Understanding Rotor Downwash: The Ultimate Pilot Guide

Rotor Downwash: The topic that is either actively avoided by pilots, or assumed to be some dark magic that no puny mortal could possibly understand.

Unfortunately it’s not a topic that gets a lot of attention from sources that make it really easy to understand properly 💡

Well, that’s what we’re going to fix today. We’re going to keep things as simple as possible, while making sure we cover the most important items.

The goal here is to understand what exactly is going on with rotor downwash, without needing a fluid dynamics PhD! We’ll also cover the biggest threats of rotor downwash, and how to manage them! 🎯

We’ve made an active effort to simplify the material and research papers we consulted. There are a lot of grey areas within complex aerodynamic models that simply do not translate well in simple language. Please keep in mind that reality is more complex and nuanced than the models and equations we’ve used in this article to aid pilots with their overall downwash awareness.

We don’t publish all our Notes from the Cockpit (like this one) publicly, some are shared only by email. Get the next one sent straight to your inbox ⤵️

What is Rotor Downwash?

Rotor downwash is a (sometimes annoying) byproduct of a rotor disc generating thrust. It can cause a lot of headaches and needs active management by pilots during flight. Throughout this article, we’ll use this downwash definition:

How is this different than outwash? Outwash is simply the downwash after it has interacted with the ground surface.

(There is a formula to calculate outwash velocity, which you can find in the source material below. We’ve deliberately left this out of this article to try keep things as simple as possible!)

Because each individual blade needs a certain amount of airspeed in order to generate lift, the rotor disc needs to turn at a specific rotational speed.

This essentially creates a giant fan that sucks air through from above itself, and flushes it down towards the ground.

This can unfortunately cause a lot of issues for pilots, as it can have destructive effects on nearby bystanders, buildings, and make things fly when they shouldn’t…

So let’s talk about how fast things can get, and how rotor downwash behaves.

How Fast is Rotor Downwash?

So how fast does the air underneath the disc really get? We’re told as pilots that helicopters can generate winds of ‘hurricane’ speeds. But how true is this really?

How does Downwash Behave through a Rotor Disc?

Imagine a rotor disc floating in the air:

If we make it start spinning, each individual blade starts generating lift, which creates rotor thrust, acting vertically upwards (perpendicular to the rotor disc). As we mentioned before, the byproduct of this is the suction of air going through.

Let’s have a look what happens to the pressure and velocity of air going through the disc.

So, we have our rotating disc that accelerates air through it. We’ll call the pressure and velocity of the airflow directly below the disc P1 and V1:

If we go further underneath the disc however, air is still accelerating because of the added energy by the rotor disc.

Where Does Rotor Downwash Reach its Highest Speed?

Both wind tunnel testing and the equations we use to calculate this stuff prove that it takes roughly 3 rotor diameters for the air to reach its highest velocity (V2) and lowest pressure (P2):

Remember Bernoulli’s principle?

This is also the reason that the shape of the rotor downwash column becomes more narrow. The pressure has reduced because of the accelerated airflow, causing the stream-tube to get slightly narrower!

We can use Bernoulli’s principle to come to the conclusion that V2 is twice the speed of V1. (We won’t show you the proof here as it gets a bit lengthy, but we will include it in the resources page below.)

After reaching its highest point at roughly 3 rotor diameters below the disc, the air starts slowing down again, until it is considered ‘dissipated’ at around 10 rotor diameters below the disc. That is where the pressure and velocity of the air has returned to their initial value (Po and Vo):

That’s the full journey of an air particle going through our rotor disc! But how fast does it really get?

What Speed Does Rotor Downwash Reach?

So how fast does the air actually go within the rotor downwash column? To calculate any rotor downwash velocity for V1 (directly underneath the disc), we can use this equation:

g (gravitational constant) = 9.81

ρ (rho) = air density

The concept of weight divided by rotor disc area is called disc loading, which is part of this equation. High disc loading increases rotor downwash! So, let’s fill it in to give us an idea about what is going on! Let’s use an S92 (which is a pretty heavy helicopter. We’ll use these variables (rounded to keep things simple):

Mass: 12000 kg
Rotor Disc Diameter: 17 m

This gives us:

30 kts! (29 actually, but we’ll use 30 for simplicity).

Now remember what Bernoulli’s principle said about V2? V2 at roughly 3 rotor diameters underneath the disc, is twice the speed at V1. So, at about 3 rotor diameters underneath an S92 (3 x 17 = 51 m = 170 ft), the speed reaches almost 60 kts! Or an 11 (violent storm) on the Beaufort Wind Scale in terms of our hurricane winds!

