1 - How to fly helicopters in X-Plane

[brett s]

Here's a quick tutorial on basic helicopter flight in the sim, as of version 9.01 when this was written – not going into detail here about how specific systems work or things that apply to all X-Plane aircraft, just some of the differences that apply to helicopters to get you started. We'll use the Hughes 500D but much of this will also apply to all helicopters.

Controls

One important consideration – you truly need full joystick controls to effectively fly a helicopter in X-Plane, including pedals & throttle (configure it to control "collective", and remember that it's backwards – raising it or pulling back = add pitch). Anything short of that limits your control severely!

First, a description of the main controls:

• Cyclic, in the pilot's right hand – controls the helicopter's pitch & roll attitude by cyclically changing the main rotor pitch. Push the stick forward & the rotor disk tilts forward, etc.
• Collective, in the pilot's left hand – controls the main rotor blade pitch. Raising it increases the pitch on all blades, lowering it reduces the pitch on all blades.
• Pedals, controlled by your feet – increases or decreases the pitch of the tail rotor blades, push your left foot fwd & the nose turns to the left.
• Throttle – in the sim it's handled automatically by a rotor governor, it will attempt to maintain rotor rpm at 100%. Most real ones do the exact same thing – older piston ones mechanically (which require some pilot input, they aren't perfect), most modern ships have true governors.

Gauges

The single most important gauge on the panel for a helicopter pilot is the rotor tachometer. If you lose rotor rpm in flight, very bad things start happening – below a certain point the rotor blades will stall, it's generally not recoverable as they can start to diverge from their normal plane of rotation & start hitting things.

Most helicopters have a dual tachometer with both engine & rotor needles, in normal operation the needles overlap & appear as a single needle – makes it very easy to see if the engine has failed. On this particular one the inner needle is rotor rpm & the outer one is engine rpm.

Most of the other gauges are also applicable to airplanes, so we're not going to talk about them here.

Flying

In the real world, student pilots are trained by an instructor that would normally start you off with easier tasks like straight & level forward flight, followed by some turns & then maybe a few minutes of hovering starting with one control at a time at the end of the lesson. The instructor would also save you from certain death (repeatedly) at first as most students lose control pretty rapidly especially in a hover!

Helicopters take practice, they aren't naturally stable – another thing that takes time is learning how the controls all interact with each other. The average pilot in the US takes about 65 hours to get a private pilot rating, 15-20 hours before they even solo (if training in a Robinson, 20 hours in the minimum to solo per FAA regs these days). So don't expect to instantly be perfect at everything

Adjusting any one control almost always requires compensation with one or more others, with practice you'll start doing that without having to think about it. You're almost always making small corrections, especially in a hover – the controls are very sensitive, most people will over control at first. There's also a slight lag between making an input & when the ship responds that adds to that tendency.

Performance

It takes more power to hover than in low/medium speed forward flight, and it takes less power to hover near the ground within about one rotor diameter (in ground effect – IGE) than it does to hover out of ground effect (OGE) – these are important things to keep in mind. Detailed performance charts & theory are way beyond the intended scope of this basic tutorial, there's a wealth of information available online for those interested.

Hover

In X-Plane you don't have the luxury of an on-board instructor – so we'll go ahead & start off on the ground, ready to lift off. You should have the cyclic & pedals centered, collective fully down, and rotor rpm at 100% with the governor "on". Wind – always know where the wind is coming from, takeoff & land into it as much as possible.

Slowly start raising the collective – as the ship gets light on the gear it may start to drift or spin, letting you know to make a cyclic or pedal correction to compensate. It should take you several seconds to lift off, think "smooth". Our example helicopter will require some left pedal & left cyclic as you get light.

Note that the helicopter is tilted slightly to the left even in a stationary hover – the tail rotor thrust, used to compensate for main rotor torque, actually pushes the entire ship to the right & compensating for that requires a bit of left roll.

Once you're off the ground, pedals are used to maintain heading, cyclic your position, collective your altitude – at first, you probably won't be able to keep it from moving around quite a bit. Don't feel bad, you'd go through the same thing in a real one!

