• hob63
In summary: Thank you.In summary, a symmetrical wing at 0 degrees inclination does not produce a resultant force. However, at an inclination angle of 10 degrees, the lower surface experiences an unfavourable pressure gradient due to the flow being pushed downwards by the upper surface. This results in a decrease in pressure towards the trailing edge. The curvature of the surface also plays a role, with curving upwards being associated with a favourable pressure gradient and curving downwards with an unfavourable pressure gradient. While the contribution of ρg(Δh) does have an impact, it is not the main reason for the unfavourable pressure gradient on the lower surface.
hob63
For a symmetrical wing (NACA 0012 - due to wide data avaialble) at 0 deg inclination the following Cp to x/c relationship exists:

The upper and lower surfaces produce the same Cp and hence a symmetric wing with no inclination doesn't produce a result force (i'm happy with this).

Now at an inclination angle (10 deg) the following happens:

My question is for the lower surface we have Cp = 1.0 (stagnation point, this is ok), however, this decays to freestream (Cp = 0) close to trailing edge.

Is the lower surface actually an unfavourable pressure gradient? it is a symmetric wing and so I have a hard time seeing the lower as unfavourable (in terms of geometry). Why is it that curving upwards = favourable pressure gradient and curving downwards = unfavourable pressure gradient? there is a small contribution (ρg(Δh)) in terms of pressure due to the actual height, is it simply due to this? Or is it that the stagnation points dominates the lower flow resulting in a high->low gradient and thus an unfavourable gradient (not resulting from the actual geometry but due to the location of the stagnation point).
Cheers for any help :)

hob63 said:
Is the lower surface actually an unfavorable pressure gradient?
Why do you call the lower surface gradient unfavorable? As the flow approaches the trailing edge, it should be approaching ambient pressure. Also, the lowest pressure above the wing in the graph seems to occur unusually close to the leading edge.

Hi there,

The lower surface of the wing does indeed experience an unfavourable pressure gradient at an inclination angle of 10 degrees. This is due to the fact that the flow is being pushed downwards by the upper surface, causing the air to accelerate and create a low pressure region. This decrease in pressure is what causes the Cp to decay towards the freestream value close to the trailing edge.

In terms of geometry, it is true that curving upwards is generally associated with a favourable pressure gradient and curving downwards with an unfavourable pressure gradient. This is because the flow naturally wants to follow the curvature of the surface, and when the surface is curving upwards, the flow is being pushed towards the surface, resulting in a higher pressure. When the surface is curving downwards, the flow is being pushed away from the surface, resulting in a lower pressure.

The small contribution of ρg(Δh) does play a role in the overall pressure distribution, but it is not the main reason for the unfavourable pressure gradient on the lower surface. It is mainly due to the effects of the upper surface and the flow being pushed downwards.

I hope this helps to clarify your question. Let me know if you have any further questions.

An adverse pressure gradient on a wing is a situation where the air pressure on the upper surface of the wing decreases while the air pressure on the lower surface of the wing increases. This creates a pressure difference that can negatively affect the aerodynamic performance of the wing.

An adverse pressure gradient on a wing can be caused by a variety of factors, including changes in air density, changes in wing shape or angle of attack, and the presence of obstacles or obstructions near the wing. It can also occur when the wing is operating at high speeds or in turbulent air.

## 3. What are the effects of an adverse pressure gradient on a wing?

The effects of an adverse pressure gradient on a wing can include reduced lift, increased drag, and decreased stability. It can also cause the wing to stall, which can be dangerous for aircraft. In extreme cases, it can lead to loss of control and potentially a crash.

## 4. How can an adverse pressure gradient on a wing be mitigated?

There are several ways to mitigate an adverse pressure gradient on a wing. One approach is to design the wing with a more gradual change in curvature, known as a laminar flow airfoil. Another method is to use devices such as vortex generators or boundary layer control systems to manage the airflow over the wing. Pilots can also adjust their flying techniques, such as reducing speed or increasing angle of attack, to minimize the effects of an adverse pressure gradient.

## 5. What are the implications of an adverse pressure gradient on a wing for aircraft design?

An adverse pressure gradient on a wing is an important consideration in aircraft design, as it can significantly impact the performance and safety of the aircraft. Designers must carefully consider factors such as wing shape, airfoil design, and airflow management systems to minimize the effects of adverse pressure gradients. Additionally, the potential for adverse pressure gradients should be taken into account during the testing and certification process for new aircraft.

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