Dressing a prop 101

[iflyforpie]

This is mostly for apprentices but also for those like me who had to teach themselves the easiest and most correct way to dress a metal prop in spite of being 'taught' numerous times to do it and having the required standards to make it airworthy.

The reason I bring this up is because I still run across planes with incorrectly dressed props, and still run across people who either don't know how to do it properly or are actively preaching the wrong way to do it (like an old crusty DOM I worked under who told us to peen over the damage with a flat screwdriver).

Going online reveals lots of standards and advice, but none get to the meat and potatoes of dressing. Manuals only give standards to achieve, not methods to achieve them.

So when I see a prop that has a v-shaped notch in it.. approximating a 10:1 ratio with damage still at the bottom and the airfoil only somewhat restored... I feel the need to post stuff like this.

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Prop dressing 101:

1) Assess the damage. Pretty basic. If it exceeds the limitation in the manual, you are done. You need at least a 10x magnifying glass to properly inspect the damage and probably sand paper or emery cloth to remove the paint, primer, and surface imperfections that might hide damage. If the metal has been peened over... you will need to file it away to reveal cracks if there are any there. Some dye penetrant--even if you are not an NDT AMO--is very helpful to ensure you haven't missed anything.

2) Remove the damaged area. This is strictly damage removal.. we don't care about the 10:1 contour or the aerodynamic contour at all. I prefer a round file between 1/8" and 1/4" in diameter (depending on the size of the damage) because it removes metal fast but it leaves a rounded bottom unlike a triangular file or edge file. File the visible damage away, use some emery cloth around the file to remove the file marks or peened metal, and reinspect as per 1) to ensure all of the damage is removed. Then measure the depth and check with the maintenance manual to ensure you are within limits.

3) Contour the damaged area. For most props, it is 10 times the depth of the damaged area we have to contour on either side of the damage. Get a depth micrometer or callipers and measure, and mark out the area to be contoured. All we are worried about at this time is making it contoured when looking at the blade from the front or back... the leading edge should be as flat as a pancake when we are done. I prefer a half-moon straight-cut file for this, though a bastard file will work less efficiently in a pinch. We draw the file along the length of the prop for the best results using the rounded side near the damage and the flat side near the edges of the repair. The goal is to have an ogive shaped (look at the leading edge of the Concorde wing for an idea) contour on each side of where we removed the damage... with a gentle curve between them over the previously damaged area (at least the radius of the half-moon file, perhaps more). Again... we do not give one single crap about the aerodynamic profile at this point... the leading edge should be very blunt. Run an emery cloth over the repair to remove file marks.

4) Restore the aerodynamic shape. Now we use our file to get that leading edge shape back... typically using a straight cut file drawn along the length of the leading edge at the various angles we need to achieve it. Again, use emery cloth or sandpaper to removed file marks and smooth the repair, and make a final inspection with a 10x magnifying glass to ensure all damage was removed.


5) Restore the surface finish. I'm not super picky about that. Some primer and flat black paint applied to the face (flat) side with a brush is fine after using Wax and Grease Remover, MEK, or even Isopropyl Alcohol to clean it.

We've removed a lot more material than most AMEs and pilots would be comfortable with.. but we've ensured that there is no remaining damage with a poor contour that only serves to amplify the stress riser.

TL;DR.

Assess the damage.

File out the damage.

File the contour.

Restore the aerodynamic profile.

Do one at a time... not all four at once while not actually doing any of them...

[comfail]

I lurk here not only for the occasional drama, but for gold such as this. Thanks Pie.