#heliseries

An airplane or helicopter uses a particular shape in its wings or rotors known as an airfoil to generate lift.

To put it simply, airfoils generate lift as a function of airspeed (how fast the air is traveling over it) and angle of attack (AoA, the angle that the airfoil is traveling through the air). Airplanes travel forward and adjust their speed and pitch angle (nose up or down) to change the amount of lift whereas a helicopter spins its rotors around at a constant speed and adjusts the blade angle to change the amount of lift. But there is another factor at play with a helicopter - induced flow.

Induced flow is the vertical component of airflow through a helicopter's rotors. This is the air getting sucked down into the blades from above and being expelled below. It is strongest during a hover and at low airspeeds.

Because the airfoils of the rotors are traveling "forward" from their perspective they experience airflow in the opposite direction of rotation (the rotational relative wind). And because of the induced flow, these two components combine to make the oncoming airflow into the rotors at a slightly downward diagonal angle.

This diagonal "resultant relative wind" effectively reduces the rotor blade's angle of attack and the amount of lift it can generate. So an even higher angle of attack must be used to compensate for the loss of lift which has a penalty of more engine power required. This is why it requires more power to hover than to fly with speed.

As the helicopter accelerates into forward flight the downward induced flow passing through the rotors gradually transitions from a downward vertical angle, to a diagonal angle, and finally to a nearly horizontal angle.

Now there is much less induced flow and the rotor blades have a much larger angle of attack, generating significantly more lift. This requires less engine power and an increase in aircraft performance.

Here are a few screenshots from Microsoft Flight Simulator which does a decent job representing this phenomenon to demonstrate the performance increase.

In the first set the helicopter is hovering at standard temperature and pressure at 2500lbs gross weight near sea level. On the left side there are eight smaller gauges in two columns of four. The top right of these instruments (right below the highlighted blue square) is the torque meter which provides an indirect indication of engine power. Its currently at about 58% to hover just a few feet above the ground.

This next set was taken after accelerating to forward flight. I've kept the power at the same 58% torque and the result is now the helicopter cruising at a steady 60 knots and a 500 foot per minute climb rate.

A bit of speed made that much difference with the exact same power setting used to hover just a few feet above the ground.

This is why helicopters often take off like an airplane whenever possible. Accelerating from a hover over a short distance before climbing away.

But once people take a moment to think about it they become very concerned whether the rotors will collide with each other. Short answer is - they wont. They're mechanically synchronized via a gearbox and cannot get out of sync without a catastrophic mechanical failure. But thats not what this post is about. Its about the next thing that usually crosses people's minds when they watch the KMAX for long enough. How does it turn?

The answer is pretty interesting and it is actually three things going on that allows the KMAX to turn in place or "yaw". But before that I think its worth explaining how a traditional helicopter yaws.

The engine produces torque (a rotational force) to spin the main rotor. The torque force has a side effect of trying to spin the rest of the helicopter around the opposite direction. So a tail rotor is used to produce sideways thrust to counter that rotation. When the tail rotor thrust exactly equals the torque force of the engine, the helicopter remains facing forward. If the tail rotor thrust increases then the helicopter will yaw one direction (to the right with a clockwise main rotor and to the left in a counterclockwise main rotor). And if the tail rotor thrust decreases then the torque force naturally rotates the helicopter the other way.

This action is done via the left and right pedals in the cockpit which will yaw the helicopter accordingly.

The KMAX still has pedals yet no tail rotor. The intermeshing rotors spin opposite directions at the exact same rpm and so the torque force between them is neutralized. Without that variable tail rotor thrust to provide yaw, the KMAX must rely on three other methods to accomplish the same movement.

The first is it simply has a rudder.

There is a moveable rudder on the tail at the trailing edge of the vertical stabilizer. As either pedal is pushed, the rudder will swing to the left or right and deflect the airflow around it to produce a yaw reaction. This functions exactly like an airplane rudder. However its only effective at higher airspeeds. In a hover or at low speed there is little to no air flowing over the rudder which will have no effect on yaw.

For low speed flight the second and third methods are necessary. One of which is called differential cyclical pitch. As the rotor blades spin they create a continuous disc of lift. By tilting the rotor disc you can direct this lift to fly in in any direction.

Because the KMAX has two main rotors, each with their own cyclical pitch control, you can angle the discs in separate ways.

For example if the left main rotor tilted forward and the right main rotor tilted rearward the helicopter would yaw to the right around the center of mass between the two rotors. This is sort of like the motion of your hands as you steer the handlebars of a bicycle to pivot it.

The third and last method is probably the trickiest to understand and has to do with differential collective pitch. Collective pitch refers to the pitch angle of the rotor blades in unison. When the blade angle increases collectively the lift will also increase across the rotor disc.

