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Old Nov 13, 2011, 05:26 PM
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NASA says lift from air shoved down - can we trust NASA?

Here's a cut-n-paste from the NASA Glenn Reasearch Center's educational section devoted to college level and below.

Note that NASA states that it's the air being shoved down that causes lift.

The URL is

http://www.grc.nasa.gov/WWW/k-12/airplane/lift1.html

Here's the text, copied and pasted here, without change ...

"Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air. Lift is generated by every part of the airplane, but most of the lift on a normal airliner is generated by the wings. Lift is a mechanical aerodynamic force produced by the motion of the airplane through the air. Because lift is a force, it is a vector quantity, having both a magnitude and a direction associated with it. Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction. There are several factors which affect the magnitude of lift.

HOW IS LIFT GENERATED?

There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments. To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail.
Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

NO FLUID, NO LIFT

Lift is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It is not generated by a force field, in the sense of a gravitational field,or an electromagnetic field, where one object can affect another object without being in physical contact. For lift to be generated, the solid body must be in contact with the fluid: no fluid, no lift. The Space Shuttle does not stay in space because of lift from its wings but because of orbital mechanics related to its speed. Space is nearly a vacuum. Without air, there is no lift generated by the wings.

NO MOTION, NO LIFT

Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion. Drag acts in the direction opposed to the motion."

My personal observations and experiments cause me to agree with the above. Why did I post this - because I've become concerned with the propagation of the wrong versions of lift (basically, what we were all taught up through 1990) - and as this is a modeling science forum, I thought there should be one post that's got this right.
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Old Nov 13, 2011, 05:55 PM
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As far as it goes NASA's part of that is all perfectly reasonable. Of course it fails to explain how or why the "moving flow of gas is turned by a solid object" and that's always where most of the debate comes in .

The rest of the debate is caused when people paraphrase things incorrectly e.g. converting the accurate statement "Lift occurs when a moving flow of gas is turned by a solid object" into the incorrect "it's the air being shoved down that causes lift". That's isn't what NASA says nor is it what they mean .

Steve
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Old Nov 13, 2011, 06:40 PM
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Hi Tim, I will be nice this time...I promise Please, for the sake of the discussion, rather than completely shutting everything off, open your mind a bit to the whole picture. Never once did we disagree with you about it being shoved down, we all agreed that does take place, but there is a lot more to it than that.

For Example, since you like NASA links, here is one from the same guys as your link that say downwash reduces lift, rather than causes it.
http://www.grc.nasa.gov/WWW/k-12/airplane/downwash.html
Cut-n-paste from this link
"The wing tip vortices produce a downwash of air behind the wing which is very strong near the wing tips and decreases toward the wing root. The local angle of attack of the wing is increased by the flow induced by the downwash, giving an additional, downstream-facing, component to the aerodynamic force acting over the entire wing. The downstream component of the force is called induced drag because it faces downstream and has been "induced" by the action of the tip vortices. The lift near the wing tips is defined to be perpendicular to the local flow. The local flow is at a greater angle of attack than the free stream flow because of the induced flow. Resolving the tip lift back to the free stream reference produces a reduction in the lift coefficient of the entire wing".

This one is neat, also from your same source. This one is called The Lift Equation, and what do you know, this "official" NASA lift equation has not just the velocity of the air, but it also takes into account the dynamic pressure, and it says that that portion of the equation is taken from Bernoulli's equation...wow!!
http://www.grc.nasa.gov/WWW/k-12/airplane/lifteq.html
Cut-n-paste from that link:
In the equation given above, the density is designated by the letter "r." We do not use "d" for density, since "d" is often used to specify distance. In many textbooks on aerodynamics, the density is given by the Greek symbol "rho" (Greek for "r"). The combination of terms "density times the square of the velocity divided by two" is called the dynamic pressure and appears in Bernoulli's pressure equation.


