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hao apple 39 people agreed with this answer
As an aircraft engineer, here is a good answer to this question.
Let ’s first understand the lift principle
Suppose that the airflow blows over the wing surface. The airflow has two forces on the wing surface: one is the pressure F due to the air pressure perpendicular to the wing surface , and the other is the tangential force F due to the air viscosity. The sum of the forces on all wing surfaces S is equal to: a force F and a moment M.
Force F + Moment M = Force F acting on a specific point p
This specific point p is called: pneumatic center . The force F is parallel to the component of "inflow", called "wing resistance" Fx , and the component perpendicular to "inflow" is called " wing lift " Fl . Taking the direction of " incoming flow" as the positive direction of the x-axis, an xy coordinate system is established, called " incoming flow coordinate system", and the force F can be expressed as F (F_x, Fl).
F is always relative to the "coming current coordinate system". Understanding this and understanding the nature of rising resistance is the key to understanding induced resistance.
For a wing of limited length, due to the pressure difference between the upper and lower surfaces of the wing (the lower height is still low), a "wing tip vortex" will form at the wing tip . The wingtip vortex will affect the entire flow field and cause downwash. The direction of the incoming flow has relatively changed.
The aerodynamic force F is relative to the "incoming flow coordinate system", so that because of the downwash, the incoming flow coordinate system rotates, and a part of the original lift component Fl becomes resistance-this is induced resistance !
Because of the viscosity of the air, the intensity of the wing tip vortex is the strongest near the wing tip. The closer to the center of the wing, the weaker the downwash, the weaker the induced resistance.
A large aspect ratio wing, the entire wing, is affected by the vortex of the wingtip, which is relatively lighter than a small aspect ratio. Therefore, the induced resistance of the large aspect ratio is smaller.
The induced resistance is not only related to the aspect ratio, but also to the overall shape of the wing. At low speed ( <0.3Ma), the estimated formula of induced resistance is:
λ is the aspect ratio of the airplane, and c is the correction factor.
Flying speed is greater than 0.3Ma, subsonic speed, supersonic speed, induced resistance will be reduced, the specific value should be carefully calculated by 3D CFD method.
Induced resistance accounts for 50-70% of the total resistance . In order to reduce the resistance of the wing, as long as the strength of the wing is allowed ..., the aspect ratio is generally increased as much as possible, and the trapezoidal or elliptical wing is used (it is difficult to make). ( Note: This is only the case for aircraft, and the basic frictional resistance of underwater vehicles accounts for 70% -90%)
Excellent answerer on topics of computational fluid dynamics ( CFD) and fluid mechanics
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Since it is "science popularization", the extreme example is the situation where the aspect ratio tends to be infinite. According to the cause of induced resistance, the induced resistance is equal to zero.
The aeronautical engineer's teacher / band keyboard player / lost this one in order to make up three. .
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In traditional textbooks, the explanation of induced resistance always starts from the wing tip vortex. I have always said this in class, but I have thought about a problem recently when writing something. In fact, when not considering the previous reasons, this problem should start from underwashing, which causes lift deflection, and the backward component after deflection is induced resistance. Looking forward from the bottom wash, did the wingtip vortex cause the bottom wash alone? No, if we look for an airfoil flow field, we will find that after the two-dimensional flow field bypasses the airfoil, downwash occurs. Anderson's Understanding Flight said that it is precisely because the airfoil gives the air a downward force, the air can deflect and wash (Niuyi), and the reaction force of this force is lift. Can the airfoil in the two-dimensional flow field be regarded as a straight wing of infinite length? Isn't there a wash here? So I think there will be induced resistance.
So I am a little confused now. Please discuss it. I think there are two possibilities. One is that the definition of induced resistance in the textbook is not perfect. It should not be “resistance induced by lift” but wingtip vortex induction Out of resistance. The second is that when writing textbooks, due to the complexity of the previous definition of lift, the underwash caused by the lift is ignored. In fact, this downwash should be greater than that caused by the wingtip vortex.
Regarding the problem of large aspect ratio, I would like to say that the concept of "induced resistance" may be inaccurate. Should it be the induced resistance coefficient, because the aerodynamic coefficient discards the interference of the area, so the wing tip vortex in the large aspect ratio The induced resistance that comes over should be evenly smaller.
Sorry, you should ask questions, but because there are many experts here, so come up with a discussion.
Engineering dog who likes to draw ...
The induced drag is called induced drag because it is induced by the induced velocity. The
induced velocity is the free vortex induced by the free vortex behind the wing.
The reason is that there is a fluid that can be generated from the limited length of the wing.
Having said that, infinite length has no induced resistance, just because there is no fluid wing , there is no free vortex),
so of course, the larger the aspect ratio, the smaller the effect of the slight effect on the whole ~
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