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 |
#161
|
|||
|
|||
aerodynamics of gliding
Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or turbo, a glider IS essentially gravity powered. The resultant force of gravity plus wing lift, angled very slightly forward, opposes drag. Thus a glider runs down a very slight slope through the air. The less drag there is, the flatter the glide angle becomes. Nicely worded answer. On several occasions I've had (non glider pilot) friends ask me why does it help when we make our gliders heavier with water ballast. Seems counter-intuitive. I'm thinking that a proper explanation is in terms of the gravitational force in a similar fashion to what you describe. Higher mass = higher gravitational force (F=MA). Hence the glider is "pulled down the slope" by a higher force. The glide angle is no better, but we can glide faster at essentially the same glide angle which is an advantage (normal caveats about thermal climb ability trade-off). A more complete answer might also discuss the higher drag at higher speed interplay, but that could probably be left out as a simplification. Perhaps a further discussion of the classic experiment where in a vacuum a feather and a rock will fall to earth at the same rate because the acceleration of gravity is a constant (I know, but within limits it *is* a constant). But in the presence of air the "air-drag" on the feather is relatively high compared to the relatively low gravity "down-pull" due to its low mass. Comments on this explanation are welcomed/sought. I thought I would find a well worded description of this in Reichmann but it isn't there that I can see. TIA Regards, -Doug |
#162
|
|||
|
|||
aerodynamics of gliding
On Mar 18, 8:20*am, Doug Hoffman wrote:
Derek Copeland wrote: In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or turbo, a glider IS essentially gravity powered. The resultant force of gravity plus wing lift, angled very slightly forward, opposes drag. Thus a glider runs down a very slight slope through the air. The less drag there is, the flatter the glide angle becomes. Nicely worded answer. On several occasions I've had (non glider pilot) friends ask me why does it help when we make our gliders heavier with water ballast. *Seems counter-intuitive. I'm thinking that a proper explanation is in terms of the gravitational force in a similar fashion to what you describe. *Higher mass = higher gravitational force (F=MA). *Hence the glider is "pulled down the slope" by a higher force. *The glide angle is no better, but we can glide faster at essentially the same glide angle which is an advantage (normal caveats about thermal climb ability trade-off). *A more complete answer might also discuss the higher drag at higher speed interplay, but that could probably be left out as a simplification. *Perhaps a further discussion of the classic experiment where in a vacuum a feather and a rock will fall to earth at the same rate because the acceleration of gravity is a constant (I know, but within limits it *is* a constant). But in the presence of air the "air-drag" on the feather is relatively high compared to the relatively low gravity "down-pull" due to its low mass. Comments on this explanation are welcomed/sought. *I thought I would find a well worded description of this in Reichmann but it isn't there that I can see. *TIA Regards, -Doug Remembering that the power for the glider is coming from the gravitational potential energy, so it is correct that a higher mass glider has more energy and this is where the increased L/D does come from. However you can't do an analysis quite like that to explain the results. A good discussion of this for L/D for powered aircraft is is in "Mechanics of Flight" by Phillips, if you read the "Power Failure and Gliding Flight" chapter you will get a pretty good picture of what happens, even if not really discussing glider wind loading. Google books has extracts on line at http://books.google.com/books?id=6-_...nics+of+flight and available from Amazon. http://www.amazon.com/Mechanics-Flig.../dp/0471334588 It's expensive but very good. Darryl |
#163
|
|||
|
|||
aerodynamics of gliding
On Mar 18, 6:45*am, Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or turbo, a glider IS essentially gravity powered. The resultant force of gravity plus wing lift, angled very slightly forward, opposes drag. Thus a glider runs down a very slight slope through the air. The less drag there is, the flatter the glide angle becomes. Both airbrakes and large positive flap angles increase the drag, so the glider has to run down a steeper slope to maintain speed. Airbrakes both reduce lift, by disrupting the airflow over part of the wing, and increase drag, so the answer to that question is obvious. * Large amounts of positive flap increases lift, but also increases drag to a much greater extent. Derek Copeland At 12:30 18 March 2009, Bob Cook wrote: Well, at least I did learn a lot from the previous thread, thanks mostly to Ian who helped me "tidy up" some of my conceptions. Yes this is Jr High physics. *I think if we took the final exam, Ian would get an "A", while I would get a "C" and most of the guys would do worse! Since everybody liked my "gravity" question so much, here is more from my pet peeve department. Q) Two identical gliders on final approach. *Glider A has spoilers closed. Glider B opens spoilers. *Glider B will make a steeper approach because "spoilers reduce lift". *True or false? *And why. Q) Two identical gliders on final approach. *Glider A has flaps retracted. Glider B has flaps extended. *Glider B will make a steeper approach because "flaps increase lift". * True or false? And why. Cookie Actually the amount of lift in both cases is unchanged (once the approach is stabilized) Total lift equals the weight of the glider - otherwise the glider would experience a vertical acceleration. There are some vector effects from the angle of the glideslope, but I'm pretty sure they are