Contact   Imprint   Advertising   Guidelines

how can we ride 40-50 mph in only 20-25 mph of wind?

forum for kitesurfers


User avatar
spork
Very Frequent Poster
Posts: 2512
Joined: Sat Jun 26, 2004 7:41 pm
Kiting since: 0
Brand Affiliation: None
Location: Mtn View, CA (S.F. Bay)

Postby spork » Fri Jun 09, 2006 4:39 am

Well I don't think I'm getting the $100K action. But, because I'm a complete sucker, and can't wait for people to tell me I'm an idiot and breaking every law of physics, I suppose I'll post a few of the ways to do this.

First way: already posted. Put sails on the spokes of the wagon wheels and articulate them so they deploy only when beneath the axle. Not terribly practical, but works in theory.

2nd way: Imagine a big yo-yo. Wrap the string around the axle until it reaches half the radius of the yo-yo. Set it on edge so it can roll - with the string exiting beneath the axle and parallel to the ground. Put a parachute on the string. Wind pulls the parachute at near wind speed. Yo-yo rolls up the string as it's being pulled. Yo-yo approaches 2X wind speed. This one is more "illustrative" as it comes to an end when the yo-yo has rolled up all the string and reached the parachute. It also has the downside that the parachute can be considered part of the vehicle, and it never exceeds the wind speed. But it helps some people see the idea.

3rd way: tack downwind with an ice-boat. Yes - tacking downwind can yield a downwind component faster than true wind speed. It's easy enough to post the vector analysis if need be. Well that's not "directly" downwind you say. True - so build a big light frame around your ice-boat so the frame goes straight downwind while the boat tacks inside of it. Still no good you say because the C.G. isn't going directly downwind. Fine - put two ice-boats in the frame and keep them on opposite tacks.

4th way: build a land vehicle with a propellor on back that is geared to the wheels such that the tangential speed of the blade tips is equal to the speed the vehicle goes downwind. This will yield wings that are in a continuous spiraling "tack" 45 degrees downwind. This one is really peculiar. The prop doesn't cause the wheels to turn. The prop pushes the vehicle, which causes the wheels to turn, which turns the prop, thereby constraining the tips to thier 45 degree downwind tack. And NO, this is not a perpetual motion machine. It will only work so long as the wind blows. And NO, the vehicle will not just keep going faster and faster.

5th way: You kind of have to look at this one upside down and backwards, but this one happens everyday. Imagine you're in a sailboat drifting with the current on a day with 0.0 mph wind. But the current is 10 knots. Well that gives you an apparent wind of 10 knots over your bow (current pushing you through still air). If you want to go where the current is already taking you, but you want to get there more quickly, you can raise your sails, and tack into the apparent wind. The current is "pushing" you - but you're beating it to it's destination. Since the sail and the keel are both just wings in a fluid it works the same right-side-up or upside-down. Given that the constant 10 knot current is a valid inertial frame like any other, we know this is no different than tacking upwind in 0-current and a 10-knot wind. So anytime we tack upwind it's equally valid to see the water as a constraint while we tack into the wind, or the air as a constraint as we tack downstream faster than the stream.

People frequently think it's impossible to "outrun" the motive force of the wind, but we have to keep in mind the energy we are extracting is that of the relative velocity of the air over the ground or water - not over our vessel. We do have to get creative to come up with ways to do it however.

Head down - flamesuit on. I know the drill.

gdorfman
Medium Poster
Posts: 122
Joined: Sun Oct 17, 2004 8:44 am

Postby gdorfman » Fri Jun 09, 2006 4:54 am

i would say that apparent wind is both an effect of and cause of vessel speed. it's a virtuous circle. i don't see anything inconsistent about that.

i agree that ice sailors are the ones who by far must have the most practical experience with these phenomena. there are also skiff sailors for whom this is real. these concepts are more foreign to kiting where most people dont think about sailing angles anyways, at least not explicily.

to summarize more precisely these questions:

can a vessel travel (speed through water) faster than the wind? absolutely.

can a vessel travel faster than the wind while maintaining a course more than 90 degrees off the wind? absolutely, these are probably some of the best angles.

can a vessel make a downwind VMG faster than the true windspeed? they would do this by gybing back and forth on the best VMG angle. i don't know, but it seems it may be possible with the right conditions. ice sailors would know in practice.

can a vessel make a direct-downwind course that is faster than the true wind? i think no. because at this point, the apparent wind would be totally head on, and you can't sail in that.

so as a vessel goes faster and faster and keeps bearing off and bringing the apparent wind forward, at some angle this no longer works because it gets too head on, and they slow down to a speed which puts the ratio of their speed and the true wind speed, and their direction, in balance. also, as they bear off more deeply, the true wind begins getting subtracted from apparent wind, instead of adding to it or just changing direction.
Attila wrote: Second, I’d like to point out something that a lot of people talked about but doesn’t sound write: “using” the apparent wind. The only difference between the AW and TW is the fair wind, which is opposite direction to the direction you travel. FW is an effect of travel speed not a cause. Therefore talking about using AW as opposed to TW to create speed doesn’t make sense.

