How do airplanes fly with heavy weight and air resistance?

[rcgldr]

Is there some transition between the way a helicopter wing and a fixed wing works, then? The difference in efficiency is just a difference in detail - it doesn't have to he 'in principle'. In fact, how can it be? The only difference is surely that the helicopter blade effect is hundreds of times more on the same local region of air around it.

There are several reasons for helicopter rotor inefficiencies. One issue is that the relative speed at the inner part of the rotor is much less than the outer speed. Another issue is that a cambered airfoil produces a downwards pitching torque that would put too much stress on a rotor blade and it's support, so a helicopter rotor uses a nearly symmetrical airfoil. Similar to a propeller, there is washout, and the washout near the outer tips is set to reduce lift and the associated vortices that would otherwise be generated.

A hovering helicopter has to deal with it's own induced wash, and for some helicopters, it's unsafe to vertically descend into the downwash because there's not enough power to stop the descent. For a helicopter in forward flight there's much less induced wash, and forward flight takes less power than hovering.

The core principle is the same, lift is the result of accelerating air downwards, via diversion of the flow relative to a rotor blade, propeller blade, or a wing.

energy gradually dissipates.

but not the impulse. The impulse may get spread out over a large area, but the magnitude of the impulse does not diminish over time or distance until some other force or impulse opposes it. As pointed out in several posts, the average force that the atmosphere applies to the surface of the Earth is the sum of the weight of the atmosphere and the weight of any aircraft (or hovering balloons) that the atmosphere is supporting, it's a closed system.

behaviour in a wind tunnel though.

Wind tunnels that are too "short" prevent vertical air flow, and essentially model a wing in a combination of ground effect and "ceiling" effect (air prevented from flowing downwards from above).

 

[boneh3ad]

...then as the pressure increases again, the flow transitions into turbulent flow and separates somewhat from the surface.

As an aside, this is true but misleading. The change in the sign of the pressure gradient and the transition to turbulent flow often do occur in the same region but the relationship is not necessarily causal. In this case, the adverse pressure gradient tends to destabilize certain instabilities in the boundary layer and help speed transition along, but this is not necessarily always the case. The primary instability mode on a swept wing is actually stabilized by an adverse pressure gradient, oddly enough. Also, a turbulent boundary layer won't separate from the surface until quite a bit later than were the boundary layer laminar the whole way.

Calculating drag is a bit more complicated, partly because one set of streamlines end at the leading edge stagnation zone, and another set begins at the trailing edge of a wing (where the stream lines from above and below merge).

That's not really the reason that drag is more difficult. Really, it comes down to a couple things. For one, many times people just grab the drag spit out by inviscid solvers using a panel method, which gives you induced drag but ignores other forms of drag, particularly viscous drag. Viscous drag can often be the dominant form of drag on an airfoil section moving at subsonic speeds, so it is quite important to get it right and many codes ignore it for the sake of simplicity. The next complicating factor is the fact that the location of transition is currently impossible to predict in general. The only truly accurate way to get drag is to do wind tunnel testing at this point in time and scale it. You can, of course, use wind tunnel tests to determine the laminar-turbulent transition properties of a wing and then use that knowledge to feed back into your codes and get a much, much better idea from your codes, and that happens a lot.

You just have to know the limitations of your tool. Some, like XFOIL, go a bit beyond simple inviscid panel methods in order to model the boundary layer and correct for its effect, which improves accuracy in drag prediction. Even XFOIL still uses a very rudimentary transition prediction criterion (though it is about as good as it can get without solving the full Navier-Stokes equations given current knowledge on the subject).

Wing chord can be a straight line or it can be a curved line that is 1/2 between the upper and lower surfaces of a wing. The curved line gives a better idea of the coefficient of lift versus angle of attack for a cambered air foil.

No, wing chord is the straight line from leading edge to trailing edge. The line half-way between the two surfaces is the camber line.

Wind tunnels that are too "short" prevent vertical air flow, and essentially model a wing in a combination of ground effect and "ceiling" effect (air prevented from flowing downwards from above).

If the experiment is designed properly, this isn't an issue. Most of the time in a wind tunnel, either you use a wing small enough to avoid this problem or else you use a wing at such an angle of attack that it produces zero lift (which serves multiple purposes, though funding agencies don't like it much since it is "unrealistic"). Generally, more fundamental studies tend to be in zero-lift situations, negating the issue entirely, and more realistic studies are carried out with a combination of large enough wind tunnels and small enough models assuming the researchers are competent.

 

[Traz 0]

Reduced density requires higher actual air speed for most dynamics. In an aircraft you're probably going by Indicated Air Speed (IAS), which already compensates for air density. For example, stall speed is based on IAS, which takes air density into account.

Actually, density altitude is used to calculate expected aircraft performance, such as, will my plane be flying when I reach the end of this runway or will I be driving into those trees. Also, rate of climb, fuel use, etc.

