Pushing Beyond Vne? Physics Will Push Back – Here’s Why

If you talk to someone with fixed wing experience about the effects of airspeed on a helicopter, you will get looked at like you’re from a different planet 👽

That’s because most fixed-wing aircraft are inherently stable. Helicopters? Not even close.

This means most answers to aerodynamics-related questions just create more questions 🤷🏽

Here’s the thing: airspeed doesn’t hit the whole rotor disc evenly. The faster you go, the weirder things get.

But why that number? Why is it 140 knots on one aircraft, 167 on another, and 120 on a third? Where does Vne come from, and what are the actual reasons we can’t go faster? 💡

Let’s break it down!

Vne is based on several very real, very physical limitations that start kicking in as helicopters go faster. None of them are things you want to mess with in flight…

Let’s take look 👀

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What is Dissymmetry of Lift?

The most fundamental reason airspeed limts are slightly more complicated for helicopters compared to planes is dissymmetry of lift.

Unfortunately, a change in forward airspeed does not affect most of the aircraft equally. Helicopters have a giant rotating disc attached to them, which complicates things a little.

To start, imagine a counter-clockwise rotating disc in still air with a tip speed of 400 kts:

If we move forwards, the disc can be divided into the advancing side (blades move into the airflow across the disc) and the retreating side (blades move with the airflow across the disc):

Then, let’s say we increase forward airspeed to 50 kts. This affects each side differently due to the blade travel direction.

We add the 50 kts to the airspeed of the advancing side, and subtract it from the airspeed of the retreating side:

If we just left the disc like this, the advancing side would constantly generate more lift than the retreating side, and the entire helicopter would consistently change bank and pitch, i.e dissymmetry of lift!

Let’s talk about what we can do about this, and how this affects forward airspeed limits.

What is Flapping to Equality?

To counteract dissymmetry of lift, the blades flap (or sometimes slightly bend) up and down.

Flapping is the vertical movement of a blade relative to the plane of rotation (the rotor disc).

The blade on the retreating side flaps down, which increases the angle of attack:

Why does this happen?

Well, a blade with less airspeed generates less lift, which results in the blade flapping down. When it flaps down it has more air hitting it from below, which increases the angle of attack, compensating for the reduction in airspeed.

The opposite is true for the advancing side. The blade flaps up due to an increase in airspeed, reducing the angle of attack, which compensates for the airspeed increase.

Do you remember the lift equation? We’re using TAS and CL to equalise lift across the entire disc:

With these two variables compensating each other, we keep the disc level and ready to be used by pilot inputs without all these effects ruining our day. Tadaa, we have “fixed” with dissymmetry of lift!

This technique is called flapping to equality.

The blade entering the advancing side starts flapping up from the 6 o’clock position, and reaches its highest flap up rate in the 3 o’clock position. At this point it has the lowest angle of attack, and therefore the lowest CL.

The highest position is the 12 o’clock, and the highest flap down rate is at 9 o’clock on the retreating side.

This equalises the lift distribution across the disc area.

Now that we’ve discussed where it comes from and what we can do about it, let’s have a deeper dive into the effects it causes on a flying helicopter.

While flapping to equality is a nice way to solve dissymmetry of lift, it doesn’t solve all of our potential rotor disc problems…

What is Airflow Reversal?

Yea unfortunately we’re not done with all the aerodynamic shenanigans just yet. Airflow reversal, also called Reverse Flow, is another effect that needs dealing with that happens at higher airspeeds.

Remember that we had to add the aircraft airspeed to the advancing side and subtract the aircraft airspeed form the retreating side?

Well, keep in mind that the highest blade airspeeds are present at the blade tips, while the blade roots are going much slower. This is because it covers a smaller distance in the same rotation as the blade tips.

This means that when we subtract our aircraft airspeed from the root’s airspeed of the retreating side, we might end up with a negative airspeed value! What does this mean?

It means that the airflow is no longer going from leading edge to trailing edge, but instead goes the opposite way: from the trailing edge to the leading edge. At this point the blade isn’t doing much for us and just turned into a massive drag generator.