With practice you'll be able to shrink the size field required to contain your hovering – one tip that applies both in the sim as well as the real world is don't look down at the ground right next to you. Look further out, it's much easier to judge small attitude changes before you start moving. You're at a disadvantage in the sim to begin with because there's no "feel" or peripheral vision so it's hard work.

Departure

Ok, now let's transition to forward flight – from a hover, add a small amount of forward cyclic to start moving. You only want to nose down a few degrees & start gradually accelerating. Maintain heading with the pedals, you shouldn't need to add any collective at this time – as you pass through about 15-20 kts airspeed you'll notice a large increase in lift (known as ETL, effective translational lift).

Continue to accelerate, keep your altitude relatively low until you reach about 50 kts & then adjust your attitude to start climbing. The exact target climb speed varies by type, we're going to use 60 kts here. Keep the ship in trim with pedals, the yaw string is your friend here.

Cruise

As you reach the desired altitude, accelerate to the desired speed (we'll use 80 kts here) & then reduce collective as needed – which will require you to also adjust the pedals & cyclic of course! Use cyclic to adjust your attitude & in turn your airspeed.

To turn, just apply a bit of cyclic in the desired direction– that's all you'll need in a shallow turn. Steeper turns will require either collective or aft cyclic to avoid descending.

To accelerate, you'll need a combination of collective & forward cyclic; the opposite to slow down. In either case you'll have to make a pedal adjustment to stay in trim. Try accelerating to 120 kts, then slow back down to 80 kts again – as always, do it smoothly & things are much easier. Note the power required at those speeds – much higher at 120 kts than 80.

Approach

Ok, so now you're ready to head back down & land – unlike airplanes, a helicopter final approach isn't done at a constant speed. You're continuously slowing down as well as descending, aiming to end up passing back through ETL at around 10' above the ground. A "normal" approach angle is also steeper than airplanes would use, about 10 degrees.

Once intercepting your desired approach angle, lower the collective to start descending (as you do that it'll require a touch of aft cyclic to keep the nose from dropping & pedal to stay in trim). Use the collective to control the angle of approach – one way to help judge the approach angle is by aligning the touchdown point with a spot on the windshield, it should remain constant. If it's moving up you're going to undershoot & need to raise the collective a bit, if moving down you're going to overshoot & need to reduce collective a bit.

Use cyclic to maintain your rate of closure – the idea is a nice gradual deceleration, not a huge flare at the end. One trick is to keep your apparent rate of closure at what appears to be a fast walking pace.

As you near the touchdown spot & slow through ETL, the loss of lift will require a noticeable amount of collective increase – as always, compensate as needed with pedals & cyclic. If you planned everything out right you'll end up in a nice stable 5' hover right over your spot, gently set it down 

Good approaches take practice, and it's harder in the sim than in real life because of the limited visual cues – so don't get too discouraged at first. Keep on practicing & it'll all come together!

H/V curve, emergencies, etc.

So, what happens if the engine quits while in cruise flight? Helicopters are capable of autorotation – the collective is lowered to maintain rotor rpm, the rotors are driven by airflow passing through them as you descend. Glide angles are much steeper than an airplane, in the 3-4:1 range typically but the pilot retains full control.

Landing without power is just an exercise in energy management – flare to reduce speed & rate of descent, cushion the landing by raising collective & touching down before you run out of rotor rpm.

A successful autorotation, meaning you walk away & can re-use the aircraft when you're done, is only possible within a certain range of airspeed & altitude – these limits are derived from actual flight testing & called the height/velocity curve. Stay out of the "avoid" area unless necessary, a power failure would mean almost certain damage and possible injury or death.

Each helicopter type has a unique H/V diagram, but they all share the same basic shape – note how the takeoff & landing profiles we used earlier fit in here.

Autorotation demo

Here's a straight-in touchdown autorotation from 500' AGL, using 60 kts as our target approach speed. When lined up & at the appropriate angle to hit our target, collective down & throttle rolled off to enter autorotation – the engine/rotor tach needles are split, showing that the engine isn't powering the rotors.

