# COMMUNITY : FORUMS : SQUARE PEGS

## Principles of aerodynamics

The forces and counter forces of flight

CameronSS7 years ago
Paper airplanes don't generate lift in the same way as full-sized aircraft. In a full-size airplane, the wings are curved and shaped in order or deflect air downward and to create a low pressure zone above the wing. In a paper airplane, the airfoil is flat or mostly flat, and the lift is created because the air is deflected downwards. Newton's law: the action of deflecting air downwards pushes the wing upwards. However, this creates a lot of induced drag, which is why paper airplane wings are so inefficient compared to full-scale airplane wings.
Prometheus7 years ago
To get back on topic: The forces that create flightr are simply as follows: Lift is generated by the suction above the airfoil and the pressure below it, hence the shape of the commonly-associated "airfoil" design... Such airfoil design is implied on helicopters as well as simple aircraft. As with simple fan-dynamics, lift/pressure is generated by the force generated by the airfoil and how it it is imposed on the surrounding (ambient) airspace. The simple idea of flight is answered in the dynamics of the basics of proopeller-type technology, which led to winged flight, which led to jet-propulsion-coupled with winged-flight, and hence our technology today....The evolution of the airfoil has gone a long way since it's conception....
SoDDiggerCpl (author)  Prometheus7 years ago
I said simply put, not x-nasa language. YOur explination could do with a diagram showing the forces/counterforces of flight. Drag is also a factor in wing design, if you cease the wing surface in the wrong place, the eddies swirling off the wing increase in size, and consequently the amount of drag on the wing also increases.
SoDDiggerCpl (author) 7 years ago
Sorry for not posting recently. The point of this post is to explain to people who know that a paper airplane flies, but not how, in terms that make common sense, not EX-NASA language. Might do me a bit of good to have things explained to me too!
7 years ago
Disclaimer: I haven't done any analysis on this, nor do I plan to (primarily due to my laziness tonight), so all of what follows is pure speculation on my part. I suspect that a paper airplane's "wings" function in a similar fashion as a symetric airfoil so when you throw it forward, earth's gravity causes the airplane to start down towards the ground, this induces an angle of attack in the wings causing the airplane to gain lift. Now you might be asking about this a little more and wondering about the terms. Well, a symetric airfoil is just that, symetric. The top surface is shaped the same as the bottom surface. Ok, so imagine a line passing from the very front of the airfoil to the very rear of it. Now imagine the wind acting as a line towards the airfoil. The angle between these two lines in the "angle of attack". Got those two terms and concepts? One of the properties of symetric airfoils is that they produce 0 lift at 0 angle of attack and it's a linear increase until stall. So when you throw the airplane it goes down and then causes lift to be generated. If there's interest I can explain why a lot of paper airplanes pitch up and then dive rapidly. As an aside: From the front edge of the airfoil to the rear edge is known as the "chord". Symetric airfoils are classified based on a ratio of their thickness to chord (t/c) as a percentage. The common range would be ~8%-18%, the most common being the 10% and 12% which are often used for vertical stabilzers on airplanes.
7 years ago
Im no expert, but being gliders, dont paper airplanes create no lift? The curve of other wings makes the air on the top of the wing go faster than below it, right? Since paper airplanes have no reason for the top air to go faster, they just go, with the wings acting kind of like a parachute.
7 years ago
Lift is not that same as being able to ascend.

Say you have an object falling through the atmosphere, if that object is falling less than gravity, it's producing some drag and/or lift. If the object is falling slower than terminal velocity then there *has* to be lift being produced.

Let's consider a free body diagram in the vertical direction for a glider: in the -Y direction you have gravity or the weight. In the positive Y direction you have a component of the drag and a component of the lift. Sum all of these and you get a force which is equal to m*a. In level flight a=0 so the forces sum. Now, the force of gravity is equal to m*g where g is the gravitational acceleration. If m*a is less than m*g then there has to be some positive upward force. This comes from a component of the drag and the lift.

As I posted earlier, a symetric airfoil at 0 degrees angle of attack (AoA) produces 0 lift, but at positive AoA it produces positive lift. If AoA is negative, it will produce negative lift, that is a force downward.

You might be confused about airfoils as they are conventionally drawn. The typical airfoil is drawn with a rounded top and flat bottom. This is known as a "cambered airfoil". Basically a cambered airfoil produces lift at 0 AoA (the trade off is that they often stall earlier), but there is a negative AoA where there is 0 lift produced, which we sometimes use to compare the lift-curve slope with other airfoils. (Lift-curve slope is the graph of lift vs. AoA, the slope of which is often 2*pi [don't ask me why]. To explain further, in aero we often use C_l; coefficient of lift. Now C_l=C_l_alpha*alpha where alpha is the AoA, so that C_l_alpha is the lift curve slope (this was for a symetric airfoil, for a cambered airfoil, there's a constant in there)). That was longer and probably more confusing than I intended, if you have a question, just ask.
7 years ago

I also forgot to mention that a flat plate *can* generate lift when it sees a positive AoA.
7 years ago
Out of curiosity... Is an oval paper air plane considered a symmetric foil? Hrmm.. I think I just answered my question - but is there anything special about that design?

I mean this type: http://www.instructables.com/id/EV3GW00JZ4EYF8LZG3/ -- not the kind that you spin (for boundary layer effects).
7 years ago
No idea, sorry. While I'm thinking about, the kind you spin probably has a stability advantage due to the gyroscopic motion.
Weissensteinburg7 years ago
And what about non-planes? ...cars...people...chairs on wheels?
SoDDiggerCpl (author)  Weissensteinburg7 years ago
If you can get people to fly without wings, and power them with no more than a fart, you'd be a millionaire.
7 years ago
Cars have aerodynamics too.. Aero is for being in the air
SoDDiggerCpl (author) 7 years ago
Lets stop bickering and get to the point of this thread!
7 years ago
Very basic... A wing, being curved on top, bends the air thereby thinning it. With the air presure is higher below the wing than above, you acheve lift.
trebuchet037 years ago
Newton's Second Law (reaction lift, thrust, etc.) Bernoulli Effect (Foil Shape Lift) Drag (skin friction, Vortex Shedding and Geometry, etc.) And other cool little things like flutter :) Do you have a question?
SoDDiggerCpl (author)  trebuchet037 years ago
Jesus man, where did you get your PhD?
7 years ago
Hehe... I don't have a PhD (but the people behind instructables do)... I'm in school for mechanical engineering -- not exactly aerospace stuff, but we still need to know it (well, some of it).
SoDDiggerCpl (author)  trebuchet037 years ago
You just became my best friend! Is that space ship one as your pic? I'm guessing that legos were your favorite toy.
7 years ago
I got all excited when I saw this thread, and then I read it and now I'm just confused as to what the point of it is.

Anyways, as long as I'm posting, in level flight Thrust=Drag and Lift=Weight.
7 years ago
I'm just confused as to what the point of it is.

Me too :P
7 years ago
Mainly I'm just sad that I didn't get to claim to be the resident aerodynamicist.
7 years ago
:P Oh, you are ;) From our previous discussions, you're light years ahead of me :) I'm just at the point in my education where I can finally come up with answers for some questions that I could never get a straight answer to - like why modern machining equipment is so damn heavy and continues to get heavier :P