Aerodynamics Discuss the concepts of aerodynamics here
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Flitetest join the downwind turn debate!

Old 11-07-2011, 07:39 PM
  #26  
JetPlaneFlyer
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Originally Posted by HX3D014 View Post
so then that would mean that the V^2 would need to be divided by the Radius?
Good catch!.. corrected.
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Old 11-07-2011, 09:09 PM
  #27  
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Wow, so many see it and just still dont' understand it!

I guess maybe because i've sailed I know that there is windspeed, boat speed and apparent wind.

WIth the anemometer on the plane what they were measuring was apparent wind - what the wind fells like if you opened the window of a plane an stuck your head out. Or did it in a car or bicycle or boat.

You move 20 mph and stick your head out and you fee 20mph wind, right?

Now if you have a 10 mph head wind you'll feel 30 mph with your head out the window.

If you have a 10mph tail wind you'll feel only 10mph wind on your face with your head out the window.

On a windy day if you stand still and face the wind you feel only the wind. If you run (or fly your plane) into this wind you feel the wind plus whatever speed your going. They add together for what you feel, windwise. (apparent wind)

Now stop and put your back to the wind - same wind, but now if you run just as fast (or fly) the wind in your face is not the same as your actual speed of movment - if you have a 10mph tail wind you have to go 10mph before you feel no wind at all.

So if you plane needs 25mph to maintain lift and you fly into a 25mph wind you will have lift with no engine and no forward speed. If you turn around the plane won't fly unless you can make it go 50 mph (ground speed). You need 25mph of wind over the wings to get lift - with 25mph on your tail you have negative air speed. You'll need to get to 25mph just to get to zero wing speed then add 25 more to get lift.

This is why planes get blown downwind and can't fly back - they can't fly faster than the wind.
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Old 11-07-2011, 09:31 PM
  #28  
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Prof,

There is a very important and fundamental difference between boats and planes. Boats (and cars, and people) are connected to the ground (or to water). The ground (or water) therefore has a big effect on boats, cars and people.
Planes in flight on the other hand have no physical connection to the ground so the ground cannot effect them at all. If a plane is moving at 50mph, 0pmh or even backward relative to the ground makes absolutely no difference to the plane. The only thing that is important to a plane is it's motion through the air that surrounds it, a.k.a. 'airspeed'.

Once this is accepted then you can see that wind cannot effect a plane in flight because wind by definition is the movement of air over the ground, as the ground is incapable of effecting the plane then the fact that air may be moving relative to the ground also cannot effect the plane. As far as a plane in flight is concerned the ground might as well not exist and so the concept of wind becomes meaningless.

So if you plane needs 25mph to maintain lift and you fly into a 25mph wind you will have lift with no engine and no forward speed.
I'd challenge this point. First 'no forward speed' is meaningless without stating what that speed is relative to. The plane actually does have a 25mph forward speed relative to the only thing that matters to the plane(the air). Also it's untrue to say 'no engine' the plane will not stay in flight without the engine. The plane is flying at 25mph airspeed and it needs the engine to provide enough power to overcome the drag that the plane produces at 25mph. This is EXACTLY the same power as the plane would need to fly at 25mph airspeed in the opposite (downwind) direction. the fact that relative to the ground it's doing 0mph in one direction and 50mph in the other makes not the slightest difference to the power required.


Steve
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Old 11-07-2011, 11:58 PM
  #29  
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Yes, planes are not connected to the ground - but that is totally beside the point and has no bearing on what this is about.

A lifting body (wing, whatever) produces lift when air travels across it. The faster the air the more lift. The less the air, the less lift.

In a wind tunnel the wing does not move at all in any way shape or form. The air moves - if it moves at 25 mph over the front of the wing you get lift. Agree so far?

Now if the wing is propelled forward into the wind (the moving air) then the apparent wind speed over the wing changes - you add the wind and the forward motion together - so if you move the wing forward at 10 mph you now have a total of 35mph of air moving over the wing. Yes? You see where I get this from, yes?

