Coffin Corner, Explained: Boldmethod Live

[Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] [Music] hi I'm Alex Idris and thanks for joining us tonight for bold method Pro live tonight we're gonna be talking about some transonic effects specifically Mach tuck in coffin corner and this is kind of a follow up to the last bold method Pro Live where we started to talk about critical Mach number but don't worry if you missed that or if you're not familiar with critical Mach number we're gonna intro by kind of getting into some of that tonight we've got Corey chimeric on chat so if there's anything that I confuse you with or lose you at or if you want me to go into more detail throw that out into the chat Corey will grab that and send it over to Colin Cutler who's our technical director tonight he'll work it into the presentation don't wait for the end keep the questions coming all the way throughout it keeps this much more interesting and so with that we're actually going to jump in pretty quickly here and we're gonna start as I said by understanding the concept of the critical Mach number and Mach itself so with that we'll start with the speed of sound and we'll look at what happens there and then from there we're gonna talk about shockwave formation and how that can affect the aircraft's control and it's pitching and possibly rolling moments and then we're going to talk about how to recover from some of those effects and then we're gonna finish by looking at coffin corner so we'll start by jumping into the iPad and talking about the speed of sound the speed of sound is essentially there we go okay this there we go the speed of sound is affected by one thing and that's the temperature of the air okay so here we've got some incredibly warm air 50 degrees Celsius and you can see how fast those molecules are moving around and how much they move but as that air cools down notice that they're moving much slower so watch that again so here when the air is hot the molecules move far and they move quickly but as the air starts to cool down those molecules move only a little bit and they move very very slowly and so okay what does that have to do with Mach and the speed of sound when we think about the speed of sound all we're really thinking about is how fast air can move pressure forward okay if I was to clap my hands and you hear that clap well all it's happening is sound waves are rippling out from my hands and that's happening because the molecules of air are just bumping into each other and pushing those waves further out the faster the air molecules move the farther they go in less time the faster those pressure waves can travel that is the speed of sound and so the one thing it's not density it's it's not altitude that affects the speed of sound it's just simply the temperature of the air the warmer the air is the faster the speed of sound because air those air molecules are moving faster and they can transmit pressure waves faster okay so with that you're going but wait a second I know that the speed of sound changes with altitude and it does because air temperature changes with altitude so if we go back to the iPad this is today's temperature sounding in Denver and what you can see if you look at look at the ground where we start okay we're about I think we're about 25 degrees Celsius when we took this you can see that air temperature drops off quickly up to 9000 and then a little slower and then it continues dropping all the way up to 39,000 and had this sounding gone up into the stratosphere you would have noticed that this would have probably started to level out and maybe even increase okay so that is also showing you the change in the speed of sound in this case the speed of sound would be the fastest right at the surface in Denver and as you go up the speed of sound will start to slow down okay it's gonna slow down pretty quickly here up to 9,000 feet and then it continues to slow down as we go up to 12,000 but not as fast and then again since the temperature drops quickly up to 18 it'll slow down quite a bit and then it just follows that temperature curve and if we got to the stratosphere you might notice that the speed of sound actually starts to even out it becomes constant it doesn't change much and then as we go up a little higher you'll notice that as the temperature slowly increases and we're talking about very very cold temperatures maybe like you know in the negative 60s or negative 50 Celsius so this is extremely cold but you'll notice that the speed of sound will start to increase along with that temperature as well so when we're talking about the speed of sound it's easy to overcomplicate it to talk about you know well what altitude and what speed but but just start by thinking about it is simply the warmer the air is the more the out molecules bounce around the faster they can transmit those pressure waves and so the speed of sound is always going to follow the temperature profile of the air ok so let's talk about how that affects our airplane we'll go back to the iPad and if you tuned in on our last Pro you've probably seen this but it's a good thing to review this right here is a basic airflow and in this case we're flying at about Mach point 5 so half of the speed of sound what that basically means is that the airfoil is moving slower than what air could transmit pressure waves so for example if I was to clap the clap would move out ahead faster than the wing would move and so essentially you know you can think about this as a car you know if you or even an airplane you're sitting in an airport you look at an airplane on final you can hear the airplane because the airplane is moving slower than the speed of sound the pressure waves from the airplanes engine the air flowing over the wing all of that that noise is being transmitted out at the speed of sound and as the listener okay I'm hearing those before the airplane actually passes me so that's the that's the interesting thing about the speed ascend you can really see it or think about it just when you watch something a car airplane you can hear them because the noise is moving at the speed of sound but they're moving much slower okay let's go back to the iPad and see what happens as the airplane starts to speed up and approach the speed of sound you'll notice that those pressure waves start to pack in together and right at the speed of sound boom they basically form what you could call a bow wave and we actually call this a shock wave and there's several different kinds of shock waves but what's happened here at Mach 1.0 is that the airplane or in this case the airfoil is moving exactly as fast as air pressure can move so all those pressure waves pile up right on the very front of the airfoil or the airplanes nose and if you're you to look at an actual aircraft you'd see these pressure waves at Mach 1 all over the airfoil the nose on the tail on the wings and the other interesting thing is if you were to watch an airplane approach at Mach 1 or faster you wouldn't hear anything and the reason you wouldn't hear anything is because the sound that the aircraft's emitting is moving forward at the same speed that the airplane is so the first time that you would hear the airplane is as it passes right over you that's