Lesson 4: Flying Helicopters (I)
We started out discussing types of helicopters, types of rotor systems, and elements of control systems. There are a few more technical things to discuss, but we will wait until they become truly necessary. The basics that we will discuss in this first column on really flying consist of: straight and level flight, turns, acceleration and deceleration, and ascents and descents. These maneuvers in helicopters are really similar to those in fixed wing aircraft, but the control motions are much more sensitive and you will need to coordinate throttle application with the collective stick and anti-torque with the pedals. Of course, X-Plane will apply engine power as you raise collective. A full suite of controls is highly recommended (particularly a throttle).
First, select an aircraft to fly. I recommend a larger aircraft such as the S-76 to start. Set up your joystick for minimum deadband and set the aircraft up at an altitude of about 2000 feet and an airspeed of at least 60 knots. Location and heading are your choice, but you can use any heading 3 miles east of MMK to be sure the area is somewhat usually clear. This is a designated practice area for helicopters operating out of Meriden Airport in Connecticut, for those of you interested in realism. Remember that it is essential to keep rotor speed at 100%, although with power applied X-Plane will handle this for you quite well.
Straight and Level Flight
It might be convenient to set the altitude hold feature in the autopilot for your desired altitude, just to ensure some pitch stability. Adjust your stick to stay on a constant heading. Adjust your throttle/collective until airspeed is somewhat constant (try to stay between 60 and 100 knots). Add some pedal to center the slip/skid ball. Work with this for a while until you are comfortable with the sensitivity in roll. Play with your pedals a bit just to get an idea of how they will respond (actually, it might be convenient to change the instrument panel to a conventional one in Plane Maker to get the slip/skid ball). Note where the horizon crosses your screen for the airspeed you select to fly. If this is too difficult, work with the attitude indicator, but dont spend a whole lot of time with it.
Select the heading hold feature on the autopilot and de-select the altitude hold. Now try to hold altitude and airspeed. Note the sensitivity of the aircraft to your pitch inputs. Again, reference your outside horizon or the attitude indicator, or better yet, work with the vertical velocity indicator (unlike airplanes, trainer helicopters have extremely minimal instrumentation I didnt see a VVI until my fourth aircraft and quickly fell in love with it).
If you are feeling comfortable (and ambitious), de-select heading hold and continue on a constant heading with constant altitude and airspeed.
OK, youre doing great time for some turns. Re-select altitude hold on the autopilot and get comfortable on a heading. Try a 30 degree turn to the left. Note that you dont have to pull the nose up or add any pedal to keep altitude and turn coordination. Its OK to cheat here keep an eye on the attitude indicator and put the needle on the first mark to keep the turn shallow (steeper turns will require a more noticeable application of power). Watch the outside horizon and/or the turn coordinator and keep the view steady. Roll out of your turn by applying some right cyclic and stabilize yourself. Then try a turn to the right. Again, if you are feeling ambitious, de-select altitude hold and try the turns again. As long as you have some forward speed, this shouldnt be too difficult. If you are still feeling ambitious, try a steeper turn. If you are still not humbled, try maintaining a standard rate turn by monitoring the turn coordinator.
Ascents and Descents
The difficulty with ascents/descents and acceleration/deceleration is the coordination of the throttle/collective and the pedals. Since we have already worked on holding airspeed, we will put off acceleration and deceleration for a few minutes. Start by selecting heading hold on the autopilot.
While holding airspeed, slowly lower the collective a bit. As you do this, you may note that the nose swings to the left so slowly add some right pedal until the slip/skid ball centers. You also may have to adjust pitch attitude slightly. Continue lowering the collective until you have achieved a 500 foot per minute descent while maintaining your selected airspeed. Now apply up collective until you have arrested your descent. Remember to apply left pedal.
Now pull in some more collective (and add some more left pedal). Watch your airspeed and try to achieve a 500 fpm rate of ascent. If you have run out of collective or engine power, pull back slightly on the cyclic stick to slow your airspeed (but not to less than 60 kts). When you are near your desired altitude, you can begin lowering collective to level off. If you reduced airspeed to achieve your climb rate, you might try pushing the cyclic forward a bit to your original airspeed before lowering collective.
While it may seem easier to move the cyclic to achieve your climb or descent, this is considered bad form (and in some aircraft it may result in serious damage or injury details later). For small changes in altitude (50 to 100 feet), judicious cyclic changes in lieu of power changes are not that unacceptable, but this is no time to develop bad habits.
Continue to practice altitude changes, and if you desire more challenge, de-select the autopilot.
Accelerations and Decelerations
The coordination required for acceleration and deceleration is similar to that required for ascents and descents except that more deliberate application of cyclic will be necessary. Select the autopilot as desired and slowly lower collective a bit. Take a quick glance at the VVI, and as soon as you note a slight loss of altitude, apply a bit of aft cyclic stick. Continue to slowly lower collective and move cyclic aft while ensuring that you are maintaining constant altitude. As you approach your desired airspeed (try for something 20 to 40 kts less than your original speed), slowly move the stick forward and add collective (and pedal!!!) as required. Note that there may be some lag in the response of the airspeed indicator, and be prepared to adjust airspeed a bit after you have leveled off.
