Landing forces at aircraft touchdownequations?

[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..!

Many thanks in advance for any help you can give!

 

[anorlunda]

ping @jrmichler

 

[jrmichler]

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: .