## چند سوال ایرودینامیک

rohamavation

نام: roham hesami radرهام حسامی راد

محل اقامت: فعلا تهران قیطریه بلوار کتابی 8 متری صبا City of Leicester Area of Leicestershire LE7

عضویت : سه‌شنبه ۱۳۹۹/۸/۲۰ - ۰۸:۳۴

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### چند سوال ایرودینامیک

i really apologize if my english is weak because my main language is farsi. huppa kids i wrote in english this time.
my instructor asked me this question during my lesson and i couldnt come up with any answers. he asked why do I
need to look at the indicated airspeed rather than ground speed when in takeoff roll or as soon as we touchdown?
T an airspeed indicator is actually more of a dynamic-pressure meter with the dynamic pressure converted to airspeed.
dynamic pressure is $q=\frac{1}{2}\rho V^2$ where ρ is density and V is airspeed.
my article on lift coefficient explains that lift is proportional to dynamic pressure wing area and lift coefficient which in simpler terms is a function of angle of attack$L=\frac{1}{2}\rho V^2 S C_L(\alpha) = qS C_L(\alpha)$
the takeoff speed is the speed at which you will have enough lift to get the airplane off the ground at the angle of attack that the airplane will have post-rotation. so for a given aircraft at a given weight wing area post-rotation angle-of-attack and lift curve you will be able to take off at a particular dynamic pressure.
in fact all the aerodynamic forces on the aircraft are proportional to the dynamic pressure. so thats why stall speed never exceed speed above which the aerodynamic forces could start damaging the aircraft structure etc. are all given in indicated airspeed - because its a proxy for dynamic pressure.
true airspeed is the speed of the aircraft relative to the air mass its flying through and can be calculated from indicated airspeed by correcting for density and temperature. your ground speed is then the true airspeed added to the wind speed.
the second question
does the engine's thrust not contribute to the lift?
here are my thoughts:
there are two types of aircraft resistance one is resistance created to create lift which is called lift resistance and the other is resistance. why is there lift resistance?
since the wing has an angle of attack there will be a region in the direction of the wing speed so there will be lift resistance.
engine thrust balances lift and other resistances to get the plane flying. so does thrust only help overcome resistance and not lift?
because drag contains lift resistance and overcoming lift resistance creates lift. if lift resistance is not overcome the aircraft cannot move and the wings cannot generate lift. furthermore even if the aircraft is moving if there is no engine thrust to overcome the lift resistance the inertia will overcome the lift resistance so the speed of the aircraft will continue to decrease and the lift will continue to decrease. so to create lift there must be engine thrust to overcome lift resistance.
a glider does not have an engine but actually uses the component of gravity as a thrust force to overcome the resistance of lift thus creating lift. Therefore to create lift the glider had to lose altitude.
the wing is similar to an inclined plane. we all know that there must be a thruster to keep the object stationary on the inclined plane. it is wrong to say that thrust does not contribute to an object at rest on an inclined plane.
why is the pressure-to-weight ratio smaller than ? since the wing is an inclined plane we all know that the smaller the angle θ the smaller the force f that keeps the object fixed on the inclined plane. the same is true for wings. the angle of attack of the wing is generally small so the thrust-to-weight ratio of the aircraft is less than . of course this angle θ is not the angle of attack of the wing. In a wing this angle is the aerodynamic angle of the wing. in general the angle of attack is not equal to the aerodynamic angle. but there is an inner connection between them.
am i righta
drag consists of two part:
parasitic drag caused by friction with the wind. this can be minimized by making the airplane's shape as streamlined as possible. such an airplane is called slippery.
Induced drag caused by part of the lift vector pointing backward as you intuited . this can be minimized by maximizing the aspect ratio the ratio between wing length and wing width. thats why gliders have long skinny wings. this drag correlates with wingtip vortices that drain energy. anything that reduces the strength of the wingtip vortices also reduces the induced drag.
now try to understand this: airplanes are not always flying level at constant altitude. sometimes they are ascending and just like a car climbing a hill they need more power or thrust.
sometimes they are descending and just like a car descending a hill they need less power or more drag.
every aircraft with particular weight balance and trim has a favorite speed which the pilot can adjust with the trim control . if the pilot does nothing the aircraft will travel at that speed up down or level depending on the amount of power being applied. a paper airplane or glider descends at that speed because no power is being applied.
the angle at which it descends is determined by the ratio of lift (L) to drag (D) called the L/D ratio. this determines how far it can glide if there is no engine power.
in a typical small airplane like a cessna 172 the L/D ratio is about 9 at slow speed meaning it can glide about 9 miles for every 1 mile of altitude lost.
for a typical jet transport like a boeing 737 the L/D ratio is more like 25 so it can glide 25 miles while losing 1 mile of altitude. this means for level flight the amount of thrust needed is about 1/25 of weight.
notice one problem with being too slippery. its hard to come down. thats why airplanes with a high L/D ratio need speed brakes or spoilers which they deploy when descending or landing.
so why are jets so powerful? the answer is so they can climb at a steep angle when taking off.
why do they need to climb at a steep angle? safety thats why. if they lose power for some reason the higher they are the farther they can glide and find a good place to land.

عضویت : شنبه ۱۳۸۴/۲/۲۴ - ۱۹:۱۷

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### Re: چند سوال ایرودینامیک

لطفا در تالارهای فارسی فروم، از ارسال پست‌هایی به زبان‌های غیرفارسی خودداری کنید.
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