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Topic: Direct Down Wind Faster Than The Wind (Read 24243 times) previous topic - next topic

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Re: Direct Down Wind Faster Than The Wind
Reply #1225
Static thrust (fixed prop at a fixed RPM) is the maximum thrust as illustrated here: [snip]
One frequently sees this statement (that static thrust is the maximum thrust), but that's always(?) stated in a context where negative airspeed is not even considered.  Intuitively, I'd expect the thrust in such a case to be even better than static.  (It still wouldn't be of much practical use for aircraft, for the reasons you've already given, but it's clearly applicable to the cart!)  Are there any good writeups anywhere about propellers with negative airspeed?  The usual model of how a propeller interacts with the air wouldn't even make sense in that case, so some of the equations I've seen wouldn't apply.

It's a good question and one would guess that it generates even more thrust in a negative windstream up to some point.  I haven't seen any data on it.  Spork may have some.  (after note:  I see Spork did have some info but I think where we might find some actual info is from the helicopter folk.  There is a state called a "Vortex Ring State" that is effectively negative airflow (I think) through the rotor.  I do know the that the lift is greatly reduced from the static thrust state so I believe there is the changeover point but again, I'm just guessing without a lot of hard data and smarts backing me up on this.)

In the DDWFFW cart, I don't think it matters all that much because it has been reported that the cart doesn't really "hit it's stride" until it is above windspeed because unlike a static thrust test the prop is geared to groundspeed.  Again, Spork may have some insight on that.  My assumption (pure guess) is that the prop needs to get  out of any self generated turbulence from parts of the blades being stalled and into a more laminar region but again that is just a guess.   This would depend upon gear ratios and other parameters.   I wouldn't hang a hat on it.

 :grin: Windgrins
  • Last Edit: October 06, 2017, 01:41:26 PM by windgrins
Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

Re: Direct Down Wind Faster Than The Wind
Reply #1226


The reason we take off and land into the wind is to minimize ground speed for flying and landing because during the takeoff, the resistance decreases as soon as the wheels clear the ground.  And during the landing, you are trying to minimize speed at which you contact the ground in case you lose control and run into something.

Another trivial addition.  One of the other benefits of landing as slowly as possible is tire wear and less stress on the landing gear.  Since the tires are not normally spun up for landing.  They undergo a short skid (the "skirty dirk" sound you hear when a plane touches down and the tire smoke on landing from larger jets) until they are spun up to speed.  The landing gear also experiences transient forces from this which are minimized the slower one lands.  On takeoff, the wheels are turning at takeoff speed so no major forces are felt except for unloading of the gear as the tires no longer need to accelerate.

Landing and taking off in crosswinds introduce another whole can of worms so speed is not normally minimized for this or gusty conditions.


 :grin: Windgrins

Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

Re: Direct Down Wind Faster Than The Wind
Reply #1227
Good to see you again Windgrins!

Glad to be back, MikeB.  Thanks.

 :grin: Windgrins
Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

Re: Direct Down Wind Faster Than The Wind
Reply #1228
Since the last time I was here (I think)  I received my glider rating in addition to my powerplane ratings.
Lunatic Fringe, I know you're out there.  You've got to blame someone for your own confusion.

Re: Direct Down Wind Faster Than The Wind
Reply #1229
What a guy! Glad to see you back and still working on your bucket list, Grins.  :wave:

  • A.T.
Re: Direct Down Wind Faster Than The Wind
Reply #1230
DDWFTTW post by Mark Levi, professor of mathematics at the Pennsylvania State University, writing for the Society for Industrial and Applied Mathematics:

https://sinews.siam.org/Details-Page/downwind-faster-than-the-wind

  • F X
  • The one and only
Re: Direct Down Wind Faster Than The Wind
Reply #1231
"I thought people would say, 'That's cool,' but they didn't. They said, 'Wow, you're an idiot.' So we decided to build a full-size one."
"If you pick up a starving dog and make him prosperous he will not bite you. This is the principal difference between a dog and man."
― Mark Twain 🔭

  • Brother Daniel
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Re: Direct Down Wind Faster Than The Wind
Reply #1232
Being somewhat serious here, Heinz should open a basic Physics thread first.  The DWFTTW portion is a distraction to his more basic mistakes of the past.
Still kinda tempted to make one of these.

  • F X
  • The one and only
Re: Direct Down Wind Faster Than The Wind
Reply #1233
Being somewhat serious here, Heinz should open a basic Physics thread first.  The DWFTTW portion is a distraction to his more basic mistakes of the past.
Still kinda tempted to make one of these.
There was one, it got merged with this thread
"If you pick up a starving dog and make him prosperous he will not bite you. This is the principal difference between a dog and man."
― Mark Twain 🔭

Re: Direct Down Wind Faster Than The Wind
Reply #1234
But then I remember the tides ...

Speaking of which, remember that oceanography textbook you quoted while flailing around trying to support your FysiX?  The new edition has been corrected.

  • F X
  • The one and only
Re: Direct Down Wind Faster Than The Wind
Reply #1235
Sidetrack avoided

For the never ending tides go here
"If you pick up a starving dog and make him prosperous he will not bite you. This is the principal difference between a dog and man."
― Mark Twain 🔭

  • Brother Daniel
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Re: Direct Down Wind Faster Than The Wind
Reply #1236
Heinz,
Please don't ignore this point.  This post reveals the direct contradiction between what you claim and what everyone else is saying.  There is no ambiguity here, or mixing of terms or definitions.
You claim the ground engaged cart will go backward, others claim it moves forward.  Don't try to explain WHY the carts in the video move forward, support your claim that the cart SHOULD MOVE BACKWARD.