Where there is lift, there is also drag and an increase in lift will also increase drag. This drag will try to slow the rotor rpm so the engine must increase power (and torque) to compensate. As mentioned above torque has an effect of trying to rotate the helicopter the opposite direction of the main rotor's spin.

And just like with differential cyclic pitch, the two main rotors have independent collective pitch control. One rotor can increase collective pitch while the other rotor decreases collective pitch. The torque on the higher pitch rotor will increase and the torque on the lower pitch rotor will decrease. This has an effect of yawing the helicopter opposite of the higher pitch rotor's direction of spin. Clear as mud?

So to recap with an example. A hovering KMAX wants to make a turn to the left. The pilot presses in the left pedal to begin the turn.

1) The rudder swings out to the left but because the helicopter is hovering there is no airflow over it to assist with the turn.

2) The left rotor tilts rearward and the right rotor tilts forward to rotate the helicopter around its center.

3) The clockwise spinning right rotor increases pitch and the counterclockwise spinning left rotor decreases pitch. This creates a torque imbalance that forces the nose to turn left.

You can see from this diagram that the two outer vertical fins (stabilizers) mounted on either side of the horizontal stabilizer near the tail rotor, are both clearly angled to starboard. Here is another angle of it from a BK 117

When the helicopter is facing straight on you can see the stabilizers are not. But when the stabilizers appear to be straight the helicopter is clearly at an angle.

The reason for this has to do with the tail rotor. In a hover and at low speeds the tail rotor is responsible for providing sideways thrust to prevent the helicopter from spinning uncontrollably due to torque. But when the helicopter accelerates into forward flight the stabilizers on the tail cause a weathervane effect and try to align themselves with the airflow.

By angling the stabilizers you can increase the force of this weathervaning. This aids the tail rotor in fighting the torque and ends up contributing so much that the tail rotor may not even be necessary at certain airspeeds and power settings. The tail rotor thrust can be greatly reduced and that power that would have been consumed by the tail rotor is now freed up to be used by the main rotor instead. This can allow for slightly higher speeds or more carrying capacity.

Several other helicopters take advantage of this and have it engineered into their design as well, like this Robinson R44

Designed in the 1980s by Bell helicopters it was used primarily as a VIP transport and later used pretty widely as a emergency medical transport.

Just on looks alone its a very aesthetically pleasing airframe with its sleek lines and sponsons to contain the retractable main landing gear.

But almost counter to the rest of the design is it's massive two bladed main rotor which has a crazy 28 inch chord length (even larger than UH-1 Huey blades at 21 inches) that give it an incredible chopping sound in flight.

I love the cockpit layout that feels like you're sitting in an airliner and not a helicopter.

But the strangest part is the collective. In the right PIC seat the collective handle is turned horizontally and held like an airplane throttle. Pulling up and rearward to raise the collective and down and forward to lower it.

Meanwhile the left SIC seat has a traditional collective.

Sadly my chances of ever getting to fly one are slim. Bell only produced around 200 of them and less than two dozen are still airworthy in the US. Flying for a few decades with life limited parts and a shutdown production line meant it was only a matter of time before they all bacame ground and replaced. And being an older design without much improvement (aside from the 430 which isn't quite the same) prevented it from competing with other helicopters of a similar class such as the A109 or EC135.

So I'm here to explain exactly how something like that could happen and how to avoid getting a messy haircut when entering or exiting a running helicopter.

Lets start with just being around one in general. The whole rear half of the helicopter is just completely off limits. There's the tail rotor which has been responsible for many many accidents, hot engine exhaust, and you are not in view of the pilot. Approaching from the sides or from a forward diagonal angle instead is best.

But why is approaching directly from the front not generally the preferred method?

Because the rotors can tilt.

When the pilot moves the cyclic control forward, aft, or sideways, the rotor disc will tilt in the corresponding direction. If the cyclic is pushed forward then the front of the rotor disc will dip lower than usual. Even larger helicopters can have the blade tips dip below 5-6 feet which is enough to hit most people.

To exacerbate this, many helicopters also have rotor masts that are pretilted forward a few degrees, even with the cyclic in a neutral position.

This is why you should prefer to approach diagonally and not directly from the front where the rotor blades could potentially be at the lowest point, while avoiding the hazards to the rear.

Another thing to consider is the slope. Plenty of accidents have happened because of people approaching from or leaving to an upslope.

A helicopter landed on a slope (even a seemingly shallow one) will touch down sideways to it and use lateral cyclic input to help hold it into the upslope side.

Up close to the side of the helicopter or beyond the rotors you are safe but attenpting to cross the blade tips could put you close enough to them to cause injury.

