Rotor Wash Force

[dakota99]

Does anyone here have any experience calculating the force of rotor wash from a helicopter on to the side of a building that is adjacent to a landing pad? I would think I should be able to calculate the vertical force by taking the weight of the helicopter times an acceleration factor and dividing it by the area of the circle created by the blades. I am just not sure how to determine the horizontal thrust that would be projected on to the side of the building. Once determined, would I add that to the full 90 mph wind load or something less that would be more realistic for a helicopter to be landing in? I have paged through the ASCE 7 and have not found any guide lines.

 

[ishvaaag]

This goes on decks.

http://www.hse.gov.uk/research/otopdf/2001/oto01072.pdf

http://www.hse.gov.uk/offshore/guidance/helideck.pdf

http://www.imt.eu/industrial_lighting_downloads/english/CAP437.pdf

Respect loads on close façades maybe a CFD study for the configuration could be a start for the thing; a watered down pressure from the downwards exerted pressure; however always dynamic and fatigue detail.

 

[SteelPE]

I don't know where you are located, but heliports and helistops are covered in section 1605.4 of the IBC 2009. They don't really make any reference to wind loads on the side of the building so I'm not sure it would be a concern.

Is this for a hospital? I find it hard to believe that they will be landing a helicopter on top of a building during a full 90mph wind event unless it there was an emergency. Maybe this is why there is no mention of the wind loads in IBC 2009.

 

[MiketheEngineer]

You CAN'T land a helicopter in 90 mph winds -- heck - some of them won't even go 90 mph!!

You have to check w/ the mfgs but I am guessing 30 mph would be the upper limit.

 

[racookpe1978]

The force sideways out of the rotor wash will be greatest at the lowest helicopter height (just as the copter is taking off at its maximum instantaneous vertical acceleration.)

So, all of the air going down (like you wrote) is then forced to go sideways out of the cylinder defined by the rotor diameter and the height of the rotor when the copter is just touching the pad. Divide that net sideways air force by the portion of the cylinder blocked by the building wall at the radius of the building wall from the cylinder of downward moving air..

Add that net force to the maximum wind you are going to design for: the pilot will want zero. Emergency circumstances will (sooner or later) force him to land in a 20 or 30 knot wind - but I don't know the real limit.

 

[msquared48]

Even 20 or 30 knot winds can get real hairy when landing with channeling and eddying effects, particularly from adjacent structures.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[a2mfk]

I'd be shocked if the lateral pressure from a helicopter would exceed that of a 90mph wind event, especially give the minimum clearance to any vertical element of a building.

But my former marine coworker who has jumped out of a few choppers said the bigger ones the military for transporting troops and trucks uses can push you over because of the lateral pressure.

Reminds me a of a great Mythbusters where they made a school bus go airborne in back of a 747, but that is a completely different beast.

 

[bradleyelwood]

dakota99,

The disc loading of a Sikorsky Skycrane is about 47,000 pounds divided by its 72 ft diameter disc, or about 11.5 pounds per square ft. A horizontal component of wind occurs as the helicopter lands or takes off and it is highest near the edge of the rotor disc (when the helicopter is close to the building). Its magnitude is slightly less than the vertical value. Having worked around Skycranes, the value is roughly 70 mph horizontal and can have a pulsating effect to the force, shaking things like pickup trucks. The rotor wash can move loose things like 4X4 lumber, outhouses etc.

Smaller helicopters create less rotorwash than the Skycrane, and if you need to get the value, just divide the helicopter gross weight by its rotor disc area.

If you design for normal wind loading on structures, the helicopter rotor wash is probably smaller than UBC values.

 

[msquared48]

Sounds like moving the outhouse might have been a personal experience? Should have been in a scene from MASH...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask