Estimated reading time 24 minutes, 28 seconds.
Revolutionary is defined as constituting or bringing about a major or fundamental change. The argument over whether or not the Bell-Boeing V-22 Osprey is a revolutionary step in aviation continues to be fought. While the truth may lie somewhere in the middle, the final verdict is probably years down the road.
Writing a story about the V-22 without delving into the controversies may sound impossible, but that is exactly my plan. There have been plenty of articles published detailing the development and introduction of the Osprey, but they fail to tell the reader one key thing: what its like to fly the airplane. Thats the intent of this article to give you an inside glimpse into the flying qualities of the Osprey and with it Ill throw in both the good and bad of the airplane. So, if you want to read about the politics youll have to look elsewhere, but if youre interested in learning what its like to pilot the V-22, then hop in for the ride.
First Things First
There are several important points to keep in mind about flying the Osprey. First, notice in the paragraph above that I describe the Osprey as an airplane, not a helicopter. Most people think of the Osprey as a rotorcraft, but the majority of a V-22 pilots flight time is spent thinking, flying and operating in an airplane environment. Consequently, the V-22 should be more precisely categorized as an airplane that just happens to take off and land vertically, or better yet, as an official VTOL (vertical takeoff and landing) airplane. Even the United States Federal Aviation Administrations official powered lift categorization of the Osprey is misleading, because it implies that lift is always produced by the rotors, when in fact this is only a small portion of the flight profile.
Secondly, the key to unlocking the performance of the Osprey lies in understanding the powerful concept of controlling the thrust vector by moving the nacelles. This is a unique concept, but is somewhat similar to a thrust-vectored airplane like the Hawker Siddeley Harrier.
Finally, the Osprey is flown by establishing a nacelle angle that generally corresponds to a specific airspeed. There are three distinct modes of flight: 1) a helicopter or VTOL mode, where the nacelles are at 85 to 96 degrees; 2) a conversion mode, where the nacelles are between one and 84 degrees; and 3) an airplane mode, with the nacelles at zero degrees or on the downstops. The terms transitioning and converting are used when describing changing the flight mode. Transitioning the Osprey means to change from helicopter mode to airplane mode and converting is just the opposite. For reference, a 30-degree nacelle setting roughly corresponds to 130 knots calibrated airspeed, or KCAS, 60 degrees will give you 110 KCAS, and 80 degrees equates to about 80 KCAS. These will vary slightly depending on environmental conditions.
The Front Office
Climbing into the front seats of the Osprey definitely does not produce the most graceful entrance: it requires some contorting around the armrest, center console and overhead panel. The cockpit is dominated by four night-vision-goggle-compatible, six-inch-square multi-function displays that allow access to flight, navigation and system information. Mechanical flight controls consist of a center control stick, thrust control lever (TCL) and rudder pedals. The control stick functions as cyclic control while in conversion and helicopter modes, but steadily fades into a traditional airplane control stick as the nacelles transition to airplane mode.
The TCL moves fore and aft just like an airplane throttle, unlike a helicopter collective. It does operate as a collective control, however, and becomes a traditional throttle during the transition. While this may seem counter-intuitive to helicopter pilots, it actually makes a lot of sense, because regardless of the mode of flight, youre always doing the same thing: controlling the thrust vector. Forward on the TCL in helicopter mode is the same as raising the collective in a helicopter, and vice versa. The first couple of hours for helicopter pilots transitioning to the Osprey highlight a bit of collective dyslexia, but very quickly it never becomes a further problem. A spring-loaded, knurled rotary knob on the TCL that lies in contact with the pilots left thumb controls the nacelles. Roll the thumbwheel aft and the nacelles rotate to the vertical, roll it forward and the nacelles continue to the downstops. The controls follow the hands on throttle and stick, or HOTAS, concept, and have all the controls necessary on them to control multiple systems on the airplane.
Once the auxiliary power unit is running, the engines are started by placing the corresponding overhead engine control lever in the start position. With the FADEC (full-authority digital engine control) system, the start sequence is uneventful, but is usually announced by a large belch of oil smoke out of the exhaust, caused by oil leaking through various seals.