And here is yet another one from your very same source, which explains hw both the pressure difference/Bernoulli theory and turning flow/Newton theory are BOTH correct, if applied correctly.
the quote:
"So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force. "
The link
http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

And here is another one from them same fella's that explains in detail Bernoullli's principle, his equation, and how it relates to lift. It also states how you can use his equation to calculate the TOTAL AERODYNAMIC FORCE on the airfoil....here is that direct cut and paste quote....
"Along a low speed airfoil, the flow is incompressible and the density remains a constant. Bernoulli's equation then reduces to a simple relation between velocity and static pressure. The surface of the airfoil is a streamline. Since the velocity varies along the streamline, Bernoulli's equation can be used to compute the change in pressure. The static pressure integrated along the entire surface of the airfoil gives the total aerodynamic force on the foil. This force can be broken down into the lift and drag of the airfoil."
And the link...
http://www.grc.nasa.gov/WWW/k-12/airplane/bern.html

So my friend, it seems as there is a heck of a lot more going on than simply, air being shoved down, as your own guys pointed out in that really cool website they have. So, does shoving air down produce lift...yup. Does the pressure difference produce lift...yup.

Quote:
My personal observations and experiments cause me to agree with the above
I know we went through this before (at great length) and I know you won't want to listen, but come on man, your experiments do not prove anything to do with lift. Myself, Bruce Matthews, and a couple others made that pretty clear. Don't even start with them again.
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Old Nov 13, 2011, 06:51 PM
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Thanks for starting this topic.

I did not see anything I disagree with. The description is not rigorous, but I would say it captures some of the major points. Would anyone else care to help make it more rigorous?

I'll start by saying we are considering a Newtonian fluid, not just a gas.
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Old Nov 14, 2011, 07:23 AM
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Originally Posted by stardustertoo View Post
Hi Tim, I will be nice this time...I promise Please, for the sake of the discussion, rather than completely shutting everything off, open your mind a bit to the whole picture. Never once did we disagree with you about it being shoved down, we all agreed that does take place, but there is a lot more to it than that.

For Example, since you like NASA links, here is one from the same guys as your link that say downwash reduces lift, rather than causes it.
http://www.grc.nasa.gov/WWW/k-12/airplane/downwash.html
Cut-n-paste from this link
"The wing tip vortices produce a downwash of air behind the wing which is very strong near the wing tips and decreases toward the wing root. The local angle of attack of the wing is increased by the flow induced by the downwash, giving an additional, downstream-facing, component to the aerodynamic force acting over the entire wing. The downstream component of the force is called induced drag because it faces downstream and has been "induced" by the action of the tip vortices. The lift near the wing tips is defined to be perpendicular to the local flow. The local flow is at a greater angle of attack than the free stream flow because of the induced flow. Resolving the tip lift back to the free stream reference produces a reduction in the lift coefficient of the entire wing".

This one is neat, also from your same source. This one is called The Lift Equation, and what do you know, this "official" NASA lift equation has not just the velocity of the air, but it also takes into account the dynamic pressure, and it says that that portion of the equation is taken from Bernoulli's equation...wow!!
http://www.grc.nasa.gov/WWW/k-12/airplane/lifteq.html
Cut-n-paste from that link:
In the equation given above, the density is designated by the letter "r." We do not use "d" for density, since "d" is often used to specify distance. In many textbooks on aerodynamics, the density is given by the Greek symbol "rho" (Greek for "r"). The combination of terms "density times the square of the velocity divided by two" is called the dynamic pressure and appears in Bernoulli's pressure equation.


And here is yet another one from your very same source, which explains hw both the pressure difference/Bernoulli theory and turning flow/Newton theory are BOTH correct, if applied correctly.
the quote:
"So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force. "
The link
http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

And here is another one from them same fella's that explains in detail Bernoullli's principle, his equation, and how it relates to lift. It also states how you can use his equation to calculate the TOTAL AERODYNAMIC FORCE on the airfoil....here is that direct cut and paste quote....
"Along a low speed airfoil, the flow is incompressible and the density remains a constant. Bernoulli's equation then reduces to a simple relation between velocity and static pressure. The surface of the airfoil is a streamline. Since the velocity varies along the streamline, Bernoulli's equation can be used to compute the change in pressure. The static pressure integrated along the entire surface of the airfoil gives the total aerodynamic force on the foil. This force can be broken down into the lift and drag of the airfoil."
And the link...
http://www.grc.nasa.gov/WWW/k-12/airplane/bern.html

So my friend, it seems as there is a heck of a lot more going on than simply, air being shoved down, as your own guys pointed out in that really cool website they have. So, does shoving air down produce lift...yup. Does the pressure difference produce lift...yup.



