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Old 09-16-2006, 07:04 PM   #1
Twizter68
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Default Laws of Aerodynamics- all 3 of them!

Also included Bernoulli's principle, the driving principle behind lift. Sources are USN "Airman" Rate Training Manual, and Enlisted Aviation Warfare Specialist PQS. Enjoy! -Twiz


CHAPTER 3


PRINCIPLES OF FLIGHT




PHYSICAL LAWS AFFECTING AERODYNAMICS


LEARNING OBJECTIVE:
Identify the physical laws of aerodynamics to include Newton's laws of motion and the Bernoulli principle.
Aerodynamics is the study of the forces that let an aircraft fly. You should carefully study the principles covered here. Whether your job is to fly the aircraft and/or to maintain it, you should know why and how an aircraft flies. Knowing why and how lets you carry out your duties more effectively.

LAWS OF MOTION
Motion is the act or process of changing place or position. Simply put, motion is movement. An object may be in motion in relation to one object and motionless in relation to another. For example, a person sitting in an aircraft flying at 200 mph is at rest or motionless in relation to the aircraft. However, the person is in motion in relation to the air or the earth. Air has no force or power other than pressure when it's motionless. When air is moving, its force becomes apparent. A moving object in motionless air has a force exerted on it as a result of its own motion. It makes no difference in the effect whether an object is moving in relation to the air or the air is moving in relation to the object. The following information explains some basic laws of motion.

Newton's First Law of Motion
According to Newton's first law of motion (inertia), an object at rest will remain at rest, or an object in motion will continue in motion at the same speed and in the same direction, until an outside force acts on it. For an aircraft to taxi or fly, a force must be applied to it. It would remain at rest without an outside force. Once the aircraft is moving, another force must act on it to bring it to a stop. It would continue in motion without an outside force. This willingness of an object to remain at rest or to continue in motion is referred to as inertia.

Newton's Second Law of Motion
The second law of motion (force) states that if a object moving with uniform speed is acted upon by an external force, the change of motion (acceleration) will be directly proportional to the amount of force and inversely proportional to the mass of the object being moved. The motion will take place in the direction in which the force acts. Simply stated, this means that an object being pushed by 10 pounds of force will travel faster than it would if it were pushed by 5 pounds of force. A heavier object will accelerate more slowly than a lighter object when an equal force is applied.

Newton's Third Law of Motion
The third law of motion (action and reaction) states that for every action (force) there is an equal and opposite reaction (force). This law can be demonstrated with a balloon. If you inflate a balloon with air and release it without securing the neck, as the air is expelled the balloon moves in the opposite direction of the air rushing out of it. Figure 3-1 shows this law of motion.


BERNOULLI'S PRINCIPLE
Bernoulli's principle (fig. 3-2) states that when afluid flowing through a tube reaches a constriction ornarrowing of the tube, the speed of the fluid passingthrough the constriction is increased and its pressure isdecreased.


AIRFOIL TERMINOLOGY
The shape of an airfoil and its relationship to theairstream are important. The following are commonterms that you should understand before you learnabout airfoils. Leading edge The front edge or surface of theairfoil (fig. 3-3). Trailing edge The rear edge or surface of theairfoil (fig. 3-3). Chord line An imaginary straight line fromthe leading edge to the trailingedge of an airfoil (fig. 3-3). Camber The curve or departure from astraight line (chord line) from theleading to the trailing edge of theairfoil (fig. 3-3). Relative wind The direction of the airstream inrelation to the airfoil (fig. 3-4). Angle of attack The angle between the chord lineand the relative wind (fig. 3-4).

AIRFLOW AROUND AN AIRFOIL
The generation of lift by an airfoil depends on theairfoil's being able to create a special airflow in theairstream. This airflow develops the lifting pressureover the airfoil surface. The effect is shown in figure3-5, which shows the relationship between lift andBernoulli's principle. As the relative wind strikes theleading edge of the airfoil, the flow of air is split. Aportion of the relative wind is deflected upward and aft,and the rest is deflected downward and aft. Since the upper surface of the airfoil has camber to it, the flowover its surface is disrupted. This disruption causes awavelike effect to the airflow. The lower surface of theairfoil is relatively flat. The airflow across its surfaceisn't disrupted. Lift is accomplished by this differencein the airflow across the airfoil.The shaded area of figure 3-5 shows a l ow-pressurearea on the airfoil's upper surface. This low-pressurearea is caused by the air that is disrupted by the camberof the airfoil, and it is the key t o lift. There is lesspressure on the top surface of the airfoil than there is onthe lower surface. The air pressure pushes upward o thelower surface. This difference in pressure causes theairfoil to rise. Now, you know that lift is developed by the difference between the air pressure on the upperand lower surfaces of the airfoil. As long as there is less pressure on the upper surface and more pressure onthe lower surface of an airfoil, an aircraft has lift. Lift isone of the forces affecting flight.

