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Why exactly shouldn't you run a 914 without a prop?

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Gilles.Thesee(at)ac-greno
Guest

Posted: Tue Apr 13, 2010 12:00 pm Post subject: Why exactly shouldn't you run a 914 without a prop?

Hi Ron,



Please allow me to chime in.



 	  Quote: 	
		   I have run plenty of motorcycles and cars that

 are a lot hotter tune than a 914 and have yet to over rev one in neutral.

   

Cars and motorcycles are *designed* to run in neutral. They have 
	


carefully designed flywheels, bobweights, balance masses to dampen out 

torque vibrations.

Aero engines rely on a prop with specific inertia range to achieve the 

same goal. Just think of the press fitted crank, timing gear, reduction 

gear.



 	  Quote: 	
		   Running without

 prop is a lot safer if you needed to check something, like intermittent

 miss where you could observe one of those transparent spark plugs like

 Bing sells? Had a nagging charge problem that would allow you to have a volt meter hooked up and aggressively wiggle and polk at connections? Wanted to check exhaust for leaking by blocking exhaust pipe and listening? If you had a nagging coolant or fuel or oil leak that only dripped with engine running? Start up I think can be a bit harder without flywheel which could stress spraigue I imagine, but sure do like the idea of keeping ones self at less risk compared to performing mentioned checks with spinning prop close by.

   

Treating such an expensive piece of hardware the way the manufacturer 
	


forbids is not necessarily the best way to ensure reliability.

As for the simple troubleshooting you describe - not likely to be needed 

in a properly assembled homebuilt- chock the airplane, inspect the 

engine, run it up, stop it, re-inspect, that's all.



Best regards,

-- 

Gilles

http://contrails.free.fr

Gilles.Thesee(at)ac-greno
Guest

Posted: Wed Apr 14, 2010 5:59 am Post subject: Why exactly shouldn't you run a 914 without a prop?

AmphibFlyer a écrit :

 	  Quote: 	
		    I ran that very engine three days ago myself with no prop and it idled perfectly smoothly.


	


How do you qualify a "perfectly smooth" idle ?

My engine also idles perfectly smoothly (with a prop attached, though) 

between 1100 and 1800 rpm, and yet Rotax advises against idling below 

1400 rpm, due to stress and chatter in the reduction gear, which to date 

I have never been able to detect.



Did you take special measures to ensure acceptable stress in the 

reduction gear, or do you simply *think* things are okay ?



BTW our project uses *no* springs for the throttles, but push-pull 

controls. And of course idle and full throttle stops.



Best regards,

-- 

Gilles

http://contrails.free.fr

Gilles.Thesee(at)ac-greno
Guest

Posted: Thu Apr 15, 2010 1:24 am Post subject: Why exactly shouldn't you run a 914 without a prop?

Hi Ron,

 	  Quote: 	
		   Was turning over my 914 today to move around a little lubrication and figure I should make the point that the Rotax 914 does in fact have a flywheel. Gilles a while back you posted a picture of a crankshaft stating there is no flywheel. You didn't mention the picture that attaches to the aft (opposite the prop) end. #19 which is called by Rotax the Flywheel hub, and #1 Magnito ring that is a big hunk of metal that is attached to the outside OD of Flywheel hub. Together these components are in fact a substantial piece of rotating mass (flywheel).

You are referring to the Rotax alternator, which certainly plays a role 
	


in torsional vibration modes, but its inertia cannot be compared to that 

of a prop.

An engine doesn't just need "a flywheel".

Its torsional modes are subject to rigorous testing and adjustments 

before it can achieve reliability. Most backyard mechanics don't have 

the equipment to make the appropriate measurements and prolonged bench 

fatigue tests, so better follow the manual's dos and don'ts.



 	  Quote: 	
		    That said if you were to mount a 914 on a motorcycle, it would probably be OK as is, but for easier drivability, you may want more flywheel when operating at lower RPMs.

 For sure if you used a new engine it would be one costly motorcycle!

   

Running it against the manufacturer advice might make it an expensive 
	


aircraft engine as well 



Best regards,

-- 

Gilles

http://contrails.free.fr

lucien

Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM

Posted: Thu Apr 15, 2010 5:46 am Post subject: Re: Why exactly shouldn't you run a 914 without a prop?

 	  Gilles.Thesee(at)ac-greno wrote: 	
		  AmphibFlyer a ï¿½crit :

How do you qualify a "perfectly smooth" idle ?

