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| Author |
Message |
N:dlzc D:aol T:com (dlzc)
science forum Guru
Joined: 25 Mar 2005
Posts: 2835
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 Posted: Fri Jun 23, 2006 1:13 pm Post subject:
Re: New German 4 seater gets 157 MPG
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Dear Bret Cahill:
"Bret Cahill" <BretCahill@aol.com> wrote in message
news:1151047380.307271.291050@y41g2000cwy.googlegroups.com...
| Quote: | The obstacles are safety-related.
Only in a demolition derby do you need a heavy
vehicle to be safe.
|
You've driven in Arridzona. It *is* a demolition derby, on some
days.
And no, I have a very small car that will suffer badly if
impacted with a truck or SUV.
David A. Smith |
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Richard The Dreaded Liber1
science forum beginner
Joined: 23 Jun 2006
Posts: 1
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| Posted: Fri Jun 23, 2006 8:05 pm Post subject:
Re: New German 4 seater gets 157 MPG
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On Fri, 23 Jun 2006 06:13:42 -0700, N:dlzc D:aol T:com (dlzc) wrote:
| Quote: | Dear Bret Cahill:
"Bret Cahill" <BretCahill@aol.com> wrote in message
news:1151047380.307271.291050@y41g2000cwy.googlegroups.com...
The obstacles are safety-related.
Only in a demolition derby do you need a heavy
vehicle to be safe.
You've driven in Arridzona. It *is* a demolition derby, on some
days.
And no, I have a very small car that will suffer badly if
impacted with a truck or SUV.
|
You might have a better traffic record if you consider it more of an
obstacle course.
Good Luck!
Rich |
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Eeyore
science forum beginner
Joined: 22 Jun 2006
Posts: 31
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| Posted: Fri Jun 23, 2006 11:09 pm Post subject:
Re: New German 4 seater gets 157 MPG
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Bret Cahill wrote:
| Quote: | The obstacles are safety-related.
Only in a demolition derby do you need a heavy vehicle to be safe.
In real life there are more intelligent ways to be much safer.
Supposedly Ford and Microsoft are working on collision avoidance
systems.
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I wouldn't trust either of those companies very much.
Graham |
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Robert Clark
science forum Guru Wannabe
Joined: 30 Apr 2005
Posts: 129
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| Posted: Mon Jul 17, 2006 6:31 pm Post subject:
Re: Using lift to increase speeds.
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The idea was to use the lift force to increase the forward velocity of
the craft. However, I have been informed by email that since lift is
always perpindicular to the velocity it can not be used to increase the
forward speed. Nevertheless the idea of having the craft travel in a
circle could still work by using the lift force to counteract the large
acceleration implied by the formula a = v^2/r.
However, there may be a way to use aerodynamic forces to increase the
forward speed. The phenomenon of "tacking into the wind" in sailing
allows a sail boat to actually have a forward velocity component that
goes *into* the wind. Indeed using this method the boat's speed can
actually exceed the wind speed. This page provides a good explanation:
The physics of sailing.
http://www.phys.unsw.edu.au/~jw/sailing.html
The method is used with boats that have a keel that extends into the
water. The basic idea is that if the boat is sailing at an angle to the
wind then the wind is pushing the boat at an angle. This causes the
keel to push on the water at an angle which means the water is
providing an equal and opposite force on the boat. Note now though this
force on the boat from the water does have a *forward* component. And
the sum total of the vector forces of the wind and the water on the
boat also has a forward component.
This causes the boat to move at an angle into the wind. To arrive at a
course direction directly into the wind, the boat is made to move first
to the right of the wind and to the left of it alternatingly in zigzag
fashion.
Could this idea be applied to hypersonic waveriders? At hypersonic
speeds, the density of the air within the shock wave is many times the
density of the ambient air, as water is many times the density of sea
level air. Then the idea would be to have a "keel" that extends into
the shockwave and a vertical airfoil (a "sail") that extends through
the shock layer into the surrounding low density air.
See the last image on this page:
Hypersonic Flow.
http://www.aerospaceweb.org/design/waverider/flow.shtml
You see the shock layer is close to the craft on the bottom of the
craft and extends further out at the top. Then the "keel" would extend
from the top in this case to remain within the shock layer, and the
airfoil "sail" would extend from the bottom into extend into the
ambient air.
This page gives a formula for the stagnation pressure of the shock
layer at least initially:
STAGNATION PRESSURE
http://www.ae.utexas.edu/courses/ase120k/ase120k_stagpres_temp.html
So at Mach 20 using a ratio of specific heats gamma of 1.4 for air,
the pressure increase would be by factor of 4783 of the pressure
initially in the shock layer over the pressure of the ambient air. This
is greater than for example the ratio of the ratio of the pressure of
the water on a keel than the pressure of the sea level air on a sail.
This pressure within the shock layer though would decrease as you get
further from the front of the vehicle so you would want the "keel" to
be close to the front.
The question, would the addition of these extra structures result in
just an increase in the overall drag of the vehicle?
As discussed on this page the addition of winglets to aircraft has a
similar effect of producing extra thrust, so reducing the overall drag:
How Things Work: Winglets.
"The airflow around winglets is complicated, and winglets have to be
carefully designed and tested for each aircraft. Cant, the angle to
which the winglet is bent from the vertical, and toe, the angle at
which the winglets' airfoils diverge from the relative wind direction,
determine the magnitude and orientation of the lift force generated by
the winglet itself. By adjusting these so that the lift force points
slightly forward, a designer can produce the equivalent of thrust. A
sailboat tacking sharply upwind creates a similar force with its sail
while the keel squeezes the boat forward like a pinched watermelon
seed."
http://www.airspacemag.com/ASM/Mag/Index/2001/AS/htww.html
It would appear from Newton's second law you could not get more
forward acceleration than from the craft operating in vacuum under the
propulsion method used. Nevertheless, effectively you get more forward
acceleration in air than without these extra structures because of the
overall drag reduction. Just as importantly, the overall reduction in
drag would result in an increase in the lift to drag ratio.
