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Old May 11, 2012, 10:45 AM
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Originally Posted by richard hanson View Post
frankly this idea is hogwash. -always was always will be yes I have heard various guys claim this - never seen it .
you can get close -but mother nature can not be fooled .
the trim for upright must fight gravity
turn it over and unless there is a miracle at work - a different trim(altho it can be slight) is required.
The world of aeronautics is full of crackpot ideas.
It's easy to believe these ideas because some want to believe em.
Put it to the test .
I will post to the RC slope forum and ask that they put it to the test. Steve
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Old May 11, 2012, 10:48 AM
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Originally Posted by richard hanson View Post
I don't believe in miracles- and hands off zero trim setups
OR
downwash on the tailplanes as having any effect or even existing!.
Well, that pretty much definitively shows, how much weight we should give your opinion on this subject.

Understanding the decalage settings, of say, free-flight model airplanes, is pretty much impossible without considering the effect of the wing's downwash on the tail. Particularly the models that are set up to climb near vertical (very low a-o-a) under power and then glide near min sink (high a-o-a) power-off, all with no change in elevator position.

Steve
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Old May 11, 2012, 10:48 AM
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Interesting --
I went outside and jumped up n down as hard as I could -and the patio never jumped back at me - I yelled at the patio and asked why it wasn't attracted to me -
Now I feel bad.
What am I doing wrong?
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Old May 11, 2012, 10:50 AM
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Well, that pretty much definitively shows, how much weight we should give your opinion on this subject.

Understanding the decalage settings, of say, free-flight model airplanes, is pretty much impossible without considering the effect of the wing's downwash on the tail. Particularly the models that are set up to climb near vertical under power and then glide near min sink power-off, all with no change in elevator position.

Steve
FWIW we went thru this stuf meeeeeny years ago with free flights and never worried about downwash on the tail as quite frankly it is not relevant.
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Old May 11, 2012, 10:55 AM
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I will post to the RC slope forum and ask that they put it to the test. Steve
OK-and just exactly how will a slope glider demonstrate this ?
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Old May 11, 2012, 10:59 AM
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Originally Posted by BMatthews View Post
Could it be that these pressure changes above and below the wing mimic this same air pressure gradient caused by the balloon? ...Note that no downwash is needed in this way of looking at the how of producing lift.
The difference between a ballon and an airplane, is that the balloon is no heavier than the air that would occupy the same space if the balloon were not there. Therefore the balloon's gravitational upward pull on the earth is no larger than the upward pull that would be exerted by the air molecules that would otherwise occupy the same space. Therefore the balloon does not attract the earth upwards, even if it exerts no downward force on the earth, (above and beyond the down force that would be exerted by the weight (pressure) of the air molecules occupying that space).

In contrast, the airplane weighs much more than the air molecules it displaces. So its gravitational pull does attract the earth upwards. Except for the fact that the airplane is, on average, exerting a 1-G downforce on the earth. Because the "push" the airplane exerts on the air is, on average, 1-G downward, aimed at the earth. The airplane can push the air in other directions too-- like when creating earthward lift when inverted at the top of a loop-- but the average push is aimed at the earth and exerts a downward force on the earth equal to the plane's weight. This prevents the earth from being attracted upwards toward the aircraft.

Steve
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Old May 11, 2012, 11:00 AM
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OK-and just exactly how will a slope glider demonstrate this ?
By demonstrating pitch-stable flight inverted as well as upright, with no pitch-trim change and no inputs on the pitch stick. Steve
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Old May 11, 2012, 11:03 AM
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FWIW we went thru this stuf meeeeeny years ago with free flights and never worried about downwash on the tail as quite frankly it is not relevant.
Sure, you can make the plane fly without understanding all this.

But it is relevant if you want to actually calculate the various pitching moments-- see Frank Zaic's "Circular Airflow and Model Aircraft". Pages and pages worth of examples.

I can't believe anyone is trying to argue that the downwash from the wing exerts a negligible effect on the tail's upforce/ downforce...

Steve
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Old May 11, 2012, 11:05 AM
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Interesting --
I went outside and jumped up n down as hard as I could -and the patio never jumped back at me - I yelled at the patio and asked why it wasn't attracted to me -
Now I feel bad.
What am I doing wrong?
Yes but it did push back at you. Otherwise you would be currently accelerating downward toward the center of the earth.

Steve
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Old May 11, 2012, 11:21 AM
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Sure, you can make the plane fly without understanding all this.

But it is relevant if you want to actually calculate the various pitching moments-- see Frank Zaic's "Circular Airflow and Model Aircraft". Pages and pages worth of examples.

I can't believe anyone is trying to argue that the downwash from the wing exerts a negligible effect on the tail's upforce/ downforce...

Steve
Believe it -
In many cases it does not apply
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Old May 11, 2012, 11:23 AM
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Yes but it did push back at you. Otherwise you would be currently accelerating downward toward the center of the earth.

Steve
I always thought this old example was weird as a clockwork grapefruit .
Just because it is an old textbook example does not mean it is a good one.
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Old May 11, 2012, 11:27 AM
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Believe it -
In many cases it does not apply
What is the proof of this assertion or reasoning behind this assertion?

Steve
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Old May 11, 2012, 11:29 AM
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I always thought this old example was weird as a clockwork grapefruit .
Just because it is an old textbook example does not mean it is a good one.
I'm not reading any textbook at the moment, just using common sense. You really don't believe the floor is pushing up on your feet as you stand? How else can the net force on you be brought to zero?

Sorry this is too obvious to talk about, let's go back to pitch stability.

Steve
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Old May 11, 2012, 11:36 AM
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What is the proof of this assertion or reasoning behind this assertion?

Steve
EZ- demonstrate the physical arrangement where this does have an effect
I am sure there are arrangements where the disturbed air from the wing (wings) screws up the flow seen by the horizontal and vertical stabilizers.
been there - proven it
However - this is not a constant - it happens at certain AOA and speed and sizes of airframes
I have proven that disrupting flow to the vertical stabalso does mess up yaw stability ( many of us have seen this ).
Honestly - you should try some applied info . It really is helpful.
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Old May 11, 2012, 11:39 AM
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I'm not reading any textbook at the moment, just using common sense. You really don't believe the floor is pushing up on your feet as you stand? How else can the net force on you be brought to zero?

Sorry this is too obvious to talk about, let's go back to pitch stability.

Steve
Ah- the old net force argument .love it
It's just a model -
pitch stability? -- have at it - I deal with this all the time -in actual practice .
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