![]() |
Quote:
|
Yep
That?s a fact...
|
Well said below
Quote:
|
Two biggest concerns with RV designs
I have two main concerns over the RV engineering.
First is the location of the fuel tanks which can easily impact a post or tree and rupture during an off field landing. Other designs put the tank behind the main wing spar providing better protection. Second is the tendency of the fuselage to buckle behind the cabin but in front of the shoulder harness anchor point during a hard landing causing the occupant to be more likely to impact the glairshield. I love flying my -9a. It is an amazing machine. Those are the only two design details that give me some pause when contemplating an off field landing. |
Quote:
A second example to cite is the A-300 that shed its vertical tail in a high sideslip overshoot that was arguably pilot-induced, but the politics glossed over the fact that the incident occurred at a speed lower than V_a (maneuver speed) and the fin should have stalled before exceeding any structural limit at that speed. Designed to meet the narrow letter of the regs, but failed to meet the spirit and intent of the regs, and a whole bunch of people died. |
Quote:
RV-3 VH-BEM, RV-6 VH-TXF, RV-6 ZK-VBC and likely RV-6 VH-OAJ as well. |
Quote:
"Examination of the failed propeller blade A fatigue crack had initiated near the leading edge of the blade 216 mm from the blade tip. Crack growth had occurred as a result of alternating thrust loads, and had propagated along the thrust face (rear surface) of the blade. The characteristics of the crack indicated that it had grown under constant amplitude loading. There was no evidence of flight by flight striations. The propeller material was of the correct type and no damage or other reason for the crack to initiate was found. The pilot fitted a new engine and propeller to the aircraft during construction. The engine was modified to improve and balance the airflow through the valves of each cylinder to enhance engine performance. In an apparent further attempt to improve engine performance, the pilot replaced one magneto with an electronic ignition system that was capable of varying the ignition timing in response to changes in engine RPM and manifold pressure. That variation contrasted with the fixed timing ignition provided by the other "standard" magneto fitted to the engine." "All propellers are subject to alternating thrust loads during normal operation. Propellers are designed so that those loads will not exceed the design value, thus preventing the development of fatigue cracks during operation. The firing of each cylinder in a reciprocating engine produces torsional vibrations. That means that the crankshaft momentarily speeds up at each firing stroke, and then slows down again prior to the next firing stroke. The vibration leads to alternating thrust loads in the propeller. Examination of the engine connecting rod big-end bearings revealed distress on the bearing surfaces. That distress was indicative of firing loads exceeding the designed capacity of the bearing lubrication. For optimum operation of spark ignition engines, the peak pressure developed by the combustion of the fuel air mixture should occur approximately 15 degrees after the crank has located past top centre. Ignition timing was a critical factor, influencing engine power, fuel economy, and the operating condition of the engine. Timing depended on the rate of propagation of the flame front through the fuel-air mixture. Increased or advanced ignition timing resulted in increased combustion chamber pressures. Magneto timing was fixed and was optimised for the operating range of the engine. If the response of the electronic ignition system to reductions in manifold pressure created by part throttle opening was to advance the timing of ignition, that could increase cylinder head pressures and increase the magnitude of torsional vibration." Did they just say that the prop and engine bearing damage was because he was using an electronic Mag with a standard one? Will start looking closer at my Catto prop during pre-flights. Have an EMAG and a Slick. Best regards, Mike Bauer |
Quote:
Now the leading statement that is NOT necessarily true - that advancing the ignition point causes increased chamber pressures - that statement is only partially true. If the advancement occurs at full manifold pressure, then yes it is true. Most EI's will not go to full advance at full manifold pressure - because doing so puts you into the detonation "red box" where you can damage the engine. If the manifold pressure is lower, such as at altitude in cruise, then you can advance more on the timing and you are still outside the detonation area, so you are fine. At lower manifold pressures, advanced timing does NOT produce a peak cylinder pressure greater than the standard full manifold pressure levels with standard timing - so the logical leap that advanced timing automatically means exceeding cylinder pressure limits is faulty. The finding is worded to point at torsional vibrations from the EI firing as a suspect cause. Now the critical factual statement that is not mentioned - that bearing distress evidence indicating excessive cylinder head pressures is commonly caused by detonation, rather than torsional vibration. Torsional vibration can easily cause the propeller blade fractures, if it's bad enough for long enough - but it likely did not cause the bearing damage on the engine connecting rod - that was almost certainly caused by detonation. Now the key part of this is taking the anectodal apart from the factual - we know the electronic ignition was installed, but we don't know what model/manufacturer it was, and we don't know what the timing map looked like versus the manifold/RPM on that unit. If the user had played with the factory maps and gotten the engine into a detonation area, that would explain bearing damage and MAY explain the torsional vibration producing the propeller blade crack. Without knowing what the installed ignition map looked like, and the operating conditions at the time, there is no reason to assume the EI is at fault (no matter what brand it was) - and it may very well be a case of the operator taking the EI outside its normal parameters in a hunt for more power, and finding the limits of doing so. |
Quote:
|
Propeller testing done in conjunction with Van's, by Hartzell has produced empirical data that shows that advanced ign. timing can have a detrimental effect on fatigue life of propellers in certain operating modes.
There is info on Hartzels web site. Detonation could definitely have an impact as well (consider it to be an order of magnitude worse than just highly advanced ign timing). The issue with this case is that there appears to be no evidence to indicate what amount of timing advance the engine may have been operated at. Only that there was evidence of engine damage that could have been caused by extreme combustion pressures, and the non standard ign system is the only thing to point a finger at as a possible cause (though they don't seem to do that directly). |
| All times are GMT -6. The time now is 03:16 AM. |