Ok, now lets turn the wing around and blow air from behind it at. You get no lift - no air if flowing over the wing to give it lift (flowing from the back won't do that, for this discussion at least). So to get lift you need to move the wing forward faster than the wind from behind, yes? You follow me so far? If you need 20 mph of air movement over the wing (from the front) to get lift you need to move forward at 45 mph (the 25 from behind plus the 20). If you only move forward at 20 mph you still have 25 coming from behind you have a -5 mph wind over your wing and you will not have lift.

This is why all planes take off and land into the wind instead of with a tail wind - the added wind speed (added to the airplanes forward speed) gives more lift allowing shorter takeoffs and slower landings.

With boats you have to be moving faster than the current of the water to maintain steerage - water moving over your rudder. If you are drifting with the current you cannot steer the boat. Same for a plane - you must fly faster than the wind to remain in control.
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Old 11-08-2011, 01:05 AM
  #30  
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Agree with all you have said, but what you have said isn't how a plane in flight behaves. If a car drives at 25mph into a 10mph wind then, yes, it will 'feel' a 35mph 'apparent wind' and if it drives at 25MPh with a 10mph tailwind it will only 'feel' a 15mph apparent wind. While this is obviously true for a car it's not for a plane.

Planes are not driven through the air by ground based traction, like wheels on a car.

A plane flying into the wind will not have the wind speed added to it's flying speed and a plane flying away from the wing will not have wind speed subtracted from it's flying speed. Regardless of if a plane is flying toward the prevailing wind or away from it, the plane's airspeed is TOTALLY dependant on the trust produced by the prop and the drag produced by the airframe. These things are dependant only on the speed of the plane through the air, not the speed of the plane over the ground.

An example; lets say that at 50% throttle our plane in level flight flies at 25mpg airspeed. It will fly at 25mph airspeed irrelevant of any wind that may be blowing over the ground. If it flies into a 10mph headwind then it's airspeed will not magically increase to 35mph, it cant because the engine isn't making enough thrust for 35mph. The plane will simply continue to fly at 25mph airspeed into the headwind (for a 15mph ground speed)
Now our plane turns 180 deg, still at 50% throttle.. It will continue to fly at 25mph airspeed, the 'tailwind' will not be subtracted from this, all that will occur is that the planes speed over the ground will increase to 35mph.

So what I'm saying is wind changes the speed a plane moves over the ground, not the speed a plane moves through the air. It's only the speed though the air that effects how a plane flies. The ground speed is irrelevant.

Take off and landing are a different matter. When the plane is physically in contact with the ground then obviously groundspeed is an issue and movement of the air over the ground (aka wind) can be exploited to give the plane a some initial speed through the air and reduce the speed it needs to roll over the ground at at in order to lift off. But once the wheels leave the ground the ground no longer influences the plane one jot.

Steve
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Old 11-08-2011, 01:05 AM
  #31  
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Ground or not, if a plane is flying at 60 mph IAS, and 20 mph of that is attributable to air moving toward the plane (wind), that means, then, without increasing throttle, it makes a 180 degree turn (even ignoring the reality of loss of lift due to the wings no longer being parallel with Earth), now the plane is flying at 40 mph IAS. If stall is 50 mph IAS, its going to stall. This is due to the difference in wind direction relative to the airplane. Got nothing to do with the ground.
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Old 11-08-2011, 01:10 AM
  #32  
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The proof is in the video in the first post. Headed into the wind the airspeed was 41. After turning 180˚ and heading down wind the airspeed was 41. They expected something different but what the test showed was that airspeed did not change. Maintain airspeed and you maintain the same flight characteristics. It matters not what the air is doing relative to the ground.
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Old 11-08-2011, 01:19 AM
  #33  
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Earth,

No, if a plane is flying at 60mph IAS then none of that is due to 'wind' it is ALL due to the thrust of the engine driving the plane through the air. when it turns 180deg nothing changes, thrust is the same, drag is the same, so IAS will be the same.