because the airplane is now traveling Mach 1 or faster okay looks like we've got a question okay Damon's got a good question based on that chart with the temperature soundings he says this so what if the temperature is 60 minus 60 degrees outside and I clap my hands is the speed of the sound that I'm making with my hands gonna be a lot slower you were exactly right it will and when we talk about a lot slower it's still very very fast but it will be a lot slower a sound will transmit more slowly on a cold frigid winter day than it will on a hot day and if you're able to be able to time it and you could actually think about this we do notice the speed of sound when you start to think about something like lightning and thunder because you see the lightning then you hear the thunder right well that's the speed of sound at work right the the light from the Lightning is traveling at the speed of light and the sound from the Thunder the pressure really the Thunder is just the pressure expanding from the heat of the lightning well that pressure expansion is happening at the speed of sound much much slower so if you're to have a thunderstorm in the summer and it was five let's say it's five miles away you see the Lightning and then you hear the Thunder okay now imagine you got thunder snow it's in the winter and it's still five miles away you're gonna see the lightning but it will take a lot longer for you to hear the Thunder because the speed of sound is so much slower and that's that's kind of a way to think about it and when we think about a shockwave on an airplane that really is just air flow moving at the speed of sound so if you go back and you look at the iPad here essentially at Mach one what you end up with is this big shockwave because all the pressure that's being pushed forward by this airfoil can't travel any faster than the airfoil itself so it just piles up and angles off okay that doesn't happen immediately and if you think about an airplane you know the airfoil or the the airflow accelerates as it starts to move over a nail airfoil right here we could be subsonic but then as we go up and we go over the airfoil that air is accelerated that's what's generating lift so it becomes faster and at some point if we're close enough to Mach 1 that airflow could become supersonic now in this case we've drawn it on an airfoil it could be the wing it could be detail but it doesn't necessarily need to be an airfoil it could be the fuselage itself but at some point in time as you approach Mach 1 in an airplane somewhere along that airframe the airflow will start to accelerate to supersonic speeds that's what we call the critical Mach number the critical Mach number is the speed at which at some place on your airplane you start to get some supersonic flow you're starting to generate supersonic air movement and again it could just be just a tiny area on the wing or on the top of the fuselage or in the wing route but that at that speed will happen slower than Mach 1 so on old airplanes that could happen maybe Mach 0.85 with new designs we've been able to get a lot of airplanes to the point where they can get very very close to Mach 1 before they start to accelerate air into supersonic speeds but it will always happen at a speed earlier than Mach 1 so when you talk about a critical Mach number that's always going to be a number slower than Mach not one okay so let's go back to this picture and take a look at this because this is what we call a normal shockwave and there are different kinds of shockwaves we'll do a video on that one day but the interesting thing about a normal shockwave is if you look at the air flow as the air flow passes through the shockwave it does not change direction but just about everything else with that air flow does start to change and let's talk about that air is able to accelerate to supersonic speeds very easily it's completely smooth there's no turbulence it accelerates just fine and the problem becomes where it starts to slow down to subsonic speeds as soon as the air flow starts to slow down below Mach 1 it generates this big pressure wave and that's because all the you know all the pressure is essentially backing up it can no longer move as fast as the airfoil is moving that pressure wave causes a lot of problems it loses a ton of energy okay so basically we're going to go in this supersonic area it's moving extremely fast so we're gonna go from very low pressure to back in fact I'll do that in blue so it makes more sense that's not blue okay we're going to do that in very low pressure over here and then as soon as we cross that shockwave the air gets too high pressure and that change happens almost instantaneously and because of that it bleeds all of the energy out of the air maybe not all put a lot of it okay if you think about a stall what happens in the stall will the pressure gradient along the wing is adverse you're moving from low pressure and as you get towards the trailing edge of the root your your the wing you're going to move towards higher pressure that's what we call an adverse pressure gradient you're going from low to high there's naturally wants to go from high to low okay but if it goes from low to high it's taken energy out of the air and eventually it takes so much energy out of the air that it no longer stays attached to the wing and it starts to separate and that separation can cause a loss of lift and as that separation grows up the back of the wing that could start to cause our total amount of lift to decrease and when the total amount of lift decreases we've entered a stall okay so essentially when we start taking energy away from the air when it's moving across an adverse pressure gradient and it's losing that energy it can separate from the wing and it can cause a stall okay so now we look at a shock wave and the problem here is this is like a thousand times worse than what you would normally get off of just a normal pressure gradient because the change is so aggressive as it crosses the shock wave it's so drastic that it can quickly cause airflow to separate and so boom all of the airflow here now separates off okay so if we think about a normal wing I've got a slide here typically this is a basic airfoil and if we think about what's normally happening on a wing the air comes up it goes over and then it washes down okay and then you've got some air that goes over the bottom as well and if you think about what's going to happen here it speeds up right we're getting faster and faster and faster and we reach this point of maximum low-pressure right here okay so after it crosses maybe this point now it starts to slow down and it gets back to I'm gonna put an N here for normal pressure okay it's essentially getting itself back to normal pressure we start at normal pressure and then we move towards low pressure and then we move back towards normal pressure again okay when you think about that what that means is we get this lifting shape on the wing and it looks something like this I'm gonna clear this and draw it again so you can kind of see it the entire wing is generating lift though you'll notice most of the lift shows up right here and that point is about 25% of of Mac Mac means mean aerodynamic cord the average cord of your airfoil and when we talk about cord we'll go back and take a look at that when we talk about cord cord is the distance from the leading edge to the trailing edge that's what cord is it's basically and you just measure from the tip to the tip and you would measure your entire airfoil and then average those distances out and then you end up with Mac or mean aerodynamic cord and in subsonic flight this point right here what you can call the center of the center of lift or the center of pressure that's going to happen right about 25% of mean aerodynamic cord the problem is when we start to add a shockwave so let's go back to our shockwave slide everything gets messed up okay we used to have this lifting force which looked kind of maybe like this and trailed off well all that's gone now because you're not generating lift generally behind that shockwave so now all the sudden the airplane loses a massive amount of lift okay can only generate lift basically right here and that lift distribution is fairly even so as opposed to the center of pressure being maybe right here it might start to move back to right here but the big problem is we can quickly start to lose lift okay we're gonna get into this in depth in just a second but it looks like we've got another question and I'm gonna carefully dress berry up as I hit the wrong button okay we got a question from Daymond again here and actually there's a follow-up question from James Damon's question is this is a sonic way is the sonic wave related to the sonic boom that you would hear from an airplane and to follow that up I'm just gonna say James is here yeah what he wants to know is is the point the point where you hear the sonic boom the moment where you exceed the speed of sound that is a great question if you're in the airplane you'll never hear the sonic boom that's the first thing you'll just ride along with the airplane they don't they don't they don't hear the boom but the people on the ground will so let's take a look at this again this is a massive airplane but let's just imagine what's happening how that bow wave works and actually there's a trailing wave here and you end up with these kind of reverse and waves that kind of go underneath there and we'll talk about that in detail on another day but if you're this person standing right there okay that's you that's a nice drawing by the way that's why I'm not the graphic artist okay so that's you and you're sitting there on the ground okay you're gonna hear this sonic boom when that wave passes over you okay you essentially boom when those waves pass over you that's when you're gonna hear the sonic boom and that's what you're really tet you're hearing is the the bow wave or that night not even the bow wave but the pressure waves on the airplane that's what's that's what's reaching you and you may actually see the airplane pass overhead before the sonic boom reaches you and that's because those sonic boom waves are trailing out okay so think about this if you're in the airplane you would constantly if you if those right waves were crossing you you would just constantly hear the pressure change and you don't you won't even notice it so the airplane crosses the sound barrier you don't get a boom and it's not people think the sonic boom is a one time thing is I punch through the barrier I get a boom nope the sonic boom is a continuous thing essentially is just a wave right over the ground and everybody on the ground is that wave passes over them they hear the boom and the boom is just constantly moving that's that's what a sonic boom is so you know if you were able to move faster than the airplane and to jump back in front of the wave and back in front of the wave you'd hear the boom each time so the boom is really just a continuous pressure wave it doesn't happen once it happens the entire time that that aircraft's exceeding the speed of sound okay looks like we got another question okay next question has to do with the shock wave over the top of the wing and that is how does the pressure pass the shock wave change instantly or how does it change so quickly or why does it change clean essentially that's a good question and I'm going to try to explain it the best I can without doing more research but but essentially what's happening is those pressure waves are backing up that that once you get subsonic the airflow is moving the air flow pressure waves can no longer move forward let me let me do this let me do a little more research on that because there's there's also there's also oblique shock waves expansion waves and I'm missing anything normal oblique expansion I think that's it I think I got all of them let me do this I'm gonna answer that in more detail on a on a video on pressure waves because I want to do a little more research before I jump in too much as to why that's piling up that way otherwise I might give you something that's overly simplified not really right okay let's go back and talk a little bit about what's happening with this shock wave again we've talked about the fact that once you generate the shock wave even though the airplane is not going supersonic as soon as that shock wave shows up we can start to run into problems because we can generate separation we also talked about the fact that as soon as the shock wave shows up lift that at one point in time might have be might be centered here can start to move backwards possibly on the wing the interesting thing I'm going to go back to the slides the interesting thing is if you look at a subsonic wing you're that's not 25% that's more like 25% that would be your subsonic lift but our supersonic lift is essentially at 50% so subsonic lift is 25 twenty-five percent Mac and supersonic lift moves back to 50 percent Mac okay if we think about this this causes a big trim change right and that's because the center of gravity and the center of pressure or the center of lift work together to create some torque a pitching moment down and what counters that well it's our tail our tail down forces what counters that and so if that center of pressure starts to move suddenly now all the sudden the aircraft's going to start to pitch we're going to need completely different control pressures we're getting any completely different trim and if that change happens rapidly or nearly instantaneously it can be very very difficult to control okay so let's talk about that as we start to see the shock wave form on the airplane we're not always exactly sure where it will form first first of all it depends on the aircraft's design it also depends on our maneuvering or whether we have any control surfaces deflected all of these things can change where supersonic flow starts to happen first but when it does happen it can very quickly change how the aircraft starts to fly and a lot of people have heard of the term Mach tuck Mach tuck is where an airplane flying level starts to enter what we call the transonic range it may not be too Mach one yet where it might be just above so the transonic range is where you start to see supersonic flow in the airplane maybe 0.8 or 85 it depends on each airplane and the flight conditions and then kind of ends wants full supersonic flow has been established so just slightly above Mach one so as we get into that transonic range