A co-worker claims that headwinds INCREASE stop distance on a take off roll because when rolling on the ground the aircraft has higher lift on the wings (because of headwind) for the same speed over the ground (the aircraft's wheels are still touching the runway).
Since there is more lift there is reduced breaking efficiency and therefore a longer stop distance in the case of a rejected take off.
I've tried everything to convince him otherwise, maybe you can do better.
Thanks in advance.
Solve this issue and help me save my breath
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corporate joe
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Solve this issue and help me save my breath
The 3 most important things to remember when you're old:
1) Never pass an opportunity to use a washroom
2) Never waste a hard on
3) Never trust a fart
John Mayer
1) Never pass an opportunity to use a washroom
2) Never waste a hard on
3) Never trust a fart
John Mayer
If he's under the opinion that headwinds increase Accelerate Stop Distance, just show him the statement on the charts where it says something to the effect "each 10kts headwind decreases distance by 10%".
If he thinks that it just lengthens the stopping portion, have him explain to you why then do headwinds shorten landing distance required.
Having him answer his own questions is often the most effective way of getting him to see his error.
Good luck.
If he thinks that it just lengthens the stopping portion, have him explain to you why then do headwinds shorten landing distance required.
Having him answer his own questions is often the most effective way of getting him to see his error.
Good luck.
Re: Solve this issue and help me save my breath
corporate joe wrote:A co-worker claims....
Which airline do you lads work for, I don't feel to safe flying with you guys....
Actually this could be interesting. I am thinking about not the accelerate part but just what distance it takes an aircraft to slow down from a groundspeed of, let's say 100 knots to complete stop in a twenty knot headwind compared to when there is no wind. Or a plane touches down at 120 knots, 1000 feet down a runway and the next day touches down at 100 knots on a no wind day with everything else the same.
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snaproll20
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Option a.)
With the inherent huge lift, (per your co-worker's theory), haul back on yoke to perform a loop. Once "over the top', the aircraft will drift downwind while inverted in the (former) headwind and be able to land safely.
Option b.)
The severe pressing of brakes attendant to terror and the input of full forward yoke will compress the nosewheel such that the angle of attack will be reduced, thus severely limiting the nasty lift forces. As a precaution, perform full shut-down sequence and scream at passengers to "Brace".
Option c.)
If it is an F/O take-off, Captain Magic calmly says "I have control", well-trained SOP F/O states "You have control." (See TSB report for follow-up)
Option d.)
To be calculated before departure. (Obviously, this cannot be calculated in the middle of a 'reject')
Stop distance = V x wing coefficient of lift - 50% of headwind
divided by MGTOW x CRFI + % runway slope + 10% for
unimproved surface length, less 70% factor x strip length
( no allowance for TORA, nor reverse thrust.)
where:
i.) the PIC has completed 3 take-offs and 7 rejects in the previous
90 days, or
ii.) has not;
A.) completed a full course of reject decision-making in the
previous 6 months, and
B.) has declined to donate a rat's ass, or
C.) has declined to give a rhesus's fornication
Note: Rejects do not count where performed at local tavern.[/u]
With the inherent huge lift, (per your co-worker's theory), haul back on yoke to perform a loop. Once "over the top', the aircraft will drift downwind while inverted in the (former) headwind and be able to land safely.
Option b.)
The severe pressing of brakes attendant to terror and the input of full forward yoke will compress the nosewheel such that the angle of attack will be reduced, thus severely limiting the nasty lift forces. As a precaution, perform full shut-down sequence and scream at passengers to "Brace".
Option c.)
If it is an F/O take-off, Captain Magic calmly says "I have control", well-trained SOP F/O states "You have control." (See TSB report for follow-up)
Option d.)
To be calculated before departure. (Obviously, this cannot be calculated in the middle of a 'reject')
Stop distance = V x wing coefficient of lift - 50% of headwind
divided by MGTOW x CRFI + % runway slope + 10% for
unimproved surface length, less 70% factor x strip length
( no allowance for TORA, nor reverse thrust.)
where:
i.) the PIC has completed 3 take-offs and 7 rejects in the previous
90 days, or
ii.) has not;
A.) completed a full course of reject decision-making in the
previous 6 months, and
B.) has declined to donate a rat's ass, or
C.) has declined to give a rhesus's fornication
Note: Rejects do not count where performed at local tavern.[/u]
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sky's the limit
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corporate joe
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LOL! Nice post!snaproll20 wrote:Option a.)
