Selection of airfoil and sizing of propeller

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Discussion Overview

The discussion revolves around the selection of airfoils for propellers and the sizing of propellers based on thrust requirements for an aircraft. Participants explore mathematical methods for evaluating propeller performance and the relationship between propeller characteristics and engine torque, as well as the implications of advance ratio and efficiency in propeller design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants inquire about mathematical methods for selecting the best propeller among those with different airfoils based on thrust and efficiency graphs.
  • There is a discussion about the importance of considering the engine's torque characteristics when selecting a propeller, as it affects thrust production.
  • One participant suggests an ideal scenario where the best engine is always available for the selected propeller, prompting debate about the practicality of this assumption.
  • Another participant argues that real-world constraints must be considered, emphasizing that the propeller must be compatible with the engine's torque output at its operational speed.
  • Some participants express uncertainty about how to proceed with propeller design if engine characteristics are not known, questioning the feasibility of building an efficient propeller under such conditions.
  • Concerns are raised about low efficiency due to low flight velocities, which lead to small advance ratios and insufficient thrust when reducing propeller diameter.
  • One participant mentions making simplifications in their analysis, assuming optimal engine conditions for the propeller.

Areas of Agreement / Disagreement

Participants generally agree on the importance of the relationship between propeller selection and engine characteristics, but there is no consensus on how to proceed without knowing the engine's specifications. Multiple competing views on the ideal approach to propeller design remain unresolved.

Contextual Notes

Participants express limitations in their ability to design propellers effectively due to uncertainties regarding engine characteristics and the implications of low flight speeds on propeller efficiency.

shaft
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Two problem to solve.
1) I have 4 propellers built with different airfoils. I have the graphs of coefficient of thrust and efficiency in function of advance ratio. There is a mathematical method to select which one is the best or the only way is just look at graphs
2)I know the maximum thrust necessary at my aircraft so I need to found the right dimension (diameter) and number of blade for a propellers .
I can calculate the thrust produced by the propeller but this change with the advance ratio and the pitch angle so really i don't know how to select the best diameter and number of blade.

I thank you in advance for the answers
 
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shaft said:
Two problem to solve.
1) I have 4 propellers built with different airfoils. I have the graphs of coefficient of thrust and efficiency in function of advance ratio. There is a mathematical method to select which one is the best or the only way is just look at graphs
2)I know the maximum thrust necessary at my aircraft so I need to found the right dimension (diameter) and number of blade for a propellers .
I can calculate the thrust produced by the propeller but this change with the advance ratio and the pitch angle so really i don't know how to select the best diameter and number of blade.

I thank you in advance for the answers
A propeller is a device which converts torque from an engine into thrust. Selecting a propeller is more than just picking one which produces the right thrust. You also must consider the engine driving the propeller and what its torque characteristics are.
 
SteamKing said:
A propeller is a device which converts torque from an engine into thrust. Selecting a propeller is more than just picking one which produces the right thrust. You also must consider the engine driving the propeller and what its torque characteristics are.
but in the ideal case that I can have always the best engine appropriate for the propeller ?
 
shaft said:
but in the ideal case that I can have always the best engine appropriate for the propeller ?
That's not the point. You are selecting a real propeller driven by a real engine to add thrust for a real plane.

If the propeller can't absorb the torque output by the engine at its speed of revolution, then the propeller also won't produce the amount of thrust you think that it will.
 
yes you have right, but in the case that I can select the engine after the selection of propeller, in this way it's the propeller that give the characteristics to the engine.
 
shaft said:
yes you have right, but in the case that I can select the engine after the selection of propeller, in this way it's the propeller that give the characteristics to the engine.

Good luck with that approach then.
 
SteamKing said:
Good luck with that approach then.

so if i don't know the characteristic of the engine I can't built a propeller ?
 
shaft said:
so if i don't know the characteristic of the engine I can't built a propeller ?
You can build any number of propellers, but you want to build (or select) the propeller which is the most efficient at turning the available torque produced by your engine into thrust for your plane.

This article discusses the key topics in selecting the best propeller for your aircraft:

http://www.experimentalaircraft.info/articles/aircraft-propeller-selection.php
 
the problem is that i have already small efficiency of the propeller because the velocity of fly is max 30m/s this mean a small advance ratio and a small efficiency and if i decrease the diameter of prop i don't have enough thrust .
It's a loop where I don't found a solution for this I am doing some semplification like I can have the best engine for the propeller and considering only the condition where it spend the major part of time.
 

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