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HomeOff Topics › Downwind faster than the wind
04-07-2010 07:22 AM  8 years agoPost 1201
spork

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Mountain View, CA

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Cool spork, good to see your still around, you still flying some rc these days to?
Hardly fly R/C at all lately. I've still got my T-Rex sitting there waiting to be played with, and just got one of those little tiny P51 foamies, but between work and the downwind cart (and flying paragliders at the coast) I don't seem to get them out these days.

But over the last 30+ years, it's come in waves. I'll probably jump in head first again at some point.

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04-09-2010 02:02 AM  8 years agoPost 1202
GimbalFan (RIP)

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Big Coppitt Key, FL

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come in waves.
This is either a metaphor to live by, or a prophetic lyric to a new hit song.

op-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-thwop-t

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04-20-2010 07:57 AM  8 years agoPost 1203
spork

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Mountain View, CA

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We spent Saturday with the folks from the Discovery channel. They're doing a segment on the downwind cart for a Canadian program called "The Daily Planet". It won't air in the U.S. but I hope to be able to post the video after it airs in Canada.

In the meantime, we've been hard at work adding aerodynamic components to the vehicle since we were hitting higher speeds than we initially anticipated. We also slapped a coat of paint and our sponsors' logos on it just in time for the Discovery channel crew.

With the new slightly less ugly look, we felt it deserved a name change from BUFC (big ugly…cart) to "Blackbird".

We're working with NALSA to establish the new records category for this class of vehicle - and we hope to be taking a shot at the new record in the coming weeks. There are still some more improvements to be made, but we're beginning to be optimistic about an official record of 3X wind speed or more - directly downwind.

As always, you can follow our progress at http://www.fasterthanthewind.org

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04-20-2010 12:47 PM  8 years agoPost 1204
SSN Pru

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Taxachusetts

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how bout a pod behind the prop, a spinner, and a headrest!

Stupidity can be cured. Ignorance is for life!

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04-20-2010 01:22 PM  8 years agoPost 1205
spork

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Mountain View, CA

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You're one step ahead of us. All those things are planned.

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04-20-2010 05:14 PM  8 years agoPost 1206
SSN Pru

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Taxachusetts

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sweet, too bad Discovery isn't airing the show in this country.

Stupidity can be cured. Ignorance is for life!

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05-05-2010 03:20 AM  8 years agoPost 1207
Brokenlink

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Oakdale

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Wow,that thing is coming along nicely.You guys are going to have to take it to Bonneville to have enough room.When it's all done,is it going on display anywhere?

Jamie Griffith

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05-06-2010 08:15 AM  8 years agoPost 1208
spork

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Mountain View, CA

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Discovery Canada aired the segment on our downwind cart last night. We couldn't get it here, but they did post it on their website:

http://watch.discoverychannel.ca/da...010/#clip298008

It wasn't 100% technically accurate, but we liked the piece nevertheless. Check it out if you're interested.

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05-06-2010 09:36 AM  8 years agoPost 1209
DKNguyen

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Ive been digging through this thread from the beginning and there's so much irrelevent back and forth that I don't have any idea what's going on anymore. I don't know anything about sailboats or iceboats so it doesn't seem help much to have those analogies.

Could you just put a direct (no analogy of other systems) description of how it's supposed to work so I can muddle my way through it?

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05-06-2010 10:25 PM  8 years agoPost 1210
spork

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Mountain View, CA

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Alrighty - here's as simple as I can make it. Feel free to ask further questions...

First off it's critically important to understand that the wheels turn the prop - always. The prop does not turn the wheels - ever.

Now...

Power = force x speed.

So if I have a retarding force of F on the wheels and the vehicle is going at a speed of Vc, we know we're putting F x Vc power into the vehicle through the wheels. If there are no losses (rolling resistance, aero drag, etc.) we get to use all of that power.

Now if the prop is producing a thrust T, and is moving through the air at Vp, we know it's putting out T x Vp power.

