downwind turn
03-12-2014, 05:57 PM
#26
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You calculated the momentum wrong as the plane is traveling in an opposite direction then one of your calculatious should have been negative. If you subtract the difference the momentumer difference will be 400 no matter if you are using ground speed or air speed. The energy came from the engine and lift from the wing when banking in the turn.
03-12-2014, 05:58 PM
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Lift = (1/2) d v[SUP]2[/SUP] s CL
- L = Lift, which must equal the airplane's weight in pounds
- d = density of the air. This will change due to altitude. These values can be found in a I.C.A.O. Standard Atmosphere Table.
- v = velocity of an aircraft expressed in feet per second
- s = the wing area of an aircraft in square feet
- CL = Coefficient of lift , which is determined by the type of airfoil and angle of attack.
03-12-2014, 06:41 PM
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Yes - momentum will mean that the aircraft will "fight" against the change in wind direction, and in theory, momentarily reduce airspeed, and thus lift, and result in a slight nose drop.
However, our models are extremely light weight for their size. They are influenced considerably more than a full sized aircraft by the air mass, and the change will be extremely slight.
Also, don't forget that to get the full effect of the "downwind turn", you'd need to skip out the crosswind leg altogether and make a heck of a sharp turn.... I'd be willing to bet that making this turn is an area that is likely to cause a stall/crash, especially when you have the visual illusions of uncoordinated flight from the wind. (thus perpetuating the myth!)
To put this into comparison: The other day, I was flying a full scale two seat aerobatic aircraft with a pretty high wing loading. One of the manoeuvres I was taught is a steep turn at around 45 degrees. The wind was substantial as well, up to about a steady 35 knots at altitude. From my perspective turning downwind, the only effect the wind had was that my constant rate turn appeared elongated from the track on the ground. Turning back upwind, the aircraft "appeared" to tighten up the rate of turn and stay stationary over a single point.
Throughout the exercise, the airspeed remained constant.
The point is, the aircraft doesn't care what it's flying in, and if turning downwind doesn't effect a 2000 lb brick with wings, it's not going to effect our five lb models either.
However, our models are extremely light weight for their size. They are influenced considerably more than a full sized aircraft by the air mass, and the change will be extremely slight.
Also, don't forget that to get the full effect of the "downwind turn", you'd need to skip out the crosswind leg altogether and make a heck of a sharp turn.... I'd be willing to bet that making this turn is an area that is likely to cause a stall/crash, especially when you have the visual illusions of uncoordinated flight from the wind. (thus perpetuating the myth!)
To put this into comparison: The other day, I was flying a full scale two seat aerobatic aircraft with a pretty high wing loading. One of the manoeuvres I was taught is a steep turn at around 45 degrees. The wind was substantial as well, up to about a steady 35 knots at altitude. From my perspective turning downwind, the only effect the wind had was that my constant rate turn appeared elongated from the track on the ground. Turning back upwind, the aircraft "appeared" to tighten up the rate of turn and stay stationary over a single point.
Throughout the exercise, the airspeed remained constant.
The point is, the aircraft doesn't care what it's flying in, and if turning downwind doesn't effect a 2000 lb brick with wings, it's not going to effect our five lb models either.
03-12-2014, 06:58 PM
#31
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I believe the oposite is true. A change of 40 mph caused by a 20 mile wind will have much more effect on a 5 pound AC than it will a 2000 pound one.
Also the turns we fly with models is much steeper than you do in a full size one
Also the turns we fly with models is much steeper than you do in a full size one
03-12-2014, 08:33 PM
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dirtybird, you're trying to disprove aerodynamic facts. There is NO climb turning upwind, and NO descent turning downwind. It simply doesn't happen, RC or full scale.
The aircraft is not on a string, like a control-liner, which in fact might react the way you expect.
The aircraft is not on a string, like a control-liner, which in fact might react the way you expect.
