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Why aft CG increases TAS

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B-J-J,


I'll try and tackle your drag question. Basically, there are two main types of drag on an aircraft. They are parasite drag and induced drag.

Induced drag is that drag caused by the production of lift. We all know about the pressure differences between the upper and lower surfaces of the wing. The air "spills" over the tips to fill in the void caused by the lower pressure above, thereby creating wingtip vortices. Looking at the wing spanwise (at the tip looking towards the fuselage), as the flow comes up and over the tips, it imparts a downward velocity on the flow leaving the trailing edge. This downward velocity component has the effect of tilting the lift vector rearwards (since lift is perpendicular to the relative wind). So, the lift from the wing now has a vertical component (to offset weight) and a horizontal component opposite the direction of flight. This horizontal component is drag. This drag force is not constant though. It depends on the lift coefficient (angle of attack), the planform shape of the wing, and the dynamic pressure (density and speed) of the airflow. Remember the FAA's "heavy, clean, and slow"? Well, the heavier the aircraft and the slower the airspeed, the higher the AOA will have to be to sustain the necessary lift. Flaps produce vortices of their own that tend to dissipate the main vorticies, so a clean wing will usually produce greater wake turbulence, but it varies with each wing and flap structure. "Ground effect" is something that we're all familiar with. As the aircraft approaches the pavement, the ground tends to dissipate the vortices, causing the lift vector to tilt forward. This reduces the drag and increases the lift at the same time. Some people will tell you that it is a "cushion of air" that causes ground effect. No such thing.

Basically, parasite drag is everything that is not induced drag. One exception is wave drag, but that's for another time. Parasite drag consists of skin friction drag (air viscosity), form drag, and several other types of less-significant drag. Form drag is the result of the shape of the aircraft and the size of the wake it produces. Flat plates held perpendicular to the airflow will exhibit lots of form drag, but very little skin friction drag. The opposite is true if you align the plate with the airflow. The classic example is sticking your hand out the window in a moving car. If your palm is forward, you will have much more drag (mostly form drag) than if your palm is down (mostly skin friction, but a much lesser value).

As a pilot, it's important to know when the aircraft will have the most drag. Parasite drag increases with airspeed (exponentially I might add!) and induced drag will decrease (mostly due to the lower angles of attack found at higher speeds). But as the aircraft slows, the parasite drag will decrease but the induced will increase. When the induced drag far overshadows the parasite drag, this is called the "backside of the power curve", meaning that more power will be required to sustain level flight as the aircraft slows.

Anyway, enough for now. This is a good discussion, let's have some more answers!

SuperD
 
Flaps

Flaps generally cause the nose to rise because of an increase in downwash on the tail. It's not that the wing is making any more lift with the flaps down, but that the lift is redistributed so more of the lift (and thus downwash) is made inboard right in front of the tail and less outboard. The wing itself actually wants to pitch nose down with flap extension, but the effects of downwash are usually stronger. There are aircraft that pitch nose down with flaps, but I can't think of any right now.

Scott
 
It's not that the wing is making any more lift with the flaps down, but that the lift is redistributed so more of the lift (and thus downwash) is made inboard right in front of the tail and less outboard.

Huh? Redistributed lift and downwash???

I wont swear to it, but....I vaguely recall something about flaps increasing the camber, AOA, and on some types, (i.e. Fowler) wing area resulting in an increse in total lift produced by the airfoil. The resultant shift of the center of pressure in relation to the CG of the aircraft causing a pitch-up or pitch-down tendancy, but maybe I'm wrong.
 
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Re: Flaps

sstearns2 said:
It's not that the wing is making any more lift with the flaps down, but that the lift is redistributed so more of the lift (and thus downwash) is made inboard right in front of the tail and less outboard.

Huh? The flaps do increase lift by increasing the angle of attack. They also add drag.
Not sure about the tail theory.

My understanding is that the force of drag upon the flaps acts above (or below) the CG of the airplane, thus rotating the airplane around that axis, causing a pitch up (or down).
 
Drag

The previous post on drag was good, but here's a little different way of looking at it (the way I think of it anyway).

The way I think of it, there are 3 kinds of drag...

1- Parasite or 'skin friction' drag.

2 - Induced drag or the drag caused by making lift in whatever direction.

3 - Wave drag or the drag cause by air decellerating thru the speed of sound (shock waves).

They are all really the same basic thing, the acceleration of the air in some direction. Skin friction drag is caused by the entrainment of air in the boundary layer (the layer of air right next to the skin of the airplane). Basically skin friction drag is caused by the air your just dragging along with you as you fly along. Induced drag is caused by the acceleration of the air in some direction to make lift in the opposite direction. The wing accelerates the air down to make lift up.

