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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


The Prop Doctor steals an idea: airfoil sections


Have I got a treat for you this month! The idea for this month's topic comes from Bruce De Chastel, up dere in de Sunshine State. If you don't know Bruce, not only is he a very talented pylon flier, but he's a very sharp cookie indeed. The only problem is that he makes props in competition with me, so we don't often get to swap ideas. On this occasion I was complaining about the problem of converting a favourite propeller to CNC coordinates on the milling machine.
You all know how I feel about prop airfoils, from my previous efforts in ACLN. Often the only way to improve a prop is to make sure the airfoils are the right shape, whatever that is. If you want to compare props, you need to know the camber and camber high point to determine the zero-lift angle and hence the experimental pitch. Now these two variables are hard to pin down with any accuracy: they are just plain hard to measure. Well Bruce had the answer to this, and in an unguarded moment shared it with me, so old big-mouth me is now going to tell you.

This is what you do. Borrow a prop. Cut a slice out of the prop; say a mm wide. A good place to start is at the 3/4 radius point on the blade, as this is representative of the way the whole prop behaves. Drop it on the platen of your computer scanner (mine is an Optic Pro 9630P) and scan it at a resolution of say 300 dpi. The result is stunning. That tiny airfoil can be blown up to 6" wide, where all its imperfections and graces can be examined. 

Even more stunning, my "MicrografX Photo Magic" software lets you put a ruler over the image. In fact, You can put an arrow on the edge of the profile and the screen shows the co-ordinates of the section at that point accurate to 50 microns! You can even select "View/Info" and the co-ordinates are displayed neatly to the side of the screen. With these data, you can get the high point and camber very readily, with all the accuracy you ever dreamt about.

Well, I couldn't wait. 6 destroyed props later, I had six little airfoils sitting on my scanner platen, with the P100 warmed up and ready to go. The first scan was not too good. It had big shadows on one side, and the Tornado yellow nylon did not show up so good. The airfoils were re-arranged so that they pointed down the platen, after they were coloured-up with black Texta (all except the APC). This time the results, with the contrast turned up a bit, were really pretty good.
There should be a copy of this scan somewhere near this bit of text. Take a quick peek at it. I'll deal with the results one at a time.

Propeller cross sections

The Tornado 12X6 airfoil is really very good, much like Clark Y. If anything, it may be a bit too thick, but that is forgivable in a nylon prop where strength is a consideration.

Next is one of my CNC aluminium moulds, a pylon prop for the AT-6 Texan/Harvard 1.2 cu.in. class. Prop is carbon-fibre, you can find it on my Website at www.space.net.au/~props. The airfoil is NACA 6311. The 6 means the camber is 6%, the 3 is the high point at 30%, and the section is 11% thick. This is what is called a "NACA 4-digit section". The thickness form is that of Clark-Y, although it has been thinned a little, as Clark-Y is 11.75% thick. The thing that is interesting about this airfoil is that the high point has been moved forward to 30% (Clark-Y is at 40%).

The forward high point has the same effect as leading-edge flaps, or the droop-snoot type of airfoil. This feature allows the airfoil to generate high lift at high angles-of-attack, very useful where good acceleration is required, such as in F2C, F5B and F2A. The scan shows that it has been reproduced faithfully, as you might expect from a machine accurate to 5 microns.

The APC prop is next. It appears to have a circular camber line, which puts the high point quite a long way back. It reminds me of Go795, an excellent low drag airfoil, but not so good lift-wise. It is not really the section you would choose for a propeller, but its accurate profile is going to guarantee better performance than most hand-shaped airfoils.

Last month I mentioned some props had the airfoil on backwards. That is the case with the Master Airscrew 9X6 wood, although I am not sure I would really call this an airfoil. It felt bad slicing up this prop, as it was a beautiful piece of timber, well lacquered and most attractive. But it didn't feel so bad after I saw the section, glad I never put it on an airplane.

The Strato 9X8 dates back to the 1950's, an Australian-made prop of good quality, very popular at the time. Paul Turner used Strato 10X5's on his OS Max 3 powered Thunderbirds, so I did likewise. Problem was my Max 3's never ran as well as Paul's, to this day I wonder what his secret was. The trailing edge is a bit thick, as you will find this with nearly all wood props, including the full-size variety. It is simply a strength matter; thin wooden trailing edges are vulnerable to handling and usage damage. Otherwise, this is a good section, Clark-Y with a bit of extra forward camber. 

The improved performance of aluminium alloy props, or in our case nylon and composite, is almost entirely due to the thinner trailing edges and thinner sections these materials make possible. 

Contrast the Master 9X6 nylon with its wooden counter-part. The section is well suited for props, and Master Airscrew nylon props deservedly have a very good reputation, despite their homely appearance. 

Well there you have it. Is that a neat trick or is it not? To a single-minded fellow like myself, it is very exciting indeed. Time to sign off. To you chaps who have recently been kind enough to comment favourably on my series of articles in ACLN, I thank you. The rest of you can go ..... oops, not that, Rosemary has told me I must change my image!

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