aircraft can experience what's called Mach tuck where they suddenly pitch over and if you think about that that causes a real problem because if we didn't want to go faster than Mach one if we ended up too fast and got into this transonic regime by mistake and exceeded the critical Mach number by mistake we don't want to go any faster and Mach tuck on the other hand now that pitches is down next thing you know you're going a lot faster so we'll talk about why that happens now and we'll look at some some different conditions on the airplane and the key thing with this is each of these conditions could happen some airplanes will experience all or some and and you could experience different conditions depending on whether you're maneuvering whether you're banking so really you don't always know what to expect as the aircraft enters the transonic range if it was never meant to do so if you're flying an airplane that was meant to fly through the transonic range completely different design but if you're flying something like an Embraer 145 or a phenom or a 737 these airplanes were not meant to fly through that range you're never exactly sure what's going to happen okay so the first thing we're gonna take a look at is what happens if we end up at a shock with a shock wave at the root of the aircraft first okay so essentially by the route of the aircraft I mean you might start to form a little shock wave maybe right here okay that would be at the root okay and let's just say this is happening symmetrically it's happening on both wings at the same time so we've got this little area of supersonic flow and by little I mean right here in front of it this is sped up to maybe supersonic okay everything else on the airplane is subsonic but now we have separation happening coming off right here well first of all this is going to change the aircraft's center of lift and if you think about it the route generates quite it's got a lot of surface area it generates a lot of lift okay but now we've cut away a significant part of that so we've taken away a lot of the lift contribution from the aircraft's wing route and if you look at the swept back wing look where it is it's all behind the aircraft's wing route right so in this scenario okay even discounting the fact that shockwaves can move the center of pressure a little aft in this scenario you would find that the shock where that the center of pressure on an airport might move from here back to there you could see that essentially draw that with a smaller arrow the center of pressure is moving back okay so what does that mean well let's take a look at this 7:37 you know bold methods made it where we have our own custom painted 737 okay so let's start with that tail down for or take a look at just the basic concept of CG here right you have the center of gravity forward of the center of pressure also known as the center of left that wants to pitch the nose down we're balancing that with a tail downforce we have an upside-down wing essentially on our horizontal stabilizer that's keeping the nose up if this lift suddenly starts to jump back we have a different arm between the center of gravity and the center of pressure let's look at those so that's if we look at the original arm fairly short the new arm longer okay so basic leverage right with basic leverage is we increase the length of the arm you end up with more torque right and the problem with changing the center of pressure and its interaction with the center of gravity is that there is a lot of force at the center of gravity okay so very little changes in arm can cause some pretty big problems that's why when you look at especially a light aircraft you've fairly Marinero CG envelopes maybe a foot and a half two feet you know transport category aircraft there's CG envelope is still very narrow okay they that's why they ever see everyone's father to move passengers around to keep aircraft in the CG envelope so if your center of pressure was to jump back several feet you could end up with a massive change in nose down pitching moment it would aggressively start to pitch down okay and the problem that you start to run into is not necessarily that your tail can't generate enough tail downforce to keep the nose up its that by trying to use your elevator to solve the problem you could quickly create supersonic flow over the elevator and completely remove the elevators effectiveness and you essentially can't stop that nose from pitching down okay so let's take a look at the elevator and kind of see what's going on here remember I said the elevator is an upside down wing right it's a wing that's meant to generate lift going down that's what it does okay so you've got the relative wind will draw the relative wind right here kind of coming through okay that's our relative wind and if we want to generate a little bit of lift more lift going down what would we do well we would deflect that elevator I'm gonna draw that a little further back we dislike that up and the reason we do that is because that essentially creates you can kind of see at the very end of the video a positive angle of attack right and that positive angle of attack would increase the magnitude of our left okay so by by pulling back on my yoke and my 737 okay what am i doing I'm increasing the tail downforce I'm increasing the tail down lift on my elevator okay so as I increase that lift I'm increasing camber I'm getting a bigger pressure difference between low and high you know when we think about a wing lows on top highs in the bottom but when we think about a stabilizer a horizontal stabilizer the low is on the bottom and the high is on the top and as I continue to pull back I'm accelerating the air more and more and eventually I could form a shock wave on that horizontal stabilizer okay so let's go back and you can see what happens as soon as that shock wave forms not only may I lose surface area to generate lift so I don't have the ability to generate nearly as much lift or tail downforce off the elevator but the other problem is look what happens with separation the air flow separates behind that shockwave and or our elevator is when I'm possibly moving my yoke the only part on the tail that on light aircraft in and some other aircraft that would move is the elevator itself and now the elevator is behind separated air so one of the problems you run into with a conventional horizontal stabilizer with an elevator on the back is if you start to experience Mach tuck what's gonna happen is as you bring that yoke back to try to solve the problem you'll end up with supersonic flow on the bottom of your horizontal stabilizer not only does that decrease the amount of tail downforce you can generate because you have a smaller lifting area but it's blanketing your elevator the only thing that you can move and it's preventing you now from changing the aircraft's angle of attack so essentially your tail just became fairly useless and this is why when you look at aircraft that are meant to operate close to transonic speeds or supersonic Lee you'll notice that they use stable aiders or all flyable horizontal stabilizers as opposed to an elevator section in the back the entire thing pivots in the airflow that way you don't lose your control effectiveness