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 Post subject: Angle on rear wing
PostPosted: Mon Jun 23, 2014 1:56 pm 
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Hi guys
Just a quick question for the experts I have made my own foil and is riding well, but I can only get a 10 second ride at a time before touching the nose down and wiping out the wind here at the mo is winter gusty and does pull me off balance quite a bit.
The AoA is 0° on the rear wing, and once on the foil it seams quite balanced, even weight on both feet.
Should I trim The rear wing so that it will want to rise slightly. So that I have to put a little more weight on my front foot.
or should it take me more than a 8 hours of riding to feel at home and in control of when I touch down.

Thanks for your help
Steve


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 Post subject: Re: Angle on rear wing
PostPosted: Mon Jun 23, 2014 4:12 pm 
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Joined: Sun Feb 24, 2008 6:15 pm
Posts: 168
Location: France
A O A of rear wing can really be personal choice.
I have always ridden 0 front and 0 back but thats just me.
I know of some who will shim ultrathin and test and repeat untill it just feels right to them.
I bet if you measure almost all of the producttion foils have some if not a lot of negative stab, rear wing A O A.
Some like to ride "hot" that is with the stab adding more neg thereby making foil want to rise needing slight front foot pressure to hold it down.
Standard rule is 1-2 ° negitive, guitar picks, washers or thin peices of plastic work well to adjust.
I have heard that if you feel you have to constantly apply rear foot down pressure then you do not have enough neg AOA, but for me it seems I just move my rear foot further back to find my set ups trim stance.
I have always questioned stance placement vs stab, rear wing AOA and still do not know what to think? I feel you need to be a very skilled flyer to know the differance.
Just yesterday a friend spent an afternoon tuning just this and he is a high level rider so for him it is important and he claims he can feel less than 1/2 °.
It is easy to do so test on your board and see what you feel!
R H


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 Post subject: Re: Angle on rear wing
PostPosted: Mon Jun 23, 2014 6:26 pm 
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Location: Ukraine
there is still a fact, if the angle is not zero, then the more your speed, the stronger the influence.


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 Post subject: Re: Angle on rear wing
PostPosted: Mon Jun 23, 2014 8:12 pm 
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vvs wrote:
there is still a fact, if the angle is not zero, then the more your speed, the stronger the influence.


In general i agree.
but i believe the fact is that each profile will generate zero lift at some specific angle of attack.
if the aft foil is not symmetric the required angle may not = zero.

the only reason i mention this is a comment from the carbonic boat guys.
(http://carbonicboats.blogspot.com/)
they suggested the use of profiles for the aft foil which generate lift at a faster rate than the forward foil.

the intent is to increase the 'sweet spot' by having the aft foil attempt to counteract undesired pitching moments before they get out of hand.

-bill


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 Post subject: Re: Angle on rear wing
PostPosted: Mon Jun 23, 2014 8:25 pm 
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yes, sorry, I incorrectly expressed his thoughts. did not need to talk specifically about the zero angle of attack.


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 Post subject: Re: Angle on rear wing
PostPosted: Mon Jun 23, 2014 11:43 pm 
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A bit puzzled about all those "zero degree" AOA posts ?

Is that from centerline front wing to centerline stabilizer ?

If so, it only shows that you have some lift on the wing, even when the stabilizer is not lifting up nor down.

All front wings have lift at zero degree AOA (which is NOT parallel to the bottom of the wing)
Often named the "Beta" angle, which is the negative AOA the wing should have, in order to produce zero lift.

The angle of the fuselage and board means nothing (apart from parasitic drag and handling) - it is only the AOA difference between the front wing center line and stabilizer center line that matters here :rollgrin:

Dont know if off topic or not - but I find it confusing talking about zero degree AOA without naming exactly what front wing profile that has been used.
Also a bit sceptical, IF the front wing really has zero degree AOA, as I think a few might call it "zero" if the bottom of the wing is "flat" on the table so to speak - eventhough the centerline has positive AOA, and when you add the Beta angle, you actually got loads of lift now :thumb:

8) PF


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 Post subject: Re: Angle on rear wing
PostPosted: Tue Jun 24, 2014 6:40 am 
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Peter_Frank wrote:
A bit puzzled about all those "zero degree" AOA posts ?

