Some controversy has arised about the "Jet Blast" effect near the Pentagon. How could a B 757 jet approach the Pentagon, flying over rd 27 at ~10 to 20 ft height - deduced from lamp poles damage - with engines at full rev - deduced from witnesses accounts - and not blow about cars which were right under it on rd 27 ? Jerry Russel and Richard Stanley argue about that in their web pages "five sided fantasy islands", on a page named Eyewitnesses and the plane bomb theory. Here are their arguments.
Further evidence of the validity of our argument was recently provided by David MacGowan, who posted this video
http://www.davesweb.cnchost.com/simulation.mpeg
which is a computer-animated video showing an automobile being blown about like a tumbleweed by the blast of an aircraft engine exhaust. As NASA explains at:
http://asrs.arc.nasa.gov/directline_issues/dl6_blast.htm
Before a crew can say "powerback," jet engine blast can up-root trees, flatten building structures, shatter windows, lift and propel heavy objects, weathercock braked airplanes, blow over lift trucks, shift unbraked baggage carts, and create other havoc on airport ramps, taxiways, and runways.
Although the diagrams don't say so, jet blast can also injure or kill crew and passengers who happen to cross its path.
Yet we are expected to believe that the 757 flew over a highway overpass at about 10 feet altitude, clipping antennas with its engines, passing near billboard-style highway signs, and passing within 6 feet of Frank Probst, all without catching anything in the blast of its turbine exhausts. We found ourselves wondering if perhaps the forward speed of the plane might paradoxically have mitigated the effects of the engine's blast, by carrying the air mass forwards around itself (relatively speaking). We can investigate this further by using the engine modeling tool at:
http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html
We chose the GE CF6 engine model (which is a similar engine to the RB211 used in the 757, although with a slightly lower bypass ratio) and set the altitude to sea level, and the airspeed to zero, thus duplicating the conditions shown in the video clip. We find that the engine exhaust velocity, right at the exit nozzle, is 2281 feet/sec (that is, about 1500 mph).
What happens if the plane is moving at high speed? At sea level, and 400 mph airspeed -- the exhaust velocity declines only by the tiniest smidgen, to 2242 feet per second. That's relative to the velocity of the plane, so the ground speed of the exhaust would be reduced to about 1100 mph -- which is still plenty fast enough to send Probst spinning like a pinball. Not to mention the issue of wake turbulence and ground effect, as the weight of an 80-ton jetliner must be supported by exerting downward pressure on the air squeezed between its wings and the ground.
Here is a copy of my answer to this controversy, sent on a mailing list :
Nice video. I hope it was a staged demo with nobldy on board this vehicule... However things were completely different on rd 27 that was crossed by the 757 about one second before hiting the Pentagon.
1 - This is a 747 with four engines. A 757 has two engines.
2 - A 747 is an older jet, with a lower bypass ratio, thus the stream output of it's engine is faster than the stream output from a 757's engine, which has a higher bypass ratio.
3 - The 747 is stopped on a runway, then the relative speed of the jet engine streams regarding to the hit vehicle is maximum. The 757 overflew rd 27 at ~350 mph (if speed released from black boxes is accurate). Thus the speed of the engine stream relative to the ground is less important. This effect combines with effect N° 2 to give a much lower absolute engine stream speed regarding to the ground.
4 - The engines of the 747 in the video are very close of ground (~2m), and the engines of the 757 which flew over rd 27 were probably at least at ~5m.
5 - There is a ground effect, with a smooth ground which concentrates the stream of the engines and makes a much higher pressure on the vehicle. On rd 27, there were many natural or artificial obstacles to the streams of the engines : road signs, security barriers, embankments...
6 - The vehicule in the staged demo doesn't drive fast, and thus stays a long time in the engines stream, say several seconds. It is thus submitted to a strong acceleration during a long time, which allows it to take a high speed on the deadly path where it is pushed. The time during which the vehicles on rd 27 were submitted to the engines stream of the 757 was very short, say something like one or two tenth of a second, and they coulnd't take such a lateral speed.
For all these reasons, the effects of the 757 engines were much weaker and, it is normal that the vehicules on rd 27 were just shocked (a witness said his vehicle was pushed aside on the rd), and not blown over security barriers like in this demo.
Jerry Russel and Richard Stanley bring some precisions : "the engine exhaust velocity, right at the exit nozzle, is 2281 feet/sec (that is, about 1500 mph)." I think that this should be refined in two ways :
The plane was said to fly at ~350 mph when it hit the Pentagon. Thus, I think that when the cars on rd 27 were blown by the blast of the engines, this blast had a relative speed of ~150 mph regarding to the ground. As explained above, this did not last a long time. Cars were shocked and moved a little sidely. As they were all stopped or nearly stopped in a traffic jam, this did not cause any accident by itself.
Some witnesses who were inside cars on rd 27 said that their car was rocked and pushed sidely on the road. This limited effect is coherent with the overfly of a Boeing 757 plane flying at approximately 350 mph with engines at full throttle. The case presented in the video quoted above is completely different. This "no cars were pushed over the safety barrier of rd 27" statement is by no way an argument to say that the Pentagon was not hit by a B 757.