Cruise during flight is a fairly low effort phase of flight you point it in the direction you want to go you keep it at the same altitude and you leave it alone for a few hours but there are a few little things to consider along the way [Music] hi I'm Grant and welcome to the seventh class in the performance Series today we're going to be taking a look at the cruise which is a phaser flight we're going to spend a lot of time in as Pilots it's not as complicated as take off Landing or even climbing or descending but it's still important and there are a few little performance things that we need to have a look at in steady level flight the forces are balanced lift equals weight and thrust equals drag in a simple drawing of the four forces they all act through the center of the aircraft which is fine for rough calculations in reality though the four forces act in slightly different locations if we look at the aircraft in a bit more detail we can see that the lift acts through the center of pressure the cfp which is where the aerodynamic forces of the wing is said to act the wheat then acts through the center of gravity and this is where the aircraft will rotate around it's the fulcrum point it's that middle of the Seesaw if you like as you can see from this diagram the center of gravity is in front of the center of pressure this means that the lift pulls the aircraft up from behind the center of gravity and causes a downward pitching moment this arrangement of center of gravity in front of the center of pressure is normal for commercial transport planes as it is quite good for stability if you want more information check out my video on stability that I did in the principles flight series so the lift weight couple causes nose down pitching moment which we then need to counteract by creating a an opposite an opposing nose up pitching moment and we do this by creating down Force at the tail of the aircraft this means that we counter the rotation which is good but we will also now have to make more left to counteract for this added downforce so we can now say that lift is equal to weight plus the downforce in most modern jet aircraft the engines are mounted below the wings meaning that the thrust acts slightly below the center of pressure and the center of gravity while the drag acts through the center of drag which is quite a hard point to Define and it's different depending on the configuration of the aircraft and it's generally quite difficult to calculate where that exact point is but because we know induced drag is an aerodynamic Force we can see that the drag roughly acts also through the center of pressure and due to the engines being below this point and below the center of gravity it means we have a little moment arm and a couple that creates a bit of rotation and this rotation is a nose up rotation this helps to counteract some of the moment caused by the lift weight couple and we would need less down Force at the tail as a result and therefore less lift we could fly a bit slower we don't have to have the flaps out it's generally quite good this is partly why this is such a common setup for airliners under slung engines as they're called reduced downforce required at the tail so for steady level Cruise flight we can say that lift equals weight plus downforce and thrust equals drag and also the positive and negative moments must be balanced the convention around naming moment is a bit weird and it's based on clockwise anti-clockwise and depending on the way the aircraft is facing it can sometimes mean that a pitch up moment is referred to as a negative moment which to me is just bizarre because that is a positive pitch angle so I always say nose up is a positive moment and nose down is a negative moment so the downfall produced at the tail of the aircraft requires lift to be larger than weight in order to balance out this extra downforce so lift is equal to weight Plus downforce this extra lift that we generate means we also produce more induced drag because Energy Drive is generated by flow over the wings or by producing more lift we therefore produce more drag the horizontal stabilizer generates downforce by basically using an upside down Wing or you can think of it as a spoiler that you would get on like a fast car and this also produces a bit of aerodynamic induced drag because it is using the flow of air to generate a force so we have more induced drag caused by more lift and more induced drag caused by the horizontal stabilizer and we call this additional drag added on only because of these things trim drag trimming is the process of adjusting the position of the horizontal stabilizer to generate just the right amount of downforce that we need to balance out the moments that are involved in the aircraft so by adding downforce and creating this extra trim drag it means we now have to balance out this extra drag by adding more thrust so thrust equals drag plus trim drag which is just total drag but I'm highlighting the fact that there is extra drag added by the fact we're creating downforce and we need more lift so by having a center of gravity in front of the center of pressure we therefore burn more fuel because we're needing to balance out this extra trim drag with more thrust so the benefits involved of adding this added stability is not good for fuel consumption for fuel burn for efficiency of the aircraft but the benefits of stability outweigh the fuel burn in the designer's eyes the center of gravity can be within a certain range of positions where the aircraft remains in a safe stable and controllable state but depending on its position within this range we get different levels of fuel burn so if we have a center of gravity at the most forward limit then we have a long distance between the center of gravity and the center of pressure