Downwind turn Myth
01-18-2014, 10:44 PM
#1176
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5-15kt winds, so 10kt difference between lull & gust. Your stall speed is 35kts. You are 5kts above stall (40kts) & ok in the crosswind, turn into the wind & encounter a gust & your indicated airspeed will momentarily indicate 50kts (you are fine), but turn into the downwind & encounter the gust from behind & you will momentarily indicate 30kts which is 5 below your stall speed & it will be a bad day. Because of the laws of physics the mass of the aircraft takes time to accelerate to the gust factor & can put your wing into a stalled state during high alpha/low speed flight. Not the biggest deal because we all bump up our speed to accommodate for gust.
Best airplane to play with this would be a trainer or clipped wing cub. Do a sharp turn into the wind & it's hard to get in down (porpoises & climbs) but do a sharp turn to downwind & it will fall on its face & loose altitude until you regain airspeed. In steady state wind this doesn't apply.
This is my theory, I'm NOT saying I've tested or read it anywhere, just drawing from experience
Best airplane to play with this would be a trainer or clipped wing cub. Do a sharp turn into the wind & it's hard to get in down (porpoises & climbs) but do a sharp turn to downwind & it will fall on its face & loose altitude until you regain airspeed. In steady state wind this doesn't apply.
This is my theory, I'm NOT saying I've tested or read it anywhere, just drawing from experience
01-18-2014, 11:10 PM
#1178
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Ah, I think I get why you misunderstand me. I'm talking about a lull BELOW prevailing wind speed. If prevailing wind speed is 15 kt, with 5 kt gusts, I'm saying that the wind speed goes up to 20 kt in a gust, and DOWN to 10 kt in a lull. This has to be the case. You can't have a gust above prevailing wind speed without having a nearby lull below prevailing wind speed. It's like waves on water. For every wave crest above sea level, there is a trough BELOW sea level.
If it's dead calm, you only get a bit of stirring of the air due to thermals. That's maybe 0-5 kt tops. To get real gusts like we're talking about, there has to be a prevailing wind that causes turbulence over land features, trees, buildings, mixed with the thermals too. I think thats the only way you could get a really gusty day like we're talking about.
So a lull is not just back to prevailing wind speed, a lull goes lower by the same amount on average that gusts go above prevailing. So do you now see why the upwind turn is just as treacherous as downwind on gusty days?
If it's dead calm, you only get a bit of stirring of the air due to thermals. That's maybe 0-5 kt tops. To get real gusts like we're talking about, there has to be a prevailing wind that causes turbulence over land features, trees, buildings, mixed with the thermals too. I think thats the only way you could get a really gusty day like we're talking about.
So a lull is not just back to prevailing wind speed, a lull goes lower by the same amount on average that gusts go above prevailing. So do you now see why the upwind turn is just as treacherous as downwind on gusty days?
Last edited by RZielin; 01-19-2014 at 12:10 AM.
01-18-2014, 11:54 PM
#1179
Airspeed is everything ground speed is nothing to a plane flying. Just look at the birds- they've been doing it far longer than we have, by millions of years. They always head into wind for gliding. Any bird of prey when hunting will hold station in the sky as it looks for a source of food will head into wind.
I suggest you read this
I suggest you read this
01-19-2014, 07:29 AM
#1180
Let's consider a plane in a constant 60 degree bank while flying in a high steady wind, You will feel a constant 2G force throughout the turn. But the ground path is not a steady circle, A car following this ground path will not have a steady state G force but will increase as the paths turn sharpens and decreases, That is the G force is relative to the wind not the ground, so gusts in a turn has the same effect upwind as it does downwind.
01-19-2014, 09:32 AM
#1181
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Let me start by saying there are many people who have posted here that are more educated than me so Im just trying to understand what is being said and what I've experienced. I submit situations where both sides could be correct. Can we agree that ground speed is changed in transition from upwind to downwind in any substantial wind condition? My theory is that during that transition is where tha arguement is. If the turn is gradual enough there is no noticeable lost in lift similar to full scale scenarios. But if the turn is very abrupt there is a lag in which the airframe is adjusting to directional forces causing a momentary change in lift. I know people will say that groundspeed is irrelevant but in an abrupt turn groundspeed in not instantanious. There is a time period of adjustment for the airframe. Is that not a factor in this arguement?
01-19-2014, 09:56 AM
#1182
But if the turn is very abrupt there is a lag in which the airframe is adjusting to directional forces causing a momentary change in lift. I know people will say that groundspeed is irrelevant but in an abrupt turn groundspeed in not instantanious. There is a time period of adjustment for the airframe. Is that not a factor in this arguement?