FORCES AFFECTING FLIGHTLEARNING OBJECTIVE:
Recognize thefour primary forces acting on an aircraft.

An aircraft in flight is in the center of a continuousbattle of forces. The conflict of these forces is the key t oall maneuvers performed in the air. There is nothingmysterious about these forces—they are definite andknown. The direction in which each acts can becalculated. The aircraft is designed to take advantage ofeach force. These forces are lift, weight, thrust, and drag.

LIFT
Lift is the force that acts in an upward direction tosupport the aircraft in the air. I t counteracts the effectsof weight. Lift must be greater than or equal to weight ifflight is to be sustained.

WEIGHT
Weight is the force of gravity acting downward onthe aircraft and everything in the aircraft, such as crew,fuel, and cargo.

THRUST
Thrust is the force developed by the aircraft'sengine. It acts in the forward direction. Thrust must begreater than or equal to the effects of drag for flight tobegin or to be sustained.

DRAG
Drag is the force that tends to hold an aircraft back.Drag is caused by the disruption of the airflow about thewings, fuselage (body), and all protruding objects onthe aircraft. Drag resists motion as it acts parallel and inthe opposite direction in relation to the relative wind.Figure 3-6 shows the direction in which each of theseforces acts in relation to an aircraft.Up to this point, you have learned the physical lawsof aerodynamics, airfoils, and the forces affectingflight. To fully understand flight, you must learn aboutthe rotational axes of an aircraft.

ROTATIONAL AXES LEARNING OBJECTIVE: Identify thethree axes of rotation and the terms relative to the aircraft's rotation about these axes.

LONGITUDINAL AXIS
The longitudinal axis is the pivot point about whichan aircraft rolls. The movement associated with roll isbest described as the movement of the wing tips (one upand the other down). Figure 3-7 shows this movement.This axis runs fore and aft through the length (nose totail) of the aircraft. This axis is parallel to the primarydirection of the aircraft. The primary direction of afixed-wing aircraft is always forward. Figure 3-8 showsthe longitudinal axis.

LATERAL AXIS
The lateral axis is the pivot point about which theaircraft pitches. Pitch can best be described as the upand down motion of the nose of the aircraft. Figure 3-7shows this movement. The pitch axis runs from the leftto the right of the aircraft (wing tip to wing tip). It isperpendicular to and intersects the roll axis. Figure 3-8shows the pitch axis and its relationship to the roll axis.

VERTICAL AXIS
The vertical axis runs from the top to the bottom ofan aircraft. It runs perpendicular to both the roll andpitch axes. The movement associated with this axis isyaw. Yaw is best described as the change in aircraftheading to the right or left of the primary direction of anaircraft. Figure 3-7 shows this movement. Assume youare walking from your work space to an aircraft located100 feet away. You are trying to walk there in a straightline but are unable to do so because there is a strongwind blowing you off course to your right. Thismovement to the right is yaw. The yaw axis is shown infigure 3-8.

AMA 869130/LSF 8042

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Old 12-20-2006, 11:46 PM   #2
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haha good info!

Dont forget with enough Thrust anything will fly!
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Old 12-21-2006, 03:31 PM   #3
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Originally Posted by Tinman View Post
haha good info!

Dont forget with enough Thrust anything will fly!
Very true indeed! But somewhere comes the point where you're no longer talking about aircraft design but about rocket design

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Old 12-21-2006, 03:58 PM   #4
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Bernoulli's principle is often overemphasised in discussions of lift. See Stop Abusing Bernoulli - How Airplanes Really Fly by Gale Craig. http://regenpress.com/ Available from the AMA and Amazon.

Ron Stewart
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Old 12-21-2006, 07:52 PM   #5
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That description of lift is so wrong it's downright funny. So all of NACA's hundreds of symmetrical airfoils don't create any lift because they're the same shape top and bottom ? Well I never .

Bernoulli's Principle works well for what it is intended for.....flow in a venturi. When was the last time you saw an airfoil flying inside a constricted tube ?

Still it is true that Newton's Laws tell you all there is to know if you can just work out how to apply them.

Steve
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Old 12-21-2006, 07:55 PM   #6
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Slipstick, which description are you knocking?

