Stopping without the brakes
Thrust reversers are popular fixtures on the jet aircraft landscape, and for good reason. They help to shorten landing distances while saving wear and tear on brakes. Their decelerative effects are magnified on slick runways, where brakes may be only marginally effective, or even completely useless. But, as with everything else an aircraft designer must consider, thrust reversers are not part of some aerodynamic free lunch; their use implies design compromises. The up-front cost is an ongoing weight and parasitic drag penalty that must be amortized over an entire flight but which provides a benefit only during the last half-minute. Like a good insurance policy, however, the cost of all those regular premiums is quickly forgotten when thrust reversers are really needed.
There are two primary kinds of thrust reversers. One is the target type, which employs an external clamshell door stowed in a faired position on the engine nacelle until needed. Once activated, the two halves of the door extend to block the engine exhaust, deflecting the hot gases forward and away from the engine. This type is often used with low bypass engine designs such as the Pratt & Whitney JT8D series found on Boeing 737-200 series aircraft. Low bypass jet engines combine hot core combustion gases with cooler bypass air (air which flows around the hot section to cool it) inside the exhaust nozzle. The combined flows exit at one location, allowing a single door to block all of the engine's thrust effectively.
Although target reversers are fitted to some high bypass engines, a second design — called the cascade vane reverser — is more commonly used.
Because the exit points of the fan bypass air and hot combustion gases are physically separate on some high bypass engines, a single clamshell door cannot effectively block both flows. Cascade vane thrust reversers therefore incorporate a series of internal vanes that move to block the rearward flows at several different points within the engine, directing them forward. On some engines, only the fan bypass air is blocked.
Because the reverse thrust is diffused and angled from the aircraft's direction of motion, reverse thrust typically represents only 30 to 50 percent of the engine's forward thrust.
Thrust reversers are most effective when used during the early part of the landing roll at high power settings. At idle power, a jet engine still supplies significant forward thrust, on the order of five percent or so of maximum. Thus an engine rated at 10,000 pounds of thrust is still producing 500 pounds at idle. Selecting idle reverse thrust on such an engine, presuming a reverser system that is 50 percent effective, will result in only 250 pounds of reverse thrust being applied. Although helpful, this will not decelerate the aircraft nearly as well as if the pilot uses the full 5,000 pounds of reverse thrust that is available. In everyday practice, many pilots seem to prefer intermediate power settings that still get the job done but are easier on passengers' ears and nerves.
Thrust reversers are used in concert with ground spoilers (if available) and brakes. Ground spoilers assist in placing the aircraft's entire weight upon the wheels more quickly than if the pilot merely waited for the wing to stop flying on its own. This, in turn, makes the full capacity of the brakes available if a maximum effort stop is necessary. In some aircraft with tail-mounted engines, such as the McDonnell Douglas MD-80, the pilot must first wait until the nose gear has touched down before "opening the buckets," to prevent dragging the clamshell reverser doors on the runway. This is possible under certain combinations of high body angle and full landing gear strut compression, as could occur during a hard landing.
Reverse thrust operating technique is easy to master. On most aircraft, the same throttles that control forward thrust also control reverse thrust. Throttles are first retarded to the idle forward thrust position during landing, and at the appropriate moment the pilot raises reverser levers that are an integral part of the throttles themselves. This, in turn, releases mechanical interlocks (which serve to prevent reverse thrust activation when forward thrust is desired) and opens the thrust reversers. Reversers are usually powered hydraulically, but some systems employ high-pressure engine bleed air instead. Once the reversers have opened, the pilot may then increase reverse thrust to the desired level by pulling back farther on the same levers used to initiate the process.
On many jets, particularly those with wing-mounted engines, it is important that the pilot not allow large degrees of asymmetric reverse thrust to develop. On wet or slushy runways, asymmetric thrust can exacerbate the effects of hydroplaning or skidding, resulting in further directional control problems. Since each engine may have a slightly different spool-up time, the proper technique is first to establish approximately equal levels of low-power reverse thrust; then, once the engines are spooled, simultaneously increase power on all engines to higher levels if desired.
