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Thread: downwind turn


  1. #1

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    downwind turn

    OK I have my flame suit on.
    I have a question for those that say there is no loss of altitude when turning from upwind to downwind.
    First look this over
    :http://en.wikipedia.org/wiki/Inertia

    You will find momentum defined as P=MV
    Where
    M= mass
    V= Velocity

    Lets say out aircraft weighs 4 lbs
    If flies 50MPH
    The wind is 10MPH
    When flying upwind its momentum is 4x(50-10)=160
    After it turns downwind its momentum is 4x(50+10)=240.
    Its momentum has increased by a factor of 80.
    Where dos the energy come from to increase the momentum?
    dirty old men need love too.

  2. #2
    tacx's Avatar
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    Don't fly until you figure that out. LOL

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    radfordc's Avatar
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    You are calculating momentum incorrectly. You are using the surface of the earth as your reference point. You should be using the surface of the sun.

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    tailskid's Avatar
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    Planes don't fly because of momentum..... they fly because MONEY makes them fly

    Next chance you get to fly in a full scale airplane, put the "hood" on look at the floor and have the pilot do a 360 then look up (now you are dizzy) THEN watch the airspeed indicator while he does another 360....and answer the question, "Which way was the wind blowing"? If he did those two 360 at a 60 degree bank - you can now throw up
    Last edited by tailskid; 03-11-2014 at 10:51 PM.
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    radfordc's Avatar
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    Quote Originally Posted by dirtybird View Post
    You will find momentum defined as P=MV
    Where
    M= mass
    V= Velocity
    OK, momentum is mass x velocity. Imagine yourself floating in a hot air balloon with your model. As you float along in the air you don't feel a breath of wind since the balloon is moving at exactly the same speed as the air mass. Now launch your model and fly in circles around the balloon...do you notice any change in velocity? Now fly out a way and then turn and fly directly toward the balloon and fly just under the basket. Do this from all four major directions; N, S, E, and W. Is the speed of the plane the same in all cases?

    The answer is yes, the speed of the plane through the air is exactly the same no matter which direction the plane is headed. The momentum of the plane with respect to the air molecules is the same and this is the only thing that matters to a plane in flight. It's the reaction of the plane moving through air molecules that produces the forces on the plane.

    In your original post you described the velocity of the plane changing depending on how it was oriented to the surface of the earth. What the surface of the earth is doing (or the surface of the sun) doesn't matter at all. As another mind experiment, imagine somehow making the earth totally disappear so that all that's left is a big ball of atmosphere orbiting the sun. Do you think that the direction that the plane is traveling with respect to the sun makes it's velocity change?

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    Moderator daveopam's Avatar
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    Good answer ^^
    I never want to see a crash. But I don't want to miss one either.

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    I have never been in a balloon and I expect you have not either. If you do, and the wind is blowing, try it. I expect you will find the airplane will rise when it flies the direction oposite of the wind and fall when going with it. Unfortunately I dont think you will get a chance as usually balloons dont fly in the wind.
    The object (airplane) is flying thru space with the earth because it is attracted to it by gravity.. The earth is not a big ball of atmosphere, its a large mass that possesses a large mass attraction called gravity. Thats why you can stand on it and not be thrown into space, The reference point is the center of the earth.
    The airspeed is the same no matter which way you fly, but you cant ignore the other physical attributes no matter how small they might be.
    dirty old men need love too.

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    radfordc's Avatar
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    Yes, I have flown in a hot air balloon.

    Sounds like you have it figured out. I defer to your wisdom.

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    Quote Originally Posted by radfordc View Post
    You are calculating momentum incorrectly. You are using the surface of the earth as your reference point. You should be using the surface of the sun.
    Absolutely. He also forgot the Tim Taylor rule of RC aircraft which is that any model airplane with less than a 2:1 power to weight ratio is not worth having.
    RC Aircraft/ Three Laws Safe.

  10. #10
    DeferredDefect's Avatar
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    Howdy:

    The momentum of the aircraft will change in relation to the ground with a shift in wind. That's a given.

    Imagine a model hitting a brick wall at full throttle in a no wind situation vs having a 50 knot tailwind. Obviously, in relation to the ground, the tailwind situation is going to make the aircraft appear faster. It is also going to do a lot more damage to the wall it hits because of that increase in kinetic energy created by the wind in relation to the ground.

    This doesn't mean that the aircraft is behaving any differently within that body of air. The model is travelling through a fluid, and all comparisons to "airspeed" need to be using the air as the frame of reference. Once the model is in the air, the ground has absolutely no bearing on the model (with the exceptions of turbulence caused by trees, slope lift, etc).

    A turn from downwind directly into the wind is effectively reducing the kinetic energy of the model, but not signifcantly in relation to the air, only in relation to the ground. We can see that the aircrafts movement within the air is the only factor to consider when determining lift, so the change in what is perceived as a decrease in energy is not actually relevant to the model.
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    The aircrafts relation to the ground is called altitude. The kinetic energy change as the aircraft turns must come from or go somewhere. So the aircraft loses altitude as we turn downwind or gains as we turn upwind.
    dirty old men need love too.

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    Oh, NOOOOOO! Not again! This question was murdered in the thread Downwind turn Myth. Four years and over 1300 posts, it kept coming back up. Please go back and read it.

    The turn downwind does not affect the airplane unless the turn is mishandled by allowing the airplane to . . . just read it.

