OK to Paint Prop?

OK to Paint Prop?

Hi
I have nick in the front face (the side that faces the pilot) of my prop from a stray rock and I have to sand it out. It will leave an unpainted area that will reflect sunlight at me, and of course will be unprotected. Is it OK to paint it myself - I really don't want to go to a prop shop - the closest one is 300 miles away and I'll be down for some of the best time of the summer waiting for it. Is there a special type of paint that I should use?  Anything else that I need to know?

Thanks

Tony
C-GICE

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Re: OK to Paint Prop?

yep, just put a bit of flat black on it, you can brush it, or use p/pack, just don't use water based paint.
tempo make an excelent  flat black lacquer in p/pack.

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Re: OK to Paint Prop?

I have a hard time beleiving that this can be leagally done.  The prop has to be balanced, if you get a thicker coat of paint on one side the prop will be out of balance.

   Dave

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Re: OK to Paint Prop?

Here's a cut and paste from George Horn's post dated 12-02-02 on "prop care"
***********
    Re: Prop Care      George Horn         12-02-02 11:50

Re-painting the non-glare paint on the back of a prop does not require the removal of the prop. It is a simple matter to either chemically strip it, mask and repaint it using non-gloss, flat-black lacquer in an aerosol can.
Be careful to do this with the airplane tail into the wind to avoid overspray onto the aircraft. (Don't spray your neighbor's airplane, either.)
If you do not mind a slightly less perfect job, it is also permissible to simply spray over the currently damaged paint without stripping the old paint. There is a negligible amount of paint actually being applied. (Remember, you're not trying to add a thick coating of paint, you're merely trying to reduce glare.) Most owners simply use flat-black lacquer available in aerosol cans from local hardware stores. Lacquer does not build up, and also drys quickly, and will not hide cracks in the base metal like an enamel would. It can be easily removed with MEK or paint thinners.

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Re: OK to Paint Prop?

If you believe a propeller is statically balanced this close have I got a deal for you.

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Re: OK to Paint Prop?

john britting wrote:

> If you believe a propeller is statically balanced this close
> have I got a deal for you.



No kidding....I wonder if he realizes it will be more "out of balance" if he splats a June bug and gets more guts on one blade than the other!

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Re: OK to Paint Prop?

If you're talking about repainting just the spot where the maintainance was done on the nick...I don't think that calls
for repainting the whole prop.  A quick "phsssft" on the
exposed metal should do it.  The amount of paint shold
be pretty darned inconsequential, I would think.

Michael

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Re: OK to Paint Prop?

There is no problem in repainting the prop.  The issue at hand, though, is whether the damage is negligible or not?  Are you just covering up damage with fresh paint?

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Re: OK to Paint Prop?

Under FAR43, you can paint the prop but are not allowed to dress the prop unless you are a certified mechanic.

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Re: OK to Paint Prop?

Not only is Bob correct about an A&P certificate being required to legally dress the prop, but NEVER use regular, old sandpaper on a prop.  The prop is one of the few things on an airplane that can kill you quickly if it fails.

Touch-up painting it is usually not a big deal.  Painting the whole prop could leave it out of balance.

Be reeeaaly careful messing around with props.  Once you've seen a prop blade separate because a nick was improperly dressed, it will really get your attention as it rips the engine off of the airplane.

Walter Atkinson
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Re: OK to Paint Prop?

<...rips the engine off of the airplane.> 

Seems like this should be the design criteria for the engine mounts. 1/2 Prop separates at full throttle.  Anyone know what it is?

Makes the case for wearing a parachute!

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Re: OK to Paint Prop?

*Seems like this should be the design criteria for the engine mounts. 1/2 Prop separates at full throttle.*

Hmm?  How heavy do you think that kind of engine mount would be??  A 1/4" STEEL firewall to mount it to??  A big truss??  Think it would fly?

Walter

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Re: OK to Paint Prop?

[Hmm? How heavy do you think that kind of engine mount would be?? A 1/4" STEEL firewall to mount it to?? A big truss?? Think it would fly?]

Don't know what the forces would be without a calc to answer that question. 

The real question is do you want to fly a plane where the engine can be ripped off by a very plausible and possible failure mechanism for an active component.  Nevertheless, seems like this kind of thing can and does happen.   I wasn't aware that this was a real risk and I am very surprised!

Has there been any operating experience reports of this happening?

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Re: OK to Paint Prop?

Barry,
A Sensenich fixed pitch propeller on a small aircraft at 2600 rpm generates @ 13 Tons of centrifugal stress per blade. Losing a large portion of a blade will usually cause such an immediate catastophic vibration that retarting the throttle can not happen fast enough to prevent the engine from being torn from the airframe..
Propeller blades are designed so that the propeller can have an infinite number of stress cycles before it endurance limit is reached. However,  nicks and scratches in a blade create stress risers that lowers the number of stress cycles before failure...  Look at how glass is cut - the roller cuts a notch and that is where the glass will break... Or a rock chip in a windshield starts the same thing... Jack

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Re: OK to Paint Prop?

Jack,

A 3 lb mass spinning at 2600 rpm 2 feet from the center of the rotating shaft can generate that amount of force on the shaft.  I believe you!

What I'm surprised about is that I did not understand the risk associated with this type of failure.

The next time I preflight, I'm going to spend a little more time looking for stress concentrations on the prop!