Acceleration through the turn is the same also regardless of wind.

Acceleration is defined as change in velocity over time. velocity is scalar (ie it has direction).
The formula for linear acceleration is: (V2-V1)/t
where:
V1 = initial velocity
V2 = final velocity
t = time

A calm day... a plane is flying north at 20m/s ground speed and it turns 180deg to a southerly heading at 20m/s ground speed. it performs the turn in 5 seconds.

Lets call North positive vector and South negative.

average acceleration = (V2 -V1) /t
= (-20 - 20) /5
= -8m/s^2

Now lets say that there is a 15m/s north wind blowing
V1 goes to 5m/s
V2 goes to -35m/s

average acceleration = (V2 -V1) /t
= (-35 - 5) /5
= -8m/s^2

So even when measuring ground speed the wind makes no difference to the acceleration the plane goes through in performing a turn.. proved my math

Steve

PS.. when you say "This is due to the difference in wind direction relative to the airplane. Got nothing to do with the ground." please define what wind is without referring to the ground.. can wind exist without ground being there to measure from?.. wind has EVERYTHING to do with the ground.

Last edited by JetPlaneFlyer; 11-08-2011 at 08:41 AM. Reason: fixed units of acceleration
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Old 11-08-2011, 01:40 AM
  #34  
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Originally Posted by prof_fate View Post
Yes, planes are not connected to the ground - but that is totally beside the point and has no bearing on what this is about.

A lifting body (wing, whatever) produces lift when air travels across it. The faster the air the more lift. True The less the air, the less lift. the less the air density ? True, or the less the air speed over the wing True
In a wind tunnel the wing does not move at all in any way shape or form. The air moves - if it moves at 25 mph over the front of the wing you get lift. Agree so far? Yep

Now if the wing is propelled forward into the wind (the moving air) then the apparent wind speed over the wing changes - you add the wind and the forward motion together - so if you move the wing forward at 10 mph you now have a total of 35mph of air moving over the wing. Yes? Yes You see where I get this from, yes? Yes
Ok, now lets turn the wing around and blow air from behind it at. You get no lift - no air if flowing over the wing to give it lift (flowing from the back won't do that, for this discussion at least). So to get lift you need to move the wing forward faster than the wind from behind, yes? Yes You follow me so far? Yes If you need 20 mph of air movement over the wing (from the front) to get lift you need to move forward at 45 mph (the 25 from behind plus the 20). If you only move forward at 20 mph you still have 25 coming from behind you have a -5 mph wind over your wing and you will not have lift. true

This is why all planes take off and land into the wind instead of with a tail wind - the added wind speed (added to the airplanes forward speed) gives more lift allowing shorter takeoffs and slower landings. not true. its to reduce the ground velocity and the breaking through the wheels when the plane is on the ground. also to reduce runway length.
With boats you have to be moving faster than the current of the water to maintain steerage - water moving over your rudder. If you are drifting with the current you cannot steer the boat. Same for a plane - you must fly faster than the wind to remain in control.
the problem is If an aircraft is only capable of doing say 20mph and the wind is a steady 10mph how can that aircraft get an Apparent wind (as you called it) of 30mph. you see when an aircraft is only capable of 20mph that is apparent wind speed while at the same pressure altitude most of us live in.

if an RC aircraft is only capable of 20mph, then it is the speed relative to the air it is flying through.

if the aircraft is flying with the wind at full throttle level flight. it is only doing 20mph relative to the air. but relative to the ground it is doing 20mph air speed + 10mph wind speed = 30mph ground speed, Good to know when you are traveling somewhere.
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Old 11-08-2011, 02:07 AM
  #35  
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The danger of turning downwind (only when flying VERY near stall in an airplane with sufficient inertia such that the wind cannot impart its energy to the plane within the period of the 180 degree turn) is exactly that the plane (and pilot) can not sense the component of airspeed attributable to the wind! That can only be determined from a fixed frame of reference, such as the ground.

