Why do maximum L/D and max. lift not occur at the same angle of attack?

In summary, the highest lift-to-drag ratio occurs at an angle of 12 degrees where as the maximum lift is at 15 degrees.
  • #1
v_arsha
7
0
Hello,
I did a wind tunnel test with a NACA 0012 airfoil. And my results showed that the highest lift-to-drag ratio occurred at an angle of 12degrees where as my maximum lift is at 15degrees. I even read somewhere that these values do differ but there was no such explanation as in why is that so..

Can someone please help me out..
Thanks
 
Physics news on Phys.org
  • #2
Can't really word it better than the wiki entry so here it is:
The peak L/D ratio doesn't necessarily occur at the point of least total drag, as the lift produced at that speed is not high, hence a bad L/D ratio. Similarly, the speed at which the highest lift occurs does not have a good L/D ratio, as the drag produced at that speed is too high. The best L/D ratio occurs at a speed somewhere in between (usually slightly above the point of lowest drag). Designers will typically select a wing design which produces an L/D peak at the chosen cruising speed for a powered fixed-wing aircraft, thereby maximizing economy. Like all things in aeronautical engineering, the lift-to-drag ratio is not the only consideration for wing design. Performance at high angle of attack and a gentle stall are also important.

http://en.wikipedia.org/wiki/Lift-to-drag_ratio
 
  • #3
That is a very good question,
We actually try to calculate maximum lift of the aircraft or aerofoil to know the maximum lift is how far and the next point of the increase in the Angle of attack (AOA) causes downfall of the lift and increase in drag and tends to stall.
And when coming to L/D ratio or gliding ratio we take into account of the lift how many times greater than and this ratio is important for a stable flight and ideal performance. For example: in emergency landing the aircraft has to land quickly and we have a problem of insufficient fuel if L/D ratio is 12 at for ‘X’ AOA then the pilot tries to get the aircraft to that point or at particular AOA so that he can save some fuel for emergency landing and he knows that lift is 12 times greater than drag and pilot knows what to do.
 
  • #4
physixlover said:
That is a very good question

....

OK, but that doesn't answer why the highest L/D ratio isn't necessarily the maximum lift point. Again, for that I refer you to the wikipedia entry for the best description.Here goes for my attempt at an explanation:

The optimum L/D ratio is the point where you get the most lift possible with as little drag as possible.

Above this point, you may get more lift but you get significantly more drag.

Below this point, you may have significantly less drag but you don't have much lift.

In your example, your optimum L/D ratio may be 10 (Lift=20/Drag=2) but your maximum lift L/D ratio could be 5 (Lift=25/Drag=5)*. Although in the latter case you are getting more lift than the previous, you are also getting significantly more drag. Making the AoA less efficient to use.

* Numbers purely for demonstration.
 
  • #5
Thanks, it really helped :)
 

1. Why is the maximum lift not achieved at the same angle of attack as the maximum L/D ratio?

The maximum lift and maximum L/D ratio occur at different angles of attack because they are two different performance measures of an airfoil. The maximum lift is the angle of attack at which the lift force is the highest, while the maximum L/D ratio is the angle of attack at which the lift-to-drag ratio is the highest.

2. What factors contribute to the difference between the angles of maximum lift and maximum L/D ratio?

The difference between the angles of maximum lift and maximum L/D ratio is primarily due to the balance between lift and drag forces. At the angle of maximum lift, the lift force is at its highest, but the drag force is also high, resulting in a lower L/D ratio. At the angle of maximum L/D ratio, the lift force is slightly lower, but the drag force is significantly reduced, resulting in a higher L/D ratio.

3. Can the angle of attack at which maximum lift and maximum L/D occur be adjusted?

Yes, the angle of attack at which maximum lift and maximum L/D occur can be adjusted by changing the shape and design of the airfoil. By manipulating factors such as camber, thickness, and curvature, the angle of attack at which maximum lift and maximum L/D occur can be shifted to different values.

4. Why is it important to have a high maximum L/D ratio?

The maximum L/D ratio is an important performance measure for an airfoil because it indicates the efficiency of the airfoil in generating lift while minimizing drag. A high maximum L/D ratio means that the airfoil can produce a significant amount of lift with minimal drag, resulting in better fuel efficiency and overall performance of an aircraft.

5. How does the angle of attack affect the stall behavior of an airfoil?

The angle of attack plays a critical role in the stall behavior of an airfoil. As the angle of attack increases, the lift force also increases until it reaches the maximum lift. Beyond this point, the airflow over the airfoil becomes turbulent, causing a sudden decrease in lift and an increase in drag, leading to a stall. Therefore, it is essential to operate the airfoil within its maximum lift and maximum L/D range to avoid stalling.

Similar threads

  • Aerospace Engineering
Replies
5
Views
3K
  • Aerospace Engineering
Replies
4
Views
5K
Replies
4
Views
1K
  • Classical Physics
Replies
30
Views
3K
  • Engineering and Comp Sci Homework Help
Replies
9
Views
2K
  • Mechanical Engineering
Replies
5
Views
5K
  • Aerospace Engineering
Replies
6
Views
21K
  • Mechanics
Replies
7
Views
2K
Replies
2
Views
3K
  • Mechanical Engineering
Replies
4
Views
3K
Back
Top