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Old May 10, 2012, 05:50 PM
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Originally Posted by Brandano View Post
. Overall, the air that gets disturbed stays on average in the same place, like for a normal wing section.
I've seen contrails split into two portions-- one portion contains the entrained tip vortices, visible as 2 distinct cores. Another portion is more of a diffuse sheet. The portion containing the tip vortices clearly sinks away from the other portion, often ending up quite significantly below. The effect is most visible with 4-engine aircraft. Certain specific weather conditions are needed to make both portions linger long enough to create a clearly visible separation.

I'm having trouble with the idea that air, on average, "stays in the same place" for a normal wing section.

The effect of wing downwash on tail pitching moments is very well known.

Steve
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Old May 10, 2012, 05:58 PM
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Originally Posted by aeronaut999 View Post
....The effect of wing downwash on tail pitching moments is very well known.
In my experience with models this has not been an issue. I've not noticed any need to modify a model going from a low to T tail.

This may change when the wing is able to be more heavily loaded such as on full size aircraft and where wing wash is stronger and likely longer lasting.
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Old May 10, 2012, 06:05 PM
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1) How can what happens behind the wing make any difference?
From the earth's point of view it makes a difference-- it is part of the way that the plane pushes down on the earth, so that a scale under a block of air containing the plane will feel the plane's weight (or more precisely, will feel the plane's (weight x G-load.)

But, I'm no longer arguing that the downwash is the only way that the plane can exert a downward push on the earth.

As far as whether what happens behind the wing makes a difference to the wing-- I'm really not saying that downwash is the "cause" of lift. I'm just saying that the wing cannot make lift without exerting a downward force/ shove/ push on the air. Since the air is a fluid not a bunch of bb's in a vacuum,. the resulting momentum transfer to the air is affected (decreased) by many variables. These complexities allow the downward "push" of the plane against the air to occur in a way that is not entirely translated into downward momentum. Some of the "push" is instead absorbed by the shearing forces within the air. Regardless of the exact form that the wing's "push" against the air takes, if the "push" happens to be aimed at the ground (e.g. normal upright flight), then the ground will "feel" the push, i.e. a scale under a vat of air containing the plane will register the plane's G-load. This will not be the case if the "push" is aimed in some other direction (e.g. the plane is inverted at the top of a loop.) Over the long run, the fact that the ground does feel the downward push of the wing, cancels out the upward gravitational attraction that the plane exerts on the earth.

Steve
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Old May 10, 2012, 06:08 PM
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Originally Posted by ShoeDLG View Post

2) Surely you aren't suggesting a wing can create lift without deflecting air downward. Next you'll be saying you can stand behind a turning propeller and not feel the momentum. There have been several threads where people have insisted downward momentum is required for airplanes to fly, so it's true. The physics behind lift must be simple (I mean you can actually feel the momentum). Look how many words you've already had to use to suggest otherwise.
Was this aimed to me? Generally it seems to me that the creation of lift does indeed involve the downward deflection of air. I'm just saying that the downward momentum transfer that is required to create a given amount of lift is less if we are flying in honey (high shear forces) than if we are flying in air (low shear forces.)

Steve
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Old May 10, 2012, 06:17 PM
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Originally Posted by BMatthews View Post
An odd train of thought occurs to me in connection with this.

In some other thread a while back the case of a lighter than air ship came up in connection with the "how" of why it floated in the air. The upside was that the envelope filled with the lighter than normal gas in the floatation bag provides lift by being big enough vertically that it uses the natural pressure gradient of the atmosphere to end up with the pressure at the top side being enough less than the pressure from below. So the air simply pushes the balloon or blimp upwards.

Let's look at our airplane again. We've got a low pressure zone above the wing and a higher pressure below the wing. Could it be that these pressure changes above and below the wing mimic this same air pressure gradient caused by the balloon? In effect the wing produces a "lighter than air" zone above the plane and a "heavier than air" build up of pressure below that mimics the effect of the pressure gradient found operating on a LTA airship? I don't have the math to show this idea one way or the other but it sort of makes sense.
Certainly a diagram of the pressure field around an airfoil shows more high pressure below, and more low pressure above-- if it were the other way, the direction of lift would be reversed. Yes, regardless of what is happening behind the airfoil-- but I still think there is some unavoidable connection between the pressure field around the airfoil and the downwash behind it. I don't think that a lifting airfoil can be making an upwash. Sure if the TE is reflexed we might have a little upwash but it will be followed by a larger downwash. But I don't really want to get into splitting hairs over whether the downwash is somehow "causing" the lift-- I'd just say that it is unavoidably associated with the creation of lift. But less so in fluids that are highly viscous (high shear forces.)

