起落架是飛機(jī)上眾多關(guān)鍵組件

1、起落架是飛機(jī)上眾多關(guān)鍵組件之一，飛機(jī)起落架的基本功能是在地面上實(shí)現(xiàn) 的，包括滑跑、起飛和著陸。在上述中、最重要的一部分就是著陸部分，因?yàn)樗?包括許多能量的轉(zhuǎn)換，并且在一些情況下系統(tǒng)一定要足夠穩(wěn)定。一些約束條件和 需求對于起落架的性能來說，包括耐久性、平順性、重量、高度、沖程、起落架 收上和轉(zhuǎn)向。以上所述的性能在地面操作上實(shí)現(xiàn)合理的性能必須做出恰當(dāng)?shù)膬?yōu) 化。雖然有很多不同的起落架模型，但是傳統(tǒng)的模型還是包括輪胎部分、緩沖器 部分、支承結(jié)構(gòu)部分。輪胎部分包括一個主要車輪裝配連接機(jī)身和一個前輪裝配 連接到機(jī)頭。有三種常見的起落架布局：后三點(diǎn)式，前三點(diǎn)式、串聯(lián)式起落架。 本論文主要研究前三點(diǎn)式起落架。

2、在著陸期間，主輪也就是后排機(jī)輪首先與地面 接觸，然后機(jī)頭部分的輪胎也就是前輪與地面接觸。一般有三種為前起落架所采 用的幾何圖形布局，包括套筒式、鉸接式、和半鉸接式。如圖1.1。圖1.1起落架車匕胎布局套筒式輪胎布局設(shè)計在運(yùn)動學(xué)上來說很簡單，但在幾何學(xué)上來講得不到大的 支撐力量。自從支撐在表達(dá)沖擊設(shè)計兩端鉸接，它僅產(chǎn)生的軸向力，并減少軸頸摩 擦力,卻使得運(yùn)動變得復(fù)雜。沖擊支撐在半鉸接設(shè)計中只是一端鉸接，但性能可以 通過選擇適當(dāng)?shù)凝X輪參數(shù)進(jìn)行優(yōu)化。兩個主要的起落架性能：（a）降落過程中的性能。（b）飛機(jī)滑跑、著陸和隨 后的緩沖中軌道粗糙度對勵磁性能的影響。高水平的瞬變激發(fā)在著陸期間必須控 制的平滑

3、，為了實(shí)現(xiàn)穩(wěn)態(tài)在合理的時間內(nèi)。1.2起落架領(lǐng)域的發(fā)展飛機(jī)起落架的設(shè)計和發(fā)展有著大量的科研成果，大部分的科研成果都是在某 些特定的情況下做出來的。在起落架設(shè)計的各個環(huán)節(jié)中，Currey通過實(shí)驗(yàn)成果 與經(jīng)驗(yàn)總結(jié)，很好的描述了起落架的性能需求和緩沖器的設(shè)計。來自美國國家航 空宇宙航行局的Jocelyn也細(xì)節(jié)的解釋了起落架動態(tài)特性，尤其是在擺動和制動 過程的振動，并且很好的總結(jié)了工作文件在過去的10年里，突顯了解決振蕩問 題上的最新成果。依照他們的工作，起落架振蕩包括由自身引起的振動稱為擺動 和制動過程的振動?？赡苄缘脑?qū)τ跀[動是低扭轉(zhuǎn)剛度，在裝置中有過多的空 轉(zhuǎn)，車輪的不平衡或磨損的部件。制動過程

4、包括裝置行駛，尖叫和振蕩等由摩擦 特性導(dǎo)致的，介于剎車系統(tǒng)旋轉(zhuǎn)和非旋轉(zhuǎn)部分。這篇文章也解釋了 Moreland在 飛機(jī)起落架動態(tài)理論中所做的工作。Moreland發(fā)現(xiàn)為了更加準(zhǔn)確的描述系統(tǒng)和 振動現(xiàn)象，數(shù)學(xué)模型的建立需要5個自由變量：輪胎偏差、旋轉(zhuǎn)角度、支撐偏差、 阻尼器的聯(lián)系應(yīng)變和機(jī)體運(yùn)動。盡管這些自由變量數(shù)據(jù)看起來比較難以處理，但 是他們對于充分理解實(shí)質(zhì)性的工作條件發(fā)揮著相當(dāng)重要的作用。W. Kruger et al在寬廣的理念中備份了飛機(jī)起落架動態(tài)特性，涵蓋了在 控制方面的著陸和地面操縱。有兩個控制觀念對自動控制系統(tǒng)很有益：（a）在一個完全活動的懸浮執(zhí)行機(jī)構(gòu)提供了動力，它被直接稱為一個控

5、制 法則。（b）在一個半主動懸浮中，只有阻尼力在瞬間減震器取代方向上被調(diào)整去 控制阻尼系數(shù)。他們的工作分析了設(shè)計要求的問題并對所有變量的系統(tǒng)進(jìn)行仿 真。由于缺少高性能計算設(shè)備，為了減少結(jié)果方程的復(fù)雜程度，大多數(shù)早期的模 型都是非常簡單的，D.Yadav和R.P.Ramamoorthy在Yadav和Kapadia所研究的 舉模型的基礎(chǔ)上進(jìn)行了更加深入的分析。這個飛機(jī)模型通過兩輪減震器彈簧被理 想化的作為剛性梁。大多數(shù)的輪子和其中的聯(lián)系是集中在各自近似為軸輪和簧載 質(zhì)量，被認(rèn)為鉸鏈和縱搖的自由度。舉模型忽略了縱擺自由度及其耦合模型，而舉投模型合并這些效應(yīng)分析數(shù) 據(jù)。在著陸沖擊時，舉模型運(yùn)用了兩輪鉸

