Flying Helicopters

Helicopters are sufficiently different that they merit a FAQ page (at least) on their own. Most of the material on the page is based on a series of postings in the Tech List from guys with serious helicopter experience that know what they're talking about. Special thanks go to Jim McNeill, Norm Lagasse, and Todd Fredricks.

How accurate is the X-Plane helicopter flight model?

Based on comments from those who know, it's very good. The significant helo effects, such as the fundamental instability, interaction of control inputs, ground effect, transition between hovering and forward flight, etc., are all done accurately.

How do the controls work?

The remainder of this discussion applies to the traditional helicopter design with a main and tail rotor. Twin rotor designs are a little different...

The basic idea behind a helicopter is that the main rotor acts like a large propeller and provides lift. (Yeah, OK; add in ground effect, forward airspeed, and all the other effects and things get a lot more complicated. More on all that later.) The tail rotor is a sideways prop that counters the torque of the main rotor. In flight both rotors are normally driven at constant speed and control input comes from varying the blade pitch. Most helicopters have an automatic throttle control that compensates for the varying torque requirements.

Do I need to change my X-Plane control setup?

No. Your stick becomes the cyclic control and the pedals (or stick twist grip or whatever) control the tail rotor. Your throttle becomes the collective but it works backwards: Full forward is minimum collective and you pull back to increase. Note to self: While your helicopter is loading up in X-Plane, push the throttle all the way forward (minimum collective) so that when the flight model starts you stay on the ground. Starting with maximum collective usually makes for a very short and violent flight.

I don't have any pedals

You can get away without pedals if you check the "Auto-coordinate" box in the Joystick setup menu, Center tab (called Null Zone in earlier versions). X-Plane will automatically adjust the tail rotor to keep you straight. There's just some things you won't be able to do, such as turn in place in a hover.

Wow, this is hard! How do I even get started without crashing instantly?

Yeah - it really is hard. Helicopters are fundamentally unstable. The adage among pilots is "An airplane wants to fly. A helicopter wants to crash." You need constant corrective inputs to keep it where you want it. The lack of "seat of the pants" feel and peripheral vision in a sim make it that much harder. There's a couple of things you can do to make things easier to start with:
The details...
OK - that's the basics. How let's hear from the experts.

The rest of this material is taken from various postings on the subject in the Tech list. I haven't done a lot of editing here - treat it as pieces of an ongoing conversation.

Norm Lagasse:

These words from my first helicopter instructor have served me well for the last 22 years: Small, Frequent Inputs. It doesn't matter if you're flying a fully manual heli (Bell 47), a UH-1 or AH-1 (hydraulic boost) or a Sea King or Blackhawk (Automatic Flight Control System with stabilization).

Without a stabilization system in the helicopter (i.e., full manual controls) every movement of one control (cyclic or collective) will in fact require an adjustment to another control (cyclic, collective or pedals).

Once you discover the hover "bug" you'll truly enjoy the flying and the challenges. Just keep this in mind; Flying a normal approach to landing is one of the hardest things to learn to do well in helo flying. When you master that you'll be able to land the helos on oil platforms, building tops and mountain ridges.

Jim McNeill:

The short answer to your basic question is that x-plane does a very decent job of replicating helo flight. The biggest limitation is the limited field of view (FOV) that one monitor provides. In real life peripheral vision is very important in maintaining slow speed/hovering flight, and that is lacking in X-Plane for the most part.

Two comments:
  1. No real helicopter pilot in his or her right mind would attempt hovering flight solely by reference to instruments unless he/she was using a coupled autopilot with hover hold capability... it just ain't possible to do.

