Isonic Theory ?? Anyone??

[Dan]

Originally Posted by Floyd

Hi Dan
Think you`ve mixed trailing/leading up in your reply but I understand all you are saying and agree with most points but in the blurb starboard claim/make the premis that adding cutaways decreases drag at tail of board.
This is contrary to current fluid dynamics theory.Cutaways or any other manipulation of trailing edge by sheer maths must increase that edges "edge length" (#see point later) This may well be advatageous for other reasons.(ie widening board to increase leverage available for fin but keeping tail area down for top end control) But none of that is mentioned.

#There is a current (yet unproved) theory that applying "saw tooth" shape to trailing edge of a wing helps lift /drag ratio but this is to do with generating tip vortices accroos span of wing and not as a direct way to reduce drag.)But tip vortices and their effects have nothing to do with planing surfaces.Boards plane through momentum exchange between water and board.(ie a board is not a wing/hydrofoil)


With a staight leading it is mathematically impossible for trailing edge to be shorter.A boards leading edge is inevitably straight.(Boards` planing surface is virtually flat in almost every instance) The minimum trailing edge would be another staright line parallel to leading. This would give minimum drag but probably be unsailable ????

Right you are, I called the leading edge the trailing edge in my example. I think what Starboard was trying to convey is not that the trailing edge is shorter in its absolute length (for the reasons you point out) but shorter in the way that one end of a rhombus is shorter than the other parallel edge. In other words, ignore the distance of the actual trailing edge as it curves and cuts away, but measure the widths of the board at the planing surface's leading and trailing edges. In this way the trailing edge can be said to be shorter than the leading edge.

-Dan

[andretsin]

Hello everybody,
I read someone here saying that the planing surface of the board is not working like a lifting shape (wing for example). I would like to find some similar things between the two shapes.
-We all now that the centre of effort of a wing is placed in more or less the first third of the shape, isn't it? An also that if it is narrower there is less vortex in the tip, so it's more efective.
-Imagine you take a ruler and use is like a planing board. When you are sailing the water is comming from front to the board and because of the positive pitch of the board the water is pushed to go down so the board goes up to the surface. But with the ruler in longitudinal position (like a board) a big amount of water is scaping by the sides instead of being pushed down, so this amount of water is not helping to planing, so we need biger board or higher speed to get the lift. With the ruler in transversal position (wide boards) there is fewer amount of water scaping by the sides, so more water is helping to planing, so we need less surface to get the same amount of lift. And less surface mean less friction so less drag. So this is similar to wings, isn't it?
-Now, do you know the Bernoulli formula? Anyway, do you know that incresing the speed of a fluid the pressure decreases? So, now draw in a paper a flat surface of the sea, and a board with positive picth. Now draw the flow lines under the hull. Can you see how the lines are more separated in under the leading edge than at the end of the board? More compressed lines mean higher speeds. So there is more pressure under the leading edge than under the end of the board. So the lift is concentrated in the front part like in wings, isn't it?
So you see wings and planing surfaces have things in common.
Any way, what I think is that the comment at the begining of these post was not very well writen so we started to argue. I will try to explain it better.
After the explanation i said you can imagine that we are looking for wide boards, to have planing surfaces which looks more like the transversal ruler than the longitudinal one, to reduce the surface we need to get the lifting force (i'm not going to talk about control in high speeds). While you make the board wider and wider, the leading edge (first line of the board wich touch the water) is moving backwards while the trailing edge can't move forward than our rear feet (if we don't use cutaways). Finally the leading egde arrives to the limit when it's near to our centre of gravity on the board, so that if it moves even backwards then the nose falls down, so the trailing edge moves forward again. Is the limit. So if we want to continue reducing the surface we have to do it by the back with the cutaways. In the extreme limit we would have a planing surface that is almost like the transversal ruler, just under our center of gravity (between our feet more or less). But why don't we make only one cutaway in straight line from one side to the other? Because then the root of the fin would be out of the water so the ventilation of the fin would apear and then spinout. Do you remember the flapper of the first wide boards arround 1998? It was to avoid this! Nowadays we are using cutaways instead of flappers, but if you think you will see that the final shape is quite similar. I don't have no idea why we are not using flappers. But there would be an other solution. Why don't we move the fin forward to place it just between our feet? Then everything in the position of body and sail would have to change to keep the balance between the force of the sail, footmast and fin. It would be maybe too unconfortable. And also, if the fin is not after the planning surface the directional control would be almost impossible.
I hope you understand everything i wrote or at least something.

