For Everyone
For SABC Members
 |
Sport Aircraft Builders Club (SABC) |
 |
Reprinted with thanks to Steve Ashley from West Coast Propellers. Steve was the guest speaker at the August 2002 SABC meeting.
The Propeller is the most stressed component found on any propeller driven aircraft. It not only has to absorb all the horsepower the engine it is fitted to produces, but also accept Torsional, Centrifugal, Twisting and Vibratory Stresses without failing.
Back to Top of Page
The Propeller/Engine combination must be acceptable and provide enough ground clearance for your aircraft to operate safely. Cruise or takeoff performance parameters have to be considered along with Propeller type and tip configuration. Many different blade sections can be employed and this too is dependent upon many factors including the number of blades to be utilised.
Operational and service requirements have in the recent years seen propellers in common General Aviation reach efficiency factors of nearly 97% in some applications.
Back to Top of Page
It is important to remember that the rotating mass on the front of your aircraft engine is most likely to be one of the most neglected parts of your aircraft.
Sitting in a popular Piper Navajo on the runway awaiting takeoff, you have full power applied, propellers in fine pitch, mixtures set as required and the aircraft starts to roll. At this point when full power is delivered there is approximately 50 000lbs of force trying to tear each blade out of the propeller hub socket. In terms we can relate to - this is the equivalent of say the weight of 13.14 Holden Commodores, or 26.74 Mitsubishi Lancer Coupes or 24 Hyundai Excels hanging out of EACH blade socket!! Gets you thinking doesn't it!
Yet with the above in mind we have few major fatalities related to catastrophic propeller failure.
Even more surprising - we never see most privately operated propellers in a recommended repair/servicing facility for checkups until absolutely necessary.
The Manufacturer is always updating ideas, fuelled by field information and defects, etc. This is why many Manufacturers suggest overhaul periods for their products. These periods are more frequent than CASA require. This allows the internal as well as external condition of your propeller to be checked. It keeps your propeller up to date and current with Service Bulletins, Letters and Advisories. In short - it keeps the Propeller as safe as possible.
Back to Top of Page
All propellers should be statically balanced after overhaul in approved repair facililities. These balances are within the manufacturers tolerances and are "good" enough for run of the mill use in many aircraft applications. However Dynamic Balancing has been availalble for some years now. This further enhances the balance/vibration characteristic of the propeller and can be utilised very efficiently.
Please remember that when a dynamic balance is performed it must not be employed to hide another problem, like uneven engine cylinder pressures, manifold leaks, and worn out engine mounts, etc. It is an ongoing process and when any rotating or reciprocating component is changed, or if any major blade rework is carried out, it must be re-dynamically balanced. Please also note that when you dynamically balance the propeller it does not always indicate the "propeller" is out of balance. In most instances it is the combination of Engine/Propeller that is fine tuned. Apart from comfort levels, we have seen a large reduction in baffle, oil cooler, and cylinder cracking on certain aircraft after dynamic balancing has been carried out.
Back to Top of Page
We live in a less than perfect world. Most propellers operate in less then perfect environments. Hence they do suffer from anything ranging from poor use, maintenance, all the way through to neglect. A pity really as they cost and do so much.
There is simply no way to overcome fatigue and stress exposure when a propeller is operated. Even just leaving one idle without due care can allow corrosive attack which impacts upon stress more severely when the propeller is operated.
The propeller is dynamic. The blades do resonate and flex more than most people appreciate and can be induced to flutter like wings and control surfaces in certain circumstances. It is for these reasons that work performed on any propeller should be carried out timely, correctly, and safely; and by properly trained personnel.
Many propellers can be subjected to overspeed conditions and both manufacturers can testify that when overspeed occurs, the stress factor on the blade and hub unit can cube. Relate back to the Navajo and think, at maximum power the blades are exerting a force equivalent to the weight of 24 Hundai Excels on each blade socket, exceed the maximum rated RPM by say 200rpm and this now increases from 24 to 13 824 Excels weight trying to pull your propeller blades out of the hub!!!
Not trying to scare, but this puts perspective into what is going on and makes one appreciate just how much load the propeller hub unit and blades have to absorb. If operated correctly and within the parameters set, all is fine. The analysis for stress etc seems to work very well, but exceed the laid down criteria and you are in uncharted territory.
The fact is we are very lucky that we do not see more blade failures in Australia when our operating conditions and general propeller care is considered.
Simple in service inspection, maintenance and protection of the blade surfaces during the operational life of the propeller can go a long way in promoting both the safety and longevity of any propeller. There are required practices in place for leading edge damage to blades that must be followed closely and correctly.
We have seen a tip separate from a blade 32 hours after some leading edge damage had been noted by the pilot and fogotten about at the time. The owner/pilot of the aircraft stated as soon as the tip separated and the engine increased rpm his memory swung straight back to a pre flight check he had done some weeks back whereby he remembered the nick/notch in the blade leading edge. Incidentally this pilot was some 20 minutes out from land over the Indian Ocean at the time.
Back to Top of Page
Many propeller shops operate under commercial pressure like you the owners and commercial operators do. There is no fixed golden role on how to operate every propeller as the conditions they fly in, the frequency they are used and the maintenance they receive in service varies.
However one thing is for sure, the Propeller Overhaul requirement is vital. We are constantly amazed by some people who seek a "cheap" overhaul as the "aircraft hardly flies" or "it has worked all right up until now". The fact is the overhaul has to be carried out in accordance with laid down data supplied by the manufacturer. In most cases propellers that have not been used much and have been on the aircraft for more years than the manufacturer recommends are in poorer condition than thosed used frequently or to those fitted on commercially operated aircraft.
Although it can be proved in many cases that more regular overhauls can actually reduce costs many owners want to believe this not to be the case. When was the last time that you really got something for nothing? Propeller operation and overhaul is no exception.
Manufacturers cannot control world-wide authorities but do nominate, audit and recommend "approved" facilities. Rule of thumb law allows us to understand that such approved shops provide better product knowledge, assistance, and in most cases service, to owners and operators. These shops have manufacturer back up, support and staff have to be trained on an "ongoing" basis with the more recent developments in our field. Overhaul costs therefore may be a little higher than just a CASA approved shop due to the work being carried out strictly in accordance with the manufacturers laid down prrocedures.
During overhaul, a propeller requires rework in many ways. Propeller blades do require grinding to remove the fatigued or aged surface from them. You cannot simply paint strip and repaint the blade. Other inspections like Dye Penetrant, Magnetic Particle and even Eddy Current have to be employed. In some cases - tasks like blade cold rolling and profile inspections have to be performed. Of course there are parts that have to be replaced that are work out, corroded, or are mandatory retirement items.
At overhaul, your propeller should be updated to the latest configuration / specification. In fact your propeller should, for all intent and purposes, be as good as new.
Back to Top of Page
Costs should be calculated regardless of the hourly use of an aircraft/propeller. As stated earlier, affects such as corrosion can seriously affect operating costs when work has to be performed upon your propeller which has not been expected.
All manufacturers do notate and STRONGLY recommend repair and overhaul criteria and intervals, both hour and calendar related, that an owner/operator should comply with to maintain their propellers in safe and efficient condition.
If you would like to enquire or discuss any of the above please feel free in contacting us at your convenience. We can be reached
West Coast Propellers
westcoastprops@bigpond.com
work hours phone/fax 08-9354 4113If urgent assistance is ever required you can use our after hour mobile contacts which are
0417 945 367 Steve Ashley or
0419 049 866 Dave Chapman
Back to Top of Page
Back to Tips & Tricks