No, Ground speed has no factor. Only the airspeed matters. Of course there is a moment of transition at the start of the turn, but it has the same effect upwind or downwind,
s
01-19-2014, 10:14 AM
#1183
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Let me start by saying there are many people who have posted here that are more educated than me so Im just trying to understand what is being said and what I've experienced. I submit situations where both sides could be correct. Can we agree that ground speed is changed in transition from upwind to downwind in any substantial wind condition? My theory is that during that transition is where tha arguement is. If the turn is gradual enough there is no noticeable lost in lift similar to full scale scenarios. But if the turn is very abrupt there is a lag in which the airframe is adjusting to directional forces causing a momentary change in lift. I know people will say that groundspeed is irrelevant but in an abrupt turn groundspeed in not instantanious. There is a time period of adjustment for the airframe. Is that not a factor in this arguement?
Once an a/c has broken ground, steady state winds have no effect on airspeed (hence, lift) whatsoever, and airspeed does not change simply because one is flying upwind, downwind, crosswind, or even when alternating between any combinations of the above. Period. That is the beginning, middle, and end of the story.
This is absolutely correct. There is no argument there.However there are others factors involved. It turns out ground speed does matter.
The earth is spinning on its axis. An airplane is traveling with the movement of the spinning earth. Its sort if like an object spinning around you and being held up by a rope and cylindrical force.. In the case of an airplane it is being held there by gravity in opposition to the lift provided by its wings. This gravity is referenced to the center of the earth.
The aircraft possesses what is defined as moment of inertia.
One of our great philosophers (Newton I think) defined moment of inertia as being equal to Mass times the radius squared.
When an aircraft changes from going up wind to downwind in relation to the ground its moment of inertia is changed. Since the mass is not changed,the only other place the aircraft can get the energy to change its MOI is from the radius. The result is a loss of altitude.
01-19-2014, 12:31 PM
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01-19-2014, 12:51 PM
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Keep thinking-your on to something.
The aircraft possesses what is defined as moment of inertia.
One of our great philosophers (Newton I think) defined moment of inertia as being equal to Mass times the radius squared.
When an aircraft changes from going up wind to downwind in relation to the ground its moment of inertia is changed. Since the mass is not changed,the only other place the aircraft can get the energy to change its MOI is from the radius. The result is a loss of altitude.
The aircraft possesses what is defined as moment of inertia.
One of our great philosophers (Newton I think) defined moment of inertia as being equal to Mass times the radius squared.
When an aircraft changes from going up wind to downwind in relation to the ground its moment of inertia is changed. Since the mass is not changed,the only other place the aircraft can get the energy to change its MOI is from the radius. The result is a loss of altitude.
01-19-2014, 02:43 PM
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Of course its true whenever you change direction. It also depends on your turn in relation to the earths rotation. If you turn downwind in the direction of the earths rotation you will get more of a change than if you turn against the rotation direction
01-19-2014, 03:10 PM
#1189
Keep thinking-your on to something.
Once an a/c has broken ground, steady state winds have no effect on airspeed (hence, lift) whatsoever, and airspeed does not change simply because one is flying upwind, downwind, crosswind, or even when alternating between any combinations of the above. Period. That is the beginning, middle, and end of the story.
This is absolutely correct. There is no argument there.However there are others factors involved. It turns out ground speed does matter.
The earth is spinning on its axis. An airplane is traveling with the movement of the spinning earth. Its sort if like an object spinning around you and being held up by a rope and cylindrical force.. In the case of an airplane it is being held there by gravity in opposition to the lift provided by its wings. This gravity is referenced to the center of the earth.
The aircraft possesses what is defined as moment of inertia.
One of our great philosophers (Newton I think) defined moment of inertia as being equal to Mass times the radius squared.
When an aircraft changes from going up wind to downwind in relation to the ground its moment of inertia is changed. Since the mass is not changed,the only other place the aircraft can get the energy to change its MOI is from the radius. The result is a loss of altitude.
Once an a/c has broken ground, steady state winds have no effect on airspeed (hence, lift) whatsoever, and airspeed does not change simply because one is flying upwind, downwind, crosswind, or even when alternating between any combinations of the above. Period. That is the beginning, middle, and end of the story.
This is absolutely correct. There is no argument there.However there are others factors involved. It turns out ground speed does matter.