03-12-2014, 08:51 PM
#34
03-12-2014, 08:56 PM
#35
Yes - momentum will mean that the aircraft will "fight" against the change in wind direction, and in theory, momentarily reduce airspeed, and thus lift, and result in a slight nose drop.
03-12-2014, 08:59 PM
#36
The momentum change occurs during the turn and is taken on by the wings and engine. Done correctly there is no loss of altitude in the turn and is no differeent than a turn in calm air.
03-12-2014, 11:12 PM
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Dirtybird, I do see your point completely, the inertia of the aircraft and momentum relative to the ground changes when flying upwind or downwind.
This is totally correct.
But the aircraft does not climb or descend as a result.
falcon_56 quotes the lift formula, notice that the V is referring to "air speed" not the ground speed. Ground speed is not a variable in the lift equation.
So the lift produced by the wing is only affected by airspeed changes, not ground speed changes.
Since airspeed is constant in a 360 turn in a steady moving air mass, there is no change in lift and the plane will not climb or descend.
The real key to understanding this is defining your reference point for measuring speed / momentum.
Relative to the ground? Relative to space? The earth is moving at 1000 Mph east at the equator and 60,000mph through space around the sun.
The momentum of a plane can be calculated from many reference points and each will be significantly different.
Here is another example. You make a paper aeroplane, you throw it at 10 mph and it glides for 30 feet.
Now imagine you are in a 747 travelling at 500 Mph north. Throw the paper plane towards the front and it will glide 30 feet within the larger aircraft.
Throw it towards the back and it will glide exactly the same distance, Since your perspective is inside the 747 moving with it, you see no difference.
However, compared to the ground, the air inside the 747 is moving at 500Mph and when you throw the paper plane, it will move many hundreds of feet across the ground before landing on the floor of the 747.
It depends entirely on your frame of reference.
Another example. How much momentum and inertia does (did) the Space Shuttle have when sitting on the launch pad?
You might say "Zero" but when it launches and flies east, it has already received a significant boost to orbital speed from the rotation of the earth, so it must have already been carrying that energy while sitting motionless on the launch pad.
The answer is the same in our wind scenarios, IE it depends entirely on your "Frame of Reference" be it ground or Space, or air?
Last edited by Rob2160; 03-12-2014 at 11:19 PM.
03-13-2014, 12:21 AM
#39
Right, I'm no maths or physics genius just your average flier who is interested in the mechanics of flight.
From where I see it, can I make my observations...
E.G.
A plane is flying at 50 mph in completely still air- no movement at all : therefore plane airspeed 50mph. Wind speed over the wings 50mph.
A plane is flying at 50 mph with a headwind of 20 mph : plane airspeed 50 mph. Wind over the wings 70 mph.
A plane is flying at 50 mph with a tailwind of 20 mph : plane airspeed 50mph. Wind over the wings 30 mph.
So, am I correct in thinking that it is the speed of the air over the wings that makes the plane have lift? It has nothing to do with the actual air speed of the aircraft.
If all above is true would this explane why my little vintage Junior 60 will quite happily sit still ( no ground speed)in a strong head wind and not move forward. The speed of the wind over the wings is sufficient for enough lift. And as the prop is providing thrust over the airframe the plane still has airspeed?
Is it the case that some people are confusing plane airspeed with airspeed over the wing?
From where I see it, can I make my observations...
E.G.
A plane is flying at 50 mph in completely still air- no movement at all : therefore plane airspeed 50mph. Wind speed over the wings 50mph.
A plane is flying at 50 mph with a headwind of 20 mph : plane airspeed 50 mph. Wind over the wings 70 mph.
A plane is flying at 50 mph with a tailwind of 20 mph : plane airspeed 50mph. Wind over the wings 30 mph.
So, am I correct in thinking that it is the speed of the air over the wings that makes the plane have lift? It has nothing to do with the actual air speed of the aircraft.
If all above is true would this explane why my little vintage Junior 60 will quite happily sit still ( no ground speed)in a strong head wind and not move forward. The speed of the wind over the wings is sufficient for enough lift. And as the prop is providing thrust over the airframe the plane still has airspeed?
