chris330 0 Posted February 7, 2006 Dear All, This is no way related to anything I have, or am going to build or do with OFP. Nor is it related to anything I am doing in my work or private life right now. It just been bugging me for a while. I was watching a documentary which detailed the diabolical problems faced by developers and test pilots who were the first to build and test aircraft which could go faster than Mach 1. They said that as soon as you went over Mach 1 the aircraft suffered massive structural problems. They did not explain why this was however. Here's a guess: I assume that the air the plane is flying into could be considered stationary, compared to the velocity it is being impacted by from the plane. My main question is, due to the fact the the speed of sound is basically the maximum speed vibrations can travel through air as sound, does this also mean that this is the maximum rate a pressure front can move through the air? If this is so (which it may well not be - it's only a guess) I would imagine that this would cause HUGE pressure differences across the front to the back of aircraft (principally the wing area) due to enormously sized vacuums being built up behind the aircraft because the atmospheric pressure cannot drive the air behind the aircraft quickly enough to fill the void left created by the aircraft's body and wings as it is travelling above Mach 1. Is this what's responsible for the structural issues on the aircraft's frame? I'm sure there are people on this board easily smart enough to tell me if I'm right (and more likely where I'm totally wrong). I would appreciate it if you would share your knowledge with me. I'd just love to know  Share this post Link to post Share on other sites
BlackScorpion 0 Posted February 7, 2006 Wiki link about Mach number. If I read it correctly, when the fluid (or in this case, air) crosses the shock wave (which is caused by supersonic velocity, the object compresses air in front of it etc.), it comes hotter and denser and thus creates much more stress on the structures... eventually rips the object apart as/if they fail. Share this post Link to post Share on other sites
Tigershark_BAS 0 Posted February 7, 2006 This was very cool. I learnt something today. Thanks for the question and for the answer! Share this post Link to post Share on other sites
shinRaiden 0 Posted February 7, 2006 I'm by no means an expert in the area, most of these ramblings are anecdotal etc. First off is the B-47. Most of the B-47 variants were at risk in a high-speed high-altitude box. This is because the wing was designed more for a single flight profile rather than mixed profile. The result was that the airspeed margin between super-sonic concussion and stall speeds became extremely narrow. When you move into the hyper-compressed pressure cone, the air pressure on the flight surfaces is radically different than in low-pressure lower-speed flight. Canard forward stabilizers become more common, as they're aerodynamically more effective than in the rear vortex or cavitation (?) shadow at those atmospheric profiles. Simply put, lower speed wings are generally designed to optimally suck up as much lifting atmosphere as possible, while supersonic and hypersonic wings are generally geared more towards deflecting the pressure wave and channeling air to the engines. Transsonic wings are far more tricky to develop, the frustrations of development and disasters of the first several decades led to the strong emphasis in the previous generation on swing-wings (F-14, F111, B-1A and B, first Boeing SST prototype, Tu-160, etc). However the complexity and costs associated with that methodology have generally been impractical for commercial usage, and even marginally justifiable in military administration. What has happened is that there has in the military been a more honest evaluation of the true operational envelope of general military aviation being primarily sub-sonic for manueverability reasons. The pie-in-the-sky folks are responding with various unmanned concepts that could theoretically be more transsonic than current manned systems, however honest evaluations of the additional remote piloting and communications infrastructure would put that back towards the dubious practicality category. Commercial civilian usage is even more tenuous. There is a narrow barely sub-sonic window of increased atmospheric efficency that small private executive jets can exploit, but it's been an on-going challenge to take advantage of that in larger commercial transports. Additionally, there are significantly increased costs due to low-volume, exotic materials, compensations for significant fuselage deformation inflight, absurdly extravagent fuel consumption, and environmental mitigation for the NIMBY anti-human activists. Share this post Link to post Share on other sites
harley 3 1185 0 Posted February 7, 2006 Quick answer from me, as I certainly can't say I'm an expert on avionics. This I know; the problem with aircraft is the damage inflicted when they hit the sound barrier; after that things are relatively hunky-dory. It's all the pressure building up just under the speed of sound, and then even hitting it, which destroyed a lot of test planes back in the forties. I'd look at the Wiki, but do I know it's true (raises eye-brow)? As to the B-47 - that was an "insane" aircraft - tandem cockpit, only six engines and an easily stretched wing - it took brave men to pilot those for extended periods of time. But it did the job rather well until the B-52 came along. Share this post Link to post Share on other sites
chris330 0 Posted February 8, 2006 Thanks very much for the replies. I think that pretty much covers it! With the exception of ShinRaiden's posts however. They were just too technical for me to understand Share this post Link to post Share on other sites
soul_assassin 1750 Posted February 8, 2006 There are some very important things I learned when studying aerodynamics: - first off u can devide airflow speed in these states: - low-speed subsonic (till around 300 km/h) - high-speed subsonic (300< x <M1) - transsonic (+- M1) - supersonic (M1< x < M5) - and hypersonic (x > M5) Just a small side note for those interested the difference between the low and high speed subsonic is that for below 300km/h air can be considered incompressible which simplifies calculations alot. Also take note that M1 is diffrent depending on what height you are flying at. As the air gets less dense with higher altitude the speed of sound goes down. Now to your question. Air behaves very differently below and above the M1 transition point. When u hit the S.O.S. the aircraft creates a cone (shockwave, and if u look at it in 2D the it makes a triangle with the tip of the aircraft and the outer radii of the last soundwave created, as you know sound waves travel outward). As you move faster the sound wave gets less time to spread making the cone narrower and narrower. What is important to know is that air behaves ALOT differently within the cone then it does outside. Aerodynamics is based on the basic variables: Pressure, Temperature and Density of air. Of of those values within the cone are different to those outside. The difference is so huge it can cause alot of structural stress. For example if your wings are not swept enough backwards when the cone becomes narrower than the angle of the wings and if they start crossing the boundary of the shockwave they will experience tremendous stress which they are not designed for. Thats also why Delta wing is very popular for high speed aircraft (eg, Concorde). Another thing is control surfaces with small area (elevators). I mentioned that the pressure is significantly different and control surfaces depend on pressure differences to produce lift in order to break equilibrium. Elevators that you see on subsonic craft will not function in supersonic flight! So the aircraft becomes uncontrollable. The solution they found was that instead of the horizontal stabilizer just having elevators they had to have the whole stabilizer pivoting to produse any difflection. This is why all the early pioneers had so many problems.They just had no way of knowing what will happen when they hit the shockwave, what kind of fundamental differences would occur in the airflow. Hope thats understandable, tried to explain in Layman's terms. Share this post Link to post Share on other sites
chris330 0 Posted February 8, 2006 Very well explained! Thanks a bundle One question though can you tell me why the triangle is formed between the tip of the wing and the outer radius of the last sound wave produced? Also why it is such a structural issue when the cone becomes so narrow that it goes 'inside' the horizontal cross-sectional area of the wing? Share this post Link to post Share on other sites
Mr ThunderMakeR 0 Posted February 8, 2006 Supersonic aircraft also have to overcome a third form of drag known as 'wave drag' that is related to the Work an aircraft does on the air when a shock wave is created. To overcome this drag, more power is needed and so supersonic aircraft require more powerful engines. One way wave drag is reduced is by reducing the angle of the shockwave, attempting to make it as near horizontal as possible. This is why high-speed aircraft have very sharp leading edges and nose sections, as this reduces the angle of the shock wave. Also, wave drag is a function of the density of air, which is why supersonic aircraft fly at high altitudes where the air is thinner. Share this post Link to post Share on other sites
soul_assassin 1750 Posted February 8, 2006 Very well explained! Thanks a bundle One question though can you tell me why the triangle is formed between the tip of the wing and the outer radius of the last sound wave produced? Also why it is such a structural issue when the cone becomes so narrow that it goes 'inside' the horizontal cross-sectional area of the wing? Ok i'll try to answer your questions : 1. U misunderstood me when i ment tip of the plane i meant the nose cone. Maybe this picture will help you understand the theory behind the Mach wave: <span style='color:red'>Explanation</span>: Suppose a beeper (a sound emmiting device) is at point R at some time t=0. its traveling at some speed V which is > than speed of sound (a). Now suppose it creates a sound wave which travels in all directions. Lets say some time dt after that the beacon has traveled the distance V*dt while the sound traveled a distance of a*dt but since we know that a<V thus a*dt also < V*dt. This means that the beeper is outside the soundwave it just created. Ok this is clear so far. now imagine that one instant (unit) after the 1st wave a second is created at time dt and later we see that its wave radius is always a*(t-1). Now try to picture these infinately many wave circles between the points R and S on the image i showed, each new one smaller than the previous. U can draw an analogy with a skipping stone on water. If you observe then every new wave made on a hit will always stay smaller compare to the previous one at any one moment. Oh and Mr ThunderMakeR is absolutely right induced wave drag is also a very important issue. Share this post Link to post Share on other sites
