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take off speed

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grigorib
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take off speed

Postby grigorib » Wed Aug 16, 2017 6:49 pm

relevant to the discussion about hydrofoil lowend I wonder about couple of things. I guess the area matters the most, the AR matters and the profile depth/shape matters. I'm omitting fluid viscosity and avoid mentioning Re number since it's not easy to find it out for various wings on the market. So
- what exactly and to what extent (are there numbers available) impacts take off speed?
- what was wrong with large area freeride foil wings (pre-2014 MHL, first LF foil, Naish/Cabrinha foils)? They felt great for freeride and would take off at low speeds without stalling...
- what are the terms describing sub-foil speed when riding looks as Pugachev's Cobra / Harrier jet takeoff / ground effect flight - you start riding at a high AoA and then at certain speed the wing actually gains lift so it doesn't need to serve as a reflector anymore? What's that speed called/considered?
Last edited by grigorib on Wed Aug 16, 2017 9:35 pm, edited 4 times in total.

grigorib
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Re: take off speed

Postby grigorib » Wed Aug 16, 2017 9:09 pm

the speed point between "reflection lift" and "glide lift" - I guess it's called a stall speed and is defined by Re number, right?
What's the best term of above-zero < pre-stall < foiling speed?

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Re: take off speed

Postby irwe » Wed Aug 16, 2017 9:19 pm

IanFoil.jpg
IanFoil.jpg (43.27 KiB) Viewed 873 times
I have also found lift off speed is quite different when comparing kiting to being pulled behind a boat for the same hydrofoil set up.

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Peter_Frank
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Re: take off speed

Postby Peter_Frank » Wed Aug 16, 2017 10:17 pm

I am pretty sure the lift off speed is the same, if you are being pulled behind a boat with a tower so the rope is same angle as your kitelines.

You reduce the needed lift hugely this way, thus a lot lower take off speed.


- what exactly and to what extent (are there numbers available) impacts take off speed?
Total load on the foil versus overall foil lift impacts take off speed.
Total load is total weight minus vertical lift from the kite.
And to be more precise, the take off speed is proportional to the squareroot of (total foil load divided by total foil lift), meaning if you press on the board with 4 times as much load (heavier rider and maybe smaller kite or flown lower), your takeoff speed will only be doubled.
And the other way around, if you have a wing with 4 times more area, your take off speed will only be half as fast, and not one quarter of the small wing take off speed.
Overall lift is mostly dependant on area * lift coefficient, the latter of course how much camber, and if too low AR lift is lost also.
Numbers ? Can be calculated yes, but you will have to know the support from the kite, which is not easy - as the bigger the kite, and the higher it is flown, the lower take off speed you get.


- what was wrong with large area freeride foil wings (pre-2014 MHL, first LF foil, Naish/Cabrinha foils)? They felt great for freeride and would take off at low speeds without stalling...
Nothing was wrong with these IMO, and many use these today.
Difference is just, that there are so many choices today, racing, waveriding, beginners, freestyle, jumping, etc etc, that fewer will choose a "beginner" wing, eventhough many use these later for freestyle and waves here.


- what are the terms describing sub-foil speed when riding looks as a Harrier jet takeoff or rather "ground effect vehicle" - you start riding at a high AoA and then at certain speed the wing actually gains lift so it doesn't need to serve as a reflector anymore? What's that speed called/considered?
Stall speed. This is when you have max lift and the lift starts to drop fast or instantly at higher AoA, and only drop slightly at lower AoA.
But this is not how you start foiling normally IMO (deflection), it is simply by increasing speed/pull and NOT AoA, you get to the lowest takeoff speed.
If you push with your rear foot at too low speed, the wing will ride stalled and almost no lift remaining and it will take even longer to regain lift, maybe never happen - of course you can increase your speed (using the kite) till you get sufficient flow to get the needed lift eventually in some cases, but this will be a very very bad and inefficient way - much better to ride with low AoA and no deflection really, and then when the speed increases you can (with loads of experience) find the balancing point where you can ride the slowest possible speed on the verge to stalling, or maybe semistalled.


8) PF

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Re: take off speed

Postby gbrungra » Wed Aug 16, 2017 10:43 pm

Does anyone have a boat with a tower tall aenough and rope short enough to match the angle of pull of a kite?

Maybe a really short rope?
Or normal rope from the mast of a cruising catamaran?

grigorib
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Re: take off speed

Postby grigorib » Wed Aug 16, 2017 10:49 pm

Peter_Frank wrote:
Wed Aug 16, 2017 10:17 pm
...
8) PF
Thanks!