One very important event that takes place during the initial engine start is when both proprotors start turning. This validates the single-engine capability of the Osprey is working as advertised: the interconnect driveshaft in the wing transmits power from the operative engine to the gearbox on the opposite side.
With the engines started and all preflight checks complete, the Osprey is taxied using the nacelles only, and steered with the power-assisted nosewheel.
To Hover Is Divine
Smoothly and steadily pushing the TCL forward brings the Osprey into a very stable, solid hover at 20 feet. The normal range of motion for the TCL is only four inches, so very minor movements are all that is necessary in order to climb or descend.
The Osprey hovers at a nacelle setting between 86 and 88 degrees, which allows for a level deck attitude, although its possible to adjust the nacelles forward or back about five degrees, with a corresponding pitch attitude change (an especially helpful trait during longitudinal slope landings). Maneuvering in hover mode requires the same control inputs as a helicopter and the handling feels very similar to a large, cargo-class rotorcraft.
Hover taxiing can be accomplished in two ways, either with the cyclic or the nacelles. There is no magic number differentiating when to use one over the other, but its much more efficient to use the nacelles if moving more than just a couple of aircraft lengths.
The first noticeable trait of the Osprey is its hover stability, due in part to the automatic flight control system. On a no-wind day, its very possible to trim the nacelle angle and establish a hands-off hover. If thats not good enough, there is a coupled hover mode in the flight director that will maintain either a position over the ground or a ground speed. (Earlier Ospreys suffered from a phenomenon known as lateral darting, where the rotor downwash would impact the fuselage when hovering below about 15 feet and cause unwanted wobbling. Much of this issue was factored out of the aircraft through software updates in the flight control system; a slight bit of this still remains but is very manageable.)
The Osprey hovers with a level platform, which is a nice feature when operating the rescue hoist or conducting either rappels or fast-rope operations from the tail ramp. While it is possible to open the right entrance door and place a crewmember in that position, the rotor downwash outside is severe, which is why these operations are done from the tail.
Obviously, going fast in Osprey terms doesnt mean much to jet jockeys, but to the pure helicopter pilot, breaking the 200-knot banner is pretty exciting stuff. Cruise speeds range from about 170 to 240 KCAS with a never-exceed speed of 280 KCAS. Typically, 170 KCAS is used when flying in the instrument pattern prior to starting the approach and during holding patterns. Tactical cruise speeds range from 210 to 240 KCAS, depending on the mission.
Accelerating the Osprey at the maximum nacelle movement/tilt rate (eight degrees a second) will push you back into the seat. It can also knock a crewmember down, or worse, throw them out the back of the aircraft if theyre not secured in the cabin. So, announcing your intentions is crucial!
The transition process is a game of finesse, gradually raising the nose attitude while simultaneously rolling the nacelles to the downstops. Because the landing gear has a 140 KCAS speed restriction, it is standard practice to not go lower than 60 degrees on the nacelles until the gear is up and locked. Once the nacelles hit zero, the next step is to autobeep the rotor r.p.m. to the cruise speed of 84 percent by briefly flipping the nacelle thumbwheel forward and letting it spring back into position. The sound and vibration levels drop significantly at this point and the Osprey is flying with about a five- to seven-degree nose high platform while accelerating through 200 KCAS all in about 15 to 20 seconds.
During transition, the flight controls switch from a helicopter to an airplane based upon a speed schedule contained in the flight control computers. Swashplate movements are reduced and the flaperons rise up to become very large ailerons. (One thing to note is that the TCL doesnt change its function; it still controls the thrust vector by collectively changing the proprotor pitch.)
At this point, the Osprey is cruising along essentially as a twin-engine turboprop airplane, flying at the same speeds, altitudes and flight rules as traditional turboprops. The primary difference is the lack of ability to fly with one proprotor feathered, (which is one of the major training obstacles of multi-engine airplane transitions). Should a proprotor gearbox fail in airplane mode, causing the related proprotor to stop, the only recourse is to shutdown both engines and conduct a power-off glide and emergency landing; the adverse yaw is just too great for the rudders to overcome, leaving few options.
