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anhedral or winglets

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zfennell
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Re: anhedral or winglets

Postby zfennell » Wed Apr 23, 2014 6:55 pm

interesting.
many , if not most of the major foil manufacturers are using anhedral with their front wings.
more than a few resons have been offered for this use, (i.e. yaw stability, reduce wave/surface interaction, reduce rear wing interference) But i dont believe anyone has claimed increased lift efficiency.
i dont really know the true reason.

if increased lift efficiency is desired , the use of winglets is a proven concept.
particularly for wings with finite/reduced aspect ratio.

regarding the use of vortex generators:
they certainly work. however i believe the design operating condition is when angle of attack (AOA) is already high or near stall conditions.
i can see the benefit for commercial airplanes attempting to avoid premature stalling before landing/takeoff or the need for a fighter jet to operate at extreme AOA.
Even the bumps on the leading edge of whale fins have been shown to generate vorticity which maintains lift at high AOA.

however all of these conditions are typically quite far removed the the low AOA an 'efficient' wing would operate at to maintain optimum lift/drag ratio.
i dont believe any of the above vortex generators have proven benefits a low AOA where separation or stall is not likely.
perhaps as peter sugested, you may find benefits at extreme operating conditions of your foils where its more important to generate lift desipte any drag peanlity

regards,
-bill

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Re: anhedral or winglets

Postby dlprince69 » Wed Apr 23, 2014 7:34 pm

zfennell,

The vortex lift generated by the leading edge root extensions (fore-body) is exactly what the hydrofoil was missing. The high angle of attack needed to early foil is a real game changer and the leading edge root extension do not cause addition drag when angle of attach is reduced during high speed.

I have ridden the Spotz, Sword and MHL race foil. All three need extremely high flying speed. The vortex lift during takeoff solves this issue.

Here is my most recent version. DP v8 - LERX with thin high AR main wings. 5mm tip droop and -4 degree tip washout.
Attachments
DP v8.jpg

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Re: anhedral or winglets

Postby revhed » Wed Apr 23, 2014 8:41 pm

Hello P F
The photo is the rear wing or stab as the French like to call it.
h20 flowing from left to right.
Stabilizer seems a good semantic choice as I believe it does this in all three flying terms, pitch, roll
and yaw.
Having test flown flat stabs with no vertical element as in wing tips angled either up or down and no TT style fin seem to lack yaw stability, but they turn fast.
This also has to do with where the strut is place on the fuse.
As far as I know F K was one of the first to use upswept stabs with the thinking of keeping the wing tips out of the front wings washout, vortex. and adding yaw control.
It seems at least for now that front wing anhedral mixed with rear wind dihedral is being used with success by many top builders. And at the same time their are those out there making completely flat stabs and using a vertical fin either under or over it.
Srange to see the "maybe" new gastraa foil with no rear yaw control element?

And another interesting idea is from S A wherby both wings are 100% flat and both a forward and rear vert fin is used.
Christoff
H4-1.jpg
H4-1.jpg (38.84 KiB) Viewed 1092 times
This may be worth considering as for garage builders flat wings are a hell of a lot easyer to make using glass as a mold starter and hot wire for dense foam profile cutting!
R H
Last edited by revhed on Fri Apr 25, 2014 7:53 am, edited 1 time in total.

zfennell
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Re: anhedral or winglets

Postby zfennell » Wed Apr 23, 2014 9:20 pm

dlprince69 wrote:zfennell,

The vortex lift generated by the leading edge root extensions (fore-body) is exactly what the hydrofoil was missing. The high angle of attack needed to early foil is a real game changer and the leading edge root extension do not cause addition drag when angle of attach is reduced during high speed.

I have ridden the Spotz, Sword and MHL race foil. All three need extremely high flying speed. The vortex lift during takeoff solves this issue.