With the inherent huge lift, (per your co-worker's theory), haul back on yoke to perform a loop. Once "over the top', the aircraft will drift downwind while inverted in the (former) headwind and be able to land safely.
Option b.)
The severe pressing of brakes attendant to terror and the input of full forward yoke will compress the nosewheel such that the angle of attack will be reduced, thus severely limiting the nasty lift forces. As a precaution, perform full shut-down sequence and scream at passengers to "Brace".
Option c.)
If it is an F/O take-off, Captain Magic calmly says "I have control", well-trained SOP F/O states "You have control." (See TSB report for follow-up)
Option d.)
To be calculated before departure. (Obviously, this cannot be calculated in the middle of a 'reject')
Stop distance = V x wing coefficient of lift - 50% of headwind
divided by MGTOW x CRFI + % runway slope + 10% for
unimproved surface length, less 70% factor x strip length
( no allowance for TORA, nor reverse thrust.)
where:
i.) the PIC has completed 3 take-offs and 7 rejects in the previous
90 days, or
ii.) has not;
A.) completed a full course of reject decision-making in the
previous 6 months, and
B.) has declined to donate a rat's ass, or
C.) has declined to give a rhesus's fornication
Note: Rejects do not count where performed at local tavern.[/u]
On a more serious note, that formula you offered is kinda where I was going with him to explain why it was folly. However I was going the "V1 increases with headwind" route.
The 3 most important things to remember when you're old:
1) Never pass an opportunity to use a washroom
2) Never waste a hard on
3) Never trust a fart
John Mayer
1) Never pass an opportunity to use a washroom
2) Never waste a hard on
3) Never trust a fart
John Mayer
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corporate joe
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- Joined: Mon May 29, 2006 8:18 am
- Location: the coast
co-joe wrote:Hmmm...so less weight on the wheels = longer stopping distance? nope just put him out of his misery and shoot him now.
Actually less weight on the wheels means less breaking efficiency. Now this is where he gets his theory: less braking efficiency automatically means longer stoping distance doesn't it?
(being the devil's advocat to be fair).
The 3 most important things to remember when you're old:
1) Never pass an opportunity to use a washroom
2) Never waste a hard on
3) Never trust a fart
John Mayer
1) Never pass an opportunity to use a washroom
2) Never waste a hard on
3) Never trust a fart
John Mayer
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Dust Devil
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mellow_pilot
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Dust Devil
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One way would be to discuss the extremes of the situation.
For example, if the wind were 0kts I would suggest that the both of you would agree that 0 wind has 0 effect.
The other extreme would be to say if the wind was 200kts. If the aircraft was just sitting there, (just barely heavy enough to not blow away), it would clearly want to go with the wind and not against it.
Take a bunch of your buddies with heavy weights on themselves so that they too don't blow away and give them a nice tow rope. Which direction will be easier to pull the aircraft in this situation, with or against the wind?
Clearly, pulling the aircraft into the wind at 1kt will be quite a task, and as per your friends comments, the aircraft will want to fly (becomes lighter), but that's alot of weight for your buddies to hang onto if it decides to take flight and go in the opposite direction. The opposite is for your buddies pulling the aircraft with the wind. Once it gets moving...
To stop the aircraft, your buddies pulling the thing into the wind got a pretty easy job, just stop pulling. However, the guys pulling the aircraft with the wind are probably now schmushed by the aircraft.
Therefore it is easier to stop the aircraft when pulling it into a 200kt wind versus with it.
So, with a wind from 1kt to 200kts, does this situation reverse at any point where it is easier for your buddies to stop the aircraft while pulling it with the wind? In other words, which magical number between 1 and 200 inclusive is it easier to stop the aircraft while pulling it with the wind and why???
This magical number does not exist. The results become harder to measure with lower wind speeds, but the physics do not change. If the number did exist, I'm pretty sure Mr. Einstein would have to modify his theory of relativity
Therefore it is easier to stop an aircraft at any speed while it is going into the wind vs. with the wind.
For example, if the wind were 0kts I would suggest that the both of you would agree that 0 wind has 0 effect.
