RE: Reflexed Airfoil and Flying Wings - Downwash?  
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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 2:45:55 AM   
Ben Lanterman



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Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline 		Tim, It might represent velocities of the air over the wing at that point at which time the velocity along the wing is broken into one horizontal and one vertical component. Velocity I'll buy.

It sure as heck isn't a lift vector. Give me a break about it being on the CG. If it were the downwash lift vector wouldn't it be larger at the trailing edge? Measurements show this to be true by the way I believe. That would mean the lift is maximum at the trailing edge of the wing according to you.

And that simply isn't the case.

Your figure must represent the lift on the wing at the right point and in the right direction. It must correlate to and reflect the other know physical things that exist on the wing while lift is being produced. Your figure has neither. Remember I am not disputing the existence of downwash. It has been mapped and investigated for many years. It effects horizontal tail effectiveness and so on.

But for lift to be the result of downwash you must show more than just a weak drawing. You have to show the force interfaces that is appropriate for every source of force on the wing. You simply can't do it when the approach is from the downwash.




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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 10:17:06 AM   
adam_one


 

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Status: offline 		One of the fundamental laws of physics states:
Any force has an equal force in the opposite direction.
And this is clearly stated in the NASA site as following:
The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction.

It is also clearly stated in the site you have referred to:
the whole purpose of the wing is to impart some downward motion to the air.

For an airplane in steady flight, the forces must balance. We know from the Newton’s third law that for every force there must be an equal and opposite force somewhere.

If you don’t accept all those facts above, there is nothing that can convince you about anything.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 2:40:22 PM   
Crane



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From: Socorro, NM, USA
Status: offline 		Specifically, our topic relates to air deflection by reflexed airfoils. Below are two pleasing airfoils be Pescara - the Eiffel 400 and 401. The Plank shown above used the Eiffel 401. Both airfoils have the same L/D characteristic. The 400 design incidence angle is -1 degree. For the 401 it is +5 degrees. The only difference between the two sections is the chordwise pressure distribution. The trailing edge droop of the 400 generates no more lift at the design AOA than the reflexed TE of the 401. The 400 has a greater max Cl but that has to do with the zero lift angle.




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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 3:34:23 PM   
Crane



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From: Socorro, NM, USA
Status: offline 		Using the lift from down wash analogy it would be difficult to explain the avantage of the V formation of birds. You can observe that this formation provides an advantage to both birds even when flying only in pairs. I believe the bound vortex simply extends to encompass the span of both wings. In large formations the bound vortex extends spanwise over the entire formation. Drag on the lead bird is reduced as long as the angle of the V is not too steep or shallow.

Downwash?

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 3:45:02 PM   
KenLitko


 

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quote:

ORIGINAL: adam_one

One of the fundamental laws of physics states:
Any force has an equal force in the opposite direction.


There is a perfect balance of forces between the pressure on the skin of the wing and the air all around it.

My coffee cup sits on my desk, my desk pushes up on my coffee cup... perfect balance of forces. My coffee cup doesn't need small rockets attached to it in order to stay there. A solid surface performs this feat... so can a moving fluid.

quote:

And this is clearly stated in the NASA site as following:
The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction.


It's a little simplistic. I could also say that the wing actually sucks air from above at the front and shoots it straight out the back... action / reaction and we have lift.

Downwash exists.. I don't dispute that... what I dispute is that it imparts upward momentum to our wings. Pressure and momentum are two different things...

quote:

It is also clearly stated in the site you have referred to:
the whole purpose of the wing is to impart some downward motion to the air.

For an airplane in steady flight, the forces must balance. We know from the Newton’s third law that for every force there must be an equal and opposite force somewhere.

If you don’t accept all those facts above, there is nothing that can convince you about anything.



And you still haven't answered my question...

quote:

ORIGINAL: KenLitko
...what is replacing the parcels of air that are shoved downwards? Where are they coming from and what is their effect on the lift of the wing?


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 3:51:54 PM   
LouW



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Status: offline 		Excellent summary on the topic. From your curves, it is obvious that at a given lift coeficient, the reflexed airfoil has more drag. For instant at a lift coeficient of 16, the reflexed airfoil has a drag coeficient of .059 compared to the chambered airfoil with a drag coeficient of .043, about 30% more.

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 6:25:33 PM   
Ben Lanterman



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Status: offline 		"One of the fundamental laws of physics states:
Any force has an equal force in the opposite direction. "

We agree and it works fine with a nozzle, which is a perfect example.

"And this is clearly stated in the NASA site as following:
The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction.
It is also clearly stated in the site you have referred to:
the whole purpose of the wing is to impart some downward motion to the air. "

I should have said the site was good escept for that comment. They are doing the short cut I have talked about and mixed up vectors and magnitudes. Equal magnitudes (lift and downwash) does not mean opposite vector quantities.

