RE: Jef Raskin article on aerodynamics in Fly RC Magazine  
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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/19/2005 8:58:48 PM   
adam_one


 

Posts: 398
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline 		Here it goes:

First you should bear in mind that an aircraft's NP (Neutral Point) is the position through which all the net lift increments act for a change in angle of attack.
The major contributors are the main wing, stabiliser surfaces and fuselage.

Referring to the picture below:
It shows a wing "sweeping" through a volume of air in unit time.
The air (or the wing) moves with a velocity V, and as a result of the action of the wing, the air is given a downward velocity w in that unit of time.
The force that the wing exerts to make the air change its momentum downwards is the same as the force lifting upwards.

The force, F is equal to the mass of the air, m times its downwash velocity, w.
The mass of air is given by its volume times its density, r (rho), and the volume in unit time is V times the "swept" area A.



Note that the lift L is only approximately equal to the force F on the wing due to the air being "pushed" down because the force of the air being pushed down moves the wing both upwards and rearwards where the rearwards action corresponds to drag.

For those who have difficulty visualising the dynamic effect of the wing moving through the air, just look at the picture below:

See how the air above the wing is deflected down and aft, while the air below the wing is deflected down and forward.
Conclusion:
Wings move the air mass downward (action) which makes them being pushed upward (reaction). That's what makes lift. All the rest is just secondary details.

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< Message edited by adam_one -- 2/21/2005 9:31:22 PM >

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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/19/2005 9:18:22 PM   
dick Hanson



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Status: offline 		Well -it looks like the obvious has once again flogged to pulp----

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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/19/2005 11:53:45 PM   
Johng



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quote:

ORIGINAL: dick Hanson

Well -it looks like the obvious has once again flogged to pulp----


OK, call it a day. I'm buyin' netbeer to celebrate the fact that I more or less sidestepped this one while only post twice - well 3 times now.

Aren't you glad you spent that time here instead of building something in the shop?

BEERBEERBEER
BEERBEERBEER
BEERBEERBEER
BEERBEERBEER



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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 2:21:03 AM   
Siefring



Posts: 152
Joined: 10/22/2003
From: Owings, MD, USA
Status: offline
quote:

Kudos to all and everyone take a bow

I agree Bruce. My background is Electrical Engineering and Physics. Six months ago I did not know squat about aerodynamics. For me, this forum has been like a series of intense college courses and it cost me nothing. It does require filtering to get the wheat separated from the chaff, but if you participate, you usually get corrected pretty quickly . I personally have had a lot of light-bulb moments following the discussions here. Most recently Adam's animated gif made me realize some things about flow in the non-wing coordinate system.

I was originally bothered in this thread by Ben's seemingly hard attitude about not putting downwash and force in the same sentence. I’ve changed my mind. If you are going to explain a concept with words and not mathematics, you have to be extremely careful about the wording and definitions. If you say “downwash force” or “downwash lift” it is wrong. Downwash is “flow”, “velocity”, “ a ripple in the local wind”, “a motion”, or “momentum”. It’s okay to say, “Lift and downwash are the result of the action/reaction forces on the wing”. If you say downwash is “created by” or “results in” lift it’s simply wrong. You’re basically saying force is directly proportional to velocity etc. Blah, Blah

quote:

OK, hang on a second while I don my Nomex underwear....


Bruce, since you’ve got the Nomex on, please avoid sentences like “downwash energy can also be shown to be equal to the lift”. It’s just as bad to make the same mistake with Energy and Force.

I’ve been thinking about your energy arguments. Normally it is easier to follow the energy than the momentum in a system. Not a vector quantity after all. In a fluid flow, if you get rotation you can get kinetic energy stuck in rotating vortices. So mathematically, I don’t think you can write a nice potential energy to kinetic energy relationship that works everywhere around the wing. Not very rigorous, but I just hit the end of my fluid theory understanding, please correct me if I am wrong.

Carl

John the beer was a big help.

(in reply to BMatthews)
       Post #: 129

RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 2:23:14 AM   
Ben Lanterman



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From: St. Charles, MO, USA
Status: offline 		good try Adam, but all you have done (which was agreed on 4 pages ago) is show downwash is equal to lift ( lift = downwash ) in magnitude only, how about direction and point of application?

Your neat figure doesn't say either how the downwash force got from going down aft of the wing to pushing up on the wing does it?

That is the question to be answered.

Remember the little Lift vector in the figure is not pulling up on the wing except in our vector convention, real air cannot pull on anything - period. First law of aerodynamics (or should be). Air cannot pull. The real physical fact is that lift is actually under the wing pushing up at the 25% chord point. Agreed?