How effective are thrust reversers? It's safe to say that on a dry runway, thrust reversers will typically shorten landing distances by 10 to 15 percent. But it's on contaminated runways where they really shine. If poor braking conditions or hydroplaning is encountered, thrust reversers can become the pilot's primary means of initially slowing the aircraft to a safe speed. When computing landing distances for a jet, however, thrust reverser use is never taken into account, since a landing may be necessary with one or more failed engines or hydraulic systems, resulting in no reverse thrust's being available. That explains why thrust reversers may be inoperative according to an aircraft's minimum equipment list, but brakes, thankfully, may not.
Thrust reversers are not intended for use in flight, except on a few aircraft such as the Douglas DC-8, where they serve as speed brakes.
Thrust reversers can be especially valuable during aborted takeoffs. A heavy-weight, high-speed abort in a jet aircraft places extreme stresses upon brakes and tires, to the point where they may fail before the aircraft comes to a stop. If this occurs, reverse thrust can mean the difference between stopping on the runway or somewhere else.
A less dramatic use for thrust reversers is the powerback, a maneuver in which an aircraft is backed up under its own power. As a matter of procedure, pilots are cautioned not to apply brakes while powering back, as this may cause the aircraft to settle on its tail. Stops are made instead by first applying forward thrust and then braking normally once forward motion begins.
All things considered, the positive benefits of thrust reversers probably outweigh their ongoing weight and drag negatives. It's no wonder they remain such a popular option on many jet aircraft.
Next-Generation Thrust Reversers
The Dee Howard Company and Calcor Aero Systems, Incorporated, have separately announced plans for new families of thrust reversers that will, for the first time, enhance aircraft in-flight performance rather than degrade it. Both will make use of variable- size engine exhaust nozzles, instead of fixed-size nozzles, as is the norm for business and commercial jet aircraft.
Varying the size of the exhaust nozzle is not a new idea. Some military aircraft, afterburning fighters in particular, have used the concept for many years. Doing so allows the engine to be optimized for performance and fuel efficiency during different phases of flight. At takeoff, for instance, it is desirable for an engine's exhaust nozzle to be at its widest possible circumference. This allows the engine to produce more thrust than it otherwise could. In cruise, where lower thrust levels are called for, a smaller nozzle size results in better fuel efficiency. This is somewhat analogous to the benefits of a constant-speed versus a fixed-pitch propeller. Fixed-pitch props are designed to be most efficient during one phase of flight, usually cruise, where an airplane spends most of its time. So, too, jet engines with fixed-area exhaust nozzles are optimized for best cruise performance. Everywhere else, they perform less efficiently. Because of the added complexity of variable-size nozzles, however, engine manufacturers have favored fixed nozzles instead.
The new Calcor system permits infinitely variable nozzle area changes of up to 15 percent. It incorporates a target-type thrust reverser as an integral part of the system. Depending upon the particular engine and aircraft combination to which it is adapted, Calcor claims, takeoff thrust can be increased as much as eight percent, while cruise fuel flows can be reduced by two percent.
The Dee Howard Variable Geometry Nozzle design has a two- position nozzle (one for takeoff, the other for cruise) that is also mated to a target-type thrust reverser. The takeoff position may be up to 10 percent larger than the cruise position. Dee Howard predicts that a seven-percent increase in nozzle area for takeoff will boost thrust six to seven percent. Installed on a typical mid-size business jet, the system will allow a five-percent increase in payload — which, the company estimates, translates into a 15-percent (or more) increase in range if used for carrying additional fuel.
Both the Dee Howard and Calcor designs, when certified, will be adaptable to engines in the 2,000- to 30,000-pound thrust ranges. — VC
