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Dihedral stabilizers

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lovethepirk
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Dihedral stabilizers

Postby lovethepirk » Tue Nov 14, 2017 4:14 am

I think this is my first post here, hi all. I'm without a foil while my board is getting fixed so I have time to think about a foil design and build.

I've seen reviews on foils where there is a dihedral stabilizer, and let's not get confused b/c the stabilizer is at a negative angle of attack...the stabilizer's wing tips are curving upwards to the rider.

With a positive angle of attack, dihedral provides some rolls stability due to slip, but on a stabilizer at a negative angle of attack this does the opposite.

Here are my thoughts...1st I read here from some of you with sick experience and you've stated the wing is not like a normal airplane b/c the kite is pulling you away from the slip angle. I agree with that. Do you think the stabilizer being dihedral is then helping stability by means of that force and how do you separate that from the front wing thought.

I've been thinking about wings for months and it's like the more you learn about them the more questions there are, it's like gravity. Thanks.

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Re: Dihedral stabilizers

Postby tegirinenashi » Tue Nov 14, 2017 4:25 am

Stabilizer main function is pitch stability. For that matter, yaw stability is the duty of the mast, and roll stability is responsibility of the main wing. Dihedral (or anhedral) stabilizer makes no sense whatsoever.

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Re: Dihedral stabilizers

Postby lovethepirk » Tue Nov 14, 2017 4:51 am

tegirinenashi wrote:
Tue Nov 14, 2017 4:25 am
yaw stability is the duty of the mast
teg,

Thanks...that brings me to another question I always had..with such a large force created by the mast is the small vertical stabilizer on some older designs below the fuselage necessary from your experience or are they just additional drag?

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Re: Dihedral stabilizers

Postby Kamikuza » Tue Nov 14, 2017 1:06 pm

tegirinenashi wrote:
Tue Nov 14, 2017 4:25 am
For that matter, yaw stability is the duty of the mast, [...] . Dihedral (or anhedral) stabilizer makes no sense whatsoever.
I would have thought the length of the fuse (and the drag from the stab) would have more effect on yaw stability.

Isn't it Levitaz that had the V-tail? What the reason for that?

Foils with flat wings will probably need the addition of a vertical stab, unlike foils with winglets on the wing and/or stab. The LF foil noticeably improved its yaw stability with the longer fuselage, the SS HG is overly yaw stable for me (winglets) and the Axis Ride foil is just about perfectly neutral for me.

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Re: Dihedral stabilizers

Postby tegirinenashi » Tue Nov 14, 2017 4:23 pm

Are we talking the same terms?
https://en.wikipedia.org/wiki/Aircraft_principal_axes
I never tried short fuselage (will try DYI extra long with JNJ wings next Spring), but would speculate that fuselage length is affecting pitch, not yaw stability. I'd further suggest that second generation LF improved yaw stability is due to increased mast chord length. Another way to increase yaw stability is having a design where the mast is positioned few inches backwards on the fuselage (windfoil-like). Therefore, yes, vertical stabilizer doesn't make any sense too.

I don't know what is the purpose of winglets in HF, but for aircraft they are designed to stabilize the laminar flow near the wingtips. This effectively reduces the required wing span. Their effect on yaw stability should be proportional to their size (i.e. negligible).

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Re: Dihedral stabilizers

Postby BWD » Tue Nov 14, 2017 6:30 pm

around what point does the foil rotate in yaw?
With reference to this point, what is the moment developed by the vertical foil elements (angled wings, vertical fins, mast, winglets, fuselage...)?

=>foil rotates around a point close to the CE of main wing.
main wing: small horizontal force component, short distance from center of rotation = small moment
mast: large horizontal force component, very short distance from center = bigger moment, especially when immersed, very small moment flying high
fuselage: usually small horizontal force component, symmetrical distributed for and aft of center = small moment
stabilizer: very small horizontal force component if flat, but large distance from center of rotation, so moment can be large if significant dihedral/anhedral, winglets, or vertical tail fin.

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Re: Dihedral stabilizers

Postby lovethepirk » Thu Nov 16, 2017 2:47 am

Not sure if this answers your question, but what I did here was make a dirty model with an angle of attack on the front wings and negative on the back. Both wings have dihedral and anhedral so you can see the theory. I'm still learning so this is new to me.

Anyway, from what I've read in planes if there is a sudden banking to one side from outside forces(ie kinda what we do b/c we don't have rudders and we can move entire weight distribution at will) the plane banks to one direction and then there is a NEW apparent wind angle slamming up against the plane and wings b/c the plane is side slipping. I've put a 30 deg bank angle and 20 deg side slip as an example. You can see some of the wings lose surface area for lift, etc.

The bottom picture is the flow of air particles headed dead nuts toward the plane. Same as in the top pic. Those particles of air cannot collide with the underside of some of the wings given the new apparent wind.
Image

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Re: Dihedral stabilizers

Postby lovethepirk » Thu Nov 16, 2017 3:01 am

Speaking of sideslip, lol...so I'm wondering a couple more things...A plane sideslips, BUT it doesn't have a massive fucking kite pulling it downwind. So is it possible a hydrofoil is sideslipping in the opposite direction a plane does? Furthermore, attack angle is based on apparent wind and chord...we all can go crazy speeds downwind relatively level, but when we go slighly slow but still fast as hell upwind the pressure on the board and legs is insane.

What do you think the attack angle on a 30knot downwind tack is vs the attack angle on a 22knot hard core angle into the wind?

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Re: Dihedral stabilizers

Postby Kevin Brooker » Thu Nov 16, 2017 1:37 pm

Trying to follow this thread but the terminology is being used in a way I am unfamiliar with. My background with foils and flight is with models and full sized aircraft. Most of the models are 1.5 meter span dicus launched gliders wherevyou hold the model by a wing tip and spin to generate speed and fling it skywards to about 50
Meters.

With aircraft there are 3 axis of control: pitch, roll, and yaw. There are basically four forces acting on the aircraft: lift, drag, thrust and gravity. The same is true for the foils we use.

Lift is generated by almost anything moving through a fluid. Stick you hand out a car window and you will experience lift and drag. For the sake of discussion, Lift is generated perpendicular to the plane of the wing. A wing being a main wing,rudder (fin,singlet etc), stabilizer, and even the rounded fuse generates some lift. Drag is a by product of generating lift (induced drag) and disturbing the medium the craft was s moving through ( parasitic drag). For now, drag is just a force acting towards the rear of the craft trying too slow it down. Gravity is the force acting in the opposite direct of lift. Thrust is s provided by a vector from the kite to the rider and must move the foil forward in a order to generate lift and from tears motion. There is a thrust component generated by pumping a foil. But we'll ignore this for now.

Pitch is a movement along the long axis of the fuselage and essentially parallel to the mast regardless of the orientation of the craft. Roll is a movement around the long axis of the fuse regardless of its orientation. Yaw is a movement parallel to the plane of the wing.

Thanks illustrate these axis point your index finger straight out and you thumb straight away to make an "L". Your middle finger should point perpendicular to your index finger. The tip of your index finger is ge dir cation of movement and the fuse of out model. The joint by your hand is the center of gravity where all movement takes place. The center of effort will be ignored for now but it is just behind the cg of the craft will be stable. Rotating around the middle finger is pitch (the fuse moves parallel to the last); the roll is around the pointer and yaw is around he thumb. No matter how you hold your hand these axis of rotation never change in relation to each other.

The mast will have little to face the on yaw as it is too close to the cg to have any moment to correct the fuse from being out of alignment with the direction of movement. The mast will have the ability to effect roll and will never effect pitch.

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Re: Dihedral stabilizers

Postby Kamikuza » Fri Nov 17, 2017 12:40 pm

I think I've been abusing the term "winglet". I use it as anything that pokes up or down at the wing tip, but perusing the literature it seems there's a core or people that insist it has a specific use. I can't be bothered parsing the nuances though :D

Basically, as BWD said: it's a lever. A little drag and lift far away from the CoE is the same as a lot close to it. And any vertical surface will add to yaw stability.

The long LF fuse was noticeably more stable in the yaw (less fish tailing) and slower to react to pitch. It has two settings, and the longer one was still noticeably "better" than the shorter one.

A plane side slips by a bootfull of rudder and some aileron input. I don't think it's something KBHF needs to worry about...? If you can push your back foot out you might be able to "skid" the foil, but what's the point?

I rode a pre-production foil, and the holes for the vertical stab had been hand drilled and tapped so they were a tiny bit wonky. This set up a gentle snaking at very high speed but otherwise wasn't a huge issue.


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