I am not saying that Flite Test had it right on their episode. I'm just pointing out a ridiculously narrow set of parameters that could lead to a crash due to turning downwind.
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Old 11-08-2011, 07:26 AM
  #36  
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Earth,

Again, and hopefully to ber very clear on the point; for a plane in flight there is no "component of airspeed attributable to the wind", none, zero! Airspeed is a result of the plane's thrust minus it's drag.

Thrust pushed it forward, drag holds it back, when the two balance the plane has reached it's steady flying airspeed. Wind over the ground doesn't come into it in any way, shape, or form. If you think the wind we feel on the ground can effect a planes airspeed then please explain how it does so with physics.

But i do agee that turns in wind can be dangerous. Where the turn can become dangerous is if the pilot does not focus on airspeed but instead focuses on ground-speed. This is not so much a problem in real planes as they have airspeed indicators and usually fly high above the ground, but for models it's a real problem because the flyer ('pilot') is stood on the ground and has no direct airspeed indication. This can cause him to fly too slow when going downwind because the plane 'looks like' it's going fast.
This problem is very real but it's nothing to do with the physics of flight, it's purely pilot error.

Steve
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Old 11-08-2011, 12:02 PM
  #37  
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I am saying that the engine is at idle in my very narrow little scenario, such as an airplane on approach. And I don't agree that an airplane in flight has no component of airspeed associated with wind. That component exists, but can't be determined from the airplane's frame of reference.
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Old 11-08-2011, 12:43 PM
  #38  
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[
Originally Posted by earthsciteach View Post
I am saying that the engine is at idle in my very narrow little scenario, such as an airplane on approach. And I don't agree that an airplane in flight has no component of airspeed associated with wind. That component exists, but can't be determined from the airplane's frame of reference.
Sorry for the last post there Steve (the one with all the red text in it). I did have internet connection problems, and did not know if anyone had posted. I finally left the PC on and waited as long as I could before I had to go to work, but it was still not finishing the Submit Reply process that I could see. I feel As though I was just echoing part of what you were saying.

Earth.
you need to work it out in your own head. Be scientific about it. IE prove it wrong for your self. and if you can prove it wrong then get some one to check it, and see if you can prove that proof to be at fault or wrong. if not then you may have solved it.

For now. all we can do is try to help out with the understanding of it all.

We can throw mind games at you or pure math. but we are only guessing what you must be thinking to believe there is a component of Wind relative to the ground that is a portion of an aircrafts final Air speed.

So work it out on paper with different directions and different maximum speeds (or throttle settings if you prefer)

compare the results and make conclusion as to weather the theory holds.

get back to us if you have questions or are stuck somewhere.

PS. I personally feel that maybe the sailing world of two to three different reference points may be over complicating your view of aircraft in the air?
I am not sure because I don't know you, but for now it is just a guess.

PS
just to make another echo.
for a plane in flight there is no "component of airspeed attributable to the wind", none, zero! Airspeed is a result of the plane's thrust minus it's drag
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Old 11-08-2011, 02:23 PM
  #39  
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Originally Posted by earthsciteach View Post
I am saying that the engine is at idle in my very narrow little scenario, such as an airplane on approach. And I don't agree that an airplane in flight has no component of airspeed associated with wind. That component exists, but can't be determined from the airplane's frame of reference.
But the plane is only flying in its own frame of reference, none other. And the plane's airspeed only exists relative to that same frame of reference. So if it can't be determined from the plane's frame of reference then it isn't airspeed you're talking about.

If you're really just saying that a pilot can get into trouble by turning without checking his airspeed and correcting for changes then of course you're right. It just has nothing to do with wind affecting the plane's airspeed and everything to do with changes in lift and drag when banking and turning....either into or out of the wind.

Steve
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Old 11-08-2011, 02:33 PM
  #40  
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Is the earth a factor? I'm standing on it flying the plane...upwind (as in the wind I feel) vs downwind very definitely has an effect from what I see and have to do and consider to keep the plane flying.