  2. When you're making a visual approach in a helicopter, don't try to fly it down to the ground with a constant forward airspeed, like you would in an airplane, and then rapidly decelerate into the hover... that's why it's getting "squirrelly" on you. From the time you lower the collective to begin the final approach, you should be slowly decelerating so that you gradually go through the translational lift point into hovering flight as you reach hover height (normally 2 - 3' skid height above the ground).
Norm Lagasse:

Jim hit the high points on the difference. Helo flying is a lot of seat of the pants flying. The only time you really look at the gauges is for quick cross-check of altitude (radar altimeter), airspeed and power setting (torque). The limited FOV IS the real limitation for normal helo flying. When you're flying approaches to landing or hover, you start at known parameters like 500'AGL and 70 knots. Reduce power (torque) to (try 15-20% torque or 5" manifold pressure) establish a 500 fpm rate of descent and then set pitch to bleed airspeed to maintain that "sink" rate. At 100' AGL you're looking for a sink rate of about 300 fpm and about 40 knots on the airspeed indicator. Just realize that once you get to about 30 knots or so the airspeed indicator isn't reading all that accurately (unless you have an advanced flight director/air data computer/navigation system interlink).

This is where that peripheral vision is key. The rate of closure to your intended landing spot is about the pace you would "walk your dog". The rate is determined by looking out to the side about 45-60 degrees to the right (for a right seater pilot). The other important cue that you're missing in the sim is the seat of the pants feel of when you're transitioning from forward flight to translational lift (about 15-25 knots) to hover. In the real helos you can feel the "burble" as the vortex ring (generated by lift generation within the rotor system) translates through the disc and you feel the support of ground effect hover. As Jim pointed out you're looking to reach hover just as you leave trans-lift.

To challenge your flying skills and learn to shoot power managed approaches you should determine the power it takes to hover. Reduce power about 5% torque or 2" or manifold pressure and use that setting as your "not to exceed" power for the normal approach to a touchdown. If you can consistently fly that approach without pulling in a bunch of torque at the end to "save it", then you're ready for landing on building tops and mountain peaks.

From a simulation hardware perspective I would recommend a separate throttle placed at your left side in a slightly declined position (leaned forward about 60 degrees). This will provide a more natural feel to increasing and decreasing power settings, i.e. pull up for more power, push down for less. Pedals would certainly help "decouple" the hand and control inputs for yaw control (tail rotor). For US made helos, you'll need a touch of left pedal and slight right cyclic for hovering. In European helos, it'll take a touch of right pedal since their blades tend to rotate in the opposite direction.

There's great fun to be had maneuvering a helo at low altitude in close quarters. Have fun!!

Kerry:

It has also been great hearing from people who have flown real helicopters, my guess is that a desktop joystick doesn't give an accurate feel for what it would be like to use the real controls of a helicopter, a slight nudge of the joystick can throw the helo into a spin where I'm sure there would be more subtle feedback in a helicopter.

Jim McNeill:

Kerry, there is feedback, but if anything the joysticks we use with XP are much *less* sensitive than real cyclics. The major difference is that the helicopter hydraulic systems, and some specialized force-feedback systems, can generate an artificial feel. The basic rule that we used to teach new students was to "think" the move... the mere thought of making a move induces enough muscle contractions to create the proper input without over-controlling. Collective levers, by their very nature, generally require more muscle but still demand a delicate touch.

Matt Bailey:

I think Kerry is referring to the short throws of today's PC joysticks. Real helos are probably more "sensitive" in the sense that you make a whole lot more tiny corrections... i.e., ideally you would be making as small a correction as practical (hence "thinking" the move instead of waiting until the necessary correction becomes large enough to require signficant cyclic movement). Short-throw PC sticks don't exactly lend themselves to high precision... a correction that seems like barely a movement of the PC joystick is likely to equate to a comparatively large and wandering deflection of the controls in the real helo/plane. So you *can't* achieve this level of control on a typical PC joystick.

Norm Lagasse:

Fully concur with short throw controls. Having normal length controls (both cyclic and collective) achieves a couple of things for you. For the cyclic you're able to rest your right arm across your right leg to support the arm and that leaves your control touch to just fingertips for movement. It is significantly easier to finesse the control inputs when you're not supporting your arm with the grip of the stick. As for the collective, having a console just to the left of the collective allows you to rest your left pinky & ring finger on a horizontal surface and now you can flex the remaining fingers on the collective. Very easy to make small inputs and corrections that way.