Want to think about it some more. You can't use anything that says the magnitude of the two are equal, we already know that. You are getting a vector that doesn't pull, and is located at the 25% chord point on the airfoil.

Oh yes the delta increase in pressure under the wing that you say is due to the wing moving forward is a small fraction of the total pressure lift (oops, I said pressure lift again). I remember seeing that it was about 15 or 20 percent of the total but I might be mis-remembering the exact number. The bottom line is that it doesn't account for the total lift on the wing.

Go for it :-)




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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 2:30:42 AM   
BMatthews



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quote:

ORIGINAL: Tall Paul

......One reason the old c.p. idea vanished about 1935 was the mathematical reality that the c.p. had to move to infinity in some conditions to satisfy the math.
As this can't happen, the pitching moment was quantified as the force reacting (nominally) at the 25% chord point.


But, but, but.......

For most real world situations the CP method worked fine and the pitching moment we use now is a reflection of the real Cp moving around within some reasonable confines. Supposedley a stall was defined back then as the point where the Cp moved strongly towards the leading edge and then suddenly back as the air speparated fully IIRC. I'm curious about what case would make the Cp move to infinity and what the effect would be on the Pm. I'm guessing it had something to do with the sudden Cp travel during a stall? After all if the true Cp can move to infinity then I would think it stands to reason that the Pm should suddenly spike to infinity as well. Mathematical tricks notwithstanding.

I agree Dick and John. It's Miller time everyone. Other than my one little Epiphany late in this thread I see no movement here on our own obvious personal fervor in what constitutes the primary cause of lift that the wing sees.


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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 3:35:02 AM   
Tall Paul



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Joined: 6/23/2002
From: Palmdale, CA, USA
Status: online 		An airfoil at its zero-lift angle, such as a Clark-y, still has a pitching moment while the c.p. is rapidly going... somewhere...chart 1
The c.p. at this condition is a result of the Cm divided by the lift... the lift is zero.
Division by zero isn't allowed.
The c.p. can't vanish and then reappear as an airfoil passes thru zero.
The pitching moment OTOH remains in effect at zero.
The idea of c.p. relied on the assumption that lift was responsible for the pitching moment.
For symmetrical airfoils, the c.p.'s motion due to the divsion by zero appeared as shown in chart 2, ... a gap at zero, with Cl and Cd continuous thru the region.
It was noted that symmetrical airfoils at zero lift have zero pitching moment, around the 25% chord point.
Dividing zero by zero yields zero, therefore it was determined that Cm operates around the 25% chord point, with symmetrical airfoils have no pitching moment regardless of alpha. This was predicted by Kutta and Jukowsky in the first decade of the 20th century.
This lead to the discovery that any cambered surface has a pitching moment about the 25% chord, and this Cm is usually constant.
NACA adopted Cm in lieu of c.p. in the 1930's.
C.p.is still mentioned in some aero discussions, but is an obsolete explanation..
I've seen an animated demonstration recently on the 'net where the c.p. begins ahead of the wing, moves across and off the trailing edge with changes in alpha. Ahead of the wing, and behind it there's nothing for the c.p. to be acting on, in reality.
*
From Martin Simons, "Quiet Flight International 15" August '95

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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 3:58:18 AM   
adam_one


 

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From: Stockholm, SWEDEN
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quote:

Your neat figure doesn't say either how the downwash force got from going down aft of the wing to pushing up on the wing does it?

Ben, one should see the downwash as a reaction to the wings' action.
The static vector forces are shown on the figure below.
As you know a force upwards requires a force downwards (refer to Mr. Newton).

The fact that the downwash is mostly visible towards the wing's TE is simply because the wings are moving through the air mass that has inertia which delays the air's downward deflection.
Actually, the wings' down force is applied to the air mass long before the air reaches the trailing edge.

Attachments
Click to see fullsize image.
Click for fullsize


< Message edited by adam_one -- 2/20/2005 3:01:13 PM >

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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 11:33:13 AM   
SST



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From: Mt. Morris, MI, USA
Status: offline 		AAAAAAARRRRRRRRRRRRGGGGGGHHHHHHHHH!!

Returning to this post is like picking scabs...I don't want to do it, it doesn't do anything good for me, but I have this sick compulsion to see just how far into the ground this dead horse can be beaten.

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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 12:21:40 PM   
JimTrainor


 

Posts: 1308
Joined: 10/25/2002
From: Ontario, ON, CANADA
Status: offline 		I don't understand all the fuss...

The plane can't stay up with out a force....

The air flow can't change direction or speed without a force acting on it...