as you enter transonic and supersonic speed ranges now you still may see some tabs back there and people isn't there a really small elevator back there those are usually trim tabs or control tabs but but on most transonic or supersonic aircraft that elevator would no longer be effective and so instead we use all flyable stable aiders and you might see them on like a piper cherokee that's not because the airplanes meant to go supersonic it's just because that's the way the designers decided to do it for air damage or aerodynamic reasons okay so what happens you end up with this wing root shockwave first which moves your center of pressure aft now you got a bigger arm between the center of pressure and the center of gravity the nose starts to tuck down and you can't pull back on the yoke because if you do and you try you end up stalling out the tail and now the nose just keeps going down your airspeed keeps going faster how do you get the airplane out of this obviously bring the thrust levers back that might not be enough the airplanes going really really fast so what can you do to slow the jet down before things start to come off the side of it well we can deploy our spoilers we could possibly deploy our flaps and we could deploy our landing gear people go away to second if you're above critical Mach putting your landing gear down you're gonna have some serious problems you will lose the landing gear doors most likely but the landing gear themselves will probably remain welded down they're incredibly strong in fact you probably will be able to land on them no problem so inadvertent mock talk essentially you're gonna start putting drag devices out there into the air flow to try to generate enough drag to get the airplane to slow down in an airplane that was not meant to fly in the transonic regime oftentimes if you experience muck the only way to get the nose back up is not to try to use your control surfaces but to try to slow the airplane down using your drag devices once you slow it down beyond below the point where you have supersonic flow you can regain control surface surface effectiveness and start to fly the airplane out okay there's a less known version of this and you could really think of this as mock pitch up as opposed to tucking down you could end up in a case where the aircraft starts to pitch up okay so let's take a look at that I'm going to pull up that slide what would happen if we ended up with the shock wave at the tip of the wing first maybe an aileron a little later on deployed there and accelerated the airflow something happened and all of a sudden we have this little shock wave the forms out here on the tips okay well what's going to happen is the center of pressure is going to start to move again and what might have been the center of pressure here will start to soar and just changing the ink will start to move up here it's going to move essentially forward so what does that mean when we look at the airplane well our old arm between the center of gravity and the center of pressure is going to get smaller and so now essentially you end up with too much tail down force and the aircraft's nose is going to start to pitch up on you this is less known it's less talked about but if your wingtips were just all first this is something that you can end up with okay there's another reason that the airplane could start to tuck down and if you take a look at this loss of downwash on tail let's let's think about what normally happens is air flows over the airplane okay normally you have your relative wind that comes up crosses the wing and then it flows down smoothly and that smooth airflow flows over your horizontal stabilizer allowing it to generate lift as a tail downforce but in this scenario if we have a shock wave that's formed on the wing and let's say there's a shock wave that's formed right here now we've created all of this turbulent airflow coming back and that can blanket your horizontal stabilizer so now as opposed to having the ability to generate a lot of tail downforce it may not be able to generate tail downforce at all and even if the center of pressure has not changed location you've lost essentially that right there and so the aircraft starts to pitch nose down so even if the center of pressure doesn't move around in the wing because of a wing root stall first if the if the separation behind the shock wave starts to blanket the horizontal stabilizer now you lose the ability to generate tail downforce and the airplane wants to tuck under as well okay next question okay next up Bama pilot wants to know do these swept wings help reduce the shock waves okay great question so let's take a look at the swept wing I'm gonna let's see we're gonna zoom in on this one here and pull it over I should work so what we've done on a swept-wing is essentially we've changed the direction of the airfoil so if you were to look at a non swept wing and I'm just going to kind of draw one out quickly if you just looked at your rectangular Piper wing looks something like that okay an old Cherokee and we want to look at the camber lines look at that you see it now okay perfect okay so if we look at the camber lines here they're gonna go like this and if you think about where that relative wind is coming from it's coming just like this okay it's coming essentially parallel to those camber lines okay so now let's take a look at what we've done to the camber lines on our swept wing so you can see the the swept wing there or the core I sorry I'm calling camber lines and I should be calling them cordon lines it's my fault cord lines if we look at these cord lines here they've been turned at an angle okay so the cord lines have been turned at an angle but if we look at the relative wind the relative wind is still coming straight it's still coming basically right down the airplane so essentially your aircraft's cord is no longer aligned with the relative wind so let's let's go through and just take a look at one of those so here's a cord line here here's a relative wind I'm gonna draw that a little smaller something like that now essentially what we do is we break that cord into two components or the wind into two components so let's do that with the wind we end up with one component of wind that is flowing parallel and one that is perpendicular okay so sorry just like that so if you look at these okay so here is your parallel component of relative wind and here is your perpendicular component of relative wind we call this right here chord wise flow so if I draw that out you'll see chord wise chord wise flow and span wise flow and the chord wise flow because we've broken the relative wind into two different components if you think back to a triangle right a squared plus B squared equals C squared well C square is always going to be bigger than a and B so as we start to break the relative wind into components okay the chord wise and the span wise components those components are always smaller than the total relative wind so by sweeping essentially the wing back what we're doing is we're effectively slowing down the wind over the over the wing because the court or the camber of the wing only accelerates the component of air that is flying parallel to the chord line and so essentially because