Kerry, sounds like you've been working the hover mode extensively. The pitch up around 20 knots is very accurate and easily detectable in an actual helo. It does take forward cyclic to overcome the pitch up in the disk. If you care to understand why it does this, drop me a line and I'll exceed the layman's language slightly to explain it.

Anyway, here's an exercise to use that hover & transition practice and take it a step further. If you're flying a skid equipped helo, get the bird stabilized in a 4-5' hover. Note your power setting. Now slowly accelerate forward without changing power and accelerate through trans lift and get about 50 knots on the A/S indicator. Now ease the cyclic back to climb at 50 knots with hover power and clear a 100' obstacle.

Start a good ways back initially and work your way closer and closer to the obstacle. This technique is excellent practice for Max gross weight takeoffs from confined areas. When you've worked your way very close to the obstacle and can consistently clear it, move in a little closer and try applying right pedal in the climb.

If you're flying a wheeled helo, take the hover power minus 10% torque or 2" of manifold pressure. Start the helo rolling on the ground with that adjusted power setting and accelerate past trans lift to about 30 knots. The bird will lift off a little after that and then you can continue accelerating to 50 knots. The remainder of the maneuver is the same. This exercise provides good training for taking an wheeled helo off a runway at max gross weight and high DA. (Not enough power available to hover).

Jim McNeill:

I'll add one thing to your dissertation, Norm. You can do the same running takeoff exercise with skid-mounted helicopters. Raise the collective just enough that the helicopter gets light on the skids then maintain that amount of power with the collective and veeeeery slowly feed in forward cyclic. As the helicopter accelerates it will start to climb. Feed in enough forward cyclic to prevent the climb without descending back into ground contact.

The helicopter may occasionally skip off the ground but that's ok. At somewhere between 15 and 20 knots you'll begin feeling the translational lift shudder and nose pitch at which time you will accelerate into forward flight. With practice you'll find that you can make takeoffs at power settings as much as 5 pounds below hover torque. And one nice thing about doing it in sim... you don't wear out skid shoes ;-)

Norm's absolutely correct about getting a good "weld" with your cyclic forearm. It's a major problem for most new helo pilots, and because of differences in pilot stature and seat position, etc, there's no one position that works for everybody... you have to experiment to find a position that works for you.

Norm Lagasse:

You can also do this exercise on the water with amphibious helos, Sea King, Jolly Green Giant, Jet Ranger on Floats. Some of the best fun I've had was making water landings in H-3's (AF version of Sea Kings with retractable trike gear and rear ramp). I need to go drop the XP version in the water and see if I can shut it down. The two 360's during shutdown are always a treat!

Jim McNeill:

Ground effect has the same effect on rotary wing aircraft as it does on fixed wing ones. Helo pilots talk of HIGE, Hover In Ground Effect, and HOGE, Hover Out of Ground Effect, power to describe it. In essence, translational lift is the point at which the helo "translates" from requiring HIGE to HOGE power due to it's accelerating off the high pressure "bubble" or, more accurately, vortex ring of air it's been riding on. It's at this point that the vortex starts to flow up through the rotor rather than remaining under it; and that is what causes the shudder and pitch change. There is also an accompanying yaw change that occurs at this point.

Norm, did I leave anything out?

Norm Lagasse:

Pretty close. The vortex ring is actually on the outside of edge of the rotor disk. The vortex ring encased, ground effect "cushion" can be thought of in simple terms with the dynamics of an air hockey puck. The edge lip would be the vortex ring and the air trapped in the center of the puck the lift air affected by decreased downwash due to ground effect. As you said Jim, the shudder is in fact caused by the vortex ring translating through the rotor disk. Think of it as wingtip vortices sliding through the rotor disk and causing turbulence on each "wing" as it passes through. The pitch up is actually a function of the rotor blades in the forward half of the disk being affected by the induced airflow over the "wing" (combination of rotational airflow plus forward motion induced airflow). On the advancing blade, increased airflow - on the retreating blade decreased airflow. Due to rotational dynamics the force is applied at the 3 & 9 o'clock positions and manifests itself 90 degrees beyond in the rotation. This results in the disk lifting in the front and lowering in the back. The aircraft will follow due to the pendulum action. Ok, now does anyone's brain hurt this early in the morning?

