Operational definition of "glide"

[JHern]

Glide can be measured as the angle of descent of a disc after apexing in flight. I have a mathematical analysis of glide that will be published in my book on Disc Physics.

 

[sidewinder22]

Glide can be measured as the angle of descent of a disc after apexing in flight. I have a mathematical analysis of glide that will be published in my book on Disc Physics.

^ This. Glide is how much the disc maintains loft after it hits the apex, the disc is no longer rising.

 

[bikinjack]

You are correct in that if you draw a free body diagram of a parachute falling, there's certainly a force vector pointing up due to the resistance of the air on the parachute. If you want to call that lift go ahead.

I was referring to lift in the sense that a disc, like a wing, discs get lift from the differential in pressure caused by air passing over it the upper and lower surfaces at different speeds as it travels forward.

Sorry for any thread drift, and I know that this could come across as me being a smart ass, and I don't mean it that way, but...

As for the first paragraph I quoted, what else would you call the upward force acting on a parachute?

And the second paragraph, is actually not entirely correct, at least according to some current thinking on aerodynamics of airplane wings, at least according to this.

At any rate, this thread is an interesting discussion, and a good read.

 

[BogeyNoMore]

You're not coming across as a smart @ss at all. I was recalling the conventional physics explanation of how a wing works/generates lift and equating it to a disc, which shares many of the same attributes, but admittedly isn't an identical situation.

I don't think older style parachutes quite act like airfoils in terms of generating lift, they just work on air resistance - in much the same way a piece of falling paper has air resistance. But now that I think about it, I have a feeling the air pressure under the falling paper probably is greater than the air pressure above it, just not greater the weight of the paper as well.

I just enjoy thinking about this stuff - thanks for the link - makes sense.

Unfortunately, discussions like this really work better in person, where we exchange could back and forth quicker and more easily - sometimes thoughts come to me after I've posted and realize my previous statement was incomplete or flawed in some way. Fun nonetheless!

 

[bikinjack]

OK, I've been thinking about this since I posted that link yesterday evening. What makes different discs of the same mold fly differently when new? The parting line height does. Could the higher PLH, which makes the disc fly more overstable, be causing the leading edge of the disc to force more air downward, in turn causing the center of lift to be more forward of the center of rotation, which in turn causes an earlier and/or harder fade?

Basically could a higher parting line just indicate a "steeper" wing? Could the massive overstability of discs like super flat Champ FierBirds just come from a lift bias towards the leading edge of the disc due to more lift on the front? All along I've thought that discs like that lacked "glide" because they lacked lift. Could it really be because they have too much, at least at the leading edge?

To tie this thought in with the OP, could a working definition of "glide" be how well "balanced" a disc flies?

 

[bradharris]

OK, I've been thinking about this since I posted that link yesterday evening. What makes different discs of the same mold fly differently when new? The parting line height does. Could the higher PLH, which makes the disc fly more overstable, be causing the leading edge of the disc to force more air downward, in turn causing the center of lift to be more forward of the center of rotation, which in turn causes an earlier and/or harder fade?

Basically could a higher parting line just indicate a "steeper" wing? Could the massive overstability of discs like super flat Champ FierBirds just come from a lift bias towards the leading edge of the disc due to more lift on the front? All along I've thought that discs like that lacked "glide" because they lacked lift. Could it really be because they have too much, at least at the leading edge?

To tie this thought in with the OP, could a working definition of "glide" be how well "balanced" a disc flies?

I'll repeat what I posted earlier. In an overstable disc, precession takes over much more drastically, causing the disc to tip in the air. This causes the lift vector to move so that the lift it's generating is no longer in direct opposition of gravity.

 

[bikinjack]

I'll repeat what I posted earlier. In an overstable disc, precession takes over much more drastically, causing the disc to tip in the air. This causes the lift vector to move so that the lift it's generating is no longer in direct opposition of gravity.

Yes, but what causes the precession? The center of lift being not the center of rotation does. The further apart the two centers are, the stronger the precession, therefore the more overstable (or understable, if the lift center is behind the center of rotation) the disc. Also, any time a disc flies in an orientation that isn't entirely parallel to the horizon, the lift vector isn't going to be directly opposing the gravity vector.

 

[BogeyNoMore]

Glide can be measured as the angle of descent of a disc after apexing in flight. I have a mathematical analysis of glide that will be published in my book on Disc Physics.

When can we expect this to be available? :hfive:

 

[Dan Ensor]

As for the first paragraph I quoted, what else would you call the upward force acting on a parachute?

Buoyancy?

 

[knettles]

The flight ratings are relative to disc speed so they are indeed reflected. It's like saying a Boss is an overstable disc. Also you shouldn't throw a putter nose up for it to glide far, that's actually counter productive. A putter nose up will glide further than a driver nose up, but that's because a putter has a less sharp nose. If you throw a putter and driver with the same amount of power, say 25% power, the putter will glide out further than a driver.