    SM
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    Yes I read that thread.
    Its all about wind speed. They did not consider the effect of momentum or inertia that I could find.
    All I am asking is where does the AC get the energy to change its speed?
    dirty old men need love too.

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    radfordc's Avatar
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    It does not change airspeed. The only effect that the earth provides is gravity. Gravity acts vertically downward and has zero effect on a plane's speed when in level flight. The only thing that causes a plane in level flight to change airspeed is power and drag.

    http://www.aviation-history.com/theory/force.htm
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    radfordc's Avatar
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    Quote Originally Posted by Sgt. Meyer View Post
    Oh, NOOOOOO! Not again!
    You are right, there are a million references that explain the physics of this issue.

  16. #16
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    Quote Originally Posted by tailskid View Post
    Planes don't fly because of momentum..... they fly because MONEY makes them fly

    Next chance you get to fly in a full scale airplane, put the "hood" on look at the floor and have the pilot do a 360 then look up (now you are dizzy) THEN watch the airspeed indicator while he does another 360....and answer the question, "Which way was the wind blowing"? If he did those two 360 at a 60 degree bank - you can now throw up

    Good example, now, when you do this, look at the nearest landmark on the ground before you put the hood on. Look out from under the hood when the first 360 is completed and look again when the second 360 is complete. If there is no wind, you should be in relatively the same place in relation to the land mark each time you complete a circle, but if there is any significant wind, say 15mph or more, tell me, are you are still in relatively the same place in relation to the landmark after each circle?

    Everyone here probably already knows the answer to that question. So the issue is not "miss handling" a downwind turn. The issue is that, as RC pilots, we are more concerned with position relative to a landmark (ourselves) than we are making a perfect 360 degree circle in a moving air mass. Yes, I can probably make perfect turns on a windy day without losing any airspeed or altitude, but my ground track will be no where near the flight pattern for my flying field. Therefore, I adjust my inputs to achieve the same ground track, which is needed to keep the plane in the proper place relative to the landmark (Me).

    Yes, If I do this incorrectly, I will lose altitude and possibly crash, even if I do it correctly I may still lose some airspeed or altitude, but if I tried to fly the plane like a full scale would and make a standard turn and not loose any airspeed or altitude, my plane would probably be out of range of my radio or my eyes very quickly.

    There is no "MYTH", there is just pure science and fact that is being confused by people trying to compare apples to oranges.
    Jerry
    AMA -922698 Nomal people scare me, but not as much as I scare them...

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    POSTED BY hugger - "So the issue is not "miss handling" a downwind turn. The issue is that, as RC pilots, we are more concerned with position relative to a landmark (ourselves) than we are making a perfect 360 degree circle in a moving air mass. Yes, I can probably make perfect turns on a windy day without losing any airspeed or altitude, but my ground track will be no where near the flight pattern for my flying field. Therefore, I adjust my inputs to achieve the same ground track, which is needed to keep the plane in the proper place relative to the landmark (Me). . . .

    Yes, If I do this incorrectly, I will lose altitude and possibly crash, . . . "



    You are absolutely correct, hugger, but this is exactly the Mishandling I was talking about. If you did it incorrectly, you mishandled it. Right?
    Last edited by Sgt. Meyer; 03-12-2014 at 03:41 PM.
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    Popcorn stocks go up when threads like this get started.
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    Quote Originally Posted by dirtybird View Post
    Yes I read that thread.
    Its all about wind speed. They did not consider the effect of momentum or inertia that I could find.
    All I am asking is where does the AC get the energy to change its speed?
    It does not change its airspeed. It is flying within a moving air mass. All reference is to that airmass.

    And btw, that thread is full of discussions about mass and momentum and inertia. Inertia comes into play in event of a gust or a windshear. The myth assumes a steady wind. It has all really been discussed. Wind shear, downbursts, fatal airline crashes, even instant 180 degree turns caused by alien spacecraft. Full scale vs. models. It's all in there. To continue this is to repeat over a thousand posts.

    SM
    Last edited by Sgt. Meyer; 03-12-2014 at 03:59 PM.
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    It does not change its air speed.
    But it changes its ground speed.
    Inertia and momentum are referenced to ground speed.
    Please provide a link to where this is discussed in the long thread
    dirty old men need love too.

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    seven.

    the answer is seven.
    Don S

  22. #22
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    Quote Originally Posted by dirtybird View Post
    OK I have my flame suit on.
    I have a question for those that say there is no loss of altitude when turning from upwind to downwind.

    Where dos the energy come from to increase the momentum?
    The increased energy comes from the moving air mass. This still has nothing to do with the aircraft flying through that same air mass loosing or gaining altitude in a turn. It only makes a difference when you hit the ground at full throttle. Your crash will be more impressive with the wind than against it due to the increased momentum :-)
    When everything seems to be going against you, remember that the airplane takes off against the wind, not with it.
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  23. #23
    Sport_Pilot's Avatar
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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.
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    Lift = (1/2) d v2 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.


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    Quote Originally Posted by Neverlost1 View Post
    The increased energy comes from the moving air mass. This still has nothing to do with the aircraft flying through that same air mass loosing or gaining altitude in a turn. It only makes a difference when you hit the ground at full throttle. Your crash will be more impressive with the wind than against it due to the increased momentum :-)
    If the energy comes from the moving air mass then that means the air speed must change. But everyone agrees there cannot be a change in airspeed
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