Once you are stabilized, add a bit of forward cyclic stick. Again, as you note a slight loss of altitude, add in some collective. Continue to slowly add cyclic and collective until you have achieved your desired airspeed 20 to 40 kts greater than when you started. Unlike a deceleration, the sticks should end up pretty close to where you want them at the end of the acceleration, although you should be prepared for some minor tweaking to hold your desired settings.
It is obviously convenient to combine climbs (or descents) with turns. The goal is to maintain consistency in both rate of turn and rate of descent and finish both the turn and the climb (or descent) at the same time. A useful exercise is to descend 1000 feet and turn 360 degrees. At a standard turn rate of 3 degrees per second, descend at 250 feet per minute. You can try the same with a climb.
There are three considerations to successful completions of these exercises. These are: control application, power available/power required, and dissymmetry of lift.
Control application is applicable to both helicopters and fixed wing aircraft. Try for smoothness in application of your controls. Make small and slow adjustments, and note how the aircraft responds. Avoid rapidly pulling back on a control if you really want to push it forward but have found that you may have gone too far. Work on coordinating application of power and pitch (and pedal). Remember when we tried to decelerate the helicopter? First we reduced power but then waited a bit before applying cyclic. We cant feel the altitude change in our seats in a simulation, so we have to simulate that the inertia of the aircraft will keep it from immediately responding to the power change, must be vigilant in anticipating this response, and be prepared to apply pitch control at the right moment to achieve the desired result. The goal here is to achieve some measure of precision in the control of the vehicle. We want to change altitude without changing heading or airspeed, change airspeed without changing heading or altitude, and change heading without changing airspeed or altitude. When we do desire a change, it should be at a consistent rate (as measured on the instruments) and should not result in a change in some other parameter.
Power available/power required is a somewhat technical concept, but also applies to airplanes as well as helicopters. Power required for a given flight condition is a function of drag. There are several components of drag, two of which work in opposite directions to each other. One of these is called form drag, and is caused by the speed of the aircraft. Form drag increases proportionately to the square of the speed of the aircraft. It can be affected by lowering the landing gear, extending flaps, using external fuel tanks, or deploying spoilers or speed brakes. Another manifestation of drag is called induced drag and decreases with the square of the speed of the aircraft. If you add these two drags, the resulting curve will be somewhat U shaped. This total drag is related to the power required to maintain the aircraft in a given flight condition. Density altitude, gross weight, and angle of bank can also affect power required.
Power available is that power capable of being produced by the engines. It can be affected by density altitude. The difference between power available and the power required for a given condition is your excess power available and determines how fast you can go, how fast you can climb, how much weight you can carry, and how steeply you can bank. The bottom of the U in the power available curve relates to your best endurance airspeed (how long you can stay in the air), gives the best rate of climb, least rate of descent, and allows the best maneuvering airspeed for steeper turns. A line drawn from the zero airspeed/zero power point of your graph to a point tangent to the U curve will give you the best range airspeed and the speed for longest glide (consider that fuel flow is proportional to power, and so miles per hour divided by gallons per hour results in miles per gallon this tangent line will have the shallowest slope that intersects the power required curve, or the best miles per gallon figure). Note that the best range speed is greater than best endurance speed. Approximate numbers for the least power required point are between 55 and 80 kts. For the best range speed, approximate numbers are between 70 and 100 kts.
It is also important to note that at some point it takes more power to go slow than to go fast. It also takes more power to hover than to move forward. Remember this, as we will make use of it when we discuss approaches and departures, as well as hovering.
Dissymmetry of lift only applies to helicopters and is less pronounced on teetering rotor systems. The phenomenon results from the fact that the advancing side of the rotor disc (typically the right side) will be moving faster through the air than the retreating side. This is because the air over the advancing side is moving at the sum of the speed of the rotor rotation plus the forward motion of the aircraft, while on the retreating side the air is moving at the speed of rotor rotation minus the forward speed of the aircraft. As the aircraft moves out of a hover into forward flight, this will show up as a tendency to roll (typically to the left, because there is increased lift on the right side of the rotor disk). Some aircraft have a lateral trim beeper switch to minimize this effect on the stick forces (of course, you must remember to trim back to the right as you decelerate). There will be an area on the retreating side of the rotor disk where the net airflow will result in local stalling. As airspeed increases, this area increases until a serious portion of the retreating side of the rotor disk is no longer providing sufficient lift, resulting in what is called retreating blade stall. This is one possible consideration for a limit on the forward speed of a particular helicopter (others include remaining control margins or an undefined structural limitation).
Because a teetering rotor system has blades that are rigid in flapping but free to teeter at the top of the rotor mast, this rolling effect is usually transparent to the pilot. Retreating blade stall is still a factor, however.