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三点

2011-02-11  偈飞

后三点式:后轮一个,“角”向后
前三点式:前轮一个,“角”向前
最先实用化的喷气机ME262原型机是传统螺旋桨机的后三点式,发现水平尾翼处于机翼的尾流区里效率低(低速飞机的问题不大),于是后来给改成前三点了。
“后三点”易拿大顶,前三点还容易拖尾擦地。
三点中,横向的两点就是主起落架了,是最主要的受力区,也是整机的重心所在,而机冀也是重心所在,所以这两点布置在机冀两边是最科学的。
关键是剩下的一点如何布置。
如果布置在后面,降落时的前冲力如何消除?起飞时,前起落架未离地前的仰角如何解决?没有仰角,升力不够,主起落架就不能离地。
如果布置在前面,当达到一定的速度,可以抬起前架,让飞机有更大的仰角,得到更大的升力,在此基础上飞机才离地。降落时,主起落架先着地,产生阻力,但阻力也会使飞机有前倾,这时前架会阻此飞机前倾。
(参见 http://forum.xitek.com/showthread.php?threadid=718315 有图哦)
 
 
 
 
 
Most aircraft today have three landing gear.
许多现代飞机使用三点式起落架。
Two main landing gear struts located near the middle of the aircraft usually support about 90% of the plane's weight while a smaller nose strut supports the rest.
重心附近的两个大的主轮,承担约 90% 的重量,小轮子承担余下部分。
This layout is most often referred to as the "tricycle" landing gear arrangement. However, there are numerous other designs that have also been used over the years, and each has its own advantages and disadvantages. Let's take a closer look at the various undercarriage options available to engineers.
目前的飞机以前三点起落架为主,让我们来回顾一下后三点起落架及其优缺点。(意译)
Tailwheel or Taildragger Gear
后三点尾轮式与后三点尾橇式起落架
Though the tricycle arrangement may be most popular today, that was not always the case. The tailwheel undercarriage dominated aircraft design for the first four decades of flight and is still widely used on many small piston-engine planes.
虽然前三点起落架比较普遍,但是在几十年前的飞机,及当今的许多小型飞机是使用后三点起落架的。
The taildragger arrangement consists of two main gear units located near the center of gravity (CG) that support the majority of the plane's weight.
后三点起落架,由两个在重心靠前位置的主轮支持大部分的飞机重量。
A much smaller support is also located at the rear of the fuselage such that the plane appears to drag its tail, hence the name.
一个非常小的尾轮装置在机身,看上去这个小轮子是被拖着走,所以,英文 Taildragger 也因此而得名。
This tail unit is usually a very small wheel but could even be a skid on a very simple design.
它即可以是一个小尾轮,也可以是一个尾橇。
 
What makes this form of landing gear most attractive is its simplicity.
后三点起落架最吸引人的优点就是“简洁”。
The gear are usually relatively lightweight, and the two main gear can also be easily encased in streamlined fairings to produce low drag in flight.
后三点起落架通常重量都比较轻。两支主轮子可以配置整流置用来减小飞行中的阻力。
Another potential advantage results from the fact that the plane is already tilted to a large angle of attack as it rolls down the runway.
另一个潜在的优点,是飞机在跑道滑行时,机翼已经有一个较大的迎角。
This attitude helps to generate greater lift and reduce the distance needed for takeoff or landing.
合适的大迎角,有利于在较低的速度下产生足够的升力,以便于减少起飞滑跑距离。
This attitude is also an advantage on propeller-driven planes since it provides a large clearance between the propeller tips and the ground. 对于牵引式螺旋桨飞机,机头向上抬起,使螺旋桨不会接触跑道。(意译)
Furthermore, taildragger planes are generally easier for ground personnel to maneuver around in confined spaces like a hangar.
同时,后三点起落架的飞机,更便于地勤人员把飞机拖到停机棚等窄小的空间里。
However, the greatest liability of this landing gear layout is its handling characteristics.
另一方面,后三点起落架也有一个最大的缺陷——着陆时的操纵(handling characteristics).
This design is inherently unstable because the plane's center of gravity is located behind the two main gear.
造成此天生缺陷的原因,是因为两只主轮在重心的前面。
If the plane is landing and one wheel touches down first, the plane has a tendency to veer off in the direction of that wheel.
如果着陆时起落架只有一只主轮前触地,这时,飞机有向此轮子一边转弯倾向。
This behavior can cause the aircraft to turn in an increasingly tighter "ground loop" that may eventually result in scraping a wingtip on the ground, collapsing the gear, or veering off the runway.
严重时,这种转弯的倾向(ground loop)将可能使翼尖触地、起落架受损、滑出跑道。
Landing a taildragger can be difficult since the pilot must line up his approach very carefully while making constant rudder adjustments to keep the plane on a straight path until it comes to a stop.
这就要求在飞机着陆触地前,需要使用方向舵修正自己的航线,使它能够笔直的滑翔直到安全触地。
Many taildragger designs alleviate these handling problems by fitting a tailwheel that can be locked instead of swiveling on a castor. Locking the tailwheel helps keep the plane rolling in a straight line during landing.
使用尾橇,能减轻这种现象。(意译)

 
Another disadvantage of the taildragger is poor pilot visibility during taxiing since he is forced to peer over a nose that is tilted upward at a steep angle.
后三点起落架的另一个缺点,许多飞机在地面滑跑, 机头向上翘起时,会影响飞行员的视线。(意译)
It is also often difficult to load or unload heavy cargos because of the steep slope of the cabin floor.
对于货机,飞机的迎角不利于货物的装卸。
Similarly, pilots and passengers are forced to walk uphill during boarding and downhill after arrival.
此问题,对于乘客来说,也是一样的道理。(意译)
Many aircraft also rely on gravity to bring fuel from tanks to the engine, and some planes have been known to have difficulty starting the engine because it is uphill from the fuel supply.
发动机抬起,有时也不利于供油,造成启动困难。