meaning that we have a larger nose down moment produced because remember moments equals force times distance so the distance is increased and that larger nose down moment will be have to be balanced by adding more downforce this means that more lift needs to be produced and we also get more trim drag as a result meaning more and more thrust is required to counteract this increase in Drag which obviously burns more fuel if we have a center of gravity towards the rear limit that is the opposite a smaller balance arm weaker pitching moment requiring less down force and less extra thrust so a lower fuel burn so if we're at the rear limit we're at the best possible fuel burn and at the forward limit would be the worst possible fuel burn and uh in some long-haul aircraft for example the tail might have a fuel tank in it so either in the cruise you can Pump Fuel into the back therefore moving the center of gravity backwards a bit and that improves uh fuel burn when we're in the cruise in theory we want to cruise at a speed close to if not at the maximum speed we possibly can in practice though we fly according to something called a cost index which will dictate our maximum speed by flying faster we burn more fuel so there's a compromise between flight speed and fuel burn that the companies that we're flying for find a sweet spot for where we don't burn too much fuel but we also don't fly too slow to make the flight unnecessarily long this compromise is given a numerical value which is the cost index and we put that into the computer on board the aircraft and it tells us a cruise speed to fly where we're um getting the desired amount of fuel burn versus flight speed but when we're looking theoretically we want to know where our maximum speed is it's quite simple to find because we know that in steady level flight thrust equals drag where drag is made up of the drag of the aircraft plus any trim drag so we just need to compare our thrust available versus our total drag curves where the lines cross over would be where we have no more thrust available to balance out flying any faster and increasing the drag of the aircraft so there it is on a jet and in a propeller it's there notice it doesn't occur where our level of drag is the least or a vmd which can be a bit weird to think about you would think that where drag is the lowest speed would be the highest but this is where we would have the largest level of acceleration if we think about it in a simple equation f equals m a and rearrange for acceleration we can say that acceleration equals the force over mass and the forces on the aircraft is going to be thrust minus drag over Mass and the biggest difference between thrust and drag is in here so that's where we're going to get the largest acceleration not necessarily the largest speed and anything that influences our thrust available line will have an impact on the maximum speed stay say for example we're High upper altitude then we would have less thrust available maybe a line something like that and that means that our crossover point is lower and that's obviously a slower maximum speed which sort of makes sense less thrust you're not going to go as fast okay so nice and quick there the next two classes we're going to jump into a bit more detail about efficiency by looking at range and endurance but for now just Cruise in general we have the four forces in Balance but because they act in slightly different locations we get moments introduced the lift and weight couple usually creates a nose down moment which we then use downforce for to create a nose upward moment therefore lift has to equal weight plus this added downforce and thrust equals drag but remember that drag now is consisting of a bit of added trim drag because the downforce production creates a little bit of drag and also because we're creating more lift to balance out the weight plus down Force creating more lift makes more induced drag so thrust still equals drag but remember there's that little trim drag section now to think about it and we also have to have the moments in Balance which is why we have the tail in the first place so this is what I'm explaining here lift has to be larger than weight therefore the drag has to increase for those reasons I just explained and that causes more fuel burn and the position of the center of gravity makes a difference so if we have a very forward center of gravity then at the moment arm is longer I've actually drawn this wrong way around this should be different weight positions not different lift positions but it's the same Theory the moment arm is a different length and basically if it's towards the forward limit that means you're gonna have a longer moment arm more of a moment that's pitching us down therefore requiring more downforce to pitch us back up that means we get more drag more trim drag therefore needing more thrust to balance it out more fuel burn less efficient but the stability benefits are pretty good conversely if we have it towards the rear limit of the safe range we've got quite a small moment arm that means a weaker nose down moment less down Force required a less and lower amount of trim drag added to the total drag therefore needing less thrust good for fuel and the maximum speed that we fly in theory is where the lines cross over so thrust available versus drag which is thrust required is what I've heard it referred to a couple times which is quite a good way to think of it thrust available and thrust required and the lines crossing over would be our maximum speed anything that reduces our level of thrust say we're at altitude and our thrust available line goes down that means our speed would go down as well because the lines intersect at a slightly earlier point a notice it doesn't occur at vmd vmd is where we get our maximum acceleration