How does a headwind affect speed and power output?

Commuting, Day rides, Audax, Incidents, etc.
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: How does a headwind affect speed and power output?

Post by Jdsk »

Nearholmer wrote: 13 Nov 2022, 11:33pm Yes. So, what is the point under discussion?

Put another way, what is it that is currently not agreed?
Eventually the point is Jon's question.

But I was trying to get there step by step as a way of reaching agreement. That approach clearly hasn't worked.

This is not disagreement with the third equation cited upthread for when air is moving relative to the ground. In my approach we just haven't got to that yet. But that step is about air moving relative to the ground and using the appropriate frame of reference, not about any effect of means of propulsion on drag.

Jonathan
Nearholmer
Posts: 3898
Joined: 26 Mar 2022, 7:13am

Re: How does a headwind affect speed and power output?

Post by Nearholmer »

Hmmm …… I’m still not sure I understand why this is still going round.

An approach to answering Jon’s question is in what I said yesterday.

If we’ve moved on from men on bikes to aeroplanes, we’ve still got the same two vectors, wind and motion of the plane, both having speed and direction, both measured relative to the ground.

Simplify by assuming the plane is heading direct into the wind, then find the difference between the two and we have the resultant air speed at the plane, which we can use to calculate drag/resistance if we know the coefficient to apply based on the shape of the plane.

The power that the plane needs to output will depend upon the speed that it is required to achieve relative to the ground, and the resultant air speed, plus all sorts of stuff to do with the efficiency of propellers, turbo-fans, and jet engines which is well outside of my knowledge.

I was watching a light aircraft fighting a gale last weekend. Wind speed was probably as much as 60mph at 10 000ft judging by the way clouds were racing along, and the engine of the plane was working at full power, it sounded as they do when accelerating at take-off, yet the plane was stationary in the sky. I was with several others watching a village football match, and everyone was staring up at the plane, wondering whether it was going to blow a gasket or run out of fuel and get blown a county away, but the wind eventually dropped and it began to inch forward, engine still roaring. The pilot must have regretted taking it up.

Birds are more interesting, because they vary their shape not only to change the amount of lift they create, but also their drag, which is impressive stuff. They can make headway against the wind by using it to lift them high, then adopt a low-drag mode and go into a swoop that carries them a long distance forward.
roubaixtuesday
Posts: 5801
Joined: 18 Aug 2015, 7:05pm

Re: How does a headwind affect speed and power output?

Post by roubaixtuesday »

Chris Jeggo wrote: 13 Nov 2022, 10:51pm Screenshot from 2022-11-13 22-46-10.png
It is the second equation that applies here.
C'est exact.
User avatar
Chris Jeggo
Posts: 571
Joined: 3 Jul 2010, 9:44am
Location: Woking, Surrey

Re: How does a headwind affect speed and power output?

Post by Chris Jeggo »

Jdsk wrote: 13 Nov 2022, 11:43pm ...
But I was trying to get there step by step as a way of reaching agreement. That approach clearly hasn't worked.
...
Jonathan
!!
roubaixtuesday
Posts: 5801
Joined: 18 Aug 2015, 7:05pm

Re: How does a headwind affect speed and power output?

Post by roubaixtuesday »

Chris Jeggo wrote: 14 Nov 2022, 10:17am
Jdsk wrote: 13 Nov 2022, 11:43pm ...
But I was trying to get there step by step as a way of reaching agreement. That approach clearly hasn't worked.
...
Jonathan
!!
It's been like an Escher painting in thread format.

But at least it's good natured.

And, I think everyone now agrees with your formula from wiki - thanks for that, I couldn't find a reference.
User avatar
Chris Jeggo
Posts: 571
Joined: 3 Jul 2010, 9:44am
Location: Woking, Surrey

Re: How does a headwind affect speed and power output?

Post by Chris Jeggo »

Someone has edited Wikipedia and made it wrong:
Screenshot from 2022-12-03 14-27-29.png
Only a part of the second equation has been changed; everything else is the same.
mattheus
Posts: 5030
Joined: 29 Dec 2008, 12:57pm
Location: Western Europe

Re: How does a headwind affect speed and power output?

