Jet engine sim for testing 9/11 planes

Originally posted by HLR53K
I found this site to be an interesting read. What do you guys think?

Pretty much what i already knew from working on the flight line.

Apparently jet blast is only a concern on the ground.

Originally posted by HLR53K
Apparently jet blast is only a concern on the ground.

Well mainly a concern on the ground, but that does not mean that jet blast stops when the plane is off the ground.

Originally posted by ULTIMA1

Well mainly a concern on the ground, but that does not mean that jet blast stops when the plane is off the ground.

I will give you from ground to 20 feet for an airplane in flight as a concern (well, 20 feet above the top height of an object).

I will give you 50 feet directly behind exhaust nozzle while the airplane is in flight at any altitude, provided you can overcome the wake turbulence and get that close to the engine in the first place.


This is a better representation:
www.tc.gc.ca...

But it's for airplanes on the ground only. It's kind of hard finding diagrams that show the danger area for an airplane in flight.



Here's another graph for a B737:
www.geocities.com...

[edit on 6-5-2008 by HLR53K]

I hate double-posting, but I had to create another post for this:

www.youtube.com...

A French KC-135 passing very low over the desert. Comments?

[edit on 6-5-2008 by HLR53K]

Originally posted by HLR53K
I will give you 50 feet directly behind exhaust nozzle while the airplane is in flight at any altitude, provided you can overcome the wake turbulence and get that close to the engine in the first place.

Well as stated wake turbulence decrease as speed increases.

I have only found the 1 good source that talks about jet blast from a plane flying over cars or obsticles.

Originally posted by ULTIMA1

Well as stated wake turbulence decrease as speed increases.

I have only found the 1 good source that talks about jet blast from a plane flying over cars or obsticles.

Well, not exactly. It's also about how much surface area is facing perpendicular to the airstream. Even if you're going very fast, you can have a huge wake turbulence if you have a large surface area facing into the airstream.

But that should be obvious.


I just thought about that statement a bit. We can agree that wake turbulence is a by-product of lift, correct? As in, the larger the lift, the larger the wake turbulence.

If lift is a function of the square of velocity (assuming the other wing factors are kept constant), doesn't the math say that wake turbulence increases as velocity increases?

Velocity goes up -> Lift goes up -> Wake turbulence goes up

Of course, there is a maximum velocity point where the wings aren't creating lift anymore as much as they become giant control surfaces.


I remember a lecture from my Introduction to Aerospace professor. Air is still made up of many molecules, yes? So we can agree that when an airplane is flying through the air, it's displacing those molecules which help generate the wake turbulence (displaced air).

As an airplane goes faster, it's pressing against this "wall" of air molecules and they're gathering faster on the surface area facing the airsteam than they can roll off. So now the airplane is fighting more and more drag and this causes more turbulence to form.

It's like if you move your hand like a knife underwater slowly and then quickly. If you move it slowly, there's less drag and turbulence and it'll glide smoothly through the water. But if you move it really quick, you'll feel the water slowing you down and it might even create a wake on the water's surface if you're close enough to it.

Water's only a thicker fluid than air, but the principles should be the same.

Anyone think I'm crazy?

[edit on 6-5-2008 by HLR53K]

I've rechecked all the witness accounts and only one states that the landing gear was lowered while all the others making mention of it specifically state it was not lowered. That one witness, who was awarded the Airman's Medal for his actions after the event, suffered a nasty head injury when the explosion of the plane knocked him against a pole so he can be excused for what seems to be at odds with other observations. A head injury can blur the distinction between reality and imagination especially when it comes to fine details. It in no way speaks against his good character that he's very possibly mistaken about some of the details he remembers.

As for the subject of the plane's height above the roadway and the nose-down attitude (supported by observers) I again submit this picture which gives a very good view of the flight path in relation to the poles, the road and the building. Nose-down implies the jet blast being angled upward at a slight angle so the main blast would have missed the vehicles safely and what they felt would, at most, be the turbulent zone around the fringe of that blast which lasted for maybe 100 milliseconds.

Originally posted by HLR53K
I just thought about that statement a bit. We can agree that wake turbulence is a by-product of lift, correct? As in, the larger the lift, the larger the wake turbulence.

