Lately, power generation using windmills has been planned by the government for the electrification of islands and rural mountaneous areas in our country. Also, in many other foreign countries research is being conducted on this subject.
The purpose of this investigation to show an appropriate reinforcement measure to the windmill propeller, which is the most critical part in a wind power generator system, by exploring the stress distribution on the propeller both theoretically and experimentally.
For theoretical development, following assumptions were held:
1. The hollow rotating propeller is viewed as a cantilever beam.
2. The propeller has been untwisted and set at zero angle.
3. The forces acting on the propeller are aerodynamic forces and contrifugal force.
The simple blade theory is applied to compute the net bending moment by means of a tabular method. This yields the bending stress which is superposed with the centrifugal stress due to the centrifugal force, to constitute the total stress.
The result of the stress analysis helped point out the weakest portion on the propeller. This led to the possibility of decreasing the stress concentration in the portion by providing an appropriate inforcement to that area.
A strain gage balance was designed and made in order to obtain the data for the purpose of experimentally obtaining the lift coefficient ($C_L$) and the drag coefficient ($C_D$) of the propeller. A physical model, 1/4 of real size, was then made and satisfactory values for these coefficients were obtained.
A static stress test device was next designed and constructed in order to measure the bending stress due to the net bending moment. Strain gages were attached to a selected portion on the propeller and the values were measured using the vishay indicator.
Approximately 20% of difference was noted between the experimentally measured and theoretically calculated values of stress distribution in the propeller portion of radius between 48cm and 102.6cm.
The experimental analysis of the reinforced propeller revealed that an overall reinforcement, determined by the reinforcement theory, is much more desirable than a partial reinforcement.
A propeller with a corrugated type of interior surface is recommended for this purpose.
And the fracture analysis revealed that the fracture had occurred due to the repeated load, which was created by the instant inertia moment due to the impact present during the operation of the pitch control device.
Therefore, a proper modification is called for in the design of the pitch control device.
最近, 우리나라에서는 島嶼와 山間僻地의 電化를 爲해서 風力發電을 計劃하고 있고, 外國의 여러 나라에서도 風力發電의 硏究가 遂行되고 있다.
本 硏究는 風力發電機에서 重要한 部分인 프로펠러에 對하여 定常作動時의 應力分布를 理論 및 實驗的으로 說明하므로써 適切한 補强을 提示하는 것을 內容으로 한다.
回轉하고 있는 프로펠러 (속이 비어있음) 를 하나의 Cantilever beam으로 보고 몇가지 假定, 卽, 프로펠러는 비틀려 있지 않고, 裝着角度는 零이며, 프로펠러가 받는 重要한 힘은 航空力學的인 힘과 遠心力으로 생각하며, 單純날개 理論을 適用하여, 二次元的으로 基本 資料를 計算하고, Tabular Method를 利用하여, Net bending moment를 求하고, 여기에서 나온 굽힘應力과 遠心力에 의해서 생기는 遠心應力을 重量(Superposition) 함으로써 風車 프로펠러에 걸리는 全體應力을 求하여 應力解析을 하였다.
應力 解析結果, 프로펠러의 가장 脆弱한 部分이 指摘되었으며, 適切한 補强에 依해서 最大應力을 크게 減少시킬 수 있음을 보였다.
應力計算을 爲한 基本資料를 求하기 爲해서 넓은 範圍의 迎角에 對한 揚力係數($C_L$) 抗力係數($C_D$)가 必要하였으므로 Strain gage에 의한 Balance를 設計製作하고, 風車 프로펠러의 1/4되는 模型을 만들어 滿足스러운 값을 求하였으며, Net bending moment를 날개에 주어 굽힘應力을 求할 수 있는 靜的應力實驗裝置를 設計製作하여, KFC-5-Cl-11 과 KFC-2-Cl-11型의 Strain gage를 選定된 프로펠러 地点에 附着하고 Vishay 1011 Indicator를 通하여 實驗値를 얻었다.
理論과 實驗結果에서 應力分布는 半徑 48cm에서 102.6cm사이에서는 約 20%의 差異를 갖어 왔으나 比較分析한 結果, 理論의 假定에서는 프로펠러의 內部 補强材料가 完全히 附着되었다고 했지만, 實際는 間隔이 많이 있었기에 實驗에서 荷重을 주었을 때는 外部材料에만 많은 힘을 받기 때문에 큰 應力値가 나왔다고 본다.
補强 프로펠러의 實驗結果에서 局部的인 補强보다는 補强理論에 의한 全體的인 補强이 要求되었으며 內面이 Corrugate型 프로펠러가 推遷되었다.
그러나 正常狀態에서의 應力은 銑의 許容應力보다 훨씬 낮었기에, 破裂은 다른 要因으로 보고 硏究한 結果 프로펠러의 破裂은 핏치 調整裝置의 作動時 衝擊에 의한 瞬間的인 慣性 모오멘트의 反復荷重이 가장 危險하므로, 衝擊을 招來하지 않도록 핏치 調整裝置가 設計되어야 함을 發見하였다.