Here is my most recent version. DP v8 - LERX with thin high AR main wings. 5mm tip droop and -4 degree tip washout.
very cool.
so you use it to transition up to flying speed?

i'm sure there are codes that can compute/predict the flow field around such a geometry.
i'm wondering who actually has access to such a tool?

otherwise, how do you know which parameters to modify for each design iteration?
even though i do agree that vorticity will be generated in your prototype there is also a significant amount of projected area that must also contribute lift at any angle of attack. (as opposed to delaying separation at high AOA)
how do you tell the diffenence?

still very cool.
thanks,
-bill

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Re: anhedral or winglets

Postby Peter_Frank » Wed Apr 23, 2014 10:10 pm

zfennell wrote:interesting.
many , if not most of the major foil manufacturers are using anhedral with their front wings.
more than a few resons have been offered for this use, (i.e. yaw stability, reduce wave/surface interaction, reduce rear wing interference) But i dont believe anyone has claimed increased lift efficiency.
i dont really know the true reason.

if increased lift efficiency is desired , the use of winglets is a proven concept.
particularly for wings with finite/reduced aspect ratio.

regarding the use of vortex generators:
they certainly work. however i believe the design operating condition is when angle of attack (AOA) is already high or near stall conditions.
i can see the benefit for commercial airplanes attempting to avoid premature stalling before landing/takeoff or the need for a fighter jet to operate at extreme AOA.
Even the bumps on the leading edge of whale fins have been shown to generate vorticity which maintains lift at high AOA.

however all of these conditions are typically quite far removed the the low AOA an 'efficient' wing would operate at to maintain optimum lift/drag ratio.
i dont believe any of the above vortex generators have proven benefits a low AOA where separation or stall is not likely.
perhaps as peter sugested, you may find benefits at extreme operating conditions of your foils where its more important to generate lift desipte any drag peanlity

regards,
-bill
Agree, REALLY interesting Bill !

In my eyes, anhedral would be superior in terms of efficiency, no doubt at all when talking airplanes at normal AOA's:rollgrin:

But WHY is it not used much in airplanes then ?

Because of dynamic and practical handling :wink:

A glider with dihedral has so much better turn characteristics, with much less waste and a way better overall efficiency because of that.
So eventhough it in perfect level straight flight would perform better, it does not in real life where thermal curves, and handling and straight out stability too is what it is all about.

For passenger aircraft another practical issue arises - if you had anhedral, you would need extremely long and heavy landing gears, besides really high entry points for passengers.
So because winglets has an approaching similar effect, without these downsides - they are chosen :thumb:
Furthermore, you dont get the downsides of anhedral, namely a totally corrupt way of handling when turning, especially if turning with a slight yaw :o

8) Peter

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Re: anhedral or winglets

Postby zfennell » Wed Apr 23, 2014 11:07 pm

thanks Peter,
i'm not sure i know the answer yet.
but you've done a good job of describing the big picture.
-bill

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Re: anhedral or winglets

Postby Scuba » Thu Apr 24, 2014 5:26 am

Is it important with a front anhedral to have an epler shape say for instance 817 ?
than just a flat bottom profile with a foil on the top edge?
On the tail wing should this be only foiled on the under side? Or should this also be an epler shape? Tilted a few degrees down.
Thanks

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Re: anhedral or winglets

Postby Hawaiis » Thu Apr 24, 2014 6:37 am

Scuba wrote:Is it important with a front anhedral to have an epler shape say for instance 817 ?
than just a flat bottom profile with a foil on the top edge?
On the tail wing should this be only foiled on the under side? Or should this also be an epler shape? Tilted a few degrees down.
Thanks
Foil shapes will definitely be better than flat bottom shapes.

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Re: anhedral or winglets

Postby windfreak74 » Thu Apr 24, 2014 8:59 pm

dlprince69,
Nice wing design!
with the cufed design how much área comes from the wing and how much from the fore body.
do you have a pic to see the anhedral on the wing?
Pedro

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Re: anhedral or winglets

Postby revhed » Fri Apr 25, 2014 8:46 am

Scuba wrote:Is it important with a front anhedral to have an epler shape say for instance 817 ?
than just a flat bottom profile with a foil on the top edge?
On the tail wing should this be only foiled on the under side? Or should this also be an epler shape? Tilted a few degrees down.
Thanks
Yes, if you want to fly fast, or in very low wind. :!:
That said, myself and MANY friends learned with flat bottom, curved radius,anhedral, low A R front wings.
The top profile was also very basic, a hand shaped front L E to match most NACA s and a steeper angled T E.
The stab, rear wing was of course inverse, meaning flat on the top with profiled bottom and dihedral.
But of course a "real" naca profile like 817 would be better, but how much?
And if you want to fly in radical conditions, strong gusty wind and ,or waves I would use a lower performance foil any day for more fun.
There are those who say your stab needs a few degrees of negitive, probalby correct with fully profiled shape, but I run 0° and 0° flys just fine for learning.
The problem with this is that based on your level you will out grow a simple foil fairly fast.
But you can always let your learning friends have a go! :thumb:
R H


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