The earth is spinning on its axis. An airplane is traveling with the movement of the spinning earth. Its sort if like an object spinning around you and being held up by a rope and cylindrical force.. In the case of an airplane it is being held there by gravity in opposition to the lift provided by its wings. This gravity is referenced to the center of the earth.
The aircraft possesses what is defined as moment of inertia.
One of our great philosophers (Newton I think) defined moment of inertia as being equal to Mass times the radius squared.
When an aircraft changes from going up wind to downwind in relation to the ground its moment of inertia is changed. Since the mass is not changed,the only other place the aircraft can get the energy to change its MOI is from the radius. The result is a loss of altitude.
This is why I keep bumping into the western walls of my home as I walk about it, even as I have trouble reaching the eastern ones. I Made the mistake of jumping once whille too close to a wall on the west side and BAM! Lucky I didn't break something. Ever notice how loudly the tires on your car squeal when attempting to turn right while heading north? :-)
Sorry my man. I mean all of the above in the friendliest way, but you may want to re-examine your premise.
01-19-2014, 03:19 PM
#1190
Ah you did not take into account the factoid of living in the Souther hemisphere I keep bumping into the walls on the East side of my house and my tires squeal much louder when turning left while heading South?
Mike
Mike
01-19-2014, 04:09 PM
#1192
It seems to me that most of the misunderstanding stems from the fact that modellersare looking at the aircraft from a FIXED POINT. In a zero wind condition there is no difference in the appearance of the model’s performance. Given aheadwind, everything changes from the perspective of that fixed point.
As an example, when heading into wind in a climb, theaircraft appears to climb promptly. During the transition to downwind, theclimb appearance gradually lessens.Downwind the apparent climb lessenseven more fooling the brain into thinking the aircraft’s performance isdeteriorating. In reality the actual performance is identical. Visualy, not somuch…
The converse is true when turning upwind, the apparentperformance miraculously improves. But no one ever mentions that!
Gusts and microbursts clearly can and do have an effect if one isunlucky enough to be in a turn just as one chooses to arrive. Other than these occasions, have faith in phyisics!
Dennis
As an example, when heading into wind in a climb, theaircraft appears to climb promptly. During the transition to downwind, theclimb appearance gradually lessens.Downwind the apparent climb lessenseven more fooling the brain into thinking the aircraft’s performance isdeteriorating. In reality the actual performance is identical. Visualy, not somuch…
The converse is true when turning upwind, the apparentperformance miraculously improves. But no one ever mentions that!
Gusts and microbursts clearly can and do have an effect if one isunlucky enough to be in a turn just as one chooses to arrive. Other than these occasions, have faith in phyisics!
Dennis
01-19-2014, 04:18 PM
#1193
Let's consider a plane in a constant 60 degree bank while flying in a high steady wind, You will feel a constant 2G force throughout the turn. But the ground path is not a steady circle, A car following this ground path will not have a steady state G force but will increase as the paths turn sharpens and decreases, That is the G force is relative to the wind not the ground, so gusts in a turn has the same effect upwind as it does downwind.
01-19-2014, 05:30 PM
#1194
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Everyone arguing about this that doesn't understand an airplane has no !idea that it is windy please read the Book Stick and Rudder! And use telemetry. Bet you will find out your plane stalls at the same airspeed(zero G's) everytime no matter what direction its flying.
01-19-2014, 06:17 PM
#1195
Correct, but it will not have a steady speed, and the airplane will have a constan air speed. So the difference remains the same,.
01-19-2014, 06:27 PM
#1196
It also depends on your turn in relation to the earths rotation.
01-20-2014, 01:23 AM
#1197
01-20-2014, 10:39 PM
#1198
Hey all you true believers in the Myth:
Several professional pilots in addition to myself have described our real-life experiences to you which completely dispel the Myth (as opposed to the seemingly never ending stream of "thought experiments" which supposedly "prove" it's validity) and we've repeatedly pointed out that if the Myth were a real phenomenon we'd be dead many times over, having lost airspeed when there was none to lose and then crashed.
Several of you have countered with "...but a slow and lightweight (name the plane) would have fallen out of the sky when turned down wind because (whatever theory)"
Today I took my new Christmas toy - a UMX P-40 - up for the first time. This little wonder weighs 1.6oz. (That's right, one-point-six OUNCES) and at nearly full power was hovering in front of me in a stiff head wind nearly equal to the little P-40's max speed. If any airplane ever were going to fallout of the sky for having made a turn to down wind, then this would be the airplane and the occasion.
So what happened when I rolled into a nearly vertical bank and reversed direction 180 degrees from a hover to a tailwind in under a second? Did this micro-light and under-powered little gem fall to earth right then and there for having been instantaneously starved of airspeed and lift? Nope. In fact, it didn't lose even an inch of altitude.
