rmh

Ok- no controline guys -a couple of 3D guys - some sport flyer guys and plenty of don't fly nothin guys .
Time to bust out the referrence books .

BMatthews

True up to a point Siefring. At very low velocities the laminar flow may even be maintained the whole way around the wire. I suppose the proof would be to look at the drag of a wire and an airfoil of the same thickness.


Way back up there somewhere I posted about how I think the air interacts with objects. I'm still wondering about that. The source of the wing's lift still comes down to how the air actually interacts with the actual surface of the wing.

At this point I think we are all comfy with the idea that the aifoil interacts with the air to produce an acceleration of air downwards and a localized air pressure over the wing's surface simultaniously.

So here's the question. What is the nature of how energy is transferred to an object by moving air? As we've shown air is not a stream of bullets. I think it's safe to say that air interacts with an object moving through it by moving around it and zones of low and high pressure are created as the air piles up and moves from areas of higher relative pressure to lower relative pressure. The movement itself does not actually do anything to the object. Only the transfer of energy via these higher and lower relative pressure zones is seen by the object.

If no one has an issue with that then I would suggest that it is thus the pressure that provides the lift even though the energy required to support that lift comes from the downwash. The downwash air movement does not actually push, pull, suck or blow anything as it flows over the wing but it does interact with the wing through the pressure distribution created by the proximity of the wing and the downwash. Going back to that fabric covered airplane with the upward bulging fabric. Only a pressure difference between the air inside the wing and the outside of the wing would generate that bulging. And it's that bulging that is lifting the airplane.

Oddly enough it is the passage of the wing though the air that first forms the airflow and pressure distribution so that the wing can then use that energy flow. It still seems like the wing is pulling itself up by its own bootstraps. The wing induces the flow that it derives the lift from. Inducing that flow must take up energy but it then gets some of that back. It must because the lift force is always more than the drag force.

rmh

It is far easier than everyone makes it out to be .

Air is just like water --in that it takes the path of least resistance
air just compresses surrounding air when an obstruction is pushed thru it.
On tiny slow wing - it is easy to compress and slide around it, so very little air gets trapped into doing work.
on big wing - the larger plate /chord whatever - the air can't easily compress and get out of the way - so it has to take the next path --over / under the wing -whichever is easiest.
The fancy foot work -downwash/hogwash/ whatever -is simply air once trapped into unequal pressure zones -which takes the easiest path back to equalibrium.
The wing which produces the most UNBALANCE in pressure at a given speed is the most efficent.

adam_one

Bruce,
How about that like the chicken and the egg or is the cup half full or half empty?
Here's my half cent:
Hold a piece of paper so that it curves downwards, then blow across the top - the paper will rise. However, most people don't realize that the paper would not rise if it is flat, even though you are blowing air across the top of it furiously...
So, is not enough that the air on the top of the wing is moving faster than at the bottom to create lift, but it has to accelerate downwards around the curve of the paper.
Now we all know that the air always flows toward areas of lower pressure and away from areas of higher pressure. This means that pressure has to be less than the ambient on the top of the wing (where the air curves toward the wing) and greater than ambient at the bottom of the wing (where the air curves away from the wing).
The air over the top will accelerate as it enters the lower pressure region, while the air under the wing will slow down as it enters the higher pressure region.
Thus, the top of the wing ends up being the major lift contributor, usually producing twice as much lift (downwash) as the bottom of the wing.

KenLitko

Adam_one,

It works just fine if it is flat. I have done this plenty of times myself. The lift is more for a curved piece of paper and the paper rises more.... this doesn't mean that the flat piece has no lift, just that it has less. Which is true for flat plate airfoils versus cambered airfoils.

Try doing it with a tissue... which has no curvature that you can put into it... it gets pushed into the low pressure region of the faster moving air.

Ken - www.litkoaero.com

rmh

Ahem -- the flat plate has the BEST profile for very light aerobats flying at speeds up to 30 mph. (wing loadings of 4 oz per sq ft.)
If anyone can physically demonstrate how there is any improvement in using double curved surfaces ( symmetrical) for small aerobats - I will be extremely surprised .
No theory- no opinion -just demonstrate it.
This will involve building two identical models , one curved "airfoil", one flat "airfoil".
I have tried it -and simply can't find any value in the curved .
They are more work - which I don't mind - but they don't do any maneuver any better -including flying at the edge of a stall .
A big fat airfoil is a loser .
We are using airfoils which have a chord of 8" and a thickness of .120 (3mm)
The best airfoil is always the one which does the job best for a given application .
I have never seen how the top or bottom of the wing contributes more than the other !
If one moves so does the other .
If what you claim is true - then airfoils would in some cases and it would surely have happened by now --simply rip into two pieces --or become smashed -depending on which theory(?) is being addressed .
It is classic YIN/YANG

BMatthews

This is the point I'm trying to ask about. The air can accelerate all it wants to but what is the mechanism that transfers the reaction of that acceleration to the actual paper or wing? The paper or wing can't see this acceleration except through the pressure variations that are induced by the airflow's behavior in close proximity to the surface. As I see it that is the pressure acting on the paper. Hence although the energy for the lift may be seen in the acceleration of the air mass downwards it's the pressure variations that are coupled with that acceleration that actually lifts the paper or the wing.