Ron Stewart
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Old 12-21-2006, 08:44 PM   #7
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Subsitute "Bernoulli" for "Big Enders" and "Newton" for "Small Enders"... and
read "Gulliver's Travels..Lilliput"
It's the same endless discussion.
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Old 12-24-2006, 09:06 PM   #8
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Originally Posted by slipstick View Post
That description of lift is so wrong it's downright funny. So all of NACA's hundreds of symmetrical airfoils don't create any lift because they're the same shape top and bottom ? Well I never .

Bernoulli's Principle works well for what it is intended for.....flow in a venturi. When was the last time you saw an airfoil flying inside a constricted tube ?

Still it is true that Newton's Laws tell you all there is to know if you can just work out how to apply them.

Steve

Symmetrical airfoils indeed don't create any lift at zero incidence.

For a description of Bernoulli's Principle have a look at Wikipedia

http://en.wikipedia.org/wiki/Bernoulli's_principle

It is not possible to describe how an airfoil creates lift using only Bernoulli. Reality is a bit more complex. A venturi is a classic example to make Bernoulli's Principle easy to understand, it is however by far not the only application. I'm not going to annoy you with a complete description of how lift is created. If you are interested again have a look at the article on Wikipedia, it is well written.

http://en.wikipedia.org/wiki/Lift_%28force%29

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Old 12-24-2006, 09:35 PM   #9
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Bernoulli's equation was derived from Newton's laws of motion and is nothing but the laws of motion applied to fluid flow. It is really important to remember that the reason that the fluid loses pressure as it speeds up is because it experienced acceleration.
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Old 04-08-2007, 09:49 AM   #10
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Seems like someone went through a whole lot of effort to start a fallacy.

http://regenpress.com/

Frank

Have to wonder why?
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Old 04-08-2007, 11:57 AM   #11
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This sounds like another crazy idea from the people that say the world is not flat.
What are they going to try to get us to believe next?
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Old 04-08-2007, 01:34 PM   #12
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I used to fly a Great Planes Cub 20 a lot and one time a curious spectator started talking to me while I was flying it. With a straight face, I told him that because of this airplane's flat bottom airfoil, it was impossible to fly it inverted, the lift of that airfoil would pull it straight down if I tried that. The whole time I was saying that, my Cub was contradicting everything I was saying by flying the pattern inverted!

He got the joke BTW.
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Old 04-08-2007, 05:33 PM   #13
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I was watching a helicopter hovering inverted the day before yesterday.. with the blades rubbing on the runway!
We were shouting at the guy to go lower.. but he declined.. saying he was as low as he could get!
The blades would be at some negative pitch angle to provide lift, but with them touching the ground, the air on the bottom side wasn't paying a whole lot of attention to Bernoulli.
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Old 04-08-2007, 05:34 PM   #14
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Great Planes 20 Cub.. Love mine!
With a VT21 4-stroke, it's got scale power for real!
I changed mine to barn door ailerons, to get the full Cub adverse yaw experience.
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Old 04-08-2007, 06:01 PM   #15
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Bernoulli's observations were incomplete! Sure the pressure is less above a foil- but the pressure is HIGHER below it- if there is a positive Angle Of Attack! Angle of Attack will cause lift on a wing whether its a flat bottom, sym or a flatplate.

Msr Bernoulli's effect was demonstrated when 2 ships pulled alongside one another (while under way) to transfer supplies and the lessened pressure between them caused the ships to be pulled together with great damage! Ouch! They keep their distance these days!

'Stick and Rudder' is still the best book I've ever read on how an airplane flies. That is where I got the explanation for how airfoils and dihedral work that made so much sense, I wondered why it wasn't more obvious before!

Keep the shiny side up!
TW
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Old 04-08-2007, 06:37 PM   #16
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Originally Posted by TeslaWinger View Post
Bernoulli's observations were incomplete! Sure the pressure is less above a foil- but the pressure is HIGHER below it- if there is a positive Angle Of Attack! Angle of Attack will cause lift on a wing whether its a flat bottom, sym or a flatplate.

Msr Bernoulli's effect was demonstrated when 2 ships pulled alongside one another (while under way) to transfer supplies and the lessened pressure between them caused the ships to be pulled together with great damage! Ouch! They keep their distance these days!

'Stick and Rudder' is still the best book I've ever read on how an airplane flies. That is where I got the explanation for how airfoils and dihedral work that made so much sense, I wondered why it wasn't more obvious before!

