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flyadive(at)gmail.com
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 Posted: Wed Apr 14, 2010 1:04 pm Post subject: Why exactly shouldn't you run a 914 without a prop? |
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Guys:
Why are you becoming Theoretical Physicist and reinventing the wheel?
Here is two hunks of information that are prevalent and should be followed:
1 - In all my training with internal combustion engines in 45 years...
You NEVER run an engine with out a flywheel of some type or a load.
The prop is both a flywheel and a load.
Now if you don't believe this jump to item 2...
2 - Call up ROTAX and ask them: if they will warranty the engine if
you tell them that you ran it without a flywheel or a prop?
The reason for the flywheel or load is so you reduce or eliminate the
possibility of the engine RUNNING AWAY... Over reving. Have you ever
been to the race track and the drive shaft shears and WHAM.... One
hell of a runaway situation. That is one reason why they put
explosion blankets around the flywheel.
Barry
On Wed, Apr 14, 2010 at 9:51 AM, Gilles Thesee
<Gilles.Thesee(at)ac-grenoble.fr> wrote:
| Quote: |
<Gilles.Thesee(at)ac-grenoble.fr>
AmphibFlyer a écrit :
>
> 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
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flyadive(at)gmail.com
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| Posted: Thu Apr 15, 2010 1:20 pm Post subject: Why exactly shouldn't you run a 914 without a prop? |
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It never ceases to amaze me .... 1.5 Million Years to evolve from
Neanderthal to Modern Man.
And No Matter how many times you tell people the fire is hot - You
will get burned ---
They Just Got To Put Their Hand In The Fire!
What the hell does it prove if YOU can get a ROTAX to run without a prop?
Ya want a pat on the back - - - OK, Here it is. Pat - Pat.
Now, give every one your N-Number and Serial Number of the engine so
they know to stay
away from them when you go to sell.
Barry
On Thu, Apr 15, 2010 at 5:15 AM, Gilles Thesee
<Gilles.Thesee(at)ac-grenoble.fr> wrote:
| Quote: |
<Gilles.Thesee(at)ac-grenoble.fr>
Hi Ron,
>
> 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.
> 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
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occom
Joined: 26 Aug 2006
Posts: 404
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| Posted: Thu Apr 15, 2010 4:46 pm Post subject: Why exactly shouldn't you run a 914 without a prop? |
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This started as a question and a conversation and is rapidly becoming
something ugly. Why do people feel it necessary to "win" a discussion?
The first couple of replies identified the potential downside of operating
the engine without a prop and so does the install manual, there is no
mention of operation sans prop in the operation manual that I could find.
Here is every instance of operation without propeller I can find in the
manual:
- Never run the engine without a propeller as this will inevitably cause
engine damage and present a hazard of explosion.
â–² WARNING: Never run the engine without a propeller installed as engine
would suffer severe damage by overspeeding.
That is the sum total of what they have to say on the matter. Clearly an
engine operated in a no-load state could quickly overspeed or over-rev if
you like. Beyond that possibilibility, there doesn't appear to much
likelihood of a problem. It's very likely a bad idea because over-revving
can occur quickly, but all this talk of flywheel, torsion, gearbox, etc in a
no load situation doesn't seem likely to me. Indeed the video of one running
at low RPM looked quite happy to me, humming along smoothly. So, can we
agree that it's not a good idea because the engine may over-rev and leave it
at that, no pats or flames required.
---
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AmphibFlyer
Joined: 29 Dec 2009
Posts: 25
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| Posted: Sat Apr 17, 2010 5:39 am Post subject: Re: Why exactly shouldn't you run a 914 without a prop? |
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| flyadive(at)gmail.com wrote: | It never ceases to amaze me .... 1.5 Million Years to evolve from
Neanderthal to Modern Man.
...
What the hell does it prove if YOU can get a ROTAX to run without a prop?
Ya want a pat on the back - - - OK, Here it is. Pat - Pat.
Now, give every one your N-Number and Serial Number of the engine so
they know to stay
away from them when you go to sell.
Barry
|
Dear Barry:
If you want information, please try to be civil. If you want to be abusive, go find a cave and make up stories for the Neanderthals there, who might be entertained. Science requires light, not fantasy or invective.
I posted the information that Russ Garner has run a Rotax 914 without a prop in order to let the the h. Sapiens here know that is it possible, even though Rotax advises against it in several documents. Whether it is advisable to run the engine without a prop, I can't say--can only report that Russ has run that particular engine for several hours with no apparent damage.
The engine has a flywheel; that is clear if you look at the Illustrated Parts Manual, which is available online. Let's please stick to what we observe and can verify, and not cloud the picture with untested hypotheses and snotty remarks.
=Don
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rickofudall
Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA
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| Posted: Sat Apr 17, 2010 12:23 pm Post subject: Why exactly shouldn't you run a 914 without a prop? |
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Well, I ran the Rice King for about 50 hours and I didn't find it the panacea you want to make it, but then I don't run a sea plane or a tow plane. It DID reduce the glide of my Kolb Mk III to somewhere between 1 to 1 and 2 to 1. As long as I did my approaches with the engine at 3000 RPM it made no difference. If I chopped the throttle I got Space Shuttle descents. I traded it for flight time at Wallaby Ranch and went back to stock. I've had engine outs twice since and with the prop stopped it still has about twice the glide it had with the RK.
Rick Girard
On Sat, Apr 17, 2010 at 12:16 PM, lucien <lstavenhagen(at)hotmail.com (lstavenhagen(at)hotmail.com)> wrote:
[quote] --> RotaxEngines-List message posted by: "lucien" <lstavenhagen(at)hotmail.com (lstavenhagen(at)hotmail.com)>
occom wrote:
> Lucien, this is extraordinarily interesting to me. I have always understood
> that when the engine is stopped, the prop will accelerate wildly and
> generate a huge drag effect. Are you saying that this is not the case? I am
> very interested in what is observed and not conceived. Please feel free to
> email off list if you care to.
>
"accelerate wildly" is of course silly and not the case, no. It does windmill in the relative wind but the magnitude of the drag effect is also usually overblown, again by folks who've never flown a clutch equipped plane. On my FSII, for example, the drag added by my 68" WD 3 blade freewheeling wasn't too much more than the added drag of the 66" TPI 2 blade stopped on a dead engine on a friend mine's FSII. My glide ratio was a little worse than his, but not the "drag chute" myth spread by the guys who don't use a clutch.
They also don't understand that the drag that does get added can actually be useful (i.e. if you need extra drag such as when high/hot on final).
>
>
> BTW the "E" box is not a candidate for the clutch as you would be unable to
> start the engine, I'm sure you already knew that but you made such a case
> for it with the electric start that I thought someone might not be aware.
>
> ---
No, I was talking about the concern that you couldn't hand prop. My point was the E box kills two birds with one stone there - you have the prop fixed to the crankshaft for hand propping if desired and you can also retain the pull start on the mag end in case of an electrical problem.
LS
--------
LS
Titan II SS
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