 

[Traz 0]

Flight school teaches you to fly. Just like a seagull, if you spent your time worrying about the Physics of how you stay up there, you'd fall out of the air.
Many years ago I did a parachute jumping course, along with a group of fellow (very bright) research Engineers. The (Army) instructor just couldn't cope with our constant and very interested technical questions. ("I'm afraid I'm losing you guys.") In the end, we just did it by numbers - as he wanted - and we got on fine.

Silly flight school. They wanted us to understand aerodynamics. Lol

Also, that wing cross-section diagram posted a few entries ago does show "normal force" as acting perpendicular to the chord line, which IS as I was taught. And, so, again my initial question: Is this still considered the proper model for diagramming wing function? Or is it a rough approximation that's good enough for government work?

 

[cjl]

Silly flight school. They wanted us to understand aerodynamics. Lol

Also, that wing cross-section diagram posted a few entries ago does show "normal force" as acting perpendicular to the chord line, which IS as I was taught. And, so, again my initial question: Is this still considered the proper model for diagramming wing function? Or is it a rough approximation that's good enough for government work?

Notice that it has a normal force and an axial force in that frame of reference though - the total force is not perpendicular to the wing. That's just a different frame of reference (which is seldom used - the lift and drag components are usually more useful for calculations). If you really wanted, you could break the force down into any pair of normal vectors you wanted - you could define one as 45 degrees from the chord line for example (and it would be completely correct, just not very useful).

 

[rcgldr]

The change in the sign of the pressure gradient and the transition to turbulent flow often do occur in the same region but the relationship is not necessarily causal.

I had the impression that an adverse pressure gradient usually (but not always) triggers a transition to turbulent flow (assuming turbulent flow hasn't already begun due to other factors).

drag - partly because streamlines end, new ones begin ...

That's not really the reason that drag is more difficult.

I thought that the streamlines ending and beginning were an issue for profile drag (the "partly" part), like a bus traveling down a highway, where the stagnation zones front and rear do not have the smae pressure, and most of the profile drag is usually due to the lower pressure aft of an object (depending on the shape).

No, wing chord is the straight line from leading edge to trailing edge. The line half-way between the two surfaces is the camber line.

I thought I fixed that. It's fixed now.

 

[boneh3ad]

I had the impression that an adverse pressure gradient usually (but not always) triggers a transition to turbulent flow (assuming turbulent flow hasn't already begun due to other factors).

Like I said, it is a related pair of phenomena but not one in the same. At the risk of digging too deep into this topic, I will try and clarify fairly succinctly here what I mean. As an example, the boundary layer on a flat plate will transition to turbulence without the action of an adverse pressure gradient. Boundary layers are essentially very complicated, nonlinear dynamical systems, in many ways like a mass-spring-damper system, only more complicated, and instead of being governed by Hooke's Law and some damping, they are governed by the Navier-Stokes equations.

Much like their simpler counterparts, they have various instability modes that can grow and eventually get large enough to transition to turbulence. It turns out on a flat plate, the instability mode that leads to turbulence is a streamwise wave called a Tollmien-Schlichting wave. On a flat plate, these will eventually grow large enough to transition to turbulence. As it also happens, they are dominant on a 2-D wing. They are also remarkably unstable to an adverse pressure gradient, so when you get farther downstream on a wing and they encounter the adverse pressure gradient, they grow much faster than usual and it leads to early transition. They don't need the adverse pressure gradient in order to cause transition, but the adverse pressure gradient does speed the process along. In this case, your understanding is correct.

For a swept wing, Tollmien-Schlichting waves exist but are not dominant. In those situations, you have what is called the crossflow instability that dominates. That instability, it turns out, is actually made more stable by an adverse pressure gradient. However, on most practical swept wings in service, it transitions well before the adverse pressure gradient occurs anyway. In other words, most commonly-used swept wings actually transition independently of the adverse pressure gradient, and the pressure gradient itself would actually delay the transition somewhat.

I thought that the streamlines ending and beginning were an issue for profile drag (the "partly" part), like a bus traveling down a highway, where the stagnation zones front and rear do not have the smae pressure, and most of the profile drag is usually due to the lower pressure aft of an object (depending on the shape).

That's true enough, but that only occurs on an airfoil when you have separation. Otherwise there is no issue of the streamlines not meeting up neatly. On a bus, you have that massive separation.

 

[sophiecentaur]

Silly flight school. They wanted us to understand aerodynamics. Lol

Also, that wing cross-section diagram posted a few entries ago does show "normal force" as acting perpendicular to the chord line, which IS as I was taught. And, so, again my initial question: Is this still considered the proper model for diagramming wing function? Or is it a rough approximation that's good enough for government work?

I think so. They can hardly expect a specialist flier to be a specialist Physicist at the same time (or vice versa ).
The meaning of the word 'understanding' is very wooly. I am sure that the flight school course didn't require you to do more than be 'comfortable' with the techinical stuff at a reasonable level. What they told you would probably not have been sufficient for you to have designed a wing, for example.