Bob Clark
Robert Clark wrote:
| Quote: | William.Mook@gmail.com wrote:
Orbital speed is where centripetal force equals gravity force and is
given by;
v = sqrt(GMe/r)
Which can be derived from the following three equations;
F = G*m*Me/r^2 - gravitational force
a = v^2/r - centripetal acceleration
F = ma - relating mass and acceleration
a = F/m = GMe/r^2 - gravitational acceleration
a = v^2/r - centripetal acceleration
Setting the two accelerations equal
v^2/r = GMe/r^2
v^2 = GMe/r
v = sqrt(GMe/r)
If we increase velocity by 41.4% we double the centripetal
acceleration, which means that if we were to fly an aircraft at Mach 33
we'd need wings to hold it in the atmosphere! Since wings lift
aircraft all the time against gravity, it seems reasonable to believe
that wings could hold an aircraft down. Everything would seem quite
normal to the occupants, except down would be up to them, and the lift
would be directed toward the Earth's center.
The vehicle if possible would be capable of circumnavigating the Earth
in 60 minutes - and delivering payloads to targets anywhere in 30
minutes or less.
Would such a craft be possible?
Yes. I speculated about this possibility for the use with beamed
propulsion:
From: Robert Clark
Date: Sat, Nov 19 2005 2:23 pm
Email: "Robert Clark" <rgregorycl...@yahoo.com
Groups: sci.astro, sci.physics, sci.math
Subject: Math question for the trajectory of beamed propulsion.
http://groups.google.com/group/sci.astro/msg/71a00732000ef7f7
This would also be applicable to the scenario where electrical power
for propulsion is transmitted though long cables:
From: Robert Clark
Date: Fri, May 27 2005 12:10 pm
Email: "Robert Clark" <rgregorycl...@yahoo.com
Groups: sci.astro, sci.space.policy, sci.physics,
sci.electronics.design, sci.electronics.misc
Subject: Re: Long cables to power "ioncraft" to orbit?
http://groups.google.com/group/sci.astro/msg/5a2b09463e87dde6
The problem is that though the height to orbit might be 100 km, the
horizontal distance travelled might be 2000 km in order to build up
sufficient speed for orbital velocity.
The proposals for beamed propulsion I've seen though do not use
lifting surfaces for the craft:
Riding Laser Beams to Space.
http://www.space.com/businesstechnology/technology/laser_propulsion_000705.html
However, the lift to drag ratios at hypersonic speeds suggest we might
be able to increase the thrust and therefore the acceleration by
several times if the craft was designed for aerodynamic lift. See the
graph showing lift to drag ratio versus Mach number here:
Waverider Design.
http://www.aerospaceweb.org/design/waverider/waverider.shtml
With airplanes you have the thrust directed horizontally to overcome
the drag force against forward motion and the lift provides the force
to keep the airplane aloft. Since subsonic L/D ratios can be 15 to 1
and higher the thrust required from the engines is much less than the
actual weight of the plane.
However, with beamed propulsion a key problem is the dimunition of the
power with distance, which decreases with the square of the distance so
you want to keep the distance short. The idea then in this case using
aerodynamic lift would be to use the thrust produced by the beamed
propulsion to overcome gravity and drag and use the lift force to
provide the higher acceleration to reach orbital velocity in a shorter
distance. Essentially the craft would be pointed upwards so that the
wings/lifting surfaces provide the "lift" in the horizontal direction.
The graph on the "Waverider Design" page shows the L/D ratio can be
about 7 to 8 at hypersonic speeds. For instance if the beamed
propulsion provided a thrust of 1 g to counter gravity plus 4 g's
against drag for a total of 5 g's in the vertical direction, then the
horizontal acceleration could be as much as 8*4 = 32 g's.
Note though it would be important to keep the craft oriented so that
so that the velocity vector is always pointed through the forward
centerline of the craft. When lift and drag calculations are made it's
always in regard to the craft moving so the airstream is flowing more
or less parallel over the wings/lifting surfaces, according to angle of
attack. If instead the airstream was flowing perpindicular to the plane
of the wings the lift would be much less and drag would be much greater
so the L/D ratio would be severely reduced. The aerodynamic control
surfaces would be used to keep the craft properly oriented.
Estimates for beamed propulsion are about 1 megawatt of power to send
1 kilogram to orbit. If say such beamed propulsion provided thrust for
5 g's of acceleration then the lifting force could provide 32 g's, or a
factor of 6 more. So the distance required would be smaller by a factor
6. This means the power required would be smaller by a factor 6^2 = 36.
Then 36 times greater mass could be lifted for the same power. This is
dependent though on how much acceleration beamed propulsion could
provide. If it were 7 g's then the lifting acceleration would be 8*6 =
48 g's, about a factor of 7 more. Then the power required would be less
by 7^2 = 49, and 49 times greater mass could be lifted.
There are apparently megawatt class lasers already in operation:
Mid-Infrared Advanced Chemical Laser (MIRACL).
http://www.fas.org/spp/military/program/asat/miracl.htm
Let's say they are at the 10 megawatt stage now. Then this could
accelerate 10 kilos to orbit. Then with aerodynamic lift it could lift
perhaps 360 kilos to orbit, which is the size of a small sized
satellite.
