#heliseries

[rotorsexual]

I often see Mi-24s posted and being misidentified by the variant. So I put together a Hind variant guide for telling apart the major production models.

The first and most obvious is the Mi-24A Hind A/B. These were the earliest variant and had only a short production run in the early 1970s. Its easily identified by the "greenhouse" canopy design that was soon redesigned into the version found on all other Hinds.

Next is the Mi-24D Hind D. This one featured the redesigned cockpit mentioned above but shared a lot of the same technology of the Mi-24A. It entered service a year later in 1973.

The Mi-24D is often the most confused variant with the similar looking but upgraded Mi-24V Hind E that would enter service in 1976.

These two variants can be distinguished by the radio transmitter located under the nose on the portside. The earlier Mi-24D has a smaller and oblong shaped transmitter that is able to rotate left and right. The Mi-24V has a larger and more rounded transmitter that is fixed in place.

The change in design between these two transmitters is due to the change in armament between the two variants, specifically the guided anti tank missiles. These are usually mounted on the wingtips and are another way to tell at a glance which variant it is. The Mi-24A/D was equipped with the older AT-2 Swatter missile which sits mounted atop a guide rail and has four large stabilizing fins.

Mi-24V's were equipped with newer AT-6 and later AT-9 Spiral missiles which required a new guidance system, hence the redesign. These missiles are much narrower and contained in a tube prior to launch.

The Mi-24P Hind F is essentially a Mi-24V with the 12.7mm gun replaced with a twin barrel 30mm cannon mounted on the starboard fuselage.

The lack of a chin turret is fairly obvious to spot when identifying the Mi-24P. It also saw a reorganized front cockpit but that cant be seen from the exterior.

The final and most modernized Mi-35M is quite different from the earlier variants. It has new avionics and optics, a new 'X' tail rotor, shortened wings with only two pylons instead of the usual three, a new cannon mounted in the nose, and non-retractable landing gear.

Its nearly a new helicopter compared to the older V and P variants.

I'd also like to point out that there are Mi-25 and Mi-35 (not to be confused with the Mi-35M) designations thrown around too. The Mi-25 was essentially a Mi-24D that was produced with slightly different avionics and intended for export to allied countries outside the USSR. The Mi-35 is a Mi-24V intended for the same.

So as a quick summary. The Mi-24A has a greenhouse canopy. The Mi-24D has an oblong transmitter and AT-2 missiles. The Mi-24V has a round transmitter and AT-6/9 missiles. The Mi-24P has a 30mm side mounted cannon. And the Mi-35M has new sensors, short wings, X tail rotor, and fixed landing gear.

[rotorsexual]

So this is going to be the first part in a collection of posts about helicopters that I'm putting together. They may cover topics including aerodynamics, systems, type history, or whatever else I feel like typing up, in no particular order. I'll try to keep things simple for beginners to be able to follow, but some more complex concepts may require a more technical background.

For the Part One I think it would be fitting to introduce principles of flight and basic aerodynamics. Simply put - how a helicopter flies.

Any aircraft whether its a plane, helicopter, airship, etc, will manipulate the four forces of flight. These are lift, weight, thrust, and drag.

Lift is the upward force that supports an aircraft in flight and it opposes weight. When lift exceeds weight an aircraft will climb and when weight exceeds lift it will descend. When lift exactly equals weight the aircraft will maintain it's altitude.

Planes and helicopters generate lift using a specialized shape called an airfoil. Airfoils come in as many shapes and sizes as the aircraft that use them but they are typically shaped like an elongated teardrop with the thicker side on the leading edge and the thinner side on the trailing edge.

This shape has the benefit of maximizing lift while minimizing drag.

The way an airfoil generates lift is twofold - using newton's third law and bernoulli's principle.

Newton's third law states that for every action, there is an equal and opposite reaction. When an airfoil travels through the air at an angle it will deflect the airflow around it. The action of deflecting airflow downward has a reaction of pushing the airfoil upward, creating lift. The steeper the angle, the more lift is created (up to a certain point).