This little thing doesn't have any power to spare, and can barely do a loop from level flight. Did it fall out of the sky for lack of airflow over the controls at the top of a loop when at the top of that loop the nose was pointed downwind? Did it perform better at the top if the loop when it was initially entered from downwind? Nah. No difference. None. Zero.
Game over. Thanks for playing.
Hopefully this thread will hibernate again for a long time.
Good night,
Don.
Several professional pilots in addition to myself have described our real-life experiences to you which completely dispel the Myth (as opposed to the seemingly never ending stream of "thought experiments" which supposedly "prove" it's validity) and we've repeatedly pointed out that if the Myth were a real phenomenon we'd be dead many times over, having lost airspeed when there was none to lose and then crashed.
Several of you have countered with "...but a slow and lightweight (name the plane) would have fallen out of the sky when turned down wind because (whatever theory)"
Today I took my new Christmas toy - a UMX P-40 - up for the first time. This little wonder weighs 1.6oz. (That's right, one-point-six OUNCES) and at nearly full power was hovering in front of me in a stiff head wind nearly equal to the little P-40's max speed. If any airplane ever were going to fallout of the sky for having made a turn to down wind, then this would be the airplane and the occasion.
So what happened when I rolled into a nearly vertical bank and reversed direction 180 degrees from a hover to a tailwind in under a second? Did this micro-light and under-powered little gem fall to earth right then and there for having been instantaneously starved of airspeed and lift? Nope. In fact, it didn't lose even an inch of altitude.
This little thing doesn't have any power to spare, and can barely do a loop from level flight. Did it fall out of the sky for lack of airflow over the controls at the top of a loop when at the top of that loop the nose was pointed downwind? Did it perform better at the top if the loop when it was initially entered from downwind? Nah. No difference. None. Zero.
Game over. Thanks for playing.
Hopefully this thread will hibernate again for a long time.
Good night,
Don.
01-21-2014, 05:07 AM
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Aaaarrggh,
Blue Bus, you're not listening. You keep bringing up gusty conditions which have NOTHING to do with the "downwind turn myth". The fact that you think gusts are relevant demonstrates that you don't even get the gist of what the "downwind turn myth" is. None the less, whiteout understanding what it's even about, you SEEM to believe the myth is true. Pretty confused.
I have, against my better judgement, engaged you on the gusty topic, which is a whole different scenario, and you also fail to discern my point on those posts either. You misunderstand what a "gust" is and how air currents move. The "gusty" scenario is interesting and valid and something that must be understood in order to be a good pilot. But it IS separate from the Downwind Turn in STEADY wind scenario which HighHorse was just referring to. You must understand the theory and reality of the steady wind scenario before you can understand the more complex situation where gusts are superimposed on it. You understand neither.
When someone makes a clear point, you jump into a different scenario to dispel it, which is not very logical nor intellectually honest. I must question you motives. Honestly, I can't see how someone with your mind set gets to be an instructor. Please go back to the start of this thread and read it through before you rehash stuff that was thoroughly debunked 30 pages ago. And remember, EVERYTHING that HighHorse and I have said in this thread is carefully worded and iron clad true. NOBODY has shown any credible evidence to the contrary.
Blue Bus, you're not listening. You keep bringing up gusty conditions which have NOTHING to do with the "downwind turn myth". The fact that you think gusts are relevant demonstrates that you don't even get the gist of what the "downwind turn myth" is. None the less, whiteout understanding what it's even about, you SEEM to believe the myth is true. Pretty confused.
I have, against my better judgement, engaged you on the gusty topic, which is a whole different scenario, and you also fail to discern my point on those posts either. You misunderstand what a "gust" is and how air currents move. The "gusty" scenario is interesting and valid and something that must be understood in order to be a good pilot. But it IS separate from the Downwind Turn in STEADY wind scenario which HighHorse was just referring to. You must understand the theory and reality of the steady wind scenario before you can understand the more complex situation where gusts are superimposed on it. You understand neither.
When someone makes a clear point, you jump into a different scenario to dispel it, which is not very logical nor intellectually honest. I must question you motives. Honestly, I can't see how someone with your mind set gets to be an instructor. Please go back to the start of this thread and read it through before you rehash stuff that was thoroughly debunked 30 pages ago. And remember, EVERYTHING that HighHorse and I have said in this thread is carefully worded and iron clad true. NOBODY has shown any credible evidence to the contrary.