Is it the case that some people are confusing plane airspeed with airspeed over the wing?
03-13-2014, 01:23 AM
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Right, I'm no maths or physics genius just your average flier who is interested in the mechanics of flight.
From where I see it, can I make my observations...
E.G.
A plane is flying at 50 mph in completely still air- no movement at all : therefore plane airspeed 50mph. Wind speed over the wings 50mph.
A plane is flying at 50 mph with a headwind of 20 mph : plane airspeed 50 mph. Wind over the wings 70 mph.
A plane is flying at 50 mph with a tailwind of 20 mph : plane airspeed 50mph. Wind over the wings 30 mph.
So, am I correct in thinking that it is the speed of the air over the wings that makes the plane have lift? It has nothing to do with the actual air speed of the aircraft.
If all above is true would this explane why my little vintage Junior 60 will quite happily sit still ( no ground speed)in a strong head wind and not move forward. The speed of the wind over the wings is sufficient for enough lift. And as the prop is providing thrust over the airframe the plane still has airspeed?
Is it the case that some people are confusing plane airspeed with airspeed over the wing?
From where I see it, can I make my observations...
E.G.
A plane is flying at 50 mph in completely still air- no movement at all : therefore plane airspeed 50mph. Wind speed over the wings 50mph.
A plane is flying at 50 mph with a headwind of 20 mph : plane airspeed 50 mph. Wind over the wings 70 mph.
A plane is flying at 50 mph with a tailwind of 20 mph : plane airspeed 50mph. Wind over the wings 30 mph.
So, am I correct in thinking that it is the speed of the air over the wings that makes the plane have lift? It has nothing to do with the actual air speed of the aircraft.
If all above is true would this explane why my little vintage Junior 60 will quite happily sit still ( no ground speed)in a strong head wind and not move forward. The speed of the wind over the wings is sufficient for enough lift. And as the prop is providing thrust over the airframe the plane still has airspeed?
Is it the case that some people are confusing plane airspeed with airspeed over the wing?
Air speed is the speed that the aircraft is moving through the air. This is the speed of the wind over the wings.
Only your first example is correct. Airspeed 50mph, air over wings is 50 mph.
Your second example. Airspeed is 50 Mph, air over wings is still 50 mph, the 20 mph headwind only means you Ground speed is now 30 mph
Your third example, airspeed is 50 Mph, air over wings is actually 50 mph, the 20 mph tail wind means your ground speed is now 70 mph.
To be correct, your three examples would read like this.
A plane is flying at 50 mph airspeed in completely still air- no movement at all : therefore plane groundspeed 50mph. Wind speed over the wings 50mph. (in nil wind airspeed and ground speed are the same)
A plane is flying at 50 mph airspeed with a headwind of 20 mph : plane groundspeed is 30 mph. Wind over the wings 50 mph.
A plane is flying at 50 mph airspeed with a tailwind of 20 mph : plane groundspeed is 70mph. Wind over the wings 50 mph.
03-13-2014, 02:02 AM
#41
Ok, so I am confusing airspeed and ground speed. I have done some searching and come up with this..
"A comparison that I like to make is that it's like swimming in a river. If you're swimming upstream your "groundspeed" will be your swimming speed minus the speed of the water current. Let's say you're a strong swimmer and can make 4 mph. If you swim into a 3 mph current you will only make 1 mph along the riverbank. If you turn around and swim with the current your speed along the bank will be your swimming speed of 4 mph plus the speed of the current (3 mph) for a total speed of 7 mph. The speed at which you can swim has not changed, the speed along the riverbank has. The exact same principle applies with airspeed & groundspeed. Well, almost... When you're talking airspeed there are several other "types" - indicated, calibrated, true, etc. I'm running out of time so if you're still confused, do a search - there's been a lot of threads on this subject in the past.""