My engine also idles perfectly smoothly (with a prop attached, though) 

between 1100 and 1800 rpm, and yet Rotax advises against idling below 

1400 rpm, due to stress and chatter in the reduction gear, which to date 

I have never been able to detect.



	




Like you said, tho, that doesn't mean everything is necessarily ok and there won't be long-term problems. 

In fact, I was in a nasty thread a while back on another group where someone was advocating a shutdown procedure that introduced the possible hazard of running the engine below the specified minimum if it weren't carried out just exactly right. The claim was that it came from Eric Tucker or something like that but was otherwise backed up with nothing else but that heresay. 

My contention is, at 20 large (and continuing to go up) a pop, the 912 is a poor platform for testing these oddball procedures gotten off the Internet or somewhere, unless you really have a lot of money to burn.



As for running the 912 series without a prop, it's also very likely that not a lot of field testing has been done with the 912 in that particular configuration .



So basically, these oddball ideas are done at one's own risk, both financially and in terms of support from Rotax and dealers, etc. On something relatively inexpensive like an auto or motorcycle engine you're not likely to be out a whole lot of cash if something breaks, but not with our 912's. 



Let's be careful out there,



LS

_________________
LS
Titan II SS

rlborger(at)mac.com
Guest

Posted: Thu Apr 15, 2010 10:23 am Post subject: Why exactly shouldn't you run a 914 without a prop?

 	  Quote: 	
		  Scott,



My answer to your first question is "Darned if I know why anyone would do that."  But I'm sure there are lots of folks out there who would and they can tell you why they would.  Maybe safety when making tests around a running engine?



Best source for initial learning about your Rotax engine is Rotax-Owners.com.  Go and sign up.  Then hit the downloads page and download every document pertinent to your engine; owners manual, maintenance manual, etc.  If you pay the big buck for membership, you can have access to some good video courses on the engine.  I'd also recommend you attend one of the Rotax owners/maintenance courses that are offered around the country.  I took the one at Lockwood Aviation in Sebring Florida.  They are very good sources of information and practices.  You can check my Europa builders site ( http://www.europaowners.org/forums/gallery2.php?g2_itemId=60232 ) for my gallery on the course I attended.



The Rotax engines are fine little machines but they do have their own ways and you want to treat them like Rotax engines and not Continental or Lycoming engines.  



Good luck and have fun building and flying your RV-12,

Bob Borger

Europa XS Monowheel, Intercooled Rotax 914, Airmaster C/S prop - Flying

Little Toot Sport Biplane, Lycoming YIO-320, Sensenich wood prop - under construction

On Thursday, April 15, 2010, at 12:43PM, "Scott Schmidt" <scottmschmidt(at)yahoo.com> wrote:

 	  Quote: 	
		  	
   I just joined this group so I can start learning about the Rotax.  Just started an RV-12. 

 
	


I may have missed something discussed earlier about this thread, but why would you ever run this motor before mounting it on the plane with a prop?

What is the advantage of running it without a propeller?  When I built my RV-10 I had to be very careful not to bet my CHT's over 200 during my 3 test runs before the first flight.  I just made sure the mag and ignition system worked, the oil pressure was up and the prop cycled.  But I don't know the break-in procedures for the 912 yet so that is why I am asking. 

 

Thanks. 

  

 Scott Schmidt  scottmschmidt(at)yahoo.com 

  

 

  

    From: lucien <lstavenhagen(at)hotmail.com>

 To: rotaxengines-list(at)matronics.com

 Sent: Thu, April 15, 2010 7:46:16 AM

 Subject: Re: Why exactly shouldn't you run a 914 without a prop?

  

 --> RotaxEngines-List message posted by: "lucien" <lstavenhagen(at)hotmail.com (lstavenhagen(at)hotmail.com)>

 

 

Gilles.Thesee(at)ac-greno wrote:

 	  Quote: 	
		   AmphibFlyer a ï¿½crit :

 How do you qualify a "perfectly smooth" idle ?

 My engine also idles perfectly smoothly (with a prop attached, though) 

 between 1100 and 1800 rpm, and yet Rotax advises against idling below 

 1400 rpm, due to stress and chatter in the reduction gear, which to date 

 I have never been able to detect.

 

 

 
	


 

Like you said, tho, that doesn't mean everything is necessarily ok and there won't be long-term problems. 