Bob Clark |
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William.Mook@gmail.com
science forum addict
Joined: 06 May 2006
Posts: 50
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| Posted: Mon Jul 17, 2006 7:27 pm Post subject:
Re: Using lift to increase speeds.
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|
Robert Clark wrote:
| Quote: | The idea was to use the lift force to increase the forward velocity of
the craft.
|
Nope. You've misunderstood it totally. Thrust propels the aircraft,
lift keeps it airborne under normal circumstances. This is inverse
lift . I suppose you could say you use lift to maintain altitude
against centripetal force. The lift doesn't increase speed in any way.
In fact staying in the air induces drag and then there's drag induce
lift. Negative lift in this case allows you to operate at higher
speeds than you might, even though the these losses are present. Lift
doesn't contribute to your thrust at all.
| Quote: | However, I have been informed by email that since lift is
always perpindicular to the velocity it can not be used to increase the
forward speed.
|
The person who informed you was right. You are totally confused about
what's going on.
| Quote: | Nevertheless the idea of having the craft travel in a
circle could still work by using the lift force to counteract the large
acceleration implied by the formula a = v^2/r.
|
You are even more confused than you were a second ago! But so close!
Forget about what you're thinking and think this.
The circle you're travelling in is a great circle route around the
surface of the Earth. If you complete an orbit your r = 6,366,198
meters, and your centripetal acceleration at 11,000 m/sec is 19.64
m/s/s - double that of gravity. An object constrained to travel at
this radius at this speed would experience 2 gees of force directed
away from the center of rotation. Now, if that center of rotation
happens to be the center of the earth, then, 1 gravity pulls them back
toward the center, leaving one gravity directed away from the center.
So, someone sitting in the aircraft would have the aircraft 'lift'
pulling holding it at its altitude, and the persons sititing inside
would be seated looking out the window, feeling nothing unusual, except
the Earth would be ABOVE their heads, and the sky below.
| Quote: | However, there may be a way to use aerodynamic forces to increase the
forward speed.
|
No, aerodynamic forces will always slow an aircraft. That's the nature
of aerodynamic forces. If you want to add energy to the flow somehow,
that would be called thrust, and it involves the expenditure of energy.
| Quote: | The phenomenon of "tacking into the wind" in sailing
allows a sail boat to actually have a forward velocity component that
goes *into* the wind.
|
Word hash. This is a trick with vector sums and relies on the relative
speed of wind and water. There are two fluids here moving relative to
one another. The boat takes advantage of both, with the keel operating
on the water and the sail operating in the air.
http://www.phys.unsw.edu.au/~jw/sailing.html
???
Tell me did you read this? I mean did you see the part about the keel?
You know that part in the water? Sheez.
An aircraft isn't floating in water. Its floating in air. I suppose
if you had two bodies of air moving at different speed relative to one
another you could take advantage of that difference to extract thrust
from it. But that's quite different than extracting thrust from
aerodynamic drag - which is not what I'm talking about.
So, you are proceeding from error to error each one building on the
next. lol.
| Quote: | The method is used with boats that have a keel that extends into the
water.
|
Yes. And there is no keel and no water in a free flying aircraft.
| Quote: | The basic idea is that if the boat is sailing at an angle to the
wind then the wind is pushing the boat at an angle. This causes the
keel to push on the water at an angle which means the water is
providing an equal and opposite force on the boat. Note now though this
force on the boat from the water does have a *forward* component. And
the sum total of the vector forces of the wind and the water on the
boat also has a forward component.
|
Look at the freakin' pictures - its a problem of vectors. You're not
sailing directly into the wind, you're sailing at an angle across the
wind, and taking advantage of how the vectors sum on the keel and the
sail to extract thrust from the difference.
| Quote: | This causes the boat to move at an angle into the wind. To arrive at a
course direction directly into the wind, the boat is made to move first
to the right of the wind and to the left of it alternatingly in zigzag
fashion.
|
The boat doesn't move directly into the wind, it moves at an angle
towards the wind, it has to tack - as you say, and while it can move
faster than the wind across the wind, it cannot move faster than the
wind into the wind. Check it out. Take the hypotenuse of the vector
triangle, and project it into the wind - its less than wind speed dude.
| Quote: | Could this idea be applied to hypersonic waveriders?
|
No.
| Quote: | At hypersonic
speeds, the density of the air within the shock wave is many times the
density of the ambient air, as water is many times the density of sea
level air.
|
Ever hear of the Hugoniot relations? They accurately describe
compressible supersonic flows. And they demonstrate that what you are
proposing to do is impossible in normally constitute fluids.
http://www.pma.caltech.edu/Courses/ph136/yr2004/0416.2.K.pdf
Now there are exceptions to everything and in this case its if the
compressible flow detonates - then one of the assumptions is violated,
and you can produce thrust. But that's a totally different thing than
you're talking about - and ALL of this is a totally different thing
than what I talked about originally.
| Quote: | Then the idea would be to have a "keel" that extends into
the shockwave and a vertical airfoil (a "sail") that extends through
the shock layer into the surrounding low density air.
See the last image on this page:
|
The keel is creating the shockwave dude. Read that chaper I provided
and when you understand it, then come back and post. What you are
posting here is drivel.
| Quote: | Hypersonic Flow.
http://www.aerospaceweb.org/design/waverider/flow.shtml
You see the shock layer is close to the craft on the bottom of the
craft and extends further out at the top. Then the "keel" would extend
from the top in this case to remain within the shock layer, and the
airfoil "sail" would extend from the bottom into extend into the
ambient air.
|
Nope it don't work that way. Any object in the stream creates its own
shockwave. The angles and momentum transfer is such that it always
produces drag - for a normal fluid like air. Now, if you have an
explosive, that happens to release energy based on density, then you
might have something. But that's not a normal fluid. That's a fluid
that generates energy - a combustible mixture or something.
