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Actual G-Force Data...LDT vs. MDT vs. HDT


CrazyCooter

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Over the past 10 years or so reading here I have seen a lot of discussions regarding the potential violent nature of hitch connections on HDT's vs MDT's vs LDT's. I also know that lots of stuff on internet forums is just regurgitated over the years by the masses and can become "The Law" to many without putting any thought into what is really happening and no facts to support the claims. We however are a safety, comfort, oriented, and technical group with members from all professions, therefore lots of input for technical discussions.

 

Has anyone done any ACTUAL testing with instruments that collect/record data that would support the need for these air hitches? Or maybe just a simple glass of water still being left on the counter after a day of driving?

 

This question comes from my recent disgust following the install (and now removal for refund) of a "Lippert Trailair Flex" that was obviously defective as delivered.....see here:http://www.rvnetwork.com/index.php?showtopic=119003&page=9 Even while towing with my Dodge 3500, I felt the need to treat not only my 5th wheel, but also the occupants in the tow vehicle to a cushioned ride on the concrete slabs we were driving on the west coast. I originally was concerned about the off road driving we were doing as boondocking is our primary camping style.......But the highways are much worse on us than the gravel roads due to the higher speeds involved and the more square edged bumps/holes found on CA's broken highway slabs if what is felt in the seat has anything to say about it.

 

Do I even need an air hitch? The HDT rides much better than the Dodge, but this is obviously masked by an air cab and seats. What about the rear axle riding solely on air? Most of our trucks are singled and running at at least 50% of rated axle/bag capacity, mine is closer to 90% when load up heavy for a big trip.......This has to have an impact as well. Just by looking at the different rear suspension designs, Mack, KW, and Pete's Flexair designs look like they would ride better than Pete's Airleaf and Volvo's setup. I'm sure there are others that are better/worse.......

 

I know running the so called "Commercial" hitch can be hard on things since it doesn't rock side/side and takes a fair amount of force backing into it to be sure it's latched....something my B&W head does not require. The commercial hitch is an obvious bad idea for RV use.....So how far must we go to ensure our investment if you will (we all know they depreciate), doesn't get pounded into wrecking yard status with more than the average consumer use?

 

I searched the net for data and found nothing. Even if there was, I know this is a case by case scenario....Singled, tandem, low pro tires vs. standard, air pressures, hitch location, wheelbase, shock valving, un-sprung weight, etc..........You get the idea.

 

I just had a brainfart while I was swapping my Morryde back onto the 5er in prep for shipping the Trailair back to Lippert..... Why not use my smartphone or tablet by downloading a "G-Force meter app", and fastening it to the pinbox....Make a few road tests in the Dodge and then again in the Pete while trying to duplicate the runs as close as possible on the same road/speeds, then compare the data? Decide then what is needed........if anything.

 

Calling Henry, bmzero, and anyone else who uses or builds these things for input. The hamsters in my brain are 110% throttle right now....too much coffee this afternoon!

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Instead of fastening it to the pin box why not just put it on the floor in trailer above the pin box or even at the back of the trailer as a second set of numbers. We feel the very rear of our trailer did a lot of bouncing with our dually but not as much now with the HDT but as you say we have no real numbers.

Dave

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Over the past 10 years or so reading here I have seen a lot of discussions regarding the potential violent nature of hitch connections on HDT's vs MDT's vs LDT's. I also know that lots of stuff on internet forums is just regurgitated over the years by the masses and can become "The Law" to many without putting any thought into what is really happening and no facts to support the claims. We however are a safety, comfort, oriented, and technical group with members from all professions, therefore lots of input for technical discussions.

 

Has anyone done any ACTUAL testing with instruments that collect/record data that would support the need for these air hitches? Or maybe just a simple glass of water still being left on the counter after a day of driving?

 

This question comes from my recent disgust following the install (and now removal for refund) of a "Lippert Trailair Flex" that was obviously defective as delivered.....see here:http://www.rvnetwork.com/index.php?showtopic=119003&page=9 Even while towing with my Dodge 3500, I felt the need to treat not only my 5th wheel, but also the occupants in the tow vehicle to a cushioned ride on the concrete slabs we were driving on the west coast. I originally was concerned about the off road driving we were doing as boondocking is our primary camping style.......But the highways are much worse on us than the gravel roads due to the higher speeds involved and the more square edged bumps/holes found on CA's broken highway slabs if what is felt in the seat has anything to say about it.

 

Do I even need an air hitch? The HDT rides much better than the Dodge, but this is obviously masked by an air cab and seats. What about the rear axle riding solely on air? Most of our trucks are singled and running at at least 50% of rated axle/bag capacity, mine is closer to 90% when load up heavy for a big trip.......This has to have an impact as well. Just by looking at the different rear suspension designs, Mack, KW, and Pete's Flexair designs look like they would ride better than Pete's Airleaf and Volvo's setup. I'm sure there are others that are better/worse.......

 

I know running the so called "Commercial" hitch can be hard on things since it doesn't rock side/side and takes a fair amount of force backing into it to be sure it's latched....something my B&W head does not require. The commercial hitch is an obvious bad idea for RV use.....So how far must we go to ensure our investment if you will (we all know they depreciate), doesn't get pounded into wrecking yard status with more than the average consumer use?

 

I searched the net for data and found nothing. Even if there was, I know this is a case by case scenario....Singled, tandem, low pro tires vs. standard, air pressures, hitch location, wheelbase, shock valving, un-sprung weight, etc..........You get the idea.

 

I just had a brainfart while I was swapping my Morryde back onto the 5er in prep for shipping the Trailair back to Lippert..... Why not use my smartphone or tablet by downloading a "G-Force meter app", and fastening it to the pinbox....Make a few road tests in the Dodge and then again in the Pete while trying to duplicate the runs as close as possible on the same road/speeds, then compare the data? Decide then what is needed........if anything.

 

Calling Henry, bmzero, and anyone else who uses or builds these things for input. The hamsters in my brain are 110% throttle right now....too much coffee this afternoon!

Good evening Crazy......

 

Below is a post that I made in April regarding bumper-pull hitches and a few vibration measurements that I had made in the past.......

 

++++++++++++++++++++++++++++++++++

Posted 01 April 2015 - 09:21 PM

 

Bigred,

 

I agree with Bronchauler that bumper hitch HDT is a good tow platform for conventional trailers.

 

We tow a 30ft /10K toy-box with a single removable horse module in a slant configuration in the last 3ft of our "garage / barn" and the horse enjoys the ride very well (Dolly-the-paint-horse-PET) is the kind of horse that is very low-key but IF the trailer is uncomfortable she lets us know!!

 

I have utilized some industrial vibration testing equipment to compare our one ton pickup vs our FL Century and the Freightliner is far more stable and does not stress the toy-box EXCEPT when I get club-footed on the clutch.

 

My fear of the Dolly-moma (DW) makes my clutch foot very smooth whenever the horse is in tow.......

 

One thing that helps out tow somewhat smooth is that we often carry several hundred gallons of fuel and water as well as a ton or more of horse gear plus up to a ton of horse feed for our extended trail-riding boondock camps. While our toy-box could hold 200 gal of water we only tow with almost all of the water on the truck and maybe just 10 to 20 gal in the trailer tanks. keeping the trailer below gross helps the trailer tires and the added weight on the truck helps smooth the truck ride so it is a win / win. Our toy-box axles are set well back so we carry a fair ball hitch wt of 900 to 1100 lbs so that also helps dampen the ride as well.

 

Also note we remain tandem since we often carry more bed weight than suitable for a single, sometimes tandems ride good and other times (washboard roads) not-so-good .....so..........tandem or single...... the stress mostly depends on the road surface conditions......

 

Happy travels,

 

++++++++++++++++++++++++++++++++++

 

I have access to a fair amount of vibration test equipment however..........most Industrial-Grade-Vibration equipment is intended to test fairly "intense rotational-event" thus Industrial-Grade-Vibration equipment is configured to test higher frequency events that you are likely desire to measure at your hitch location..........The "hitch-events" you desire are Low-Frequency / High-Amplitude events .......this type of vibration sensor is called a LVDT or Lineal-Variable Differential Transformer. The letters LVDT are an acronym for Linear Variable Differential Transformer, a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±20 inches

 

The signal self-generated by a LVDT is easily plotted in a time and amplitude graph to allow viewing and recording of long-time-test-samples..... Of course Simple tests often lead to............not so simple tests that tend to morph into..........complex tests...........so be careful how much Dr. Pepper you consume during .........TESTING..........

 

Drive on..........(Try to keep one eye on the road during testing...........)

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Instead of fastening it to the pin box why not just put it on the floor in trailer above the pin box or even at the back of the trailer as a second set of numbers. We feel the very rear of our trailer did a lot of bouncing with our dually but not as much now with the HDT but as you say we have no real numbers.

Dave

 

great idea. Maybe stick it to the nightstand in the bedroom as it's the only hard surface up there and may be more accurate to transmitting forces to the tablet. The rear of the trailer it pretty smooth with the addition of the third axle, Dexter EZ Flex, and Kyb monotube shocks. I used to notice things getting tossed around back in the garage, but not anymore.

 

 

 

Good evening Crazy......

 

Below is a post that I made in April regarding bumper-pull hitches and a few vibration measurements that I had made in the past.......

 

++++++++++++++++++++++++++++++++++

Posted 01 April 2015 - 09:21 PM

 

Bigred,

 

I agree with Bronchauler that bumper hitch HDT is a good tow platform for conventional trailers.

 

We tow a 30ft /10K toy-box with a single removable horse module in a slant configuration in the last 3ft of our "garage / barn" and the horse enjoys the ride very well (Dolly-the-paint-horse-PET) is the kind of horse that is very low-key but IF the trailer is uncomfortable she lets us know!!

 

I have utilized some industrial vibration testing equipment to compare our one ton pickup vs our FL Century and the Freightliner is far more stable and does not stress the toy-box EXCEPT when I get club-footed on the clutch.

 

My fear of the Dolly-moma (DW) makes my clutch foot very smooth whenever the horse is in tow.......

 

One thing that helps out tow somewhat smooth is that we often carry several hundred gallons of fuel and water as well as a ton or more of horse gear plus up to a ton of horse feed for our extended trail-riding boondock camps. While our toy-box could hold 200 gal of water we only tow with almost all of the water on the truck and maybe just 10 to 20 gal in the trailer tanks. keeping the trailer below gross helps the trailer tires and the added weight on the truck helps smooth the truck ride so it is a win / win. Our toy-box axles are set well back so we carry a fair ball hitch wt of 900 to 1100 lbs so that also helps dampen the ride as well.

 

Also note we remain tandem since we often carry more bed weight than suitable for a single, sometimes tandems ride good and other times (washboard roads) not-so-good .....so..........tandem or single...... the stress mostly depends on the road surface conditions......

 

Happy travels,

 

++++++++++++++++++++++++++++++++++

 

I have access to a fair amount of vibration test equipment however..........most Industrial-Grade-Vibration equipment is intended to test fairly "intense rotational-event" thus Industrial-Grade-Vibration equipment is configured to test higher frequency events that you are likely desire to measure at your hitch location..........The "hitch-events" you desire are Low-Frequency / High-Amplitude events .......this type of vibration sensor is called a LVDT or Lineal-Variable Differential Transformer. The letters LVDT are an acronym for Linear Variable Differential Transformer, a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±20 inches

 

The signal self-generated by a LVDT is easily plotted in a time and amplitude graph to allow viewing and recording of long-time-test-samples..... Of course Simple tests often lead to............not so simple tests that tend to morph into..........complex tests...........so be careful how much Dr. Pepper you consume during .........TESTING..........

 

Drive on..........(Try to keep one eye on the road during testing...........)

Sounds like you have some experience with this! Figured somebody had done some testing in the past.

 

I was thinking the grabby clutch was a bigger concern too as I do get a bit of chatter on an uphill take off just like the double disk in the Dodge. This is where the Morryde shines and the "Flex" part of the Trailair was supposed to work.

 

I was thinking IF I don't have the gnarly up/down forces then may just stick the Morryde. If air is needed then so be it.

 

One thing I have noticed is that even though the truck can toss me around on a really rough road, it's not the hard hits the Dodge delivered. The 5er and truck can wobble all they want as long as it's controlled, I just don't want it to feel like it's getting dropped on the ground with every bump.

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Anyone try a wireless baby camera in the back with the monitor up front? Then you could SEE what was going on. A couple with two smart phones could also leave one on in the back making a video call to the front.

 

I have a Wifi Gopro that I have done that with, but it doesn't have an accelerometer in it for the shock data. I was thinking if I start recording with the smartphone and GoPro at the same time, I could compare what is seen with what is felt by the accelerometer in the phone.

 

I'll see about uploading a screenshot tonight so you guys can see what the data would look like.

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We have used a camera in the cargo compartment to watch our vehicle but the detail is not there to see much.

 

We have also used a camera in the living quarters area for but again the detail is not there but we also were not using it to monitor vibrations so wasn't watching for that. Probably could have set up a glass of water on the table to see what it did but it doesn't give you actual measurable numbers.

Dave

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IIRC, Mark Shelley and Henry were involved with some stress testing in the beginnings of the ET hitch design. They only measured vertical forces and not shear forces. A much bigger concern for me has always been the sheer forces, particularly with HDTs and MDTs and people that try to drive them like they have gas engines.

 

The torque of an HDT is enough to twist up a driveshaft. That is why on start off, no accelerator pedal should ever be used. The engine computer will handle the throttle control as needed and no more.

 

The pinbox of a commercial trailer is the whole apron under the front and not just the spot where the pin sticks down. The whole frame of a semi trailer is designed as part of the pinbox. On RVs, no such complete design is in place. The pin box is usually vendored out and is an add-on. WIth two vehicles of approximately the same mass but hard connected together, most of the forces that one experiences will be shared with the other. Inertia can be a hard taskmaster, though.

 

My commercial hitch on my HDT is very easy to latch as long as I properly line up the pin with it in 3 planes before I try to engage it. I have many times hooked up and never felt it or see the trailer move until it clicked. The hard latching one typically sees on commercial rigs is due to the driver, not the hitch.

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My commercial hitch on my HDT is very easy to latch as long as I properly line up the pin with it in 3 planes before I try to engage it. I have many times hooked up and never felt it or see the trailer move until it clicked. The hard latching one typically sees on commercial rigs is due to the driver, not the hitch.

 

Agreed. Well maintained equipment, when aligned properly gives almost zero loading before you hear the "clank" from the hitch locking.

 

OTOH, I have lifted a 100,000lb+ coil tubing trailer four inches onto the hitch plate by taking a 20' run at it full speed in reverse in the snow. One of the annoying things about a coil trailer of that type is no landing gear at all. So the only way you can align pin height is by dumping air, and if the tractor plate height is still different things can get exciting fast. Much sound and fury, but we didn't break anything.

 

I would have liked to see what the accelerometers would have reported on that one!

 

Geo

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Agreed. Well maintained equipment, when aligned properly gives almost zero loading before you hear the "clank" from the hitch locking.

 

OTOH, I have lifted a 100,000lb+ coil tubing trailer four inches onto the hitch plate by taking a 20' run at it full speed in reverse in the snow. One of the annoying things about a coil trailer of that type is no landing gear at all. So the only way you can align pin height is by dumping air, and if the tractor plate height is still different things can get exciting fast. Much sound and fury, but we didn't break anything.

 

I would have liked to see what the accelerometers would have reported on that one!

 

Geo

So that's what high range reverse is for, hitching up trailers!

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Lol! And tire chains are for scraping away the top 6" of hardpack snow down to bare frozen ground. Took an hour or so and a few broken links, but we were still 4" too high at the plate even with the air dumped.

 

You had to be there. I don't commend it as a way to hitch up an RV. I don't commend it as a way to hitch up a coil trailer either, but sometimes needs must.

 

And nobody got any video. :( There were a lot of bad words said about the crew who had shored up (not very well) the coil trailer prior to unhitching the previous tractor. We got away without being dinged for any downtime by the oil company, but it was 'a near run thing'.

 

This unit is really close to the rig we were working on that night. It was about 3am before we got hitched.

 

 

Jewel-1side-view-800x208.jpg?

 

Geo

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FlyGuy: Post#6

Anyone try a wireless baby camera in the back with the monitor up front? Then you could SEE what was going on. A couple with two smart phones could also leave one on in the back making a video call to the front.

 

Sometimes what us humans "see" is.........not actually what is happening........ remember the old "shoot-em-up-western-movies where the old stagecoach wheel spokes seemed to reverse direction as the robbers chased the coach down the road before the bad guy leaped onto the lead horse and yanked the leads to stop the horses..........What you were "SEEING" was a effect caled "Aliasing"........this is also known by the slang term as "strobe-effect-out-of-phase"..........Whew........anyhow what was happening with the stagecoach wheel spokes was that the "spoke-pass-frequency was very near the same frequency as the motion-picture-camera-film-frame-pass-frequency so.......ithe "effect" was that the film frames were able to shoot the spokes at or nearly static position .......in effect the motion-picture-camera had become a rather low-grade "freeze-frame-camera" since the spokes were "passing" at nearly the same rate as the film frames were......

 

So you ask.........what does this "geezer" example have to do with putting a camera on a truck hitch.........well stay tuned it is time to pass on a "vibration-experience" that will you make you laugh and cry and laugh and cry some more......

Budd: Post#9

IIRC, Mark Shelley and Henry were involved with some stress testing in the beginnings of the ET hitch design. They only measured vertical forces and not shear forces. A much bigger concern for me has always been the sheer forces, particularly with HDTs and MDTs and people that try to drive them like they have gas engines.


The torque of an HDT is enough to twist up a driveshaft. That is why on start off, no accelerator pedal should ever be used. The engine computer will handle the throttle control as needed and no more.


The pinbox of a commercial trailer is the whole apron under the front and not just the spot where the pin sticks down. The whole frame of a semi trailer is designed as part of the pinbox. On RVs, no such complete design is in place. The pin box is usually vendored out and is an add-on. WIth two vehicles of approximately the same mass but hard connected together, most of the forces that one experiences will be shared with the other. Inertia can be a hard taskmaster, though.


My commercial hitch on my HDT is very easy to latch as long as I properly line up the pin with it in 3 planes before I try to engage it. I have many times hooked up and never felt it or see the trailer move until it clicked. The hard latching one typically sees on commercial rigs is due to the driver, not the hitch.

I am spit-balling here but I would imagine Henry jotted down some serious numbers before he started hacking and welding some iron into his prototype hitch........and it might seem likely that he did some testing to confirm the performance of his design........history seems to be the final judge of mechanical designs and it seems that history seem content with Henry's designs.........

 

My example posted here is a example of where vibration testing can lead to far different conclusions that anyone thought at the start of the testing program............

 

Too many moons ago I had way too many airplanes but I had a very gifted master-airframe & powerplant mechanic (A&P) with "Inspection Authorization" (IA)..........Being a Master mechanic, Chuck found his shop was very sought after by clients and his largest client was a "nameless govt agency" that owned & operated a fair-size fleet of light-twin-recip-engine-pressurized six place aircraft for "utility-duty" and transport of mid-level gvt folks........

 

One day I stopped by Chucks hanger and he was elbow-deep in removing the right engine in gvt twin #23.......I said uh-oh looks like I better pay my taxes because this looks expensive..........Chuck says, not smiling......."oh no not to worry this is a "free engine......on the house"........this engine has under 50 hours and it has a persistent vibration that no amount of prop balancing has helped.......... I said thats odd.......maybe it's not the engine"..........Chuck responds "oh really......"if you saw that prop spinner orbit of about 6 inches you would not believe you eyes" .............I respond, "wow, still seems odd"........

 

Next day I start a delivery trip to Central America and hide-out for a month for some sun and fishing......,when I arrive home I taxi past Chucks hanger and guess what..........yep you got it........Chuck is still elbow-deep inside the Right engine nacelle of old #23..... I shove my beast into my hanger and then wonder over to Chuck and say........."wow, I've been gone a month and your still milking this engine swap"........Chuck smiles a evil smile and says......."hey do me a favor, go over to the bottom drawer of my big Snap-On roll-away and get my big wrench"........, being a bit dense I say " what size wrench?" ..........Chuck says........"Well......pretty-boy just any big-wrench will do since I will only use it to beat you to a pulp.........you see while you were sipping cold drinks on the beach......I was swapping engines in and out of this wreck this is #3 engine and #5 prop.......this is NO FUN!!"............I respond, "wow, thats no fun at all, sorry" ........Chuck says " ya my hands feel like I have been sorting wild-cats and of course no one will want to pay the parts & labor bills on this mess"........I drive home and think......sure glad I am for once, not involved in "Chucks-mess"..........motto: be careful what you wish for.........

 

Fast forward........next morning I open my hanger door and start cleaning the "Jungle-bugs" off of the beast when all of a sudden "Rick" from the "agency" pulls up to Chucks hanger and I see them talking and once in a while they point my way.............I think.......what is that about........ Rick drives away.......and........with a evil smile on his face Chuck walks over to my hanger and with a BIG smile on his face he says.........GOOD MORNING COMRADE.......your government needs YOU!!...........I no longer smiling blurt out ......"NO"..........Chuck smiles BIGGER and says "OH YES COMRADE........DUTY AND YOUR .............GOVERNMENT CALLS YOU!!!".........Then Chuck says....."Listen up IF I have to go get my BIG wrench.....no problem.....now stop whining.....your now part of........"the team"........enjoy!!"

 

I said "why me"........ Chuck responds......"well rumor is out that you have had some vibration testing on aircraft and now the engine and prop and "agency" wants a third-party involved..........I respond, "well this is not my kind of party".........Chuck responds......."stop your whining, it's party-time' then he walks away........gulp......

 

So I drive over to Chuck M. vibration & balance shop and tell him ......."COMRADE......WE......have been drafted by our government".......Chuck M. says........"NO COMRADE.....this is your first time drafted.....I have been drafted before and me and the government have parted ways"....... I smile and say "DUTY CALLS COMRADE.......tag....your it...."

 

One day later #23 is ready to fly.....again.... so Chuck M. and I affix two LVDT sensors to the front of the right engine and route the cable into the pressure vessel / cockpit and then connect to the recorder........ Chuck M. declines to ride in #23 so we operate the recorder for a short ground run........new recip aircraft have to be power-loaded in flight very soon after a short ground run so that the piston rings can start to "break-in".........

 

So the "agency-pilot" (Ted) arrives and we preflight #23 together and then I brief the pilot on the vibration test program.......

 

We strap-in and light the fires and off we go and by the time the landing gear was retracted ole #23 was SHAKING HARD!!! I could not believe my eyes......the darn Right prop spinner tip was tracing a orbit about the size of a dinner plate!!! Ted glanced over and smiled a evil smile and said ......" well your eyes seem a bit bugged-out......how long do you want to best this thing??" .........I looked down at the plotter and the engine and prop plots were silk-smooth...... I reached over and pointed at the gear switch and said to Ted do you mind if you mind if I drop the gear ....Ted say be my guest......I drop the landing gear and point at the runway and said ...."Test#1 complete"........ Ted smiled and said ......I had a hunch we might no be gone long.......and we landed and as soon as the wheels touched the runway the prop spinner tip stopped the orbit......

 

After review of the plots it was obvious that indeed the engine and prop were PERFECT and VERY SMOOTH.........Chuck M. smiled and I smiled......

 

Next we removed one of the LVDT sensors from the engine and rerouted the sensor back into the cockpit AND then firmly mounted the LVDT sensor to the copilot seat rail.......

 

Ted was briefed for test #2 but I could tell he was not happy about flying again and he ask what we had changed and I told him that we had moved one of the sensors so we needed to make another short test #2..........I said "this time let's just leave the landing gear down and just make as short pattern and land.......Ted paused and then said 'well.....ok ...a short pattern......

 

Ted and me strap-in and light the fires and sure enough as soon as the wheels the are off the runway ole #23 starts to shake HARD...... Right spinner tip is in full orbit but the engine/prop plot is smooth but the copilot seat rail sensor plot was a sine-wave full-scale plot of about 2 Hz (two cycles per second)........no wonder the screws were falling out of my glasses!!

 

We landed and climbed out and Ted did not look happy at all and said......."well I don't know about you but as far as I am concerned I did not sign as a test pilot and I am DONE flying this soon-to-be-wreck......and they will NOT find me in the wreckage".....then her hopped in his government car and drove off ......and to this day I have not seen Ted.........

 

Chuck M. said "well COMRADE we have a problem......

 

Shortly Rick dropped by and ask if the tests were complete and I replied......”well actually it looks like one more flight will answer most of the questions but......it looks like Ted has retired from being our test pilot” Rick said “ Yes Ted said he is not interested tn flying #23 anymore”........Rick then said …...'the only pilot I have to fly #23 might Willy but he is a crusty geezer near retirement and tends to be pretty grumpy” ….....I told Rick that I had had a check-ride with Willy a few years ago and we did fine........

That afternoon we installed a third LVDT sensor......this time on the Left engine.......

Shortly Willy arrived and ask what what the test would involve.....I said that #23 would start shaking as soon as the wheels were off the runway and I would like to 5,000 above the airport in a fairly close orbit....once we were level I would like to cool the Right engine and shut it down and feather the prop to allow recording vibration data collection on the dead engine..... Willy paused and then ask do you want to fly the shut-down and ten the restart? …......I smiled and said “sure”.......

So.......Willy and I strap-in and light the fires and off we go in a shaking orbit climb up 5,000 and then level off......Willy takes his hand off the wheel and says it's yours........,.,I start reducing power to cool the Right engine a bit and then start the shut-down and feather the prop............as the prop ran down to the static position the shacking of the aircraft continued but he prop spinner stopped the orbit and then started jerking around in a random bobbing action..........so we fly along on the left engine for a minute or so letting the plotter record both engines and the airframe vibrations..........so I look over a Willy and say ….....”well you know what I really need to do now is shut-down the Left engine........ Willy slowly looks out at the Left engine and then slowly turns to look at me and says.......,”Ok.....so....IF you do shut-down the Left-engine then......we would have NO engines turning ….Right?”........I respond …....Ya thats right …...that way I will have a nice vibration plot of the airframe without engines vibrating........” Willy grins and then says, “so IF we shut-down the Left engine and feather it......then.....you will get your airframe only data right......” I respond “thats right”........Willy grins again and says.......”so IF you have both engines feathered.....,and …...after you get your airframe you can't get them started again what is your......plan.....” I grin and say “well it looks like we will be landing a bit sooner than normal” Willy grinned and said “ya that was my plan also.....” so.........I carefully pulled back the left engine power while holding 120 knots and started the shut-down and feather of the Left engine........,the prop slowly wound to a stop and we slowed to 100 knots and it was pretty nice with very little slipstream noise but........the airframe kept shaking just as bad as ever..... and both stopped spinners were bobbing around in a random manner.......... after gliding along for a while and losing about 2,000 feet of altitude I had enough airframe data plots and so I un-feathered the Right engine and started it and then unfeathered the Left engine and restarted it..... I looked over a Willy and he was …..asleep.....so I entered the pattern and lowered the landing gear and Willy woke up and took the wheel and after we landed we were taxing to the hanger Willy looked over at me and said if any one ask me I only seeing only one engine at a time shut-down today.....right? …..I smiled....and replied “right”

Soon as we got out of the plane......Willy quickly go in his car and said if anyone ask tell them he was a little-air-sick and was going home”

Shortly mechanic-Chuck, Rick, and 4 or five Gov-geeks and two engine geeks come over and ask what the tests have "proved".........Chuck M smiled and said " oh it will take a couple of days to "refine-the-HUGE-test-data-points and then we will determine the next actions........All smiled and said that they all eagerly await the test results............Chuck M. smiled as he watched the engine and gov-geeks drive off to dinner and then mechanic-Chuck turned and said......."OK you two bums what-was-that-stall all about"..........I placed my arm around Mech-Chucks shoulder and said....."Well COMRADE........YOU have a very smooth set of engines and props installed in a REALLY SHAKY AIRFRAME!!"

 

Next I fly up to Seattle with the plots and meet with Aero-Geek "Too-Tall-Paul" and he looks at the plots and says ........"Crap......Ted is right get away from this wreck........it's about to fix-itself..........FLUTTER......"

 

FLUTTER is a "aero-elastic" action where if NOT corrected will likely become 'Divergent" and the airframe will become overstressed and fail catastrophically...........GULP.........

 

Paul said look at ALL control surfaces and trim tabs to assure balance and NO sloppy play in linkages or pivot points.......

 

I flew back and told Mech-Chuck what Paul said......so we checked every surface, trim tab and pivot point ......AND ..........everything was within book tolerances ..............BUT the Left elevator hinge was at the loose end of tolerance as was the trim tab and the trim tab linkages ......we called and told him what we found he said "REPLACE every tolerance point with new parts and then confirm to tight tolerance.

 

The next morning I arrive at mech-Chucks hanger and he and the two-engine geeks are running both engines and are re-checking BOTH props balance..........I ask “why are you guys running a re-balance on the props …..my plots say they are fine......mech-Chuck says …..”oh......fine is not good enough.....we need …...perfect” ….....Then he smiled and said …...”Perfect Comrade”.....

 

Shortly Willy arrived to fly #23 again.....so we strap-in and as the wheels leave the ground.........oh so......smooth.......smooth as silk and all three plots are so.........normal.......so Willy asked “so what do you want to do?.....” I said well lets get this thing back on the ground before it wears-out again and starts shaking again....”

 

So the next day Rick calls a big meeting and confirms that Willy reports that now that the prop-balancer is working well again it was decided that #23 will be returned back to service as soon as TWO NEW ENGINES AND PROPS are installed to replace the....... two“test-stressed-(new)-engines”.............

 

And that fellow tax-payers is how a couple tons of our tax $$$$$ were spent that year........

 

So Crazy and forum members here is a quiz question.........

Why were the engine vibration plots of a higher amplitude when BOTH engines and props were feathered (stopped) in the glide-testing, than when both engines were running?

 

Correct answer gets a cold Dr. Pepper

 

My point is that vibration testing can be a …....adventure sometimes.......

 

Drive on ….......(Glide with care........)

 

 

 

 

 

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I am new with hauling my bumper pull RV trailer with a HDT. When towing with the Dodge RAM gas engine PU it was rough on me, and any passengers that were in the truck. Had the equalizer hitch, and it still was tough, the trailer would sway, and of course the steering "technique" needed when being passed by a semi is a phenomenon most of us have experienced when towing with a PU truck. In the mirror I could see all the road bumps were transmitted into the trailer, and the resulting shifting of items in the trailer were a tell tale. The maiden voyage of my Volvo HDT was to the Rally in KS. I got the Air Safe Class 6 for bumper pull installed on my HDT. Of course my route from TX took me all the way through OK and their wonderful roads. In the mirror I saw no movement of the trailer. It was rock steady. No bobbing up and down, and no sway whatsoever. On every check of the trailer enroute, there was nothing out of place. No, I didn't place a glass of water on the counter, but what I did notice is nothing in the fridge, or the spices in a cabinet had much if any movement! It was a joyful surprise! To add to the joy the ride in the truck was wonderful. No scientific testing done, but by observation it looked to be no comparison to the pick up. I am going to try the air hitch on the Dodge and see if it improves it any. I will post info on that when I do it.

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So Crazy and forum members here is a quiz question.........

Why were the engine vibration plots of a higher amplitude when BOTH engines and props were feathered (stopped) in the glide-testing, than when both engines were running?

 

Correct answer gets a cold Dr. Pepper

 

 

 

 

Just a guess here but I'd have to say that the gyroscopic stabilizing effect of the nicely balanced running engines had a calming affect on the aerodynamically shaking airframe, hence the shift in amplitude when they were NOT running.

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I'm in the gyro effect group too especially the mass and eaero forces on the propellers.

 

Texasdually: we use an AirSafe hitch on our Dodge ton truck and it definitely makes a difference. If you have a chance to ride in your trailer you will find it rides pretty smooth it is the tow vehicle that provides the roughing up.

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Ok group........it seems a couple cups of morning Joe gets the ole-brain-waves into a high state of amplitude & frequency so.........

 

Question #2...........Why ......when BOTH engines were turning did the Right engine spinner tip "orbit-in-a-6"-circle"?

 

Maybe more Dr. P will need to be iced.......

 

 

Drive on.............(Morning tests are.......best....)

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my measurement "kind of"

 

we picked up our HDT about a month before our toy hauler arrived; so we towed our previous 5er on our first 200 mile round trip excursion (we had used this 5er for 8 years; so lots of miles behind the pickup)

this first trip the HDT didn't have an air hitch or any cushion on the pin box of the 5er

 

I put a cooler of beers in one of the toolboxes on the hdt before we left

 

100 miles of not bad interstate travel and we had arrived at the campsite

- 80% of the bottles had their caps burst off the beers

- there was a significant amount of sawdust on the floor of the 5er around each of the roof vents (this hadn't been noticed before)

 

After seeing this I decided to put an Air Safe hitch on the HDT (and the Reese was evil to hook and unhook).

After the air hitch was installed the saw dust didn't happen anymore.

 

Not very scientific but that was my experience.

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I haven't done any "numbers testing" but:

 

When I bought our truck and RV, it had the OEM rigid semi hitch. FIrst couple trips we took ALL the clothes would be on the floor in the from closet, and drawers slid out of their spots, etc. More tellingly. every trip would pop the caulking seams around the front nose area of the trailer. While traveling, I have a camera I can see the hitch and trailer front. You could SEE the jar and jerk on the trailer nose.

 

Then I built and installed my own air ride hitch. Now the camera shows the hitch spending it's time floating in a couple inches--the front of the trailer never "shudders" like before. I've never had clothes on the floor, the drawers stay in place, and once a 3/4 full glass of lemonade sat on the floor for 120 miles without moving or spilling (that one surprised me, actually, I don't drive that smooth!) The caulking is no longer popped immediately.

 

All of those things make really practical "vibration test objects." Consider the Ancient Chinese earthquake tester...balls perched just so in a vase sorta thing. No "numbers", but it worked for intensity and direction! Just so--clothes on/off the closet rod...0-100% makes a pretty compelling measurement, no?

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Our experiences have been very different but we are tandem axles. About the only things that seem to move out of place are on very pitchy roads. Rough roads are just rough. Did I say, I hate dips!

 

When things like mismatched road/bridge sections bottom out my air ride seat in the truck when the steer axle hits them but things still don't move in the 5th wheel, it has to be more about mass, inertia and suspensions than about how things are connected to the tow vehicle. Clifford has air bag suspension and my pin weight is only 4500lbs on my 21000 lb 5th wheel so the mass of the truck pretty much matched that of the RV and they move together pretty well.

 

Yes, I would like a great air hitch but so far, this combination has performed very well. No clothes off the front closet rods and no popped beers riding on the floors.

 

The beer and sawdust sounds like you have a traveling Hofbrau House.

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Instead of fastening it to the pin box why not just put it on the floor in trailer above the pin box or even at the back of the trailer as a second set of numbers. We feel the very rear of our trailer did a lot of bouncing with our dually but not as much now with the HDT but as you say we have no real numbers.

Dave

 

I fooled with the original hitch, a commercial Holland, on my MDT and saw a good deal of frame flex on my fiver, about an inch crossing some rough train tracks. I'd think that measuring vibration and/or movement would need to be done at the pinbox to get a clear look at the situation without the fiver's frame flexing complicating things.

 

Getting numbers from both the pinbox and a solid spot inside the fiver would give some interesting numbers on how much flexing the frame was doing which might convince someone that a bit more cushion under the pin would be a good thing for the fiver's frame life.

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