Induced Drag

[pipertommy]

Hi,
If you were teaching a student about induced drag how would you explain the tilting aft of the total lift force? Forming the angle for Induced Drag?

I understand the high to low airflow resulting in vortex at wing tip and the downwards part of the vortex acting on the trailing edge of the wing. This produces a downwash.

The part which I`m loosing it, is the average relative airflow. Why is this formed? Due to the trailing edge downwash?

I`ll have to dig out the ATPL notes if all else fails:ugh:

Thanks

[sdelarminat]

I suppose you're asking about the relative airflow resultant of the downwash?
If so, it has a name (I don't know how is it in english, but a spanish translation would be "relative airflow of the aerofoil "). And it forms an angle of attack different to the free relative airflow. If there's no downwash (only with wingspan= infinite), then there will be no "relative airflow of the aerofoil".

You have then the chord/free-airflow angle (the geometric angle of attack), the chord/"relative airflow of the aerofoil" angle (the aerofoil angle of attack), and finally the free-airflow/"relative airflow of the aerofoil" angle (the induced angle of attack).

The greater the induced angle of attack, the greater the induced drag.

Hope it helps ;)

[Mr Optimistic]

Your question started me wondering too. The explanations on the web aren't very convincing. The idea that wing tip vortices are responsible sounds a bit odd and the argument that the efficiency of longer wings proves it would require the effects of wing loading (reduced circulation around the wing) to be considered.

The rotation of the total force vector implies that the axial force increases more quickly than the normal force but for an inviscid fluid I am unclear as to the physical source of the axial force given that pressure can only act normal to the surface. So the origin of induced drag relies on the viscosity of the fluid (well that it is not zero at least).

Perhaps using momentum theory where the force on the wing is the reaction to the force applied to deflect and slow the air, the separation of the boundary layer at the trailing edge, and the path this opens for air at the edge to be less deflected as it moves upward.

Wouldn't buy the wing tip theory except as an extra inefficiency but if that's the right book answer...........

[pipertommy]

Thank you for the replys, seems the more I read the more the subject develops:ok:

[HercFeend]

Wing tip vertices, responsible for 80-90% of ID, create a downwash. This raises the angle of the RAF at the leading edge due to wave form. Lift acts at 90 degrees to the RAF thus is tiled back - leading to the tilting back of the TR.

The difference between the lift you SHOULD get and the lift you DO get is your ID.

[pipertommy]

Excellent:ok: Firm belief in simple explanations, thanks.

[HercFeend]

No worries - hope it helps. Currently studying for Flight Instructor quals in NZ so am living and breathing Principles of Flight........ Constantly being told to - 'KISS' :uhoh:

[pipertommy]

Ye, lol. Funny thats what I was thinking when I wrote the reply, KISS everytime!
Thanks again.

[Pull what]

You could say induced drag is proportional to the sine of the lift vector and hope your student knows little about trig.