Flight Safety for the Ka-50: Retreating Blade Stalls

Writing about "flight safety" for an aircraft that is supposed to fly right into the teeth of enemy anti-aircraft guns and missiles sounds like an oxymoron. However, attack helicopter pilots are taught and practice the principles of helicopter flight safety. This short series of "Flight Safety for the Ka-50" is intended to raise some awareness on the topic. The extended DCS Black Shark manual has great information on this topic, make sure to check it out.

Ever flew the Ka-50 straight and level around 295 Km/h, right when all alarms start buzzing, seconds before your blades disintegrate and you plummet into the ground? I have to confess that in all my ignorance I thought that this blade clipping was just the strength of the forward airspeed bending the blades downward, sometimes forcing the top blades into the bottom ones (doh! the top and bottom blades can't collide with each other!)

UPDATE 10/30/09: The top and bottom blades CAN hit each other. Thanks to faithful reader Loke for the correction (see "comments" below). Also, take a look at the following links:
Thread at the official DCS forums
News article on a Ka-50 crash due to top and bottom blades collision
SimHQ article on coaxial rotors aerodynamics

Doh! Rotor blades clipped at high forward airspeed. Image is clickeable.

What actually happens is that a stall of the rotor blades makes the blades flutter and fail structurally.

Stalls, the helicopter version
Every airfoil can stall and helicopter's ones are no exception. Actually there are two main types of stalls for helicopter airfoils: retreating blade stall and settling with power. In this entry, I will briefly mention retreating blade stalls. Settling with power will the topic of a future entry.

I'm no airfoil guru, so I will make this very simple. Imagine the simplest helicopter, with just two blades in its main rotor. The airspeed at each blade is what generates lift. When the helicopter is moving at a considerable forward airspeed, an interesting thing happens: at every turn of the main rotor there is a point where the instant airspeed at the blade moving forward is bigger than the one at the blade moving backwards (retreating blade).

A simple, two blade helicopter with the blades rotating counter-clockwise (the red arrows circle represents the rotation of the blades). The total airspeed at the advancing blade is the airspeed generated by the rotation of the blades plus the airspeed generated by forward movement. The total airspeed at the retreating blade is the airspeed generated by the rotation of the blades minus the airspeed generated by the forward movement. Click the image to enlarge it.

Eventually, if the forward airspeed is very high, the retreating blade will stall! Fortunatelly, retreating blade stalls are easy to avoid (don't exceed the maximum forward airspeed), easy to get out of plus and as frequent as stalls in fixed wing aircraft.

However, keep in mind that in the case of the Ka-50 the vibration generated by a retreating blade stall will eventually cause the blades to fail structurally. This has catastrophic consequences most of the times.

Dissymmetry of lift, de la Cierva and how this sim got me officially freaked out
Now, if you remember that in helicopters the airspeed at the blades is what generates lift you will realize that if the forward airspeed of the aircraft goes up, the advancing blade will generate more lift than the retreating blade. The result is that right half of the rotor disk will generate more lift than the left side. This is called dissymmetry of lift. The guy who solved this dissymetry of lift problem was the Spanish engineer and aviation pioneer Juan de la Cierva. De la Cierva invented the so-called flapping hinge, a device that allows the advancing blade to flap up (see third figure in this page).

This flap up of the advancing blade at high speeds is modelled in DCS Black Shark. See the screenshots below.

Flying at 60 Km/h IAS. The advancing blade in the bottom blade assembly has almost no "flap up". Click the image to enlarge.

Flying at 295 Km/h IAS. The advancing blade in the bottom blade assembly has an evident "flap up". Click the image to enlarge.

The level of detail in this simulation is un-freaking-believable!

Cheers,