How can I calculate the diameter and strength of a helicopter's downdraft?
September 23, 2012 3:36 AM   Subscribe

How low can a helicopter fly before people on the ground are affected by its downdraft? Is there a standard way of calculating downdraft strength and diameter at various heights?

I am involved in an event which might be filmed by TV news choppers. For safety reasons, it is important that any downdraft felt by the participants below be no stronger than a light breeze. Ideally, they should feel no downdraft at all. I would like to give clear instructions to media organisations requesting that they keep their helicopters a safe distance away from the event. How can I calculate what that distance should be?

(I don't know the precise helicopter models involved, but let's assume we're dealing with small to medium commercial craft).
posted by embrangled to Travel & Transportation (7 answers total)
Best answer: I think you need to worry less about downdraft than about the FAA minimum altitude laws. Google "FAA helicopter minimum altitude" and you'll see that most likely the lowest they'll be able to go is 1,000 feet. There will be no noticeable downdraft at that height. TV News helos should be aware of this. This assumes that the TV crews wouldn't have to follow some commercial FAA rules.
posted by Farce_First at 4:25 AM on September 23, 2012

The 1000 foot rule applies to fixed wing aircraft; helicopters have special rules, see FAR 91.119 d.

I don't have the answer for you, embrangled, but can you communicate to the media helicopters your safety requirements? That pilot is constantly working on maintaining a safe altitude and if you have a rotor wash safety concern they should be able to accomodate you without you doing a bunch of math for them. In particular the pilots should already be very sensitive to rotor wash stirring up debris on the ground.
posted by Nelson at 8:10 AM on September 23, 2012

Yeah, I'd be far less worried about wind than noise. I mean sure, a bunch of air gets blown around, but helicopters close enough that you have wind issues are pretty loud. Even circling at 500', if you have special communications requirements (director talking to tech folks) or other audio issues (PA system), those are far more of a concern than the wind.
posted by straw at 8:50 AM on September 23, 2012

requesting that they keep their helicopters a safe distance away from the event.

The pilot's #1 concern at all times is maintaining an altitude and position that is safe from obstructions, seen or unseen. This is almost invariably going to be several hundred feet, which is far too high for any downwash effects to be felt on the ground.

If you pay attention to that mythbusters episode, they actually show that the downwash is significant near the edge of the rotors, but less significant when right up next to the helicopter body. You can also plainly see the force on Tory and Grant's clothing, even though they're not underneath the helicopter (the downwash is being blown radially outwards by the ground below the helicopter).
posted by kiltedtaco at 9:44 AM on September 23, 2012

Helicopter pilot and safety officer here.

If the helicopters are more than 100' over the ground, no one will feel anything more than a light breeze. You may be able to work with media in order to keep them away, but if the cooperation isn't there then you're SOL - they have as much of a right to be overhead as you do. (I'm assuming there isn't a temporary flight restriction or other complicating factor associated with this event.)

For comparison, here's some video of a medium helicopter taking off from a ship. Notice that the wind noise diminishes considerably as the helicopter moves off of the deck and the sleeves of the enlisted member launching the helicopter stop flapping as well.
posted by squorch at 1:03 PM on September 23, 2012 [2 favorites]

The standard heuristic for ground effect is one rotor diameter for helicopters, and one wingspan for fixed-wing aircraft.
posted by phliar at 3:43 PM on September 24, 2012

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