Now, you might think “well, that’s an S92, surely a much smaller helicopter’s downwash is significantly lower than this right?”

Well, if you have a look at the equation we used, you can see mass is above the divide line, and rotor area underneath it.

This means that mass increases the speed, but the smaller the diameter of the rotor disc, the higher the rotor downwash becomes!

This means that while smaller helicopters have indeed a much lower mass, they also have a smaller rotor disc, which keeps the speed pretty high still. Let’s take a Cabri G2 for instance (figures are rounded again to keep it all simple):

Mass: 700 kg
Rotor Diameter: 7 m

If we fill in the equation:

So we’re still left with 17 kts, which means 34 kts at V2!

As a summary: if you want a lower downwash velocity, you need a helicopter with a low mass, and a massive rotor diameter. This doesn’t really exist.

The opposite does exist however: The V-22 Osprey.

The Osprey is known for its ridiculous downwash velocity, because the rotor diameter is only 12 meter, while its mass can go up to 24000 kg!

We’ll spare you the equation again, but this comes down to roughly 80 kts at the point of 3 rotor diameters below the disc (instead of the 60 kts for the S92)!

This is one of the reasons there was a recent incident in Cambridge, UK, where the Osprey completely destroyed a helipad that wasn’t designed for it:

So how does Rotor Downwash get influenced by the wind and other environmental factors? That’s what we’ll discuss next.

How does Wind Influence Rotor Downwash?

When we’re trying to increase our downwash awareness, we should take into account the wind. Wind will shift the column of air that we’re pushing down!

Imagine the same column we discussed earlier, but with a 30 kts crosswind. It will look something like this:

But how much will this column of air shift? Can we as pilots think of a rough estimate of how much the downward column of air will shift?

We can!

In a very simple way (please keep in mind that these figures are heavily rounded, to give an idea of what to think about).

If we take our S92 example again. We said the maximum rotor downwash was 60 kts, remember? Now obviously this isn’t the average speed of the downwash (the average is more like 45 kts). But we’ll use 60 kts anyway, as it helps with a simple rule of thumb later on.

Imagine the wind is 30 kts from the left during an approach into a tight spot during an air ambulance mission.

Most downwash starts to become an issue around 200 ft or lower. So, to understand how much this air column shifts across the ground from 200 ft, we need to know how long the air roughly takes to go from the disc to the ground.

Then we just apply the wind speed for that time period to get the amount of displacement. You following so far?

For a 60 kts downwash, it takes roughly 2 seconds to travel 200 ft from the disc to the ground.

This means that for a period of 2 seconds, the air gets pushed horizontally by the 30 kts wind!

Now, 30 kts is roughly 15 meters per second, so:

Distance = Speed x Time = 15 x 2 = 30 meters

So if you want to remember all of this in an easy way:

At 200 ft: Your downwash column displacement in meters, is at least the wind speed in kts.

Meaning: 20 kts of wind would result in a 20 m displacement

At 100 ft: Your downwash column displacement in meters, is at least half the wind speed in kts. (because the air then only takes 1 second instead of 2 seconds to reach the ground).

Meaning: 20 kts of wind would result in a 10 m displacement

Here are the assumptions we made, and why this is only a ROUGH estimate:

• This is based on a 60 kts downwash, remember that most downwash is slower and therefore the amount of displacement is larger
• This example is assuming the hover. If you’re on an approach, downwash speed is lower due to the lower power setting (and therefore lower rotor thrust)
• The average downwash speed is usually less than 60 kts (the average between V1 and V2)
• Rounding errors for simplicity

What are the Threats of Rotor Downwash?

The UK CAA has revealed that the amount of safety reports related to downwash have increased recently. It takes a pro-active attitude to identify threats before they happen. Downwash threats are no different. What are the main threats here?

Foreign Object Debris (FOD)

FOD that gets blown around can create harm very quickly as well. Sand, dust, parasols, plastic bags, basically anything that weighs less than a couple of kilograms, depending on the shape.

Wind-catching items such as trampolines, parasols, kites or anything else with a large surface are an even bigger threat to harm bystanders once they start flying around.

But the other way FOD can cause problems is when it gets sucked into the rotor disc. Even small and soft objects hitting a blade can cause more damage than you might initially think, depending on the circumstances.

Brownout / Whiteout

Sand, snow, and dust all visualise the rotor downwash circulation really well, which looks pretty right? Well yes, but not so great if you care about visibility.