So? Read about how compressible flows work and then come back.
| Quote: | So at Mach 20 using a ratio of specific heats gamma of 1.4 for air,
the pressure increase would be by factor of 4783 of the pressure
initially in the shock layer over the pressure of the ambient air. This
is greater than for example the ratio of the ratio of the pressure of
the water on a keel than the pressure of the sea level air on a sail.
This pressure within the shock layer though would decrease as you get
further from the front of the vehicle so you would want the "keel" to
be close to the front.
|
The sailboat was a good example of how to do vector sums. That's it.
Could you do a vector sum of what's happening to the shock wave here?
Well, Chapter 16 will help you. You will find that you can't get
thrust this way - with air as the fluid that is.
| Quote: | The question, would the addition of these extra structures result in
just an increase in the overall drag of the vehicle?
|
Yes, definitely. Because they create their own shock waves If the
boat didn't have a keel, it couldn't tack into the wind. The shockwave
is part and parcel of the air that forms it.
| Quote: | As discussed on this page the addition of winglets to aircraft has a
similar effect of producing extra thrust, so reducing the overall drag:
How Things Work: Winglets.
"The airflow around winglets is complicated, and winglets have to be
carefully designed and tested for each aircraft. Cant, the angle to
which the winglet is bent from the vertical, and toe, the angle at
which the winglets' airfoils diverge from the relative wind direction,
determine the magnitude and orientation of the lift force generated by
the winglet itself. By adjusting these so that the lift force points
slightly forward, a designer can produce the equivalent of thrust. A
sailboat tacking sharply upwind creates a similar force with its sail
while the keel squeezes the boat forward like a pinched watermelon
seed."
http://www.airspacemag.com/ASM/Mag/Index/2001/AS/htww.html
|
Winglets do not produce thrust. They reduce drag by reducing wingtp
vortices. See, the wing causes a deflection of the air downward,
creating lift on the wing's surface. The air just outside the tip of
the wing isn't deflected. This causes a rotation of the air, and
increases drag slightly. A winglet at the tip of a wing, reduces this
vortical motoin, and thus reduces drag.
| Quote: | It would appear from Newton's second law you could not get more
forward acceleration than from the craft operating in vacuum under the
propulsion method used.
|
That is correct.
| Quote: | Nevertheless, effectively you get more forward
acceleration in air than without these extra structures because of the
overall drag reduction.
|
You made a logical error if you're connecting these two. You compared
an aircraft in vacuum to an aircraft in the air - and then switched to
say if you have more surface area you must get more drag. The shape of
the surface has an effect, and the total area involved is small while
the effect is large, since the vortice is concentrated in a small
volume around the aircraft;
http://www.nasa.gov/centers/dryden/pdf/89234main_TF-2004-15-DFRC.pdf
| Quote: | Just as importantly, the overall reduction in
drag would result in an increase in the lift to drag ratio.
|
Depends on the details. But generally speaking winglets on the
wingtips of aircraft are a good thing performance wise. They're not
producing thrust. They're not violating laws of physics.
Bob you have proceeded from error to error and ended up in a bog. I
think I'll call you Bog Clark.
| Quote: |
Robert Clark wrote:
William.Mook@gmail.com wrote:
Orbital speed is where centripetal force equals gravity force and is
given by;
v = sqrt(GMe/r)
Which can be derived from the following three equations;
F = G*m*Me/r^2 - gravitational force
a = v^2/r - centripetal acceleration
F = ma - relating mass and acceleration
a = F/m = GMe/r^2 - gravitational acceleration
a = v^2/r - centripetal acceleration
Setting the two accelerations equal
v^2/r = GMe/r^2
v^2 = GMe/r
v = sqrt(GMe/r)
If we increase velocity by 41.4% we double the centripetal
acceleration, which means that if we were to fly an aircraft at Mach 33
we'd need wings to hold it in the atmosphere! Since wings lift
aircraft all the time against gravity, it seems reasonable to believe
that wings could hold an aircraft down. Everything would seem quite
normal to the occupants, except down would be up to them, and the lift
would be directed toward the Earth's center.
The vehicle if possible would be capable of circumnavigating the Earth
in 60 minutes - and delivering payloads to targets anywhere in 30
minutes or less.
Would such a craft be possible?
Yes. I speculated about this possibility for the use with beamed
propulsion:
From: Robert Clark
Date: Sat, Nov 19 2005 2:23 pm
Email: "Robert Clark" <rgregorycl...@yahoo.com
Groups: sci.astro, sci.physics, sci.math
Subject: Math question for the trajectory of beamed propulsion.
http://groups.google.com/group/sci.astro/msg/71a00732000ef7f7
This would also be applicable to the scenario where electrical power
for propulsion is transmitted though long cables:
From: Robert Clark
Date: Fri, May 27 2005 12:10 pm
Email: "Robert Clark" <rgregorycl...@yahoo.com
Groups: sci.astro, sci.space.policy, sci.physics,
sci.electronics.design, sci.electronics.misc
Subject: Re: Long cables to power "ioncraft" to orbit?
http://groups.google.com/group/sci.astro/msg/5a2b09463e87dde6
The problem is that though the height to orbit might be 100 km, the
horizontal distance travelled might be 2000 km in order to build up
sufficient speed for orbital velocity.
