Disc flight for noobs: fact check?

[Phil Esra]

Your quote from rybob is correct.
The disc will see a high angle of attack as it starts to fall back toward the ground. Even if the disc is still flat relative to the ground, if it's descending at a 10deg angle, it sees the air hit it at 10deg. Therefore it's flying nose up (and will also be at a lower speed) and the center of lift will be pulled forward.

Thanks a lot for the post! In particular, this quoted part is a very clear description of one of the trickier, more dynamic parts of the puzzle.

Bernoulli still holds, despite the popular backlash against it. ...
Bernoulli and Newton are both legitimate methods to calculating how much lift is created. Neither of them really explain the why question, and it shouldn't be taught that they do. ...

This whole backlash thing blew my feeble little mind when I first read about it--which was in one of those old physics threads, a few months ago. With the NASA slides and everything. To me as layperson, though, it is really more of a big asterisk than a complete teardown/rebuild.

 

[ThrowBot]

This from a person with a reasonable background in physics 'n' whatnot:

I've never seen the "Center of Lift vs. Center of Mass --> Creates Torque in the change-of-pitch direction --> results in rotation in the change-of-tilt direction" line of reasoning. Makes a lot of sense, also I love that it explains both high speed turn and low speed fade depending on the misalignment of the lift and weight vectors (resultant from angle of attack). For some reason I've always had the misconception that there were two different mechanisms at play, but really it's two sides of the same coin.

 

[ThrowBot]

Bernoulli still holds, despite the popular backlash against it. On a streamline (or within a flowfield with constant initial conditions), if the air speeds up, its static pressure will decrease. This is an absolute 100% fact.
On an airfoil, the air over the top is moving faster than the air over the bottom. If you can calculate the velocity field, you can use Bernoulli's equation to determine the pressure field, and from that you can calculate the lift. This is again a fact.
A Newtonian approach also works. If you can calculate the amount of downwash off the trailing edge, you can perform a momentum balance to calculate the upward force on the airfoil.

I think that the backlash against the Bernoulli explanation is that an oversimplification often occurs where an "airfoil" is shown with angle of attack of zero, and the streamlines upstream and downstream are identical. If that oversimplified diagram is taken to be literal, then it looks like conservation of momentum is being violated.

 

[ToddL]

I think that the backlash against the Bernoulli explanation is that an oversimplification often occurs where an "airfoil" is shown with angle of attack of zero, and the streamlines upstream and downstream are identical. If that oversimplified diagram is taken to be literal, then it looks like conservation of momentum is being violated.

A lot of interpretations of Bernoulli are indeed incorrect, and much of that comes from oversimplification. Two statements are often made that have no basis in reality:
1) The upper surface is longer so the air has to travel a longer distance in order to
2) meet back up with the air from the lower surface (equal transit time)

Check out this video. It's a symmetric airfoil, so the upper surface isn't inherently longer than the lower surface (but it's at a positive angle of attack, so the stagnation point moves away from the geometric leading edge, so the "upper surface" does indeed become longer.)
Note that the flow over the upper surface is incredibly fast compared to the lower surface. Not only does the flow not meet back up (equal transit time), but the upper surface blows well past the lower surface.
So we have the upper surface going faster (thus lower pressure), and we also have downwash off the trailing edge (Newtonian balance pushes airfoil upward).

So is Bernoulli wrong? No way.
Is he being taught incorrectly? Maybe, I dunno, I guess. I don't think I ever heard a teacher say that either of those are true, but I don't think I ever had a teacher even talk about why lift is generated until my Intro to Flight class in college.

(and another neat video from the same guy)

 

[ThomasOrion]

I like Tuttles.



and discs



and disc golf



and puppies

 

[Jukeshoe]

Here we go again...

Ho boy...


The Physics: <---Strike Back.



:| :|

 

[SonicGuy]

A lot of interpretations of Bernoulli are indeed incorrect, and much of that comes from oversimplification. Two statements are often made that have no basis in reality:
1) The upper surface is longer so the air has to travel a longer distance in order to
2) meet back up with the air from the lower surface (equal transit time)

Check out this video. It's a symmetric airfoil, so the upper surface isn't inherently longer than the lower surface (but it's at a positive angle of attack, so the stagnation point moves away from the geometric leading edge, so the "upper surface" does indeed become longer.)
Note that the flow over the upper surface is incredibly fast compared to the lower surface. Not only does the flow not meet back up (equal transit time), but the upper surface blows well past the lower surface.
So we have the upper surface going faster (thus lower pressure), and we also have downwash off the trailing edge (Newtonian balance pushes airfoil upward).

So is Bernoulli wrong? No way.
Is he being taught incorrectly? Maybe, I dunno, I guess. I don't think I ever heard a teacher say that either of those are true, but I don't think I ever had a teacher even talk about why lift is generated until my Intro to Flight class in college.

(and another neat video from the same guy)

I will maintain that Bernoulli is not applicable to explaining disc physics. Yes, the airspeed is higher on the top edge, but that is a result of lift, not a cause. The Bernoulli principle is a direct result of Newtonian lift.

In addition, specific to this thread, the airspeed in question is not relative to the surface, it is relative to the airfoil. You cannot throw a conveyor belt spinning a million miles an hour and generate lift vie Bernoulli principle.