Also consider that speed - any speed - is measured relative to what? The earth! "Miles per hour" is distance over time - what distance are we taking about? Distance over the earth. So you can't really disconnect the plane or any measurement from the earth.

And if windspeed was irrelevant from the planes perspective we could fly any plane in any amount of wind - as it would always be relative to the plane, right? So fly slow and you get 10 mph wind at the plane or fly fast and get 80 mph. But when the windspeed (as measured from the earth) hits 20 mph my planes are not capable of flying - and I don't mean just because they can't fly 'upwind' - they just can't handle the wind period.
If wind was always relative to just the plane then that wouldn't be so.
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Old 11-08-2011, 02:48 PM
  #41  
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Sometimes it's easier I think to work in the wind's reference frame, instead of the plane's.

Dave
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Old 11-08-2011, 09:55 PM
  #42  
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Originally Posted by prof_fate View Post
Is the earth a factor? I'm standing on it flying the plane...upwind (as in the wind I feel) vs downwind very definitely has an effect from what I see and have to do and consider to keep the plane flying.

Also consider that speed - any speed - is measured relative to what? The earth! "Miles per hour" is distance over time - what distance are we taking about? Distance over the earth. So you can't really disconnect the plane or any measurement from the earth.

Staying connected to the earth is only needed during take off and landing, but if you are flying low near obstacles it is also a need to be aware of the different ground tracking of an aircraft in wind when doing maneuvers (such as circuits), if you are navigating, then of cause earth is a relevant thing, you want to get somewhere else on earth so you need to take it into consideration. We measure distance and the time it took to travel that distance im MPH or KPH or M/s or F/s but when we look at the lift formula, it has Velocity, which is the true airspeed, Air Speed is the speed of a particle of air being passed by the aircraft relative to the aircraft. that particle may itself have a Speed relative to the earth, but that is irrelevant to the lift formula.


And if wind speed was irrelevant from the planes perspective we could fly any plane in any amount of wind - as it would always be relative to the plane, right? So fly slow and you get 10 mph wind at the plane or fly fast and get 80 mph. But when the wind speed (as measured from the earth) hits 20 mph my planes are not capable of flying - and I don't mean just because they can't fly 'upwind' - they just can't handle the wind period.
If wind was always relative to just the plane then that wouldn't be so.
it would be like setting a small RC boat out into the choppy surf. you could fly a small indoor flyer in some pretty fast winds, but the wind is usually a bit turbulent near the ground, and that is a true hazard. but if for example you took a balloon flight , took you indoor flyer with a top speed of say 20mph and flew it from the gondola while the balloon was tracking at altitude at say 30mph, you could fly the aircraft around the gondola (no not inside the gondola, or inside the balloon). Pick a relatively nice day, not some choppy surf type day, or else the ride would be bumpy.
Bryce

Last edited by HX3D014; 11-08-2011 at 10:11 PM.
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Old 11-09-2011, 12:52 AM
  #43  
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You can fly many planes in winds you might not think likely...it's mainly that us pilots are ground referenced and for some reason we tend to feel uncomfortable flying a plane that's happy enough in its own environment but from our point of view is disappearing downwind backwards at 20 or 30mph .

It's not usually the plane that can't handle the wind it's the pilot .

Steve
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Old 11-09-2011, 07:45 AM
  #44  
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Prof,
Of course there are also the issues of turbulence, gusts, wind sheer etc. that can make flying in the wind tricky. These effects plus the obvious problem of getting blown backward relative to where you stand, plus the previous mentioned problems of perception, all come together to make flying in the wind more challenging than flying on a calm day.

For the purposes of this thread the assumption was theoretical case where the wind was perfectly steady.

Also consider that speed - any speed - is measured relative to what? The earth! "Miles per hour" is distance over time - what distance are we taking about? Distance over the earth.
i find this an interesting claim Whats your thinking that all speed is relative to the earth? For instance 'airspeed' by definition is speed measured relative to the air (clue in the title), airspeed would be airspeed regardless of what the earth was doing or if the earth even existed. Airspeed is NOT a measure of time taken to cover distance over the earth, it's distance through the air.

