Calculating force of impact and maximum moment

[rspicer]

We have the following problem we're trying to solve:

A helicopter is landing at 6 ft/sec, with its nose tilted up 15 degrees. We're designing a bumper that sits under the tail that has to absorb enough energy to reduce the maximum moment experienced by the tail to 350,000 in. lbs. Our bumper is sitting 338 inches behind the helicopter's center of gravity.

To absorb the energy, we have a crushable aluminum material that is a cylinder 7 inches long, with radius 3.25 in. We need to pick the exact material by finding the pressure (in psi) that the material should crush at.

We started by finding the initial KE of the helicopter, which is 1/2mv^2=1/2(26,500lb)(72 in/sec)^2 = 68,688,000 lb in^2. There must be a way to use the maximum moment to find out how much energy our bumper needs to absorb, and from there since W=Fxd, F=W/d=W/7 in. What are we missing here?

Thanks

 

[LightHero]

for your help!We need to use the conservation of energy. The initial kinetic energy of the helicopter must be equal to the energy absorbed by the bumper. We can calculate the energy absorbed by the bumper as the product of the 350,000 in. lbs. and the distance (338 in.) it moves before coming to a stop. This gives us an absorbing energy of 117,300,000 lb in^2. Now that we know the energy that needs to be absorbed, we can calculate the pressure needed to crush the material by dividing the energy by the volume of the material: 117,300,000 lb in^2/((3.25in)^2π7in)= 2,796.6 psi.