The very first sentence in your source says "jet blast AND/OR wake turbulence." Or did you just read "jet blast" and stop? There are a LOT of signs that are hit by jet blast that ARE behind the planes that get damaged. However, in that article you sourced it talks about BOTH causing damage. Which is especially true for the runway signs. They are 35 feet back from the runway, being hit by wake turbulence from low speed, heavy aircraft. It's going to cause damage.

[edit on 4/12/2008 by Zaphod58]

Originally posted by ULTIMA1

Originally posted by Zaphod58
And again, you are talking about a plane that is on the taxiway, sitting on the ground, where jetblast WILL damage objects and cause problems like that.

How does jet blast damage a sign that is not behind it, maybe when the plane flys over it close then the jet blast will hif the signs?

Originally posted by weedwhacker
In flight, of course, what is encountered is the 'wake turbulence'....this is a common term to describe the wingtip vortices, produced while in flight.

But wake turbulence decrease with speed.


[edit on 12-4-2008 by ULTIMA1]

No, ULTIMA....the strength of the wingtip vortice is directly related to the amount of lift being generated.

So, a very heavy airliner, with slats/flaps extended generates a lot more wake vortice than when in the 'clean' config.

See, slats/flaps extended means the wing is changed, to provide more lift, because the chord of the wing is altered.

More lift, more drag (induced drag, not to be confused with parasitic drag).

Natually, the wing shape is needed for lower speed flight, such as take-off and landing. This is when the vortices are strongest.

Trust me, I have flown behind airplanes in cruise, and encountered the vortices....they are sharp, and tight, and very easy to maneuver out of, at altitude.

Look, a jet, at high speed, clean will leave tight vortices in its wake, vortices that dissipate fairly quickly. Low, slow and heavy (slats and flaps), makes a different wake.

I used the water example, and that was a mistake, since you can see water. Air tends to be transparent, unless sufficient water vapor is present, and condenses out to form a contrail....but that delves into a whole other subject......

WW

Originally posted by weedwhacker
No, ULTIMA....the strength of the wingtip vortice is directly related to the amount of lift being generated.

But speed also plays a part.

Originally posted by ULTIMA1
Did you see what i posted about the airport signs?

Yes I saw that but airports are windy places so I don't see it as definitive proof in the context you're attempting to put it in. Aircraft on the ground produce drafts at ground level, aircraft in the air cause much less draft at ground level but of course we could split hairs over it as usual.

There's an expressway near here and a number of the large signs (mounted on multiple 2" galvanised steel poles) were damaged about a week ago on a particularly windy night. It was a strong wind.

Originally posted by Pilgrum
Yes I saw that but airports are windy places so I don't see it as definitive proof in the context you're attempting to put it in.

Well i am just quoting the site about jet blast and turbulence causing damage to signs. So it does go along with my OP.

Umm...going back to the original Java applet.

I put in the following:

Altitude: 60 ft.
Engine: Turbofan
Throttle: 99.93% (as high as it will let me go)

I raised the airspeed from 0 mph to 800 mph (Mach 1.052), as you said to do ULTIMA1. Not once along the way did I get the "temperature warning" message on the diagram of the running engine like you did. Are you sure you didn't just hit a bug in the program?


I did got it only when I did this:

1. I switched to a ramjet engine just to see its performance
2. Since a ramjet can't operate that low/slow, it changed the speed/altitude
3. Changed back to turbofan, but it kept the higher ramjet values
4. Temperature warning until I reduced the speed below 1000 mph

Can some of you other guys do it too and tell me if you get this "temperature warning" message?

Edit: Forgot to load CF6 model. Interesting...

[edit on 13-4-2008 by HLR53K]

Whoops, forgot to load the CF6. But at 60 feet, 99.93% throttle the engine does not overheat. 500 it will, but at the parameters in the OP the engine doesn't overheat. But even if the engine really DID overheat, it doesn't matter any, because it would have only needed to hold together long enough to hit the building. It's not like it overheats and instantly explodes or fails.

[edit on 4/13/2008 by Zaphod58]

If you use the "Load My Design" instead of the CF6 design, you can recreate the RB211 turbofan. Does anyone know its component specifications?

I'm sorry, ULTIMA1, but just picking the CF6 because it's "closest to" the RB211 isn't enough. Two separate engines, two different sets of component specifications. All the small designs between the two engines can add up to a lot in terms of performance differences.

The CF6 is comparable to the RB211-524 engines which were not used in the B757-200. If you want to compare similar engines, you have to use the P&W PW2000 engine, which is similar to the RB211-535 series, which were used in the B757-200.

I know that if someone else used an analysis of the CF6 to compare to the RB211 against one of your points, you would have issues that he didn't actually actually analyze the RB211. Can we not say the same thing from the other side?

[edit on 13-4-2008 by HLR53K]

[edit on 13-4-2008 by HLR53K]

Originally posted by Zaphod58
Whoops, forgot to load the CF6. But at 60 feet, 99.93% throttle the engine does not overheat. 500 it will, but at the parameters in the OP the engine doesn't overheat.

If you set the altitude at 60 feet and run the speed up, as soon as you hit 480 you get temp warning.

Also why would the hijackers risk burning up the engines before they hit thier targets?

Originally posted by ULTIMA1

Originally posted by Zaphod58
Whoops, forgot to load the CF6. But at 60 feet, 99.93% throttle the engine does not overheat. 500 it will, but at the parameters in the OP the engine doesn't overheat.

If you set the altitude at 60 feet and run the speed up, as soon as you hit 480 you get temp warning.