Dual-rated individuals out there will find very similar flight characteristics to most turboprops. Climb capability is where the Osprey excels, with rates approaching 4,000 feet a minute depending on environmental conditions.
The biggest airplane-mode limitation of the Osprey is the lack of pressurization. This means the crew and passengers must wear oxygen masks above 10,000 feet: not a big deal for an hour or two, but with air-to-air refueling, the Ospreys duration is limited only by crew duty-time restrictions. Also, as you ascend through 18,000 feet up to the cruising altitude limitation of 25,000 feet, the risk of developing the bends becomes greater during the descent due to the lack of 100 percent oxygen available, although with precautions this is still a minor risk.
Typical of most ship-based aircraft, the Osprey utilizes an on-board oxygen- and inert-gas-generating system that takes ambient air and extracts oxygen for breathing and nitrogen for inerting the fuel system. The system is a constant-flow variety, meaning that oxygen flows through the masks regardless of whether they are being utilized. A pressurized cabin and cockpit would of course greatly increase the capabilities of the Osprey, but this is a highly doubtful future improvement.
In the low-level environment, the Osprey handles very well, cruising as low as 200 feet above ground level (AGL) by utilizing a variety of sensors. The U.S. Air Force CV-22 Osprey has the addition of a multi-mode, terrain following/terrain avoidance (TF/TA) radar derived from the Boeing F-15E LANTIRN (low altitude navigation and targeting infrared for night) and Boeing MH-47E TF/TA systems. This provides the Osprey crew with the ability to safely fly at 200 feet AGL in very low visibility conditions, like fog or rain. The radar simply looks ahead, identifies key terrain obstacles along the flight path, and then calculates a climb command based upon aircraft conditions to ensure obstacle clearance. This command is relayed to the pilots via a horizontal command bar on the MFDs, all they have to do is match the flight path vector to the command bar and the Osprey will ride along at the selected set clearance plane. The system also calculates a dive command that ensures the Osprey doesnt highlight itself above the terrain, thereby reducing the enemys ability to find the aircraft.
The Osprey is also equipped with a forward-looking infrared (FLIR) system that is slaved to the flight path vector calculated by the mission computer. The FLIR will gimbal in the direction the aircraft is going during climbs, descents and turns, providing the pilots another visual indication of what is in their flight path. This is extremely helpful on very low illumination nights when night-vision-goggle acuity can be limited.
If Things Go Bad
Nearly every system in the Osprey is triple-redundant. There are three flight control computers, three lightweight inertial navigation systems, three hydraulic systems and four generators. The V-22 only requires one of these systems to safely continue flying; the others exist for safety purposes, yet have identical functionality. This doesnt mean that things cant go wrong as with any aircraft, there are obvious emergencies that require learned procedures to overcome.
Stalling the Osprey is a definite possibility when youre flying in the low end of the airplane-mode flight envelope. Typical stall speeds occur around 105 to 110 KCAS, depending upon aircraft conditions; fortunately, its rare to be flying that slow without having converted. One situation that can be encountered, however, is an accelerated stall, because the stall speeds can increase upwards of 140 KCAS as the bank angle increases.
Normal stall characteristics in the V-22 are very benign: about the only indication that the airplane is stalled is the increase in descent rate on the display. Because the Osprey exhibits blown wing characteristics, it is very difficult to develop a full stall, thereby making the effects less dramatic. Continuing into a full stall will result in a nose-down pitching moment, but the effect is not nearly as dramatic as with some airplanes. Recovery is the same as with any airplane: reduce the control stick backpressure and apply full power: the Osprey will break the stall almost immediately. On the primary flight display, a stall meter is displayed below a 35-degree nacelle setting, showing a dynamic percentage of the stall to assist the pilot.
Probably the most discussed issue with the Osprey is the lack of autorotational ability. Of course, the Osprey spends the overwhelming majority of time flying as an airplane, so its easy to see that the need for autorotation is pretty minor but as Murphys Law states, when you least expect it, things can indeed go bad very quickly.