The proposals for beamed propulsion I've seen though do not use
lifting surfaces for the craft:
Riding Laser Beams to Space.
http://www.space.com/businesstechnology/technology/laser_propulsion_000705.html
However, the lift to drag ratios at hypersonic speeds suggest we might
be able to increase the thrust and therefore the acceleration by
several times if the craft was designed for aerodynamic lift. See the
graph showing lift to drag ratio versus Mach number here:
Waverider Design.
http://www.aerospaceweb.org/design/waverider/waverider.shtml
With airplanes you have the thrust directed horizontally to overcome
the drag force against forward motion and the lift provides the force
to keep the airplane aloft. Since subsonic L/D ratios can be 15 to 1
and higher the thrust required from the engines is much less than the
actual weight of the plane.
However, with beamed propulsion a key problem is the dimunition of the
power with distance, which decreases with the square of the distance so
you want to keep the distance short. The idea then in this case using
aerodynamic lift would be to use the thrust produced by the beamed
propulsion to overcome gravity and drag and use the lift force to
provide the higher acceleration to reach orbital velocity in a shorter
distance. Essentially the craft would be pointed upwards so that the
wings/lifting surfaces provide the "lift" in the horizontal direction.
The graph on the "Waverider Design" page shows the L/D ratio can be
about 7 to 8 at hypersonic speeds. For instance if the beamed
propulsion provided a thrust of 1 g to counter gravity plus 4 g's
against drag for a total of 5 g's in the vertical direction, then the
horizontal acceleration could be as much as 8*4 = 32 g's.
Note though it would be important to keep the craft oriented so that
so that the velocity vector is always pointed through the forward
centerline of the craft. When lift and drag calculations are made it's
always in regard to the craft moving so the airstream is flowing more
or less parallel over the wings/lifting surfaces, according to angle of
attack. If instead the airstream was flowing perpindicular to the plane
of the wings the lift would be much less and drag would be much greater
so the L/D ratio would be severely reduced. The aerodynamic control
surfaces would be used to keep the craft properly oriented.
Estimates for beamed propulsion are about 1 megawatt of power to send
1 kilogram to orbit. If say such beamed propulsion provided thrust for
5 g's of acceleration then the lifting force could provide 32 g's, or a
factor of 6 more. So the distance required would be smaller by a factor
6. This means the power required would be smaller by a factor 6^2 = 36.
Then 36 times greater mass could be lifted for the same power. This is
dependent though on how much acceleration beamed propulsion could
provide. If it were 7 g's then the lifting acceleration would be 8*6 =
48 g's, about a factor of 7 more. Then the power required would be less
by 7^2 = 49, and 49 times greater mass could be lifted.
There are apparently megawatt class lasers already in operation:
Mid-Infrared Advanced Chemical Laser (MIRACL).
http://www.fas.org/spp/military/program/asat/miracl.htm
Let's say they are at the 10 megawatt stage now. Then this could
accelerate 10 kilos to orbit. Then with aerodynamic lift it could lift
perhaps 360 kilos to orbit, which is the size of a small sized
satellite.
Bob Clark |
|
|
| Back to top |
|
|
George Dishman
science forum Guru
Joined: 08 May 2005
Posts: 963
|
| Posted: Mon Jul 17, 2006 7:48 pm Post subject:
Re: Using lift to increase speeds.
|
|
|
<William.Mook@gmail.com> wrote in message
news:1153164469.124673.83120@h48g2000cwc.googlegroups.com...
| Quote: |
Robert Clark wrote:
....
The phenomenon of "tacking into the wind" in sailing
allows a sail boat to actually have a forward velocity component that
goes *into* the wind.
Word hash.
|
If you think about it for a moment, he is describing
how birds propel themselves forward by flapping their
wings. Compare a side view of a bird to a bird's eye
view of a yacht. It won't revolutionise spaceflight
though ;-)
George |
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Robert Clark
science forum Guru Wannabe
Joined: 30 Apr 2005
Posts: 129
|
| Posted: Mon Jul 17, 2006 11:10 pm Post subject:
Re: Using lift to increase speeds.
|
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Since as you say this is not related to what your were proposing, I
continued it in a different thread.
Bob Clark
William.Mook@gmail.com wrote:
| Quote: | Robert Clark wrote:
The idea was to use the lift force to increase the forward velocity of
the craft.
Nope. You've misunderstood it totally. Thrust propels the aircraft,
lift keeps it airborne under normal circumstances. This is inverse
lift . I suppose you could say you use lift to maintain altitude
against centripetal force. The lift doesn't increase speed in any way.
In fact staying in the air induces drag and then there's drag induce
lift. Negative lift in this case allows you to operate at higher
speeds than you might, even though the these losses are present. Lift
doesn't contribute to your thrust at all.
However, I have been informed by email that since lift is
always perpindicular to the velocity it can not be used to increase the
forward speed.
The person who informed you was right. You are totally confused about
what's going on.
Nevertheless the idea of having the craft travel in a
circle could still work by using the lift force to counteract the large
acceleration implied by the formula a = v^2/r.
You are even more confused than you were a second ago! But so close!
Forget about what you're thinking and think this.
The circle you're travelling in is a great circle route around the
surface of the Earth. If you complete an orbit your r = 6,366,198
meters, and your centripetal acceleration at 11,000 m/sec is 19.64
m/s/s - double that of gravity. An object constrained to travel at
this radius at this speed would experience 2 gees of force directed
away from the center of rotation. Now, if that center of rotation
happens to be the center of the earth, then, 1 gravity pulls them back
toward the center, leaving one gravity directed away from the center.
So, someone sitting in the aircraft would have the aircraft 'lift'
pulling holding it at its altitude, and the persons sititing inside
would be seated looking out the window, feeling nothing unusual, except
the Earth would be ABOVE their heads, and the sky below.
However, there may be a way to use aerodynamic forces to increase the
forward speed.
No, aerodynamic forces will always slow an aircraft. That's the nature
of aerodynamic forces. If you want to add energy to the flow somehow,
that would be called thrust, and it involves the expenditure of energy.