Adjust pedals to stay in trim, cyclic to maintain 60 kts – monitor rotor rpm, if it's getting too high use collective to keep it within limits (if turning steeply you'll almost always need to do this to keep from overspeeding). At around 40' AGL we'll start to flare, reducing our speed & slowing our rate of descent – as we near touchdown fwd cyclic to level the ship & collective to cushion, pedals to stay aligned. A short ground run is acceptable if the surface permits, but it's critical to have the ship level & aligned before touchdown – note that from about 500' agl we were on the ground in just over 30 seconds.

Hovering autorotation

In a hover things are bit simpler – you're simply trading off rotor rpm to cushion your landing. You have a limited amount of energy, this is the reason for the lower part of the H/V curve – most types limit you to around 10' AGL.

Here's what it looks like – in a nice steady 8-10' hover I'll kill the engine (throttle response in the sim is slower than reality when rolling off, so the needles won't quite split sometimes). That'll require an immediate pedal input to keep from spinning, since all that torque is now gone – rotor rpm also immediately start winding down, as you near the ground cushion your landing. Touchdown happens pretty quick 

Votex Ring State, also known as settling with power – at low airspeed in a powered descent the rotor can be descending through it's own downwash, the turbulent flow causes the rotor to lose efficiency. This drives the power required up sharply, most ships don't have enough excess to overcome the resulting rate of descent – if near the ground that means a bent helicopter. In the incipient stages there will be vibration building up, the sim doesn't model that.

The actual descent rate & airspeed range full blown VRS is possible at all depends on the rotor downwash velocity, which in turn is dependant on the disk loading. A light piston ship like the R22 would require a powered descent rate of 700+fpm for incipient VRS & in the 1000 fpm at less than 20 kts airspeed for full blown VRS – ships with higher disk loading require higher rates of descent & the upper airspeed range is higher (an extreme example would be the V-22). Recovery is by either gaining airspeed or entering autorotation, both of which will require altitude. The moral of the story is it's something to avoid altogether, rather than having to try & recover from.

Low RPM

It's possible to demand more power than is available in most types, the rotor rpm will start dropping – this causes several things: less lift, even less power available in piston engines, and loss of tail rotor effectiveness. Since the tail rotor turns much faster than the main rotor, it's available thrust goes down faster as rpm drops.

The only way to regain rpm in this case is by lowering collective, not a natural act as the ground is approaching!

Low G Load

Helicopters don't like low or negative G loading – teetering rotor (most 2 bladed ones) control authority is proportional to g load, at 0 g you have no control at all. Once the airframe & rotor start doing their own thing very bad things start happening – the rotor head can contact the mast (usually followed by it's departure from the helicopter), or blades going through the airframe. No low G flight at all in these - avoid abrupt forward cyclic inputs at high speed.

Articulated & rigid rotor systems still have control authority at low G, but are still limited by the airframe & rotor system. For example, an AH-64 is a pretty aerobatic helicopter – but it's G limits are -0.5 to +3.5, still pretty low compared to a fixed wing.

Known issues

• As of this writing ( VRS is simulated but the actual effect on the aircraft is incorrect – rather than the rotor's induced drag going up & power required doing the same, the sim induces a rate of descent & unloads the rotor instead. If you've ever been descending at low speed & started having the rotor rpm spike up or you start to red-out from negative g loading – you just triggered VRS. The range it's triggered in is pretty close, hopefully the effect will get fixed eventually.
• ETL is rather abrupt – too much at the lower speed end, with a large dip in lift just prior to it starting.
• Power requirements in a hover are a bit high, and in high speed cruise is a bit low.

The nice thing about X-Plane is Austin really does care & listens – most of the above are on the list to get more attention

Edit - some of the above stuff has been addressed, VRS is modeled better but the huge dip in lift when passing through ETL is still there; and the random cyclic inputs while hovering IGE are gone.