Is that from centerline front wing to centerline stabilizer ?

If so, it only shows that you have some lift on the wing, even when the stabilizer is not lifting up nor down.

All front wings have lift at zero degree AOA (which is NOT parallel to the bottom of the wing)
Often named the "Beta" angle, which is the negative AOA the wing should have, in order to produce zero lift.

The angle of the fuselage and board means nothing (apart from parasitic drag and handling) - it is only the AOA difference between the front wing center line and stabilizer center line that matters here :rollgrin:

Dont know if off topic or not - but I find it confusing talking about zero degree AOA without naming exactly what front wing profile that has been used.
Also a bit sceptical, IF the front wing really has zero degree AOA, as I think a few might call it "zero" if the bottom of the wing is "flat" on the table so to speak - eventhough the centerline has positive AOA, and when you add the Beta angle, you actually got loads of lift now :thumb:

8) PF


Great questions,
We need a referance point when talking A O A of wings for sure.
So what we call 0° A O A is centerline if you want.
Attachment:
e818.gif
e818.gif [ 12.06 KIB | Viewed 356 times ]

I would agree fully that in this 0° AOA centerline that there is lift here.
BUT,
we now have a referance for the rear wing using the same 0° AOA centerline
Attachment:
e222 LINE.gif
e222 LINE.gif [ 12.21 KIB | Viewed 356 times ]

Sure we know the rear wing is mounted 180° to shown diagram.
Anyway I try to keep it simple for my garage builds.
90° strut to board and 90° strut to fuse
Because of this you measure 0° AOA centeline which is the center of the LE of wings and point of T E to the fuse parallel.
I would have no way of finding true NO lift A O A of wings in my garage.
Attachment:
3v.png
3v.png [ 74.04 KIB | Viewed 356 times ]

Peter,
I know you fly a production kiteboardhydrofoil, if you have a minute could you measure your wings centerlines, AOAs as compared to the fuse AND board as they are probably not 90° angles.
I also know you understand and study aerodynamic concepts, I only throw my extensive rc helicopter experience at this hydrodynamic kiteboardhydrofoil game so any and all new info much needed.
Another question for you to ponder?
Do you think when a foil flys that the rear wing is giving down force, may seem like a stupid question as it would seem that even with profile inversed that the positive AOA when flying would force it to generate lift.
R H


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 Post subject: Re: Angle on rear wing
PostPosted: Tue Jun 24, 2014 2:51 pm 
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I've recently been working through this same thing with my foil - the rule of thumb is as listed above - 0 degrees for front wing and 0 to -2 degrees for rear wing. This is actually a pretty poor generalisation because the lift curve of the profiles differs depending on what profile is used - for example the Eppler 818 above have a lift co-efficient of 0.4 at 0º, other profiles might have 0.3 or 0.25 or something fairly different, the zero lift angle for Ep818 is maybe -4º .

When I did the numbers on my foil set up to "rule of thumb", I found my wing was lifting about 2 times body weight and the stab pulling down 1 bodyweight, which are very big forces. This explains why fuselages need to be so solid and strong and resist bending and twist. This is good for stability, but poor for drag because both wings are working hard and the induced drag goes up with the square of the lift co-efficient - so ideally you want each wing to only be doing as little work as possible (low Cl)

So bottom line - the rule of thumb is ok and probably good for low and medium speeds, but to get the best performance you actually need to understand the profile characteristics and how the interplay of forces and geometry works. (the different profiles also have very different nose over moments - Eppler 817 and 818 have very high moment coefficients)

This is just where I'm at at the moment - by next week I may have changed my mind :)


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 Post subject: Re: Angle on rear wing
PostPosted: Tue Jun 24, 2014 4:01 pm 
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You can define AoA in two ways.
1) The geometric angle of attack is the angle between the chord line of an airfoil and the vector representing the relative motion between the body and the fluid through which it is moving.
2) Another choice is the fluid dynamic AoA which uses the zero lift axis instead — zero angle of attack corresponds to zero coefficient of lift.
The two are the same for a symmetric profile, unfortunately our foils are not!


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