"A comparison that I like to make is that it's like swimming in a river. If you're swimming upstream your "groundspeed" will be your swimming speed minus the speed of the water current. Let's say you're a strong swimmer and can make 4 mph. If you swim into a 3 mph current you will only make 1 mph along the riverbank. If you turn around and swim with the current your speed along the bank will be your swimming speed of 4 mph plus the speed of the current (3 mph) for a total speed of 7 mph. The speed at which you can swim has not changed, the speed along the riverbank has. The exact same principle applies with airspeed & groundspeed. Well, almost... When you're talking airspeed there are several other "types" - indicated, calibrated, true, etc. I'm running out of time so if you're still confused, do a search - there's been a lot of threads on this subject in the past.""
03-13-2014, 04:36 AM
#43
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dirtybird, why do you think that you have suddenly discovered something that no pilot has observed, and no aeronautical engineer or physicist agrees with? Does it not occur to you that maybe you are wrong and millions of very highly trained people over more than a century of flight might be right?
03-13-2014, 05:02 AM
#44
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This is an interesting thread, in a way, because most of the people who buy into the downwind-turn myth fall for it because they think a plane's airspeed changes when it turns into or away from the wind direction. Here we have someone who sees that the airspeed won't change, but who still thinks the plane will gain or lose altitude when turning in a mass of air that is moving with respect to the ground. Still wrong, but in a novel way.
03-13-2014, 05:02 AM
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Ok, so I am confusing airspeed and ground speed. I have done some searching and come up with this..
"A comparison that I like to make is that it's like swimming in a river. If you're swimming upstream your "groundspeed" will be your swimming speed minus the speed of the water current. Let's say you're a strong swimmer and can make 4 mph. If you swim into a 3 mph current you will only make 1 mph along the riverbank. If you turn around and swim with the current your speed along the bank will be your swimming speed of 4 mph plus the speed of the current (3 mph) for a total speed of 7 mph. The speed at which you can swim has not changed, the speed along the riverbank has. The exact same principle applies with airspeed & groundspeed. Well, almost... When you're talking airspeed there are several other "types" - indicated, calibrated, true, etc. I'm running out of time so if you're still confused, do a search - there's been a lot of threads on this subject in the past.""
"A comparison that I like to make is that it's like swimming in a river. If you're swimming upstream your "groundspeed" will be your swimming speed minus the speed of the water current. Let's say you're a strong swimmer and can make 4 mph. If you swim into a 3 mph current you will only make 1 mph along the riverbank. If you turn around and swim with the current your speed along the bank will be your swimming speed of 4 mph plus the speed of the current (3 mph) for a total speed of 7 mph. The speed at which you can swim has not changed, the speed along the riverbank has. The exact same principle applies with airspeed & groundspeed. Well, almost... When you're talking airspeed there are several other "types" - indicated, calibrated, true, etc. I'm running out of time so if you're still confused, do a search - there's been a lot of threads on this subject in the past.""
He does touch on something I avoided for simplicity, Close to the ground where most RC aircraft fly (IE under 1000 feet) Airspeed is the same as the speed of air molecules moving over the wing.
Not wanting to complicate the issue but flying in a real aircraft at 45,000 feet, the air is much thinner. Your Airspeed (as per the flight instruments) can be less than half of the speed of the air molecules actually passing your wing. This is called Indicated Airspeed vs True Airspeed and it changes because indicated airspeed is really a measure of air pressure, As air becomes thinner at altitude you need to move faster through it to achieve the same pressure.
However that is high altitude aerodynamics and not really relevant to (most) RC aircraft.
03-13-2014, 05:16 AM
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Love it....was hoping to buy popcorn before this started, but....
