# Helicopter rotorwash

At ground level what is the wind speed of the rotorwash of a Blackhawk helicopter engaging in a fast rope insertion? I’m assuming that the helicopter would be about 50 feet above ground level but if anyone has better knowledge of fast roping please correct the height.

Not sure how acurate, but they seem to work on models for this.

At about 50 feet, it think they are saying it would be about 100 km/hr

Wow, that seems like a lot. Having fast roped from CH-46 and CH-53, I don’t remember it being that strong. But that was a long time ago.

Since you already have a good answer, may I ask if you are in Africa or Europe, and how large is the coconut you are carrying?

You can coarsely estimate it from fundamental fluid mechanics. From Blackhaw specs, the main rotor area is 210 square meters, and let’s assume it’s at the max takeoff weight of 9979 kg. That works out to a main rotor pressure of 466 Pa. That’s a small enough pressure change (in the context of an atmospheric pressure of 101,325 Pa) to regard the air as being constant density, so we can use Bernoulli’s principle to calculate the downwash velocity. Waddya know, I came up with 100.3 KPH, which agrees closely with what @SoToasty estimated from their reference, although the reference shows that there’s a lot of variation depending on whether you’re being shielded from the downwash by the “shadow” of the helicopter, or whether the tail rotor is redirecting some of the downwash away from you.

The higher the heli is, the more the downwash plume gets mixed and diffused by surrounding air, resulting in a broader area of ground being impacted by a lower velocity flow. But given that the Blackhawk’s rotor is 50 feet in diameter, an altitude of 50 feet (as in the OP’s case ) is practically nothing.

I have to say I’m impressed with myself. My WAG to the person who asked me was about 50 knots. If it is 100 kph that would be 54 knots.

Good work, Mr Helo dude! I’m impressed with yourself too.

Been a lot of years since I did that but my WAG was not far from yours. Once can stand in the wash under a hover, but it’s a PITA and crap is flying around like mad. Makes me thing tropical storm force winds. Not too violent to stand up, but too violent to be anything close to comfortable.

I got dropped on a cable from the belly of a CH 47 (SAR training). The downwash was significant. Lets just say, don’t wear a hat.