I know we went through this before (at great length) and I know you won't want to listen, but come on man, your experiments do not prove anything to do with lift. Myself, Bruce Matthews, and a couple others made that pretty clear. Don't even start with them again.
Regarding your accusation that I don't listen - I find your posts vacuous, with lots of words used as a substitute for logic. So, please stop telling everyone I don't listen, because I do. I love logical discussion - but broadsides of useless verbiage aren't useful to me.
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Old Nov 14, 2011, 07:43 AM
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Originally Posted by slipstick View Post
As far as it goes NASA's part of that is all perfectly reasonable. Of course it fails to explain how or why the "moving flow of gas is turned by a solid object" and that's always where most of the debate comes in .

The rest of the debate is caused when people paraphrase things incorrectly e.g. converting the accurate statement "Lift occurs when a moving flow of gas is turned by a solid object" into the incorrect "it's the air being shoved down that causes lift". That's isn't what NASA says nor is it what they mean .

Steve

NASA isn't quite as specific as I'd like, on the turning vs shoving air down either. Their turning link refers to a Newtonian response to turning a fluid, not a Newtonian response to shoving air down.

Can we trust them?

Or do we have to look at the "helicopter won't lift if a flat plate is attached to its skids" experiment?

Because to me, what NASA's saying, is that the lift is generated at the rotor, which means a chopper with it's downwash blocked should still rise - but it doesn't.

There are scientists claiming that lift is both - Newton response to downwash at low speeds, and Bernoulli at high speeds.

The chopper experiment intrigues me no end though - and I've yet to see a Bernoulli only, or a turning only, demonstration of lift in a chopper.
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Old Nov 14, 2011, 07:48 AM
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Originally Posted by DPATE View Post
Thanks for starting this topic.

I did not see anything I disagree with. The description is not rigorous, but I would say it captures some of the major points. Would anyone else care to help make it more rigorous?

I'll start by saying we are considering a Newtonian fluid, not just a gas.
Experiments would help. An experiment showing that lift is solely in the chopper's rotors due to turning the air, and not as a response of the chopper to the air its throwing down.
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Old Nov 14, 2011, 09:13 AM
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Quote:
Regarding your accusation that I don't listen - I find your posts vacuous, with lots of words used as a substitute for logic. So, please stop telling everyone I don't listen, because I do. I love logical discussion - but broadsides of useless verbiage aren't useful to me.
Whatever. We tried logic in the last thread, me and a bunch of other guys. All we got back from you was constant contradictions. You would say 2 different and opposite things in the same post. You would not answer anyone else's questions, or explain your "logic".

Done. I will admit, I did have fun. I may not have convinced you to open up your mind a bit, but at least if someone if someone else looks at this thread who truly does want to learn the whole lift story, they will see my post and realize that there is a whole lot more to the story than what you are trying to shove down everyones throat.
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Old Nov 14, 2011, 09:35 AM
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Tim, I couldn't quite decipher your last comment, but in regards to some things you've said in the past, let me say this:
Now that this discussion is purely about lift, we need to stick to it's definition. Lift is the force exerted on a body by the fluid it is moving through that is perpendicular to this relative motion (and perpendicular to a reference span direction).

In the case of a propeller/rotor positioned horizontally (hover), the flow is moving through the plane of the propeller (prop disk) at the same time the prop is spinning. So if you consider the relative velocity between a section of the prop and the flow that moves past it, you will find that this velocity is neither vertical nor horizontal. This means the lift produced by the section is neither vertical nor horizontal. As a matter of fact, the relative velocity is different for different sections of a prop (because the sections further out are spinning faster), which means they have local lift vectors that are pointing in different directions. Therefore, it does not make sense to discuss "the lift of the rotor". What you have been calling "lift" is "thrust".