Bernoulli's principle has to do with the speed and pressure of a fluid (air). As the speed of a fluid increases, the pressure it is exerting on a surface decreases. An airfoil will take advantage of Bernoulli's principle by accelerating the airflow over its upper surface, causing a low pressure area. With a relatively low pressure above an airfoil and a relatively higher pressure below, a lifting force is applied.

Using these two concepts, lift becomes a function of airspeed (how fast its traveling through the air) and angle of attack (the angle of the airfoil to its relative wind or direction of travel). Change one or both of these and the amount of lift will change.

Weight is the the most familiar of the four forces. Weight opposes lift and will cause an aircraft to descend when it exceeds lift. Weight is variable depending on how much fuel, payload, or passengers are being carried.

Every aircract will have an empty weight and a maximum gross weight. If you take the maximum gross weight and subtract the empty weight from it you are left with the useful load (ex: GW 3000lbs - EW 1700lbs = UL 1300lbs). This is how much the aircraft can carry as a combination of fuel, payload, or passengers.

Weight doesn't just change with loading but will also change in flight. As fuel is burned by the engine(s) it will be exhausted overboard and the aircraft will become progressively lighter.

Thrust opposes drag and when it exceeds drag an aircraft will accelerate. Thrust is very similar to lift and is created in much the same way. An airplane's propeller is just a wing turned vertically to create lift in a forward direction. Whereas a helicopter's rotor is able to tilt forward, aft, left, and right, to direct some of its upwards lift in those directions. This creates a diagonal lift/thrust resultant that both supports a helicopter and pulls it in that direction. The more a helicopter tilts its rotor to increase thrust, the more lift must be created to compensate to maintain altitude.

Finally there is drag which opposes thrust. When drag exceeds thrust an aircraft will decelerate. There are several different types of drag but they all stem from air resistance. The size, shape, and speed of an object moving through the air will affect the amount of drag it is experiencing. Drag can be useful and be manipulated using control surfaces or other means. Drag also comes as a side effect of creating lift and an increase in lift will often be accompanied with an increase in drag.

Managing energy and manipulating the four forces of flight is the basic principle behind all flight. Every basic and advanced flight maneuver comes from a result of understanding these fundamentals.

[rotorsexual]

Heliseries part 7 off airport landings.

During the first long while of helicopter training, you spend your time learning how to fly and operate within an airport environment. Taking off and landing from runways and taxiways, communicating with ATC, generally flying the exact same way as other fixed wing general aviation traffic. Its not until later in your Private Certificate that you finally get to do some real "helicopter shit" as we like to call it.

Landing off airport comes with its own hazards and a procedure to help mitigate the risks associated with them. In an airport environment you have a large open area that is structured specifically for aircraft to safely operate. However when you go off airport you have to rely on your own decision making and assessment of a landing site, even if one has been partly prepared for you.

We start with an orbit at around 500 feet over a potential landing spot and do a high reconnaissance check. For this we have an acronym called SWOPP. If the landing spot fails any requirements of SWOPP then a new one must be selected.

S is for Suitability which has several sub-meanings here. The first are size and shape. If you cant fit then it's obviously a no go. Next is Slope. Helicopters can tolerate landing on only a few degrees of slope and anything greater than that quickly becomes dangerous. Pretty much every off airport landing is going to be sloped somewhat so picking the apparently flattest spot is best.

Finally there is Surface. Different surfaces can come with their own hazards and ideally you'd want a clear paved surface to set down. Rocks are uneven and can shift beneath the landing gear when weight is applied. Dirt and sand can do the same plus can be blown by the rotor wash and cause a loss of visibility in a brownout.

Snow and ice can do the same and theres a chance of it being stuck to the landing gear when taking off again.

