Why aft CG increases TAS

[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

 

[profile]

Well, a couple of points:

First, it's not hard to see why most aircraft pitch forward with flaps, as the CP moves aft. A pitch up indicates that the flow in the vicinity of the horizontal stab has been altered by the flap addition to the relative AoA results in a pitch up.

I do have a bit of a "nit" with "Bobby" above. It appears that Bobby is implying that there is some difference between "Newtonian lift" and "Bernoulli". While there is some application of "impact lift" as applied to hypersonic vehicles, in conventional aerodyanamics, they are just two ways of describing the same thing. Actually, it is all based on the theories of Newton, meaning Bernoulli's equations derive from Newtons.

 

[A Squared]

Parasite drag increases with airspeed (exponentially I might add!)

SuperD

Actually, it doesn't vary exponentially with airspeed, it varies with the airspeed raised to the power of 2 (squared). If it was an exponential funtion it would vary with some value raised to the power of the airspeed. An exponential function of some variable will increase much more rapidly than a squared function of the same variable.

Regards

 

[SuperD]

A Squared,


I was merely trying to point out that the relationship between parasite drag and airspeed is nonlinear and increases at a higher rate with increasing speed. The word "exponentially" was a bad choice. Thanks for the info!

SuperD

 

[sstearns2]

.....

This was an interesting thread.

One observation...

Most of these post are correct, just answering slightly different questions. This happens all the time. Two people will be going round and round in a heated discussion only to realize they're both giving the correct answer to differnet questions.

It's good to be aware of this tendency. When you read something your think is totally wrong make sure really understand what is being said and then let them have it!

Scott

 

[sstearns2]

Wake vorticies

Someone asked what I think about Wake vorticies...

The common explaination of wake vorticies is that they are formed by air leaking around the tip of the wing, from high pressure below the wing to low pressure above.

The problem is that this is not the whole story. The leakage of air around the wing causes the air on the bottom of the wing to flow slightly toward the tip and the flow on the top of the wing to flow slightly toward the root of the wing. When the air from the top of the wing meets back up with the air from the bottom of the wing they are going in slightly different directions. This 'shear' at the trailing edge of the wing causes a vortex to be formed along the entire trailing edge of the wing contributing to the vortex formed at the wing tip.

Scott

ps- I have a feeling that someone is going to use the 'Kutta condition' to agure against this. The Kutta condition is only valid in 2-D flow not in real life.

 

[TurboS7]

On a 727 at cruise if you pull the LED CB you can crack the flaps 2 degrees, this unloads the stabilizer and results in higher IAS and lower fuel burns. Just make sure your FE knows you have the CB pulled. Essentially you are moving the CG aft in cruise.

 

[profile]

That bit on the 727 is absolute rubbish. If it were true, Boeing would have designed a mechanism to do that, or designed the wings that way at the outset. Should also note that adding flaps would more probably increase the load on the horiz stab.

 

[chperplt]

Profile

By your comments, I assume you haven't heard the true story of the UAL crew that did that.. TurboS7's remark about the CB was in reference to that crew not letting the FE, who had gone to the lav, know what they did. He retuned to his panel, saw the CB out, and popped it back in. The LEDs deployed and the aircraft had a few problems... the CVR was erased... and so on and so forth.. Very interesting incident report.

 

[profile]

Chpr

By YOUR comments, I assume you really haven't read the TRUE story of that event yourself! For started, it was TWA, and the whole notion of the CB being intentionally pulled, etc., is pure speculation. ALPA magazine had a good article about it all several years back, perhaps you should read it?

 

[chperplt]

Profile,

Thank you for correcting me..

And yes, I have read the true story. I've also read the interviews with the NTSB and Boeing.

Pure speculation or not.. We ALL know what happened. No, I wasn't there as I'm sure that was your next question.