The phenomenon of "tacking into the wind" in sailing
allows a sail boat to actually have a forward velocity component that
goes *into* the wind.
Word hash. This is a trick with vector sums and relies on the relative
speed of wind and water. There are two fluids here moving relative to
one another. The boat takes advantage of both, with the keel operating
on the water and the sail operating in the air.
http://www.phys.unsw.edu.au/~jw/sailing.html
Indeed using this method the boat's speed can
actually exceed the wind speed. This page provides a good explanation:
The physics of sailing.
http://www.phys.unsw.edu.au/~jw/sailing.html
???
Tell me did you read this? I mean did you see the part about the keel?
You know that part in the water? Sheez.
An aircraft isn't floating in water. Its floating in air. I suppose
if you had two bodies of air moving at different speed relative to one
another you could take advantage of that difference to extract thrust
from it. But that's quite different than extracting thrust from
aerodynamic drag - which is not what I'm talking about.
So, you are proceeding from error to error each one building on the
next. lol.
The method is used with boats that have a keel that extends into the
water.
Yes. And there is no keel and no water in a free flying aircraft.
The basic idea is that if the boat is sailing at an angle to the
wind then the wind is pushing the boat at an angle. This causes the
keel to push on the water at an angle which means the water is
providing an equal and opposite force on the boat. Note now though this
force on the boat from the water does have a *forward* component. And
the sum total of the vector forces of the wind and the water on the
boat also has a forward component.
Look at the freakin' pictures - its a problem of vectors. You're not
sailing directly into the wind, you're sailing at an angle across the
wind, and taking advantage of how the vectors sum on the keel and the
sail to extract thrust from the difference.
This causes the boat to move at an angle into the wind. To arrive at a
course direction directly into the wind, the boat is made to move first
to the right of the wind and to the left of it alternatingly in zigzag
fashion.
The boat doesn't move directly into the wind, it moves at an angle
towards the wind, it has to tack - as you say, and while it can move
faster than the wind across the wind, it cannot move faster than the
wind into the wind. Check it out. Take the hypotenuse of the vector
triangle, and project it into the wind - its less than wind speed dude.
Could this idea be applied to hypersonic waveriders?
No.
At hypersonic
speeds, the density of the air within the shock wave is many times the
density of the ambient air, as water is many times the density of sea
level air.
Ever hear of the Hugoniot relations? They accurately describe
compressible supersonic flows. And they demonstrate that what you are
proposing to do is impossible in normally constitute fluids.
http://www.pma.caltech.edu/Courses/ph136/yr2004/0416.2.K.pdf
Now there are exceptions to everything and in this case its if the
compressible flow detonates - then one of the assumptions is violated,
and you can produce thrust. But that's a totally different thing than
you're talking about - and ALL of this is a totally different thing
than what I talked about originally.
Then the idea would be to have a "keel" that extends into
the shockwave and a vertical airfoil (a "sail") that extends through
the shock layer into the surrounding low density air.
See the last image on this page:
The keel is creating the shockwave dude. Read that chaper I provided
and when you understand it, then come back and post. What you are
posting here is drivel.
Hypersonic Flow.
http://www.aerospaceweb.org/design/waverider/flow.shtml
You see the shock layer is close to the craft on the bottom of the
craft and extends further out at the top. Then the "keel" would extend
from the top in this case to remain within the shock layer, and the
airfoil "sail" would extend from the bottom into extend into the
ambient air.
Nope it don't work that way. Any object in the stream creates its own
shockwave. The angles and momentum transfer is such that it always
produces drag - for a normal fluid like air. Now, if you have an
explosive, that happens to release energy based on density, then you
might have something. But that's not a normal fluid. That's a fluid
that generates energy - a combustible mixture or something.
This page gives a formula for the stagnation pressure of the shock
layer at least initially:
STAGNATION PRESSURE
http://www.ae.utexas.edu/courses/ase120k/ase120k_stagpres_temp.html
So? Read about how compressible flows work and then come back.
So at Mach 20 using a ratio of specific heats gamma of 1.4 for air,
the pressure increase would be by factor of 4783 of the pressure
initially in the shock layer over the pressure of the ambient air. This
is greater than for example the ratio of the ratio of the pressure of
the water on a keel than the pressure of the sea level air on a sail.
This pressure within the shock layer though would decrease as you get
further from the front of the vehicle so you would want the "keel" to
be close to the front.
The sailboat was a good example of how to do vector sums. That's it.
Could you do a vector sum of what's happening to the shock wave here?
Well, Chapter 16 will help you. You will find that you can't get
thrust this way - with air as the fluid that is.
The question, would the addition of these extra structures result in
just an increase in the overall drag of the vehicle?
Yes, definitely. Because they create their own shock waves If the
boat didn't have a keel, it couldn't tack into the wind. The shockwave
is part and parcel of the air that forms it.
As discussed on this page the addition of winglets to aircraft has a
similar effect of producing extra thrust, so reducing the overall drag:
How Things Work: Winglets.
"The airflow around winglets is complicated, and winglets have to be
carefully designed and tested for each aircraft. Cant, the angle to
which the winglet is bent from the vertical, and toe, the angle at
which the winglets' airfoils diverge from the relative wind direction,
determine the magnitude and orientation of the lift force generated by
the winglet itself. By adjusting these so that the lift force points
slightly forward, a designer can produce the equivalent of thrust. A
sailboat tacking sharply upwind creates a similar force with its sail
while the keel squeezes the boat forward like a pinched watermelon
seed."
http://www.airspacemag.com/ASM/Mag/Index/2001/AS/htww.html
Winglets do not produce thrust. They reduce drag by reducing wingtp
vortices. See, the wing causes a deflection of the air downward,
creating lift on the wing's surface. The air just outside the tip of
the wing isn't deflected. This causes a rotation of the air, and
increases drag slightly. A winglet at the tip of a wing, reduces this
vortical motoin, and thus reduces drag.