W is for wind. You really want to avoid landing with a tailwind and ideally with a headwind. Tailwinds push you toward the landing spot which requires a slower, steeper approach with higher power requirements. They can also blow your own rotorwash forward into your descent path which is hazardous since it is a recipe for vortex ring state (which I'll cover in a future post). Headwinds, on the other hand give you more lift and stability. They allow for shallower approaches and less power required. Depending on the obstacles surrounding the landing spot you may not always be able to land with a direct headwind but you should always do what you can to avoid a tailwind.

O is for Obstacles. This is another big hazard when landing to an unprepared area. Trees, powerlines/wires, buildings, fences, people/animals, and pretty much everything else can be an obstacle in and around a landing spot. Making sure to avoid them when landing and departing is crucial to avoiding an accident. Identify what the obstacles are and how they could affect the flight. For example ive seen tarps blown by rotor wash and sucked into a rotor. Or when it comes to powerlines you should always pick a tower and fly over the top of it, rather than between where the lines could sag.

When finally touching down to a landing spot you should always do so to the last 1/3 of the area or so. With the cockpit located at the front of the helicopter its easy to forget how much is actually behind you. Pilots have clipped their tails landing to confined areas or performing a hovering turn within one. Always looking before you turn your tail should become habit.

The first P is for both Path In and Path Out. Once youve identified the spot, wind, and obstructions, you should plan a landing path into it and a takeoff path out of it. Determining both before approaching will allow you to ditch to your path out in a go-around if anything doesnt look right. The path in should also ideally provide a safe place to land if a power failure were to occur during the approach.

And the second P - Power. Helicopters are power limited and various conditions like weight, altitude, temperature, and wind will affect how much power you have available. If a landing spot confined enough to require a steep or vertical approach into it, then you will need to choose a new spot when very power limited. You can roughly gauge how much power will be required just from experience with different conditions but you can also check by performing a high altitude hover away from the ground. If you can hover comfortably out of ground effect then you can definitely hover in ground effect to land.

So now you've determined all of the requirements of SWOPP and its time to attempt a landing. You'll exit your 500ft orbit and begin flying a downwind leg to get some distance from the landing spot. Pre landing checks are performed and a descent is started. When the spot appears to be at a rearward 45° angle off your shoulder a turn back inbound is made.

Now the process begins again but in an abbreviated version. Monitoring power gradually increasing as you get lower and slower, checking again for any obstacles that weren't spotted before, and making sure the path in and out is still clear. Assume a normal, steep, or vertical approach angle and terminate to the last 1/3 of the spot.

And thats basically how you land a helicopter.

[rotorsexual]

Heliseries part 6 will cover pedal position in a hover and in flight. This is something I've seen questioned in forums and not common knowledge to those starting out with helicopters.

Its been covered in a previous part how the flight controls work and what they are doing to the rotors to maneuver the helicopter accordingly. I will go over the pedals again here for reference.

The pedals control the pitch or angle of the tail rotor blades. With some positive pitch on the blades they will generate thrust in a sideways direction which is used to counteract engine torque and keep the helicopter from spinning. When the thrust from the tail rotor exactly equals the torque force of the engine, the helicopter will remain facing forward. By pushing in the left pedal (in a counterclockwise main rotor) the tail rotor blades will increase pitch and generate more thrust to turn the nose to the left. By pushing in the right pedal the blade pitch will decrease and less thrust will be produced, causing the torque force to turn the nose to the right.

Here is what the blade pitch looks like with the pedals in the neutral position.

Here is full left pedal.

And full right pedal.

You can clearly see how the tail rotor blade changes pitch across the full travel of the pedals.

What I'd like to point out next is that the pedals are almost never perfectly neutralized in flight. In a hover the pedals will usually look something like this.

This is because hovering (for reasons I'll cover in a later post) actually takes quite a lot of power, more than forward flight, even. With such a high power setting there is a lot of torque being produced by the engine which needs to be compensated for with extra tail rotor thrust - hence, the added left pedal.