[Floyd]

Hi Andretsin
Understand all you wrote.Dont agree with parts of it.From reading I have done lift on a planing surface is simply to do with momentum exchange of water impinging (at an angle) on board.Venturi model is flawed because with a venturi flow is enclosed .

Still dont understand how increasing "edge" length can decrease drag.That seems to contradict principles.(I thought there was a direct correlation between edge length and drag.

[Floyd]

PS
I think the centre of a lifting force of a planing surface is also its geometric centre.(Obviosly not of board but geometric centre of that part actually in contact with water)

[andretsin]

To say that a wing and a planing surface work with different principles depends only on how do you call things. If you say that the board works because of momentum exchange of the water and the wing because of venturi effect then is different. But if now you say that the board goes planing because there is higher pressure in the bottom of the board than in the deck then is the same than a wing. Or if you say that the wing is flying because it is sucking the air from up (because of venturi effect) and throwing it downwards this is momentum exchange of the air, so is the same as the planing surface of the board. I would say that the only big difference between both is that the wing is working mainly with the sucking face and the board with the high pressure face. But even working like this both of them are looking for a short and wide shape.
As I said in the previous message, when the shape of the planing surface is short and wide is more effective because there is less water scaping by the sides so there is more water exchanging it's momentum by going down so pushing up the board. So you get the same lift but with less surface. And less surface mean less friction so less drag. BUT!! the friction per squared cm is higher next to the leading edge than at the trailing edge. This is because of the boundary layer. The speed of the particles next to the surface of the board is decreasing because of friction while they travel towards the end of the board. And the fricition is proportional to this speed. So the friction is lower at the trailing edge. So, if you have a wide and short surface of planing you have less surface to produce the same lift (because is more efficient) but the average of friction per squared cm is higher than the one you would have in a large and narrow planing surface. (All this is also valid for wings). The conclusion is that there must be an optimum of the aspect ratio width/length. In higher ratios the drag starts to increase againg. So this agree with what you thought. If you increase the lenght of the leading edge you increase the drag (but as you see this is only true after you overpass the optimum ratio). Ah, and is always true if you are talking about a surface which don't have to produce any lift (for example the body of an airplane).
I can't imagine now any clear example to say that the centre of lifting force is forward of the geometric centre of the surface, and even more i'm not sure at all. But I would say is like this.
Finally I wanted to say that in formula boards they use cutaways instead of the shape of the I-sonic because the rear foot is just in the corner of the board, so you can't cut that part of the board. And that they don't do it in narrow boards because the planing surface is so narrow that is unefficient and it needs all the surface, so you can't cut any part.

[Floyd]

Arndetsin
You just ant make anology towards a wing and assume what works/theories for one apply to other.They just dont.
Well above half of lifting force for a wing comes from lower pressure above it and with virtually no momentum exchange involved.(If momentum exchange was going on situations approachingin a at zero angles of attack a wing would produce negative lift (ie air striking upper surface)

Hydrodynamic bearings are a closer anology to a planing surface than a wing is.
There is no actual evidence showing centre of lift is closer to leading edge; its an assumption made with similar analogies to ones offered.
Look at underneath shape of your board.Calculate its geometric centre and see where this relates to weight it is carrying at planing speeds. (shape of board moves geometric centre forward.If what you were saying is correct we would not have such control problems at high speeds ,(when tail produces excessive lift;hence cut outs, to move centre of lift forward)
Look at centre of gravity of you;board;rig in sailing position.That must be overcentre of lift of board otherwise board would porpoise)
Take care

[Erik Loots]

Originally Posted by andretsin

-Now, do you know the Bernoulli formula? Anyway, do you know that incresing the speed of a fluid the pressure decreases? So, now draw in a paper a flat surface of the sea, and a board with positive picth. Now draw the flow lines under the hull. Can you see how the lines are more separated in under the leading edge than at the end of the board? More compressed lines mean higher speeds. So there is more pressure under the leading edge than under the end of the board. So the lift is concentrated in the front part like in wings, isn't it?