And certainly the pressure around the surface of a lifting gas envelope (balloon) must be higher on the bottom than on the top-- the difference being related to the weight of the air that is displaced--
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Old May 10, 2012, 06:24 PM
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Originally Posted by ShoeDLG View Post
Is it possible for a wing to fly without adding any net momentum to the air? Absolutely. Just seal a hangar and fly a plane inside it. Because a sealed hangar prevents the CG of the air from translating, the net momentum of the air in the hangar must remain zero at all times.
Yes. The reason that the net downwash is zero, is that it is hitting the floor of the hangar and imparting a downward force to the floor. The floor is pushing up on the downwash, bringing its momentum to zero, and the downwash is pushing down on the floor, which prevents the earth from being attracted upward by the plane's gravity. Yes, on the planetary scale, there is no net downwash. If the scale we are talking about is large enough to include the earth's surface, there is no net downwash.

Does a scale under the hangar floor feel the plane's weight? Yes. And this is in some part (I'm no longer saying entirely) due to the downwash impacting the floor, and giving up its energy by imparting a downforce on the floor.

Steve
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Old May 10, 2012, 06:29 PM
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effect of wing downwash on tail

Quote:
Originally Posted by BMatthews View Post
In my experience with models this has not been an issue. I've not noticed any need to modify a model going from a low to T tail.

This may change when the wing is able to be more heavily loaded such as on full size aircraft and where wing wash is stronger and likely longer lasting.
The effect of wing downwash on tail pitching moments is well known to modellers. Frank Zaic's book "Circular airflow and model aircraft" is chock-full of examples, and I'm sure that Martin Simons treats the subject in "Model Airplane Aerodynamics".

Take a model with a symmetrical wing airfoil, and a symmetrical tail airfoil, and zero decalage. Assume the elevator (if there is one) is centered. How can such a model be stable in pitch? Because the tail is in the wing's downwash, effectively creating decalage, so that the angle-of-attack of the tail is smaller than the angle-of-attack of the wing. Flip the plane inverted and it will still be stable-- the tail will still be in the wing's downwash, still creating effective decalage, even if the geometric decalage is zero.

Steve
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Old May 10, 2012, 10:24 PM
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Originally Posted by aeronaut999 View Post
The effect of wing downwash on tail pitching moments is well known to modellers. Frank Zaic's book "Circular airflow and model aircraft" is chock-full of examples, and I'm sure that Martin Simons treats the subject in "Model Airplane Aerodynamics".

Take a model with a symmetrical wing airfoil, and a symmetrical tail airfoil, and zero decalage. Assume the elevator (if there is one) is centered. How can such a model be stable in pitch? Because the tail is in the wing's downwash, effectively creating decalage, so that the angle-of-attack of the tail is smaller than the angle-of-attack of the wing. Flip the plane inverted and it will still be stable-- the tail will still be in the wing's downwash, still creating effective decalage, even if the geometric decalage is zero.

Steve
No-
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Old May 11, 2012, 02:52 AM
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Sounds like a great time for a simple all sheet test glider.

Actually I guess many free flight modelers and I have already done this. With contest handlaunch gliders we build them to be 0-0 then warp in just a skosh of up trim on the trailing edge of the stab. If we don't then they simply are not stable enough to fly and not end up doing something crazy. So if there is some amount of downwash induced decalage it is extremely minimal.
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Old May 11, 2012, 07:01 AM
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I don't believe in miracles- and hands off zero trim setups
OR
downwash on the tailplanes as having any effect or even existing!.
Built too many models ----
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Old May 11, 2012, 10:01 AM
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effect of wing downwash on tail

http://www.av8n.com/how/htm/aoastab....eyond-decalage

"The tail flies in the downwash of the wings. This reduces stability, again because it reduces the steepness of the tail’s lift versus angle of attack curve. The air flowing off the back of the wing tends to flow straight off the trailing edge, regardless of the angle at which it approached the wing. Also, when the airplane’s overall angle of attack changes, the aft wing can move in or out of the forward wing’s wake. This changes the lift curve of the stabilizer in ways that are hard for designers to predict. Further, any change in the downwash pattern can move the angle of attack to a new equilibrium point. Therefore, on most aircraft, extending the flaps perturbs the trim speed, as discussed in section 5.5."

Also:

http://www.allstar.fiu.edu/aero/axes33.htm

"On most airplanes, the stabilizer appears to be set at an angle of incidence that would produce an upward lift. It must, however, be remembered that the tailplane is in a position to be in the downwash from the wings. The air that strikes the stabilizer has already passed over the wings and been deflected slightly downward. The angle of the downwash is about half the angle of attack of the main airfoils. The proper angle of incidence of the stabilizer therefore is very important in order for it to be effective in its function."