So why not just make the neutral pedal position with more blade pitch so you can hover that way? Because hover power isn't consistent. Being loaded with different weights, air temperature, humidity, altitude, wind, and more are going to significantly affect how much power is required to fly so you can't engineer a single solution.

In forward flight the tail rotor is more effective and aided by the vertical stabilizer so even with a higher power setting, less left pedal is necessary. Neutral pedal or even a bit of right pedal may be required to keep the helicopter coordinated in high speed flight.

So if the helicopter is biased against left pedal why is there so much right pedal travel available? Because the helicopter is not always hovering or in accelerated, climbing flight. When the power is reduced to slow the helicopter and/or descend, then you must trade left pedal for right as the collective is lowered. At a low power setting the right pedal will be pushed in somewhat because there is little power required to keep the rotor rpm up and therefore little torque applied.

In the event of an engine failure the right pedal may need to be pressed nearly all the way which would actually produce a bit of thrust in the opposite direction. This is because there is some mechanical drag in the drive system that must be overcome, otherwise it would cause a yaw movement.

There isn't a one size fits all setting or solution to pedal input. Anticipating whats required and adjusting accordingly is just a reality of flying helicopters.

[rotorsexual]

Heliseries part 5 will cover empennage stabilizers. Check out previous parts for more information.

You may have noticed these fins (stabilizers) on the tail of helicopters, but have you ever wondered what they are for?

An airplane uses these stabilizers to balance aerodynamic forces in flight and manipulate the control surfaces on them to maneuver.

But a helicopter maneuvers using its main and tail rotors and has no need for control surfaces, right? This is correct - a helicopter doesn't need them to fly. In fact, the first production helicopters designed in the 1940s and early 1950s didn't have any stabilizers at all.

Sikorsky R-5

Sikorsky S-55/H-19

By the mid 1950s practically all helicopters were adding stabilizers. Early ones were designed without them because the engineers correctly figured they weren't necessary. If you dont want to go fast, that is.

A helicopter is basically suspended beneath its main rotor and will react accordingly. In forward flight the main rotor disc is tilted forward and the fuselage follows along in a nose-down attitude. Having the lift and thrust of the main rotor up high and the drag of the fuselage down low has an effect of pushing the nose down lower at higher airspeeds. This becomes an aerodynamic limit to high speed flight in helicopters. Engineers fixed this by adding a horizontal stabilizer.

A horizontal stabilizer works by using an airfoil shape to create lift just like a rotor blade or airplane wing. Except its upside down. This has an effect of pushing the tail down, and by extension, the nose up. Being out on the tail puts the stabilizer far from the center of gravity where it has a lot of leverage which reduces its required size.

Note the curved lower surface of this horizontal stabilizer indicating its designed for downward lift.

And just like how a higher airspeed will create more drag and lower the nose, it will also increase the downward lift of the stabilizer to lower the tail. This creates a self correcting mechanism to keep the helicopter stable in pitch. The fuselage will still be a bit nose down in cruise flight but at a much less extreme angle.

Ok so what about the vertical stabilizer?

A helicopter has a tail rotor to produce sideways thrust against the engine torque and prevent the helicopter from spinning around opposite of its main rotor. This torque exists in a hover and in forward flight so it must constant be resisted by the tail rotor.

The vertical stabilizer is there to assist the tail rotor in forward flight. It is just like the horizontal stabilizer in that it is an airfoil out on the tail. Only it is turned vertically to create sideways lift in the same direction as the tail rotor thrust.

Note the leading edge of this vertical stabilizer is angled to the right. This assists the tail rotor in pushing the tail to the right/nose left.

In a hover there is no airflow over it so the tail rotor is fully responsible for anti-torque. But the vertical stabilizer will begin to unload the tail rotor as the helicopter accelerates. This means there isnt as much pedal required to keep the helicopter flying straight which frees up a little power that can then be used to accelerate more or climb.

In the event of a tail rotor failure many vertical stabilizers are designed to permit limited controlled flight at a high enough airspeed and/or low enough power setting.

