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.

 

[profile]

>>We ALL know what happened. No, I wasn't there as I'm sure that was your next question.<<

As an experienced accident investigator, I would say that contrary to your assertions, no "we all" do NOT "know what happened." The scenario you mention was very convenient for Boeing, though, wonder who spread that particular rumor around?

 

[chperplt]

You're right, it was very convenient for Boeing.

As an experienced accident investigator, how would you explain why the crew erased the CVR?

 

[profile]

Well, that portion of the tape would have been history anyway by the time they got on the ground (more than 30 minutes after the upset), so it's really a moot point. The procedure now is to pull the CB to preserve the pertinent portion of the tape, but in actual practice few do that. While it may not look good, the fact is that there are a variety of innocuous reasons to hit the erase button. It is habit for a fair number of pilots (at least it used to be). It could be that somebody said something in poor taste, it could be that someone mentioned a mistress, could just have been his SOP (which he stated), but doesn't indicate that. Read the ALPA report on it and then decide what you think.

 

[chperplt]

It's the point that the tape would have been history, and they still erased it, that makes me think they were messing around and it bit them in the butt.

I read the reports a few years ago. I'll give them a re-read and see if I come up with a different opinion.

Fly safe

 

[profile]

Well, it could have been anything from "this divert is going to mess up my date tonight" to just being paranoid for no real reason (a very common trait among airline pilots these days, and not without good cause!). Perhaps they didn't follow some published TWA procedure after the event, so were worried on being burned on that.

I can recall one event where the crew got blamed for not reading the entire checklist even though the smoke in the cockpit and goggle problems made it virtually impossible. Crew got the airplane on the ground without a scratch, and everyone out without any injuries. If they had hit that erase button they'd now have clean records instead of having FAA and NTSB hammer them. Capice? Kind of makes you consider hitting that erase button....

While the "procedure" described in the initial post was bantered around, I doubt very seriously that anybody did this, if performance improvement were possible this way, as I said before, the "flap 0" position would have left them with the same droop as "flaps 2" did. IOW, Boeing would have designed it into their airplanes. Considering that there have been more than one recorded event of uncommanded slat deployment at altitude since that event, it is more probable that the crew's story is accurate.