Also:

http://history.nasa.gov/SP-367/chapt9.htm

"Finally, with respect to the tail, the downwash from the wing is of ...considerable importance. Figure 136(a) shows how the air is deflected downward when it leaves a wing. This deflection of air results in the wing reaction force or lift. This deflected air flows rearward and hits the horizontal-tail plane. If the airplane is disturbed, it will change its angle of attack and the downwash angle also changes. The degree to which it changes directly affects the tail effectiveness. Hence, it will reduce the stability of the airplane. For this reason, the horizontal tail is often located in a vertical location such that it is exposed to as little downwash as possible, as shown in figure 136(b).."

It is an interesting paradox that the downwash generally reduces the tail's stabilizing effect, yet the downwash also makes possible stable flight in the case of the aircraft with symmetrical wing and zero decalage.
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Old May 11, 2012, 10:06 AM
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Originally Posted by richard hanson View Post
No-
Yes.

Are you not aware that there are some aircraft that are pitch-stable in upright flight, and also are pitch-stable in inverted flight, with no change in elevator position?


I believe that this is only possible because of the way the wing's downwash affects the flow at the tail, effectively increasing the decalage.

In general I believe that this situation is only possible if the wing airfoil is symmetrical or relatively close to it, and if the geometrical decalage of the tail surface (considering also the effect of the elevator, if deflected) is near zero relative to the wing. I might be wrong about that.

Whatever the details, I'm sure that the wing's downwash plays a key role in making this possible

Here is a link to a discussion of this on RC groups. No definitive conclusion was reached. The role of upthust/ downthrust was mentioned, but I'm sure that I've heard of this same phenomenon in RC sailplanes also. The wing's downwash, increasing the effective decalage of the tail, has to be key.

http://www.rcgroups.com/forums/showthread.php?t=1414003

I'm sure some more googling with terms like "zero decalage", "stable inverted with no pitch trim change", "downwash", etc would shed more light.

Steve
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Old May 11, 2012, 10:23 AM
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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 .
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Old May 11, 2012, 10:37 AM
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/\ /\
This.
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Old May 11, 2012, 10:44 AM
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General thoughts on creation of lift

General thoughts on creation of lift

We know for sure from Newton's laws that if the air is pushing or shoving the aircraft upwards (exerting an upforce on the aircraft), then the aircraft is pushing or shoving the air downwards (exerting a downforce on the air).

Whether or not the air accelerates downward, or how much, is another matter. Just as the aircraft need not accleerate upwards.

Models for creation of lift (or thrust from a prop, for that matter, same difference)

1) Non-viscous fluid with no molecular interaction-- We are standing on an ice rink on the surface of which are some bowling balls. We accelerate ourselves in the westward direction by scooping up bowling balls and hurling them to the east.

2) Fluid with molecular interaction- the bowling balls which are distributed about the surface of the ice are connected by long stretchy springs. As we pick up a bowling ball and hurl it to the east, we still accelerate to the west, but there is no net movement of the bowling balls. Assuming that there is some constraint at the edge of the fluid. If there is no such constraint, the whole glob of balls will be accelerated to the east a tiny bit. In the latter case, is the momentum that we originally imparted to the balls, conserved? It appears to me that it is conserved.

3) Viscous fluid- there is a 6-inch-deep layer of honey on the surface of the ice. When we hurl one ball to the east, it tends to quickly slow, and drag other balls with it (through the shear forces transmitted through the honey). The velocity quickly damps out to near zero. Is the momentum also damped out to near zero, or simply distributed among more balls? This is not obvious to me. To the extent that the shear forces end up heating the honey, I suppose some momentum is lost.

Key point-- in every case, we propelling ourselves by shoving/ thrusting/ forcing balls opposite the direction in which we are trying to create a force. And in every case, IF there is a barrier wall at the far edge of the ice ring, and our shoving/ thrusting/ forcing effort is pointed at that barrier, then that barrier will feel the force that we are exerting on the bowling balls. Whether that "feeling" comes in the form of the actual balls we have thrown crashing full-speed against the barrier, or the balls near the edge of the barrier pushing against the barrier with little or no velocity (cases 2 or 3). We don't need the barrier to create force, but if it exists, it will feel the force we are creating. Regardless of whether not the momentum we impart to the balls is conserved or dispersed into heating, etc. The barrier will still feel the force that we are imparting to the balls.

So to does the ground feel a downforce when an aircraft creates upward lift. This prevents the ground from being attracted upward, by the plane's gravitational pull.

Steve
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