Sigma-6 29 Posted February 8, 2006 Quote[/b] ]I'd look at the Wiki, but do I know it's true (raises eye-brow)? The BBC referred to Wikipedia as the best source of information in history. Wikipedia articles, whatever else their flaws, are ceaselessly and ruthlessly peer-reviewed. If you're worried about the article's veracity, usually the talk page attached to it will answer your concerns. Personally, I'd be more worried about the relative accuracy of traditional encyclopedias. Share this post Link to post Share on other sites
Guest RKSL-Rock Posted February 9, 2006 I really don’t understand why you are all making this sound so complicated :P As Soul Assassin responded, the problem is down to the amount you can compress a fluid (air in this case) at speed. The problem comes to a head when you reach transonic speeds. The entire problem can be illustrated with 3 very simple diagrams: Each of the “red cones†represents the compression wave formed when the aircraft moves through the air. You can see from the first planform that the wings extend to the Low Sub Sonic and just don’t quite fit into the High Subsonic cone. This type would have good low speed handling but poor high subsonic performance and no transonic ability. Take off would be very short and the take off and stall speeds would be quite low. This type would have moderate low speed handling but better high subsonic performance and limited transonic ability depending on the wing design (more on that later). It would require a longer runway and have a much higher take off speed requirements Finally the Delta shape, as you can see it fits inside the transonic cone completely. This is a transonic design but it comes at the cost of low speed performance. Again take off will be higher that the other planforms and low speed performance will be poor. To explain, the plane can only fly properly at higher speeds if it fits within the pressure cone of the air that it creates. The faster you fly the tighter the cone gets and if your aircraft doesn’t fit inside the pressure wave you will start to oscillate and shake the airframe apart. Conversely if your aircraft is designed for only high speeds you will sacrifice some capability and be unstable at low speeds. This is where wing design is critical. The right design will allow you to channel the airflow using air dams and vortex generators, smoothing the airflow and reducing drag. The problem is that faster you want to go the more you must sweep the wings. The more you sweep the wings the faster you need to go to generate lift. It’s a vicious circle. Arguably the best option would be a “Swing Wing†design , as ShinRaiden mentioned, quite a few aircraft use the design. By changing their wing profile they can safely transition to and from high and low speeds and retain the best performance in all speed regimes. Its all a question of designing out the risks. Now going back to your original question… Quote[/b] ]Why is supersonic flight so dangerous? It’s not so much “dangerous†anymore (it was in the early days when we were still learning) but it’s still expensive and difficult to do cost effectively. Most military aircraft are similar to the “High Subsonic†model – a compromise between the need for speed, payload performance and cost. The Tornado tried to do this with a swing wing, but resulted in a superb but limited transonic aircraft due to the weight of the swing wing mechanism. As a result it’s weapons load is smaller than it contemporaries but has fantastic low level high and low speed performance. Compare that to the F-15 a good all-rounder, excellent performance and payload but its low speed handling is said to be “twitchy†due to the short (relative to length) wing span, but the design makes up for it by using a clever wing design and lower fuselage shape to help stabilise it at moderate speeds and through transonic and into super sonic speeds. Again as ShinRaiden mentioned mixed flight profiles are where it get dangerous. The changes in air pressure mean that the controls behave differently and require constant updates, more than the average human can deal with. This was why the B-47 was a failure at the time and why no one else picked up the design for 40 years until the B-2 came along. Most supersonic aircraft use Fly-by-wire controls to get around the handling problems in transonic flight. So in summary you have three major hurdles: 1) Design – finding a stable practical design that will fly in all speed regimes 2) Control – Once you designed it can you control it 3) Cost effectiveness – Now you’ve designed it and worked out how to control it, will it be a cost effective use of your resources. Most of the major manufacturers have the capability to design and build supersonic aircraft both military and civil. It just that it cost far too much and isn’t practical. Hope that makes some simple sense. Share this post Link to post Share on other sites