There we have it: Airflow Reversal 💨

The main thing we can do to limit airflow reversal is adhere to the maximum helicopter speed restriction: Vne.

Various aerodynamic tests have shown that some helicopters at Vne have roughly 40% of their retreating blade affected by airflow reversal. But it doesn’t stop here, we have another effect going on on the retreating side ⤵️

What is Retreating Blade Stall?

So we have this retreating blade with a root area that doesn’t do much for us, reducing the total lift for this blade a lot. So what can be done to compensate for this?

Just like with dissymmetry of lift, a blade that generates less lift will naturally start flapping down more as it is no longer being pulled up by lift. The fact that we have this airflow reversal area present means the rate of flapping for this blade will increase even more.

This excessive flapping can result in blade stall. The rate of flapping will be higher at the tip as it has more speed than the root, so the initial stall tends to start at the blade tip rather than the root, like this:

Not only that, but airflow velocity impacts the stall angle as well. The graph below shows the relationship between TAS and the stall angle of attack. Basically, the higher the TAS, the easier it is for a blade to stall, which is why it tends to start at the blade tip.

So the retreating side has two potential hazards: airflow reversal and retreating blade stall. The risk for both of these increases as airspeed increases, and has a massive influence on the Vne of any helicopter.

Finally, the density altitude influences Vne as well, which we discussed here:

How to Calculate Density Altitude: A Step by Step Guide

Retreating blade stall could and has caused accidents in the past, such as this HEMS BK117 B2 that lost 4000’ in altitude after a loss of control in the cruise phase.

Flight Control System Limits

Flying faster doesn’t just stress the rotor system, it stresses the control surfaces, too.

As the helicopter speeds up, the cyclic and collective inputs have to fight greater aerodynamic loads. Linkages, servos, swashplates, and hydraulic actuators all start to experience heavier forces.

If these components are pushed too far, they can start to deform, jam, or, in the worst case: fail.

To prevent this, manufacturers look at the mechanical limits of the control system and make sure the Vne doesn’t allow speeds that would overstress them.

Rotor Tip Speed Compressibility

As helicopters fly faster, the advancing blade tip moves faster relative to the air. Eventually, its speed (helicopter forward speed + rotor tip speed) can approach Mach 1. We discussed this here:

What is a Sonic Boom, and How Does it Affect Helicopters?

That’s a problem, because airflow at near-supersonic speeds brings on shockwaves, buffeting, loss of lift, control issues, and LOTS of drag…

This usually only affects helicopters with very large or fast-spinning rotors, or those flying at high altitude (where the speed of sound is lower).

But it’s another reason manufacturers put a hard limit on airspeed.

This is why helicopters that are designed to have higher Vne’s, tend to have smaller discs, like the SB-1 Defiant:

This is also the reason many helicopter rotors blades nowadays feature a swept-back tip, which reduces the airflow component across the blade, delaying the onset of mach effects. Like this:

Aircraft Configuration and Conditions

One important detail: Vne can change depending on how the helicopter is equipped or operated.

For example:

🔸 With doors off: Many helicopters have a reduced Vne when flying with doors removed: due to unpredictable airflow or stress on the hinges and fuselage.

🔸 With external loads: Flying with a sling load, long line, or hoist may reduce the max speed for stability reasons.

🔸 Altitude and temperature: For most helicopters, the Vne is lower at higher density altitudes, where rotor RPM drops off or control margins shrink.

So the Vne you see in the Flight Manual is usually the maximum, but not always the number you’re allowed to fly at.

Conclusion

Funny how a single red indication on the airspeed indicator hides so much chaos underneath.

At first glance, Vne seems like just another figure to memorise, but behind it lives a cocktail of aerodynamic weirdness: blades flapping to equality, airflow literally reversing, tips flirting with Mach 1, and control systems quietly screaming.

It’s not one clean limit. It’s the point where everything starts to unravel bit by bit.

It’s a boundary drawn by physics, backed by flight test, and shaped by all the weird, wonderful quirks that make helicopters… helicopters!