Expert: Paul Soderman - 3/20/2009

Question
QUESTION: Hello: In reference to a recent tragedy involving a Sikorsky S-92 off Canada's east coast which cost 17 lives,I have been wondering if helicopters could be fitted with some sort of flotation system to prevent the aircraft from sinking if it has to ditch? From the news reports of this latest accident,I gather that the craft sank soon after crash landing,and escape was therefore impossible.I envision some sort of crash triggered airbag system,or foam built into hollow spaces in the airframe.
Thanks

ANSWER: Yes, just like an airplane, a helicopter could have flotation system.  But unlike airplanes, helicopters have to work very hard to stay in the air so the extra weight would be a real penalty in speed and range.  You also cannot be sure that a crash would be soft enough not to destroy the flotation system. Or the helicopter might crash at some abnormal angle.  For these reasons, I doubt the operators would put up with a flotation system.

While I was writing this I thought there might be some special cases where a floatable helicopter might be desirable such as for short hops to lakes or rivers.  But my second thought is that the downwash from a helicopter would be so strong that the water spray would be hazardous to the ship and visibility.  Helicopters and water don't mix well.

Paul

---------- FOLLOW-UP ----------

QUESTION: Hello,Paul: Thanks for your prompt answer! I am not an aeronautical engineer,but wouldn't it be feasible to fill cavities in a copter with foam,which doesn't weigh very much? Maybe this could be factory installed for ocean going variants? It was suggested in the news that the crash I referred to was survivable,as the passengers were in survival suits,but couldn't get out before the machine sank. I don't understand your second last sentence about "water spray".
thanks again!

Answer
Foam isn't as good as an empty sealed cavity, though I guess it could displace water from an unsealed cavity at the cost of foam weight.  To stay afloat, a helicopter must have buoyancy equal to weight which is also equal to the weight of water displaced.  Helicopters can weigh 5000 to 50,000 pounds.  Consider a 20,000 lb helicopter.  Sea water weighs 64 lb/ft^3.  So the sealed or foam cavities must displace 312 cubic feet of water.  That is, you need 312 cubic feet of cavities in the helicopter under water (not counting the unsealed structure which also displaces water).  The fuselage could provide that volume, but it is not water tight -  especially in a crash.  The landing struts aren't big enough so you have to attach pontoons under the machine, but we are back to the problem of extra weight.  It sounds impractical to me.

Water spray on landing comes from the fact that helicopters generate huge downwash which is how they get lift.  The downwash can kick up dust and dirt on land that is very dangerous because the pilot can lose visibility at a crucial time.  Many machines have been lost that way.  The same could happen in a water landing.

Paul