Why aft CG increases TAS

[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.