Flight Mechanics – I

hey everyone welcome back yeah it’s been a long time that I’ve actually made a video I was busy with some other Nomad minutes that I had so anyways let’s continue on we’re gonna discuss flight mechanics today alright so flight mechanics deals with how the aircraft behaves in various phases of flight alright so first comes up straight and level line alright so as simple as it sounds so the aircraft is straight and level now important thing to note here is there is a CP there is a CG so CG there is weight acting and this is the arm and from the CP there is lift acting all right so this is your CP this is your CG where the weight is actually acting and all of us know thrust is acting forward and drag is acting redwood like this alright so now in a straight and level flight what happens if this for obvious reasons in CG being a head of CP CG being a head of CP it creates a the lift weight couple creates a nose-down moment alright so now this is compensated by offsetting the thrust drag couple so the thrust line is generally arranged below the drag line so that there is a nose up tendency okay so this is balanced now another question is how how do you move the thrust line below the drag line alright so this can be done by a lowering the height of the prop or engines whatever be smaller diameter of the drop and see adjusting the crank shaft angle okay so these are the three ways you can actually move the thrust line below the drag line the crank shaft is obviously in the propellers I’m talking about okay so now we’ll discuss what are the factors factors that affect straight and level flight there are namely four general factors all right so first one is your is the most important since the density at a particular level is generally constant if your is is decreased then the CL has to be increased to maintain straight and level flight now how does this come if you remember left equation half Rho v square CL and s of course so now suppose the density is constant at a particular level correct now if your is is decreased so this is decreased then the CL has to be increased to maintain a level flight correct so so this factor straighten level flying depends on the velocity speed is indicated airspeed so if you decrease V you have to increase CL correct so CL can be increased by angle of attack correct so you increase the angle of attack to maintain a certain straight and level flying attitude ii altitude not attitude altitude all right so now altitude what is happening is loss of lift is happening at higher levels you know that the density is reduced correct so at higher the density is reduced and which has to be compensated by higher tasks all right so TAS has to be so that the total lift remains same so n equal to half Rho v square CL s now at higher level that this is decreasing so this is automatically compensated by increase in tasks so this whole thing is is has to be constant and then your left will be that is this lift is basically we maintain straight and level flying for a

given angle of attack because angle of attack is if you keep an angle of attack constant suppose we had four degrees all right so your whole if your density at higher altitude is decreased the whole the task has to be increased to keep the AI as constant to maintain level flying so higher altitude altitude density is decreasing so TAS is increased to maintain constant is and angle of attack correct all right third is weight or load factor now if you have more load it obviously requires more lift so in in case of straight and level flying so if you have a heavier aircraft so you have you need more lift to maintain certain level flying so higher load or live load to say requires more lifts this is by is proportional to all up wait okay fourth one is the L by D ratio now this is how is that maximum range maximum range is obtained obtained by is and angle of attack that is linked to the best L by D ratio okay so if you have the best L by D ratio then only you will have the maximum range alright so the next thing we are going to discuss is climbing okay the climb is a very important phase for my craft so climbing here we go it’s a climbing so this is your ground that’s the air crafts path the angle theta and suppose you have an aircraft like this sorry for my drawings guys though I mean it’s not all that great but yeah so in this case you have the same drink line you have thrust correct and you have lift and you have drag and you have weight that is acting downwards towards the earth right and then there is this component and Robert with another sketch there’s this component and this which is this angle is equal to this angle theta if you are good at math you guys you can just find out that this angle is equal to this angle due to properties of mathematics and this this will become W sine theta using trigonometry all right so now obviously aircraft requires some power to remain in the air okay so the excess power that is available is used to overcome drag so I’ll just write it down so that it will become clear so aircraft requires power to remain in air cliff second point here is important is the excess power the excess power that is available after overcoming drag is used for climb so thrust has to first of all overcome the drag and the excess thrust or the power that is available is then used for the climb this is the very important point how the aircraft will actually climbing and I as I mentioned this is your angle of climb angle of climb it is the angle between the longitudinal axis suppose this the axes longitudinal axis of the aircraft and the earth horizontal okay now what will happen if you increase the angle of attack increase angle of attack so now there are two things that will happen I forgot to write here this is w cos theta these are the if you good at trigonometry guys you just look forward cheetah the vector