Technically, the Osprey can actually enter autorotation, although the flight characteristics are extremely poor. Reduce the TCL to the full aft position with the nacelles full aft and the rotor system is being powered solely by the upward flow of air through the rotors. The greatest detriment to the autorotational capability of the Osprey is the very-low-inertia rotor system, which doesnt store as much energy as a traditional helicopter rotor system. Rotor r.p.m. will bleed off very quickly if the autorotation is not entered almost immediately, and it is very difficult to recover lost r.p.m. Stopping the nacelles at the full aft position is also critical, because any edgewise airflow over the rotor will rapidly decay r.p.m. This also corresponds to very poor qualities during the flare and touchdown portions of an autorotation. The autorotational descent rate is quite large about 5,000 feet a minute and an aggressive and rapid flare is necessary to arrest that rate. An increase in r.p.m. will be briefly noticed here; but, again, due to the low inertia of the rotor system, that gained r.p.m. will very quickly start to decay.
Autorotations are taught and practiced in simulators with varying degrees of success. The simulators are designed to indicate a crash if any structural load limitations are exceeded; most autorotations end in a red screen. The truth of whether an autorotation is survivable, though, is hard to define. Chances are that an autorotation in an Osprey would be an extremely difficult maneuver, with survival owed more to luck than skill.
The loss of both engines in airplane mode requires very similar emergency techniques as utilized in a twin-engine airplane. However, as mentioned earlier, unlike an airplane it is impossible to feather the proprotors. The glide ratio of the Osprey is about 4.5 to 1 and the rate of descent while windmilling is about 3,500 feet a minute at 170 KCAS. Landing speeds vary with aircraft weight, but a middle-of-the-envelope speed is 130 KCAS. Unfortunately, the proprotors will definitely impact the ground, and converting the nacelles is not recommended. A safety design feature of the proprotors, however, is for them to broomstraw and throw the resulting fibers away from the fuselage to minimize damage to the occupants. Unfortunately, this characteristic has been tested in accidents; fortunately, it works as advertised.
Converting Back to a Helicopter
Transitioning to airplane mode is a relatively simple matter. Converting back to helicopter mode, though, is a bit trickier, and requires more practice to get comfortable.
The first step is to increase the rotor r.p.m. by autobeeping to 100 percent, but the Osprey must be below 220 KCAS for this to happen. Deceleration begins by bringing the TCL back to near the aft stop and letting aerodynamic drag take effect. There are no speedbrakes or other devices to assist with this, so greater airspeeds require greater planning. Just as when transitioning, the flaperons are scheduled automatically, here theyre lowered to full down position as the Osprey slows down, to reduce the effects of downwash. Once the r.p.m. is autobeeped, a noticeable increase in the deceleration rate is felt due to the increase in drag from the higher r.p.m. This assists in the next phase of moving the nacelles aft as the airspeed must be below 200 KCAS for this to begin.
As the pilot moves the nacelles aft with the thumbwheel, a feature called conversion protection factors into the equation and stops the nacelles from moving too fast, as that might either damage the aircraft or cause a stall. Similar to the flaperons, the conversion protection corridor is airspeed-based and varies with altitude. At the lower portion of the corridor, nacelle movement will be modulated so the Osprey will not stall; the upper portion of the corridor protects loads on the rotor system from too much airspeed. If the pilot tries to continue moving the nacelles aft while the airspeed is to too high, the system will actually stop and move the nacelles forward by overriding the pilots inputs. The pilot has a visual indicator on the primary flight display that notes where the nacelle angle is in relation to the protection corridor; keeping the nacelle angle near the middle of the corridor during the conversion is the key to a smooth deceleration.
As the conversion is taking place, the nose is lowered to a level platform and the TCL is increased as the nacelles climb past about 45 degrees. Although the Osprey is flown nose-level in conversion and helicopter modes, a slight amount of nose-high (five degrees or less) helps the deceleration rate during the final portion of the approach. The key to a smooth and efficient conversion is recognizing that further movement of the nacelles does not produce an immediate impact, it takes a couple of seconds for the input to take effect. New pilots tend to get anxious and add more aft nacelle, which, once this takes effect, causes the Osprey to dramatically decelerate and stop well short of the intended landing spot.