We also know that Vp = Vc - wind_speed. Finally, we know that the cart will remain at a steady state speed if the net force acting on it is 0. This suggests that T and F are equal.

And this tells us that for a cart with no losses, there can be much more power coming in (F x Vc) than power going out (F x (Vc - wind_speed)) if wind_speed is greater than 0.

In the real world we do have drag, rolling resistance, etc. So it's simply an engineering matter to design a cart with low enough losses that you can cover them with the excess power described above.

Note: no sailboats were harmed in the making of this post.

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05-06-2010 11:07 PM  8 years agoPost 1211
jphilli

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Atlanta, Ga.

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Haha, you guys want to go Pi* wind speed! Rick, you're classic GT.

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05-06-2010 11:16 PM  8 years agoPost 1212
psych-lick

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Portland, OR

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Very cool. I didn't understand it until I saw the vid. Well done.

-Jeff T.

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05-06-2010 11:26 PM  8 years agoPost 1213
Brokenlink

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Oakdale

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Check some more out here.
http://www.fasterthanthewind.org/

Jamie Griffith

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05-07-2010 07:47 AM  8 years agoPost 1214
DKNguyen

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Oh, I was actually asking about the actual plane scenario, not the cart and propeller scenario. ANyways...

So if the wheels are turn the propeller but never the other way around, there is some kind of clutch-type thing?

And what exactly do you mean by the propeller never turns the wheels? As in the propeller cannot backdrive the wheels? But can still propel the cart forward via thrust (rather than backdriving the wheels)?

And which way is the propeller's "front" pointed? Forward of the cart? Or backward?

Mechanics aside, I'm having trouble following your definition of power "in" and "out". What are the losses, the power/energy being added to the system, and the energy already in the system (via movement aka kinetic energy). I took "retarding force" to mean some kind of loss but you say it's power going into the system. And then
F x (Vc - wind_speed)) sounds like power going into the system, but you say it's going out.

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05-07-2010 08:15 AM  8 years agoPost 1215
spork

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Mountain View, CA

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Oh, I was actually asking about the actual plane scenario, not the cart and propeller scenario. ANyways...
What's the "plane scenario"?
So if the wheels are turn the propeller but never the other way around, there is some kind of clutch-type thing?
We do have ratchets on the prop shaft and on the axles, but they are there basically just to make sure the prop doesn't drive the wheels (because our transmission isn't designed to load in that direction).
And what exactly do you mean by the propeller never turns the wheels? As in the propeller cannot backdrive the wheels? But can still propel the cart forward via thrust (rather than backdriving the wheels)?
The prop is turned by a chain that goes around the axle. The axle applies the torque that tightens the right hand side of the chain, and this turns the prop.
And which way is the propeller's "front" pointed? Forward of the cart? Or backward?
Forward.
Mechanics aside, I'm having trouble following your definition of power "in" and "out". What are the losses, the power/energy being added to the system, and the energy already in the system (via movement aka kinetic energy).
We aren't considering any kinetic energy in the system. None is required.
I took "retarding force" to mean some kind of loss but you say it's power going into the system.
The road is forcing the wheels to turn. The wheels apply a retarding force because turning them drives the prop - and that ain't free.
And then
F x (Vc - wind_speed)) sounds like power going into the system, but you say it's going out.
Yes. That's the thrust of the prop multiplied by the speed it moves through the air. We have to turn the prop to make that happen, so that's an energy cost.

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05-07-2010 08:38 AM  8 years agoPost 1216
DKNguyen

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Wasn't the original scenario some kind of airplane flying downwind but moving twice as fast relative to the ground as the wind was? Or was I mistaken?

TO your last reply:
Oh, I see...sort of. Somewhat, but not sure how to put it into words. SOrt of like when looking at balancing an account and not being sure what are assets, costs, losses, and liabilities (with regards to power in and power out being losses of one part of the machine but inputs to another part of the machine).