User avatar
spork
Very Frequent Poster
Posts: 2512
Joined: Sat Jun 26, 2004 7:41 pm
Kiting since: 0
Brand Affiliation: None
Location: Mtn View, CA (S.F. Bay)

Postby spork » Fri Jun 09, 2006 6:06 am

gdorfman wrote:can a vessel make a direct-downwind course that is faster than the true wind? i think no. because at this point, the apparent wind would be totally head on, and you can't sail in that.
I absolutely agree that a typical sailing craft such as a sailboat, ice-yacht, windsurfer, etc. can not run directly downwind and out-race that wind. However, I gave several examples above that allow the craft to extract the energy of the wind over the ground or water that WILL cause the craft to go directly downwind faster than the wind, and powered solely by the wind. In all cases it will slow the wind down relative to the ground or water the wind is moving over, as the craft extracts that energy.

ImagineParadise
Medium Poster
Posts: 137
Joined: Sat Feb 18, 2006 5:47 am
Kiting since: 0
Brand Affiliation: None
Contact:

Postby ImagineParadise » Fri Jun 09, 2006 6:46 am

BJZ wrote:Speed will be limited severly by board drag. Ice boats hit speeds over 150 mph by removing drag. Kites seem pretty efficient already. Work on the board and surface friction and turbulence.
Bingo.

Also, I can see a kiter go faster than the wind by a little bit. It would be somewhat difficult, here goes my explination...

Riding in the apparentwind/downwind and getting into posission BEHIND the kite while sheeting in, then pull very hard, keeping balance, this would give you a "burst" of momentum for as long as you could hold the kite in the pull. But, As you gain momentum, I see the kiter starting to inch forward past the kite so he is now infront of it a little. Then you would slowdown to wind speed as you franticly try to power your kite up as you get behind it again. Then you do it all over again, in bursts, I find it hard to believe kites can do this continuously as they are still being modified every 6 months or so. Its kind of like when you were a little kid on a swing. you swing faster by pushing your weight down when the swing is just starting to go foward.

User avatar
spork
Very Frequent Poster
Posts: 2512
Joined: Sat Jun 26, 2004 7:41 pm
Kiting since: 0
Brand Affiliation: None
Location: Mtn View, CA (S.F. Bay)

Postby spork » Fri Jun 09, 2006 6:53 am

Even if that could work, wouldn't that be powered partially by the kiter's muscles rather than purely by the wind?

gdorfman
Medium Poster
Posts: 122
Joined: Sun Oct 17, 2004 8:44 am

Postby gdorfman » Fri Jun 09, 2006 6:53 am

i wrote my reply before i saw your latest. i was also thinking of conventional vessels, not your creative ones.

however, to respond to your ideas in order:

#1: i don't believe this works, due to my point that the apparent wind will be from directly ahead, not behind. all you're doing here is using mechanical advantage (gearing) to extract greater speed than the wind power. but once you going downwind at true windspeed, there is no more power. and if you go faster it all becomes drag.

#2 first, i can't really visually how the yo yo gets pulled up the string, i think the string would just unroll. but maybe you have a diff design. anyways, grant you that, but the problem is that as the yo yo goes downwind and catches up to wind speed, the parachute falls out of the sky b/c the lines are tethered to it. again, you have reached 0 wind, or a headwind. this is like jumping, if you fly horizontal too fast, your wind over the kite would drop.

#3 actually i like this one! as long as you can get get downwind VMG greater that windspeed (my 3rd question) then you can create a box to have that vmg turned into direction by having the boats tack inside it. cool.

#4 i believe this is similar to #1. again, you're using gearing, and you're also using prop rotation (kind of like kite looping) to create greater windspeed over the foil than the vessel is actually travelling. however, again, once the vessel reaches a greater downwind speed than windspeed, you will have a headwind. okay, your props can still turn in the head wind (if they are rotated) but they are creating drag, not lift. an airplane on a runway doesn't magically move forward if a headwind blows on it and the prop is already turning (with no power source).