that's now smaller by sweeping the wing back the air it gets accelerated less and less and less so as you add more and more sweep back essentially the wing starts to feel like it is flying slower and slower and slower because the relative wind is essentially less and less direct there's two problems with this one of the problems is that the wing feels like it's flying much slower so when we're flying close to a critical Mach number that's fantastic when we're taking off for landing we run into some bigger problems essentially swept wings have very bad low-speed characteristics so you need to add slats in the front maybe expense or a big Fowler flaps on the back not only are those systems expensive but they're heavy and they're complicated and they require more maintenance but that's what you need to do because a swept wing airplane has poor slow speed performance it really feels like it's going slow so it's going to have a much higher stall speed if that wing was clean the other problem that you run into if you take a look at this again is all of that span-wise flow starts to pile up you would think that it's kind of Occulus but it isn't what it ends up doing is it thickens the boundary layer right here essentially it removes energy from the air at the tips of the wings normally we design aircrafts that they stall at the root of the wing first but when you end up with a lot of span-wise flow it tends to stall from the tip of the wing first and the problem with that is that's typically where your control surfaces are so the aircraft can be very difficult to control as it goes into a stall so that's another downside of swept wings okay let's take a look we talked a little bit about the horizontal stabilizer and let's take a look at some things that we can do to try to preserve elevator effectiveness as we start to get towards that critical Mach number because if you think about it as you started your fast if you need to apply back pressure and you're you have a conventional elevator on the back of the airplane as you start to apply more and more back pressure you're going to speed up the airflow underneath that horizontal stabilizer and you could develop a shock wave that shockwave could cause separation and that separation would then render your elevator useless so let's take a look at this l-39 Albatros which is a subsonic airplane but what you can see here are all of these vortex generators so vortex generators are just angled pieces of metal but what they do is they generate a little vortex that swirls and pumps high-energy air into the boundary layer and so those vortex generators are essentially going to keep the airflow energized and attached to the bottom of the horizontal stabilizer and so if the horizontal stabilizer starts to get close to supersonic flow the VG's will both delay the shockwave they delay the critical Mach number on the horizontal stabilizer and if a small shock wave forms they can help pump air into the boundary layer to keep it attached or minimize the amount of separation so this is unique because typically we see VG's on the top of a wing and again they're there to pump in energy on the top of the wing where the airflow is accelerating and then going through the adverse pressure gradient and you can see those on a wing to prevent shock wave formation or delay separation you can see them to decrease stall speed because again anytime you're pumping air into or pumping energy into the air you're going to delay separation so they'll decrease stall speed but if you see them on the bottom of a horizontal stabilizer and you see an elevator behind it there's a good chance they're there to prevent separation do two critical Mach number or supersonic flow okay there's one other thing to think about if you look at this 737 you can see that it's rolling and that is a symmetrical shock okay so we're rolling here to the left and the reason we're doing that is in this case we've generated a shock wave I'm gonna draw a little smaller line on this wing but there is no shock wave matching on the opposite wing and so that's going to cause a loss of lift here on the left wing and the airplane is going to want to roll towards the left so why could this happen well one of the cases could be maybe this you know most aircraft are configured now with spoilers and spoilers essentially create drag to drop a wing or get rid of lift is the right way to look at it to drop a wing as opposed to accelerating air to raise a wing a spoiler is just gonna kill lift allowing the wing to drop as opposed to traditional ailerons which generate lift on the rising wing speeding up the air and causing it to raise but if you were using a L'Orange at a speed close to your critical Mach number and you tried to lift a wing with an aileron that would accelerate the flow over the aileron and could cause that flow to go supersonic and the wing would drop so we call this control reversal and on conventionally aileron rigged airplanes that's a problem as they enter the transonic range you try to roll in the airplane ends up going the opposite direction again we don't want to accelerate airflow over the wing when we're getting close to our critical Mach number so one of the ways we essentially protect an aircraft from this as we use spoilers on aircraft that operate close to their critical Mach numbers spoilers as I said they basically kill lifts they pop up they can add drag they get rid of lifts and they cause the wing to drop so they're not accelerating airflow the same way they could accelerate it a little bit but essentially they're forcing the wing down as opposed to trying to raise away now if you look at an airliner oftentimes you'll see both ailerons and spoilers but you'll notice when you're flying in cruise flight those ailerons aren't moving they're locked into position it's just the spoilers that are controlling the aircraft's roll okay let's take this in a coffin corner what I've drawn here on the iPad is an example of two important lines our stall speed right here and then are essentially our maximum Mach number or you could look at this as the point where we start to experience supersonic flow okay Mach Buffett remember the speed of sound gets slower as we get climb higher okay so this line makes sense okay and it at sea level it's fast and it slows down as we get higher up stall speed is going to increase as we get higher up okay so essentially the air is less dense we need to fly the aircraft faster to prevent it from stalling at some point the stall speed and the over speed we'll end up meeting and this point right here we call coffin corner okay so essentially what we call a coffin corner because if the aircraft would start if the aircrafts flying at a speed right here okay and it starts to get too slow okay you have very little room to decelerate before you end up hitting your slow speed stall but you also have very little room to speed up before you were to over speed or experience Mach buffett so essentially you may be flying in a window where you only have a few knots either side of your current cruise speed and if you slow down more than a couple knots the aircraft stalls if you speed up more than a couple months you end up with supersonic flow and the aircraft expiry it's a mock muffin