My experience is that with high area / low aspect (MHL 2013) wings the stall speed is very low, so at first kite dive I'm up and foiling. When I demoed the Slingshot Hoverglide and rode the Ghost Whisper I felt I'm rather deflecting (thank you for the word!) before I gain more speed - but at the same low speed I'd be foiling already on the MHL where that "deflection" part is almost non-existent.
Any yes, no crap, transition from deflection mode to foiling through the stall point is iffy :)

So another question - since the "beginner" wings are so good at lowend (and they're so fun at the lowend!), what are the modern wing models as good as those are? Lift130? Or maybe Lift110 would do just fine?

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Re: take off speed

Postby irwe » Wed Aug 16, 2017 11:21 pm

I think there is a difference in lift off speed for a kiter vs. a boat tow. In the above picture the rope is attached to a wakeboard tower and I need at least 18.5 mph to get up on the foil.
When I am hydrofoiling with a kite. I can be up on the foil in 10 kts with a 14.5 m kite and can be going very slow.

So in my opinion the lift from the kite makes a significant difference.
Anyone else for comparison?

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Peter_Frank
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Re: take off speed

Postby Peter_Frank » Thu Aug 17, 2017 8:34 am

grigorib wrote:
Wed Aug 16, 2017 10:49 pm
Peter_Frank wrote:
Wed Aug 16, 2017 10:17 pm
...
8) PF
Thanks!

My experience is that with high area / low aspect (MHL 2013) wings the stall speed is very low, so at first kite dive I'm up and foiling. When I demoed the Slingshot Hoverglide and rode the Ghost Whisper I felt I'm rather deflecting (thank you for the word!) before I gain more speed - but at the same low speed I'd be foiling already on the MHL where that "deflection" part is almost non-existent.
Any yes, no crap, transition from deflection mode to foiling through the stall point is iffy :)

So another question - since the "beginner" wings are so good at lowend (and they're so fun at the lowend!), what are the modern wing models as good as those are? Lift130? Or maybe Lift110 would do just fine?

A bit more knowledge that might help:

Higher drag does NOT mean lower take off speed, this is a common misconception.
But of course, as both bigger and more cambered wings has more drag, some think it is connected this way, but it is just reverse :wink:

In the case above where you feel they are very different, it can be so many other causes, like the actual profile, the stabilizer, the angle etc etc, that gives you the very different feel - so I would not base experience mostly on the wing size and AR, as this will be misleading very often.

In general, there is a huge difference in low versus high AR wings:

Low AR wings will create tip vortices that change the AoA of the waterstream a lot "back" when you increase AoA, meaning you can ride with a lot higher AoA without stalling, thus they will not be pitch sensitive.
They will almost not stall but can be ridden at extremely high AoA without stalling, and they will also give you a warning (on some wings) in terms of a slight feel or vibration so you know you are about to go too slow now.
So on these it might feel like you ride on "deflection" only, but it is still the same lift as any wing uses, it is just because they can be pushed to way higher angles without stalling, that you get this idea :roll:

High AR wings are a lot more pitch sensitive, and if you push too hard with your rear foot at too low speed, they will stall over the entire or most of the wing, without warning, and it feels like an elevator where the cable snaps :naughty:


So low AR bigger wings are WAY easier to ride, they compensate for rider errors hugely and turn faster because of less span.
But high AR wings, everything else being equal, can ride in a tad less wind actually, start at a slower speed (but very little difference, hardly noticeable).
The major advantage is a lot less drag, particularly when going upwind, so because of this they can ride in even less wind and go a lot better upwind (and downwind).
They wont turn as fast as lower AR wings, so there is a big tradeoff in terms of fun yes.

You like big and early starting wings right now, but for many they can become boring pretty fast.
Why is this ?
Because, it feels draggy, you dont accelerate "freely" anymore like a faster wing does even without much kitepower, and furthermore a tad smaller not too high AR wing can turn really really fast so the best of both worlds.

Personal likings only, but when the waves and wind picks up a bit, I definitely choose a smaller also medium or low AR wing for freestyle and waves, way more fun (for me).
And in lighter winds/small waves, bigger low AR wings are the most fun.
When really light, bigger and higher AR wings are my choice, as they can ride in the least possible wind.

8) PF

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Re: take off speed

Postby Mossy 757 » Thu Aug 17, 2017 4:53 pm

Lose weight, take off sooner, live longer.

grigorib
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Re: take off speed

Postby grigorib » Thu Aug 17, 2017 9:21 pm

Peter_Frank wrote:
Thu Aug 17, 2017 8:34 am

High AR wings are a lot more pitch sensitive, and if you push too hard with your rear foot at too low speed, they will stall over the entire or most of the wing, without warning, and it feels like an elevator where the cable snaps :naughty:

8) PF
exactly that "elevator cable snap" feeling on smaller, higher AR wing! While higher speed is fun and smoother, it requires more finesse at low speeds, especially taking off and trying to not overpush on the back foot. Less forgiving I'd say.

When I gain speed first and then come afoil it's just fine, no problem but in choppy conditions and with a tiny board I'm just tempted to get afoil at the first dive, above whitecrap and keep going.


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