Imagine there is a box of still air, then a glider flies thru the box of air. Now the box of air has a velocity component in the direction of the flight of the glider caused by the glider dragging (acclerating) air along with itself and there is a vertical velocity component caused by the need of the glider to accelerate air down in order to make the needed lift. The air will also be rotating some amount because of 'wing tip vorticies'. (I hate the word wing tip vortex and disagree with the book explainations, but that's another tirade.)

Wave drag is caused by the sudden deceleration of air thru a shock wave. Air cannot decelerate thru mach 1 smoothly, it will be going Mach 1.5 for example and the hit the shock wave and slow to Mach 0.75 (or so, I don't have a mach table in front of me) in a fraction of an inch. Basically there is a big wall of air being dragged along behind the shock wave that causes a huge amount of drag.

I hope this make some amount of sense.

Scott
 
Flaps

I cast my vote with Metro Sheriff and IFly4Food. Flaps indeed add lift. Isn't that why so many aircraft require extended flaps for takeoff (and landings, of course)? When flaps are extended, camber changes, the center of pressure moves and, hence, the pitch change. Bernoulli lift. Relative wind hits the extended flaps and causes a pitching moment. Newtonian lift.

Very stimulating discussion, folks. Better that taking a ground school refresher or digging out the books. Great posts, too, on drag and aircraft stability (one of my favorite aerodynamic subjects :) ).
 
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I wont swear to it, but....I vaguely recall something about flaps increasing the camber, AOA, and on some types, (i.e. Fowler) wing area resulting in an increse in total lift produced by the airfoil. The resultant shift of the center of pressure in relation to the CG of the aircraft causing a pitch-up or pitch-down tendancy, but maybe I'm wrong.

Metrosheriff is the big winner.

You are correct.
 
Re: Drag

sstearns2 said:
The previous post on drag was good, but here's a little different way of looking at it (the way I think of it anyway).

The way I think of it, there are 3 kinds of drag...

1- Parasite or 'skin friction' drag.

2 - Induced drag or the drag caused by making lift in whatever direction.

3 - Wave drag or the drag cause by air decellerating thru the speed of sound (shock waves).

They are all really the same basic thing, the acceleration of the air in some direction. Skin friction drag is caused by the entrainment of air in the boundary layer (the layer of air right next to the skin of the airplane). Basically skin friction drag is caused by the air your just dragging along with you as you fly along. Induced drag is caused by the acceleration of the air in some direction to make lift in the opposite direction. The wing accelerates the air down to make lift up.

Imagine there is a box of still air, then a glider flies thru the box of air. Now the box of air has a velocity component in the direction of the flight of the glider caused by the glider dragging (acclerating) air along with itself and there is a vertical velocity component caused by the need of the glider to accelerate air down in order to make the needed lift. The air will also be rotating some amount because of 'wing tip vorticies'. (I hate the word wing tip vortex and disagree with the book explainations, but that's another tirade.)

Wave drag is caused by the sudden deceleration of air thru a shock wave. Air cannot decelerate thru mach 1 smoothly, it will be going Mach 1.5 for example and the hit the shock wave and slow to Mach 0.75 (or so, I don't have a mach table in front of me) in a fraction of an inch. Basically there is a big wall of air being dragged along behind the shock wave that causes a huge amount of drag.

I hope this make some amount of sense.

Scott

Dude! I think you need to lay of aerodynamic theory for a while.
Parasite drag is direct contact of leading edge surfaces with the air and friction. Did you know that there are over 200lbs/in2 on every surface at 250KIAS?

Induced drag is a by product of lift. Wake vortices are a great example (whether you believe in them or not... but I hope you do). It is what you described as parasite... dragging air behind you.

Mach drag? I won't worry about that until my airline buys a Concorde!

Who do you fly the Brakillya for?
 
....

Previous post....

>The flaps do increase lift by increasing the angle of attack. They >also add drag.


Flaps increase the ability of the wing to make lift. In order to fly staight and level lift has to equal wieght. The weight of the aircraft doesn't increase with flap extension, so the amount of lift made in straight and level flight is the same with the flaps up or down.

Scott
 
Re: ....

sstearns2 said:
Previous post....

>The flaps do increase lift by increasing the angle of attack. They >also add drag.


Flaps increase the ability of the wing to make lift. In order to fly staight and level lift has to equal wieght. The weight of the aircraft doesn't increase with flap extension, so the amount of lift made in straight and level flight is the same with the flaps up or down.

Scott

Your logic is flawed. It has nothing to do with weight.
Lift is a combination of speed, angle of attack, and the wings lift coefficient. Flaps allow you to maintain a high value of lift at a lower than otherwise airspeed. In essence, they change the coefficient of lift of the wing (by increasing camber) and increase the angle of attack (by lowering the trailing edge of the wing).
The total lift doesn't change in relation to the weight; you just get that value at a lower airspeed with flaps.

I recommend the book Aerodynamics for Naval Aviators where you can research these issues. It's also a great cure for insomnia
 
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