in the past older aircraft essentially were capable of flying faster than their critical Mach number and so this green line here you could look at is the aircraft's maximum speed okay so at sea level the airplane doesn't generate enough thrust to get faster than the speed of sound okay but at some point in time it does have enough thrust on it to get faster maybe not in the speed of sound but faster than it's over speed number it's critical Mach number so at some point a cruise altitude there's still enough thrust left on older aircraft to get past their their maximum Mach number possibly in level flight those airplanes are capable of flying in Coffman corner they've got enough power at high altitudes where they could exceed getting that necessarily the speed of sound but they could get critical Mach they could get supersonic flow somewhere on the airplane and that typically would only happen in high altitude cruise but now if you look at a modern airplane they're pretty much what we call thrust limited okay so as opposed to this green line we end up with the blue line so first of all you notice that the aircraft's power is essentially flat here until we reach a certain altitude that's because most aircraft's engines now are D rated or flat rated and we say D rated or flat rated essentially the Jets capable of producing a lot more power but the fade X system limits the amount of power that the jet can produce and there can be a couple reasons it could be aircraft and airframe limits it could be controllability limits it could be pressure limits inside the engine there could be a lot of things but typically every new modern jet engine that you see is in some way D rated it's going to limit the amount of thrust that it can produce at low altitudes but as we start to climb up it can continue to push in a little bit more fuel and a little bit more fuel and a little bit more fuel it can maintain that thrust rating because essentially even though the air is getting less dense it doesn't know it it still got headroom left to go eventually that engine is going to run out of headroom and just like a basic jet engine it's going to start to lose power with altitude too so if you look at a more modern aircraft you'll notice essentially its thrust capabilities flat and then it also starts to fall off but you'll notice here that oftentimes it can only generate enough thrust to get close to over speed but in level flight many airplanes have a difficult time actually getting to over speed or critical Mach and that's because essentially economy is more important right now than speed engines are much more efficient they consume less fuel and because of that they don't necessarily have the ability to push enough thrust to get us in level flight all the way up to our critical Mach number ok looks like we've got a question ok and I'm gonna hop in here I don't have a graphic up but just a good point someone asked about old airplanes versus new airplanes maybe I could you give us maybe an example of it like what is considered an older airplane that might have this critical Mach Buffett problem and maybe a new one that might be thrust limited sure so when we talk about old I'm talking about a dc-8 707 727 these aircraft and their traditional engines could easily get into coffin corner but when you start to talk about new aircraft 175 is in fact the CRJ series the Embraer 145 they don't have enough thrust on board and cruise flight typically to get themselves into a critical Mach scenario in level flight descending flight absolutely but in level flight these airplanes typically can't get there same thing would be true I think but I can't say for sure but I would bet with something like a 787 the modern 737s again all of these airplanes are designed to for efficiency as opposed to essentially when fuel was very cheap and they're looking for speed and so many of these aircraft again you could still experience critical Mach if you were diving if you're descending you've got the ability to generate a lot more speed but in level flight modern aircraft are typically thrust limited one of the problems that we ran up with thrust limited aircraft is they can end up essentially at a cruising speed that may not be very close to Mach Buffett but can be very very close to their stalling speed so if we go back to this chart you could end up in a scenario possibly where the airplane was flying let's say maybe right here okay it wasn't using full power yet it was flying right there and one of the things to think about is the jet engines power output depends on a lot of factors temperature density of the air but one easy thing that you can take a look at is the air becomes less dense the jet engine either needs to spin it faster essentially to try to get the same amount of compression and eventually you run out of the ability to do that so as you get to some you know point is the air becomes less and less dense the jet engine starts to lose power well as the air becomes warmer it gets less dense and I know that at altitude it's really cold like 50 below but to change from negative 60 to negative 50 still attached angel's ears density and it will still reduce the power output of a jet engine so if we go back to this drawing what can happen there is as that temperature starts to warm you'll notice right here the speed we're flying that's our maximum thrust setting and so as the air continues to warm if we maintain altitude at this temperature right here the airplane would have actually started to decelerate and would now be at our stall speed and so this can be very insidious you may not even notice it happening you're flying along in crews okay and the air starts to warm up and again it's not getting really hot it goes from maybe negative 60 to negative 45 but the reality is your jet engines power output starts to decrease and eventually your airplane starts to slow down and then eventually it can hit its stall point and that is essentially what we call kind of a ha tinh hi scenario in a jet different than density altitude hot and high and a prop it's the fact that that air starts to warm up and then all of a sudden you lose engine power and I shouldn't say all of a sudden it's happening gradually you just don't notice it and then your airspeed starts creeping back and the next thing you know you get a low-speed warning so that's more common when we talk about modern aircraft we're really we're really not talking about coffin corner the way we used to in the 80s in the 70s and the 60s and the 50s where coffin corner was a much more realistic place to fly in modern aircraft we're typically not there there are some other things that we can do one of them here let me show you is a supercritical wing so a supercritical wing changes the camber design of the wing to increase the critical Mach number and so essentially if you're to look at an old schooling ok so a traditional non supercritical wing a traditionally cambered wing you could end up with a very sharp coffin corner here where you don't have a lot of room between your overspeed and your stall but a supercritical wing essentially spreads that out it allows you to get much faster before you in before you reach supersonic flow and so now you'll notice that you have a much larger