Part 2
by Chuck Meager

L = CL S V2r / 2

Welcome back. Last month I ended with the descriptions of the important concepts that help in understanding helicopter aerodynamics. This month I will discuss how the helicopter actually fly's (or doesn't).

Hoverin' Fools

The unique thing about the helicopter is that, with the right person on the controls, it can hover. And as all of us who have learned to hover know, with the wrong person on the controls, it can't. But I'm not here to discuss pilot technique (or lack thereof), the point of interest of this article is why. Why does it hover? Why doesn't the fuselage spin around? Why does it take less power to hover in ground effect than out of ground effect?

Ah..to hover...

Whether I'm hovering at three feet or 200 feet, I feel special. Last month we talked a little about rotational velocity. Velocity created by the rotor spinning. When we pick the aircraft up to a hover the fuselage doesn't spin the opposite direction of the rotor because of anti- torque. In single rotor aircraft, anti-torque is created with a tail rotor or thanks to McDonnell Douglas, air. (In tandem rotor aircraft, torque is compensated for by having counter-rotating rotors.) So, as the aircraft increases in altitude, torque is increased; therefore anti-torque must be increased. In most US built helicopters this means adding left pedal. Also the tail rotor is working overtime because of all the disturbed air created by the main rotor.

Out of Ground Effect (OGE) Hover

In a nut shell, OGE hover looks like this ==>

Huge induced flow, creating reduced resultant angle of attack, so the pilot must increase the collective (power demand) to enlarge the angle of attack. The long version ==>

OGE hover creates a huge downwash. At the blade tips, vortexes are formed. The large induced velocity aerodynamically changes the angle of attack. So to maintain hover height, the angle of attack (power) must be mechanically increased. With the increase in power, there is an increase in torque so anti-torque must be increased. PROBLEM....Loss of tail rotor effectiveness (LTE). Under certain conditions, the aircraft might not have enough anti-torque available to prevent the aircraft from spinning opposite of the main rotor. This is where performance planning comes into play. With the charts available in the Operator's Manual, the pilot can predict when LTE may occur. Even a small wind can have a dramatic effect on tail rotor effectiveness.

In Ground Effect (IGE) Hover

A hover in ground effect takes much less power for two reasons: 1) reduced induced flow velocity and 2) reduction of wing tip vortexes. The induced flow is reduced because the ground interrupts the flow. The air molecules are not allowed to go too fast because the molecule in front of that one is going slow (kind of like rush hour traffic) This reduced induced flow means that the angle of attack is changed less. Therefore the resultant angle of attack is greater so the aircraft needs less power. This means less torque and less anti- torque required. Blade tip vortexes are also reduced because the proximity of the ground does not allow them to build.

Translating Tendency

Translating Tendency is the tendency of helicopters to drift laterally in the direction of tail rotor thrust. Or rather the aircraft is being pulled to the right during hovering because of the lift created by the tail rotor to counter torque. Some helicopters (especially 'em big uns') have there flight controls rigged to automatically adjust for this phenomenon. PROBLEM (although not much of one) This mechanical rigging does not take in to effect changes in gross weight. So if an aircraft is too light, too much compensation is made, so the pilot still has to fly - bummer.

Onward and Upward
The Winds of Change

So during hovering we see that there is a tremendous amount of "dirty" or disturbed air involved. That is air that is used again and again. As either directional flight or wind is introduced to the rotor things change. Chaos. The perfect world is gone.

Translational Flight

As forward cyclic is applied the front half of the rotor disk and the aft portion of the rotor disk are affected differently. The total aerodynamic force of the rotor disk is shifted forward so the air strikes the two halves at different angles (because of coning). The front half still has a large amount of down wash and vortexes, while the aft portion begins to use new air all the time. As the helicopter increases its forward velocity the airflow striking the aft portion is vertical in relation to the blades while the airflow in the forward half becomes more horizontal. This difference in angle creates a condition known as Transverse Flow Effect. This occurs between 10 and 20 knots. The increased induced flow velocity on the aft portion of the rotor disk creates a reduced angle of attack (less lift). Due to gyroscopic precession, this takes effect 90 degrees in the direction of rotation. End result: the aircraft wants to roll right. The pilot, feeling a little superior, adds a small amount of left cyclic.

About the same time (16 to 24 knots) the aircraft goes through another phenomenon known as Effective Translation Lift (ETL). This is the point where the aircraft falls off its bubble of increased air pressure and proceeds into completely undisturbed air. The huge vortexes are reduced. The nose of the aircraft is going to pitch up because the advancing blade has increased lift and flapping (which occurs to compensate for dissymmetry of lift) changes the total aerodynamic force aft (blowback). So at this point the pilot must add forward cyclic.

Retreating Blade Stall

So what happens when we go faster than we should? If we're traveling along at 100 knots and the rotational velocity of the main rotor is 100 knots at the blade tip, then the three o'clock position of the advancing blade is actually traveling at 200 knots. Hmmmm. So...the blade tip at the 9 o'clock position is traveling at...0 knots. Not Good. Fortunately that's a very small portion of the blade. However, lets not forget about blade flapping. Remember how blade flapping compensates for dissymmetry of lift? What do you think the angle of attack is on the retreating blade? Compare that with the velocity on the retreating blade. What do you get? STALL. Because of our friend gyroscopic precession, this takes effect 90 degrees later in the direction of rotation. The nose pitches up and the aircraft will want to roll left. High velocity alone is not the only way to get into retreating blade stall. Conditions that enhance the possibility of RBS are a high gross weight, low rotor RPM, High DA, turbulence, or steep turns.

Settling with Power

Settling with power is a condition that the aircraft (and its occupants) can get in during vertical or near vertical descent. This condition generally is where the aircraft is settling in its own downwash.

Backing up a bit.

During descent the aircraft may go through four different types of airflow patterns. These are: the normal thrusting state (which we've been talking about for the last two months), the vortex ring state, the autorotative state, and the windmill brake state. These airflow patterns are usually associated with vertical descents.

Vortex Ring State

The vortex ring state is a condition where the outer blade and the inner blade are creating and using vortexes. This occurs because the aircraft is descending faster than the downwash that the rotor produces. So in the center of the disk there is actually an upflow of air. The upflow helps produce an inner vortex. End result: too many vortexes. This means very poor blade efficiency.

Hopefully this will stop in the early stages and all we end up with is settling with insufficient power. This is because the pilot was smart enough to stop his descent by applying power. However, if our hero (the pilot) is distracted, and allows this condition to continue - WHAM!!! We find the aircraft descending at a high rate of speed. The more power applied the faster the descent! This is not a natural feeling. Of course the calm, cool and collected pilot quickly analyzes the situation, reduces power and applies forward cyclic. Our hero has now learned a valuable lesson. Avoid Settling with Power! Avoiding settling with power is easy. Never make vertical or near vertical descents greater than 300 foot per minute. By vertical or near vertical, I mean with rapid rates of descent, your approach angle should be less than 30 degrees. Also be careful in downwind descents.

Boy, is this aerodynamics dry or what? Does anyone have a glass of water? It's like a desert in here. Next month's topic: Autorotation - Theory and Technique

Until then, Chuck.