Also why would the hijackers risk burning up the engines before they hit thier targets?

Are you serious? 423 knots, 480 mph, and the terrorist was over this speed for 8 seconds, and it is a show stopper why? Oops, the plane is smashed. I doubt the overheat comes on before impact, if it does who cares, even if the engine explodes after a second, the plane is still hitting the Pentagon. Can you show me the indications seen in a 757 cockpit of an overheat?

The top ten reasons the terrorist risk the dreaded overheat! They,
10. want to go fast, damn the engines.
9 not using the engines tomorrow.
8 they do not know what the overheat means, do you?
7 don’t care, how many kids run their engine when the overheat comes on?
6 know the overheat will not damage the engine in the few seconds before impact.
5 never made it to section 3, limits and warnings.
4 are from the desert, they like heat.
3 have no idea why the warning light is on anyway.
2 are trying to damage engine so no one can use it again.
1 were in a hurry to party with virgins, and allah will take care of the overheat.

Have you had an engine overheat? I have, this is not an issue for a the terrorist hitting buildings and spending 8 seconds over 480 mph.

Originally posted by ULTIMA1

Originally posted by Zaphod58
Whoops, forgot to load the CF6. But at 60 feet, 99.93% throttle the engine does not overheat. 500 it will, but at the parameters in the OP the engine doesn't overheat.

If you set the altitude at 60 feet and run the speed up, as soon as you hit 480 you get temp warning.

Also why would the hijackers risk burning up the engines before they hit thier targets?

ULTIMA....so what if you get 'temp' warnings??

As you know, and for others to come accross this thread, the really most limiting factor, for a pilot, is EGT. It is important to know, on the various engines we operate, what the 'starting' limit is, what is the Maximum for 5 minutes, what is the maximum for continuous operation, etc. Along with this, is the Maximum EPR (not so much in today's Hi-bypass engines) and the MAX N1 and N2 (or MAX N3, if a Rolls-Royce engine).

Thing is, most critical number, is EGT.

We have, depending on the engine, a value for MAX Start temps, and MAX operating temps.

When an engine is being started, in a Boeing, the pilots monitor the start sequence. (airbuses are idiot-proof, today...)

Here's the sequence....we have the APU already running, assuming it is functional. (If it is inop, then that's why it is so hot inside the cabin)

So, assuming the APU is up and providing air and electrics, then the 'packs' are turned off (the packs are the air-conditioning machines...let's now refer to them as the ACM) because, the way a jet engine is started, is by introducing compressed air into the compressor section, in order to begin the 'spin-up'....this compressed air is usually supplied by the APU, but if the APU is inop, it can be supplied by a Ground Source, plugged into the airplane.

Back to a normal start, during a push-back, because the APU is normal, otherwise we wouldn't be pushing back....get the logic?

a 'switch' is engaged, by a pilot, to initiate the 'start sequence'. Most modern airplanes have a magnetically held switch, so that once engaged, it stays engaged until certain parameters are satisfied. On a B727, for instance, the pilot had to hold the switch until the engine reached a certain point in the start sequence, then manually release it. NOW, it is more automatic....

Either way, the start is monitored by both pilots, because a jet engine can encounter a 'hung start'....where, for some reason, it does not normally accellerate to idle, and the EGT can progress to temps that may cause damage. OR, the engine can have a 'hot start', where the EGT is increasing so rapidly, it can cause damage, and that last one would indicate a problem ith the fuel control unit, or FCU.

So you see, it is important for the pilots to know how to start their engines...and it is sad that most don't ever comprehend what is required of an airline pilot, they just think of them as glorified bus drivers.....and, I only offered insight into a small aspect of what pilots do....

You, as a passenger, should understand, and learn.

WW

Originally posted by ULTIMA1
If you set the altitude at 60 feet and run the speed up, as soon as you hit 480 you get temp warning.

Also why would the hijackers risk burning up the engines before they hit thier targets?

At 60 feet, 480 mph, 99.93% throttle, with a CF-6 engine loaded, I got NO temperature warning. Even if I did as was stated by several people SO WHAT, at that point you are only seconds from impact. An engine can run that long before any damage is going to occur. It's not like you get an instant reaction to a temp warning.

[edit on 4/14/2008 by Zaphod58]

Originally posted by ULTIMA1
If you set the altitude at 60 feet and run the speed up, as soon as you hit 480 you get temp warning.

Also why would the hijackers risk burning up the engines before they hit thier targets?

Now you, of all people, with your asserted experience in maintaining such systems should be aware of how redundant that suggestion is. An engine does not instantaneously self-destruct when the temperature alarm appears and from your own observation, the overtemp warning has only just been triggered. Your vague point may appeal to those with no knowledge of what it means in reality so I'm just trying to add to the voices of rational reasoning here.

I'm sure you understand the implications of overheating, its relation to MTBF and the old rule of thumb which suggests every 10C over rated maximum temperature halves the expected operating lifetime.

How long did these engines need to survive a slight overtemp to achieve their objective?

I'm heavily involved in operation and monitoring of very heavy turbine equipment and although it doesn't fly, the principles are the same and major concerns are temperature affecting lubricant viscosity producing accelerated bearing wear & tear resulting in excessive vibration over time. Thermal expansion can also cause fine turbine minimum clearance tolerances to be exceeded.

Temperature alarm settings are way below the point where damage occurs - that's the purpose of having an alarm, to warn the operator he's at risk of over-stressing the equipment.

[edit on 14/4/2008 by Pilgrum]