It sounds like you are way above most of us on measurement. After that you will need to do FFT and be able to interpret the results. To also be able to do the design and fab work is a really rare set of skills. Have at it.Help me with my math?
If I happen to have 45 tooth cam drive gear wheel and the engine is spinning at 6000rpm, the tooth signal is 4.5KHz. 10 bits of time resolution should be overkill? That's 4.5Mhz sampling. Ok, that 45 tooth wheel may not be adequate for high harmonics in a stiff system, but it is surely overkill in a soft system? Yes, i could build a system to do that. But there's an easier way:
Wright did not have 10GHz sampling when they found and fixed the 9th harmonic in their Cyclone. They had an analog mechanical system. I believe they had to make it themselves, though.
If I modernised the Wright system, I'd have a tangential accelerometer rigidly mounted to the shaft, along with an FM transmitter and a small battery. An analog radio receiver (cheap constant latency) would pipe that to a PC soundcard. Yes, I'd need at least two systems and a soundcard channel for each.
Recorded waveforms don't need crunching in real time. Modern electronics are wonderful! The 50th harmonic of 6000rpm would generate a 5kHz signal. A cheapo 44kHz sampling soundcard would not be fazed at all by that. A 192kHz commonly available card should do it easily. Some butchery may be needed to extend the low frequency end for startup and shutdown.
What I will say is that if you need to ask how to do measure and fix TV, you probably won't be able to...
As an aside, while my redrive is still in the doodle ideas and see how the numbers look stage, I have just ordered a bunch of data aquisition stuff for measuring IMEP and pressures in an engine intake system you'd likely laugh at.
Cheap USB soundcards. I will convert these to DC input and I have lots of cheap 22kHz analog inputs. I'm a bit dubious about the latency, but that isn't actually a big problem for my upcoming testing as phasing is not critical.
Shaking vibration can be cancelled out with a 2nd accelerometer on the shaft axis. that assumes you have the dynamic range and resolution, anyway. I haven't actually run any numbers on this idea yet, I'm not sure that I'll need it.I think strain gauges is needed. Accelerometers would be overwhelmed by normal vibration.
Another option is strain paint. Paint the crank and run it two hours and disassemble and check for strain.
If any exists, then repeat to find the rpm... or give up
If I operated my engine in the 1050-1200 rpm range for long, things would have broken long ago which is why I did the flywheel mod. This period was really severe with the wingtips and panel, a blur. You moved through it quickly. The rubber drive bushings were pretty rough at the 358 hour mark when examined.A soft system is most likely to have issues at idle, startup and shutdown. A bit of design effort and measurement of that critical lower end will likely be enough. Ross didn't even measure and has yet to break his redrive. Though I have noticed that his RV hours have been stuck at 422.7 for a suspiciously long time
Thank you for explaining why canard pushers can't fly.
You said: "I'm not sure where the notion that the weight of an aircraft should be hanging off the nose comes from."That's a terrible analogue for an airplane, and again, if you look at any jet, it has engines anywhere but the nose. GA aircraft with engines in the nose makes sense for air cooled engines using direct drive, but otherwise is merely a convention borne out of historical convenience, and the misconceptions of its contributions should be rooted out, and die horrible deaths.
They fly with stabilty using the same pyhsics that makes a dart stable.Thank you for explaining why canard pushers can't fly.
Any pickup can have noise. A magnetic sensors output will vary a bit with tooth imperfections and as the distance between sensor and gear change due to case flex. Those optical encoder discs would need extreme accuracy to not supply a lot of jitter, so using their full resolution seems unwise to me. Laser tachs are also going to be prone to vibratory position errors. Some of this stuff can be compensated for, some of it filtered.
This is why Ford, GM etc will merrily pay big bucks for one of these systems: it doesn't take a week or three to set a system up and start getting hard data.Oh yes, one of the neat features of Rotec and the other commercial products is they have the ability to recognize a skipped or extra tooth and to record a target and get its tooth variation pattern, then correct for all of these errors. Nice to have, and absolutely necessary for a target strip that is glued on a wheel, as there is a discontinuity at the overlap... Without correction for these "features", they may dominate your data and not allow extracting meaningful results...
Pictsidhe,This is why Ford, GM etc will merrily pay big bucks for one of these systems: it doesn't take a week or three to set a system up and start getting hard data.
I'd rather not use skipped tooth. If I did I'd have two sensor physically in pahse, then maybe OR the outputs into my recording thing. That still wouldn't be as nice as a synthesised waverform, though. The correction for off centre targets is going to be pure gold in real life. I may look at the maths that would take. Though I will likely have high quality metal targets to use that won't need correcting.
Post 53 of this very thread does mention this topic.One source of vibration I've not seen addressed is airflow through the propeller. Except for the very specific case of perfectly axisymmetric flow, that is that the prop's axis of rotation is parallel to the local airflow and there are no disturbances (for pushers, there are always disturbances), there will be at least one/rev and number of blades/rev vibrations.
The sticky that I wrote talked about vibration modes and resonance and forcing functions, but did not talk about what the forcing functions were. Part of the reason for that is simply that there are many forcing function possible.Post 53 of this very thread does mention this topic.