Ian Mathieson:

Is there any correlation between the change of power required or the change of lift resulting, from the transition out of and into ground effect, and the change of lift and power required, at the 20 kt transitional speed discussed earlier in this thread which gave rise to my original query? Are they interlinked, different manifestations of the same effect, or completely un-connected (I'm assuming the helo is moving out of ground effect my increasing its forward speed at a low height above ground - the situation relevant ot he previous discussions)?

Also, am I correct in inferring from your explanation, that when the helo moves out of ground effect, MORE power is required to maintain height or upward velocity because it is no longer resting on a bubble of what is in effect compressed air?

Jim McNeill:

I guess the best way to think of it is that they're different manifestations of the same thing. Read Norm's last post. The vortex ring surrounds the "air cushion" As long as your directional flight speed -- forward, lateral, or aftward -- stays below translational lift speed you'll require HIGE power to hover. If you climb vertically you'll require increasingly more power to remain at the hover until, at about one rotor span off the ground, you'll be totally out of ground effect and need HOGE power to hover. At that point, if you set up a relatively high rate of descent with a low directional speed and try to arrest the descent by adding power (collective) all you do is to accelerate the vortex ring and rapidly increase the rate of descent. This is called "settling with power" and has caused many helo crashes.

As you accelerate directionally, you start to outrun that high pressure bubble of air and it takes less power to maintain altitude and accelerate because the relative wind (caused by the aircraft's motion) is providing "fresh" air through the rotor which is contributing more to lift and less to drag (the vortex ring). If you decelerate at that point you return to the hover. IF you continue to accelerate you have excess power available to climb and increase speed.

If the wind is blowing strongly enough, it's possible to be in translational lift while still sitting on the ground.

Norm Lagasse:

Since helos can operate out of remote locations, it's important to understand the dynamics of hovering the helo and the associated power requirements. If you end up on a small pinnacle pad (less than 1 rotor disk in diameter) or on a significantly sloped area like the side of a hill, you're ability to hover may be hampered if there is insufficient power available to hover OGE. Slope or small ground space under the disk will prevent the IGE cushion from developing.

As Jim pointed out the wind can certainly affect your ability to hover. 5-10 knots of wind can "disturb" the vortex ring and prevent IGE hovering due to insufficient cushion and power limitations. 15-25 knots of wind puts you right into trans lift and your power requirements are significantly less.

That's why helo flying is always a challenge and usually a blast!!

Jim McNeill:

FYI, the FAA publishes an excellent training manual that will give you much more detailed insight into helicopter flight. It's a pdf so you'll need Adobe Acrobat or the reader and is available at http://av-info.faa.gov/data/traininghandbook/faa-h-8083-21.pdf.

Todd Fredricks:

I think Jim has covered it well, but I would add this wisdom from an Army Warrant IP that I was learning from.

The situation was that I was in an OH-58C and I had maybe 5 hours total time in helicopters (I had oodles of fixed wing but it wasn't helping that day) and the task was try and keep the thing within a two rotor disc circle. Well I had the thing everywhere but in the circle and finally I tried to swap tail for nose and with a quiet but firm "I have the flight controls." the warrant took over and the helicopter settled down to a 3 foot hover that couldn't have been more stable had the thing been welded to the ground.

I sat there, rotors spinning, frustrated as all get out, wondering how I was ever going to get the thing to work for me and the warrant said, "Quit thinking about what you need to do and just make it do what you want it too."

Sage advice.

I was trying to think about which pedal to push with power inputs and when to raise and lower collective in which turns etc, but in a helicopter with that big spinning gyroscope above everything happens 90 degrees from point of input and so by the time you try and think through what you need to do you are already behind the machine and the example of this is watching a young helo pilot (or myself at that time) oscillate and gyrate all over the place. The pilot is trying to think while they fly. You stop thinking and just push or pull whatever you need to to make the machine stay where you
want it. Small inputs if you please. Input, wait, input, wait.

