Getting Out From Under the Wing?

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wsimpso1

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I take it the difference is turbo vs. NA engine between 10 and 12? Either way: what a beautiful airplane. What can you tell us about the shaft drive and how they deal with torsional resonance? I imagine in a carbon airframe they are using a carbon fiber driveshaft??? While the drive INTO the Rotax gearbox is loaded with torque reversals 180 degrees apart I wonder if such a driveshaft could simply be light and stiff enough to let the Rotax TV dampers do the work?
Road vehicles worldwide operate with isolation between engine and gearbox, and then subsequent springs and masses - output shaft, final drive gearing, wheels and brakes.

If you wanted the shaft system to behave like it does with the prop attached to the gearbox flange, the shaft would have to be about as stiff as the joint between prop and prop flange. We will play merry hell achieving that on what looks like a 6 foot long shaft.

Most likely the resonant frequency of the prop on the shaft will have to be lower than the resonant frequency seen between engine and gearbox. In a six foot long shaft, that level of springiness while being strong enough should be pretty easily achieved. If the shaft is particularly skinny, it may have one or more support bearings along the shaft. Think tail rotor systems for helos.

Billski
 
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PMD

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Road vehicles worldwide operates with isolation between engine and gearbox, and then subsequent springs and masses - output shaft, final drive gearing, wheels and brakes.
Let's remember that the driveline of a typical FE/RWD can have 5 degrees of combined slop in driveshaft rotation because of the backlash of typical hypoid gearset, diff gearset and axle splines. I would think that should simply TV analysis/sustained resonance in that the rear axle could only provide load in one direction (decouple) within the massive amplitude of slop. Some FWD drivelines are not that far behind.

Getting back to the prop drive: would it not be possible just to make the driveshaft stiff enough to be higher resonant freqency by quite a margin? Also, do the resin matrices of a large diameter, thin wall CF shaft not contribute some reasonable amount of damping properties?
 
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wsimpso1

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Let's remember that the driveline of a typical FE/RWD can have 5 degrees of combined slop in driveshaft rotation because of the backlash of typical hypoid gearset, diff gearset and axle splines. I would think that should simply TV analysis/sustained resonance in that the rear axle could only provide load in one direction (decouple) within the massive amplitude of slop. Some FWD drivelines are not that far behind.

Getting back to the prop drive: would it not be possible just to make the driveshaft stiff enough to be higher resonant freqency by quite a margin? Also, do the resin matrices of a large diameter, thin wall CF shaft not contribute some reasonable amount of damping properties?
All that lash is not normally unloaded in road vehicles. The transmission, final drive gearing, and shafting have a lot of torsional compliance. Combine that with the modest torsional swing of the engine, isolated down to 5% of deflection from mean torque, and the swing transmitted is much smaller than the downstream elements. The lash is all taken up and there is not rattle or clunk... Very much on purpose.

Anyway, let's look at the idea of having a stiff shaft downstream:

First in a Stiff System - Resonant frequency of the engine to gearbox is high and the entirety of firing pulses is transmitted to the downstream parts. Add a stiff shaft and big inertia (props are much bigger inertia that flywheels in any one system), and your stiff system is just made bigger. The over all first mode frequency still has to be 2-1/2 octaves above max firing frequency or it will resonate and try to break stuff. If you have gear lash in the middle of a stiff system, we will be trying to unload and reload the mesh on every firing pulse. Make the lash small enough and the parts big enough, and it can stand the pounding, otherwise it will break stuff. And the bigger the lash, the bigger the parts will need to be to prevent breakage... Ev Hatch at PowerSport made the lash tiny and the gear teeth big for these very reasons and it appeared to work.

Then a Soft System - Normally a soft system has enough springiness between the inertias of the crank/flywheel and the gearset/prop, and firing pulses are isolated from the downstream components. Transmitted pulses are the swing of the engine at firing frequency times the spring rate of the spring - usually pretty darned small, but not zero - they are still there and can still drive resonance. Take that same system, remove the big inertia of the prop form the hub, bolt in a shaft and then the prop. What possibilities do we have?
  • If the shaft was infinitely stiff, it would be the same as having no shaft;
  • Now let's make that shaft have some springiness. Hmm, we still have pulses at firing frequency, and if the shaft brings resonance of engine to prop or transmission to prop into the firing frequency range, it will resonate with potentially destructive results;
  • Make the shaft torsionally soft enough so that our resonant frequencies coincide with (or are close to) the engine-gearbox resonant frequency, and they will feedback on each other, with potentially destructive results;
  • But if the shaft is torsionally soft enough to put engine-prop and gearbox-prop modes below the engine-gearbox mode frequency, then nothing will amplify except maybe during engine start.
All of this stuff should be run through the Eigen solver looking at all of the modes and frequencies to make sure that you have put resonance out of operating range while also putting engine-gearbox freq, gearbox-prop freq, and engine-prop freq all between firing freq at cranking and firing freq at idle while keeping all three modes at least half an octave from each other. This may drive a higher idle speed than you would otherwise want. Oh well...

Billski
 
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PMD

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All that lash is not normally unloaded in road vehicles. The transmission, final drive gearing, and shafting have a lot of torsional compliance. Combine that with the modest torsional swing of the engine, isolated down to 5% of deflection from mean torque, and the swing transmitted is much smaller than the downstream elements. The lash is all taken up and there is not rattle or clunk... Very much on purpose
I assume, though, that some of the driveline resonance we used to hear in old, manual trans FE/RWD cars under load would de-couple if the amplitude reached in an event was sufficiently great - thanks to the huge slop available to the off-load side.

Was wondering about the swinging prop blades of the Stemme. I assume they have a point of articulation down at the blade root that would - as per my sloppy RWD driveline analogy - uncouple harmonics (at least in one direction). A "soft" point in the system changing the dynamics somewhat.
 

SeaBee

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this is a fun read about a man trying to solve a torsional resonance problem in the BD-5 design.

I don't have any personal, or inside info about the Stemme system. I only know what I've found out about it online. Blade flapping may be a part of how the original design deals with resonance. But if I recall correctly, Embry Riddle put a fixed-pitch prop on one, and as far as I know, it didn't fly much, but it also didn't shake itself apart.
 
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Pops

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So , its the reason my old 1981 Chevy 3/4 ton truck that has a short tailshaft 4 speed trans and a 2 piece driveshaft with a large carrier bearing in the middle.
 

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Dear Tiger tim,
Canadian warbird restorer Bob Dymart did some funny things with his restorations. I suspect that the extra-long cabin as installed in his restoration of a Fairey Firefly naval airplane.

The bobsled style benches are popular in jump planes. They are standard in factory jump kits that come with Quest Kodiak and PAC750XL. They improve legroom by getting butts off of the floor ... allowing a shorter line-up of the same number of jumpers. The chief advantage of bobsled style benches is that they make it much easier for tandem instructors to attach and tighten side-straps.

Rob Warner, Strong Tandem Instructor Examiner
FAA Master Rigger (seat, back and chest)
 

Riggerrob

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What about staggered seating, like in Burt Rutan's Boomerang?
This allows both a short and narrow cabin. Shorter because legs can overlap with the seat ahead and everyone gets more elbow room than if they sat exactly side-by-side.
I have been sketching a 2 or 3 seater light flying boat. It resembles and Icon or Dornier S-Ray from the outside, but the pilot sits the farthest forward , on the left side. The passenger/co-pilot sits in the starboard seat. His seat can be moved ofrward to allow him to grab controls, but elbow room gets tight. There is also a third seat behind the pilot (left side). That third seat would only be occupied when taking off from asphalt runways at sea level. Similarly, ferry fuel tanks could only be full when taking off from asphalt runways at sea level. OTOH When returning from a high, mountain lake, you had better have burned off plenty of fuel or left lots of cargo behind. Hint! Hint!
 

Riggerrob

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Seating the pilot ahead of the wing leading edge is the easiest way to balance a twin-boom pusher. Look at how this little Cessna's wings are swept back to balance the engine. If seats were in tandem it would be easy to balance with the pilot seated well forward of the wing. Then seat the rear passenger at the center of gravity.
Weight and balance gets more complicated with side-by-side seating, but may I suggest two different lockers for tie-downs, tools and spare parts. One locker in the nose when flying solo, but move those accessories to the tail boom when flying two-up.

Rotorway Executive kit helicopter uses a similar chunk of ballast that is installed in the left forward skid or the right rear skid depending upon how many humans occupy the cockpit.
 

wsimpso1

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this is a fun read about a man trying to solve a torsional resonance problem in the BD-5 design.
Somebody else thinks so too. Cited that doc and others in this thread on torsional vibration.


I looked into the sales sheet. No info on vibration isolation in the Stemme systems there.

Note that the prop blades are spun out by centrifugal and aero drag effects. The prop blades are loaded against what appears to be some stout elastic stops by aerodrag from being powered around.

Firing pulse amplitude could cause some cyclic motion of the blades on the stops, but it would be tiny. I calculated some ball park numbers for prop shaft swing over the operating range of the engine in use - swing (one-way) is about 0.18 degrees at min engine speed, 0.07 degrees at max engine speed.

Now let's look at what that means to the vibe problem:
  • If the prop blade frequency on the elastic stops is at least half an octave below min idle speed firing frequency (soft system), the prop blades will be effectively isolated from the rest of the system, and the prop blades (usually the largest inertia in a piston engined airplane power unit) are not functioning as a flywheel to allow idle. A substantial flywheel would be needed on the engine itself to allow idling. The deflections calculated above will be about the movement that the blades make twice to engine turn;
  • If the prop blade frequency on the elastic stops is within firing frequency range of the engine operations (resonant system), the prop blade on the hub will resonate at some rpm within the operating range. I expect that the blades and/or the stops will resonate, and will be short lived indeed. Also, the prop blades will not be functioning as a flywheel to allow idle. A substantial flywheel would be needed on the engine itself to allow idling. Deflection of the blades about their pivots will large indeed - this is an amplifier;
  • If the prop blade frequency on the elastic stops is at least 2-1/2 octaves above max firing frequency (stiff system), the prop blades will absolutely rotate with the hub and function as a flywheel. Deflection of blades in hub is effectively zero.
The two extremes can both work. The resonating case MUST be avoided... I have no feel for what Stemme and Rotax did in this airplane.

Billski
 
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wsimpso1

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So , its the reason my old 1981 Chevy 3/4 ton truck that has a short tailshaft 4 speed trans and a 2 piece driveshaft with a large carrier bearing in the middle.
Pops,

The reason the truck builders use two-piece (and sometimes more) driveshafts is because that is usually the smart way to keep shaft critical rpm above max shaft rpm.

Critical speed of a shaft occurs when the frequency of the shaft in bending mode lines up with spin speed. Think of a jump rope whirling around. The longer the shaft, the lower that frequency is. Run the shaft speed at critical speed and you can bend/break/trash things... How to allow a long shaft to run fast enough? We can increase shaft diameter and thus its stiffness, which raises the critical speed, weight, cost, and takes space from other stuff to make room for the bigger shaft. Or we can keep the smaller shaft section and make it into two short shafts which have much higher critical speed. If the bearing to the frame and extra Hooke joint weigh less and cost less and have less other disadvantages over the BIG diameter one-piece shaft, the smart folks do the two-piece shaft and size the shaft diameters to put critical speed above max shaft speed. Some big delivery trucks have three-piece shafts. Helos (Bell 47's have this visible from the outside) use a long thin shaft for the tail rotor with a bunch of support bearings along the shaft to prevent that shaft from going into jump-rope modes.

In the process of having more Hooke-Joints and a smaller diameter shaft, the shaft spring rate also gets lower and thus more compliant for isolating vibrations. Win-win.

Billski
 
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Pops

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Pops,

The reason the truck builders use two-piece (and sometimes more) driveshafts is because that is usually the smart way to kept the shafts below critical speed.

Critical speed of a shaft occurs when the frequency of the shaft in bending mode lines up with spin speed. Think of a jump rope whirling around. The longer the shaft, the lower that frequency is. Run the shaft speed at critical speed and you can bend/break/trash things... How to allow a long shaft to run fast enough? We can increase shaft diameter and thus its stiffness, which raises the critical speed, weight, cost, and takes space from other stuff to make room for the bigger shaft. Or we can keep the smaller shaft section and make it into two short shafts which have much higher critical speed. If the bearing to the frame and extra Hooke joint weigh less and cost less and have less other disadvantages over the BIG diameter one-piece shaft, the smart folks do the two-piece shaft and size the shaft diameters to put critical speed above max shaft speed. Some big delivery trucks have three-piece shafts. Helos (Bell 47's have this visible from the outside) use a long thin shaft for the tail rotor with a bunch of support bearings along the shaft to prevent that shaft from going into jump-rope modes.

In the process of having more Hooke-Joints and a smaller diameter shaft, the shaft spring rate also gets lower and thus more compliant for isolating vibrations. Win-win.

Billski
Thanks, that is what I thought but now know for sure from an expert. Good engineering.

One time as a millwright, I did the maintenance on the plants cranes up to the 200 ton overhead bridge cranes. Also had a lot of grantry cranes in the tool and die dept. The grantry cranes had a cap of 35 ton. In the design, the electric motor was on one side of the main drum with a long drive shaft in the middle of the drum to the gearbox on the opposite side. When loaded close to the max cap , the drive shafts would twist and fail. Poor design that kept me busy. Kept wanting the cranes placarded at a lower cap, but never happen.
 

wsimpso1

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I assume, though, that some of the driveline resonance we used to hear in old, manual trans FE/RWD cars under load would de-couple if the amplitude reached in an event was sufficiently great - thanks to the huge slop available to the off-load side.
The basics for this whole topic were covered in a junior year engineering class I knew as ME340-Vibration. Passing a class on this topic is required for granting a certified Bachelor of Engineering Science in Mechanical Engineering. There are noteworthy textbooks on the topic, authors are Hertog, Thomson, and others. Much is to be gained in understanding of this topic by spending some quality time in one of these books.

For more specific knowledge of the whole topic, you can visit this thread:

Now to the comment. Sorry, something is wrong here.

When we drive an older manual trans car with the engine at low speeds, say in a parking lot driving, we might get the engine firing frequencies down into resonance. The inertia of the engine/flywheel is vibrating opposite the transmission with the clutch damper springs cycling in between them. What we hear is either the lash of gears and/or splines and/or some other elements unloading and then reloading with an impact. You can easily get whole trains of items unloading and reloading with clicks. You can even get elements unloading from the drive direction, traversing the lash, and loading up in the coast direction with an impact then back into the drive direction with an impact each time. Clatter clatter clatter.

During my time in the auto industry (1990 to 2015) this sort of thing was usually avoided in manual transmissions by making the first spring rate of the clutch damper low enough to take resonance well below idle, and then putting in some small friction devices to further tame the oscillations. With electronic engine controls, the engine was simply not allowed to be easily driven below idle. Idle air control can bring up torque trying to hold idle rpm, and then fly-by-wire throttles gave even more capacity to hold rpm while idling along... You could lug down into resonance on a grade or with brakes. Go back far enough, and you could get into the rattle modes, but even my folks 1964 Ford Station Wagon with three-on-the-tree would drive quietly with no gas pedal in any gear.

In automatic transmissions, the calibrators set the minimum converter clutch applied rpm to engine speeds high enough to isolate well. Try to drive the car into a speed range where the vibe gets significant, and they either downshift or open the clutch, or both.

Now, the cases I just described are low torque - near zero. Go to serious torque in the system with the prop spinning fast, and the system deforms elastically. If there is an isolating spring in the system, it is compressed and cycles by the amount that the engine speeds up during firing pulses and slows down between firing pulses. This is small, usually well below one degree, while the elastic parts deflect much more. The system stays loaded - no clatter.

The only time lash unloads a system is when average torque is very close to zero. If the lash is greater than the full travel swing from firing pulses, then the lash can swing back and forth without hitting either end. Theoretically. In reality, the driven part has some friction and the driving part smacks into it either occasionally or on most firing pulses, giving either intermittent clatter or continuous clatter. And this only works when the net power is very close to zero.

A serious point about lash closing. It is impact. Impact is a great way to excite vibration modes. NVH geeks instrument stuff and then hit them with a hammer to find out what the frequency content is. Impact in use is usually bad.

In short, opening and closing lash is to be avoided.

Billski
 
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PMD

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Somebody else thinks so too. Cited that doc and others in this thread on torsional vibration.




I looked into the sales sheet. No info on vibration isolation in the Stemme systems there.

Note that the prop blades are spun out by centrifugal and aero drag effects. The prop blades are loaded against what appears to be some stout elastic stops by aerodrag from being powered around.

Firing pulse amplitude could cause some cyclic motion of the blades on the stops, but it would be tiny. I calculated some ball park numbers for prop shaft swing over the operating range of the engine in use - swing (one-way) is about 0.18 degrees at min engine speed, 0.07 degrees at max engine speed.

Now let's look at what that means to the vibe problem:
  • If the prop blade frequency on the elastic stops is at least half an octave below min idle speed firing frequency (soft system), the prop blades will be effectively isolated from the rest of the system, and the prop blades (usually the largest inertia in a piston engined airplane power unit) are not functioning as a flywheel to allow idle. A substantial flywheel would be needed on the engine itself to allow idling. The deflections calculated above will be about the movement that the blades make twice to engine turn;
  • If the prop blade frequency on the elastic stops is within firing frequency range of the engine operations (resonant system), the prop blade on the hub will resonate at some rpm within the operating range. I expect that the blades and/or the stops will resonate, and will be short lived indeed. Also, the prop blades will not be functioning as a flywheel to allow idle. A substantial flywheel would be needed on the engine itself to allow idling. Deflection of the blades about their pivots will large indeed - this is an amplifier;
  • If the prop blade frequency on the elastic stops is at least 2-1/2 octaves above max firing frequency (stiff system), the prop blades will absolutely rotate with the hub and function as a flywheel. Deflection of blades in hub is effectively zero.
The two extremes can both work. The resonating case MUST be avoided... I have no feel for what Stemme and Rotax did in this airplane.

Billski
I am assuming that the radius of the point of articulation is considerable (to allow the blades room to tuck in) so there would be another "hard" item as a considerable flywheel mounted at the end of the driveshaft, but with reducing "stiff" inertial contribution as the blades swung out to what I also assume is a "soft" i.e. - resilient stop - if even any stop at all used. Now I am REALLY curious!!!!
 

wsimpso1

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I am assuming that the radius of the point of articulation is considerable (to allow the blades room to tuck in) so there would be another "hard" item as a considerable flywheel mounted at the end of the driveshaft, but with reducing "stiff" inertial contribution as the blades swung out to what I also assume is a "soft" i.e. - resilient stop - if even any stop at all used. Now I am REALLY curious!!!!
There are posts a little ways back on this thread with pictures. The blades appear to have a pivot and a resilient stop which aligns with my memory of the gadget. IIRC, the stop was soft, and would make the frequency in the positive torque direction kind of low And isolate the prop blade inertia from the rest of the system.
 
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