Lastly and one of the reasons for the accident with the Bell 407 in the video mentioned earlier is the landing gear.

That particular helicopter was equipped with low skids which are typically used for operations from prepared helipads and paved surfaces. They are a bit more streamlined and more convenient to enter and exit the cabin but they bring the fuselage and rotors closer to the ground.

Here is a comparison of a Bell 407 with low and high skids equipped.

(Low)

(High)

The extra few inches added with high skids makes a big difference for raising the fuselage and tail rotor from obstacles when landing off airport.

Depending on the helicopter the number of main rotor blades will vary between 2 - 8.

R44 with two blades

H125 with three blades

A109 with four blades

AW139 with five blades

SA321 with six blades

CH-53E with seven blades

Mi-26 with eight blades

Did you notice a correlation between the number of blades and the size of the helicopter? Thats not a coincidence. Generally speaking a smaller helicopter will have fewer blades and a larger helicopter will have more blades. I say 'generally speaking' because there are a few exceptions.

The MD500 is a small helicopter with a four, and in later variants, five blade main rotor.

While the Bell 214ST is a large helicopter with a two blade main rotor (the largest with two blades, in fact).

The reason for the general trend has to do with cost, complexity, and aerodynamics.

Cost and complexity is pretty straightforward. Fewer blades have a more simple rotor head with less material. More blades requires increasingly more complex rotor heads with more material and complexity. Production and maintenance becomes more intensive with more complex rotors which will add to the cost.

Then theres aerodynamics which is a bit more tricky. A two blade rotor is the most efficient design from this perspective. This is because as a rotor blade moves through the air it will leave behind a turbulent wake. The following rotor passes through that wake which reduces its efficiency. Two blades allows the most distance between the rotors for the wake to dissipate and each additional blade incrementally reduces this distance.

And with fewer physical blades there is less drag experienced by each blade as they generate lift.

So why don't all helicopters just use two blades if it has the least cost and most aerodynamic efficiency? Mostly because of structural and material limits.

As helicopters get larger and heavier they require more lift to fly. A main rotor with two blades for a very large helicopter would be physically impractical due to the length of the blades required. With such a large rotor diameter the size of the helicopter itself would need to be extended to provide clearance for the tail rotor, which adds more weight, which requires more lift, and so on. This creates a detrimental cycle that needs to be managed.

Another reason is that materials strength only goes so far. Blades need to be both strong and lightweight and an excessively long blade isn't practical for either. Adding strenght often comes with more weight, which needs more strength, and another detrimental cycle appears.

So the best solution is to add more blades. This is why larger helicopters will have more blades on their rotors compared to a smaller, lighter one. There is a penalty to aerodynamic efficiency, cost, complexity, weight, and drag, but each additional blade contributes to lift and keeping the rotor diameter down to a reasonable size. Although adding too many blades will have diminishing returns so its up to the engineers to come up with a design that balances all of these factors.

Something I've been seeing quite often in the comments under helicopter posts that make it to the broader internet spaces is discussions on autorotation. These discussions are mostly incomplete information at best and outright wrong at worst. A lot of people seem to be able to recall it as a fact about how helicopters can glide to a safe landing, but aren't aware of the actual process. So here's a guide on what an autorotation is, how its performed, and some of the nuances to it.

For the uninitiated, an autorotation is a maneuver that every helicopter is capable of performing which allows it to land safely in the event of a power failure. Even more simply put - its how a helicopter glides.

I've already made previous posts about helicopter controls and some principles of flight which I recommend checking out first if you're unfamiliar with those.

Under normal flight the engine(s) drive the rotors at a constant flight rpm and all control is made by pitching (changing the angle) The blades to make more or less lift. Essentially the same process as sticking your hand out the window of a moving car and making rise or fall in the wind. However the rotors are experiencing a lot of drag (wind resistance) which requires the engine to produce a lot of power to overcome and maintain rpm.

When an engine failure occurs there is no more power driving the rotors and the high drag will cause the rotor rpm to start to decay rapidly. If nothing is done about that then the rpm will fall so low that the rotors will stall or worse and the helicopter will fall out of the air like a rock. Thankfully we have the option to autorotate instead of that outcome.

The first thing that happens to initiate an autorotation is to fully lower the collective. This will flatten out the blade pitch and minimize the drag on the main rotor, slowing the rpm decay. As the collective is lowered the cyclic will need to come aft slightly to prevent the nose from dropping. Also the right pedal will have been pushed in as the power failure initially occured to prevent yawing.

Now the helicopter is in a steep descent and the autorotation has begun. The airflow through the main rotor has reversed from normal flight. Instead of being drawn from above and expelled downward there is a diagonally upward flow of air through the main rotor.