The other extreme would be to say if the wind was 200kts. If the aircraft was just sitting there, (just barely heavy enough to not blow away), it would clearly want to go with the wind and not against it.
Take a bunch of your buddies with heavy weights on themselves so that they too don't blow away and give them a nice tow rope. Which direction will be easier to pull the aircraft in this situation, with or against the wind?
Clearly, pulling the aircraft into the wind at 1kt will be quite a task, and as per your friends comments, the aircraft will want to fly (becomes lighter), but that's alot of weight for your buddies to hang onto if it decides to take flight and go in the opposite direction. The opposite is for your buddies pulling the aircraft with the wind. Once it gets moving...
To stop the aircraft, your buddies pulling the thing into the wind got a pretty easy job, just stop pulling. However, the guys pulling the aircraft with the wind are probably now schmushed by the aircraft.
Therefore it is easier to stop the aircraft when pulling it into a 200kt wind versus with it.
So, with a wind from 1kt to 200kts, does this situation reverse at any point where it is easier for your buddies to stop the aircraft while pulling it with the wind? In other words, which magical number between 1 and 200 inclusive is it easier to stop the aircraft while pulling it with the wind and why???
This magical number does not exist. The results become harder to measure with lower wind speeds, but the physics do not change. If the number did exist, I'm pretty sure Mr. Einstein would have to modify his theory of relativity
Therefore it is easier to stop an aircraft at any speed while it is going into the wind vs. with the wind.
"Don't stand too still..."
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snaproll20
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It's interesting how (dumb?) theories and questions get us thinking about something. I suppose it is good to be challenged and review some of the things we take for granted every day.
(Like democracy, voting, honest government....but I digress!)
I think your friend needs to realize that V1 comes sooner with a headwind, automatically giving an advantage in the total Accelerate-Stop distance over a fixed runway length. A 30 knot headwind would give an airspeed indication whereas the aircraft could have a 0 knot groundspeed (parked). (With all that lift it is 'stopped'(?) Take the brakes off, and the aircraft may well be blown backwards!) Does he remember the form-drag plate flat in an airstream, with the smoke, {Huh! ...do they even show that old movie any more in training?}
Convential wisdom says that at V1 the take-off should be continued, being less risky than an attempted Reject. Accelerate-Stop distances are developed by test pilots in certification, so whether you friend is right or wrong, any lift developed in take-off configuration is accounted for in practical demonstration.
My fear would be, having generated such a theory, he will develop a means of accomodating it and start doing things not approved in the AFM, since he has 'better' ideas.
At such time, he begins to become a menace as he will depart from the legal (and safe) means of flying the aircraft.
If you really want to get him going, ask him if retracting flaps on landing will shorten the landing roll.
(Like democracy, voting, honest government....but I digress!)
I think your friend needs to realize that V1 comes sooner with a headwind, automatically giving an advantage in the total Accelerate-Stop distance over a fixed runway length. A 30 knot headwind would give an airspeed indication whereas the aircraft could have a 0 knot groundspeed (parked). (With all that lift it is 'stopped'(?) Take the brakes off, and the aircraft may well be blown backwards!) Does he remember the form-drag plate flat in an airstream, with the smoke, {Huh! ...do they even show that old movie any more in training?}
Convential wisdom says that at V1 the take-off should be continued, being less risky than an attempted Reject. Accelerate-Stop distances are developed by test pilots in certification, so whether you friend is right or wrong, any lift developed in take-off configuration is accounted for in practical demonstration.
My fear would be, having generated such a theory, he will develop a means of accomodating it and start doing things not approved in the AFM, since he has 'better' ideas.
At such time, he begins to become a menace as he will depart from the legal (and safe) means of flying the aircraft.
If you really want to get him going, ask him if retracting flaps on landing will shorten the landing roll.
That is about the best advice I've heard on here...ch135146 wrote:Work out a couple of examples on the performance charts.
If he doesn't get it, shoot him.
It's all about groundspeed, the slower our going over the ground, the less distance it takes to slow down. pretty simple.
Following your coworkers theory, a tail wind should reduce your groundroll...
Last edited by KAG on Sat Dec 23, 2006 1:32 pm, edited 1 time in total.
The feet you step on today might be attached to the ass you're kissing tomorrow.
Chase lifestyle not metal.
Chase lifestyle not metal.
..
