Negative drag on an aerofoil possible?

  • Thread starter Jez006
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Main Question or Discussion Point

Overall the sum of the forces of drag on an object must of course be negative. But is it possible to have areas of an object that contribute to negative drag?

I have the pressure distribution over a NACA 0015 (symmetrical aerofoil about x-axis) at many angles of attack and I need to calculate lift, drag,etc..

Some of the pressure readings are indicating negative drag at multiple points at multiple angles of attack...unless I am thinking of it entirely wrong..

The picture below (not actual aerofoil!) shows a point on the aerofoil at 0 degrees AOA where the static pressure is lower than the freestream pressure. Does this mean that the resultant force on this point is the red arrow?

And as you can see from the equations this means positive lift and negative drag at this point?

Thank you in advance,

Jez
 
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  • #2
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pic

hopefully adding picture now..
 

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  • #3
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You can obviously have a negative "pressure drag" component at certain points along the airfoil, but that isn't the problem here. The problem is that you're confusing the concepts of gage pressure and absolute pressure. The gage pressure is negative, but you still have a "positive" pressure acting at such a point. Since the pressure force is opposite to the outwardly directed unit normal, it is pointing in a direction opposite to that shown in your figure.
 
  • #4
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Ok, I understand what you're saying about gauge pressure compared to absolute pressure. However, at this point there is a pressure drop due to the acceleration of air around the curved surface. This results in a pressure lower than atmospheric, resulting in a "suction force" on the top surface of the aerofoil. At this point on the aerofoil, the horizontal component of the suction force is in the negative x direction....

or am i still confused..?

You say the resultant is opposite to the one shown in my figure but that would mean that the top surface is contributing to a downforce instead of lift...

As the AoA increases the pressures on this part of the surface become larger and more negative, contributing a large factor of the lift, so I'm not sure about the resultant being opposite...?

Thanks,

Jez
 
  • #5
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The lift comes from the difference in static pressure on the two surfaces. You can't have a "negative" absolute pressure. The gas molecules impinge on the surface and produce a force directed opposite to the unit normal vector. The first chapter of Anderson's book clarifies a lot of these points.
 
  • #6
hi, Iam testing NACA 0024 (symmetric airfoil) of blade length 300mm and chord length 100mm. i have 12 pressure tabs on the surface of my blade made of steel. i have used micro-manometer to measure pressure distribution at all AOA from 0 to 360 degrees in the wind tunnel of constant velocity= 27 m/s. i have calculated drag and lift... my drag values are negative.. is this possible? what is the reason for this?
 
  • #7
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If it's a piston engine with radiators in the wings, meredith effect might be able to be harnessed.
 
  • #8
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I think there may be some interference in the test results. the force direction is right. drag at some point of the airfoil may be nagtive. but the total drag must point downward. may be the renolds is very small,so the friction drag is small. and what support did you use in the test? there may be something ignored in the test.
 
  • #9
thanks...what do u mean by support?
i dont understand.. sorry
 
  • #10
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it means where and how u fix the airfoil. some NACA series is unusual. it is not easy to measure the drag precisely. u may check the measure system carefully, including pressure measurement point, the compute procedure. wind tunnel test is very hard. u must pay attension to all details. sometimes there is problem you can not find and solve, but a wrong result.
 
  • #11
there are 12 pressure tabs in my aerofoil. but the 12th pressure tab is not working since it has got choked. and also there s no settling chamber in my wind tunnel.. will these reasons affect my pressure or force measurements?
 
  • #12
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i think u may set value for the choked point to check the effect of it. i checked some references. the result may be plus. the first reason is the way can not measure the effect of frictional force, the second reason is the result of drag is not precise, we usually use the momentum conservation law to caculate the drag. i sorry for answering u so late. i am too busy rescently for finding job.
 

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