"For an airplane in steady flight, the forces must balance. We know from the Newton’s third law that for every force there must be an equal and opposite force somewhere. "

Of course, no one is disputing that, just the selection of the forces some folks are making.

"If you don’t accept all those facts above, there is nothing that can convince you about anything. "

You have to accept only one fact going in - you can always draw a force diagram on a freebody that describes it's motion. Basic statics and dynamics courses. If forces are related vector wise, equal and opposite in magnitude and location, then you can draw them on a diagram. These are fundamental rules of static and dynamics. No bull, just fact.

It's entirely possible that in their effort to dumb down the lift comment that NASA dumbed it down too much. They violated some basic principles of mechanics in the process. They confused action-reaction relationships describing a nozzle with action-reaction relationships describing a wing. Newton is alive and well but some folks ability to pick the right forces to work with is severely limited. They would get F+ in a classroom.

There is only one real answer to what is lift. It can be drawn with vectors. It obeys every Newtonian law. It does not need downwash to describe it.

If you think downwash is needed to produce lift - draw the freebody diagram showing it in action.



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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 6:27:10 PM   
Ben Lanterman



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Status: offline 		Lou you have mentioned that downwash is a factor in lift, can you do the vector manipulation to show it?


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 6:27:21 PM   
adam_one


 

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quote:

My coffee cup sits on my desk, my desk pushes up on my coffee cup... perfect balance of forces. My coffee cup doesn't need small rockets attached to it in order to stay there. A solid surface performs this feat... so can a moving fluid.

You can have your coffee cup on your desk, but you can't have your coffee cup hanging up in the air without sending the air down by some means...


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 7:09:43 PM   
LouW



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Status: offline 		You are correct in your conclusion that in tip to tip flight, the bound vortex extends to encompass the span of both wings. The tip vortices normally associated with the two tips essentially cancel each other and the result is a wing that has an effective span wider than the sum of the effective spans of the individual wings. By eliminating two of the four vortices that would occur when the wings are operating separately, the induced drag is cut approximately in half.

From the viewpoint of momentum, the wider effective span causes the wing to effect a larger mass of air per unit of time. With this larger mass of air, less acceleration (deflection) is necessary to achieve the same lifting force. Less deflection means that the resultant force is more nearly perpendicular to the direction of flight producing less induced drag (from the rearward tilt of the resultant force).

Regardless of which viewpoint you take, the results are the same. As I have said before, pressure field theory and momentum theory are not mutually exclusive ideas. They are just two ways of looking at the same phenomenon. I am always a bit amazed at the fervor with which the arguments proceed since both approaches are valid.

It’s even more surprising when birds manage to figure all this without math or the understanding of a particular theory.

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 7:30:23 PM   
Crane



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Status: offline 		I agree. In both cases we are discussing analogies. I assume that the actual lift mechanism is very complex.

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 7:41:51 PM   
Ben Lanterman



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Status: offline 		Lou the difference is one is right and the other is wrong and based on a wrong assumption.

The mechanism of lift is simple. Air goes around the wing in a pattern determned by wing shape, angle of attack and velocity.

In the process a pressure field is developed and the summation of that pressure field is lift. It can be quantified and given a hard line vector resultant.

You can do that with downwash, notice I didn't say momentum. Of course a momentum accrounting gives pressures and velocity fields.

People are confusing momentum of the flow field with downwash. Then they say downwash is causing lift.

Unless you can manipulate the downwash vectors in some form around to the CG then the concept is wrong.

It is foolish to continue thinking a wrong concept when the correct one is right at your finger tips.

And then along comes Jef R. and says all aero guys are wrong and lift is like rubber bands and all the wing does is divert the downwash and that's where the real lift is.

It is an insult to every guy that sat in a classroom and a desk for years working in the field. We certainly were so ignorant as to have studied it wrong! There arn't different theories of lift, there is one right one and a dozen wrong ones.

It's that simple.


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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 7:54:43 PM   
Crane



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From: Socorro, NM, USA
Status: offline 		Lou,

Actually it is not that dramatic. For comparison you need to use the best L/D point as the reference. That gives:

E-401 AOA........E-401 L/D.......E-400 L/D.....E-400 AOA

..........+5...........>22................<22...............-1

..........10............15..................<17..............+4

...........0..............5.....................5................-6

The Cls are about equal. The reflex leads a bit on the left side and the 400 leads a bit on the right. There is no significant difference.

The reflex has no significant impact on lift and drag - only on chordwise pressure distribution.

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RE: Reflexed Airfoil and Flying Wings - Downwash? - 3/22/2005 7:55:05 PM