Van's over-engineering compared to certified aircraft

[paul330]

Originally Posted by rocketman1988

I wouldn?t knock that slide rule...look through aviation?s past and you will find some really impressive design work that was done...you guessed it...on a slide rule...

Most of the space program in the 60s was done on slide rules ......

[rocketman1988]

That?s a fact...

[dave4754]

Well said below

Originally Posted by cajunwings

Just my $.02 after 40+ years of maintaining and repairing light aircraft. The RV line is well engineered for the intended mission and price point. There?s no magic, they are light and clean with a good power to weight ratio. The lightness comes from using just enough metal and fiberglass to get the job done so I wouldn?t consider the RVs as durable as a similar certified aircraft. A good example of this is the landing gear. If it was built like a Cherokee it would perform like one. We just accept compromises to get the fabulous performance for the dollar that we all love so much.

Don Broussard A&P IA ATP
RV9 Rebuild in Progress
57 Pacer

[BJohnson]

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.

[scsmith]

Originally Posted by KatanaPilot

Hate to start an off-topic Boeing versus Airbus war, but as a former Boeing engineer and long time Boeing pilot, I'm going to push back a bit.

China Air 006, 747SP, pulled about 5 G's during a recovery from loss of control at 41,000 feet. Aircraft was damaged, some injuries occurred, but the airplane stayed (mostly) intact and landed safely. As you know, transport category design load factor is 2.5G's positive, with ultimate load limit of 3.8G's. I'm not aware of any early Airbus models that have "accomplished" anything similar.

Of course, the FBW Airbus models should never depart controlled flight and can't pull more than 2.5G's in normal law. So I suspect (but have no proof), that Airbus can design their FBW models with a little less margin. Whether they do or not - I really don't know.

Boeing's implementation of Fly By Wire incorporates soft and hard limits, giving pilots the ability to pull 2.6 G's (or more) if needed and aerodynamically available.

I was going to cite exactly this same case. I saw the airplane at the United depot at SFO after it survived that flight. A whole bunch of people lived because of Boeing's conservative design approach.

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.

[KRviator]

Originally Posted by BJohnson

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.

The tendency to buckle the fuselage has been noted in several accidents in Australia...

RV-3 VH-BEM, RV-6 VH-TXF, RV-6 ZK-VBC and likely RV-6 VH-OAJ as well.

[mbauer]

Originally Posted by KRviator

RV-6 ZK-VBC

First time I've seen this, quote from the linked accident:

"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

[airguy]

Originally Posted by mbauer

Did they just say that the prop and engine bearing damage was because he was using an electronic Mag with a standard one?

No, that's not what they said. Look at the factual statements, versus the leading statements, and one critical factual statement that was left out. There was a fatigue crack on the prop blade, which CAN be caused by torsional vibrations, that's true. There was an electronic ignition fitted on the engine, that's true. The timing of an electronic ignition (some models anyway) can vary depending on the manifold pressure and rpm, that's true. Bearing distress was found on the engine indicative of exceeding the stresses of the bearing lubrication - that's a fact.

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.

[RVDan]

Originally Posted by BJohnson

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.

The location of pretty much all newer Cessna and Pipers as an example, have the fuel tanks forward of the spar, typically for weight and balance reasons. Back in the 90?s Vans modified the forward tank attachment point to allow the fitting to pull away from the fuselage as the wings bent in a crash. This reduced the possibility of the root rib being torn out introducing a large leak. Aft of the spar is typically a weight and balance problem with moving CG during full burn. Bigger a airplanes can sometimes put the tank behind the spar because the CG range allows it.

[rvbuilder2002]

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).