secondary in most cases. What changes is the lift coefficient. In the case of spoilers the parts of the wing that are not affected by the spoilers operate at a higher lift coefficient to hold the glider up - this produces more induced drag, on top of the drag of the spoilers themselves. With flaps extended the lift coefficient only goes up as the speed goes down. Again the main effect on glideslope is the drag of the flaps, not a change in lifting force. For the technically inclined, the lift formula is 1/2pV^2SCl, where p (rho) is the density of air, V is velocity, S is wing area and Cl is lift coefficient. Flaps let you achieve a higher Cl at a lower airspeed by changing the characteristics of the airfoil, spoilers force you to fly at a higher Cl because the effective S goes down. If you are already at max Cl for the wing and deploy spoilers you will accelerate downward until you generate enough additional airspeed to stop it. 9B |
#164
|
|||
|
|||
Aerodynamics of Towing
On 17 Mar, 19:04, Darryl Ramm wrote:
Say what? Gravitational potential energy *increases* not decreases as the glider climbs. Sorry, typo. And that "store" is the transfer mechanism that allows the glider to glide. No it's not. The gravity force is necessary, but it's perfectly possible to soar for extended periods and distances without adding to or drawing from the potential energy store. It happens any time we fly level - along a wavebar, running a ridge, following a cloud street. There is no other mechanism. With no gravity, even with an atmosphere (which would be difficult to arrange), your glider could not glide. You miss, as so many people do, the point. Because the force of gravity is necessary for a glider to work, people assume that gravity somehow "powers" the flight. Which it does not. Guy's this has wandered into junior-high school level physics. No, it has wandered into junior high school level misconceptions about physics! Here's another one for you. Does a glider turn (normally) by (a) rolling (b) pitching (c) yawing or (d) other? Ian |
#165
|
|||
|
|||
Aerodynamics of Towing
On 18 Mar, 00:45, Nyal Williams wrote:
I'd like to turn this around since I'm not a physicist or an engineer. What force causes a ball to roll down an inclined plane? Well, not gravity, or at least not just gravity, because gravity moves things downwards, not along. So the answer is really that two forces are involved: 1. Gravity, less the vertical component of the reaction force, moves it down and 2. The horizontal component of the reaction force moves it along Ian |
#166
|
|||
|
|||
aerodynamics of gliding
On 18 Mar, 12:30, Bob Cook wrote:
Q) Two identical gliders on final approach. *Glider A has spoilers closed. *Glider B opens spoilers. *Glider B will make a steeper approach because "spoilers reduce lift". *True or false? *And why. Sort of true. All other things being equal, spoilers reduce lift, but the system adjusts itself, or is adjusted, so the the amount of lift increases again (if you're accelerating towards the ground you're Doing It Wrong!) but with increased drag. Q) Two identical gliders on final approach. *Glider A has flaps retracted. *Glider B has flaps extended. *Glider B will make a steeper approach because "flaps increase lift". * True or false? And why. Same sort of thing, really. Vertical equilibrium (ie constant descent rate) is reached with a higher drag, so more energy lost, so the descent rate is higher. But I se the paradox you're getting at - one device decreases lift, one increases it and yet they both have the same effect. The key is in drag: air brakes force you and flaps permit you to fly in a higher drag regime, and that's what makes the approach steeper. Lift never makes a difference - because it acts, by definition, at right angles to the flight path it never does or needs work. Ian Ian Cookie |
#167
|
|||
|
|||
aerodynamics of gliding
On 18 Mar, 13:45, Derek Copeland wrote:
In free unaccelerated flight with no thrust, i.e. no aerotow, winch, or turbo, a glider IS essentially gravity powered. Not true. A glider can fly perfectly happily while increasing its portential energy - exactly the opposite of being gravity powered. Ian |
#168
|
|||
|
|||
Aerodynamics of Towing
On 18 Mar, 01:38, Darryl Ramm wrote:
As I explained, Gravity provides the energy... Then you will need to explain how gravity provides the energy when the glider is climbing. Ian |
#169
|
|||
|
|||
Aerodynamics of Towing
On 17 Mar, 18:33, Mike the Strike wrote:
All of the energy to maintain a glider in steady flight through still air is derived from gravity - there is no other source of energy. And how often do we fly in still air? For that matter, how often do we build gliders on the top of hills, fly them to the bottom and then abandon them. A very little energy may be stored as gravitational potential, but gravity certainly isn't the source of the energy! Ian |
#170
|
|||
|
|||
Aerodynamics of Towing
On 17 Mar, 21:41, KevinFinke wrote:
If Bob's chicken is sliding down a hill.. what force causes it to continue sliding? Can't be gravity, or not just gravity, because chickens, like apples, fall straight down when gravity gets hold of them. How can gravity push something forwards? It is gravity. There is no forward component of lift that contributes to forward motion. What do you think overcomes the horizontal component of drag, then? In steady state gliding flight, Net Lift is equal to Weight times the cosine of the gliding angle. No it's not.You forgot something! Ian |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Seeking towplanes for Region 9 | [email protected] | Soaring | 0 | May 17th 06 12:03 AM |
US:Restricted Towplanes | Judy Ruprecht | Soaring | 8 | November 5th 04 11:27 PM |
Standard Nationals Need Towplanes | C AnthMin | Soaring | 5 | July 14th 04 12:46 AM |
Take-upReels on Towplanes | Nyal Williams | Soaring | 9 | April 21st 04 12:39 AM |
Helicopters and Towplanes | Burt Compton | Soaring | 6 | September 11th 03 05:21 PM |