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Author Topic: Runaway  (Read 1933 times)

Offline Chris Fretz

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Runaway
« on: July 26, 2017, 12:11:47 PM »
What are the common causes of Runaway?  Will vibration cause Runaway?

I was getting it last night on my Tanager. LA 46, tounge muffler, pressure, Brodak 10% 11.5 castor 11.5 synthetic, #6 plug. I would level out from the wingover and she was gone. Tried pressure on uniflow line then tried blocking off uniflow line and pressure on overflow line. That didn't change anything, then I ran out of time it got dark out.

My other Tanager did fine at Brodaks but lately it's been going crazy in the stunts,  thought it was from wind gusts. Put pantyhose on but no luck as of yet.
« Last Edit: July 26, 2017, 12:37:24 PM by #Liner »
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Offline Tim Wescott

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Re: Runaway
« Reply #1 on: July 26, 2017, 03:36:24 PM »
Using a wrong engine.  The 46LA is a right engine.

Using the wrong fuel.  It sounds like you have the right fuel.

Using the wrong prop.  Too much pitch means you try to slow it down too much with the needle, it gets "on the step" in the air, and then -- away you go, with rather an excess of line tension.

Using too big a venturi.  Too much venturi means you try to slow it down too much with the needle, it "gets on the step" in the air, and then ...

What prop?  What venturi? 

I put on nylon mesh (not pantyhose, as your PM made me realize), while leaning out for the same level lap time, until it starts getting weak in the overheads, then I back off.  You could do the same with different venturi sizes.
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Offline Dane Martin

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Re: Runaway
« Reply #2 on: July 26, 2017, 03:53:16 PM »
Tim and Brett helped me fix a similar situation.
Easy thing, remove the muffler and leave everything else alone. If that helps, drill more holes.
Then,
Different venturi. Turn yourself out a .200 ID venturi. Start there and open a little if needed

What prop are you running?

Offline Chris Fretz

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Re: Runaway
« Reply #3 on: July 26, 2017, 06:47:27 PM »
Using a wrong engine.  The 46LA is a right engine.

Using the wrong fuel.  It sounds like you have the right fuel.

Using the wrong prop.  Too much pitch means you try to slow it down too much with the needle, it gets "on the step" in the air, and then -- away you go, with rather an excess of line tension.

Using too big a venturi.  Too much venturi means you try to slow it down too much with the needle, it "gets on the step" in the air, and then ...

What prop?  What venturi? 

I put on nylon mesh (not pantyhose, as your PM made me realize), while leaning out for the same level lap time, until it starts getting weak in the overheads, then I back off.  You could do the same with different venturi sizes.
The only prop that seems to work for me, 12.25x3.75 I think it's the .280 or .288 with 3 layers of pantyhose.

Tim and Brett helped me fix a similar situation.
Easy thing, remove the muffler and leave everything else alone. If that helps, drill more holes.
Then,
Different venturi. Turn yourself out a .200 ID venturi. Start there and open a little if needed

What prop are you running?
ACP 12.25x3.75
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Offline Dane Martin

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Re: Runaway
« Reply #4 on: July 26, 2017, 07:00:56 PM »
Sounds like a BIG venturi if you got a stock OS nva or spray bar. I might have one I could send you to copy.

Offline Chris Fretz

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Re: Runaway
« Reply #5 on: July 26, 2017, 07:10:28 PM »
Pantyhose, good or bad looking Tim?

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Offline Chris Fretz

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Re: Runaway
« Reply #6 on: July 26, 2017, 07:13:50 PM »
Sounds like a BIG venturi if you got a stock OS nva or spray bar. I might have one I could send you to copy.
It is the big FP40 one.  I have the same set up in my ARF Nobler and it kicks butt, except it has a stock muffler minus the baffle and stinger slightly drilled out.

What size venturi are you running with a stock needle .273?
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Offline Dane Martin

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Re: Runaway
« Reply #7 on: July 26, 2017, 07:52:02 PM »
I ended up with a .250 , same prop and stock muffler in my nobler

Offline Tim Wescott

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Re: Runaway
« Reply #8 on: July 26, 2017, 09:09:34 PM »
Pantyhose, good or bad looking Tim?