6、鏈?zhǔn)角捌鹇浼芎吞坠苁缴炜s的主輪幾何 模型。他們也第一次學(xué)習(xí)了聯(lián)動動力學(xué)對于變形的輪胎減震支柱的影響。模型如 圖1.2所示。本文中，鉸接前起落架有三個緩沖參數(shù)一銳孔流量系數(shù)（Cd），初始空氣壓 力（Po）和氣動范圍（Aa）被選做變量。通過不同的銳孔流量系數(shù)作用于前起落 架，系統(tǒng)行為表明著陸應(yīng)該產(chǎn)生更小的振幅。這不會有主要的影響在最大振幅的反彈質(zhì)量。他們也概括了一個著陸更小的初始空氣壓力，這可以為沖擊增加時間， 能很好地應(yīng)用于減振器的主動制。圖1.2舉模型1.1Overview of Landing GearsOne of the many critical components that mak

7、e an aircraft function is the landing gear. The basic function of a landing gear on an aircraft is to maneuver it during its gr ound operations which include taxi, takeoff and landing. Of these, the most critical ph ase is the landing because it involves a massive amount of energy transfer and the s

8、ystemhas to best able enough to operate under these conditions. Some of the constrai nts and requirements for the performance of landing gears are crash survivability, riding performance, weight, height, stroke length, retraction, and steering. All of the above would have to be optimized for the rea

9、sonable performance of an aircraft on its ground operations.Although there are so many different variants of the landing gear models, the conventional one has a tired wheel unit, a shock absorbing unit and a supporting structure 1. The wheel unit has a main wheel assembly attached to the fuselage an

10、d a nose wheel assembly attached to the nose of the aircraft. There are three common types of landing gear: conventonal, tricycle, and tandem. This work has been confined to the tricycle type of landing gear. During landing, the main wheels come in contact with the ground first which is called the t

11、ouchdown and then the nose wheel makes contact with the ground.套筒式鉉接式半釵接式Fig. 1 . 1 Landing gear typesThe telescopic design is kinematically simpler but the geometry may not account for the large strut forces. Since the shock strut in the articulated design has both end hinged, it produces only axia

12、l forces and so doe minimize the journal frictional forces, but at the cost of complex kinematics. The shock strut in the semi articulated gear design has only one end hinged, but the performance can be optimized by proper selection of gear parameters.The two major concerns for landing gear performa

13、nce are:behavior during touch down impact.and performance excitation induced by track roughness during taxi, takeoff and the later part of landing runs. The high level of transients induced during touchd own have to be controlled smoothly in order to achieve the stady state in a reasonable amount of

14、 time.1.2.Developments in the Field of Landing GearsThere has been considerable research in the field of landing gear design and devel opment and most of these works are specific to a particular case under consideration. The various steps in .landing gear design, its performance requirements and sho

15、ck strut design are well described by Currey , based on experience and experiments. Jocelyn of NASA has explained in detail the landing gear dynamics, especially shimmy and brake-induced vibration and has well summarized the work documented from the last ten years to highlight the latest efforts in

16、solving these vibration problems. According to their work, the landing gear vibration includes self-induced oscillations called shim my and brakeinduced vibration. Possible causes for shimmy are low torsional stiffness excessive freeplay in the gear, wheel imbalance, or worn parts. Brake-induced vib

17、rati on includes gear walk, squeal and chatter, which are caused by the frictional character istics between the brake rotating and nonrotating parts. This paper also explains the work done by Moreland 5 in landing gear dynamics and the theory of shimmy. More land found that to precisely describe the

18、 system and the shimmy phenomena, the math ematical model required 5 degrees of freedom: tire deflection, swivel angle, strutdefle ction, damperlinkage strain, and airframe motion. Even though it may seem cumberso me to include these degrees of freedom to the specific model of study, they play an im

19、 portant role if the model has to be comprehensive enough to consider all the practical working conditions.W. Kriiger et al. 6 documented the aircraft landing gear dynamics in a wider se nse, considering both landing and ground maneuvering with control aspects. There are two control concepts which a

20、re of interest to automotive systems: (a) in a fully-active suspension an actuator provides force which is directly defined by a control law, (b) in a semi-active suspension only the damping force in the direction of the momentary damper displacement is modulated to control the damping coefficient.

21、Their work analyzed the problem from design requirements to the simulation of the system with all probable variables.Most of the earlier models were simplified to reduce the complexity of the resulting equations due to lack of computational facilities and they assumed the gear geometry to be telesco

22、pic. D. Yadav and R. P. Ramamoorthy 2 analyzed an extended version of the heave model created by Yadav and Kapadia (1990). This model idealized the air craft as a rigid beam supported by the shock absorbers over two wheel springs. The masses of the wheels and the linkages were approximated as lumped

23、 at the respective wheel axes and the sprung masses were assumed to have heave and pitch degrees of freedom.The heave model neglected the pitch degrees of freedom and its coupling with the model, whereas the heave pitch model incorporated these effects into the analysis.The heave pitch model used a two-wheeled articulated nose gear and telescopic main gear geometries during landing impact.