Category: how airplanes work

On many jets and turboprops, roll spoilers are used to assist the ailerons in banking and thereby turning the plane. Roll spoilers are flat panels mounted on the upper wing surfaces, which deploy upward into the slipstream on the down wing only, disturbing lift and thereby aiding the down-wing aileron in effecting the turn.

Roll spoilers are interconnected with the ailerons, so as to perform in harmony with them. In many aircraft roll spoilers deploy as a function of airspeed. For example, on the de Havilland Dash-8, two roll spoilers deploy on each wing with the aileron below 140 knots, but only one operates per wing above that airspeed.

The reason roll spoilers are often required on high-speed aircraft is because they operate across such great speed ranges — such planes must resolve high-speed aerodynamics with the slow flight required for safe takeoffs and landings. Planes fly fastest with small, thin wings and high wing loading. Safe take-offs and landings, on the other hand, require high-camber, high-lift wings, with low wing loading. In essence, two different airplanes are required: one that can go fast, and one that can get everybody off the ground in less than ten miles of runway!

This challenge is resolved through extensive use of big flaps and leading edge devices (LEDs) like you see on jetliners, which effectively convert the wing from a high-speed shape to a low-speed, takeoff-and-landing shape. The problem is that in order to make wing size (and drag) small for optimum cruise speeds, the flaps must extend across as much of the wingspan as possible for adequate low-speed effectiveness. With all those flaps installed, there’s little room left on the wing for ailerons. Small ailerons may be fine for high speed cruise, but they’re often too small for adequate roll response at low airspeeds, like when taking off and landing. One solution to this problem is to put multiple ailerons on each wing, separately activated as a function of airspeed. The other solution is to install roll spoilers to help the ailerons. (The Boeing 767 utilizes roll spoilers AND two ailerons per wing — the outboard ones are locked above 240 kts).

On a few aircraft with very small wings, such as the Mitsubishi MU-2, the flaps must be so big to achieve reasonable landing speeds that there’s no room left for ailerons at all! So on MU-2s, all roll control is accomplished by spoilers. Since spoilers effect roll by destroying lift, crosswind techniques for such aircraft must be modified under marginal take-off and landing situations.

Incidentally, Transport Category Airplanes must be equipped with redundant or separated primary flight controls in order to overcome any control jams. So on planes that have them, the roll spoilers usually connect to one pilot’s flight controls, while the ailerons connect to the the other. The two control yokes are mechanically linked so ailerons and spoilers work together when turning either yoke. But if either the roll spoilers or the ailerons were to jam, a clutch connecting the two systems can be released or overcome, allowing one pilot to fly via the one that still works. Pretty cool, eh?!!

©2009, 2022 Gregory N. Brown

To learn more about turbine aircraft and how they work, see Greg’s new The Turbine Pilot’s Flight Manual Fifth Edition, coauthored with Mark Holt. Available in print and ebook. (Autographed copies available.)

Ever wondered about those little exhaust pipes protruding from the tails of many jets and turboprops? Well, your eyes aren’t deceiving you— in many cases those are indeed jet engine exhausts, from small “extra” jet engines known as “APUs.”

An “APU” (Auxiliary Power Unit) is a small turbine engine installed to provide supplementary power. Often found in the tails of larger jets and turboprops, APUs serve several useful purposes.

APU generators provide auxiliary electrical power for running aircraft systems on the ground when the main engines aren’t running and no ground electrical power is available. Applications include powering environmental systems for pre-cooling or preheating the cabin, and providing power for crew functions such as preflight, cabin cleanup, and galley (kitchen) operation. Many aircraft APUs can also be operated in flight, providing backup power for the main engine generators.

On larger aircraft, APUs also generate auxiliary “bleed air”, referring to pneumatic pressure drawn from the engine’s compressor section. That’s because large jet engines like those on airliners must be started using pneumatic power. Unless a ground pneumatic source is available, the only way to start large turbine engines is from an operating APU (unless another engine is already running, of course). To accomplish this, the small APU engine is first started using an electric motor (often doing double duty as the generator). Once up and running, APU bleed air is routed to pneumatic starters on the plane’s main engines. Those, in turn, spin up the engine compressors for starting.

This schematic shows a typical APU installation. Along with providing ground power, APUs often provide backup pneumatic power for pressurization in flight, and back up environmental systems on the ground and in the air.

©2013, 2020 Gregory N. Brown

To learn more about turbine aircraft and how they work, see Greg’s new The Turbine Pilot’s Flight Manual Fifth Edition, coauthored with Mark Holt. Available in print and ebook. (Autographed copies available.)

At first glance, the “glass cockpits” found in modern aircraft may look like they come from a different planet than the round “steam gauge” instruments found in older general aviation cockpits.

But upon closer examination you can see that most primary flight displays (PFDs) actually conform closely to the “standard T” layout of round flight instruments found in older cockpits. Look closely at the illustration and you’ll see the similarities in layout.

For those who are not familiar, note that the HSI (horizontal situation indicator) may be found in both round-instrument and “glass” cockpits — it’s simply a combination instrument including both heading indicator and CDI (course deviation indicator) needles.

©2009, 2020 Gregory N. Brown

To learn more about turbine aircraft and how they work, see Greg’s new The Turbine Pilot’s Flight Manual Fifth Edition, coauthored with Mark Holt. Available in print and ebook. (Autographed copies available.)

While the reciprocating (or piston) engines that power cars and most light airplanes have a few commonalities with gas turbine (jet) engines, the two types are very different in most respects.

Both piston (left) and turbine (below) engines have somewhat similar stages of operation: intake, compression, combustion, and exhaust. But the similarities largely end there, the biggest difference being that in reciprocating engines those stages happen one at a time, while in turbine engines they are continuous.

Instead of compressing intake air with a piston, turbine engines use a series of wheels at the front of the engine known as compressors. Another set of wheels, known as turbines, is driven by exhaust gases departing the combustion section. Both compressor and turbine wheels are essentially sophisticated “fans,” composed of high-tolerance blades spinning at very high speeds inside a tightly-ducted cowl.

Much like a turbocharger, a turbine engine’s compressor and turbine sections are mounted on a common shaft. Intake air is compressed by the compressors, and forced into the combustion chamber. Fuel is continuously sprayed into the combustion chamber and ignited, generating expanding exhaust gases that drive the turbines. The turbines, through their shafts, drive the compressors, sustaining the process. The turbines also harness energy to drive accessories such as electrical generators and hydraulic pumps. Finally, the exhausting gases are accelerated through a nozzle at the back of the engine, producing thrust somewhat like the way air escaping the “nozzle” of an untied balloon propels it across the room.

In piston-powered airplanes the engine always turns a propeller to pull it through the air, but turbine engines can either operate as jets or drive propellers themselves. A turbine engine driving a propeller is known as a turboprop.

The basic gas turbine engine we’ve described is sometimes called a “gas generator,” or “core turbine engine.” Depending on how the exhaust gases are harnessed, the core turbine engine may be applied to turbojet, turbofan, or turboprop engines.

While most reciprocating aircraft engines burn gasoline, turbine engines consume kerosene. Turbine engines produce much more power for their weight than piston engines, but they burn more fuel and are far more expensive to manufacture. 

©2012, 2020 Gregory N. Brown

To learn more about turbine aircraft and how they work, see Greg’s new The Turbine Pilot’s Flight Manual Fifth Edition, coauthored with Mark Holt. Available in print and ebook. (Autographed copies available.)