All other discussions are a distraction to this point.  Solve this one and either you are correct or the "cargo cart" is. 
I am not ignoring this. You seem to be giving this more thought than the usual respondent, so I want to give you a thoughtful reply, with some math, and that takes a bit of time. I am working on it and will post when ready.
Heinz hasn't followed up this post yet, has he?

Re: Direct Down Wind Faster Than The Wind
Reply #1237
Heinz hasn't followed up this post yet, has he?

Give him a chance.  He wants to be careful to give you a well thought out explanation.  The last thing he wants to do is post something that's wrong and could make him look stupid.


  • Brother Daniel
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Re: Direct Down Wind Faster Than The Wind
Reply #1238
Ah, yes.  Good point.  :D

Re: Direct Down Wind Faster Than The Wind
Reply #1239
Actually for HH scholars his current absence was entirely predictable. As soon as A.T. posted that link the other day I could have told you we would be entering a Hyev hiatus. The length of it will depend on how long Mark Levi takes to decide Heinzyevpangloss is a raging loon. Something like this article for the Society for Industrial and Applied Mathematics will particularly enrage our resident crackpot since he knows it is a thoroughly peer reviewed scenario just as the AAPT was.

Re: Direct Down Wind Faster Than The Wind
Reply #1240
Actually for HH scholars his current absence was entirely predictable. As soon as A.T. posted that link the other day I could have told you we would be entering a Hyev hiatus. The length of it will depend on how long Mark Levi takes to decide Heinzyevpangloss is a raging loon. Something like this article for the Society for Industrial and Applied Mathematics will particularly enrage our resident crackpot since he knows it is a thoroughly peer reviewed scenario just as the AAPT was.

Nice try, Duffy Duck, but no, SIAM NEWS is just a newsletter that anyone can send an article to. Even a plagiarist like Mark Levi, who copied everything he posted, nearly word for word, from Mark Drela's wrong analysis without acknowledgement, can post there.

Society for Industrial and Applied Mathematics, SIAM, does in fact publish 16 or 17 peer reviewed journals, and you can see all of them at this link. You will also see that SIAM NEWS is not one of them, and is just a newsletter, where even plagiarists can send articles to be printed.


"When a true genius appears in this world, you may know him by this sign, that the dunces are all in confederacy against him." (Jonathan Swift)

Re: Direct Down Wind Faster Than The Wind
Reply #1241
Heinz hasn't followed up this post yet, has he?

Give him a chance.  He wants to be careful to give you a well thought out explanation.  The last thing he wants to do is post something that's wrong and could make him look stupid.


Yeah, unlike you spork, I try to get my facts right instead of rushing to conclusions, or in your case, rushing to making erroneous and ridiculous claims. The math involved in getting it right, is more complicated than your simpleton "explanation" which makes no sense at all.

Besides, I have been busy with far more important things than this thing; which has no importance to science at all since it is a farce. :yes:

By the way, have you tried my suggestion of tying the strings at a level equal to, or higher than the level of the cart, and see if the cart still advances on the treadmill?
Can you find a high frame rate camera to prove the cart isn't oscillating up the belt?
No?  :no:  I thought not! You will only do things to reinforce your irrational belief.
"When a true genius appears in this world, you may know him by this sign, that the dunces are all in confederacy against him." (Jonathan Swift)

Re: Direct Down Wind Faster Than The Wind
Reply #1242
And another thing, why don't you get off your ass and go out and build a proper big cart, so you could get out on a dry lake bed in REAL wind and see how that goes. Maybe then you'd see.

  • Brother Daniel
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Re: Direct Down Wind Faster Than The Wind
Reply #1243
I try to get my facts right instead of rushing to conclusions
This must be a new resolution, effective tomorrow.  Because it certainly hasn't been your MO thus far.

  • F X
  • The one and only
Re: Direct Down Wind Faster Than The Wind
Reply #1244
Yeah, unlike you spork, I try to get my facts right instead of rushing to conclusions, or in your case, rushing to making erroneous and ridiculous claims.
  :parrot:
"If you pick up a starving dog and make him prosperous he will not bite you. This is the principal difference between a dog and man."
― Mark Twain 🔭

  • MikeB
Re: Direct Down Wind Faster Than The Wind
Reply #1245
Yep it is "opposite day" all over again!

Re: Direct Down Wind Faster Than The Wind
Reply #1246
I very much look forward to Heinz's article in SIAM since any damn fool can publish there.

Rotational kinetic energy and frames of reference
Reply #1247
Linear kinetic energy is frame dependent.
Is rotational kinetic energy frame dependent? :dunno:

  • MikeS
Re: Direct Down Wind Faster Than The Wind
Reply #1248
Linear kinetic energy is frame dependent.
Is rotational kinetic energy frame dependent? :dunno:

Pretty sure it is.  If your frame is the edge of a rotating wheel then ....

  • Brother Daniel
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Re: Direct Down Wind Faster Than The Wind
Reply #1249
Linear kinetic energy is frame dependent.
Is rotational kinetic energy frame dependent? :dunno:
If we confine our attention to inertial frames, then rotational KE is frame independent.

Rotating frames (as Mike suggests) are another matter.