A paint only problem will manifest itself with the inability to hang enough weight on the m/r head to get the blade balanced.
Exactly Chucky,
That's called Span moment arm migration which results in insufficient mass adjustment on the head to counter significant lateral imbalance due to a change in mass distrubution across the blade. This is generally caused by either uneven paint distribution after successive paints or trapped water inside the blade. I am not suggesting that one paint job alone will make it very difficult to balance (unless it was a real amateur job). It is manifested after at least 2 or more paintings and then if not all the paint is removed and a nice even coat reapplied, this begins the process. Generally people will only sand the surface enough to allow the new paint to adhere (as you suggest in your recommendations). This is a problem in the making. Generally the leading edge and towards the tip have all the paint eroded off. Therefore we end up with an accumulation of paint (thus weight) on the inboard half of the blade. Over successive repaints this leads to a migration outboard of the blade's CofG (Span Moment arm). If all the blades on the same head are reapinted the same way, same time and retained as a set flying together, there will be no problem in dynamic balancing them, as their Span CogG will have been migrating together.
However, as soon as I replace one of those blades with a brand new blade, the head will become very difficult to dynamically balance or even impossible to. Because the span moment arm of the 3 remaining blades will have migrated quite markedly different from the OEM's original design specs.
The first thing maintainers will do is return the brand new blade as the other 3 flew perfectly well with the old blade. They will even return this brand new blade to the OEM or overhaul centre and even call it "too heavy" ......heard this before have we?? Sound familiar??? The OEM will simply compare this returned blade with his master blade, see no problem and return it back to service and bill you a fortune for the honour.
The real problem is with the 3 blades still on the head. Their Span Moment Arm has migrated (generally outboard) so much during operational use and maintenance that they now no longer become interchangeable fleetwide. We now are forced to fly our blades as "Sets" because certain blades appear impossible to dynamically balance together. Sound familiar also???
The secret is - in-service span moment arm migration. It happens to every rotor blade created but the industry has remained ignorant of its true effect on the dynamic balance solution.
This leaves only one other option - the static span weight adjustments generally located in the tip. Most helicopters have their tip weights relatively easily accessable. The 412 has their's cleverly disguised as lead shot in a bonded tip cap which are inserted via a little allen bolt in the upper surface. You can occassionallly hear them rattle if you jiggle the blade. The only way you can reduce the weight this package is by de-bonding the tip cap which really means it has to go back to overhaul. Alternatively their is scope to move by playing with the product weights. These are primarily designed to set the dynamic chord moment arm (changes tracking path of the blade tips as angle of attack is changed from flat pitch ground to hover picture) but if you look at the maintenance manual, you will see that Bell do give scope for max/mins for the individual pockets. The important thing is to maintain teh ratio of the mass between these two pockets as this determines the chord moment arm effect - afterall, that is what they are designed to affect. But if the mass of these pockets were equally reduced/increased to change the overall span moment arm, then you could counter the effect of a migrating span moment arm.
The trick is "how do I quantify my span moment arm for any one individual blade"? By this, I mean to actually provide a specificm inchlb or kgcm measurment to which I can then either reduce or increase my span weight package to bring the blade's ideal Span moment arm back into original design specs.
There is only one easy way - by using a digital static blade balancer. This is a tool that most OEMs and their approved overhaul centres still do not use although they are slowly being convinced.
Span moment Arm migtaion is your biggest problem - not trailing edge anomolies. This has been proven with data through the US Army and now an increasing number of other operators using this technology.
I suggest you read
www.rwas.com.au for greater detail. I think it may enlighten.....
PS I am not suggesting that every Tom, Dick & Harry race out and start fooling around with their tipweights. I am suggesting that the industry start demanding of OEMs the ability to adjust these tip weights at operator level ........providing they have access to a digital Static Balance tool (NOT old technology Master Blades). It is the ONLY way of knowing exactly determining the magnitude and quantifying the exact changes required to reset blades back to OEM specs and return blades back to fleetwide interchangeability and easy RTB of no more than 3-4 flights.
cheers