It's the right idea.  Don't hesitate to add more if you think you need it -- like Dane mentioned, you've got the BIG venturi on there.
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Offline Target

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Re: Runaway
« Reply #9 on: July 26, 2017, 10:46:37 PM »
See if someone near you has a smaller venturi. Hopefully a much smaller one.....
If the PH helps, especially the more layers the better, then you will know that a smaller venturi is the answer.
I would ditch the muffler pressure at that point, because you won't need it if you have better fuel draw.
But I'm a newbie, and don't know what I'm talking about. (wink).
I'm using a 11x5 Xoar on my LA46, and it seems to like it (on the ARF P-40). Not sure of the venturi size, but I would guess it is between the small and medium stock OS parts. And, I have a 4mm NVA in there. Runs perfect on uni-flow, no pressure.

Good luck regardless of what you do there. Hope you tame the beast

R,
Chris
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Offline Carl Cisneros

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Re: Runaway
« Reply #10 on: July 26, 2017, 11:03:30 PM »
Chris

I sent you a PM

Carl
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Offline Chris Fretz

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Re: Runaway
« Reply #11 on: July 27, 2017, 03:40:37 AM »
It's the right idea.  Don't hesitate to add more if you think you need it -- like Dane mentioned, you've got the BIG venturi on there.
So does the mesh look close to what you mentioned or are we still sure to stay away from pantyhose?
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Offline Chris Fretz

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Re: Runaway
« Reply #12 on: July 27, 2017, 03:41:41 AM »
 So essentially vibration doesn't cause Runaway?
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Offline RandySmith

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Re: Runaway
« Reply #13 on: July 27, 2017, 12:09:59 PM »
So essentially vibration doesn't cause Runaway?

Yes  it can cause  a lean situation, which can cause  run away, its best to try to get vibration to a minimum

Randy

Offline phil c

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Re: Runaway
« Reply #14 on: July 28, 2017, 10:26:34 AM »
Profile fuselages wear out with flights and age.  If you can see any shaking in the prop driver as you move the needle valve the nose is getting soft.  https://stunthanger.com/smf/open-forum/profile-cheek-cowl-damper-box/msg467973/#msg467973       is a post from goozgog of a nose stiffener(damper).  It's best built as a complete nose  replacement especially if you extend it back to the wing spars and tie it(small blocks) to both the leading edge and the spars.

Lots of good advice here.  An LA 46 wants to run at something over 10,000rpm in the air.  A .250 in. or smaller venturi and the 20-25 needle valve assembly are a good place to start..  The stock 3035 muffler works well at slightly higher rpms', much like the stock LA 25 can run.  I use a Master Airscrew 3-blade 10/5 prop on a Brodak Smoothie kit for ground clearane.  MA labels their props by the aerodynamic pitch, not the mostly flat rear lower surface of the blade.  Their 5 pitch is effectively 4.5+/- on a typicial anglular prop guage.

Motors with faster timing do not like to be slowed down this way.  The ASP 32 is a good example.  It wants to run at a minimum of 12-13,000 rpm.  When choked down it will almost always start backward when hand started.  The ASP 36 is not nearly as bad.

Using castor in ABC/ABN engines can cause trouble after a few lean runs.  Castor residue burns on to the piston/sleeve, the piston top and the inside of the head from overheating.  It may only be visible as a faint brown color, which is enough to cause a lot of stickiness between the piston and sleeve.  Soaking in kerosene, brake fluid, WD-40, paint thinner and some more exotic mixtures will take it off overnight.  In an ultrasonic cleaner it may only take 10-20 minutes.  Another way to clean it is by running the motor on all synthetic oil(20-22%).  Don't put a full load of fuel in until the motor starts to clean out and run better.
phil Cartier

Offline Tim Wescott

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Re: Runaway
« Reply #15 on: July 29, 2017, 01:04:37 PM »
So does the mesh look close to what you mentioned or are we still sure to stay away from pantyhose?

That mesh looks OK.  Most of the guys around here that use mesh use what looks like old pantyhose stolen from the wife.  I use a piece of old iridescent purple woven mesh stolen from a kid's craft project.  My comment in the PM was just about the difficulty of holding it on with an O-ring -- not it's suitability once it's on (and no, I did not state that well in the PM, I'm sure).

The tuning goal is to effectively close up the venturi.  Anything that does that is OK, as long as it doesn't cause other Bad Things to happen (i.e., wood shims would work, but might shake loose, RC carbs are infinitely adjustable, but unless the drum to body fit is superlative they can leak in unexpected ways, etc., etc.).

An LA 46 wants to run at something over 10,000rpm in the air.

Just as a point of interest, I launch between 9600 and 9800 RPM, depending on the plane and the prop.  IIRC, from measurements taken in the air, this turns into >10000 in the air.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.


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