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| IL-2 Sturmovik The famous combat flight simulator. |
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Thread Tools | Display Modes |
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#1
09-25-2009, 12:08 AM
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all the findings here are compliments of 609_Kahuna, i have merely copy n pasted his posts from (the old) CWOS's "Lockheed Syndicate" forum
NACA research on critical mach/compressibility: http://history.nasa.gov/SP-4219/Chapter3.html The general aeronautics community was suddenly awakened to the realities of the unknown flight regime in November 1941, when Lockheed test pilot Ralph Virden could not pull the new, high-performance P-38 out of a high-speed dive, and crashed. Virden was the first human fatality due to adverse compressibility effects, and the P-38, shown below, was the first airplane to Suffer from these effects. The P-38 exceeded its critical Mach number in an operational dive, and penetrated well into the regime of the compressibility burble at its terminal dive speed, as shown by the bar chart on page 80 .35 The problem encountered by Virden, and many other P-38 pilots at that time, was that beyond a certain speed in a dive, the elevator controls suddenly felt as if they were locked. And to make things worse, the tail suddenly produced more lift, pulling the P-38 into an even steeper dive. This was called the "tuck-under" problem. It is important to note that the NACA soon solved this problem, using its expertise in compressibility effects. Although Lockheed consulted various aerodynamicists, including Theodore Von Kármán at Caltech, it turned out that John Stack at NACA Langley, with his accumulated experience in compressibility effects, was the only one to properly diagnose the problem. The wing of the P-38 lost lift when it encountered the compressibility burble. As a result, the downwash angle of the flow behind the wing was reduced. This in turn increased the effective angle of attack of the flow encountered by the horizontal tail, increasing the lift on the tail, and pitching the P-38 to a progressively steepening dive totally beyond the control of the pilot. Stack's solution was to place a special flap under the wing, to be employed only when these compressibility effects were encountered. The flap was not a conventional dive flap intended to reduce the speed. Rather, Stack's idea was to use the flap to maintain lift in the face of the compressibility burble, hence eliminating the change in the downwash angle, and therefore allowing the horizontal tail to function properly. This is a graphic example of how, in the early days of high-speed flight, the NACA compressibility research was found to be vital as real airplanes began to sneak up on Mach one http://history.nasa.gov/SP-4219/4219-084.jpg  http://findarticles.com/p/articles/m..._n9283659/pg_2 Flight testing the P-38 disclosed that whenever the airflow over the wing exceeded Mach 1.0, compressibility effects were encountered. This result was soon predictable when this slippery fighter accelerated in excess of 0.65 Mach in dive angles greater than 45 degrees at altitudes above 15,000 feet. Cockpit-installed Mach meters had yet to be invented. George W. Grey, in his history of NACA, departed from strict engineering terms when he described compressibility effects in the P-38, saying, "The behavior of the P-38 was new to pilots, terrifying, baffling. Several men putting this two-engine fighter through its diving maneuvers experienced a sudden violent buffeting of the tail accompanied by a lunging and thrashing about of the airplane, as though it was trying to free itself of invisible bonds, and then the maddening immobility of the controls, the refusal of the elevators to respond to the wheel control." The only element he left out was the most horrifying: the nose-down pitching. Even a strongly applied aft wheel force couldn't stop the problem. The NACA High Speed Wind Tunnel team under John Stack's direction had been working on this problem and had devised a small pair of 6x40-- inch, electrically operated dive-recovery flaps to be installed on the P-38 wing's underside and outboard of the engine nacelles; they could be extended to 40 degrees. That action would rapidly pitch the aircraft up to 4G and enable the pilot to regain full control. Although Lt. Kelsey evaluated and approved this dive-recovery flap in February 1943, Lockheed did not incorporate it into production for another 14 months! By that time, 5,300 P-38s-more than half the number eventually produced-had been delivered to the USAAF. In 1943, I experienced compressibility in a Hellcat; I wonder how many of those P-38 pilots in the pursuit of the enemy dived too steeply-well beyond the critical Mach limit and into compressibility-in