Flying the final approach in an Osprey has one very nice feature: incredible visibility. Due to the level-nose attitude and the large amount of window space in the cockpit, the pilot has a great view of the landing zone. From either seat, the opposite nacelle is visible, a testament to the wide angle of view in the cockpit. The best feature is the ability to fly the approach with the nacelles aft of 90 degrees, corresponding to a nose-down approach attitude. In this configuration, the landing zone is visible in the windscreen all the way to touchdown, a very nice capability in a tight landing zone.
In a traditional helicopter, pilots worry constantly about either a tail or main rotor strike. While the Osprey isnt immune from a proprotor strike, the greatest cause for concern is a nacelle strike. On a level surface, the bottom of the nacelle sits only four feet above the ground; so, any surface slope, rocks or small trees become definite hazards during remote operations.
Another highly undesirable trait of the Osprey in remote areas is the amount of downwash from the proprotors. The high disk loading of the rotors creates a torrent of downwash, kicking up a considerable amount of debris and causing major brownout/whiteout issues. Fortunately, the Osprey has a very good hover situational display that allows pilots to conduct approaches and landings in near-zero visibility conditions.
Wrapping It All Up
This has been just a brief peek into what its actually like to pilot the V-22 Osprey, but hopefully it answers some questions and dispels some rumors about its flight characteristics. A fellow pilot of mine routinely said, The Osprey is an easy aircraft to fly, but a difficult one to fly well. This is one of the most truthful comments about the V-22 that Ive ever heard. The aircraft is loaded with features designed to reduce pilot workload, and they work incredibly well. New pilots straight from primary flight school transition quite easily, a fact that I attribute to the incorporation of technology. Because the Osprey handles so well, though, you constantly find yourself refining procedures and techniques to a minute degree, hence the difficult to fly well quote.
The Osprey is truly a 21st-century flying machine and incorporates several features more common to commercial aircraft than military. For the military pilot, this can be a difficult concept to grasp, as most military aircraft allow the pilot to utilize the entire range of system and flight envelopes, up to the point of failure. The Osprey is based on the principle that restricts the pilot from breaking the aircraft. The FADEC system on the engines controls hot starts and surges, and also stops the pilot from causing an overtemp or overtorque situation usually a good thing, but sometimes an overtemp or overtorque is a better outcome than destroying an aircraft.
Structural load limiting is another feature that essentially does not allow the pilot to over g the Osprey. This can spell disaster if a dive recovery isnt initiated with enough altitude, because the Osprey will only allow the pilot to pull back on the stick to the g limit.
These aircraft-protection features have led to another common quote among Osprey pilots: that they are only voting members when it comes to how the aircraft flies, with the flight control computers and their various components having the ultimate say. That being said, the flight envelope of the Osprey is ever-expanding as more aircraft data is collected.
Despite the widespread belief that the Osprey was in development for over 25 years, the truth is that very limited experience was gained on it due to the constant stop and start drama of the program. Basic flight characteristics were gleaned, but testing stopped well short of fully exploring the envelope of the aircraft; this only began in the mid-2000s timeframe. By then, the Osprey was in full production, showing up on the ramps of various squadrons, and the community struggled to catch up and exploit the capabilities of the V-22. Only in the past couple of years has the Osprey truly started showing promise, the result of a better understanding of how to fly the aircraft.
In a sense, the Osprey still represents more of a test bed for tiltrotor technology than a full realization of its possibilities: not the best example, mind you, but a dramatic step toward what could be a truly revolutionary mode of flight. Only time will tell if it will be instrumental in maturing tiltrotor technology, or be the sole example of it in history.
Mike McKinney is a retired USAF helicopter and tiltrotor pilot. He is a former instructor at the USAF Weapons School, where he studied and taught vulnerability and survivability principles.