Pretty much every helicopter designed in the last 70 years has had horizontal and/or vertical stabilizers because the benefits greatly outweigh the small penalty to weight, drag, and cost. In a later post I may cover some moveable stabilizers.

[rotorsexual]

Part three will cover some correlation of controls.

If you haven't seen parts one and two and you aren't familiar with the flight controls then I recommend checking those posts out first.

One of the reasons that helicopters are so notoriously difficult to fly, is that the pilot is required to use both hands and feet simultaneously to manipulate very sensitive controls. Further still, all three of the controls will have some influence over the other two.

Two controls that are very intertwinned are the collective and the pedals. Whenever the collective is raised or lowered the pedals must be manipulated as well to keep the helicopter from yawing. The yaw is coming from the change in engine torque that occurs simultaneously with collective movement.

An example of a sequence of events is as follows: collective is raised -> main rotor blade pitch increases -> lift increases -> drag increases with lift -> drag tries to slow the rotor rpm -> engine increases power to maintain rpm -> torque increases with engine power increase -> increased torque tries to yaw the helicopter (to the right in a counter clockwise rotor). If the left pedal is not applied then the helicopter will continue yawing to the right. The opposite happens when the collective is lowered except right pedal must be applied to prevent the yaw to the left. How much pedal needs to be applied is somewhat proportional to the amount of collective used. This correlation of controls is also necessary whether in a hover or in forward flight.

Speaking of forward flight. This is where the collective and cyclic are most correlated. Just as the pedals need to be manipulated with collective movement, so does the cyclic. Raising the collective will require some forward cyclic and lowering the collective will require some aft cyclic. This is because of the way that lift and drag are acting on the helicopter in forward flight.

The helicopter is essentially hanging beneath its main rotor and will behave somewhat like a pendulum. In forward flight the main rotor disc is tilted forward to provide both lift and thrust. The fuselage will follow along in a nose low attitude, in part due to drag.

When the collective is raised in this state, lift will increase and cause a nose up at the same time. If nothing is done then the helicopter will begin a climb and airspeed will decrease. By applying enough forward cyclic the nose will not raise and a shallower climb at a constant airspeed will occur. If the collective is lowered then the opposite will happen and aft cyclic will be necessary to prevent the nose from lowering.

Left or right cyclic may be necessary with pedal input but I will go into more detail on that phenomenon in a later post.

[rotorsexual]

Part two will cover helicopter flight controls. If you haven't seen part one then you may want to reference it first to be familiar with some of the concepts and terms mentioned here.

All aircraft in flight will move around one or more axis as they maneuver. These axis remain the same whether it is an airplane or helicopter but the way they accomplish these maneuvers is different.

A helicopter will pitch the nose up and down along the lateral axis, roll or bank side to side along the longitudinal axis, and yaw the nose left or right along the vertical axis.

Planes use control surfaces on their wings and tail to deflect airflow and change the amount and direction of lift to maneuver. A helicopter will vary the lift created from its main and tail rotors to maneuver. This is accomplished through one or more of the three flight controls.

First is the collective.

The collective is a lever-like control held in the pilot's left hand. It is mounted near the left side of the seat and is manipulated by raising and lowering it. The purpose of the collective is to vary the amount of lift created by the main rotor. Contrary to some belief, a helicopter's rotors do not change speed in normal flight. The rotors are held at a constant flight rpm and all control is made by changing the angle or "pitch" of the blades, to generate more or less lift.

As the collective is raised the main rotor blades will all collectively increase their pitch and lift will be increased evenly across the main rotor disc. When the collective is lowered the main rotor blades will decrease their pitch and lift will decrease across the main rotor disc.

Collective lowered with blades at a flat pitch.

Collective raised with blades at a high pitch.

This makes the collective the "up and down" control of the helicopter. Raise the collective and lift will increase until the helicopter rises or lower it and lift will decrease causing a descent. Adjust it accordingly to maintain altitude in a hover or forward flight.