Granted you can't have one without the other. They are inextricably linked but it's the pressure variation aspect that the airfoil sees and not the airflow passing around it.

adam_one

KenLitko and dick Hanson,
Notice that the flat plate creates lift only when it has a positive AOA, and when there is air flowing through both the top and bottom. It doesn't work if you only blow over the top while the air at the bottom is standing still.
You may test it using a hair dryer blowing only over the top side.
Bruce,
Sure, there is a pressure differential caused by the wing moving through the air or by the air moving through the wing (it doesn't matter which), but notice that there must be a movement for the pressure differential to take place so, no one is more important than the other. They are just like the chicken and the egg as you have already mentioned above.

BMatthews

You're right Adam, they are equally important. They go together like peanut butter and jam. But the whole gist of the orginal Raskin thread and apparently now this one is the splitting of the fine hairs that equates to the actual element that lifts the wing. Despite the fact that they are linked I'm thinking that it comes down to the pressure variations that actually acts on the wing to support the airplane. It may be the air being accelerated both horizontally to the rear and vertically downwards that provides the energy to acomplish the task but I'm thinking it's the acompaning pressure on the wing's surfaces that it feels and responds to.

My mental image of what's going on may still not be totally accurate but I think I've got a much better idea of the nature of it all now thanks to you, Ben, Ken, Tim, John and many others thanks to all the typing that's gone on up to this point.

rmh

adam- that is true of any lifting surface.
You may not agree with my simple theory of lift -but it makes sense to me . (post 168)

KenLitko

A flat plate wing will only provide lift at positive alpha... true.

Blowing air over a flat plate (piece of paper, balsa, tissue, whatever), on only the top side, works fine for creating a force to push the flat plate -into- the stream of lower pressure air. The moving air on top has a lower overall pressure than the ambient air on the bottom.

The force is small, you may need to blow harder. You may also be having trouble attaching the flow to the surface... once you satisfy those requirements it works just fine.

Ken - www.litkoaero.com

adam_one

Dick,
Both the air and the water are fluids and behave therefore in the same way despite differences in their density and viscosity. Aerodynamics and hydrodynamics are very much alike.
If I understood your description well, it sounds close to the truth, but you're just looking at only one side of the coin (pressure differential) as most people do, maybe because it is easier to measure and to understand.

adam_one

Take a piece of a flat light foam plate and blow air only over the top side (with help of a hair dryer) - it will never lift as long as you don't blow at the bottom side, I promise

rmh

I try to always reduce any "variable" to absolutely minimum usable information.
In this case - "lift" is simply, at least to me a matter of taming a common phenomena to do work.
The airfoils everybody fusses over, are shapes which are compromises using available material for a particular task.
This situation has been getting better since men first started makong aircraft -or models of aircraft or any flying device.
Imagine you had NO constraints on the strength of material to build wings to do a task.
what would the wings look like then ?
If YOUR theory is adapable - draw an ideal wing to replace the wings on -say - a Piper Cub.
This is the fun part of modeling to me .
given the new motors and super light building material I found that most of what I was told years back - can be modified - a lot.

adam_one

Dick,
It is not about my own theory, it's about physics laws.

As for the "ideal" wing, you have to tell the actual size of the plane, the weight, at what max speed it is intended to fly, stall speed, whether it should be a STOL… etc.
In fact, there's no single simple answer, it's most a compromise.

rmh

that iswhy I suggested a "typical level flight aircraft "-a Cub

adam_one

If you are referring to the full-size Cub, then I would say that I think the actual wing is a good compromise considering the plane's purpose and the power to weight ratio. But of course, it might not be the ideal wing for everybody.
Have you any suggestion for a better wing to the full-size Cub?

rmh

sure clip 6' from it and stuf in a bigger engine.

adam_one

Then I guess you'd get a faster Cub but also higher stall speed.

BMatthews

An more aerobatic. But it's been done...

Sig's scale model of Hazel's clipped wing cub.

In truth I don't think you can really come up with a better wing for the Cub. As a trainer and casual flyabout from the pre-ultralight days that continues to offer a low cost user friendly way to get closer to clouds it's pretty much a winner. Sure it's not fast or hotly aerobatic but that wasn't its mission.

Oryx

- Probably slightly faster, bit more maneuvrable (especially roll-rate due to less roll inertia) and better climb rate due to more power (although now also a bit heavier).

- Also higher stall speed, shorter duration and range (important for those that still like to do cross-country with a Cub), less forgiving, more fatique and higher stresses due to additional torque, more expensive to maintain and operate, will lose more speed in turns due to higher induced drag (higher wing loading), more care will have to be taken when opening or closing the throttle quickly (more torque) especially in ground operations and when aborting landings, heavier pitch control forces due to more forward CG, etc...

In general, it will lose many of the characteristics that Cub flyers love so much.