It would appear from Newton's second law you could not get more
forward acceleration than from the craft operating in vacuum under the
propulsion method used.
That is correct.
Nevertheless, effectively you get more forward
acceleration in air than without these extra structures because of the
overall drag reduction.
You made a logical error if you're connecting these two. You compared
an aircraft in vacuum to an aircraft in the air - and then switched to
say if you have more surface area you must get more drag. The shape of
the surface has an effect, and the total area involved is small while
the effect is large, since the vortice is concentrated in a small
volume around the aircraft;
http://www.nasa.gov/centers/dryden/pdf/89234main_TF-2004-15-DFRC.pdf
Just as importantly, the overall reduction in
drag would result in an increase in the lift to drag ratio.
Depends on the details. But generally speaking winglets on the
wingtips of aircraft are a good thing performance wise. They're not
producing thrust. They're not violating laws of physics.
Bob Clark
Bob you have proceeded from error to error and ended up in a bog. I
think I'll call you Bog Clark.
Robert Clark wrote:
William.Mook@gmail.com wrote:
Orbital speed is where centripetal force equals gravity force and is
given by;
v = sqrt(GMe/r)
Which can be derived from the following three equations;
F = G*m*Me/r^2 - gravitational force
a = v^2/r - centripetal acceleration
F = ma - relating mass and acceleration
a = F/m = GMe/r^2 - gravitational acceleration
a = v^2/r - centripetal acceleration
Setting the two accelerations equal
v^2/r = GMe/r^2
v^2 = GMe/r
v = sqrt(GMe/r)
If we increase velocity by 41.4% we double the centripetal
acceleration, which means that if we were to fly an aircraft at Mach 33
we'd need wings to hold it in the atmosphere! Since wings lift
aircraft all the time against gravity, it seems reasonable to believe
that wings could hold an aircraft down. Everything would seem quite
normal to the occupants, except down would be up to them, and the lift
would be directed toward the Earth's center.
The vehicle if possible would be capable of circumnavigating the Earth
in 60 minutes - and delivering payloads to targets anywhere in 30
minutes or less.
Would such a craft be possible?
Yes. I speculated about this possibility for the use with beamed
propulsion:
From: Robert Clark
Date: Sat, Nov 19 2005 2:23 pm
Email: "Robert Clark" <rgregorycl...@yahoo.com
Groups: sci.astro, sci.physics, sci.math
Subject: Math question for the trajectory of beamed propulsion.
http://groups.google.com/group/sci.astro/msg/71a00732000ef7f7
This would also be applicable to the scenario where electrical power
for propulsion is transmitted though long cables:
From: Robert Clark
Date: Fri, May 27 2005 12:10 pm
Email: "Robert Clark" <rgregorycl...@yahoo.com
Groups: sci.astro, sci.space.policy, sci.physics,
sci.electronics.design, sci.electronics.misc
Subject: Re: Long cables to power "ioncraft" to orbit?
http://groups.google.com/group/sci.astro/msg/5a2b09463e87dde6
The problem is that though the height to orbit might be 100 km, the
horizontal distance travelled might be 2000 km in order to build up
sufficient speed for orbital velocity.
The proposals for beamed propulsion I've seen though do not use
lifting surfaces for the craft:
Riding Laser Beams to Space.
http://www.space.com/businesstechnology/technology/laser_propulsion_000705.html
However, the lift to drag ratios at hypersonic speeds suggest we might
be able to increase the thrust and therefore the acceleration by
several times if the craft was designed for aerodynamic lift. See the
graph showing lift to drag ratio versus Mach number here:
Waverider Design.
http://www.aerospaceweb.org/design/waverider/waverider.shtml
With airplanes you have the thrust directed horizontally to overcome
the drag force against forward motion and the lift provides the force
to keep the airplane aloft. Since subsonic L/D ratios can be 15 to 1
and higher the thrust required from the engines is much less than the
actual weight of the plane.
However, with beamed propulsion a key problem is the dimunition of the
power with distance, which decreases with the square of the distance so
you want to keep the distance short. The idea then in this case using
aerodynamic lift would be to use the thrust produced by the beamed
propulsion to overcome gravity and drag and use the lift force to
provide the higher acceleration to reach orbital velocity in a shorter
distance. Essentially the craft would be pointed upwards so that the
wings/lifting surfaces provide the "lift" in the horizontal direction.
The graph on the "Waverider Design" page shows the L/D ratio can be
about 7 to 8 at hypersonic speeds. For instance if the beamed
propulsion provided a thrust of 1 g to counter gravity plus 4 g's
against drag for a total of 5 g's in the vertical direction, then the
horizontal acceleration could be as much as 8*4 = 32 g's.
Note though it would be important to keep the craft oriented so that
so that the velocity vector is always pointed through the forward
centerline of the craft. When lift and drag calculations are made it's
always in regard to the craft moving so the airstream is flowing more
or less parallel over the wings/lifting surfaces, according to angle of
attack. If instead the airstream was flowing perpindicular to the plane
of the wings the lift would be much less and drag would be much greater
so the L/D ratio would be severely reduced. The aerodynamic control
surfaces would be used to keep the craft properly oriented.