area in here you have all of this extra area to fly so supercritical wings not only allow you to fly faster but they also essentially kind of eliminate some of coffin corner ok looks like we got a question ok we actually we have a lot of questions in fact I think I have over 15 of them a lot of them are very technical and advanced I'm not gonna put you in the hot seat but what we'll try to do is with a lot of these we'll try to grab these and answer them when we post this to the site but Damon has this question is this pretty specific but I'm gonna wrap it back with maybe more of a generalization what Damon says is this at what point would 200 knots indicated air speed be equivalent to Mach 1 what I'm going to say is could you maybe walk us through just how Mach number changes as you call and maybe what's going to happen to your indicated airspeed as you climb yeah that's a great question so if you think about indicated airspeed so first of all Mach is related to true airspeed so relating it back to indicated airspeed is going to be a little bit more complicated and the reason the reason is is because essentially all Mach number is is your true airspeed over the true airspeed of the speed of sound so if your true airspeed is 300 knots and the speed of sound is 600 knots you're flying Mach 0.5 okay so when we go to look at indicated airspeed indicated airspeed is going to drop off as the air pressure decreases and the reason for that is because as we get basically the pitot tube is what's sensing your indicated airspeed and as you get higher at the same true airspeed there's just less air up there the air is less dense so it creates less force on the airspeed indicator and so because of that as we go up essentially you're indicated airspeed is going to slow down so if you think about let me see here I don't know if I've got a you see if I've got a I don't have a blank slide but we'll just draw it on the bottom of this if you think about your true airspeed your indicated airspeed is going to start to slow down as you go up so let me draw the axes here Konnor you guys able to see that pretty well okay so let's go into the highlighter let's say that our true airspeed is this magenta color okay so the true airspeed is gonna be constant your indicated airspeed essentially it's sea level on a standard day would be about the same and it's going to start to drop off okay so the speed of sound now let's draw that in a different color I'll switch over to the pen will draw the speed of sound as orange the speed of sound is going to start maybe here and it's going to go back as well so the question on where 200 knots indicated would end up matching up with the speed of sound is going to depend on a couple factors and those factors are going to be number one what's your true airspeed so what's the air density at your altitude what's the temperature at your altitude there's a lot of there's a lot of factors so it would be kind of hard to say that without looking at you know essentially a very specific condition one of the interesting these things to think about on an older an original style round dial airspeed indicator in a jet is that they had a barber pole that barber pole was their maximum Mach number essentially their maximum speed and the flight computer would actually compute it or the pressure system would compute it so as they started climbing it would figure out the speed of sound from true airspeed and then relate that back to indicated airspeed and so you would see that speed would be very high okay and then it would start to swing down and get slower as you climb and so essentially this little barber pole needle would come down that would be your maximum Mach number that's not the speed of sound it's a speed slower than that but it's representative of how close you're getting to the speed of sound on a glass cockpit aircraft the barber pole is now a barber section of the tape and that tape will also start to come down so essentially as you get higher you know at sea level you may not see the barber pole at all and in cruise altitude you might see a little bit of at the very top of your of your display and then if the temperature starts to get really really cold you might see it come down even more okay next question okay we've got time for one last question and like I said we'll try to answer all the other ones when we post this to the site or as many as we can but Jimmy wants to know this what procedure what a pilot use if they got themselves in a coffin corner situation the key thing there is to descend you you really want to maintain your airspeed but to get the aircraft out of coffin corner the best solution is to descend because let's go back to that coffin corner slide and I'm just gonna rewind this all the way to the beginning we'll zoom in a little bit okay essentially if you're flying right here and you realize wow I've got very little room to speed up or slow down I'm kind of on the verge of a stall and I'm on the verge of overspeed all you all you can really do to fix that is to get this dot lower ask ATC for a lower altitude there's a couple reasons that you could end up in coffin corner you know again the air density could change where the air temperature can change and so you know you may be flying well below your coffin corner and then all of a sudden that air density starts to change and you find that you're much higher in that coffin corner shape than you thought and your solution is to get down again air speed control don't let the air speed speed up or slow down and then you know get clearance and descend out of that take the airplane to a lower flight level okay that's all the time we've got for tonight there are a couple things ash cosh starts in two weeks a little under two weeks will be speaking Monday Tuesday and Wednesday at the EAA pilot proficiency Center each of those seminars start at 12:45 about an hour long the pilot proficiency Center is I don't know if it's air-conditioned but it is cooled is it air conditioner it's air-conditioned there you go we're the last group right before the air show so if you want to watch the air show Monday Tuesday or Wednesday before the show stop out watch our presentations the schedule is online a tal will post it at bold method as well as I said they're entirely free and the a pilot proficiency Center is kind of that main square in the Boeing Plaza it's an easy place to find we hope you see you there otherwise we're gonna be walking around Aakash all week so if you see us we'd love to say hi stop by and gravis other than that we will not be doing a live show the week of Oshkosh we'll let you know our updated schedule here shortly and with tonight's show if you like this please give us a thumbs up on YouTube it definitely helps our search rank and let us know in the comments or through email to info or support at bold method comm let us know what you'd like to hear about next thanks again for tuning in tonight have a great night [Music] [Applause] [Applause] [Applause] [Music] [Music] [Applause] [Applause] you

Channel: Boldmethod

Views: 24,176

Rating: 4.9253497 out of 5

Keywords: coffin corner, high speed aerodynamics, mach buffet, mach stall

Id: GU3fx6eQMGg

Length: 76min 37sec (4597 seconds)

Published: Wed Jul 10 2019