Extracted from Code One magazine see link for full article
http://www.codeonemagazine.com/archives/1993/articles/apr_93/stretch/index.htmlNow for the promised physical limits. Two examples are worthy of discussion. The first involves the calibrated airspeed limit on the F-16. The Dash One sets the knots calibrated airspeed, or KCAS, limit at 800 knots. Interestingly, the airspeed limit is actually based on the engine operation. With the original -200 version of the F100 engine, you almost had to dive into the point to exceed 800 knots. But just such a dive could surpass the physical limits of the engine because the -200 engine control system essentially runs open loop. That is, the control system would allow the compressor discharge pressure to increase beyond the physical limits of the engine. Therefore, the limit was established at 800 knots to ensure that the -200 engine remained within its envelope.
A tragic accident exemplifies the frivolity (read stupidity) of exceeding this limit. One of the pilots in the test squadron at Eglin (someone who certainly should have known better) took it upon himself to see just how fast the F-16 would go. On the way back from a rather mundane test mission, he climbed to 16,000 feet, turned the wick full up, and nosed over into a dive. In reconstructing the flight, we feel that he got well on the high side of 850 knots. Since the compressor discharge pressure was uncontrolled, the pressure became high enough to distort the engine case. The turbine rubbed the engine case at a ferocious rate, went through the turbine tip seals, and began eating into the engine case with equal fury.
The engine casing in this area is constructed mostly of titanium. Those of you familiar with the periodic table of elements may remember that titanium is in the same family of elements as sodium, lithium, and magnesium. The temperature to get titanium burning is very high (that's one reason it is used in the engine). Once any of these metallic elements start burning, though, you have one hell of a fire. With a fire of this intensity in this part of the airplane, you have effectively wrapped prima cord between the trailing edge of the wing and the leading edge of the horizontal tail and lit that sucker off.
The intense fire quickly burns around the entire fuselage and through both hydraulic control systems in spite of the best efforts of design engineers to isolate the two systems from collateral damage resulting from multiple combat hits. Under such conditions, all bets concerning the control of any aircraft are off. Even the most thoughtful attempt to design redundant safety systems can be sidestepped by stupid actions of a pilot. With no hydraulic pressure, you are nothing more than a passenger in an aircraft going down at almost 1,000 miles per hour. The airplane continues to descend and you don't even have a vote. That's not a feeling I want to experience.
Somehow our intrepid (but not too forward-thinking) aviator got out of the ruined airplane. But he broke both arms in the process. He ended up drowning in the Gulf of Mexico. A very bad scene from any perspective.
Those of you who may have been getting in the books or listening to the engine awareness briefings of both Pratt & Whitney and General Electric will probably now want to point out that the later -220 and -229 versions of the F100 and all models of the GE F110 engine have positive, closed-loop control systems. I'm glad you've been paying attention. The -220 engine has more thrust than the -200, once you get it moving. But it is seldom more than ten percent better. So you'll still usually have to dive the airplane to exceed 800 knots.
But the -229 and all versions of the F110 have enough power to go right through 800 knots like it isn't even there, straight and level, sometimes as high as 15,000 feet. So why not exceed the limit?
Pay attention. The newer engines control the compressor discharge pressure for the most part by rolling back fan revolutions per minute, which cascades through the engine and, thus, effectively limits the N2 (the high-pressure compressor) discharge pressure. They are not taking thrust away, but they are preventing thrust from building at the rate it would like to because of the increased ram effects. However, improvements throughout the rest of these engines result in a lot more thrust in this part of the envelope. Furthermore, this extra thrust comes without the risk of hurting the engine. So if the engine is not the limiting factor, what is?
Well, it is not the F-16's flutter limit, which is theoretically on the plus side of 900 knots. That is an impressive number, n'est-ce pas? Under test conditions, I've been as fast as 845 knots with the early GE engine. Further, it was readily apparent that the airplane was nowhere near ready to quit. It is really a ride to feel those levels of acceleration for that length of time and still know in your heart that there is a lot left. We could have easily taken that airplane (one of the old beat-up, full-scale development airplanes, F-16A No. 1) and set a low-altitude speed record with no preparation other than getting the Federation Internationale de Aeronautic to Edwards to certify the timing. But once again the political climate was not right to grant the F-16 any favorable notoriety. Plus, we were told in no uncertain terms that we were not even to talk about such an attempt. Too Bad. It would have been fun to see just how fast we could have gone. It most certainly would have been a big number.
I almost choke, however, when I hear that some pilots have had the airplane as fast as 870 knots. Even considering that the speed may inflate a little every time the story is told, I really don't like to hear about anyone exceeding 800 knots for no apparent reason.
