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Gong Hellvator - 1rst KBHF with both foils providng pos lift

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Denisesewa
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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby Denisesewa » Sat Dec 06, 2014 7:21 pm

I'd be interested to know what the relative angle of attack is between the wings, if the rear wing has a "Positive lifting foil" but a negative AOA it seems to me it would work just fine. when the front wing is at proper aoa the rear would be neutral and only provide "airfoil lift" which I think would be minimal?.
From teaching ( twintips) I know most people prefer to weight the rear foot as it seems more natural, when I bought a Mako Pro 132 it took quite a while to get used to equal weight on both feet which is what that board wanted.
And then again I could be way off the mark :P
Educate me please.

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby Europ2 » Tue Dec 09, 2014 1:33 pm

Denisesewa wrote: [...] Educate me please.
Everybody here would like to know the profiles, therefore the polars, and AoA used. :wink: :wink:
There seems to be more to this model than just sticking one Eppler 837 (FW) and one Eppler 817 (stab) on a fuselage - on the French Forum, Franckite24 has been testing many prototypes of positive lft stab for the past five years and has never found a suitable combination of front and rear profiles. However, he is very skilled in KBFH design as he developped the Sword 1 in his garage between 2009 and 2011.

FK24 probably didn't but you can use a RC design software/simulator with polar databases to assist you, play with profiles and AoA's and hopefully find the optimal combination of front and rear profiles.
- http://www.tailwindgliders.com/
- ...

If you're not ready for that, you'll have to wait a few more days to get the first technical reports and feelings of independant testers.

One expression the designer uses a lot is "The Hellvator is ardent"

What does Wiki say about that ?
"Describes the tendency of a yacht to luff (head up) when one lets the helm go."
For a horse, it means "lively".
I think that in the designer's mind, it also means "responsive"


The designer for this video on 2014 Nov 03rd wrote: "L'ours" (The Bear) on his new GONG Hellvator hydrofoil .
As you can see, it flies really well. Slow speed, high speed, moves, it does everything.
On this revolutionnary hyrdofoil, you are 100% confident. So your level improves really fast.
Remember that with 4/7 knots like on this video, flying and going upwind is a luxury. So being able to do some moves is just 100% fun and totally amazing.

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby joyrider1 » Thu Dec 18, 2014 12:37 am

Very interesting topic. Any reports/reviews yet?

Many variables seem to make it complicated. Just some amateur thoughts (I`m not an engineer or have a degree in this) …
Let’s pretend a situation which is most likely: Front wing has more surface area than rear wing and front wing can therefore generate more up/down lifting effect than the rear wing at a given speed. Rider creates between 60 and 120kg down force which has to be compensated by the wings to create overall positive lift.

In this scenario I think there are (at least?) four different situations/categories:
a) At “sub-riding” speed any AoA combination of front and rear wing creates too little lift to foil or at least one wing creates stall.
b) At the minimum speed that a foil starts to lift, both the front and rear wing would travel at uplifting AoA because the front wing alone would not have enough lift on its own. This is even the case at low/lower traveling speeds.
c) At “break even” speed one of the wings (mostly the rear one in commonly used configurations with the front wing being bigger and having more positive Aoa than the rear wing) travels at a zero-lifting AoA through the water because the other (front) wing just generates the right amount of lift on its own to keep the same flight level.
d) At even higher speeds the same wing (again mostly the rear one) travels at (slightly) down-lifting AoA through the water because it has to reduce the bigger lifting effect of the front wing and keep the combined/overall AoA of front and rear at the right angle to ride horizontally level.

Question in general: At what speed do you expect the “break even” for up to date foils with a rider of about 80kg?

Addition: Think about a configuration with the smaller rear wing having more positive AoA than the front wing.
Question: Is there practically a speed thinkable at which the front wing travels at zero-lifting AoA and the rear wing creates the needed uplift on its own?

In this whole theoretical scenario I did not consider any aspects like control, comfort or user-friendlyness. Neither was the position of cg important for me in this theoretical scenario.

Conclusions for me:
1) At lower speeds (almost?) all rear wings have to provide positive lift by their AoA, because the front wing alone does not provide enough lift to compensate the riders` weight for foiling.
2) The rear wing can very well have more positive AoA than the front one. But as long as the rear wing is (much) smaller than the front wing or has a more neutral profile than the front wing it does not necessarily generate more positive lift than the front wing.

Confusing? Wasn’t meant to be…. :bye:

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby lobodomar » Fri Dec 19, 2014 5:43 am

joyrider1 wrote: Conclusions for me:
1) At lower speeds (almost?) all rear wings have to provide positive lift by their AoA, because the front wing alone does not provide enough lift to compensate the riders` weight for foiling.
I don’t think that’s necessarily true, the front wing could provide the needed lift through a high pre-stall AoA, regardless of whether the hor stab is generating positive or negative lift. But yes, at low speeds, due to the high pitch attitude, even a horizontal stabilizer with a “negative” asymmetrical profile and/or negative angle of incidence (angle with regards to the fuselage) can generate positive lift.



joyrider1 wrote: 2) The rear wing can very well have more positive AoA than the front one.