Keep the shiny side up!
TW
Thanks for the book info. I just ordered it from Amazon. I can never have enough reference material in my library. Looking forward to reading this one.

Frank
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Old 04-08-2007, 07:10 PM   #17
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Originally Posted by Sparky Paul View Post
Great Planes 20 Cub.. Love mine!
With a VT21 4-stroke, it's got scale power for real!
I changed mine to barn door ailerons, to get the full Cub adverse yaw experience.
OS .26 four stroke on mine. Quieter than some electrics. You can hear the wind whistling through the struts while throttled down on the downside of a loop.

I'm tempted to convert it to e-power, the biggest problem is where to put the battery access hatch, I sure don't want to unbolt the wing and struts every time I change batteries.
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Old 04-08-2007, 09:08 PM   #18
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Now you guys went and done it!
Since you put Bernoulli in charge,my Slow Stick won't fly.
The distance is the same on the top and the bottom of the wing so the air speed and vacuum are the same and it doesn't have any lift.
I liked it better Gale Craig's way.
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Old 04-08-2007, 10:01 PM   #19
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Ain't no vacuum!
Natures abhors those things!
The air going over the top.. it's accelerated just by going over the top and rounding down to the trailing edge.
That alone drops its pressure, enough for the air going underneath to want to get into that area of lower pressure.. which the foam prevents.
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Old 04-10-2007, 04:03 AM   #20
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Originally Posted by Murocflyer View Post
Thanks for the book info. I just ordered it from Amazon. I can never have enough reference material in my library. Looking forward to reading this one.

Frank
Shipped already. That was fast!

Frank


The following items have been shipped to you by Amazon.com:
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Amazon.com items (Sold by Amazon.com, LLC):
1 Stick and Rudder: An Expla... $16.47 1 $16.47
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Old 04-10-2007, 11:31 AM   #21
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Sparky Paul,
The bottom is the same shape as the top, so if the air follows the bottom it is going the same distance as the top so it has the same speed and pressure drop.???

I hope you realize I am just messing with your mind.
I have been flying over 50 years, so I know what makes a plane fly.
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Old 04-10-2007, 12:10 PM   #22
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Originally Posted by groundparrot View Post
Sparky Paul,
The bottom is the same shape as the top, so if the air follows the bottom it is going the same distance as the top so it has the same speed and pressure drop.???

I hope you realize I am just messing with your mind.
I have been flying over 50 years, so I know what makes a plane fly.
This argument confuses velocity with speed. Confusing velocity with speed is the basis of all Bernoulli misconceptions.

Acceleration is any change in velocity, not nessesarily speed. Velocity is a vector, meaning that it has both a magnitude and a direction. Speed is a scalar, it only has magnitude. Changing direction is just as much acceleration as changing speed is.
The air is changing direction as it follows the curve of the wing so it is experienceing acceleration. The force needed to accelerate the air causes the pressure difference.
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Old 04-10-2007, 01:54 PM   #23
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Bernoulli is one part of one explination of lift. The Bernoulli equation is valid for frictionless, steady, incompressible flow along a streamline. If you read Gale Craig's web page you will see that his article is not against using the Bernoulli explination, it is against the concept of "Same Time". The concept that it takes the same time for the air that goes over the wing and the air that goes under the wing to reach the back of the wing. Using the Bernoulli eqn. and integrating the pressure distrubution normal to the flow across the surface will give you lift (Assuming the shear effects are nominal).

Bernoulli's observations were incomplete! Sure the pressure is less above a foil- but the pressure is HIGHER below it- if there is a positive Angle Of Attack! Angle of Attack will cause lift on a wing whether its a flat bottom, sym or a flatplate.
Bernoulli's Eqn can be applied to flow over a surface with an angle of attack.
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Old 04-10-2007, 05:23 PM   #24
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Originally Posted by groundparrot View Post
Sparky Paul,
The bottom is the same shape as the top, so if the air follows the bottom it is going the same distance as the top so it has the same speed and pressure drop.???

I hope you realize I am just messing with your mind.
I have been flying over 50 years, so I know what makes a plane fly.
.
Yeah.
It's noise.
The more noise, the further it goes. Apollo got to the moon!
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Old 04-11-2007, 12:20 AM   #25
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Originally Posted by Sparky Paul View Post
.
Yeah.
It's noise.
The more noise, the further it goes. Apollo got to the moon!

I subscribe to the theory that money is what makes airplanes fly. Want to fly a real P-51 Mustang? That's about $800/hour. Want to fly an SR-71? That's about $200,000/hour.
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