In fact, I was in a nasty thread a while back on another group where someone was advocating a shutdown procedure that introduced the possible hazard of running the engine below the specified minimum if it weren't carried out just exactly  right. The claim was that it came from Eric Tucker or something like that but was otherwise backed up with nothing else but that heresay. 

My contention is, at 20 large (and continuing to go up) a pop, the 912 is a poor platform for testing these oddball procedures gotten off the Internet or somewhere, unless you really have a lot of money to burn.

 

As for running the 912 series without a prop, it's also very likely that not a lot of field testing has been done with the 912 in that particular configuration .

 

So basically, these oddball ideas are done at one's own risk, both financially and in terms of support from Rotax and dealers, etc. On something relatively inexpensive like an auto or motorcycle engine you're not likely to be out a whole lot of cash if something breaks, but not with our 912's. 

 

Let's be careful out there,

 

LS

 

--------

LS

Titan II SS

 

 

 

 

Read this topic online  here:

 

 http://forums.matronics.com/viewtopic.pharget="_blank" href="http://www.matronics.com/Navigator?RotaxEngines-Li also available via the Web nbsp;                 -Matt Dralle,://www.matronics.com/contribution">http://www.matronics.com/contribution 

 

 

  

 

  

   	  Quote:

Gilles.Thesee(at)ac-greno
Guest

Posted: Fri Apr 16, 2010 5:55 pm Post subject: Why exactly shouldn't you run a 914 without a prop?

rparigoris a écrit :

 	  Quote: 	
		   



 Hmmm

 True not a 91X, but these guys decided to take the prop load off engine below 2500RPM:

 http://www.rotaxparts.net/Scripts/prodView.asp?idProduct=1210

   

Hi Ron,
	


Yes, they sell this device for the 2-strokes. Far less expensive, and 

far less reliable than the 91x.



Now if you were asking about starting your Rotax with no prop, I would 

strongly advise against it.

But are you really asking ?



Best regards,

-- 

Gilles

http://contrails.free.fr

Gilles.Thesee(at)ac-greno
Guest

Posted: Sat Apr 17, 2010 6:44 pm Post subject: Why exactly shouldn't you run a 914 without a prop?

 	  Quote: 	
		   The engine has a flywheel; that is clear if you look at the Illustrated Parts Manual, which is available online.


	


Don,



A flywheel has a *large diameter* and a heavy rim to provide inertia to 

the rotating crank.

The part you are referring to is the magneto/alternator rotor, which 

mainly consists of a *small diameter* aluminum rim with magnet inserts. 

Of course it looks a little like a miniature flywheel, and it has its 

own inertia, but it can't compare with that of the prop for the role of 

engine flywheel. Measuring its moment of inertia should be a 

straightforward operation.

You'll find a picture of the Rotax alternator here 

http://contrails.free.fr/elec_ducati.php.



While conducting some in-depth investigations about this particular part 

of the engine, I became fairly acquainted with its role and features, 

hence what I suggested.



Best regards,

-- 

Gilles

http://contrails.free.fr

 	  Quote:

rparigoris

Joined: 24 Nov 2009
Posts: 792

Posted: Sat Apr 17, 2010 8:27 pm Post subject: Re: Why exactly shouldn't you run a 914 without a prop?

Hi Gilles

"A flywheel has a *large diameter* and a heavy rim to provide inertia to 

the rotating crank. 

The part you are referring to is the magneto/alternator rotor, which 

mainly consists of a *small diameter* aluminum rim with magnet inserts. 

Of course it looks a little like a miniature flywheel, and it has its 

own inertia, but it can't compare with that of the prop for the role of 

engine flywheel."

I think somewhere somehow we should break down flywheel effect into two categories that are not the same. First off is needed flywheel to allow motor to run. Gilles the Rotax 914 is much closer to a motorcycle engine than a traditional cont. or lyc. or in many ways an auto engine. Many motorcycles have a similar in size of rotax 914  flywheel and they run just fine. Too light and if you miss a gear you can easily over rev in a split second, but none the less they run fine. Running a 914 without a prop should  be at low power output. This low torque is nothing compared to running motor with a load.

