Goodness, thank-you Billski, sadly I am having to go the option 2 route with this plane as I will have the reduction drive immediately after the engine- wow you do make a man's brain engage and thanks for that as I guess this is what the forum is for. My brain says redrive, clutch (centrifugal/cone/motorcycle????) damper driveshaft.Besides the already mentioned thread on torsional behaviour, you might find this one useful;
The whole topic of PSRU's and drive shafts and torsional vibration has been talked around and I keep seeing the same difficulty in the thinking of the involved folks. Since we on HBA tend to be an intelligent bunch, I got to thinking about how much I had to work to get my arms around the whole...www.homebuiltairplanes.com
Next, I can tell you that I helped some folks with a Jabiru four cylinder engine, a 44" long shaft, and prop, all in direct drive, tame their vibration. An elastomeric coupling at the engine helped with the shaft alignment issue and allowed the engine to deflect and vibrate on its mounts without overloading the shaft. This also allowed the system to operate without any lash in the system. The shaft was the primary torsional compliance having an adequately low spring rate that, when combined with a substantial increase in engine flywheel inertia, drove resonance down near engine idle rpm, and tested adequately. I modeled their design and adjusted inertia using SolidWorks and was also able to find all other nearby modes were significantly above operating input vibrations. The system tested well, and we verified no hazardous vibrations using Rotec Munich measurement system and laser tachometers pointed at targets applied to engine and both ends of the prop shaft.
Your system with a high revving outboard head will require a prop reduction system somewhere with considerable attention to torsional vibration management, particularly toward driving :
Both systems require a bunch of design and analysis work to be even close to being right when you build the first copy. The first type will be lighter and simpler, and thus easier to to make work. The second type will be more difficult to make work as you must isolate torsional vibration of the engine in a small package, then the speed reducer, then a beefier shaft, then the prop. Oh, besides Hertog's book or other ME vibration text, you should look up critical speed calcs for shafts. Whirl mode must be prevented, with several ways of preventing it, including increased shaft diameter (and concurrent increased torsional stiffness), breaking the shaft into more pieces with U-joints and supports (like are done with many trucks), and a system of support bearings as is seen in helo tail rotor shafts.
- If you run the shaft at engine speed and make it a lashless system, you may need a substantial flywheel at the engine end, allow the shaft to be small diameter (engine torque) and relatively torsionally flexible, then the gearbox at the prop hub can operate in a vibrationally clean area. This is the Allison/Airacobra system, and it worked. This system must have the shaft designed for a specific spring rate to put resonance above cranking speed and below idle speed;
- The alternative is to run the gearbox at the engine end. UGH. First you will need a significant flywheel on the engine, then a soft element such as a clutch disc center spring set or a rubber giubo or other compliant element to prevent the gearbox seeing all of the engine vibe. Then you need a U-joint of some sort to the shaft, then the shaft has to be sturdy enough to carry prop level torque and speed.
You did not pick an easy one...
Jan Kritzinger (aeronautical engineer) calculated that we should have sufficient rudder authority with the single rudder- hopefully this will prove correct- try whenever possible to keep to William Bushnell's theory of "simplicate and add lightness . It keeps in sync with my brain!!!What I said above even looked confusing to me.
Just pointing out that you need good rudder authority in a tractor gyro and I would always use a H tail to get the rudders clear of the fuselage for cleaner airflow.
I also said I "could" put triple rudders if I wanted to.
This shaft drive setup aslo works well
Goodness, thank-you Billski, sadly I am having to go the option 2 route with this plane as I will have the reduction drive immediately after the engine ... My brain says redrive, clutch (centrifugal/cone/motorcycle????) damper driveshaft.
Thanks Billski, right now what I am thinking is a cogged tooth belt from drive pulley to driven in a 2:1 ratio and possibly putting a motorcycle clutch with built in Cush dampers in the driven pulley, so I can clutch the pulley to start and then slowly (soft start) the prop and prop-shaft. Your thoughts would be appreciated and it is obvious your knowledge is far,far greater than my simple brainYour scheme - the gear box can not be allowed to see anything more than a minor piece of the major vibe orders from the engine or it will need to be an order of magnitude beefier to stand the transmitted vibe. You will need a very substantial isolator between engine and gearbox. Once you have taken off 95% of the vibe, the rest of the system should be straightforward. The big thing to do is drive the natural frequency of the engine/flywheel inertia then spring, then gearbox inertia more than half an octave below min operation firing frequency but more than a half octave above firing frequency at cranking speed. You will have your hands full with that.
A 2:1 ratio is a bad idea, the power pulses will be transmitted by the same belt teeth on every revolution, successful PSRUs always have an uneven ratio.
Hi Billski, goodness I wish I was close to you and could feed off your good sense. The difficulty i would have in placing the reduction drive in the nose is quite simply space, and without tearing the plane apart completely would be virtually impossible to do. What I have is a two bearing (for want of a better word "plumber block") housing that bolts onto the face of the nose. This supports the prop on a stubshaft approximately 300mm in length the stubshaft then bolts onto the pto which has conventional knuckles on each end- the engine side knuckles have a splined shaft as per any conventional rear driven motor vehicle. the pto is filament would carbon and is 80mm in dia and the whole sits in a 100mm dia kevlar tube for safety. The only way right now that I could see to put the redrive in the nose would be if I could find a small enough planetary gear box so as to have an in line reduction. There are dry belt driven clutches- Harley have one that drives some of their large cruisers. I totally get your comments regarding the math and physics surrounding this engineering- sadly not a strength of mine and your comments remind me that this is where I need to go. I must sound like the Mad Hatter, as I try to find a light enough, powerful enough engine and reduction drive (sub 100kgstotal weight) to drive the beast. As always thanks for your time and wisdom.A variety of belt drives have been used.
Most builders of geared engines, geared turbines, even steam turbines, as well as automotive tranny folks and turbomachinery engineers have found that whole number reductions are bad juju. Best in geared or cogged belt drives is where both sets of teeth are prime numbers. If you go with a V belt, pick pitch diameters such that ratio does the same thing. This prevents having a beat that is both annoying and destructive.
The V belt takes care of your clutching and is widely used on industrial equipment. You simply pull off the idler to disengage. Works great. Design the scheme so belt replacement is easy. LOL. In the over 100 hp arena, V-belt will likely need multiple belts of matched sizes.
The cog belt design is pretty straightforward and is used. If you are set on engaging the prop after engine start, it will require a separate clutch. If a motorcycle clutch of suitable torque capacity is available terrific. Aren't most motorcycle clutches wet clutches? That means an oil filled housing with seals. Lots of stuff to work out.
Where is the "cush" element? If it is downstream of the clutch, the clutch is still seeing engine vibe orders, and the cush is designed to protect the downstream parts from engine vibe order while the clutch is designed to take engine vibe. If the clutch is up stream, I bet that it is designed to vibrationally isolate the clutch and gearbox from engine vibe orders. Here is the rub. The stiffness of the cush section is usually designed as part of a system - it is a spring between two inertia (rotary masses) - and is designed to make the resulting natural frequency of the system both safely below idle vibe and safely above cranking vibe. When you stick this device in between your engine and your reduction drive, you doubtless will have higher inertia on both ends resulting in substantially lower natural frequencies. I worry that you may fall in resonance at cranking speed or just above, which can give difficulty getting the engine to accelerate to idle speed. Yeah, it can hang right there at resonance with all of the engine energy going into increasing the vibration amplitude. Besides making it tough to get to idle speed and above, this tends to break stuff...
The long shaft to the prop will be a fairly soft element and may be designed to put system resonance safely below operation speeds. If it does, terrific, but if it does not, you will need to add a soft element at one or both couplings. With such a system, you can usually design to avoid need for clutching a propellor. Resonance is set at least an octave above cranking firing vibe and at least an octave below idle firing vibe. With the reduction unit between engine and shaft, you will need a somewhat overbuilt cog or V belt system - which is common - as engine firing pulses and other engine orders are being seen in the reduction drive.
The lightest way to build the system is to put the reduction unit at the prop end. The reduction unit is then isolated from firing order vibe and so can be smaller, and the shaft is carrying less torque and so can be smaller and lighter too.
It should be evident that such a system will need to be engineered. Simply linking together some existing parts is unlikely to produce a successful system. An engineering text on theory of vibration must be in your future. While the books usually work in terms of masses and springs moving linearly, it translates directly to rotation inertia and torsional springs.
Please tell us why you feel that you must put the reduction drive at the engine, where things are both more complicated and heavier.
This shaft drive setup aslo works well; by keeping the engine and drive shaft seperated except for the belt.
Thanks so very much- yes I do have a test rig and will use it and follow your advise. Greatly appreciatedI can tell you from a number of people's hard experience that guessing and trial and error on this will require a separate test rig where you may tear the thing up and have to rebuild it. I personally took over vibration management on a project where they were attempting to make a direct drive shaft system work by just trying stuff without doing any analytical work, and their test rig was all scarred up from broken parts flying about and thrashing around. The airport where this project was working had exiled their test runs to a far corner of the field away from EVERYTHING else on the field. Some of their schemes lasted less than 5 minutes. I am here to tell you that I modeled their system and tamed it with one modification. They had full time folks fighting with this problem for 18 months prior to my involvement... No, this retired engineer is not for hire, but I will check your math if you want. This is not a place for trial and error.
The PTO systems you see in agricultural equipment have a big flywheel on the engine, a torsional spring set in the clutch hub, then the gearbox, then the PTO output, shaft, and then inertia of the load. This is very similar to cars and trucks, with first the engine, the spring set, the gearbox, then the output shaft, then the load. The inertia of the engine is isolated vibrationally from the inertia of the rest of the system by a purpose built device that has a spring set designed to make the resonance frequency of the system be lower than the operational firing frequencies at their ground idle.
You have already chosen the engine, prop, and shaft. That defines the minimum engine inertia, prop inertia, and shaft spring rate, and number of firing pulses per engine turn. That leaves to be defined your flywheel inertia, cog belt system inertia, spring rate of the coupling and min idle speed. This is a large design space to explore, and doing so analytically is the way to do it. Once you have the basics for your cog belt drive established, you can figure out its inertia and fix that. You will have to find or build a coupling with somewhere around the right amount of torsional spring rate and more than enough torque capacity to handle the engine output. I would start with an engine idle speed no lower than about 1600 rpm, pick the lightest flywheel that will allow that idle, and see what the available spring rates (that have torque capacities above your engine max torque) do to your resonance frequency. If it is too high, you can add inertia to the flywheel to bring it down or find lower spring rate couplings. You may also raise your minimum idle speed.
The best way to do this will involve doing all of this work analytically. Yes, you or someone your trust will have to get friendly with vibration theory to do so. It is not that hard, many thousands of engineering students do it annually in their sophomore or junior year of college every year. Doing it any other way is to basically head for random trials and error.
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