没有胆会有什么影响

Question

Looking at different airplanes most airplanes with speed brakes have similar aileron-like speed brakes. Why does the Lancair use this different design instead of the more common ones?

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Answer 1

There is a difference because you're looking at a true speedbrake and comparing it with a lift spoiler. On many aircraft lift spoilers, which are large hinged plates, also act as speedbrakes.

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As a reminder of the forces acting on an aircraft:

• Lift is the vertical component of the aerodynamic force generated (mostly) by the wing. It balances weight and acts on aircraft vertical motion (climb, descent, level flight).

• Drag is the horizontal component of the aerodynamic force. It tends to prevent the aircraft from moving and is balanced by the engine thrust to move the aircraft at a certain speed. At subsonic speed, drag has two origins: From shear between air streamlines (friction) and from an unsteady air flow (eddies).

(Source.)

Speedbrakes are different from lift spoilers. Lift spoilers are often used as speedbrakes but the two surfaces have originally a different function and a different placement:

• A speedbrake is used to increase drag and reduce speed, it can be located anywhere, not specifically on the wing. Speedbrakes are deployed symmetrically.

• A lift spoiler is used to reduce the lift coefficient of the wing, that is the efficiency of the wing to create lift, hence is located on the wing upper side. Spoilers can be used symmetrically or asymmetrically.

Both have potentially an effect on drag, lift, descent rate and velocity, it depends on how they are used.

Lift spoilers are large surfaces over the wing, forward of the flaps, which can be used below a certain speed, Typically they are used to prevent the wing to generate lift after touchdown and to compress landing gear struts. On airliners they are fully extended as soon as the aircraft has landed:

Six spoiler panels extended nearly vertically over the wing, symmetrically on each side of the aircraft (source).

On modern aircraft however the role of spoilers is mixed with other surfaces. Spoilers respond quickly to the demand and all surfaces can be used to carry pilot commands on, optimizing aerodynamic effects and fuel consumption, and allowing some redundancy, e.g. if an aileron fails. The pilot issues regular yaw, pitch, roll and thrust demands, the flight control computers decide how to use ailerons, spoilers, elevators, rudder and engine.

• A small degree of asymmetric spoilers can be used in flight together with ailerons to control roll without impact on yaw. Outboard panels are used first as they create a larger roll moment.

• A small degree of symmetric spoilers can be used to adjust the descent rate while on the glide path, without changing the pitch attitude or waiting for the engine to spool up or down. Inboard panels are preferred for their lesser roll moment.

In such configuration there is a speedbrake lever in the cockpit but there are only spoilers panels on the wing. Spoilers on an A350, used both as spoilers and speedbrakes (from maintenance handbook):

Speedbrakes are surfaces that can be fully extended at any speed, and that do not need to be associated with the wing. In fact speedbrakes are found at many different locations: On the tail (BAE 146-100 below), over the fuselage (F-15 below), as a split rudder, etc, and on the wing like in your question.

The role of a speedbrake (also called air brake) is clearly to create drag to slow down the aircraft, with as little impact on the wing as possible. In flight they can be used to reduce speed while maintaining the same engine rpm (to prevent a shock cooling).

The type used on Lancair, Mooney, Beech, Piper, etc is a one or two small panels raising from the wing upper side:

Such air brake disturbs the airflow and creates important vortices behind it. Vortices (like wingtip vortices) are synonymous of drag and are usually hunted and reduced with clever designs, but sometimes we want to add drag!

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More:

• Spoilers And Speedbrakes
• How do airplane speed brakes work and how do they differ from spoilers --