For the cart...
I googled around for explanations and I think one main thing I am having trouble with is dividing up the "part that lets you go up to the speed of the wind" and the part that "lets you go faster than the wind". SO suppose you just added a regular drag sail on your vehicle with the wheel-driven propeller. The wind is capable of pushing vehicle via the sail up to the speed of the wind as an upper limit (lower with losses). But in this action of pushing the vehicle forward with the sail via drag, it forces the wheels on the ground to roll. But the wheels are linked to the propeller which causes the propeller to spin which produces a thrust that pushes backwards against the wind further accelerating the vehicle forward? And the vehicle only stops accelerating when the drag and other losses completely cancel out the force being applied by the downwind against the sail, and the thrust being provided by the propeller?

(In reality, they both work together to bring the vehicle up to wind speed, and then the propeller part alone works to make it exceed windspeed, but let's just divide it up into the part that makes it "up to wind capable" and the part that makes it "faster than wind capable" )

If I can get that part out of the way, then I can start mulling over the two energy sources involved. The first being the speed differential between wind and vehicle (simple enough right?) and the second one which you say is the speed differential between wind and ground (the trickier one) which makes it not a PMM. Well that's not quite true. EVen with all the hand waving and whether it would actually work or not, I can see why there would be speed limits and it wouldn't continue to accelerate forever. I only consider PMMs machines that produce energy from nothing- not even machines that have 100% efficiency and continue to run forever as long as you don't try to extract energy from them (those are just 100% efficient machines that do nothing).

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05-07-2010 01:44 PM  8 years agoPost 1217
Brokenlink

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Oakdale

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Could you just put a direct (no analogy of other systems) description of how it's supposed to work so I can muddle my way through it?
Then you come up with this,see,it is easier when you look at it differently.
SOrt of like when looking at balancing an account and not being sure what are assets, costs, losses, and liabilities (with regards to power in and power out being losses of one part of the machine but inputs to another part of the machine).
SO suppose you just added a regular drag sail on your vehicle

Jamie Griffith

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05-07-2010 10:32 PM  8 years agoPost 1218
DKNguyen

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I don't see your point brokenlink. I was asking for an explanation of the system itself without analogy, especially since I thought the original scenario was supposed to be a plane flying downwind faster than the wind. But then spork started talking about this wheel-propeller thing almost like an analogy to the original scenario which just got confusing because i couldn't see how that had anything to do with a plane maintaing level flight downwind faster than the wind.

The thing about accounting and budgeting was just me trying to put into words my limited understanding of his vague definitions of power in and power out.

I said "added sail" to add an explicit drag component in. That's one thing that isn't obvious from looking at how the thing starts moving. It's not an analogy.

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05-08-2010 12:03 AM  8 years agoPost 1219
spork

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Mountain View, CA

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I don't see your point brokenlink. I was asking for an explanation of the system itself without analogy, especially since I thought the original scenario was supposed to be a plane flying downwind faster than the wind.
Where did that come from?
But then spork started talking about this wheel-propeller thing almost like an analogy to the original scenario...
The original post is mine. The original scenario IS the "wheel-propeller thing"
The thing about accounting and budgeting was just me trying to put into words my limited understanding of his vague definitions of power in and power out.
I always find it amusing that people claim I'm being vague when they fail to understand what I'm saying. I think you'll find that my energy analysis is accurate and rigorous. You don't need to use fancy and incomprehensible equations and symbols to be accurate and rigorous.

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05-08-2010 01:22 AM  8 years agoPost 1220
SSN Pru

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Taxachusetts

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Lets make this as clear as mud!

This was NEVER about an airplane going faster than the wind directly downwind.

It WAS about a wind powered LAND VEHICLE using the wind ALONE to power it DIRECTLY downwind faster than the wind.

Now, we ALL know you what you thought this discussion was about, now we all KNOW you KNOW what this discussion is about!

Stupidity can be cured. Ignorance is for life!

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