#5 i agree with your point here, but don't see the connection to going downwind faster than wind. i think you should be asking in this case if you can go *downwind* faster than the current--to go downwind relative to land. based on vmg, i think we agree it is probably possible (this is the non theoretical scenario if someone has zero wind, but travelling on a fast river, can they hold their place...). but i don't think you could ever head directly downwind, which is the point we're getting at.

actually, the more i think about it, i think the feasibility of 3 and 4 should be equivalent, because 3 seems similar to the idea of a prop where it's private apparent wind overpowers the fact that the vessel feels a headwind. hmm. i don't like 4 being true. maybe this leads to a proof that a vessel actually can't have a downwind vmg greater than windspeed. i don't know.

good topic. barely related to kiting, but better than most drivel here anyways.
spork wrote:
gdorfman wrote:can a vessel make a direct-downwind course that is faster than the true wind? i think no. because at this point, the apparent wind would be totally head on, and you can't sail in that.
I absolutely agree that a typical sailing craft such as a sailboat, ice-yacht, windsurfer, etc. can not run directly downwind and out-race that wind. However, I gave several examples above that allow the craft to extract the energy of the wind over the ground or water that WILL cause the craft to go directly downwind faster than the wind, and powered solely by the wind. In all cases it will slow the wind down relative to the ground or water the wind is moving over, as the craft extracts that energy.

User avatar
spork
Very Frequent Poster
Posts: 2512
Joined: Sat Jun 26, 2004 7:41 pm
Kiting since: 0
Brand Affiliation: None
Location: Mtn View, CA (S.F. Bay)

Postby spork » Fri Jun 09, 2006 7:55 am

gdorfman wrote: #1: i don't believe this works, due to my point that the apparent wind will be from directly ahead, not behind.
Not on the bottom half of the spokes. When the wagon travels at wind speed the point half way down the bottom spoke is going in the same direction as the cart, but at half the wind speed. So pushing on this point at wind speed will still accelerate the wagon - well past true wind speed.



gdorfman wrote: #2 first, i can't really visually how the yo yo gets pulled up the string, i think the string would just unroll.


You don't have to visualize it. Do you have a yo-yo?
gdorfman wrote:as the yo yo goes downwind and catches up to wind speed, the parachute falls out of the sky b/c the lines are tethered to it. again, you have reached 0 wind, or a headwind.


Again, when the yo-yo has reached wind speed the parachute has reached only half wind speed since it exits the yo-yo below the axle midway between the axle and the ground. Again, you can easily try this with a yo-yo. You'll see that when you pull the string with your fingers the yo-yo will be going twice as fast as your fingers. This will keep tension in the line until the parachute is going true wind-speed - and the yo-yo is going 2X true wind speed.
gdorfman wrote: #3 actually i like this one! as long as you can get get downwind VMG greater that windspeed (my 3rd question) then you can create a box to have that vmg turned into direction by having the boats tack inside it. cool.
Woohoo!!!
gdorfman wrote: #4 i believe this is similar to #1. again, you're using gearing, and you're also using prop rotation (kind of like kite looping) to create greater windspeed over the foil than the vessel is actually travelling. however, again, once the vessel reaches a greater downwind speed than windspeed, you will have a headwind. okay, your props can still turn in the head wind (if they are rotated) but they are creating drag, not lift. an airplane on a runway doesn't magically move forward if a headwind blows on it and the prop is already turning (with no power source).
This one is actually far more like #3 than it is like #1. I'm simply taking my boat that tacks downwind and constraining it to follow a continuously spiraling 45-degree downwind path (one long continuous tack). It's true that a plane won't move forward on the runway when it encounters a headwind. But it will if it encounters a sufficient tail-wind - which is how this craft works. Of course the plane won't exceed the wind speed when blown downwind on the runway, but that's because we haven't geared the prop to the wheels properly, sized things appropriately, and reduced drag sufficiently.

gdorfman wrote: #5 i agree with your point here, but don't see the connection to going downwind faster than wind.
This one isn't obvious. If a boat can tack into a purely relative wind when being pushed by the current (thus outrunning the current that pushes it), then it can similarly be "pushed" downwind faster than the wind. Because of the relative L/D of keels and sails we rarely see a boat that can achieve this feat (Steve Fosset's PLayStation is said to have polars that would make it possible). However, both the keel and the sail are simply wings acting in their own fluid. So there's no theoretical difference between the two. Any boat tacking into the wind in still water is exactly the same as a boat being pushed down-current by the water, but beating the water that pushes it. You just have to look at it from the inertial reference frame of the wind.
gdorfman wrote: but i don't think you could ever head directly downwind, which is the point we're getting at.
Yes, you're right. I mis-spoke here. This one simply demonstrates that the boat can absolutely make a VMG downwind (or downcurrent as the case may be) faster than the wind (or current). We still have to stick it in a frame to meet the original constraints of the problem.