Major wake tubulence is casued at low speed with gear and flaps down.

According to sources the turbulence decrease as speed increases.

Originally posted by ULTIMA1

Major wake tubulence is casued at low speed with gear and flaps down.

According to sources the turbulence decrease as speed increases.

First off, you've just agreed to my very first point. That increasing the surface area facing the airstream increases the turbulence. Thank you.

According to the sources I've read, wake turbulence is a direct by-product of lift.

If that's true then my hypothesis still stands (more velocity -> more lift -> more wake turbulence). I'm guessing you didn't read my post, did you? I'm starting to see why WW is frustrated.


What are your "sources" again?


Lets see what everyone else thinks. Please read my previous post and comment. I'm addressing everyone but Ultima, since it seems he prefers not to read it.

[edit on 6-5-2008 by HLR53K]

reply to post by HLR53K


HLR, if you fly fixed-wings, then you know that the strongest wake turbulence is behind an airplane in the landing configuration.

With full slats and flaps extended, it is producing max lift.

Your theory about being fast, and producing more lift, is a little off....it's all about angle of attack....low, slow and dirty, equals high angle of attack, and high lift.

Anyway, ULTIMA is so far off the mark, with this baloney of 'jet-blast'....that's not even the point of this THREAD!!!!

WW

reply to post by weedwhacker


I understand what you're saying and it's very true. I addressed that in another post where I said that anything angled as close to perpendicular to the airstream will help create more turbulence.

However, Ultima's blanket statement of wake turbulence decreasing with speed isn't completely correct.

What I wanted to say was that for an airplane at a certain AoA flight, if we keep everything else constant, an increase in speed will increase wake turbulence up to a point. Which is to say, if you take an airplane flying at a 5 degree AoA at .70 Mach and then flew the same 5 degree AoA at .8 Mach, the wake turbulence will be greater (at the same altitude too). After all, the airplane has to start moving first before the wake turbulence is created.

Of course an airplane flying straight and level will create less wake than an airplane with a high (or low) AoA.

The AoA is taken care of in the lift equation through the Coefficient of Lift. Through a decent range of AoA for a given airfoil shape, the relationship is roughly linear (the equation escapes my memory right now, I'll post it later).

L = 1/2*Cl*ro*S*v^2

But another factor is the shape of the nose of the airplane, which will determine how quickly and how much air pressure builds up in front of it, which also contributes to the wake. Do you know why the space shuttle has a blunt, round nose?

I'll have to dig out my fluid mechanics and aerodynamics books to truly verify if my hypothesis is correct.

[edit on 6-5-2008 by HLR53K]

reply to post by HLR53K


HLR....don't dig out your books, you said it already!!!

An airplane makes a wake, just as boats do....bad analogy, I know, but trying to help people understand.

I have, many times, crossed a wake from another airplane....in the 'cruise' configuration. If we are directly behind, there is a rolling moment....if we cross it obliquely, then it is a sharp 'bounce'....like turbulence, but localized, and not repeated. We know what it is, and know why it happened....and the Seat Belt sign stays off, since we know it is a one time event. That's part of the nature of knowing about flying.....

WW

reply to post by weedwhacker


Actually, I quite like the boat analogy! The boats propellers are created a thrusting force like a jet engine (sucks fluid in and expels it out the back). But when you encounter the boat, what is it that actually rocks your boat and pushes it off course? It's the wake. The bigger the boat, the bigger the wake.

You have to be closer to actually feel the "push" of the propellers.


I'm not sure if I said this, but flow separation is also a key factor. At very high AoAs and velocities, the flow around the wing actually separates from the wing itself which causes huge turbulence behind it as well.

As shown here:
www.youtube.com...

Wikipedia has a lovely picture as well:
en.wikipedia.org...


WW, I was kind of hoping to get out some hard numbers and equations. I don't know how much formal schooling Ultima has in complex partial differential equations (relating to fluid flows and aerodynamics), but numbers and equations are kind of hard to argue against. I mean, those equations have stood the test of time and have been refined multiple times to be increasingly accurate over the 100 years we've had aircraft.