the heat of combat and disappeared into oblivion. At the Joint Army/Navy Fighter Conference on October 16, 1944, I tested the P-38L dive-recovery flap well in excess of its 0.65 Mach-number limit. Upon actuation, they instantly provided a smooth, 4G recovery without pilot effort. Immediately after I evaluated these "jewels," they were installed on all Grumman 17817-1 Bearcat fighters. http://findarticles.com/p/articles/m..._n9283648/pg_4 At about 0.65 Mach, the P-38 developed heavy buffeting with a strong negative pitch ("Mach tuck"). Ordinarily, that was enough to warn the pilot of impending control lock. If the dive persisted into the transonic regime (around 0.72 Mach), the condition could become irrecoverable. Consequently, dive flaps were installed in the last 210 J models and in all Ls, and they provided a much needed speed brake. Essentially, they returned controllability to the elevators. William H. Allen flew with the 55th Fighter Group and recalls P-38 dive-- bombing missions. "Dive-bombing depended on the fuse setting; sometimes, they were three-second delays, which meant a higher release altitude, and they went up to 19-second delays, where we would drop from 10 feet in a level attitude and let the bomb skip up to the target. We would normally start our run at 8,000 to 10,000 feet and roll over, point at the target and drop when we got nervous. Dive speeds were no problem with the P-38 below 12,000 to 15,000 feet." the 45 degree dive quoted by Corky Meyer, is should be noted, only refers to sustained dive... now if we can only get the 38's climb, engine power/top speed, and low speed handling, and DM (the tail booms share ONE hitbox, meaning control surfaces can go out despite the 38's redundant control runs) correct.... Last edited by Daiichidoku; 09-25-2009 at 12:28 AM. |
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#2
09-25-2009, 12:16 AM
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more on compressibilty:
http://yarchive.net/mil/p38.html One problem the P-38 had in dealing with the Me-109, but not the FW-190 (which was more of a low and mid-altitude fighter) was the Me's high altitude performace superiority. Above 25,000 ft., cooling or supercharger impeller or turbine speeds became limiting for the Lockheed, and high speed capability started to fall off. At low altitudes, the plane could max out at about 330-340 mph. This rose to well above 400 mph between 25,000 to 30,000. As the plane approached 30,000 ft, speeds over Mach 0.60 could be sustained in level flight. Thus, manuevering could quickly give the plane compressibility problems. At Mach 0.65 (290 mph IAS, 440 mph TAS at 30,000 ft.; 360 mph IAS, 460 mph TAS at 20,000 ft.) drag began to soar as the plane began to encounter compressibility. At Mach 0.67 shock waves began forming and buffeting began at Mach 0.675. At Mach 0.74 tuck under began. Buffeting developed at a lower Mach number in any maneuver exceeding 1 g. In contrast, the P-51, had far fewer compressibility problems at speeds normally encountered in combat, including dives from high altitude. The D model was placarded at 300 mph IAS (539 mph TAS, Mach 0.81) at 35,000 ft. In a dive, the P-51 was such an aerodynamically clean design that it could quickly enter compressibility if the dive was continued (in reality, a pilot could, as a rule, catch any German plane before compressibility became a problem). But, say, in an evasive dive to escape, as the P-51's speed in the dive increased, it started skidding beyond what the pilot could control (this could be a problem in a dive onto a much lower-flying plane or ground target--couldn't keep the plane tracking on the target if speed was too high). As compressibility was entered, it would start rolling and pitching and the whole plane would begin to vibrate. This began about Mach 0.72. The pilot could maintain control to above Mach 0.80 (stateside tests said 0.83 (605 mph) was max safe speed--but structural damage to the aircraft would result). The P-51's quirk that could catch the uprepared service pilot by surprise was that as airspeed built up over 450 mph, the plane would start to get very nose heavy. It needed to be trimmed tail heavy before the dive if speeds over 400 mph were anticipated. However, in high speed dives, the plane's skidding changed to unintended snap rolls so violent that the pilot's head was slammed against the canopy. Depending on how much fuel was in the fuselage tank, on pull-out stick force reversal could occur, a real thrill that could totally flummox a low-time service pilot diving earthward at close to 1,000 ft per second trying to escape a pursuer. |
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#3
09-25-2009, 03:53 AM
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I just want to say thank you for your time and efforts on this patch.
