Less drag

[Bill]

Trick question

I've been there 500 times and the right answer is.... the one that the traffic controller says.
If they both fly at max L/D, they will both travel the same distance without wind (not quite a true statement, close enough) but, the airspeed that produces max L/D will be higher for the heavy plane and so will the descent rate. The question was time not distance.
If , however, they both descend at max airspeed and create almost the same drag. The lighter airplane has less energy to dissipate and will 'run out' faster.
The best answer is.... the pilot of the lighter 747 gets to decide.
Aloha
Bill

 

[loopy]

Hey Bill, you're a pilot?

 

[Bill]

Used to be. The US gov. pulls your licence when you turn 60. Too old and sick to even fly co-pilot. For 40 years I'd fly anything I could get a hold of from a 10 gram indoor glider to 400 tons of Boeing. Now I 'fly' in the water.
Aloha
Bill

 

[loopy]

Gee, it seems I just found myself a new idol

That's insane - I'm an aussie air force pilot, just finishing up my aero eng degree now... if I accomplish half of what you've done though, I'll be a happy man Let me know if you're ever back in Aus!

 

[Jay Styron]

Uh, yea that's what I meant.

 

[zipy]

.... the one that the traffic controller says.

rofl ha ha !

Some Pilots you are !!!

Zipy

 

[bruno]

Next wednesday i have access to a 25m pool (50m divided by a wall, which can be brought down) that is 3,5 m deep over the whole length.

I'll try "near the bottom" and "mid-water", count my strokes and measure time etc...

i'll let you know

bruno

 

[loopy]

Cheers Bruno, I think that's the best way to figure it out...

 

[Skindiver]

Yes

But its still fun warping science to suit your outcomes

skin

 

[loopy]

Hey what are you worrying about hydrodynamics for bud, you're allready at 50m

 

[cdavis]

less drag or not

Hi all

Finally got to the pool. tried multiple runs in a 25 yrd pool, total depth about 7 feet, using short stiff bi fins. I tried dolphin kick and normal bi fin kick, right on the bottom and about mid depth, multiple repititions, and found no significant difference among any of the combinations. At least for me, it feels faster on the bottom, but its only an illusion. Anybody else have any results?

 

[loopy]

I haven't had a chance to try anything out, but I'd be itnerested to see other peoples results. What I did do though is stumble across some relevant information in the pile of papers I call my lecture notes...

At a speed of 1m/s (this is what most average on a dynamic), friction drag accounts for around 3% of total drag, wave drag is about 2% and pressure drag is 95%.

At 2m/s (a fast mono speed) friction drag stays at 3%, wave drag increases to 20% and pressure drag drops to 77% of the total drag. This scale is non-linear (ie you can't linearly interpolate to find drag percentages in between).

Basically, at slower speeds, friction drag dominates. In a dynamic, this is really only important for the take off. As you move fast, wave drag plays an increasingly important role, but I don't think that the speed we use is significant enough for this. maybe if a diver was near the elite end of the spectrum, they may notice a small difference, but otherwise I think there are other ways to produce a noticable drag decrease (ie better streamlining).

Incidently, at the Sydney Olympics in 2000, Peter van den Hoogenband averaged 2 m/s over 100m... that's fast!! That's 50m in 25 seconds - not much slower than a good monofin swimmer over that distance!

 

[Ossi]

Well, actually ~10 seconds slower than a finswimmer of equal calibre... But fast anyway.

 

[Gerald]

Consider Archimedes

Originally posted by derelictp
... The most interesting was that if you have an object going in the water and the diameter of the object is "d" then you have to go down to a depth of "3*d" to get rid of the wave-effect.

In a human application it would be at about 90cm+!

That explains a part of why it's harder to do dynamic in a shallow pool. (of course it's because of a more variable boyancy too).

That 3*d Formula is very interesting, i will do some research about it.
A major reason why propulsion is easier close to the bottom of the pool is the law of archimedes combined with the law of Boyle-Mariott. Assuming a total lung volume of 7.5 Litres at the surface a dive to the bottom of a 2.5mt pool will cause the ambient pressure to increase from 1.0 to 1.25 bar absolute pressure. Relatively this is an increase by 1/4 from 4/4 to 5/4. This will cause the lungs to shrink down to a reciprocal of 5/4 = 4/5 of it's original volume = 6 Litres: 1.5 Litres less lung volume for the archimedical law to act upon, ie the diver will be 1.5 Kilos heavier close to the bottom of the pool. The most radical change in buoyancy takes place close to the surface, half of the volume is lost after the first 10 Meters of descent. From my own experience: To swim close to the ground of a pool (supposingly it' s deep enough) will just help to add this extra weight (1-1.5 Kilos) that is needed to neutralize inconvenient buoyancy.

 

[bruno]

That must be the reason.

Less lung volume will make you swim more horizontal (without neckweight you swim a little legs up, more drag).

i give you some karma for this real simple answer.

Thanks
Bruno

 

[loopy]

Hmmmm.... I agree, but I think you're missing a bit of the point. First up, if you need to be more negative at the depth you're swimming at, then you're weighted wrong. Add more weight.

Secondly, you might have to hold less pressure at 2.5m, but most pools are shallower than that - not sure if there's a rule on pool depth for dynamic in comps, but I know at Kona at least it was closer to 1m deep. Here the pressure change isn't so radical.

Finally, the reason we were debating this was from a drag perspective - true, having a narrower cross sectional area means a lower coefficient of pressure drag, but if we were to find that there's a huge increase in some other drag near the bottom, then swimming there may bot be worthwhile. The only real way to tell though is to try it

But yeah, good response have some more karma