induced drag!!!!!!

[purple haze]

dear all

i dont have many flying hrs well just four to be precise so please excuse the ignorance.

From my reading, i have found out there are 2 drags, induced drag and profile drag of which there is form drag and skin friction.

profile drag obviously increasing with air speed but why does induced drag reduce with increasing speed.

im sorry if this is a basic q.

but any answers would be appreciated.

cheers

[Tor]

Induced drag comes from the wake-turbulence the wing creates at high angle of attack. Since your AoA is decreasing with higher speeds, your wake-turbulence decreases and thus your induced drag.

By the way, I think that it's parasite drag and not profile.

Tor

[purple haze]

thank u, thats cleared it up.

[Old Dog]

The resultant lift is perpendicular to the chord of the wing section. If the angle of attack increases as speed decreases, the sine of the lift vector is adding to the total drag, while cosine of the lift vector is the useful lift you get. The larger the AOA, the larger the induced drag which is equal to sine AOA. Wing tip vortices also increases with AOA.

[This message has been edited by Old Dog (edited 21 November 2000).]

[chicken6]

Purple haze

Thought I'd clear up some bad info (bloody long for clarity, sorry in advance for boredom, other instructors please fault-find as I've got an upgrade coming next month http://www.pprune.org/ubb/NonCGI/eek.gif )

The induced drag is created by the same mechanism or system that creates wake turbulence, it is not created by wake turbulence itself.

Part of the problem is the way we define lift, i.e. "That part of the total aerodynamic reaction that acts at 90 deg to the relative air flow (RAF)". Hold this thought.

The total aerodynamic reaction around an aerofoil acts slightly behind 90 deg to the RAF (in the direction of the trailing edge).

Now our definition comes into play - with Lift acting at 90 deg to the RAF (by definition), the remaining component of the total reaction acts parallel to the RAF, this is what we call Induced Drag i.e. "Drag" induced by the production of "Lift". Really they are part of the same total reaction, we just label them as separate components to make it easier to add up total drag etc.

Now http://www.pprune.org/ubb/NonCGI/eek.gif the long way

An aerofoil creates lift by creating a difference in pressure between top surface and bottom surface (or one side and the other side of a vertical aerofoil i.e. tailfin). There is slightly higher pressure underneath the aerofoil and proportionately much lower pressure on top. The difference in pressure forces some air to try and escape to the low pressure area on top of the wing, which can frequently happen at the end of the wing where there is no longer a physical barrier to this process. This creates a component of airflow which runs sideways, along the wing, from root to tip underneath and from tip to root on top.

At slow airspeeds we need a higher AoA, so the difference in pressure between top and bottom of wing is bigger. This means a bigger spanwise (sideways) component of airflow, and when it reaches the wingtips it curls round it more drastically. As the air leaps upwards from the high pressure area to the low pressure area it is sucked inboard towards the greater area of low pressure and down towards the surface of the wing again. As it moves down towards the surface though, it finds the wing has moved on and its inertia means it returns to the high pressure area again. It is again forced sideways and out and up and in and down, and this is the process by which both wake turbulence and induced drag are created. You can also go into the change in effective angle of attack from here, as the downwash is modified by this process and the total reaction is tilted backwards. THIS is the inducing of Induced Drag, and the higher the AoA the more the difference, so the more the tilting, so the more the I.D.


(Pedantic hat on)
Old Dog

You can't sine a vector, only an angle.

Purple Haze

If this doesn't answer your question, email me privately because frankly I feel I can use the practice of explaining without pictures!



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Confident, cocky, lazy, dead.

[Old Dog]

Chicken6, thanks for the comment about sine and cosine. I agree with you. Please excuse my poor English. Let me try again.

Induced drag is the by-product of lift. The resultant force on an airfoil is slightly slanted backward (due to bending of the airflow around the airfoil), this backward force is the induced drag. The more pronounced the bending of airflow the greater the induced drag. Induced drag is proportional to sine of the effective angle of attack of the airfoil. When airspeed increases, AOA decrease, and induced drag also decrease.

Induced drag does not include drag caused by resistant to movement of airfoil through the air.

Is it clearer now ?

p/s: Some people would not include wing-tip vortices as part of induced drag, although they are closely related. One of the argument is: when AOA is zero, lift is zero, then induced drag would also be zero. But wing-tip vortices is not zero (low, but not zero). When no lift is produced (or desired), all the drag on this airfoil is profile drag.


[This message has been edited by Old Dog (edited 22 November 2000).]

[Roadtrip]

Geez, they must have been lying to me at flight school. They told me induced drag was when you forced the copilot to dress up as a woman!