Why aft CG increases TAS

[bobbysamd]

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 ).

 

[chperplt]

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.

 

[ifly4food]

Re: 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

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?

 

[sstearns2]

....

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

 

[ifly4food]

Re: ....

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

 

[sstearns2]

I'm sorry everyone, I should have stopped with the flap post, but I have to respond to this.

The previous post was

*****
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?
******

Check your math, your answer is probably in lb/ft^2 rather than lb/in^2. The dynamic pressure is on the order of 3 or 4 psi at 250 KIAS.

Induced drag is a product of lift, I agree completely.

It's not a matter of me believing in wake vortices, they clearly exist in real life. I just disagree with the book explainations of where they come from and think the term 'wing tip vortex' is a misnomer.

I fly for Skywest.

Scott

 

[MetroSheriff]

originally postes by sstearns2
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.

With a constant power setting (thrust) that is completely untrue.

Hence the reason most aircraft will tend to balloon when flaps are applied. This is why the pilot, or autopilot,will have to pitch-down (on most a/c), in order to lower the AOA and thereby compensate for the increase in total lift resulting from flap deployment.

 

[ifly4food]

I'm sorry everyone, I should have stopped with the flap post, but I have to respond to this.

The previous post was

*****
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?
******

Check your math, your answer is probably in lb/ft^2 rather than lb/in^2. The dynamic pressure is on the order of 3 or 4 psi at 250 KIAS.

Induced drag is a product of lift, I agree completely.

It's not a matter of me believing in wake vortices, they clearly exist in real life. I just disagree with the book explainations of where they come from and think the term 'wing tip vortex' is a misnomer.

I fly for Skywest.

Scott

Well, I guess picking on me for typing the wrong units is the best you can do. I'm sure I meant lbs/ft2, but that's not really the issue.
I'd love to hear your explanation of wake vortices.
Most aerodynamicists (of which one I'm not) agree that they form because of lower pressure air above the wing relative to the air under the wing. Across the span of the wing, the wing itself separates these pressure fields except at the end of the wing. With no barrier, air flows from high to low pressure, so the relatively high pressure air below the wing flows up and around the wing to meet the lower pressure air. Because of the airplane's forward motion, a vortex, or a "horizontal tornado" forms at each wing tip.

 

[skeezer]

Ok, straight from Aerodynamics For Naval Aviators :

With regards to the Flap/C.G. stuff.

"Positive Camber produces a nose down twisting moment - especially when large camber is used well aft on the chord (an obvious implication is that flaps are not practical on a flying wing or tailless airplane). The deflection of a flap causes large nose down moments which create important twisting loads on the structure and pitching moments that must be controlled with the horizontal tail.....The fowler flap causes the greatest change in twisting moment while the split flap causes the least."

So, as several have stated before, it is the change in the center of pressure that creates the pitching moment (yeah that quote doesn't say it specifically but a few pages ahead it explains pitching moments).

My question is... why do some aircraft pitch up and others down? My aerodynamics teacher in college would kill me if he found out that I forgot why but...

Seems to me that most high wing aircraft pitch up, while the low wing aircraft pitch down. For the high wing Cessnas I am willing to bet that part of the reason is increased downwash on the tail. But I know of high wing T-tail aircraft that pitch up as well. I suppose if I took the time to do some force vector analysis on a high wing aircraft I could figure it out. I will work on it, but if someone can figure it out before me, by all means do!

Skeezer

 

[SuperD]

Metrosheriff,

You explaination of why the aircrafts pitches with flap extension is correct. It has much to do with the change of pressure distribution on the wing as flaps are deployed. Whoever said that the pitching depends on the position of the wing with respect to the CG is only partly right. For instance, the BE1900D has a noticable pitch-up tendency when flaps are expended and it is clearly a low-wing aircraft (wing is below CG).

For some reason many people buy into the FAA's written test answers and continue to believe that the purpose of flaps is to increase lift. NOT TRUE! You only need a certain amount of lift for a given flight condition. One of the main purposes of flaps is to acheive the same amount of lift at a slower speed. It is true that the aircraft balloons when flaps are initially extended, but it soon returns to equilibrium with only the required lift.

SuperD

 

[MetroSheriff]

One of the main purposes of flaps is to acheive the same amount of lift at a slower speed.

SuperD,

Well put. Also the resultant lower angle of attack required to maintain level flight at a lower airspeed keeps the a/c from having to maintain excessively high deck angles on approach. Not to mention lowering the stall speed and better low airspeed handling characteristics.

BTW, great thread everyone. Quite a refreshing break from the usual intra-union and inter-carrier bickering...

 

[skeezer]

With a constant power setting (thrust) that is completely untrue.

Hence the reason most aircraft will tend to balloon when flaps are applied. This is why the pilot, or autopilot,will have to pitch-down (on most a/c), in order to lower the AOA and thereby compensate for the increase in total lift resulting from flap deployment.

I will have to disagree with you. In straight and level flight, you are producing the same amount of lift with flaps, 10, 20, 30, 45, 70, whatever. The key is "Level flight."

It is true that when you add flaps the aircraft will create more lift INITIALLY at a given airspeed till you lower the nose to reduce the AOA. Once that is done, and you are once again straight and level, the lift equals the weight. The AOA may change, but the lift will remain the same.

For an aircraft to be in level flight the vertical component of lift must equal the weight of the aircraft. If the vertical component is greater than the weight, the aircraft will accelerate upwards. Simple physics. An imbalance of forces will cause the object to accelerate in one direction.


Peace out

Skeezer

 

[MetroSheriff]

It is true that when you add flaps the aircraft will create more lift INITIALLY at a given airspeed till you lower the nose to reduce the AOA

Skeezer,

I agree. That was the point I was trying to make. Most aircraft will tend to balloon (climb) with a constant power setting, UNLESS the nose is lowered (reduce the AOA) or change the power setting (reduced thrust). That is why I would contend that there is an increase in total lift, AOA or thrust MUST be reduced in order to maintain level flight.

 

[B-J-J Fighter]

Keep the info coming.

Great Post!

 

[skeezer]

Skeezer,

I agree. That was the point I was trying to make.

Ok, Cool deal. The reason that I disagreed was in response to your disagreement with SStearns2 on his post about flaps and lift. You stated that he was wrong when I think he was trying to say the same thing that you and I agree on.

Anyway, I love this thread as well. I just glanced back and saw how we progressed from CG and TAS, to Phugoid dampning, to pitching moments with flaps, to types of drag, etc. Good stuff!

Skeezer