"Downwind Turn" Myth in RC (based only on my observations from 40 years of RC and FS flying)
a. "Downwind" means the plane is usually further away (e.g. - harder to see/discern bank angle)
b. Throttle has usually been reduced, thus causing RC model to get closer to stall speed
c. Because pilot is DESCENDING (usually), incorrect elevator inputs are given for the bank angle initiated to execute the turn
d. Bank angle and mishandled elevator now causes increase in sink rate - pilot responds by jerking ELEVATOR UP
e. Aircraft cleanly stalls outboard wing due to increase AOA from elevator input. Since throttle has not been added, aircraft stalls "faster" because the idiot hasn't been able to see what the hell is going on because all this time the freakin' airplane is going FURTHER DOWNWIND
f. Stall, snap, crash = due to the "Downwind Turn", because the idiot doesn't know enough about how to fly.
If I'm being overly-harsh, ignore it and get over it and go to your flying field and watch the next "Downwind" turn-stall-snap-crash and prove that I'm wrong.
Holy moly.
"Downwind Turn" Myth in RC (based only on my observations from 40 years of RC and FS flying)
a. "Downwind" means the plane is usually further away (e.g. - harder to see/discern bank angle)
b. Throttle has usually been reduced, thus causing RC model to get closer to stall speed
c. Because pilot is DESCENDING (usually), incorrect elevator inputs are given for the bank angle initiated to execute the turn
d. Bank angle and mishandled elevator now causes increase in sink rate - pilot responds by jerking ELEVATOR UP
e. Aircraft cleanly stalls outboard wing due to increase AOA from elevator input. Since throttle has not been added, aircraft stalls "faster" because the idiot hasn't been able to see what the hell is going on because all this time the freakin' airplane is going FURTHER DOWNWIND
f. Stall, snap, crash = due to the "Downwind Turn", because the idiot doesn't know enough about how to fly.
If I'm being overly-harsh, ignore it and get over it and go to your flying field and watch the next "Downwind" turn-stall-snap-crash and prove that I'm wrong.
Holy moly.
03-13-2014, 05:24 AM
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This is an interesting thread, in a way, because most of the people who buy into the downwind-turn myth fall for it because they think a plane's airspeed changes when it turns into or away from the wind direction. Here we have someone who sees that the airspeed won't change, but who still thinks the plane will gain or lose altitude when turning in a mass of air that is moving with respect to the ground. Still wrong, but in a novel way.
03-13-2014, 06:53 AM
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My observation of those model fliers that lose height when turning downwind, and gain height as they turn into wind, is that they are doing it themselves with the elevator because they have been fooled by the illusion of how tight the turn is, caused by the wind.
When in a turn and banked over you have some expectation of the model changing the direction of its travel. When turning from into wind to downwind, at the point the model has got crosswind it is travelling towards its canopy as viewed from the ground, the model pilot thinks he must be pulling back too hard and eases off the elevator and so the model starts to descend.
When turning from downwind to into wind, at the crosswind stage the model is travelling towards its underside as viewed from the ground so the model flier thinks he needs to pull back harder, and the climb starts.
When in a turn and banked over you have some expectation of the model changing the direction of its travel. When turning from into wind to downwind, at the point the model has got crosswind it is travelling towards its canopy as viewed from the ground, the model pilot thinks he must be pulling back too hard and eases off the elevator and so the model starts to descend.
When turning from downwind to into wind, at the crosswind stage the model is travelling towards its underside as viewed from the ground so the model flier thinks he needs to pull back harder, and the climb starts.
03-13-2014, 07:20 AM
#49
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I really have been enjoying this. I am an old fart that cant really fly eny more but I like to discuss it here. I would have res-erected the old thread but I cant seem to find it.I tried search for downwind turn and nothing came up so I started this one. I knew it would keep me entertained for awhile.I think all of you will admit that you have fun discussing it. Thats what this forum is all about isnt it?
Unfortunately there seems to be a bit of nastiness involved. Some of you dont seem to be able to accept that there is a different view than what you perceive. Lets keep it civil OK?
I enjoy reading the examples you outline but remember swimming in water or flying a full scale is not the same as a small model airplane swimming in an air stream. Water is a differ int medium and a full scale is many times heavier and flown very differently than a model.
