Understanding Flight: Pressure Distribution & the Science Behind Airplanes

[Thomas2]

Aerodynamic Lift vs. Magnus Effect

I think it is important in this context to point out the fundamental difference between the aerodynamic lift and the magnus effect. As indicated in my posts #66 and #70 (page 5), the former should exist also for a strictly non-viscous gas, but using the same argumentation as there, the magnus effect does not.
Consider a rotating ball that is moving through an inviscid gas (i.e. molecules interacting with the ball but not with each other): if the surface of the ball would be mathematically smooth, then the rotation would actually be without any effect at all because the air molecules would just bounce off like for a non-rotating sphere, but even for a realistic rough surface (obviously a surface can not be smoother than about 1 atomic radius), the overall effect still cancels to zero: the pressure on the side rotating against the airstream is higher at the front but smaller at the back (and the other way around for the co-rotating side) so overall there is no resultant force on the ball but merely a torque that slows down the rotation.
Hydrodynamics arguments (i.e. Bernoulli's principle) are therefore required to explain the magnus effect but not for the usual aerodynamic lift.

 

[PBRMEASAP]

Thus, the inviscid fluid has a somewhat twisted picture of pressure as well:
It models correctly how pressure is force per area acting strictly normal onto a surface, and that it will provide a macroscopic acceleration along its negative gradient.
Beyond that correct facet however, the inviscid fluid model completely ignores pressure's connection to the actual, random thermal motion of molecules.

Thank you for clearing that up! I had suspected this was the case, but I didn't have an argument to back it up.

 

[arildno]

Thank you for clearing that up! I had suspected this was the case, but I didn't have an argument to back it up.

OK, then I'm just about finished (unless you have some other questions).

 

[PBRMEASAP]

Yes. Apparently he only reads his own posts and no one else's.

edit: I think you've answered them all. Thanks again! (Didn't want to make a separate post)

 

[arildno]

Yes. Apparently he only reads his own posts and no one else's.

edit: I think you've answered them all. Thanks again! (Didn't want to make a separate post)

It has been a pleasure!

 

[russ_watters]

You should have a better look at the airfoil profiles. Take for instance http://www.netax.sk/hexoft/stunt/images/342.gif (which is from the page you quoted yourself in the thread https://www.physicsforums.com/showthread.php?t=66840&page=4&pp=15 recently (post #55)) : the highest point of the camber both at the top and bottom is towards the left (upstream) of the center, i.e. both the upper and lower side should produce a positive lift here. It is in fact the normal convex underside that should produce a negative lift, but since the curvature is less than for the upper side the resultant lift is then still positive.

You're changing your claims, but in any case, they still conflict with what is actually observed to occur: All cambered airfoils, regardless of where the point of max thickness occurs and regardless of if the bottom is flat, convex, or concave, produce lift at negative aoa.

You do know about LAMINAR FLOW AIRFOILS, right? These airfoils have the point of maximum thickness further back than in typical airfoils - up to 50% of the way back. Yet, they are more efficient than typical airfoils (the reason they are not used is due to flow stability, not efficiency).

What I don't get is why you don't actually test this yourself. Its relatively simple (I gave a link where a guy built a crude wind tunnel in his house, but you could also build a little model and throw it...) and you'd save yourself from being so spectacularly wrong all the time. Build a little model or wind tunnel, test it, then flip the airfoil around backwards and test again.

edit: HERE are some laminar flow airfoils (on hydrofoils, but the principle is the same) with max thickness 50% of the way back - they produce lift at zero aoa.

 

[Doc Al]

I think its time to put a lid on this thread. Some great answers here, that I'm still digesting. (Good job arildno and others!)

 

[Doc Al]

Locking of the stagnation point

arildno requested that I add this to the thread, as he didn't want to leave any gaps in his explanation:

From arildno:

Unfortunately, I glossed over a relevant topic because it is "too obvious", but on further reflection, I've found that my argument on why a viscous fluid favours downwash really becomes untenable without broaching it.
It concerns the "trivial" fact that for a viscous fluid, a stagnation point becomes locked onto the leading edge.
Clearly, that high pressure zone will give a fluid particle somewhat above the wing a horizontal acceleration component away from the leading stagnation point.
Thus, that fluid particle does not only, as I seemed to suggest, get a roughly normal acceleration onto the wing, but also a tangential acceleration down the wing, providing its "punch".

Locking the stagnation point on the leading edge effectively replaces the unphysical mechanism through which an ideal fluid effects tangential downrush:
It places its stagnation point on the downside of the wing, fluid rush up towards the leading edge, twists about, and rush downwards the upper side.
On the other side of the stagnation point on the underside, the fluid rush down to the trailing edge, twist about it, and the backflow then rush up to meet the downrush in a new stagnation point.
I.e, in the D'Alembert case, we have infinite suction pressure at BOTH edges..

Thus, the leading edge behavior in a real fluid is to replace a totally unphysical mechanism for downrush on the upper side with the mechanism of the frontal stagnation pressure, whereas viscosity's role at the trailing is to reduce upflow.

Both these mechanisms are succincntly described in russ waters' first link, i.e, that viscosity tends to DAMPEN velocity gradients.​