Here's one to ponder if you truly think "any speed - is measured relative to what? The earth":..
In 2000,000 years time the Earth had overheated due to global warming and become uninhabitable. Having long since been mined of anything useful the remaining lump of baron rock is demolished and vaporised to make way for a space parking lot.
Meanwhile far far away floating in deep space our descendants are living in giant space stations. They travel between space stations on interstellar craft. Being in a hurry and wanting to know how long the journey will take they would like to measure the speed of their craft. To do this they have a series of beacons at regular intervals along the interstellar highway, they know the distance between beacons and their systems measure time taken to go between beacons and so from this calculate speed which is displayed on the speedometer of the space craft in mph (the metric system has yet to be implemented).


So the questions for you:
  • If 'all speed is measured relative to the earth' then how does the speed they measure relate to an Earth which was in a far distant galaxy and now no longer exists?
  • Or because the Earth no longer exists is it now impossible to measure the speed of anything, did their spacecraft speedometers stop working when the Earth, many lightyears away, was demolished
  • In the vast hold of the giant interstellar craft one of the crew is flying a micro R/C model. It's equipped with telemetry and an airspeed indicator. Does the airspeed indicator now stop working?
Curious minds need to know...

Steve

Last edited by JetPlaneFlyer; 11-09-2011 at 04:07 PM.
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Old 11-09-2011, 11:07 AM
  #45  
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The airspeed of an aircraft is the speed of the aircraft relative to the air its moving in. Certainly its not directly related to the ground.

Then there are groundspeed which is the speed of the aircraft relative to the ground its moving over and windspeed which is the speed of the moving airmass relative to the ground.

Most of the problems people have with these discussions is failing to keep these three clearly separated in their heads .

And of course if you want to get really picky then none of them is really a "speed" (a scalar quantity). They're actually all velocities i.e. vectors in which direction is important .

And since I've just realised that I've got sucked into yet another of these interminable "downwind turn" confusions....I'm out of here. Have fun .

Steve
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Old 11-09-2011, 11:58 AM
  #46  
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HX3D014-I am working it out with diagrams equations and such. The complication comes down to frame of reference and that it just can't be looked at as a wing in the steady state.

I really just want to stop thinking about it. I reale I am splitting some very fine hairs here. LOL! But, as my wife says, I get a thought stuck in my head and tend to hyper-focus on it!

I'll get back to y'all. If I prove myself wrong, I will gladly admit it!
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Old 11-09-2011, 02:01 PM
  #47  
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"In 2000,000 years time the Earth had overheated due to global worming and become uninhabitable. Having long since been mined of anything useful the remaining lump of baron rock is demolished and vaporised to make way for a space parking lot."

Where's my towel
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Old 11-09-2011, 02:14 PM
  #48  
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Them global worms are making a right mess of things aren't they .

Steve
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Old 11-09-2011, 02:43 PM
  #49  
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All I can say is 42.
If that isn't the answer you're looking for then you need to rephrase the question.


Who/what invented MPH, a measure of speed? Some earthbound soul no doubt. And perhaps the air isn't connected to the earth, but air is atmosphere and that is connected to the earth, as am I when flying an RC plane. The wind affects the plane I'm flying, the windspeed I care about is relative to the earth.

And if a tree falls in the forest and I don't hear it, then as far as my reality is concerned no, it didn't make a noise.

BTW, the earth is flat. And it's round. You've all seen the pictures. It's shaped like a pizza!
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Old 11-09-2011, 03:21 PM
  #50  
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Originally Posted by slipstick View Post
Them global worms are making a right mess of things aren't they .

Steve
I laughed so hard I think I peed a little , nahhh just kidding, but I did wake the kid lol.
HX3D014 is offline  

Quick Reply: Flitetest join the downwind turn debate!


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