Post by mattheus »

Chris Jeggo wrote: 3 Dec 2022, 2:38pm Someone has edited Wikipedia and made it wrong:
Screenshot from 2022-12-03 14-27-29.png
Only a part of the second equation has been changed; everything else is the same.
That's hilarious. (I wouldn't be at all surprised if someone here did it!)

It demonstrates what I've been thinking for much of this thread (and an earlier one about h**m*ts): learning physics from Wikipedia is fraught with flaws and booby traps.
Jdsk
Posts: 24478
Joined: 5 Mar 2019, 5:42pm

Re: How does a headwind affect speed and power output?

Post by Jdsk »

Chris Jeggo wrote: 3 Dec 2022, 2:38pm Someone has edited Wikipedia and made it wrong:
Screenshot from 2022-12-03 14-27-29.png
Only a part of the second equation has been changed; everything else is the same.
It's easy to revert that if you wish: log in and one click.

If you do it's good practice to add a note in "Edit summary" and in this case probably to add a section in "Talk". That might help to avoid an unproductive series of reversals.

Jonathan
roubaixtuesday
Posts: 5801
Joined: 18 Aug 2015, 7:05pm

Re: How does a headwind affect speed and power output?

Post by roubaixtuesday »

mattheus wrote: 5 Dec 2022, 8:36am
Chris Jeggo wrote: 3 Dec 2022, 2:38pm Someone has edited Wikipedia and made it wrong:
Screenshot from 2022-12-03 14-27-29.png
Only a part of the second equation has been changed; everything else is the same.
That's hilarious. (I wouldn't be at all surprised if someone here did it!)

It demonstrates what I've been thinking for much of this thread (and an earlier one about h**m*ts): learning physics from Wikipedia is fraught with flaws and booby traps.
Wiki is generally good, but the references should be followed rather than the article depended on.

I think you can get the username of the person who made the incorrect edit quite easily - see if it matches anyone here - but I'm not wiki editing experienced and can't be bothered finding out.
DaveReading
Posts: 742
Joined: 24 Feb 2019, 5:37pm

Re: How does a headwind affect speed and power output?

Post by DaveReading »

roubaixtuesday wrote: 5 Dec 2022, 9:28am
mattheus wrote: 5 Dec 2022, 8:36am
Chris Jeggo wrote: 3 Dec 2022, 2:38pm Someone has edited Wikipedia and made it wrong:
Screenshot from 2022-12-03 14-27-29.png
Only a part of the second equation has been changed; everything else is the same.
That's hilarious. (I wouldn't be at all surprised if someone here did it!)

It demonstrates what I've been thinking for much of this thread (and an earlier one about h**m*ts): learning physics from Wikipedia is fraught with flaws and booby traps.
Wiki is generally good, but the references should be followed rather than the article depended on.

I think you can get the username of the person who made the incorrect edit quite easily - see if it matches anyone here - but I'm not wiki editing experienced and can't be bothered finding out.
Wikipedia edits are either credited to a logged-in user (by name) or to an anonymous IP address, which can't normally be attributed to an individual. In this case, it's the latter.
roubaixtuesday
Posts: 5801
Joined: 18 Aug 2015, 7:05pm

Re: How does a headwind affect speed and power output?

Post by roubaixtuesday »

DaveReading wrote: 5 Dec 2022, 9:37am
roubaixtuesday wrote: 5 Dec 2022, 9:28am
mattheus wrote: 5 Dec 2022, 8:36am
That's hilarious. (I wouldn't be at all surprised if someone here did it!)

It demonstrates what I've been thinking for much of this thread (and an earlier one about h**m*ts): learning physics from Wikipedia is fraught with flaws and booby traps.
Wiki is generally good, but the references should be followed rather than the article depended on.

I think you can get the username of the person who made the incorrect edit quite easily - see if it matches anyone here - but I'm not wiki editing experienced and can't be bothered finding out.
Wikipedia edits are either credited to a logged-in user (by name) or to an anonymous IP address, which can't normally be attributed to an individual. In this case, it's the latter.
Thanks!
Post Reply