LeSving wrote:
Nothing has changed except more power.
No, but even if this would be true, that wouldn’t lead to “20% more vibration”. You are complete ignorant of the very basic physics of mechanical vibrations.
Rwy20 wrote:
LeSving you’re being so stubborn it defies belief
It takes one to know one 
If you saw one of my first posts in this thread, it ended with a smile. The 390 is simply a 360 with 20 % more power. It’s the same engine for all intents and purposes, only more power. More power at the same rpm must result in larger torque pulses. That is the only change roughly speaking. The main dynamics of the engine has not changed much. Therefore the difference is what is left, and that is 20% more power and 20% more vibrations in the form of larger pulses from more powerful pistons.
Would that 20% larger pulses result in unwanted effects? Most probably not. A small change in propeller mass will have much larger effect than those 20%, not to mention unbalanced propellers. But those 20% are there no matter how you look at it. There is no way to increase power without it, except adding more cylinders or increase RPM.
LeSving wrote:
The 390 is simply a 360 with 20 % more power. It’s the same engine for all intents and purposes, only more power. More power at the same rpm must result in larger torque pulses. That is the only change roughly speaking. The main dynamics of the engine has not changed much. Therefore the difference is what is left, and that is 20% more power and 20% more vibrations in the form of larger pulses from more powerful pistons.
You seem to make several assumptions:
Do you know both of these assumptions to be true? And I mean “know” in the sense of understanding and not as in “common sense says so”.
Airborne_Again wrote:
Do you know both of these assumptions to be true?
Define true If you look at the 4 cylinder graph from EPI:
The graph shows the torque on the shaft during one revolution (360 degree). The engine only produces torque when the curve is positive. But, torque pulses will be oscillating around a mean torque. So, the torque will be 100% + 270% – 205% approximately (-105% to 370% and a mean of 100%). Obviously the area under the graph from 5 degree to 85 degree around 100% mean, must be equal to the area from 85 to 185 around 100% mean. The surplus energy (above 100% mean) from the power stroke of one piston, must be equal to the absorbed energy (below 100% mean) from the compression and exhaust strokes of the others. If this is not equal, the engine will increase or decrease RPM until they are equal.
The shape of the curve is purely generic. It may be different for a Lycoming. However, the shape for a 360 and a 390 must be very similar. What we want to do is to increase the mean torque to 120% (20% more power at the same RPM). The curve must be similar in shape (same efficiency, same compression ratio etc). The only way to do that is to multiply the curve by 1.2. It is stretched in y direction in both positive and negative direction.
So yes, increasing power by 20% will result in an increase in pulses by 20%. There is no other way to do it. It is important to note that it is the mean torque we want to increase. This cannot be done without also increasing (or rather decreasing) the minimum negative torque (it requires more force to compress a large cylinder than a small cylinder).
Airborne_Again wrote:
Given that the previous assumption is true, the amount of vibration is proportional to the peak instantaneous torque.
Given all else equal, yes. A flat four is a very well balanced engine, and it is the torque that creates vibrations. This torque is continuously hammering the propeller 5400 times per minute (at 2700 rpm). But, the propeller also absorbs, stores and releases energy. The energy in the propeller is equal to 1/2 * Ip * omega^2. Simply by decreasing Ip by 20% (20% “lighter” propeller), this is already taken care of. The vibrations transmitted to the airframe will be the same. This 20% is nothing that can’t easily be “fixed” by a well chosen propeller. The 390 itself is also heavier, larger cylinders, larger pistons. This will also dampen torque pulses more than the lighter 360, but it’s not 20% heavier I would think.
LeSving wrote:
However, the shape for a 360 and a 390 must be very similar.Must? Why!?
He still is neglecting every bit of vibration physics. No damping, no transmissibility, no viscous or inertia effects, no changes of eigenfrequencies by mass and design changes, no modal changes, no spectral changes… The notion that some distinct relative change in forces or momentum would result in the same relative change of a wishy-washy term “vibrations” is pretty nonsensical to anyone with a basic understanding of mechanical vibration. No, I can’t take an educated guess about the vibration properties of the IO-390 in comparison to any IO360, because I don’t know the engines well enough for that. But experience by people who actually fly these engines seem to indicate my bullshit-radar is still working.
I was really looking forward to the IO390 when it was first announced as coming out with UL fuel capability. Lycoming have decided differently in the mean time, which is a pity. It could have made a nice upgrade engine for a lot of O360/IO360 powered airplanes, but like this, not many will consider it as it puts the same problem forward as the old ones: it depends on Avgas.
mh wrote:
He still is neglecting every bit of vibration physics
He is? I think you are overthinking it. The 390 is basically a larger 360. Bigger bore, that’s all.
Mooney_Driver wrote:
not many will consider it as it puts the same problem forward as the old ones: it depends on Avgas.
Couldn’t agree more here – Lycoming missed a huge chance and remain the “Lycosaurus”.
