I begun pondering a big Delta fin for my big board with Decrepit the other day and I need to pick the brains of those who now a lot more about aerodynamics/hydrodynamics than I do.. Question - I notice the MUF Delta fins are the classic Mirage jet fighter straight leading edge shape. However Concorde had a concave curve leading edge. Whats the advantage of a straight leading edge in terms of lift/drag, if there indeed is one?

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fangman said..
I begun pondering a big Delta fin for my big board with Decrepit the other day and I need to pick the brains of those who now a lot more about aerodynamics/hydrodynamics than I do.. Question - I notice the MUF Delta fins are the classic Mirage jet fighter straight leading edge shape. However Concorde had a concave curve leading edge. Whats the advantage of a straight leading edge in terms of lift/drag, if there indeed is one?

The concord crashed and burned ...


Somtimes its not worth over thinking these things, just borrow one and try it. I've had mine for about 6 months and still yet to get it wet.

........

I have a Delta MUF for my smaller board which i find really good for where i sail which can be exceptionally shallow at times, thats why I was interested in a bigger Delta for the big board. Weight/drag/design issues aside I was just wondering when I boffinise with Decrepit whether straight leading edge or concaved edge - I can see that straight would be easier to make and certainly the one i already have is excellent for running into things and NOT crashing and burning :-)

I like the delta fins with the strait leading edge and 10deg trailing edge rake , what I don't like is the 55deg leading edge, I have modified a 15cm and 17cm to 50deg and found them to have more lift to windward and less prone to let go on bareaways, if you want a concave edge get a straightedge and wait

BTW, what do you consider your big board?

I have a 21cm yet to try it on my 100L slalom. the guys at delta reccommended going bigger to cope with my muscle weight

Let us know how you go with the research / experiment.

I am 110kgs.190 cms so I consider my 105litre board as my small board. (My big board is 140l!) I use the bog standard Delta MUF 21 on my small board and I find it it behaves pretty much as previous forum discussions. Takes longer to get planing and have to head off downwind a bit more to do so. Generally works better at higher speeds, at slow speeds I feel like I am a little ' spun out' - but as they say in the marketing blurb, the fin is still doing its thing at higher angles of attack, so even though the board is a little spun out in the tail as you pick up speed it just tucks smoothly back underneath. Not so great up wind - really have to concentrate on using the rails and not rely on the fin. Completely rubbish in chop. Really nice in smoothish water, tracks like on rails and tends to spin out gradually because of that ability to hang on a high angles of attack - which gives me a bit more confidence about pushing things a bit harder. Fantastic in shallows and weeds, not ever got caught on weed yet and bounced off a lot of unexpected real estate! I suspect compared to the hard core speed fins, its drag is too high, but I am on a bog standard Delta not the Slalom or speed versions.However I reckon the biggest factor in the fin's performance is me and most of the deficiencies in the fin are more likely down to my deficiencies as a windsurfer, so until MUF sign me up as a sponsored rider treat everything I say the same as if it was said by a politician :-)

Fangy old man, we won't even consider concave leading edge!
I was planning on convex.
Where we intend these fins for the most weed is on the bottom, there's not a lot floating around on the top.
Forget about drag coefficients in open water, we want to reduce drag through the bottom weed.
So at the surface the fin can be around 40deg, but at the tip needs to be around 55.

I also have a theory that the normal hydrodynamics that says high aspect fins have less induced drag, is not necessarily true in shallow water with weed at the bottom. To a certain extent the reduced amount of water beneath the fin, with weed in it, is going to inhibit the flow from the high to low pressure side of the fin that results in tip turbulence.

That's harsh ... Getting called 'old man' by Decrepit!

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fangman said..

I begun pondering a big Delta fin for my big board with Decrepit the other day and I need to pick the brains of those who now a lot more about aerodynamics/hydrodynamics than I do.. Question - I notice the MUF Delta fins are the classic Mirage jet fighter straight leading edge shape. However Concorde had a concave curve leading edge. Whats the advantage of a straight leading edge in terms of lift/drag, if there indeed is one?

The Concorde's delta wing like the mirage while very efficient at high speeds, suffers from problems with low speed handling. Most aircraft have wings that are modifiable in flight, to improve low speed handling, usually large flaps and leading edge slots, these devices usually require a tailplane to handle the different trimming forces created by these devices. The Concorde had no leading devices and only has very limited flaps, but it is able to make use of phenomenon called vortex lift at low speed. en.wikipedia.org/wiki/Vortex_lift

That is my understanding of why the Concorde's wing is shaped as it is, so if runway length was unlimited, a much simpler delta wind shape would have been used. The F-18 hornet's' leading edge chines also make use of the vortex lift

OK fangy, this is what I have in mind.





It will need plenty of carbon reinforcing around the base, but I think the main blade should be stiff enough with just a couple of layers of fibreglass.
Probably graft on a fibreglass tip so wear doesn't expose the timber.

DR FANG ive got a 17maui ultra delta wing on order should get it next week so some testing will be done soon in the peel im hoping it get through the snot weed !!!!!!!!!!

Stroppo, it goes through snot weed like hot knife through butter. Enjoy yourself and let me know what you think. I would really like to know what a really good windsurfer thinks about them.