What is propeller pitch and how does it differ from effective pitch?

Propeller pitch is the theoretical distance a propeller travels through air in one revolution, measured in units of length. Effective pitch is the actual distance traveled, accounting for drag forces. The difference between theoretical and effective pitch is called propeller slip, which occurs because drag prevents the propeller from achieving its theoretical distance.

How do thrust, torque, and power coefficients characterize propeller performance?

Thrust, torque, and power coefficients are dimensionless values that characterize propeller performance across different operating conditions. These coefficients normalize thrust, torque, and power supply using freestream density, propeller rotation rate, and diameter. Using these coefficients alongside the advanced ratio allows engineers to determine how a propeller operates under varying conditions, similar to methods used in multicopter aerodynamics characterizing thrust.

What are the three operating regimes of a propeller?

The propeller regime produces positive thrust and torque for forward motion. The air-brake regime occurs when thrust becomes negative while torque remains positive, slowing the system. The windmill regime begins when both thrust and torque drop below zero, with airflow controlling the propeller and producing forces the motor cannot overcome.

How does increasing the number of propeller blades affect performance?

Increasing blade number significantly raises thrust and torque production. A four-blade propeller generates substantially more thrust and torque than a two-blade design at the same conditions. However, this improvement comes at the cost of reduced propeller efficiency, as the two-blade propeller is slightly more efficient than its four-blade counterpart.

What effect does propeller pitch have on efficiency and operating range?

Higher pitch propellers produce more thrust, torque, and power for a given advanced ratio compared to low-pitch designs. Increasing pitch also extends the propeller region's range and shifts maximum operating efficiency to higher advanced ratios. This allows high-pitch propellers to operate efficiently across a wider range of flight conditions.

Why is propeller diameter less critical than pitch for efficiency?

Diameter changes have negligible effects on propeller efficiency, though the three coefficients increase slightly with decreasing diameter. This contrasts sharply with pitch and blade number variations, which significantly influence efficiency. Therefore, pitch and blade configuration are primary design parameters for optimizing propeller performance.

How is propeller efficiency calculated and when is it meaningful?

Propeller efficiency is calculated using torque and thrust coefficients along with the advanced ratio J, which normalizes freestream velocity to propeller rotation and diameter. Efficiency calculations are meaningful only within the propeller regime where both thrust and torque are positive. Beyond this regime, efficiency values become meaningless as the propeller no longer produces useful forward thrust.

螺旋桨特性表征：螺距、直径和叶片数量对性能的影响

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1. 在亚音速风洞中测量螺旋桨特性

如图3所示，使用四轴尾撑安装架在亚音速风洞中搭建螺旋桨测试台架。本演示中使用的风洞测试段截面尺寸为2.6英尺×3.7英尺，最大动压设定值为25磅每平方英尺（psf）。

用于空气动力学测试分析的无刷直流电机、螺旋桨和尾撑安装的风洞装置。
图3. 螺旋桨测试装置。请点击此处查看该图的放大版本。

将六轴测力计安装到实验装置上，用于测量推力和扭矩。
将无刷直流电机固定在装置上，然后安装第一个螺旋桨。
将直流电机连接至电子调速器和脉冲宽度调制信号发生器，后者用于控制电机转速。
连接功率分析仪，用于测量供给电机的电流和电压，并将其连接至锂聚合物电池。
使用水平仪确保桨-支杆装置在无俯仰和无偏航状态下与气流方向对齐。
关闭风洞门并开启主电源。
启动风洞，然后打开信号发生器和测力计数据采集系统。
使用虚拟仪器软件对测力计上的力进行归零。
将信号发生器设置为以10%油门运行电机。
在风洞关闭状态下开始记录零读数，并记录以下数据：
a. 螺旋桨特性——叶片数量、螺旋桨直径（in）和螺旋桨螺距（in）。
b. 转速（以信号发生器设定的油门百分比表示）。
c. 来自风洞传感器的动压（psf）。
d. 功率分析仪测得的供给无刷直流电机的电压（V）和电流（A）。
e. 来自测力计的推力（lb）和扭矩（in-lb）。
f. 螺旋桨转速（RPM，每分钟转数）。注意：RPM读数仅可在实验结束后提取。
开启风洞，并将动压从0 psf逐步增加至10 psf，步长为0.5 psf。
在每个设定点，待风洞运行稳定后，记录上述相同数据。
继续以0.5 psf为增量逐步提高动压，直至推力和扭矩变为负值。在每个增量下均记录全部数据。
将风洞动压重置为零，并关闭风洞。
将电机转速设为50%油门，重复步骤1.11至1.15。
将电机转速设为100%油门，重复步骤1.11至1.15。
对所有螺旋桨重复上述过程，确保在10%、50%和100%油门下进行测试，直至动压达到推力和扭矩变为负值的条件。
所有测试完成后，将电子调速器接入编程套件，记录所有螺旋桨的RPM数据。
关闭所有系统。

表1. 测试的螺旋桨。

螺旋桨直径 × 螺距（英寸）	叶片数量	材料
18 x 8	2	APC
16 x 8	2	APC
15 x 8	2	APC
15 x 10	2	APC
15 x 12	2	APC
18 x 8	2	Wood
18 x 8	4	Wood

请注意，本研究中使用的定距螺旋桨以其直径和螺距（单位：英寸）来定义。例如，18 × 8 螺旋桨表示直径为 18 英寸、几何螺距为 8 英寸的螺旋桨。

$$\rightleftharpoonup{xx}$$ $$\longleftharp{xx}$$, $$\longrightharp{xx}$$,

来源：Shreyas Narsipur，北卡罗来纳州立大学，机械与航空航天工程系，罗利，北卡罗来纳州

螺旋桨是一种扭曲的翼型，其弦线角度沿径向位置的变化而变化，如图1所示。螺旋桨广泛应用于飞机和船舶的推进系统中，因此需要对螺旋桨进行详细表征，以设计高性能的飞行器和舰船。

螺旋桨设计示意图，显示螺距、弦长、厚度和径向位置的测量方法。

图1. 弦长、厚度和径向位置…