Next is the cyclic

The cyclic is held in the pilot's right hand and controls the pitch and roll of a helicopter. This is the "stick" control that you may have noticed before in a cockpit. Moving the cyclic fore and aft will pitch the nose down and up respectively. Moving the cyclic left or right will roll the helicopter in either direction. This makes the cyclic control a helicopter's speed and direction over the ground.

Pushing the cyclic forward will cause the helicopter to nose down and accelerate forward as more of its upwards lift is devoted to thrust. Pulling the cyclic back will cause the nose to rise and a deceleration as thrust is reduced or even generated in the opposite direction. Applying left or right cyclic will roll the helicopter accordingly and devote some lift into sideways thrust to turn the helicopter that direction in forward flight, or slide sideways in a hover.

What the cyclic is actually doing when it is moved from the neutral position is creating a lift imbalance across the main rotor, in order to tilt the main rotor disc. The main rotor blades will increase and decrease their pitch hundreds of times per minute as they rotate around the disc. This creates more lift one one side and less lift on the opposite side. The disc will tilt as a consequence of this lift imbalance and direct the upwards lift in a more diagonal angle, providing both lift and thrust. The more the cyclic is moved from the neutral point, the more the disc will tilt in that direction.

A little bit of aft cyclic to level the rotor disk.

Forward cyclic to tilt the disk forward.

This is one reason why its a good idea to duck low when exiting or approaching a helicopter. The disk may be tilted and lower the blade tips to head height which will not end well.

Next are the anti-torque pedals. These are located on the floor in front of the pilot and control the helicopter's position around its yaw axis. The left foot is placed on the left pedal and right foot on the right pedal. When one pedal is pushed forward the other will move rearward.

In a hover the pedals will control a helicopter's heading. For example if you are in a stable hover and push in the left pedal a bit, the helicopter will begin to rotate in place to the left. If you push the left pedal in further then the helicopter will turn faster. To stop the left turn or to turn the other way just smoothly push in the right pedal until it slows to a stop and/or reverses.

What the pedals are actually controlling is the pitch (and therefore the thrust) of the tail rotor blades. In a helicopter with a main rotor that turns counter clockwise, pushing in the left pedal will collectively increase blade pitch and right pedal will decrease blade pitch.

Without a tail rotor a helicopter would spin around its main rotor in the opposite direction of the rotor's rotation, due to the engine torque. So a tail rotor pushes (or pulls depending on the main rotor's direction and which side of the tail boom the tail rotor is mounted on) on the tail to keep the helicopter stable. When the thrust from the tail rotor is greater than the torque of the engine, the helicopter will turn one direction. And when the tail rotor thrust is less, the torque will naturally turn the helicopter the other direction.

By manipulating the pedals the helicopter will maintain the desired heading in a hover and keep coordinated and flying straight in forward flight.

Finally there is the throttle. It is often located on the collective grip and manipulated by the pilot's left hand. The throttle controls the engine power and is used to maintain the proper rotor rpm.

The throttle on this collective is the brown cork foam grip. Other helicopter's may have a separate lever mounted near the collective or on the overhead panel.

Collective mounted throttles twist like a motorcycle grip and the saying goes "thumb up throttle up, thumb down throttle down".

As lift increases when the collective is raised, so does drag on the main rotor. This drag tries to slow the rotor rpm down which is not ideal. So the throttle must be increased simultaneously to maintain rotor rpm. The inverse is also true. Lowering the collective without reducing throttle would cause a rotor rpm overspeed.

Old piston engine helicopters from the 1950s and 1960s required the pilot to manually adjust the throttle as they flew. Newer helicopters and those with turbine engines have correlators and governors which automatically adjust the throttle for the pilot. Having flown both with the governor on and off I can say it is a very convenient system.

Note that these are simply an overview of helicopter controls and their functions. In the next part I will describe correlation of controls.