Andretsin thank you for the reply! I am a student at civil enginering, and do know Bernoulli very well. So far I know we use it only to calculate effect on water flow. Still I haven't thought about Bernoulli.

Bernoulli is: Energy = Pressureheight + speed + waterpressure + wriction

Now you can calculate between 2 point since energy will be the same. But I don't know how to use bernoulli for windsurfing. Because most watersurface doesn't have much energy.

But the boundery layer that is more spot on. Still have to think about that one

[andretsin]

Ok, now i searched in google and found this link:
http://www.bluejacketboats.com/planing_boat_theory1.htm
here you can see how the centre of presure is in the front part. I didn't read the text.
An other question: Momentum exchange is for sure involved in the behavior of wings. Do you thing an airplane can fly just without throwing the air downwards?? Even easier: i hope you agree the wing of an airplane works like the pelle of the propeller of an helicopter. Does the helicopter trough air downwards? Even more, just look at the flow of the air in next picture:
http://www.desktopaero.com/appliedae...s/image372.jpg
And last think. I have heard lot of times that an aeordynamic profile can produce lift in zero angle attack. The problem is how do you define which is the chord of the profile. If you define it coincident to the flat bottom of the profile then yes, the aerodymic profiles seem to be the miraculous shapes wich can produce lift at zero angle. But if you define it as the line which goes from the point where the flow is divided in the leading edge until the vertex of the trailing edge then is not so miraculous. Because what we thought it was zero angle of attack with this new deffinition is 3 to 5 degrees of attack. And of course, in this case the air is divided in two in that point at the leading edge and meeting again in the trailing edge wich is in lower position, so the air is forced to travel from up to down, so there is momentum exchange.
Erik, i'm also student of Naval Engineering

[Floyd]

Andretsn
If you read all the text you will realise the author is trying to explain why the diagram is not an acceptable model for pressure distribution.

ie) (quote)
Efficient planing therefore becomes the effort to accentuate the former and minimize the latter forces. Because the water particles hit by the leading edge of the plate are already moving away by the time the next part of the plate gets there, the added momentum to the water will therefore be reduced the further aft we go. To me, this explains the pressure diagrams shown in books although I think they are somewhat in error too.


This is the authors explanation of more pressure towards leading edge (nothing to do with venturi/wings/venturi.) Simple momentum exchange but with the assumption that (for some reason unexplained) water molecules impinge on leading part unaffected by others.(wheras towards rear front ones are now interfering with exchange)
I accept authors assumption (re momentum exchange. (ie thinking of us sailing in a sea of peas is excellent)
But I summise as board moves through water (at some angle of attack, which is always needed on planing surfaces to produce lift) each part of board is "hit" by an unaffected group of peas; being deflected down (slightly) and hence imparting lift.Put a board in a ball pool and pull it through at its normal AoA to get idea.
Its pure assumption lift migrates to leading edge and is not represented in practice. (Loads of examples of planing craft where lift is obviously not toward front) (knee boards,wake boards etc etc)
Seriously get your board out and find he geometric centre of the planing surface.It willbe somewhere between your feet or ;depending on your stance/weight distribution, straight under the craft C o G. (I`ve tried this with loads of boards and always come to same conclusion !) Have a look.

PS if this was not the case hydrodynamic bearings would not work ! (ie if lift was not distributed evenly the bearing would fail at point of least "lift". (Nearly all 4 stroke engines utilise hydrodynamic bearings where the surfaces literally "plane" over each other on a srface of oil.(BTW the oil pump pressure just supplies the oil, pressures to keep surfaces apart are extremely high (loads higher than supplied by pump)and generated by the surfaces relative movement..Not a perfect anology but same principle.

[Phill104]

Originally Posted by Screamer

You may also want to visit Boards UK forum (you're from UK aren't you?). Good equipment and spots info, and just general banter, sometimes heated discussions and abuse ;-)

Abuse Nah never

Thanks to all on this thread, it has made interesting reading.