I already had FB-AEP-PF and had not touched it in about 3 years and recently became curious about IL-2 again since the all simulations/strategy war bug struck again, and after reading about the upcoming additions of your 4.09m patch I decided to buy 1946 and also got into hyperlobby and gave mp a try. So due to your upcoming patch 1c gained a sale and hyperlobby gained a living target for your trigger finger amusement. Now I just need to hint to my woman that "Track IR" would make an excellent Christmas gift. 
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#4
09-25-2009, 09:14 AM
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Capt Stubbing .
In your reply you state that you are seeing the effects start at 300mph but don't state at what altitude. Well that could be the case depending on altitude. Its not an IAS but a Mach number problem. In the chart you post note that at 30,000feet the limit is in fact 290Kias this equating to 0.68Mach. The limit of onset values (at 1G) is solely Dependant on Mach number. This is exactly what is happening in Il2. I have the luxury of real time Mach number display in the tests I perform. So far in what I see the onset of compressibility in the P38 is almost exactly on the documented numbers. As to why the other aircraft don't have the same issue. Well many of them didn't in real life suffer quite the same problem as the P38. The design of the P38 resulted in a fairly low (by comparison with the other types) Critical Mach number (Mcrit). Further complicating this was the design of its tailplane, a large surface immediately behind and in the combined downwash of the inner wing cockpit cuploa area. So the P38 had an inherently lower Mcrit than the others types. There is for example documented cases of late model Spitfires achieving Mach 0.92.... a speed no P38 would ever approach. You also state: "The flaps we have in the game are nothing more than a Speed Brake which slows the plane down and causes some sort of lifting action. " I disagree, again the documentation on the P38 describes the effects of the Dive flaps resulted in up to a 4G pitch up raising the nose and assisting the pilots recovery. Sure the increase in Drag will assist in deceleration but the prime function of the P38 Dive brakes was to get the nose pitching up. It does exactly that in IL2 as well. (BTW don't forget that dive flaps of almost exactly the same design were fitted to late model P47D's and other types). To test in Il2 get yourself to Vmax at sea level and activate the Dive brakes what happens ? just a decel or decel + pitch up ? I do agree with you that some types P47,D9 and Tempest do end up at huge Mach numbers (1.15 in my tests) which are unrealistic. Rest assured this is being looked at. The Il2 FM was never really designed to model compressibility to the nth degree. The DT team is aware of this and is discussing this and other things. |
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#7
09-25-2009, 11:57 AM
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Hope all you want. Just don't hold your breath.
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#8
09-25-2009, 12:01 PM
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No guys, it will not be released today. The official information on the release date will be done either by Oleg or 1C.
To avoid any speculations, everything is ok with the patch. It is finished, it will be published. It is in the hands of Maddox Games and they are preparing to distribute it to everyone. There is no need to put pressur on 1C/MG or our team. Last edited by mkubani; 09-25-2009 at 12:49 PM. |
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#9
09-25-2009, 04:59 PM
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Since you're at it what about some of the other issues I mentioned? Accuracy of 50 cals on wing mounted 6 gun platforms? I think their hitting power is fine by the way. Horse Power on the F6F which seems to be anemic at best. I need to do some real testing here but I think this plane has been ignored for sometime now. Antons losing all 3 control axis from a single shot even with pushrod accuated control surfaces. Antons non self-sealing fuel leaks or what I call a fuel leak bug which empties the plane in a matter of minutes. |
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#10
09-25-2009, 06:04 PM
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FC
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