opposite to it as W sine theta because W is the arm here and this will be a w cos theta the base the base of the triangle will be on W cos theta alright so if you increase the angle of attack W cos theta will reduce so if you increase the angle of attack this this vector reduces in length all right so this means your lift requirement has gone down now with that W sine theta will increase that will demand for thrust requirement to increase so the deal is now when you increase the angle of attack this is reduced but your W sine theta is increased so at at a higher angle of attack at higher angle of attack your thrust requirement has increased okay now there are few keep speeds here V X and V by alright I’ve just discussed it on the next page VX and V Y so VX is what you will call it as VX is best angle of climb so this is the speed at which maximum altitude is gained for a given distance okay and this is also the difference greatest difference greatest difference between thrust and drag okay so this is V X V Y is your best rate of climb so this is the speed at which maximum altitude is gained enough for a given time all right suppose the ATC asks you to you know expedite your climb because they have some traffic so that means you will be at V X because at the shortest distance you will climb the maximum altitude it is a different speed now if the ATC wants you to climb fastest say in another two minutes then you switch over to V Y that’s the best rate of climb and then you will be climbing it with to the maximum altitude in a given time and this is the greatest difference between power available and power required all right now another key point to note both these speeds both these speeds are not affected by compressibility and VX is always less than V why this is important VX is always less than V Y because this your speed this speed will be lower so that because at the lower dis higher angle of attack at a given less distance suppose if you have you want to climb at the lowest port as possible distance your angle of attack will be high which will make VX low and you’ll climb fastest but you have a lot of time for a certain set time is there so you might just go something like this so this will be your V VY which will be all always higher so VX is always lower because the shortest distance okay next we’ll discuss gliding it’s fun to glide I guess let’s see gliding all right so gliding similar diagram you just the opposite way it looks gliding is something like this this is obviously the glide angle what I’m talking about here and this is our aircraft and I’m gonna use different

sketch huge again weight acting here this is your track this is your lift and another component this is important thing to notice here is the glide has only three forces acting lift weight and drag alright when it’s a power design power on descent alright when it’s not an idle power on descend then you also have some thrust that’s the only difference actually thrust and your speeds will be higher the ground speed will be higher because you’ll cover distance horizontally as well okay now so in glide in glide forces acting are forces in glide are thrust to soar your drag lift and weight only okay and the glide angle is the angle between the longitudinal axis of the aircraft and the same horizontal of the earth okay also the angle between the lift and the resultant of lift and drag see this is drag this is lift this is the resultant R of L and be correct so angle between the lift and the resultant of lift and drag is also your glide angle this is your theta is your glide angle now the now the aircraft now this is interesting here to notice this thrust here it forms one pair and this so that your aircraft is in a steady descent or glide when the forward acting acting forces are equal to R air word acting forces these are key points this is regularly as in exams so T plus W equal to L plus D these are the forward acting forces this is basically in a descent and your barrier but acting for the lift and drag all right now what are the factors that affect the glide angle and glide distance we are going to discuss that in a bit so factors affecting glide angle and glide distance okay so the first factor that is affecting is is and L by D ratio okay so the flattest glide angle is at best L by D ratio okay so any increase in drag results in higher glide angle or a steeper approach so the flattest a glide angle so this is the this is the thing and this so this flattest glide angle flattest glide angle will be at the best L by D ratio if you if any if you increase any drag on this you’ll be resulting up fly higher steeper descent so it will be something like this and your distance won’t be covered you won’t be gliding far always fly at best L by D ratio to get max gliding distance just like I just now mentioned if you have a flat glide angle you’ll go further if you have a steeper approach you may just fall short of your target higher is gives more parasite drag this I have explained in the drag chapter is the same thing lower is gives what more induced drag okay and yes now important point here higher or lower is then what BMD the same thing I discussed in the drag chapter will always increase your increase overall your glide angle and give steeper

approach okay next all up wait you will be surprised to know L by D ratio is independent of weight L by D ratio is independent of weight because you will fly at the best L by D ratio and the speed that corresponds to the particular weight so the glide distance will glide distance will remain same because you will fly L by D ratio that corresponds to particular weight all right wind what happens to wind best you will get glide angle so you have a tail tail wind will increase glide distance okay so just for the diagrams how it looks it’s something like this so a tailwind will take you for the glide distance still wind will land you here headwind will of course reduce your gliding distance so the glide angle will determine the distance that the aircraft can glide for a given change your loss of height the distance can be achieved only in still air flight distance will change when there are some winds so headwind will reduce your glide distance and tail wind will increase your flight distance glide ratio is the same as your L by D ratio so suppose this is the thing C a B so your glide ratio is given by a by B so it is the ratio of forward distance covered by an aircraft to the height lost so the forward distance covered in air is a to the ratio of the height that is lost by the aircraft for a given nautical mile all right so continue on to the next video for flight mechanics we’ll catch you up there