in which thru some structural reason could not be tapered sufficiently resulting in alot of exit turbulance in the flow. In such cases just about any tapered section (extension) added, say as a breakaway piece to avoid damage to the main appendage should improve the situation. About a week or so ago in the wee hours my brain woke me up with the crazy idea of a tapering tail piece that could mimic our marine speedsters. Over on another thread there's a big debate on the individual tasks of the right brain, left brain. Christ I wish mine would stop talking to each other and go to sleep so I can get some rest. They're like two kids at 5 AM on Christmas morning.

P.S. I see you guys are having some nice warm temperatures, If we could combine your weather with our cost of housing we'd find the perfect place to live.

 

[tspeer]

...If for some reason the appendage could not be extended in order to taper it's trailing edge within acceptable specs,but there was opportunity to fasten a fixed hard rubber extension, would this improve exit flow and reduce drag. My gut feeling says yes but considering the tail piece will flex transversley with the flow possibly it will create more problems than it solves. ...

It would depend somewhat on the purpose of the appendage and why it had a blunt trailing edge to begin with.

If the purpose of the appendage was to produce lift then the flexible trailing edge will likely be detrimental. The deflection of the trailing edge will result in a significant loss of lift, requiring the surface to operate at a higher angle of attack to restore the lift.

If the appendage was a fairing that was not intended to produce lift, the deflected trailing edge would allow it to have a larger range of angles of attack without separation. It wouldn't be as good as rotating the whole fairing to face into the oncoming flow, but it would help.

At high speed, the flexible trailing edge would contribute to singing.

In general, I'd think that if it was possible to extend the trailing edge with a rigid extension to the same chord as the rubber extension, rigid would be the way to go. Unless there was some particular requirement that drove you to make it flexible.

 

[Leo Lazauskas]

Not quite on topic, but there are some very interesting ideas for flexible-tailed aircraft at:
http://www.darkroastedblend.com/2013/04/futuristic-aircraft-update.html
The "Flagellum Oscillator" is pretty whacky!