So, does anyone know why the space shuttle's nose is blunt and round?

[edit on 6-5-2008 by HLR53K]

reply to post by HLR53K


HLR....the screw is under water, so it is overwhelmed by the 'wake' produced by the hull of the boat.....that's why we have 'no wake' buoys in certain areas (Boaters will understand this)....think of them as 'speed limit' signs, but they don't talk down to the boater....he knows how to not go so fast to produce a wake....

Back to airplanes.....what has to be taught to aspiring pilots is the concept of wingtip vortices....because, they are (usually) invisible. The vortices will form in very humid air....but a student pilot won't be flying in those limited visibility conditions.....because, very humid usually equates to IFR conditions.....

BUT....the jet engine simulator.....I have already said this, but it bears repeating.....a modern commercial jet engine is designed with 'limits'...we use a guage called 'EGT' (Exhaust Gas Temperature) as a guide....the engine designers know the actual temps inside, at each stage.....so, they tell the user (US) what the Maximum EGT should be, in order to keep the engine in good working order, and contribute to long life.

IF YOU exceed these parameters.....then, it is an emergency. This is assuming you are flying a real airplane, and trying to save lives.

If you choose to be a suicide, and fly the airplane into a building....then you really don't care about temperature limits, do you?

ps....I hope the '72 Virgins' the Hi-Jackers hoped to meet, in front of 'Allah', were all fat guys behind a computer keyboard, like the 'Comic Book Guy ' from the Simpsons......

LOL!!!!!!!

reply to post by weedwhacker


If feel like we're just throwing big facts up just to outdo each other, even though we're arguing the same thing. What I await to see is which parts of my posts Ultima will ignore. I have a feeling that I'm starting to use concepts that fly over his head (pun kind of intended ).

I'll give you the propellers under the water bit. But if they're close enough to the water's surface, it should be able to create some effect.


And I agree on your engine comment. Unless they were being dragged at that altitude and speed for the entire flight, even if a temperature light went on, it doesn't really matter.


Oh, Navy's railgun test firing to show the wake created by a very high-speed object (0:14 - 0:18):
www.youtube.com...

[edit on 6-5-2008 by HLR53K]

reply to post by HLR53K


HLR....we're on the same page!

Oh, BTW....engine overtemps on a B757....no audible alert!!!

NO ding, ding, ding, ding.....(that's the standard four dings) for a 'caution' (yellow)

Four electronic chimes, to get your attention....and you look then, at the EICAS to see what it is....

Something more serious....the Hi-Low siren.....and the RED alert on the EICAS.....pretty hard to iggnore.

The Auto-Pilot disconnect....we get the Hi-Low....but the siren can be cancelled with the same button you used to disconnect the AP ...it's like a 'double-click' on your mouse.

Simple, eh?

BUT...an Engine Fire, or APU Fire....same sound....along with the EICAS alert, and the appropriate light in the fire handle....and, the sound can be cancelled....so we can read the checklist.

Other time the Hi-Low siren will sound...is the Overspeed....along with the RED 'OVSPD' on the EICAS.....can't be cancelled....but, in the last few sedonds, the Hi-Jackers didn't care....

WW

Originally posted by HLR53K
Of course an airplane flying straight and level will create less wake than an airplane with a high (or low) AoA.

So what was the AoA of the plane at the Pentagon?

reply to post by ULTIMA1


low.

reply to post by ULTIMA1


I see that you cut out that little bit of my past posts to comment on. I really wish that you could take the time to read the rest of my posts and comment on that. I'm going to take your silence as either you don't understand it or that you're agreeing to it (since you haven't objected to any of it so far).

Anyway, I don't know the exact AoA. I would need to know the direction of the airstream too. All I can do is assume that the air was still and that the airplane was descending nose down so there's a negative AoA. That would increase the surface area being shown to the airstream and thus creates a larger wake from the drag alone.

Like WW said, it's all about the AoA. The higher or lower it is, the more wake turbulence you have. I'm sure I don't have to explain to you how nasty it would be to be behind a large airplane being shoved through the air with an AoA of 90 degrees!

[edit on 7-5-2008 by HLR53K]