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Designing and drawing a classical (and for ever) wood propeller.
Propeller design make easy.
There are some modern software to desing a propeller, It is very interesting if you want to design special propeller like ultra-high performance racing propellers with supersonic tips or others, but a classical wood propeller don't need computational models, All wood propeller have the same (approximate) structure and all propellers have good performance about 95%.
The following table have all data you need to build a classical propeller, this data don´t have any units (mm, inch) it is a adimensional data, you can design any type propeller with this table.
Chosing your propeller parameters.
The only two parameters than you have to choice are diameter and pitch. It defines completly the blade shape for a subsonic incompresible flow conditions, It mean than any point of the blade (tips) must not exceed 1/3 of sound speed.
This table haven´t unit (mm, inch). We need to select the size of our propeller to calculate our data, for example a 80-60 inch propeller, it means that our diameter is 80 inch and our pitch is 60 inch (the propeller move along 60 inch each revolution). This propeller would work well with a 134 Hp at 2700 RPM propeller speed and 125 milles/hr airplane speed.
Making calculations for your propeller size.
H/D=60/80=0.75 nominal pitch
H/D distribution= Nominal H/D * Pitch distribution/100
Angle = arc tan (H/(2*Pi*R*(r/R))) = arc tan ((H/D)/(r/R)*Pi)
You can see that pitch is not constant all along the blade, there are some very old props in early aviation days having "true helical pitch" which in itself is generally not the best way to get max efficiency from a propeller.
Drawing your propeller.
We have to draw a diferent airfoil for each blade sections, we can also select two, tree or more airfoils, for example, you can use MH126 (especific root propeller airfoil) for 20% and 30% sections, MH112 for 40% and 50%, etc. but we going to make a more simple method.
The most easy way to get diferent seccions airfoil is to select unique airfoil along all blade. There are hundreds of airfoils suitable for this aplication but I think it is beter to make a sure choice, we going to select a old shape from the days of old airfoils like Clark-Y, it is a classical airfoil used in a lot of propellers desing in the past, and also used today for simple designs.
Clark-Y is a cuasi-flat bottom airfoil, Fisrt drawing shows Clark-Y with correct zero angle of attack, but traditionally the angle of attack of flat propellers aifoils have been meassured in relation to flat-botton, the pitch resulting is called geometrical pitch and it is diferent to real pitch measures in relation to airfoil chord. Obviously, all old resources about propeller design is expressed in this way, and last table too. When you see in any propeller "80 x 60 inch", 60 is geometrical pitch. I think today it is used because it is more easy to meassure existent propeller geometry in this way.
Then, we have to rotate to coindide flat bottom with horizontal line. Second drawing shows rotated airfoil.
Next, we strech the airfoil to fit to table_2 chord and thinkness in mm, and then rotate the scheched sections to get correct pitch distribution, the rotation anglee is in table_2 too.
then, we fit the resulting airfoils to the template, all gravity centers of airfoils cross blue line. Notice than a lot of material is removed from root seccion. it have to be rounded in the finishing work.
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