Maintenance Article Issue # 3
Propeller Blade Filing: by Maurice Wills, AME & Pres
(as printed in AMU Feb/05)
In this article we are going to delve into the fine art of aluminum blade filing.
But first, lets look at what is happening to a propeller blade. When a propeller is rotating there are six forces acting on the blades, which when combined with a stress riser could lead to blade separation
• Centrifugal Force
This is the force that is trying to sling the blades out of the hub. Depending
on the rpm, size of blades etc., this could translate into 10 to 25 tons of
force.
• Thrust Bending
As load is applied this causes the blade to bend. On a tractor propeller the
blade would pull away from the aircraft
• Torque Bending
This is the force that allows the blade to lag behind the rotation of the hub.
• Centrifugal Twisting Moment
This is the tendency of the blades to want to twist to low pitch.
• Aerodynamic Twisting Moment
This is the tendency of the blade to twist to high pitch.
• Vibration
This is caused by an out of balance condition
As you can see when the propeller is working, the blades are pulling, bending, twisting and vibrating.
So why do we worry about blade filing? The answer to that would be stress risers. By inspecting and filing blades we are looking to eliminate potential stress risers and thus prevent blade separation.
For example, a customer of ours, Bob Polinuk of Selkirk Air was flying his Cessna 185 when the aircraft started vibrating violently. He says that he didn't know what was happening, but he knew he had to stop the engine. He stalled the aircraft and did a forced landing into a swamp. They knew what had happened when they saw that the last 10" of a propeller blade was missing. The engine mounts were broken and the engine was hanging on by the controls. The aircraft had to be helicoptered out and the propeller was sent to Ottawa for analysis. The report concluded that the tip separation propagated from a blade nick.
What can cause damage
Generally damage is caused by rocks or smaller stones as well as water or other foreign object strikes. For this article we will be looking at blade damage caused by nicks, gouges and water.
Water
On float operations water is one of the hardest things on propellers, and water damage is one of the quickest ways to retire a set of blades.
The easiest way to prevent water damage is directly related as to how the aircraft is handled on the water. This is beyond the AME's control and has more to do with the pilots. As in anything, sometimes you just can't help the propeller from getting spray cut by the conditions, eg. big waves, rough water.
What does the damage look like? Unlike rock damage, water causes the blades to split along their length. You will see that the leading edge looks like it is fraying apart, usually right at the tips.
Some AME's will tell you to take a screwdriver and roll the leading edge to compress the damage. This is not a recommended practice because you are not actually removing the damage or contamination but masking over it. This spray cut can sometimes penetrate up to 1/2 " into a blade.
The way to remove this type of damage is to completely dress the leading edge to remove the spray cut and reprofile the blade tip. Usually this type of damage will be the same on all of the blades.
When you are dressing the blades you are trying to maintain the original air foil and you will want to template your blades so that they are the same. If you have severe damage then you will want to send the propeller to your local propeller shop.
A little tip that we use is to tape off the area that you are dressing to restrict the dressing area and prevent the file from sliding across the blade.
Stone Damage
On this type of damage you are generally concentrating on a more local repair area.
You will want to inspect the blades for stress risers that will need dressing. I like to do a visual inspection and I also run my hands down the leading and trailing edges.
Leading & Trailing Edges
The basic rule of thumb is that you want to dress the repair by measuring the depth of damage and multiply this by 10.
"Example"
You want to maintain the original air foil and make sure that there is no foreign material embedded in the blade, eg. rock.
You can start with your file to remove the majority of the damage. You can also use air or electrical power equipment. but I find that you have a better feel with files. Once you have removed the damage you then want to remove the file marks by using fine sand paper, emery cloth etc.
Now it's time to inspect with a 10x magnifying glass and dye penetrant to make sure that there are no file marks or foreign material left.
You then proceed to treat the repair with alodine which is used for corrosion prevention and finish up with paint for protection.
Blade Face and Camber Side
These are basically similar to the leading edge repair. You want to dress over the affected area so you do not have a stress riser.
I have given you a basic overview of blade repairs. For more information you should always consult the manufacturers Owners Manual or your local propeller shop to advise you further
www.aopa.org/asf : Safety Advisor "Propeller Safety"
Maurice Wills is General Manager & Pres of Canadian Propeller Ltd..
Canadian Propeller is Western Canada 's Hartzell Propeller Approved Repair Station
and a McCauley Propeller Factory Authorized Service Station.
Maurice has been is the propeller industry and an AME for over 25 years. He
has been President of Canadian Propeller for 14 years.
For more information, contact Maurice at 1-800-773-6853
or www.canadianpropeller.com