Now the rotor rpm will begin to rise again thanks to the special design of the rotor blades. A rotor blade has an airfoil shape which is sort of like an elongated teardrop with the wider end on the leading edge. This shape minimizes drag and maximizes lift. But the blade is also slightly twisted. It has a positive pitch at the root where the blade attaches to the rotor hub which gradually transitions to a negative pitch at the tip.

Because of this twist and the difference in relative speed along the blade length (tip travels relatively faster than the root) the blades will develop three distinct regions. These are the driven, driving, and stall regions

The driven and stall regions at the blade tip and root are still producing drag but the middle driving region is actually producing lift, in an upward and slightly forward direction. This forward lift is the thrust that causes the rotor rpm to increase during an autorotation.

So now you are in a descent and recovering rpm back to the normal flight range. If you leave the collective fully lowered the rpm will now start to increase past the normal range and begin to overspeed. If the overspeed becomes too great the blades will be damaged and one could eject. Not ideal.

You have to manage the rpm manually to prevent it from becoming too low or too high. You also do this with the collective. Remember, to start the auto you should lower the collective fully to minimize rpm loss initially and then to start recovering it. As the rpm reaches the normal range the collective should be raised again just a bit to "catch" the rpm. Now you can manually adjust rpm with a tiny amount of collective movement. Rpms a little too fast? Raise it a bit. A little too slow? Lower it a bit. What this is doing is changing the size of the driven and driving regions of the blade, thanks to the twist. Lowering the collective grows the driving region and shrinks the driven region, and vice versa for raising it.

Now the helicopter is safely gliding and can be steered to a landing spot. There's not much to do until you're approaching the ground. The next maneuver will be the level and flare. The height at which you initiate the level and flare depends on the helicopter. Generally a larger helicopter will have more momentum and need to start the maneuver sooner.

Starting with the level off. You will be gliding with a high rate of descent and forward speed in an autorotation. The purpose of the level off is to drastically reduce the rate of descent. By using some aft cyclic input you will pull the nose up and put the helicopter in a level flight attitude. This causes the upwards lift of the rotor disc to act as a sort of parachute and arrest the descent.

Now with the descent rate minimal you apply more aft cyclic to pitch the nose up further and neutralize the forward speed. This is the flare and its the last opportunity to build rotor rpm in an autorotation.

Now you are just over the ground with little to no forward speed and the helicopter will start to settle and sink. Apply forward cyclic to level the helicopter parallel with the ground and use the pedals to keep the nose pointed straight ahead. Then you have whatever rpm is built up to cushion the landing. Smoothly raise the collective fully as the helicopter sinks to touchdown and the landing can be shockingly smooth.

What an autorotation really comes down to is energy. You often start at a high-ish altitude with some forward speed and this becomes the potential and kinetic energy you trade to power the rotors instead of the engine. The energy is an absolute requirement though. If you dont have enough of a combination of speed and/or altitude then an autorotation can be impossible. There are phases of flight and certain missions where you have to accept the risk of a power failure and rely on the crash-worthiness of the airframe.

The first of the long cabin Huey variants was the UH-1D. It was designed as an increased capacity troop transport to replace the UH-1B. The fuselage was stretched by 41 inches and space was made on either side of the transmission for two sideways facing seats that would often be used as door gunner positions.

(Short cabin UH-1B, notice the flat back wall with space for bench seating)

(Long cabin UH-1D with space for sideways facing seats)

The UH-1D was a successful replacement for the UH-1B and it was later upgraded to the UH-1H with a more powerful 1400 horsepower engine (up from the UH-1D's 1100 horsepower engine) for increased carrying capacity and performance. Many UH-1Hs were built from scratch but many more started as UH-1Ds and were retrofitted to the UH-1H standard.

The main exterior difference between the UH-1D and UH-1H can be identified by the position of the pitot tube.

The UH-1D has two pitot tubes that protrude from the nose, one of which can be seen in this photo.

The UH-1H had the position of the pitot tube changed to above the cockpit where it is bent at 90 degrees.

Most of the UH-1s still flying today, both military and civil, are UH-1Hs (also called Bell 205A+).

Bell would also make a twin engine variant known as the UH-1N or Bell 212. This would have the single Lycoming T53 engine replaced with a pair of Pratt and Whitney PT6T engines for increased power and redundancy. The exterior differences are more obvious with the UH-1N having a larger and bulkier engine cowling with two separate exhausts protruding and redesigned intakes.

(Single engine T53 UH-1)

(Twin engine UH-1N)

A four bladed version of the Bell 212 would also be produced as the Bell 412.