That definitely makes sense about the flight ratings being relative to disc speed. However, I'm pretty sure (horrible memory) that I saw a video where a pro was talking about driving putters. In it, they mentioned how putters are supposed to be thrown slightly nose up for distance.

 

[Dan Ensor]

That definitely makes sense about the flight ratings being relative to disc speed. However, I'm pretty sure (horrible memory) that I saw a video where a pro was talking about driving putters. In it, they mentioned how putters are supposed to be thrown slightly nose up for distance.

It might start nose up, but the nose will come down through the apex, or you're losing distance. If the nose never comes down, it stalls out.

 

[BogeyNoMore]

Buoyancy?

FWIW - Buoyancy is caused by differences in fluid density, rather than differences in pressure due to a ___foil moving through whatever medium is being discussed.


At this point, we're reallly getting more into the causes of glide rather than establishing a functional definition.
A few people have provided what I think are decent working definitions. Find the one that works for you. :|

 

[DG_player]

OK, I've been thinking about this since I posted that link yesterday evening. What makes different discs of the same mold fly differently when new? The parting line height does. Could the higher PLH, which makes the disc fly more overstable, be causing the leading edge of the disc to force more air downward, in turn causing the center of lift to be more forward of the center of rotation, which in turn causes an earlier and/or harder fade?

Basically could a higher parting line just indicate a "steeper" wing? Could the massive overstability of discs like super flat Champ FierBirds just come from a lift bias towards the leading edge of the disc due to more lift on the front? All along I've thought that discs like that lacked "glide" because they lacked lift. Could it really be because they have too much, at least at the leading edge?

To tie this thought in with the OP, could a working definition of "glide" be how well "balanced" a disc flies?

I'm a little confused:

Are you referring to the parting line as in where the mold comes together? If the disc is of the same mold wouldn't this be in the same position?

 

[jrawk]

Glide can be measured as the angle of descent of a disc after apexing in flight as it's velocity is rapidly decreasing. I have a mathematical analysis of glide that will be published in my book on Disc Physics.

^ This. Glide is how much the disc maintains loft after it hits the apex and loses velocity, the disc is no longer rising.

I added the blue parts -- to me glide is relevent when the disc begins to slow way down, yet still travel forward. I know that a disc cannot increase velocity (unless thown downhill) but glide is visually qualitative when the disc is slowing down.

i.e. The best, glidiest paper airplane in a given lot will contact the ground the farthest from where it's thrown the instant after its velocity reaches zero. The ground contact didn't cause the airplane to stop moving forward, the air did.

just my thoughts

 

[rakamkram]

Here's the issue I have with glide. People often talk about how slower discs have more glide, but Innova's flight ratings don't reflect that. More importantly, however, the discs' flights don't reflect that either. If you throw a driver and a putter on the same angle, flat, the putter will start to drop sooner. To get the putter to glide far, you have to throw it nose up/at a slight upward angle. So, I basically am no closer to being able to accurately describe glide now than I was before this thread.

I think your post here may have actually come close to what I think disc manufacturers intend with the "glide" numbers. If you were to watch a sidelong view of a disc being thrown, I'm thinking of the shape of the flight as what people mean by glide. With the putter reference there, I think a lower glide disc is one that will slow down its forward velocity more towards the end of the flight and appear to drop out of the air faster where a higher glide disc will travel further forward as it drops out of the sky.

 

[sidewinder22]

I added the blue parts -- to me glide is relevent when the disc begins to slow way down, yet still travel forward. I know that a disc cannot increase velocity (unless thown downhill) but glide is visually qualitative when the disc is slowing down.

i.e. The best, glidiest paper airplane in a given lot will contact the ground the farthest from where it's thrown the instant after its velocity reaches zero. The ground contact didn't cause the airplane to stop moving forward, the air did.

just my thoughts

A disc can increase velocity after the apex though.

 

[sidewinder22]

That definitely makes sense about the flight ratings being relative to disc speed. However, I'm pretty sure (horrible memory) that I saw a video where a pro was talking about driving putters. In it, they mentioned how putters are supposed to be thrown slightly nose up for distance.

Putters need more height to go far as they slow down faster, so they can use their glide after the apex to keep penetrating forward. Drivers rely more on maintaining their speed and they don't tend to maintain a forward penetrating trajectory well after the apex and will curve.

Try throwing a Comet and any driver off a mountain and see which one glides further. opcorn:

 

[bikinjack]

I'm a little confused:

Are you referring to the parting line as in where the mold comes together? If the disc is of the same mold wouldn't this be in the same position?

Yes, that is what the parting line is, and it's height relative to the top and bottom of discs of the same mold can vary quite a bit from run to run. This has been well documented on this forum and many other places.

 

[notapro]

As for the first paragraph I quoted, what else would you call the upward force acting on a parachute?

Drag

 

[SDMike]

Nothing to add to the conversation but I have to say I love threads like this. Thanks guys for making for some fascinating reading/discussion ::thmbup: