Monday, January 31, 2011, 03:45 PM - RUMORS1. Rumors about slinging blades now and then.
Posted by Administrator
Posted by Administrator
2. Rumors that it requires OH every 200 hours.
3. Rumors that it is not compatible with 0-320s and 0-360s because of vibration.
4. It's not recommended by the Swift Club website “The Answer Man” Montague and his co-heart Forest something or other, I think that's his name.
1. Early 2000 LAACO called me, come down to LA, we going write AD against your Aeromatic. But we can’t write AD note without U first write SB, cause we don’t want ground all airplanes with Aeromatic props on dem. I go don LAACO. They show me photos of ferule with some wood and some rusty broken screws and photo of shank end of the blade that came out of the ferule. They told me that it was slung off a F24R. They also say it was DRY ROT failure. They didn't show me photos of the blade that “slung” out of hub, just photos of the break area. ACO “engineer” said he would help me write SB. After a few visits to my factory he talk, me type he finally finished writing my SB 2000-001. At the time me new at this stuff mit der FAA ACO. ACO engineer pulled out AD 76-10-12 on Beech wood blade propellers. ACO guy make AD note for Aeromatic props look like AD note on Beech props. ACO guy send AD to FAA east for release then he go to Italy for visit.
AD note guy FAA East call me, he ask, “how many dry rot failures do you know about?” Me, “None”. He said that he would not release an AD note based on a single dry rot failure. And said he would go through AOPA and EAA to contact type clubs so he could get more data on dry rot failures of the Aeromatic blades. He gave three months to get some feedback, no response. He gave another three months and still no dry rot failures reported. He called me and said that he would not release it as an AD note but would make it a SAIB (Special Airworthiness Information Bulletin). That he did and it read virtually the exact same as the AD note that ACO guy wrote. It was released as SAIB NE-01-23. By now many months have passed. It was released May 23, 2001.
I was granted a CRS for Aeromatic propellers pm 20 Sept. 2000. So here I am inspecting propeller blades looking for dry rot. I found many broken screws but no rotten wood. So I make request to the owner of the F23R and asked him to send me the broken propeller blade so I could see what to look for for dry rot. When I got the blade the first thing I noticed was that the blade was completely severed at about half way out toward the tip. It had one hell of a hard ding at the severed place. That means that the blade took a blow of enough energy to completely sever it in two. The direction of the energy was clearly directly against the leading edge toward the trailing edge. I also noticed that the break at the shank of the blade shows that the force holding the blade against the force that severed it was from the exact opposite direction that is from the trailing edge toward the leading edge. Furthermore the blade was split from the shank spanwise toward the tip, about 12 inches long. The forces that severed the blade ripped it out of the ferule and split the blade spanwise was very substantial. A rough calculation clearly indicates that a wood blade weighing around 9# and falling from the sky could not have received enough force to completely sever it. Also the spanwise split in the blade is not at all consistent with a blade coming out due to centrifugal force. The CF* for a 9# blade with a CG of 14”, turning at 2400 rpm is close to 20,623 pounds.
If that baby was slung out of the hub with that force there is no way it could have split spanwise. It had to be broken out by the amount and direction of forces stated above. The direction of the forces and the amount of damage done clearly indicates that it was a prop strike. Further it is self evident that the engine had to be developing very close to maximum HP when it happened.
* See Machinery’s Handbook 25th Edition P 162 if you want to crunch the numbers.
OK, so much for blade slinging rumor.
2. OVERHAUL EVERY XXX HOURS.
The Swift Club web site expert, Mr. Montague says it has to be OH every XXX number of hours. Somewhere I’ve heard that the number is supposed to be about 200 hrs TBO.
There is no life limit part in the Aeromatic propeller. A propeller is just like an airframe. If it is kept in airworthy condition and the propeller is used within the known and proven operating envelope then the propeller has infinite life. An OH is not required to keep the propeller in airworthy condition. As long as wood is not stressed to the breaking point it has infinite fatigue life. The 15 lag screws which old the wood blade in the metal ferule are not stressed to the point where fatigue is a factor. Yes I have found broken screws in older blades that were manufactured by previous companies. Even so, these blades that came to me with broken screws didn’t fly off the hub. They obviously were able to handle the load with some of them broken. Some more numbers. My pull tests show that a single lag screw will break at 6000# +/- about 5%. There are 18 screws in the propeller blade mentioned above. It should be clear that the break would occur at 18 X 6000 = 108,000#. That means that the blade retention force is 5.29 times the force expected at 2400 rpm. CAR 14 and FAR 35 states that the blade shall be subjected to twice the calculated force and shall not fail. Therefore 20623 X 2 = 41246. That is 2.6 times safety above the test requirements.
Yes, I’ve found broken screws in old propeller blades manufactured by others. If the screws were not properly torqued then the screws are not equally sharing the load. If 2 or 3 screws are carrying the centrifugal loads they are being stressed into the fatigue area of the stress concentration curve. A screw can then fail due to fatigue. When they are all sharing the load then the average stress put on one single screw is 1146#.
Broken screws are not due to faulty design but faulty assembly.
3. VIBRATION on 0-320 and 0-360 engines.
First. Many years ago Univair obtained a STC for application of the Aeromatic propeller on the Beech model 23 powered with an 0-320 Lycoming.
There is a one-time STC issued to a Mr. McIntosh for his 0-320 powered TriPacer seaplane circa 1962.
There is an Aeromatic on a Piel designed experimental with 0-320 150hp up in Canada. It was put on the plane/engine combination 28 years ago and has over 500 hours on it. I talked with the owner; he inspected it and found no problem.
There have been a number of others installed on 0-320s and their comment is “it is smooth as silk”.
0-360s. Univair, in 1962 did a vibration test run on the 0-360 for the FAA. It was approved for use on that engine as far as vibration is concerned and the TCDS on the propeller shows that it is FAA approved for 180hp at 2800 rpm. I have a copy of the letter.
I am planning to put one on my 0-360 powered Bellanca Cruisair for evaluation.
4. NOT RECOMMENDED BY THE ANSWER MAN.
I’ve read all the derogatory statements made by the Answer Man on the Swift Club web site. He doesn’t offer any backup data to support his dislike for the propeller. I even contacted him posing as a Swift owner and put the question to him, “What is it about the Aeromatic that you disapprove of”? He just responded, “It’s not recommended”. He didn’t say that HE didn’t recommend it, just “It’s” not recommended.
I don’t know the web address of his site but I reckon if you want to read his diatribe you can Google it up.