The line from that website: "Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air" is meant to help middle school children picture what is going on. Lift is not defined relative to weight, to gravity, or some vertical reference. We are more sophisticated, so we can use the term "lift" as it is properly defined.
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Old Nov 14, 2011, 09:44 AM
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Originally Posted by stardustertoo View Post
Whatever. We tried logic in the last thread, me and a bunch of other guys. All we got back from you was constant contradictions. You would say 2 different and opposite things in the same post. You would not answer anyone else's questions, or explain your "logic".

Done. I will admit, I did have fun. I may not have convinced you to open up your mind a bit, but at least if someone if someone else looks at this thread who truly does want to learn the whole lift story, they will see my post and realize that there is a whole lot more to the story than what you are trying to shove down everyones throat.
Here's my contradictions you speak of ...

An object cannot have lift without throwing air down.
The direction of the thrust vector (which can be lift, if the thrust is down) follows (opposes) the final output direction of the air being ejected by an object.


Now - try to prove either statement wrong.

NOTE: Here's the link to the helicopter experiment, showing Bernoulli cannot lift a copter alone ...

http://rcuvideos.com/video/BladeCPBootStrap-wmv
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Old Nov 14, 2011, 09:56 AM
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Originally Posted by DPATE View Post
Tim, I couldn't quite decipher your last comment, but in regards to some things you've said in the past, let me say this:
Now that this discussion is purely about lift, we need to stick to it's definition. Lift is the force exerted on a body by the fluid it is moving through that is perpendicular to this relative motion (and perpendicular to a reference span direction).

In the case of a propeller/rotor positioned horizontally (hover), the flow is moving through the plane of the propeller (prop disk) at the same time the prop is spinning. So if you consider the relative velocity between a section of the prop and the flow that moves past it, you will find that this velocity is neither vertical nor horizontal. This means the lift produced by the section is neither vertical nor horizontal. As a matter of fact, the relative velocity is different for different sections of a prop (because the sections further out are spinning faster), which means they have local lift vectors that are pointing in different directions. Therefore, it does not make sense to discuss "the lift of the rotor". What you have been calling "lift" is "thrust".

The line from that website: "Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air" is meant to help middle school children picture what is going on. Lift is not defined relative to weight, to gravity, or some vertical reference. We are more sophisticated, so we can use the term "lift" as it is properly defined.
Agreed - rotors create thrust.

Not agreed - "this discussion is purely about" ... because I started the discussion, and it's about the fact that there are too many theories about lift, and they cannot all be correct. And that this should be easy to sort out with a few simple, reproducible, experiments.

Agreed - we need definitions - lift is lift - the force that keeps an object in the air.

What objects? Objects with fixed wings (planes), rotating wings (choppers) as well as fixed wing objects using their props or turbofans (rotating wings) to sustain vertical up motion (interesting, because in vertical up they're no longer using their fixed wings for lift, but purely using their rotating wings).
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Old Nov 14, 2011, 09:58 AM
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Okay sounds good. Can you explain the "Bernoulli" theory of lift as you see it so we can all be on the same page?
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Old Nov 14, 2011, 10:14 AM
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Okay sounds good. Can you explain the "Bernoulli" theory of lift as you see it so we can all be on the same page?
Text book theory ...

That the air over the top has farther to travel due to top surface being longer than bottom surface, so it speeds up before meeting at trailing edge, creating less pressure on top - voila - lift.

Issues with this theory ...

The idea here is that it speeds up because the air streams meet at the trailing edge - which is wrong. They don't meet at the trailing edge - the top air stream gets to the trailing edge first, in spite of the longer distance it travels - this theory ignores reality in this key way.

So, this leaves us wondering what speeds up the air on the top surface of the wing - well - less pressure speeds up air, therefore - the air speeds up because of the vacuum created on the top surface when either a board or a wing is given a positive AOA. The vacuum comes first. Then the air speeds up.