For those interested:
I worked out some very rough numbers to get a sense of what's happening.  When on the ground, each mount at the firewall must withstand in addition to the forces produced by the engine:

[This doesn’t account for the load being applied rapidly and using a simple geometry with just the static loads in a direction that produces the least amount of force on each!]

6500 lbf in shear  (Assuming 4 mounts 26,000 lbf / 4))

In addition, to compensate for the moment applied each of the 4 mounts at the firewall must withstand loads in tension or compression:

26,000 lbf in Tension or compression
(Assuming each mount is 1 ft from the shaft and 4 ft from the prop)

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Re: OK to Paint Prop?

just had my prop balanced last week..asked the same question.  I was told that light painting, particularly on both blades evenly, will not be significant.  ditto from above on no sandpaper.  Use chemical stripper and or Scotchbrite pads to clean, epoxy primer and "prop paint"
I didn't think I had much vibration but it measured 3.1 ips before and 0.06 after.  Smoother feeling when done but not radically so. Considering the forces involved (I have seen numbers in several ton range) I was glad to get it right.

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Re: OK to Paint Prop?

Steve,
Dynamic balancing the propeller has little effect on propeller stress and therefore propeller service life. Those little weights added to the spinner backplate do not change the bending stress created as the blades pull through air. Vibrations caused by imbalance - stress the propeller attachment and everything downstream which includes the passengers, but not the blades.. Your one ounce weight added 2" from the rotational axis only created a force of @ 21 lbs.. Your propeller blades generates some 13 tons of force so it really wasn't concerned with the 21 lbs of imbalance. Your smoothing from 3.1 to 0.6 was a nice thing to do for your airplane. It was money well spent......Many times the Static Balance from Propeller shops lacks the fine tuning that Dynamic balance accomplishes.    Another Propeller Subject - Metal Props operating off Grass Strips....easy to spot by the 1/4 inch foreward curl at the blade tips.....Now that's ugly!!!!jack

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Re: OK to Paint Prop?

good point on propeller blade stress...but if the attachment fails doesn't really matter if the prop is good or not! actually we had less weight than 1 oz: we used  15.7 grams at 120 degrees, which lined up right into an existing hole in the flywheel, better than the spinner weight location I think.  In any case the paint is usually a moot point.  What do you mean about grass strips?  I don't see why a wood prop would suffer any less on impact.  I just don't like cleaning off the grasshoppers from all over the plane!

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Re: OK to Paint Prop?

Steve,
Usually it is not the attachment that fails but somewhere within the outer 8 inches of the blade. It is the thinner part of the blade with the less strutural mass with the highest flex...So the most critical.
Yes, The Spinner is not the best place to attach weight.  The Lycoming has holes in the flywheel that can be used but you have to be careful because the added weight may not clear the passage through the flywheel/Starter slot..
Continentals - Most times the weight is added to the Spinner Bulkhead.
Grass Strips... Well, if you think about it, the grass strips very on grass depth - Freshly mowed to needing cut.  So, when you line up for take-off and you apply full power, your plane does not immediately move because of the turf drag on the tires. This is the point where the  Aluminum Propeller tips bow foreward....It's all action - reaction,  caused by the natural tendency of the Propeller.. It is designed to move foreward no matter if the plane is going or not... The Wood propellers on grass strips are not susceptible to this bow. If you bow a wood prop, it  springs back or breaks. If you bow aluminum just a bit too much, it bends.  I had a conversation with my friend who has operated a propeller shop for 30 years and he said that the Sensenich propellers are really not designed heavy enough to be operating on grass strips.. (They come into his shop all the time and he reworks them back to specs.)  He said the same thing will happen if you get the tires stuck in snow or mud.. Anytime the propeller has a high demand and the plane is not moving - then those forces cause the tips to curl.. Sight down the leading edge of your blades.. If they are curled foreward - you will see it ... It is not critical to flight but it will cause the propeller to be a little less efficient....I guess the secret is to  apply the power slowly and let the plane pick up momentum - hopefully the strip is long enough to allow this safely...  I guess I rambled... Sorry!  jack

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Re: OK to Paint Prop?

Interesting..good points!  I would never rev any prop fully until moving if possible..perfect way to pick up stones and nicks even on hard runways!  keep 'em rolling through the soft stuff! thanks

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Re: OK to Paint Prop?

OK, this has been getting my attention.

So how much "dressing" is safe for the average prop.  And how do we mere mortals evaluate whether or not our A&P has done a satisfactory job.  Even more to the point - what signs can you reasonably see that would predict a prop getting ready to fail? 

(I tap on the prop to hear that nice "tone" but have never felt I'm doing much more than a ritual here...)

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Re: OK to Paint Prop?

Tough question - my only suggestion is to send the propeller to a certified prop shop at the recommended overhaul cycle..The prop will go through a complete inspection and reconditioning -Then it is just taking care of it in the field and attempting to avoid runways with poor upkeep such as debris and rocks.....Predcting when a prop will fail has not been perfected yet.. Visit a prop shop and talk with the prop man..  he can tell some pretty good stories of what he has seen come through his front door. He will tell you what people have done to props - for one, cutting off both tips because one end had damage.....scarey!!

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Re: OK to Paint Prop?

To those that choose to post about my mentioned balance the prop, kiss off!  I simply made a comment based on what I felt and I get smacked around for it.  Geez, makes a guy NOT want to post around here!

    Dave

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