A 60 degree bank is normal for a model but would scare the hell out of a full scale.
I think we all agree that the airspeed does not change.
But consider this; if you are flying a control line the wind will change as the model see it because it is constrained by the wire.
Now your AC is not constrained by a wire but it is constrained by gravity just like the wire. The effect is very very small and will be hard to measure but its there. I was going to say trust me but you probably dont
Whee we are at the top of the most active list!
Unfortunately there seems to be a bit of nastiness involved. Some of you dont seem to be able to accept that there is a different view than what you perceive. Lets keep it civil OK?
I enjoy reading the examples you outline but remember swimming in water or flying a full scale is not the same as a small model airplane swimming in an air stream. Water is a differ int medium and a full scale is many times heavier and flown very differently than a model.
A 60 degree bank is normal for a model but would scare the hell out of a full scale.
I think we all agree that the airspeed does not change.
But consider this; if you are flying a control line the wind will change as the model see it because it is constrained by the wire.
Now your AC is not constrained by a wire but it is constrained by gravity just like the wire. The effect is very very small and will be hard to measure but its there. I was going to say trust me but you probably dont
Whee we are at the top of the most active list!
Last edited by dirtybird; 03-13-2014 at 08:12 AM. Reason: ad our position
03-13-2014, 08:24 AM
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This topic is quite interesting, and it could be valuable to explore if people could resist getting their panties in a bind!. I haven't looked at all of the 1350 previous posts, but the ones I did gave me the impression that some of the contributors don't have as good a foundation in math/science basics as they think they do. Whilst some people will say you need this to fly an airplane: I disagree!!! I put my 7 year old on Flight Simulator and didn't tell him ANYTHING except "if you move the sticks things will happen. When you crash press the red button."
In two weeks he had moved on to the helicopters. Now, this doesn't mean he can fly a real model; but I'm suspecting he could with a little time on a buddy box. So, math/science/basics of flight: very useful. But not absolutely necessary.
How's that relevant? A person could probably have the math/science wrong, but still successfully fly; and that's really what this hobby is about.
My next thought was: "maybe I THINK I know something, but I don't really know it." And the most likely culprit was "airspeed". So I thought and I thought and the more I thought the more I realized, I don't have any idea what this REALLY is!! I looked it up in Wikipedia (granted not the best source, but I'll give it a chance). It said
Vt = Vg=Vw where
Vt = true airspeed
Vg = groundspeed
Vw = wind vector (vector= speed & direction; this could be a complication for us...)
Ok, so here's a scenario.
There's a strip; an airplane on the ground; and wind blowing the same direction as the strip, on the nose of the plan.
Let's say it's blowing 10 MPH
We know the ground speed is 0 when the airplane is not moving; we know the windspeed, it's 10 mph. We've made the plane point coincident with the wind vector so we don't have to worry about vector math.
What is the airspeed? Vt = 0-10; it's -10.
That doesn't sound right! I think we have something in math that is called "sign convention", basically that means that the sign on the windspeed in this case is -, so
Vt = 0-(-10)= 10 mph
Well, I think that sounds right!!!
Now, let's turn the airplane around, so it's pointed the other way. That affects whether the windspeed vector is considered a positive or negative value, and
Vt= 0-10= =10 mph; our airspeed is -10 mph.
Does this conform to the definition for airspeed? That was "true airspeed is the speed of the aircraft relative to the wind". I'll assume that can be rephrased as "the speed of the aircraft if the wind is considered to be stationary and it is only the plane that is moving." Well, those numbers do correspond to that definition; and the equation works; but we aren't flying!!! Let's try to do that but with a flying airplane
The plane is flying over the airstrip this time.
It's moving at a groundspeed of 40 mph
The airspeed again is 10 mph on the nose
Vt =40-(-10)= 50 mph airspeed
Now, turn around. The windspeed is not going to change just because the airplane turns around; so one of these three cases happens:
- The airspeed stays the same; which means groundspeed changes
- The groundspeed stays the same; which means airspeed changes
- Both groundspeed and airspeed change...