But this is hard to prove at home - so I like the chopper experiment, as a way to get to the essentials.
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Old Nov 14, 2011, 11:06 AM
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is the airspeed across either top or bottom surfaces of the wing constant or are there sections where it is significantly greater than the average airspeed across the surface?
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Old Nov 14, 2011, 11:38 AM
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Originally Posted by Tim Green View Post
Regarding your accusation that I don't listen - I find your posts vacuous, with lots of words used as a substitute for logic. So, please stop telling everyone I don't listen, because I do. I love logical discussion - but broadsides of useless verbiage aren't useful to me.
Tim, never mind the past exchanges between you and starduster for a moment. He posted a lot of quotes directly from NASA which talks a lot more in detail about what is going on around a wing than the highly basic one you posted in the first link. What about the the information in those links and what they imply with their more detailed explanations of what is going on at the wing itself?

Quote:
Originally Posted by Tim Green View Post
.....Or do we have to look at the "helicopter won't lift if a flat plate is attached to its skids" experiment?

1)Because to me, what NASA's saying, is that the lift is generated at the rotor, which means a chopper with it's downwash blocked should still rise - but it doesn't.

2)There are scientists claiming that lift is both - Newton response to downwash at low speeds, and Bernoulli at high speeds.

3)The chopper experiment intrigues me no end though - and I've yet to see a Bernoulli only, or a turning only, demonstration of lift in a chopper.
OK, by the numbers, although I thought that we dealt with these just fine in the last thread.

1) Until you come to realize that the downwash behind the wing is the after effect of creating the lift and NOT the lift itself you're stuck. What you understand NASA to be saying about the rotor's lift occuring at the rotor is correct and is nothing more or less than we've been saying all along.

2) The airflow at the wing produces a "Yin and Yang" sort of thing with both Newtonian and Burnoullian effects acting at the same time. There is no low to high speed transition between the two. They are locked together with one producing the other and the each supporting the effect of the other. The Bernoullian pressure effects are produced by the way the air reacts to the passing of the airfoil by the Newtonian way that the air is pushed out of the way and around the wing's airfoil. But at the same time it's the Bernoullian pressure effects that hold the air and cause it to follow around the the shape to be thrown down to create the Newtonian downwash effect. So it's like the old "pulling yourself up by the bootstraps" sort of deal. The Newtonian action creates the Bernoullian effect which causes the Newtonian action. You can't separate them however much you want to. As soon as our wing or flat plate or any other object encounters moving air or moves through the air the air moves around the object in a manner which generates both effects at the very same time. .

2A) Only the simplest texts say that the upper and lower airflow meet at the trailing edge. And as you point out those are wrong. This has been known for most of the last 100 years by those that are actually in the field. But it's generally explained poorly by others that write the slimplistic texts that so many folks read. Oddly enough I understand that this simplistic error was actually published in texts used for pilot training for many years and may still be there. Which points out that we don't need to be a scientist to fly a plane.

3) Back to your blocked heli flow. OK, it's blocked because there's a plate attached. But how is this plate different from the ground for a heli that is taking off? Simply it isn't. At the point of liftoff any helicopter's downwash is blocked by the ground. Yet they all lift off. The only difference with the plate is that in that case it's trying to take the "ground" along with it. So why is this? It's NOT because the lift is blocked or the heli would not be able to take off from regular ground. It's because you tied the heli to the "ground". As I said in the last thread, and which you never commented on, if you where to attach the plate to the heli with a force measuring scale you'd see that the lift is still there and that it would be trying to rip the plate off the landing gear of the heli. But you just tied the two together into a locked system. You won't see any lift within that system from the outside. But if you reach into the system and put a force measuring device in the string that holds the plate to the heli you would be able to measure the lift force. The lift is still there, it's just being held down by the pressure effect of the plate. The pressure buildup on the plate is still a totally separate event that is relying solely on the string tieing the plate to the heli to prevent the heli from lifting. Having the plate attached to the heli is also very much the same as my astronaut and toolkit example where the toolkit hooks his foot on the way past. He generated the thrust by throwing the kit but it was negated by the tool kit hooking onto his boot and transferring all the energy back into the astronaut. But that doesn't mean that a reaction didn't occur. The astronaut was moving just fine at the moment the kit left his hand. It was the separate act of hooking his boot that stopped him from from moving. The plate tied to the heli is the same as the kit catching on the boot.
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