Now for prop flywheel effect. This is a major engineering problem. Unlike the limited weight of a flywheel on a car or motorcycle, the prop is a huge flywheel. With this huge flywheel effect it does not want to accelerate or decelerate too quickly. Now with the 91X engines you have a crankshaft, that twice per revolution gets hit with the torque blow of two connecting rods trying to accelerate the crankshaft. BTW this could be another discussion but connecting rods are more rotational weight than reciprocating, in other words they too act for most part act like a flywheel, not reciprocating weight like pistons moving up and down. OK back on thought, now you have a crankshaft when cylinders are expelling some BTUs that is in constant state of accelerating and decelerating. Now if you have a flywheel mass like the prop that does not feel too much like speeding up or decelerating as quickly as the crankshaft, enormous stress is burdening the gearbox. Cessna had a big bore motor that they tuned for making good torque/ thus higher horsepower compared to direct drive low revving cousin and put a redrive gearbox to keep prop tip speed down. The gearboxes tore themselves up, never did they figure how to get them last a long time. Rotax addresses problem by momentarily absorbing some of the connecting rods hitting acceleration, and subsequently releasing when crank begins to slow down. I will give a quick description. If we had a helical set of gears, when you begin to load that set of gears you will also begin to create thrust (sideway) loads. Now if we did our homework and put a spring to absorb sideways loading, there would be some give in the gearbox to prevent tremendous loading of gears, then when crankshaft began to slow, the spring would put back some energy. Well that is pretty close how Rotax gearbox works. They have dogs which are mini ramps that when the prop begins to load will create a sideways force, this force is absorbed by kind of sorta Belleville style spring washers. Now when crank gets hit by connecting rods and forced to accelerate and the prop which will not accelerate anywhere near as fast as the crank and total rotating mass of engine, in fact this pulse of acceleration is  absorbed by spring washers, and when engine begins to decelerate then energy will be released, in other words the prop can remain at a constant RPM where it is most happy and the engine can have crank accelerating and decelerating twice per revolution and not rip up the gearbox. Without the dogs/spring set up the gears would take the brunt because the prop would actually hold back the crank acceleration.

Now to hopeful put this to rest, if you take off flywheel mass of a rotating prop, not only is the flywheel effect gone, but there is no load on the prop shaft. You can do anything you want with motor and you are not going to hurt the gearbox/dogs. The spring washers will never get side loaded because there is no load. I think there is a bit more rotational weight but it would be like a clutch/transmission of a motorcycle turning when in neutral

I absolute agree that figuring out ramp angle of dogs and spring tension of spring washers is an engineering challenge and there are many factors to get it right, but take prop off and there is not much loading of gearbox at all.

Now if you all of a sudden increase output of motor like turbo charging of a 912S, lets all of a sudden put in a hotter cam where it now is happy at a cruise of 6000RPM instead of 5000RPM and wanted to use a much heavier constant speed prop to absorb the 190HP and hold up without not breaking, you may want to from an engineering point of view revisit strength of crankshaft (perhaps welding bits together) and ability of gears to hold up, probably needs slipper clutch redesign, dog redesign, spring washer recalculation and prop shaft and bearing strengthening. Taking prop off goes the other way and takes load off of gearbox.

I forget precise numbers, but think at 1400 RPM load on gearbox is like 10 times as much compared to 5000RPMs.

BTW if you really want to challenge the gearbox of a 914, after you just sinked the carbs forget to connect the cross tube connecting the starboard to port bank manifold. A Europa did just that and the shaking was so violent it broke the motor mount! Europa has since incorporated a mod to strengthen.

To close, I don't really have a need to run without a prop but want to understand my options.

Ron Parigoris

Gilles.Thesee(at)ac-greno
Guest

Posted: Sun Apr 18, 2010 1:02 am Post subject: Why exactly shouldn't you run a 914 without a prop?

Hi Ron,

 	  Quote: 	
		   Now with the 91X engines you have a crankshaft, that twice per revolution gets hit with the torque blow of two connecting rods trying to accelerate the crankshaft. BTW this could be another discussion but connecting rods are more rotational weight than reciprocating, in other words they too act for most part act like a flywheel, not reciprocating weight like pistons moving up and down.


	


Not sure I got everything, but one point often neglected is torsional 

*vibration *. It's what takes most of the engineering time when 

designing and testing a new engine.

My point was, there is more to engines than just *weights* etc. A 

crankshaft responds to torque variations by winding and unwinding in its 

own very complex ways, and it takes many many test and engineering hours 

to get things sorted at the targeted RPM an loads. Motorcycle engines 

have massive crank throws/wheels to add to the flywheel effect. TheRotax 

crank is much lighter.

Tempering with such a carefully balanced system is something one should 

endeavour with utmost care and knowledge if long term (unfortunately 

sometimes short term  reliability is to be maintained.

 	  Quote: 	
		   BTW if you really want to challenge the gearbox of a 914, after you just sinked the carbs forget to connect the cross tube connecting the starboard to port bank manifold.

Not willing to try this on one of the engines I'm operating 
	




Best regards,

-- 

Gilles

http://contrails.free.fr

AmphibFlyer

Joined: 29 Dec 2009
Posts: 25

Posted: Sun Apr 18, 2010 6:01 am Post subject: Re: Why exactly shouldn't you run a 914 without a prop?

 	  Gilles.Thesee(at)ac-greno wrote: 	
		   	  Quote: 	
		   The engine has a flywheel; that is clear if you look at the Illustrated Parts Manual, which is available online.



	


The part you are referring to is the magneto/alternator rotor...

	




Giles, the link you supplied didn't work for me, but we're probably thinking of the same part. It's also used for the starter motor drive. Here's the most recent Parts Manual and the others: http://www.rotax-owner.com/index.php?option=com_content&view=article&id=108&Itemid=169



I don't know the weight or mass of the rotating parts attached to the crankshaft--probably not a lot, even including the freewheel gear and the sprag clutch housing (912/914 Illustrated Parts Manual, page 6-2)--but I watched Russ's 914 run without a prop and started it, worked the throttle, and shut it down myself, several times. I was a bit surprised by how smoothly it ran--just as you can see in Russ's video. As far as I could tell, the engine wasn't banging itself to death or anything like that. I tried unbalancing the carbs slightly by manually opening one throttle at a time, but all that did was cause it to lope a bit. 



The only noticeable difference was that the propless engine would accelerate very quickly because there was almost no mass to accelerate. 



=Don

Gilles.Thesee(at)ac-greno
Guest

Posted: Sun Apr 18, 2010 6:16 am Post subject: Why exactly shouldn't you run a 914 without a prop?

Don,

 	  Quote: 	
		  

 Giles, the link you supplied didn't work for me,

It seems to be working for me, try to copy and paste in your browser.
	


http://contrails.free.fr/elec_ducati.php

 	  Quote: 	
		    but we're probably thinking of the same part. It's also used for the starter motor drive. 

   

Ah, then it must be the freewheel gear, a very thin gear with 50 teeth 
	


and large lightening holes.

 	  Quote: 	
		   I was a bit surprised by how smoothly it ran

   

The problem is, one cannot judge torsional vibrations just by eye or 
	


ear, otherwise aero engineers would not resort to sophisticated sensors 

and analysers, and their life would be easier.

Here are a few links which could be of interest to people with a 

mechanical mind (no maths) :

http://www.enginehistory.org/NoShortDays/TV.pdf

http://www.epi-eng.com/propeller_reduction_technology/torsional_vibration_issues.htm

http://www.epi-eng.com/piston_engine_technology/torsional_excitation_from_piston_engines.htm



Best regards,

-- 

Gilles

http://contrails.free.fr

Gilles.Thesee(at)ac-greno
Guest

Posted: Wed Apr 21, 2010 2:17 pm Post subject: Why exactly shouldn't you run a 914 without a prop?

Hi Ron,

 	  Quote: 	
		   Lack of differential fuel pressure on take off leads to "running" of the pilot when engine plays mellow song of "silent night". You need between ~ 2 and 5 PSI differential. If low you could get proper fuel flow but on verge of not being able to meet crack pressure of fuel pressure regulator. Problem is if you have full float bowls and begin take off run, you will be in air with perhaps not enough runway to land on when they run out. Also when cruising it gives you a heads up to close at hand failure. Most culprits of low differential pressure is clogged fuel filter or gasculator/s. If you reduce power and can attain 2 PSI then you could probably gain yourself a little more time. At altitude where the pumps have to work hardest are where you will first see lower readings, a lower altitude may allow you to reach airport. Running lean at altitude is a very bad thing to do, fuel flow may be fine at less than 2 PSI, but go too much lower and fuel flow will suffer. Differential ca!

  n give you a heads up to problem before it happens.

   


	


We made some measurements of fuel pressure/differential pressure vs fuel 

flow (on the ground). We found interesting things (circulating vapour 

bubbles, etc...), but there was no measurable delay between change of  

fuel flow and change of fuel pressure or vice versa.

So considering that fuel flow measurements take place with one turbine 

upstream of the regulator and the other downstream of it, we did not 

feel compelled to add fuel pressure measurements as both parameters are 

directly related in this engine.



Best regards,

-- 

Gilles

http://contrails.free.fr

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