道格拉斯DC-3的后三点起落架
Good examples of taildragger aircraft include the Spitfire and DC-3 of World War II.
后三点起落架的典型例子:喷火战斗机、道格拉斯DC-3。
Tricycle or Nosewheel Gear
前三点起落架
Now the most popular landing gear arrangement, the tricycle undercarriage includes two main gear just aft of the center of gravity and a smaller auxiliary gear near the nose.
前三点起落架,由两支在重心后的主轮,与一个在机头的辅助前轮组成。(意译)
The main advantage of this layout is that it eliminates the ground loop problem of the taildragger.
前三点起落架最大的优点是不会像后三点那样,着陆时容易打转。
This arrangement is instead a stable design because of the location of the main gear with respect to the center of gravity.
这得益于主轮子在重心后部的设计。
As a result, a pilot has more latitude to land safely even when he is not aligned with the runway.
这样,就算着陆时飞机没有对准跑道,飞行员还是有办法修正方向的。

 
Furthermore, the tricycle arrangement is generally less demanding on the pilot and is easier to taxi and steer.
前三点起落架更容易操纵。(意译)
The tricycle gear also offers much better visibility over the nose as well as a level cabin floor to ease passenger traffic and cargo handling.
水平的机身,使视界比较好,而且便于装货及乘客行走。
A further plus is that the aircraft is at a small angle of attack so that the thrust of the engine is more parallel to the direction of travel, allowing faster acceleration during takeoff.
机翼的小迎角,螺旋桨在水平方向产生的力,有助于飞机更快获得速度。(意译)
In addition, the nosewheel makes it impossible for the plane to tip over on its nose during landing, as can sometimes happen on taildraggers.
另外,由于前轮距重心较远,就不会出现后三点起落架容易发生的“拿大顶”现象。(意译)

The greatest drawback to tricycle gear is the greater weight and drag incurred by adding the large nosewheel strut.
前三点起落架最大的缺点是,较重的重量及较大的阻力。(意译)
Whereas many taildraggers can afford to use non-retracting gear with minimal impact on performance, planes with nosewheels almost always require retraction mechanisms to reduce drag.
另一方面,固定式后三点起落架耐冲击性能较强。而前三点起落架,需要收放式以减小飞行时的阻力。
Some planes with tricycle gear also have difficulty rotating the nose up during takeoff because the main wheels are located so close to the elevator, and there may be insufficient control effectiveness. Similarly, the closeness to the rudder reduces its effectiveness in counteracting crosswinds.
前三点起落架影响舵效。(意译)

Another critical factor when designing tricycle gear is to properly balance the load carried by the main gear versus the nosewheel. Too little load on the main wheels reduces their braking effectiveness while too little on the nosewheel reduces its steering effectiveness. Careful balancing of weight is also important to prevent the plane from tipping back on its tail while at rest on the ground.
前三点起落架对飞机重心的要求也比较高。
 

747 加油次序不当,造成机尾触地的危险。
There are many examples of aircraft with tricycle landing gear, including the F-16 and Cessna 172.
F16 与 Cessna 172 都是前三点起落架的经典案例。
Summary
总结
Landing gear serves three primary purposes--
起落架的三个主要任务:
to provide a support for the plane when at rest on the ground,
一、把飞机停放在地面。
to provide a stable chassis for taxiing or rolling during takeoff and landing,
二、起飞及着陆时提供稳定的支持。
and to provide a shock absorbing system during landing.
三、着陆时减震。
Regardless, all of these tasks are secondary to the plane's primary role as an efficient mode of travel through the air.
飞机的飞行性能还是很重要的。(意译)

To aircraft designers, landing gear are nothing more than a necessary evil since planes are designed primarily for their performance in flight rather than on the ground.
飞机的飞行性能比在地面的性能重要的多。(意译)
There have even been attempts over the years to eliminate landing gear entirely. The most extreme case was a study done by the Royal Navy to see if a jet plane could make a belly landing on the deck of an aircraft carrier coated with a rubberized surface.
有人尝试过不使用起落架,直接用机腹在航母甲板上着陆。(意译)
If successful, the method would eliminate the need for the very strong and heavy landing gear used on carrier-based aircraft. Unfortunately, the method proved impractical, but it shows the lengths some will go to while attempting to eliminate the need for landing gear!
很不幸,这个试验暂未完成成功。(意译)
We have seen that landing gear come in many varieties and each option has its own advantages and disadvantages.
我们看到,有各种各样的起落架,每一种都有它自己的优缺点。
Selecting the best arrangement for a given aircraft is a trade-off between these strengths and weaknesses as they apply to the environment the plane is designed for.
实用中,需要根据飞机及外界环境,综合优缺点考虑来进行选择。
As a result, designers try to select the simplest, smallest, lightest, and least expensive solution possible to do the job while maintaining safety.
在保障安全的前提下,最简单的、最小的、最轻的、最便宜的方案是比较好的选择。
That is why most planes only have three landing gear rather than four because fewer gear weigh less, require less structure aboard the plane, take up less space when retracted, and generate less drag.
分析现在的飞机,就是按照上面的说法设计起落架的。如:重量更轻、简洁的结构、更小的阻力、收放式。(意译)

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