gdorfman wrote: actually, the more i think about it, i think the feasibility of 3 and 4 should be equivalent, because 3 seems similar to the idea of a prop where it's private apparent wind overpowers the fact that the vessel feels a headwind.
DOH! You're ahead of me. That's what I am getting at above.
gdorfman wrote: hmm. i don't like 4 being true.
I don't like taxes. But that doesn't change the fact that they exist and I pay 'em :D

gdorfman wrote: maybe this leads to a proof that a vessel actually can't have a downwind vmg greater than windspeed. i don't know.
Well, my $100K is still available. Please get back to me sooner rather than later - because my day job is cutting into my kiting time. :D

gdorfman
Medium Poster
Posts: 122
Joined: Sun Oct 17, 2004 8:44 am

Postby gdorfman » Fri Jun 09, 2006 8:51 am

i have reviewed #1, and i believe the sails must be on the top spokes, not the bottom ones. otherwise it will go backwards and upwind. this is not complicated. are you trying to trick me? given that, the top spokes actually have less apparent wind even than the cart, and will hit a headwind first.

i do not have a yo yo. isn't it just a fixed axel, or is it complicated? i guess that you are saying that even though the kite/parachute is flying slower than the object it's fixed to, the lines don't slacken and it doesn't fall down, because the lines are being reeled in. but that entails work and i think breaks the argument. i can winch a car downwind at infinite times the wind speed.

for 3, i don't understand your "spiralling 45 degree downwind path" do you mean the path of the blades or the actual vehicle? i think you mean the blades--their path combinign the vehicle's travel and their rotation. but still, at first they have a tailwind vector added to them, and then at some point a headwind vector. doesn't this require their pitch to totally change and doesn't this threshold change all the dynamics?

i still disagree your current example is relevant. upwind is easy--the apparent wind keeps growing (although you need to point high). downwind is the exact opposite--at some point boat speed is a negative, not a positive. your point does not show you can go downwind faster (vmg) than the current--it's not at all the same thing to prove. can a fast boat in 0 wind and 10kts current, hold it's place in respect to the ground?

User avatar
Klaus (c:E
Very Frequent Poster
Posts: 687
Joined: Mon Apr 12, 2004 10:15 pm
Kiting since: 0
Style: lousy
Brand Affiliation: None.
Location: Emden, Germany
Contact:

Postby Klaus (c:E » Fri Jun 09, 2006 10:56 am

Hai,

this is a very funny and interesting discussion but may i interrupt for some calculations concerning speedkiting?

1) You have the efficency of the kite. There are measured values for L/D between 4...7 giving glide angles between 8...14°
The kite pull bears 8...14° off the direction perpendicular to the apparent wind.

2) You have the efficiency of the board, that of course is very much dependant on riding skills and conditins. No measured values here but to give You some clue:
The best speed to wind speed ratio i have measured so far (anemometer, GPS) is ~1.8 and it was achieved using a high performance kite (glide angle ~ 8°). So the glide angle of the board must have been better than 26°. This said i will set the optimum glide angle for a special speed board in good conditions to an estimated 20°.

3) The glide angles of the kite and the board add to the glide angle of the craft (consisting of board and kite). So using 20° for the board there are glide angles of the craft in the range of 28...34°

4) The optimum speed ratio that can be achieved then is 1 / sin(glide angle) and it can easily be shown that this optimum speed ratio is achieved only at the optimum course heading 90° + glide angle off the wind.

Results from this are that we can assume:
- optimum speed ratios of 1,8...2,1
- bearing 134°...128° off the wind in real speed kiting.

It would be most interesting to get some more data concerning the glide angles that are achieved by good riders in good conditions on speed boards.

Sea You: Klaus (c:E

fxeric
Medium Poster
Posts: 68
Joined: Thu Feb 19, 2004 12:24 pm
Kiting since: 0
Brand Affiliation: None

my 2 cents

Postby fxeric » Fri Jun 09, 2006 12:49 pm

you can't sail downwind faster then true windspeed but you can get downwind faster then windspeed by storing energy from the sail as forward velocity and redirecting it downwind in bursts (can't be done for more then bursts due to drag ,the less drag u have the more effectively u can use the stored energy like in ice boats to get a better vmg)

not to get off the topic but can you sail directly upwind?

fxeric


Return to “Kitesurfing”

Who is online

Users browsing this forum: Bing [Bot], Yahoo [Bot] and 23 guests

cron