Scraping the prop is beneficial for two reasons. First of all the plastic props are quite heavy. The red plastic prop assembly weighs just about 5 grams which only leaves about 5 grams to build the model. If we scrape some weight off of the blades, by making them thinner, then we can reduce the prop weight to 3 to 3.5 grams which gives you more freedom in building your model. For maximum flight times, you need to buy a plastic prop which is actually larger than 20cm and then scrape and trim it down to size and weight. The reason we use larger 9" plastic props is because they typically have a larger blade with more area on them. The larger area helps to produce more thrust and increase the flight time.

Scraping the prop is easy to do, it is just time consuming. The procedure is to take a single edge razor blade and scrape it back and forth over the prop blade. The photo sequence below shows how this is done. When doing this, you should count your strokes, do 50 on one blade then switch and do 50 on the other blade. This helps to ensure that you remove the same amount of plastic off of each blade so one is not heavier than the other which can cause the prop to shake when it spins.

The same method is used when cutting down a larger prop to 20cm. Here is an unmodified 9" prop and next to it is a 3.5 gram prop which has been scraped down and repitched. Look at how much excess plastic has been taken off near the hub.

This thicker area of the prop can be removed by using a round file or a Dremel type rotary tool.

After you have scraped the prop down to about 3 grams or so, you will have to make a prop shaft. You will need your round nose pliers, and about a 2" piece of .020 or .025 music wire. First bend a small angle as shown below, the bent piece should be about 1/16" long.

Then use the round nose pliers and start to bend the hook as shown below.

Then continue until you have the following.

Then use the round nose and bend the hook back on itself as shown below, and the prop shaft is done.

The next step is to glue the prop shaft into the prop.  The hole in the prop is .047" and the shaft you just made is .020" or .025"  diameter. Use a scrap piece of 1/16" square balsa and round it with some sandpaper, then push it into the prop. Apply a small drop of superglue on each end.

Then cut off the excess wood. Now push a piece of the prop shaft wire into the balsawood plug. Try to keep the wire going as straight as possible when pushing it into the prop.

Try to avoid making it crooked like this.

Then bend the wire back and then glue it to the prop with a drop of superglue.

The last thing to check is whether the prop is balanced. There are two types of balancing, static and dynamic. Static balancing is checked when the prop is not rotating. What you are doing is seeing if one blade has more mass than the other. This is simply done by holding one of the prop blades horizontally and then letting the blade go. If one blade is heavier, then it will rotate to the bottom. (if you are not using the red prop assembly then you will have to place the prop in the thrust bearing first before doing this test). If the prop is in balance, then the blades will stay horizontal as shown in the photo below.

If one blade is heavy, then it will fall to the bottom.

The other type of balancing is dynamic balancing. This refers to whether the prop is in balance when it is spinning. A statically balanced prop may not be dynamically balanced. If one blade has more pitch than the other, then that one blade will create more lift. The unbalanced lift will cause the prop to shake when flying, and the model will look like it is vibrating when it is flying. To check for dynamic balancing, watch the prop spin and hold the model by the motorstick and feel and look for shaking. Also watch the model fly and see if you notice some shaking when it is flying. If it does, then look at the prop from the side when it is rotating. You will see a blurred image of the prop as it rotates. If you watch carefully you can see when each of the separate blades come by as the prop rotates. Look at the tip of each blade as it comes by, each blade should follow in the same path as the previous blade. If one prop tip is ahead of another prop tip, or one blade is higher in pitch then the other then you will be able to see two separate prop shapes as the prop spins. Below is a graphic depicting this and how to correct the problems.


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