The easiest way for me to "get it" was to understand that a helicopter flies in two ways; it pushes a lot of air down and in response to Newtonian physics the machine lifts off the ground (not really flying just thrust over matter although the rotor is flying through the air in a hover and creating lift) and the second is in forward flight when the rotor disk itself begins to act like a fixed wing through the cruise envelope and then it is just like flying a fixed wing save for pedal input which is never like rudder...AHEM...but forget that and also forget about retreating blade stall which we can talk about at some other point...

Once I realized that the only 'new' flying that I needed to get a handle on was the air pushing down part and that that type of flying had nothing to do with cruise (which is just like flying an airplane but different...is this making sense yet) it got much easier.

I just realized that as I started my approach, long before passing ETL (Effective Translational Lift) my ONLY job was to pick a touch down point and do whatever it took to make that point coincide with my achieving a 3 foot hover over it. So there is the key, the touchdown point. You need to clearly see it and you need to have some visual (not instrument) cues as to your rate of closure because unlike an airplane where you fly an approach at 1.2-1.3 of your stall speed for your configuration and then you roundout/flare with the knowledge that you will touchdown at stall but never with NO velocity, in a helicopter you are going to land with 0 forward velocity (roll on landings notwithstanding).

This is why you cannot 'think' about what you are doing. You simply cannot think fast enough. You have to 'see' what you are doing because it is your eyes and butt which tell you if your rate of closure is too fast and if the two vectors, those of vertical and forward velocity are trending smoothing down toward 0. A good helicopter approach should look like a straight line to the touchdown point.

ETL is not as dramatic as you might think, but what will happen is that you will need to add a bit of collective (throttle) because the sink rate will increase as the machine shifts from flying by means of the physics of the rotor disk to depending upon a lot of air being pushed down. And ETL passage is not an instantaneous thing, you will feel it build as you decelerate and then shudder and then smooth again so as you start to feel the vibration, you will have glanced at the airspeed indicator on take-off and know about where to anticipate it on approach, you just pull in a bit of collective and continue. From then on down it is smooth, small inputs as you adjust rates of closure to arrive at a hover.

Now having a penchant for words and not wanting to drag this out forever but wanting to be helpful I would add one more thing. You might find that a way to master hovers is to practice roll ons. The one thing I like about Blackhawks (I like a lot of things about Blackhawks, namely a bunch of redundant power) is that they have wheels. So what you can do is a roll on landing which means that if you can't quite get a hover at 0 airspeed, practice landing a Blackhawk on a runway at slower and slower airspeeds until you feel comfortable with control inputs to just make the landing speed 0.

You will find that a touchdown of about 30 knots works best.

Another good exercise for budding helo pilots is to sit in your lazy boy recliner and just mentally visualize the rotor disk and the thrust vector on the disc depending upon orientation. The reason why this is valuable is that you are not just changing power in a helicopter approach but you are actually changing the thrust vector of the 'wing' itself by pulling back on the cyclic. The end result is that the horizontal component of lift in the equation is reduced while the vertical component is increased. This means you will need less power to maintain altitude and so you need to be able to move both hands independently without thinking. Roll-ons allow you to do this gradually as you develop the skill.

You can do 'roll-ons' with skids it just makes things more exciting what with the sparks and all. Very fun at night.

Jim McNeill:

Nice description and a very valid point about the thinking too much part, Todd.

The old adage about just thinking the movement you want, rather than trying to physically move the controls is still valid. When you think about making the movement, your muscles move just enough to move the control the amount you need. Anything else is over-controlling. That's probably the hardest thing that airplane pilots making the transition have to learn: Except in certain emergencies, the best thing to do is often nothing at all. Just sit there, relax and you'll find that you'll have just the amount of control authority that you need....anti-torque pedal control excepted. Right, Todd  :-)

Kezza:

Can I make one more request, a bit like talking me through a standard approach and landing and what I would expect to experience in a real helo. I'd like to practice it until I'm on cue. Say I'm approaching a landing pad at 1,000' above ground, how far from the pad would I start my descent, what speed would I be doing, Jim says an approach of 10 degrees is about right? What height would I go through the transition assuming 20 knots transitional speed?

