Impact forces of vertical landing

[alin09]

How to calculate the impact force when an lighter-than-air vehicle performs Vertical Landing? Actually, the vehicle lands vertically on the ground using air cushions. I tried to establish a speed corresponding to the impact forces. The impact forces are obtained from max ground reaction force the vehicle structure can withstand.

Besdies the simple energy equation: 1/2MV^2=Fd, are there other ways of calculating the speed?

Thanks!
Lin

 

[rb1957]

'cept that you're ignoring drag.

getting the "landing" sink rate is easy, determining the impact force requires assuming the time interval of the impact.

Quando Omni Flunkus Moritati

 

[alin09]

Thanks for the reply. Yes, drag is ignored since the speed is very small. I used static analysis of FE modle to acceleration or impact force that the vehicle can stand. Landing cushion is not in the FE model. Then, according to energy equation 1/2MV^2=Fd=M*a*d, I can calculate a speed with the acceleration and landing cushion deflection. Not sure if this simple way is correct or not.

 

[rb1957]

i'm not sure that ignoring drag is reasonable ... if the sink rate is low, then everything is small, so neglecting one small effect might not be correct (it'd be different if the other forces were large).

one thing (other than drag) is the stiffness of the air cushion (ie the air cushion will absorb some of the KE.

i guess your initial conditions are just before the cushion starts to deflect (ie close to the ground).

Quando Omni Flunkus Moritati

 

[GregLocock]

You need to know the compliance/damping characteristic of your air cushion.

Cheers

Greg Locock


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[alin09]

I don't analyze the descent condition. The FE model is used for analyze the state when the vehicle touch the ground with a constant speed and also it is a static analysis. In the model, the vehicle is given an inertia loads due to the negative acceleration (speed reduces from V to zero).

My question is how accurate if I use acceleration and cushion deflection to calculate vehicle speed 1/2MV^2=Fd=M*a*d? I have asssumed that the acceleration is constant though it is not. The time used for landing might be just around one second. The vehicle landing speed is around 0.5~2mph.

 

[WKTaylor]

Do we have any forum members with LTA eperience?

You have another element to consider: unless this is a rigid/semi-rigid airship, a blimp-like lifting bag [or cells] will have some flexibility. This means the entire airship envelope will be elastic, so total deceleration forces thru the cabin to the bag may occur over several seconds as everything smushes, then rebounds. Perhaps.

Might try making a 1/5--1/10th scale flying model: could answer some of these questions. Perhaps.

Suggest You search Google for "lighter than air vehicle design"... lots of interesting hits.

Regards, Wil Taylor

Trust - But Verify!

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[Sparweb]

Hi Alin,
The bad news is that your problem is best stated and solved with a differential equation. The good news is that DE's are fun to learn!

If you're willing to give it a go, you will find that the solution to the impact-absorption problem is a classic example of a DE. In my textbook it was in chapter 2.


STF

 

[rb1957]

"My question is how accurate if I use acceleration and cushion deflection to calculate vehicle speed 1/2MV^2=Fd=M*a*d?" ...

my answer ... not very (IMHO) ... but if v is 2mph, and t = 1sec then maybe it acceptable for your purposes. but i have to ask why an FEA of the cushion ? why build a sophisticated analysis (a palace) on such rough assumptions (in a swamp) ?

Quando Omni Flunkus Moritati

 

[IRstuff]

?? If it's an LTA, why is there that much landing force at all. Most dirigibles have a hard time reaching the ground and have to be dragged down.

I would think that any forces would be less than that of a 5-ft drop

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[Compositepro]

I must be reading a different thread. How did we go from an air-cushion to lighter than air? The physics involved is covered in course 1A. We just haven't been told anything about the characteristics of the air-cushion. It will bounce if it behaves as a simple spring. It will be designed to vent if bouncing is to be prevented. F=ma, E=Fd, and E=1/2mvv all apply.

 

[Sparweb]

compositepro
I must be reading a different thread. How did we go from an air-cushion to lighter than air?

Yes, you are. Try re-reading the first two sentences of the OP, and go from there. Nothing wrong with anything else you wrote.

STF

 

[Compositepro]

This is why I try to stay humble. Thanks.

 

[WKTaylor]

IF the point of this design is to land on mucky/marshy surfaces, why not use air-inflated pontoons

Regards, Wil Taylor

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For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[KENAT]

Is this the concept I saw a while back where the air cusion would actually be run in reverse to hold the aircraft down?

If so then the 'suction' force from the 'air cushion' may need to be taken into account.

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[alin09]

Everyone, thanks for the input!
The vehicle is different than a conventional non-rigid airship. The vehicle is equipped with air landing cushions for vertical landing. Suction force can not be applied before the vehicle fully lands on ground. The cushion pressure can be controlled to avoid "bouncing".

rb1957, I thought about building a FE model for cushion, but I don't have the software for this kind of nonlinear analysis/large deforamtion analysis/transient analysis. Try to figure out a test plan to obtain this acceleration and deflection.