I've sat through ground school.
My ground school instructors explained stuff really badly....
Were they the only ones ?
... or do hypothoses like this arise because a student grabs onto some half-truth from his GS instructor, which was never fully explained or corrected?
Or maybe he's just pulling your chain, co-joe.
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You have higher lift, but you have lower ground speed. If he says the higher lift reduces brake efficiency, you can tell him lower ground speed does not need as much brake efficiency.
Without numbers, your arguement is as good as his.
Cutting the ground speed in half, reduces the braking distance to 1/4.
Assuming "braking efficiency" (whatever that is) is linear with stopping distance, the lift would have to reduce the braking efficiency to 1/4 of original, just to break even on the original full-ground-speed stopping distance.
And that's for the whole stopping distance.
But as you start to slow down, the lift reduces and braking becomes more effective.... So therefore the lift has to reduce the brake efficiency to something way Way WAY LESS than 1/4.... for any chance at all that the reduction of efficiency will overcome the effect of reduced initial ground speed.
It is quite possible such a condition exists... but the braking efficiency probably has to be so infinitesmally small... that it is called "flying".
..
I've sat through ground school.
My ground school instructors explained stuff really badly....
Were they the only ones ?
... or do hypothoses like this arise because a student grabs onto some half-truth from his GS instructor, which was never fully explained or corrected?
Or maybe he's just pulling your chain, co-joe.
--------------
You have higher lift, but you have lower ground speed. If he says the higher lift reduces brake efficiency, you can tell him lower ground speed does not need as much brake efficiency.
Without numbers, your arguement is as good as his.
Cutting the ground speed in half, reduces the braking distance to 1/4.
Assuming "braking efficiency" (whatever that is) is linear with stopping distance, the lift would have to reduce the braking efficiency to 1/4 of original, just to break even on the original full-ground-speed stopping distance.
And that's for the whole stopping distance.
But as you start to slow down, the lift reduces and braking becomes more effective.... So therefore the lift has to reduce the brake efficiency to something way Way WAY LESS than 1/4.... for any chance at all that the reduction of efficiency will overcome the effect of reduced initial ground speed.
It is quite possible such a condition exists... but the braking efficiency probably has to be so infinitesmally small... that it is called "flying".
..
I thought that headwind creates some sort of drag or resistance (call it what you want) on the airplane??? Heck on anything that moves against it?? And if your buddy agrees on that basic principle, than that should solve the problem....the theory about the breaks losing efficiency due to lift, might have some value, but, I still think that your flying tincan rolling down the runway in 20kt headwind will stop shorter than without wind....
or maybe my FTGU textbook was all wrong?? I want my refund!!
or maybe my FTGU textbook was all wrong?? I want my refund!!
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mellow_pilot
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I think there may be something to the crappy GS instructor argument. Having seen some pretty bone-headed lessons, I can agree that students who never question the instructor are sometimes left with incorrect info.
To quote the Simpsons:
Bad firewark dispray. Bad student.
Ah, ah... bad prinprel.
Ooohhhh.
To quote the Simpsons:
Bad firewark dispray. Bad student.
Ah, ah... bad prinprel.
Ooohhhh.
Dyslexics of the world... UNTIE!
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co-joe
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Okay then take the wheels out of the equation. Let's look at a 185 on straight skis, landing on a frozen lake with no snow on it.corporate joe wrote:co-joe wrote:Hmmm...so less weight on the wheels = longer stopping distance? nope just put him out of his misery and shoot him now.
Actually less weight on the wheels means less breaking efficiency. Now this is where he gets his theory: less braking efficiency automatically means longer stoping distance doesn't it?
(being the devil's advocat to be fair).
The 185 is landing into a 20 kt headwind and therefore will have a groundspeed of what? 50 kts the only thing slowing the buck eighty five down is friction of the skis on the ice.
The same 185 landing on the same lake with no wind will have a groundspeed of wha? 70 kts.
Now think of it in terms of inertia or momentum.
Momentum = mass X velocity.
Less velocity = less momentum, and therefore = less stopping distance.
Are you gonna try to tell me that the friction (nearly non existent as it it) of a 185 with 20 kts less groundspeed is sufficiently lessened that stopping distance will be greater?
Where's my gun? Come here for a second, I'll make it quick. We're about to have a "hunting accident"...
