Helicopter Prop Wash 3

[SperlingPE]

I am designing a slab on grade heli pad for a hospital. The hospital wants a fence around the pad to defect or direct the prop wash from the helicopters.

The fence will most likely be a louvered panel to direct the prop wash downward.

Any idea on what kind of force prop wash can generate? The posts for the fence will be embeded in concrete. The force used will help determine the design of the fence panel and post spacing.

 

[MiketheEngineer]

On takeoff and landing - might be fairly high. Go measure one at your local FBO

 

[rb1957]

why direct the downwash away from the heli-pad ... it's going outwards as it is. the worst thing you can do is put an obstruction in the way out it, this can affect the helicopter performance (trapping a pocket of air under the helicopter).

build a guard rail by all means. if you're worried about downwash, build a porous platform (using mesh) to allow the downwash to escape to below the heli-pad and away from people.

 

[SperlingPE]

A little clarification.
The pad is 44' square
The fence will be 33'-0" away from the edge of the helipad. The fence will be louvered (allow air to flow through) to direct the air flow down and to block debris.

 

[InDepth]

This may not answer the load, but it could be useful. I would recommend contacting a manufacturer to see if they have any downwash load data and if this sort of design actually affects helicopter stability.

http://www.globalsecurity.org/military/library/policy/army/fm/5-430-00-2/Ch13.htm

http://www.globalsecurity.org/military/library/policy/army/fm/5-430-00-2/index.html

 

[Compositepro]

The total load of the prop wash is equal to the weight of the helicopter.

 

[InDepth]

Compositepro, that's a good way to think about it. Following along your train of thought, wouldn't it be the force developed by the mass of the helicopter multiplied by the maximum acceleration that can be developed by a helicopter on takeoff? or if they move forward a larger directional wash?

 

[MikeHalloran]

If the pad is on the roof of a tall building, you might indeed want to direct the airflow down. If the pad is on or near the ground, deflecting the air down will just stir up a lot of dust, which is not good for the helicopter's engine(s), or people's lungs, or the hospital's housekeeping. Better to direct it sort of upward.

The porous pad could cause rough landings or worse. Helicopters operate 'in ground effect' near the ground, and rely on the added lift due to compression of the air under the machine to cushion landings and to provide extra capacity for heavy takeoffs. Pilots will hate you for taking that away, especially if you do it without warning them of the hazard.

If you want to block debris kicked up by the wash, two things occur:
FOD is a serious hazard to helicopter operations, especially to the helicopters themselves, specifically the engines, which will inhale anything, and not quite digest it before dying. The pad should be policed often. I.e., it should be someone's _job_ to make sure that there _is_ no debris.

If you want to block debris that appears anyway, perhaps a fence with staggered vertical staves would be effective at stopping debris without seriously impeding the outward airflow or inducing rotational flows.

Come to think of it, there must be a design standard or recommendations for this kind of stuff. Maybe usace.army.mil has something.

<later> Nope. USACE probably has lots of stuff about pavement, but it's a big haystack, and I must have wandered into the pavement department.

However, the FAA has quite a lot to say.
Google "heliport design guide", and you'll find a bunch.




Mike Halloran
Pembroke Pines, FL, USA

 

[StructGen]

Let me add this to InDepth's statment. It will be equal to mass x (g + upward accelration)

Further when helicopter moves forward only a small amout of wash is directed towards the back, most of the wash is still down wards.

 

[SperlingPE]

Let stay on track here.
There is no mention of a porous pad.
Air flow will be allowed through the fence (saee louvered).
I am not in charge of policing the pad for FOD.
This pad is for a hospital. As such the ER people will be near by when the helicopter lands. Also there are other structures and objets that the hospital wants to protect from blow around debris. The fence will be over 60' from the center point of the pad.

Thanks for the input on the total force of the prop wash and for the links.

 

[InDepth]

Here are the UFC links. They also have pages with related documents. There are chapters on offset requirements from existing buildings.....didn't see anything in the Structural load data. It may be worth it to call a helicopter manufacturer and they may lead you down the right path.

UFC 3-260-01 Airfield and Heliport Planning and Design (11-17-2008)

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_260_01.pdf

UFC 3-260-05A Marking of Army Airfield Heliport Operational and Maintenance Facilities

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_260_05a.pdf

UFC 3-310-01 Structural Load Data, with Change 2

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_310_01.pdf

UFC 3-310-02A Structural Design Criteria for Buildings (03-01-2005)

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_310_02a.pdf

UFC 4-141-10N Design: Aviation Operation and Support Facilities (01-16-2004)

http://www.wbdg.org/ccb/DOD/UFC/ufc_4_141_10n.pdf

http://www.wbdg.org/ccb/NAVFAC/DMMHNAV/1013_10.pdf

 

[racookpe1978]

A very important secondary effect is to account for the transition parts of the landing: The chopper (pusing down at the full-loaded weight/area flight path - NOT just the small landing pad! - of the chopper/area of the blades.

So, you need to account for the full access paths from edge of property to middle of pad as the chopper is coming in from ANY allowed flight path: this direction will vary according to the winds each days/night/Sunday and Monday.

Sure, the "deflection of downwash" is easiest to solve when the chopper is pushing down dead center of the pad: but that's NOT where the chopper is as it lands.

FOD (foreign object damage) IS your responsiblity as a designing engineer. Maybe not as the "chief-in-charge-of-every-shift sweeping-and-inspecting" but you MUST plan for preventing FOD and debris clouds from blowing up: or he pilot is blinded from dust and gravel as he comes in across a dirty graveled and tarpaper roof to the pad. His death IS your responsibility then.

 

[racookpe1978]

It will be more clear if I described the pressure better:

(Max loaded Weight of biggest design load chopper)/(area of blades of that chopper) x acceleration of chopper = max expected load per square area (down, under normal conditions.) Then, add your safety factors.

Wind load (sideways!) = that maximum expected loaded weight (total maximum weight times the acceleration factor) / (area of the wind going sideways away from under the prop elevation, PLUS anyways existing maximum wind conditionos at the time of the landing. (8, 10, 15, 20 mph wind from the northwest, for example. Obviously, no sane chopper pilot is going to try to land in a 70 mph storm. Probably not, at least. Then again, no chopper pilot is sane anyway, so that might not be a good restriction.)

That is: Weight of the chopper = wind load down.

All of that wind load "down" is going to be released by the wind load sideways of the air escaping between the rotor blades and the ground. This sideways wnd load load will be at the maximum at the least elevation of the chopper blades. As the chopper goes further up, more height between the blades and the ground, less wind load sideways.

Assume a 20,000 lb chopper with a 30 foot blade diameter 10 feet above the ground, and a 1.5 safety factor.

30 ft blade dia = 700 sq ft ground area.
20000/700 = 28.5 lbs/ft square down x 1.5 = 42.5 psf live load. Down. Anywhere between the edge of the building and the center of the pad. From any "allowed" approach direction. If there's going to be a chimney or wind stock or radio tower in the way, then the chopper can't come in that direction, so there won't be a live load from that direction.

10 foot blade height at a 30 ft diameter = 942 sq ft "wind escape area" below the blades.
20000/942 x 1.5 = 2`.8 psf sideways force - at the edge of the blade diameter. Force will go down the further you get from the blade edge - IF the chopper isn't coming down over you. All the way around the pad.