Confusing? Wasn’t meant to be…. :bye:
That will depend on their respective angles of incidence and medium (since it varies spanwise) downwash angle for the given front wing’s lift coefficient ( the downwash angle is proportional to the lift coefficient of the wing, so that at lower speeds (higher Cl) the medium downwash (absolute) angle is greater and vice-versa. In other words, the AoAs of the main wing and the stab are not only different, but they also vary differently as the speed changes.

Also, the dynamic pressure (q, proportional to the density of the flow and to the square of its speed) in the downwash wake will be a bit lower than the free flow’s dynamic pressure, due mainly to friction losses at high speeds and to friction and separation losses at low speeds. In other words, at high speeds the downwashed flow will be less “de-energized” by the front wing than at low speeds. And since lift is proportional to the dynamic pressure, this effect should also be taken into account, not just the AoA.

Sorry if I confused things even more :) , and that was even without considering the drift/heeling angles, which is why your main question, regarding a certain speed, is really hard to answer, even if one knew the ClxAoA curves of both wings.

As for the product of this thread (Gong foil), I think one should not take for granted that the stabilizer will really be generating upward lift at the KBHF whole operational AoA range (minus downwash angle), regardless of the stab’s profile, specially when the angle of incidence of the stab (which seems to be adjustable) is set to minimum. At this setup, it should turn out to be a friendlier foil indeed (UDL though, using Europ2’s terminology, specially at higher speeds), as stated by the designer on one of the previous posts. But, when working on the UUL range, my guess is that it would have poor longitudinal stability, due to the low static arm (distance between the CG and the NP of the given AoA).

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby Europ2 » Mon Jan 19, 2015 12:37 am

Latest news from the Gong Forum. Not very usual, the kite is 17m with 75cm bar and 50m lines.
Escape with l'Ours just out from Hospital.

lobodomar wrote:... But, when working on the UUL range, my guess is that it would have poor longitudinal stability, due to the low static arm (distance between the CG and the NP of the given AoA).
The designer talks about it in one of his posts : the rider's CG must preferably be fore/above the front wing. If the rider's CG goes too aft, the foil crashes - a margin (smaller ?) to get used to.

Once again, I'm not related to the brand whatsoever ... just curious minded.

So, regarding Gunnar's comment on the The Foil Buyers Guide (Updated) post:

2a) 2 successful manoeuvres types missing : good point. However:
- at 1:15 a promising 10-second closeup sequence of slalom,
- a (?) jibe at 0:57 + during whole vid vid the designer is one handed due to the camera pole which is not the best setup to show his manoeuvres skills. Let's give him one more chance to edit a new video where the camera is stuck in the lines above his head so that he can use his 2 hands on the bar and show us all the possibilities of his baby ...

2b) Foot pressure and bent legs: on this new video, (1:50 and later) I can see 2 legs bending equally up and down, coherent with what has been announced (shared pressure between both legs especially at high speed).
At 2:54 taken from above, same thing plus body shifted more on the back foot (coherent with what has been announced).
What do you guys and gals see ?

PS: still according to the Forum, the 4 first foils have arrived from factory for testing. Test sessions are opened to any experienced foilers on the Gong French home spot of La Baule France right now. Worldwide plane shipping from China starts mid week so the first tests from customers and independant testers should be published very soon.

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby jaros » Tue Mar 24, 2015 11:08 am

As I can see from the Gong homepage, the foil is available for some time, the price is good. Has anyone bought / tried it? Any feedback?
I allready have a foil, but the Gong looks like a great deal to me...
Greetings,
Jaros

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby Nico » Tue Mar 24, 2015 1:00 pm

Paid and waiting : (

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby jaros » Tue Mar 24, 2015 1:57 pm

Paid and waiting : (
Waiting like normal waiting, or waiting like Carafino waiting?

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby MCN » Tue Mar 24, 2015 4:55 pm

jaros wrote:
Paid and waiting : (
Waiting like normal waiting, or waiting like Carafino waiting?

We received our Gong foils at the delivery date that was promissed. Great service from Gong during the whole process.

We are new to foiling (and kiting) and we havent tried the boards yet. But later this season we will give you our impressions from a beginner perspective.

Have experience from the boat industri so I can say that the finish work is superb, both foil and board. Also attractive price 1300 Euro foil and board (might be higher now), even foil covers were included in the package as a surprise bonus.

Last year we ordered foils and boards from Carafino so we have been there.

Gong is a serious company!

There are some early short rewievs on the Gong site: http://www.gongsup.com/forum/viewtopic. ... &start=375

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Re: Gong Hellvator - 1rst KBHF with both foils providng pos

Postby jaros » Tue Mar 24, 2015 5:08 pm

Great to hear that. The Gong foil looks really good to me. If I had not got a foil allready, this would be my choice most probably.
I am confused why there is not more talk about it here...


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