What I observe when I do this with my plane, is that the airplane seems to slow down with respect to me.
I think that means the groundspeed decreases.
It doesn't prove that airspeed stays the same; but I bet with some measurements this could be proved. I bet people HAVE taken those measurements!!!!
I don't think that there's anything in the above that is particularly objectionable but I could be surprised...
Haven't said all that, I have observed that when I turn UPWIND in a pretty heavy wind, I can sometimes bring the airplane to an almost stop!! When this happens, the airplane seems to be very sensitive to elevator, and wants to climb easily. But maybe this is all in my head and not real. By in my head; I mean:
the rise per second is exactly the same whether going upwind or downwind, but the rise per foot covered is NOT because of low groundspeed, and this is interpreted as "overly sensitive".
In two weeks he had moved on to the helicopters. Now, this doesn't mean he can fly a real model; but I'm suspecting he could with a little time on a buddy box. So, math/science/basics of flight: very useful. But not absolutely necessary.
How's that relevant? A person could probably have the math/science wrong, but still successfully fly; and that's really what this hobby is about.
My next thought was: "maybe I THINK I know something, but I don't really know it." And the most likely culprit was "airspeed". So I thought and I thought and the more I thought the more I realized, I don't have any idea what this REALLY is!! I looked it up in Wikipedia (granted not the best source, but I'll give it a chance). It said
Vt = Vg=Vw where
Vt = true airspeed
Vg = groundspeed
Vw = wind vector (vector= speed & direction; this could be a complication for us...)
Ok, so here's a scenario.
There's a strip; an airplane on the ground; and wind blowing the same direction as the strip, on the nose of the plan.
Let's say it's blowing 10 MPH
We know the ground speed is 0 when the airplane is not moving; we know the windspeed, it's 10 mph. We've made the plane point coincident with the wind vector so we don't have to worry about vector math.
What is the airspeed? Vt = 0-10; it's -10.
That doesn't sound right! I think we have something in math that is called "sign convention", basically that means that the sign on the windspeed in this case is -, so
Vt = 0-(-10)= 10 mph
Well, I think that sounds right!!!
Now, let's turn the airplane around, so it's pointed the other way. That affects whether the windspeed vector is considered a positive or negative value, and
Vt= 0-10= =10 mph; our airspeed is -10 mph.
Does this conform to the definition for airspeed? That was "true airspeed is the speed of the aircraft relative to the wind". I'll assume that can be rephrased as "the speed of the aircraft if the wind is considered to be stationary and it is only the plane that is moving." Well, those numbers do correspond to that definition; and the equation works; but we aren't flying!!! Let's try to do that but with a flying airplane
The plane is flying over the airstrip this time.
It's moving at a groundspeed of 40 mph
The airspeed again is 10 mph on the nose
Vt =40-(-10)= 50 mph airspeed
Now, turn around. The windspeed is not going to change just because the airplane turns around; so one of these three cases happens:
- The airspeed stays the same; which means groundspeed changes
- The groundspeed stays the same; which means airspeed changes
- Both groundspeed and airspeed change...
What I observe when I do this with my plane, is that the airplane seems to slow down with respect to me.
I think that means the groundspeed decreases.
It doesn't prove that airspeed stays the same; but I bet with some measurements this could be proved. I bet people HAVE taken those measurements!!!!
I don't think that there's anything in the above that is particularly objectionable but I could be surprised...
Haven't said all that, I have observed that when I turn UPWIND in a pretty heavy wind, I can sometimes bring the airplane to an almost stop!! When this happens, the airplane seems to be very sensitive to elevator, and wants to climb easily. But maybe this is all in my head and not real. By in my head; I mean:
the rise per second is exactly the same whether going upwind or downwind, but the rise per foot covered is NOT because of low groundspeed, and this is interpreted as "overly sensitive".