If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#51
|
|||
|
|||
Aerodynamics of carrying water
Reynolds number is not an aerodynamic phenomenon. It's a=20
dimensionless quantity which is useful in characterising certain aerodynamic=3D20 phenomena, principally those which involve a laminar - turbulent=3D20 transition. Perhaps I should have said "Reynolds number which characterises certain aerodynamic phenomena" and it is a fact that the slope of the lift coefficient increases with increasing Reynolds number. What do you mean by "slope of the lift coefficient"? With respect to=20 what? Ian ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~ the standard curve: plot lift coefficient against alpha (angle of attack), for the same alpha flying at a higher speed increases the Reynolds number, at this higher speed the slope is increased slightly over what it was at the lower speed. It's a well known effect. Rgds, Derrick Steed |
#52
|
|||
|
|||
Aerodynamics of carrying water
Subject: Aerodynamics of carrying water
Author: Date/Time: 17:30 16 October 2005 ------------------------------------------------------------ l/d max occurs when induced and pressure drag are the same, not at 75% induced. (interference drag is (per definition) negociated) When 75% of the drag is induced you`r flying at Vy-min (min sink) ------------------------------------------------------------ I stand corrected. Apologies. http://www.av8n.com/how/htm/4forces.html#sec-powers Rory |
#53
|
|||
|
|||
Aerodynamics of carrying water
In article ,
Rory O'Conor wrote: Subject: Aerodynamics of carrying water Author: Date/Time: 17:30 16 October 2005 ------------------------------------------------------------ l/d max occurs when induced and pressure drag are the same, not at 75% induced. (interference drag is (per definition) negociated) When 75% of the drag is induced you`r flying at Vy-min (min sink) ------------------------------------------------------------ I stand corrected. Apologies. http://www.av8n.com/how/htm/4forces.html#sec-powers Both the 50% and 75% numbers are "wrong" in the sense that either one could be correct for some particular aircraft, but neither is correct for all aircraft. The correct statement is that l/d max occurs at the speed at which a small change of speed (a small increase, say) causes an increase in the parasitic drag and an exactly equal decrease in the induced drag. In graphical terms, it is the point where the slope of one curve is the same as the clope of the other, but one is going up and the other is going down. And in the Fig 4.15 in in the link above that is pretty clearly right around 65 knots or so. And, yes, it appears that that is at the point, for that aircraft, where about 75% of the drag is induced drag. But you could make some modification to the aircraft that moved one (or both) of the curves up or down (if you could do that without changing the shape of the curve) and the minimum would still be at the same speed, but the proportion of induced to total drag could be almost anything. -- Bruce | 41.1670S | \ spoken | -+- Hoult | 174.8263E | /\ here. | ----------O---------- |
#54
|
|||
|
|||
Aerodynamics of carrying water
Bruce Hoult schreef: In article , Rory O'Conor wrote: Subject: Aerodynamics of carrying water Author: Date/Time: 17:30 16 October 2005 ------------------------------------------------------------ l/d max occurs when induced and pressure drag are the same, not at 75% induced. (interference drag is (per definition) negociated) When 75% of the drag is induced you`r flying at Vy-min (min sink) ------------------------------------------------------------ I stand corrected. Apologies. http://www.av8n.com/how/htm/4forces.html#sec-powers Both the 50% and 75% numbers are "wrong" in the sense that either one could be correct for some particular aircraft, but neither is correct for all aircraft. The correct statement is that l/d max occurs at the speed at which a small change of speed (a small increase, say) causes an increase in the parasitic drag and an exactly equal decrease in the induced drag. In graphical terms, it is the point where the slope of one curve is the same as the clope of the other, but one is going up and the other is going down. And in the Fig 4.15 in in the link above that is pretty clearly right around 65 knots or so. And, yes, it appears that that is at the point, for that aircraft, where about 75% of the drag is induced drag. But you could make some modification to the aircraft that moved one (or both) of the curves up or down (if you could do that without changing the shape of the curve) and the minimum would still be at the same speed, but the proportion of induced to total drag could be almost anything. -- Bruce | 41.1670S | \ spoken | -+- Hoult | 174.8263E | /\ here. | ----------O---------- Not true. Drag varies with 1/Cl^2+V^2. converting to V means 1/V^2+V^2. Changing that to 2/V^2+V^2 means a higher speed at which l/dmax occurs but both are half. (Just try to plot it...) Later more.. |
#55
|
|||
|
|||
Aerodynamics of carrying water
The beauty of the soapbox derby example--like Nigel's--is that it points out
that the glide slope is the same. With the added weight you have the same glide slope but a higher speed. The other way to make the soapbox derby car go faster was to reduce drag.... "Eric Greenwell" wrote in message ... Nyal Williams wrote: The best illustration used to be: Remember when you rode your coaster wagon downhill alone and also with a buddy in it? It always went faster with two people. And I suppose it would go faster uphill, with two people pulling it back: not a very good analogy, I think, since the heavy glider will climb more slowly. "Going faster" isn't the issue, but instead, why heavy is better than light. After all, both the heavy and the light glider can fly the same speeds right up to Vne. It's because the heavy glider can cruise faster for any given L/D, and (in strong conditions) loses relatively little in the climb. The aerodynamic reason it can cruise faster at any given L/D is that the angle of attack determines the L/D, and the heavy glider needs extra airspeed to generate the extra lift for that weight. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Airbus A380 water purification | john smith | Piloting | 1 | July 7th 05 02:50 AM |
Induction System Water Problem | Mike Spera | Owning | 1 | January 30th 05 05:29 AM |
Water, water, everywhere, but none for thirsty wings.... | Chris OCallaghan | Soaring | 0 | November 21st 04 03:14 PM |
Questions regarding Air/Oil Separators | Doodybutch | Owning | 6 | April 20th 04 05:56 PM |
Water Cooled Jet Engines: a possibillity then and now? | The Enlightenment | Military Aviation | 3 | December 18th 03 09:41 AM |