Estimates for beamed propulsion are about 1 megawatt of power to send
1 kilogram to orbit. If say such beamed propulsion provided thrust for
5 g's of acceleration then the lifting force could provide 32 g's, or a
factor of 6 more. So the distance required would be smaller by a factor
6. This means the power required would be smaller by a factor 6^2 = 36.
Then 36 times greater mass could be lifted for the same power. This is
dependent though on how much acceleration beamed propulsion could
provide. If it were 7 g's then the lifting acceleration would be 8*6 =
48 g's, about a factor of 7 more. Then the power required would be less
by 7^2 = 49, and 49 times greater mass could be lifted.
There are apparently megawatt class lasers already in operation:
Mid-Infrared Advanced Chemical Laser (MIRACL).
http://www.fas.org/spp/military/program/asat/miracl.htm
Let's say they are at the 10 megawatt stage now. Then this could
accelerate 10 kilos to orbit. Then with aerodynamic lift it could lift
perhaps 360 kilos to orbit, which is the size of a small sized
satellite.
Bob Clark |
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William.Mook@gmail.com
science forum addict
Joined: 06 May 2006
Posts: 50
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| Posted: Tue Jul 18, 2006 2:23 am Post subject:
Re: Using lift to increase speeds.
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To be honest, I didn't see it that way. But its a novel concept.
Flapping your wings is a way of introducing energy to the flow. So, it
could work theoretically I suppose.
Boats do this by sticking oars into the water and moving it. I guess
airboats do this with propellors attached to them.
If you had an engine that changed the relative angle between Sail and
Keel, you could flap the 'wings' of a sail boat and produce 'thrust'.
That would be cool. Airplanes do this with propellers or jet engines.
But this engine would have to burn a fuel or something to energize it.
Just as a bird or a bee needs to eat to energize its wings.
A supersonic ornithopter? A novel concept. MEMs based? Ooo... it
makes me sweat just thinking about it. Yeah, tiny shockwaves changing
at ultrasonic speeds to create thrust? Piezoelectric based supersonic
propulsion? Nifty - if possible.
This is a cool thread of conciousness, it may even be fruitful, maybe
guiding bullets in flight, or even extending the range of bullets
considerably, along with increasing their accuracy. Maybe for other
stuff as well. Who knows??
But it wasn't what I was talking about, and its as yet half baked.
And its not using the flow to produce thrust, its adding energy to the
flow to change the momentum of air passing by to produce thrust.
Something totally different. |
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William.Mook@gmail.com
science forum addict
Joined: 06 May 2006
Posts: 50
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| Posted: Tue Jul 18, 2006 2:26 am Post subject:
Re: Using lift to increase speeds.
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That's cool Robert. I made a comment there. Sometimes I say what I
feel, and people get agitated because I'm very disrespectful. lol.
Now, that I'm just as hard on myself when I make a bone-headed
statement, or find myself absolutely wrong! That's the price you pay
when you think of technical things. Sometimes, oft-times, you're
wrong. The way to progress is accept when you're wrong, understand how
you came to be wrong, and learn from it, and move on.
You seem like a creative and knowledgeable person. Read the references
I provided, they're good stuff. And understand how you were wrong and
what we're all going on about. We're not out to get you. lol. Though
it may seem like it! haha..
Good luck |
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Paul O
science forum beginner
Joined: 01 Feb 2006
Posts: 11
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| Posted: Tue Jul 18, 2006 3:31 pm Post subject:
Re: Using lift to increase speeds.
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William.Mook@gmail.com wrote:
| Quote: | To be honest, I didn't see it that way. But its a novel concept.
Flapping your wings is a way of introducing energy to the flow. So, it
could work theoretically I suppose.
Boats do this by sticking oars into the water and moving it. I guess
airboats do this with propellors attached to them.
If you had an engine that changed the relative angle between Sail and
Keel, you could flap the 'wings' of a sail boat and produce 'thrust'.
That would be cool. Airplanes do this with propellers or jet engines.
But this engine would have to burn a fuel or something to energize it.
Just as a bird or a bee needs to eat to energize its wings.
A supersonic ornithopter? A novel concept. MEMs based? Ooo... it
makes me sweat just thinking about it. Yeah, tiny shockwaves changing
at ultrasonic speeds to create thrust? Piezoelectric based supersonic
propulsion? Nifty - if possible.
This is a cool thread of conciousness, it may even be fruitful, maybe
guiding bullets in flight, or even extending the range of bullets
considerably, along with increasing their accuracy. Maybe for other
stuff as well. Who knows??
But it wasn't what I was talking about, and its as yet half baked.
And its not using the flow to produce thrust, its adding energy to the
flow to change the momentum of air passing by to produce thrust.
Something totally different.
|
William,
It is possible to propel a small sailboat by flapping the sails! This
form of propulsion (which in sailboat racing lingo is called "pumping")
is achieved by (repeatedly) quickly pulling in the sail and and then
releasing it.
Pumping to move ones boat (during a race) is generally prohibited by
most sailboat racing organizations. See:
Section 42.2 of:
http://www.sailing.org/RRS2005/Intro1-7.pdf
and:
http://propercourse.blogspot.com/2005/10/here-come-da-judge.html
Good Luck,
Paul D Oosterhout
from SAIC |
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Tom Sanderson
science forum addict
Joined: 22 Dec 2005
Posts: 55
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| Posted: Thu Jul 20, 2006 2:09 pm Post subject:
Re: "Closed" Cycle Internal Reaction Engines
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"Bret Cahill" <BretCahill@aol.com> wrote:
| Quote: | With gasoline it's an irreversible reaction.
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No, it isn't. You can generate hydrocarbons using steam and CO2 as
feedstock.
| Quote: | What's fast, highly exothermic, reversible, nontoxic and the products
and reactants liquify at 200 psi, 150 F?
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If it's fast and highly exothermic, then the reverse reaction will require
enourmous energy input. That's just energy balance at work.
Tom. |
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