It’s sometimes hard to see if a surface is dusty or simply grey or beige in colour. Pay attention around the 200 ft mark during the approach and try to detect movement of particles on the surface.

Structural Damage

FOD that gets blown around can cause damage to property, buildings and vehicles. If you’re flying over fragile structures, try to make an effort to keep as much separation as possible.

Injury to Bystanders

CASA have published wind speeds limits on certain types of people and personnel, which illustrate just how much helicopter downwash exceeds some of these (remember that 1 kt is about 1.8 km/h):

FOD can cause injuries, car doors to fold onto people, people get distracted by helicopters while driving anyway, but downwash makes it often even more chaotic.

Downwash Affecting Cars and Drivers

Cars can start rolling if the handbrake isn’t on when downwash gets stronger. Car doors can become hazards for people exiting or entering their cars as well, as they can catch a lot of wind.

On top of this, flying straight over a road during a take off or landing can cause drivers to feel the influence of the downwash as well, like this video showed during a F1 race in Silverstone, UK:

Downwash Funnelling

If you land next to a street surrounded by high buildings, your downwash can create a windtunnel that makes it even trickier to manage safely.

If you’re landing on a slope, be aware that your downwash will travel downslope and affect the surroundings there more than the surroundings upslope.

How to Manage Rotor Downwash Threats as a Pilot

So knowing all of this so far about Rotor Downwash, what can we do as pilots to manage downwash threats in the best possible way? Threat Error Management (TEM) is a huge element of being a pilot, we went deeper into TEM here if you want to read more.

Be Aware of your Downwash Profile

Like many things in aviation, this also starts with basic awareness. Ask yourself these 5 questions:

1) How heavy is my aircraft compared to most helicopters?

2) How heavy is my aircraft on this particular day?

(Heavier = more downwash)

3) How large is my helicopter’s disc area?

(Larger = less downwash, and a larger height for maximum downwash velocity on the ground)

4) Am I taking off or landing?

The approach phase has less downwash than the take off phase, due to the lower amount of thrust that needs to be generated by the disc.

5) Where is the wind coming from?

As we discussed earlier, your downwash column will shift by quite a lot depending on your height, downwash speed, and wind strength. Being aware of the direction and amount of expected displacement can help build a mental image of your downwash profile.

Plan for an Approach that Mitigates Downwash Effects

If you notice some threats in the approach undershoot, like loose objects or any of the earlier discussed things, consider flying a steeper profile or keeping your height up to a later phase in the approach.

If this isn’t an option (because of PC1 requirements for instance), you could elect to accept more crosswind in return for a more favourable approach direction that impacts less objects and surroundings.

Make sure you check your company helipad plate (if available) and conduct a thorough approach briefing.

To manage white out and brownout, aim for a zero/zero approach, or make sure you stay ahead of the recirculation until you have safely touched the ground. Keeping visual references is the highest priority, and if you no longer feel comfortable with the visibility, go around!

Secure the Landing Area

This one is often not in your control, depending on the type or operation you fly.

However, things like hospital helipad security, fire & rescue crew at elevated helipads, local police, or ground crew can often assist with securing a landing site.

If you’re not confident the biggest threats can be mitigated, it might be time for plan B – depending on the situation.

Rotor Downwash Accidents

Search and Rescue S92 Rotor Downwash Accident, United Kingdom (2022)

Fire Extinguisher Cover Fenestron FOD (2022)

Departing helicopter downwash causes arriving helicopter rotor blade to strike it’s fuselage (2013)

Australian Transport Safety Bureau: Downwash incidents at hospital helicopter landing sites

Rotor Downwash Resources

CAP2576: Understanding the downwash/outwash characteristics of eVTOL aircraft

UK CAA: Helicopter Quarterly Safety Report Mentioning Downwash

Investigation of the helicopter downwash effect on pedestrian comfort

Federal Aviation Administration: Aerodynamics of Flight

Conclusion

We made this guide to help demystify the topic of rotor downwash for pilots, while including the main scientific theories and principles that explain why things are the way they are.

Being aware of the dynamics of downwash, its speed, and the influence of environmental factors, we can make informed decisions to manage downwash threats effectively.

Recognising potential hazards like foreign object debris, brownouts, and structural damage, is a crucial element of flying a helicopter safely.

Have you got personal experience, stories, or other things you wish to share about rotor downwash? Let us know!

We don’t publish all our Notes from the Cockpit (like this one) publicly, some are shared only by email. Get the next one sent straight to your inbox ⤵️