Todd Fredricks:

Well, here is the rub, all and any approaches are okay. The approach to a carrier might be to match speed along side the thing, then translate laterally til you are over the deck and then set it down. To an oil rig to you might start the approach as a circle around the rig to recon obstacles and as you come around bleed off speed until you are moving at 5-10 knots and fly into the wind if possible over the pad and then set down. To a runway, more like a fixed wing. A building top will be start at 500 to 1000 feet above the level of the pad and orient into the wind and fly it down to the touchdown. The only real reason to start at any altitude that is much higher is so you have the opportunity to stabilize your approach. As you gain confidence and skill you will know how to configure your aircraft at any altitude to successfully arrive where you want and need to be. Just practice with runways for a while, then move to buildings and then oil rigs and finally the USS Laminar underway in heavy seas.

Jim McNeill:

I couldn't have said it better, Todd.

Kezza, just remember that the steeper the approach angle is, the slower airspeed and rate of descent you need to keep it stable.

Once you initiate the final approach, the military way is to make the approach a constant angle with a constant, gradual deceleration so that you zero out the airspeed at a stable 3 foot hover over the landing spot. With practice, you can do it every time, regardless of the type of spot to which you're landing.

Don't worry about ETL. If the winds are strong enough you may never reach it until after you shut the fuel off and the rotor RPM is winding down; or if it's a very steep approach you may go through ETL as soon as you start the collective coming down on final approach. Like Todd says, every approach is unique.

DD:

All the helo drivers I've talked to have said that they prefer high wind conditions for flight over calmer conditions. They would rather fly in 20+ knot winds than calm. However, I've never been able to get an satisfactory explanation of WHY.

Jim McNeill:

Sure. The wind hides their mistakes :-)

Seriously, the big thing is that they are constantly in ETL which means that they don't have to worry about an Out-of-Ground Effect Hover (called HOGE) which takes *much* more power than a HIGE does; and that translated into being able to carry a larger load, more fuel, or having more power in reserve if needed.

The problem with many approaches is that the wind speed decreases and changes direction as you descend below any barriers on approach (unless you're landing to a totally exposed pinnacle) so good pilots never count on making a wind-aided approach....it's just an additional bonus if it occurs.

Todd Fredricks:

I would add that due to the nature of the rotor/lift system in a helicopter, while the disk does act like a big wing, in situations of unstable air, read turbulence, the system is much less susceptible to the bouncing and uncomfortable attitude changes that occur with low wing loading airplanes. So in winds, unless you have a lot of surface shear (which can make landings interesting) you will find that circumstances which would be downright "I want to land and go home right now" in a light airplane, are a non-event and another day at work in a helicopter. Again, nothing worse than gusts and a pinnacle landing site.

Jim McNeill:

Good point, Todd, although that's a double-edged sword, especially for older fully articulated or semi-rigid rotor systems. The problem is that although the occupants don't feel the turbulence the rotor system is reacting to it and is flapping (wobbling) ever closer to the mast. Mast bumping is not something you want to encounter, as the smallest bump can deform the mast resulting in catastrophic failure.

And people wonder why helo pilots are pessimistic introverts :-)

Todd Fredericks:

The point was directed as an illustration of the differences we feel in helos over fixed wing in situations of gusts and wind. We have not even yet discussed PIOs and mast bumping. Part of the MTP return to service checklist on the OH-58 is rapid and abrupt cyclic inputs to test the dampeners. The first time I experienced that it was enough of a shock to know that I never wanted to experience full cyclic inputs and test the integrity of the system.

Having never experienced blew blade syndrome after a bump I can say that you can fly a UH-1 in some pretty stiff stuff and that was my point, the kind of air that would really make a person not want to be in a 172, and still be perfectly safe.

Point well taken.

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