Why try to even establish which is the cause, or what comes first, or is primary, or whatever, when the two are fully coupled?

(in reply to SST)
       Post #: 135

RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 12:38:22 PM   
adam_one


 

Posts: 398
Joined: 2/26/2003
From: Stockholm, SWEDEN
Status: offline
quote:

AAAAAAARRRRRRRRRRRRGGGGGGHHHHHHHHH!!

Returning to this post is like picking scabs...I don't want to do it, it doesn't do anything good for me, but I have this sick compulsion to see just how far into the ground this dead horse can be beaten.
Oh yes, the curiosity killed the cat!
This is a discussion forum where you may agree, disagree or just be indifferent.
The topic is controversial - I know, but so what?
If you don't like it, you are always welcome to ignore it.

Tim Trainor:
Well said.
It's so amazing how people just refuse to see what's so obvious!




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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 3:43:17 PM   
Ben Lanterman



Posts: 1284
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline 		Jim - the reason is for most people it doesn't make any difference just like no one cares about the details of how the surgeon that cracked my chest and did the heart operation worked. But to other surgeons and to a guy who thought that aerodynamics was an established physical science all this life might try to educate others that don't fully understand the concepts. It can be fun and it all comes together when you do it right. Understanding for it's own sake is a good thing, ignorance because someone just doesn't have enough information isn't a good thing. It is a payback for me having to suffer at a desk in a cubical for all those years :-)

"Ben, one should see the downwash as a reaction to the wings' action.
The static vector forces are shown on the figure below.
As you know a force upwards requires a force downwards (refer to Mr. Newton). "

We have said that the downwash is the action we describe as the flow moves down into the low pressure region above the wing. Only in that respect is it a result of the wing's action. Nothing more.

The static vectors in the figure are due to pressure forces only.

The vector going down is the weight, the vector up is the lift due to pressures.

"The fact that the downwash is mostly visible towards the wing's TE is simply because the wings are moving through the air mass that has inertia which delays the air's downward deflection. "

Correct

"Actually, the wings' down force is applied to the air mass long before the air reaches the trailing edge."

The wings down force (weight) in the figure is reacted by the pressures on the lower force on the wing. Action - reaction. The air pressure can be replaced by your hand - we are taught that any pressure field can be replaced by a force that sums the action of the pressure field and duplicated the action of the pressure field.

However again, show me where the down wash as a force due to moving air masses gets around to push up on the airfoil at the 25% location of the wing. In statics and dynamics we are taught that a mass can be pushed or pulled by a force vector (steel cables for example). However air can't pull. Air can only push. For the downwash to be a prime cause of the lift it must reorient it's force vector potential and come around to underneath of the wing.

Unless you can show that it is not the cause of the lift on the wing.



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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 4:32:47 PM   
LouW



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From: Moreland, GA, USA
Status: offline 		From the initial post, this thread has been about emotion and belief rather than technology. The question is not whether a wing generates lift by causing variation of pressure over it’s surface or by deflecting air downward, because both are true. It’s more a viewpoint thing, like whether one is a Democrat, or Republican.

For instance, there has been much written in the comments about flow. There is actually no flow at all. The wing passes through air that is essentially stationary. Viewing it as flow facilitates calculating pressures generated by its motion, but clouds the understanding of its effect upon the air. As the wing passes, the chunk of air affected by the wing is accelerated downward, and slightly forward (not at some shallow angle as mentioned in several posts). Maximum downward velocity occurs just after passage of the wing but the maximum acceleration occurs directly over the wing. The force accelerating the air downward is the sum of pressures generated by the wing’s passage. The equal and opposite force is the lift developed also by the same pressures generated by the wings passage. The slightly forward acceleration is reflected in the resistance to the motion of the wing (drag) that must be countered by whatever motivating force causes the wing to move through the air. Having a zero stress-strain characteristic, an inviscid fluid simply cannot develop an unsymmetrical pressure field and thereby exert a force without a corresponding acceleration of some mass of the fluid.

The statements that if the wing were an air deflector, all the work engineers do in designing to an optimum pressure distribution for increased efficiency wouldn’t be necessary, is a little ridiculous on it’s face. The pressure distribution necessarily effects the wing’s efficiency in deflecting air and is a worthy goal regardless of how one looks at the action.

The argument will never be settled as long as each side holds a fanatical belief that only their way of looking at the matter is valid, when in fact both are true.

< Message edited by LouW -- 2/20/2005 4:34:41 PM >

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RE: Jef Raskin article on aerodynamics in Fly RC Maga... - 2/20/2005 6:11:59 PM