Landing forces at aircraft touchdownequations?

• tac_r
In summary, the conversation discusses the use of fracture mechanics in investigating the forces on an aircraft structural component during landing. The equation for the effective weight in the FAA's Federal Aviation Regulations is mentioned, as well as the importance of drop tests in testing the shock absorbing capabilities of landing gear. The conversation also touches on the horizontal and sideways forces that landing gear experiences during landing, and the importance of airspeed and rate of descent. The concept of total energy at touchdown is also mentioned, but deemed less important than other factors.
tac_r
Hello All,

My first post here - I wonder if anyone can help?

I am attempting to apply fracture mechanics to an investigation of an aircraft structural component, and am having difficulty finding a suitable method of adequately calculating the total applied stress (hence 'landing forces...') in the component.

From what I have read, I was intending to use the equation for the 'effective weight' listed in the FAA's Federal Aviation Regulations, namely:

We = W [ h+(1-L)d / h+d ]

where:
We=the effective weight to be used in the drop test (lbs.);
h =specified free drop height (inches); h = 3.6( W/S )1/2
d =deflection under impact of the tire (at the approved inflation pressure) plus the vertical component of the axle travel relative to the drop mass (inches);
W=WM for main gear units (lbs), equal to the static weight on that unit with the airplane in the level attitude (with the nose wheel clear in the case of nose wheel type airplanes);
L= the ratio of the assumed wing lift to the airplane weight, but not more than 0.667.

Is this being too simplistic? Please forgive me if it is; I am a Licenced Aircaft Maintnenance Engineer, and some of this graduate stuff is pushing my boundaries..!

The situation being studied/analyised is the forces acting on the component mounting the main undercarriage legs in the wing of a tail-wheel configuration aircraft, landing in the level attitude, with the legs arranged at 78 degrees to the aircraft horizontal datum and 21.55 inches forward of the C of G (aircraft weight 1583lb). I have all of the airfoil date and lift coefficient so can work out that aspect.

I have seen another equation giving the Total Energy (E) (K.E and P.E.) at touchdown but am unsure again how this relates to the force I am Looking for.

Apologies again if this is all basic stuff, but I'm struggling to get a sensible answer..!

No, it's not too simplistic. Drop tests are a standard method for testing the shock absorbing capabilities of aircraft landing gear. The equations come from the fact that aircraft with higher wing loadings land faster and at a higher rate of descent. Drop tests are also much safer than trying to test landing gear by deliberately making hard landings.

Keep in mind that landing gear also has horizontal loads. There is a rearward force when the tires contact the runway, followed by braking forces. There are sideways forces when the aircraft is not perfectly aligned with the runway at touchdown.

The total energy at touchdown is not important, while the airspeed and rate of descent are important. The kinetic energy at touchdown is important when designing the brakes.

And, finally, here's a video of a too-hard landing: .

anorlunda

1. What are the main factors that influence landing forces during aircraft touchdown?

The main factors that influence landing forces include the weight and speed of the aircraft, the type and condition of the landing gear, the angle of approach, and the surface conditions of the runway.

2. How is the landing force at aircraft touchdown calculated?

The landing force at aircraft touchdown is calculated using the aircraft's weight, touchdown speed, and the impact factor, which takes into account the angle of approach and the condition of the landing gear. This is typically represented by the equation F = m x V x I, where F is the landing force, m is the aircraft's mass, V is the touchdown speed, and I is the impact factor.

3. Why is it important to accurately calculate landing forces during aircraft touchdown?

Accurate calculation of landing forces is crucial for ensuring the safety of the aircraft and its passengers. If the landing force is too high, it can cause damage to the aircraft's structure and landing gear, and potentially lead to a crash. On the other hand, if the landing force is too low, it can result in a hard landing, causing discomfort and potential injuries to passengers.

4. Are there any other factors that can affect landing forces during aircraft touchdown?

Yes, there are other factors that can affect landing forces, such as wind conditions, the runway slope, and the pilot's technique. Crosswinds, for example, can cause the aircraft to drift off course, leading to an uneven touchdown and potentially higher landing forces.

5. How can landing forces be minimized during aircraft touchdown?

Landing forces can be minimized by ensuring that the aircraft is properly maintained, the landing gear is in good condition, and the pilot has proper training and technique. In addition, choosing the appropriate runway based on wind and surface conditions can also help reduce landing forces. Some newer aircraft also have advanced landing systems that can adjust the touchdown angle and speed to minimize the impact force.

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