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May 23, 2019, 02:25 PM
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DPlumpe's Avatar
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Why does an engine backfire and throw off its prop in flight?


Just recently my OS FS-48 Surpass threw off its prop twice while flying. Why?

I bought it used back in 2012 and have put about 200 flights on it since then. It's mounted inverted on a 72" span Fairchild PT-19 based Chuck Hollinger's design, drives a 12x6 MA prop, and drinks 12% Coolpower with Klotz added to bring oil to 20%, with an OS type F plug. Compression, bearings and valve lash are fine.

I've been flying RC since 1955 and I don't remember seeing any other engines throw props while in flight. I have seen many throw props when starting, and I've assumed the engines were over-primed.

It's possible my engine was a running a bit rich. I didn't check the needle valve before the flight and the change of seasons (colder to warmer here in the NW corner of S. Carolina) would call for slightly less fuel. Unfortunately, I can't easily check it by flying now since the backfire tossed off half the 2-piece fiberglass cowl, formerly attached with magnets, and it'll be awhile til I lay up another.

So what do you think - was it just that it was an over-rich mixture in an inverted engine, or are there other possible causes?

-Dave
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May 23, 2019, 02:27 PM
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You set the needle too lean.
May 23, 2019, 03:33 PM
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turbonut's Avatar
Too lean or over heating is common cause..Not tight on the prop nuts..Must be tighter than you would expect..Wrong glow plug is possible..
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May 23, 2019, 03:42 PM
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Thanks for that, guys. Makes sense when you think of it as preignition. It just contradicts my long-held assumption of overpriming. Time to change my assumption.

-Dave
May 23, 2019, 03:45 PM
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1QwkSport2.5r's Avatar
Too lean. The backfire comes from the advanced ignition due to the lean mixture and the backfire causes the prop nut to come loose and the prop departing the plane prematurely.
May 23, 2019, 03:53 PM
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mike3976's Avatar
Too rich low speed and too lean high speed is the common error
May 23, 2019, 07:09 PM
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Had a magnum 25 throw a plug in flight once . Some idiot forgot to tighten it up , Guess who the idiot was ?
May 24, 2019, 07:59 AM
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Cougar429's Avatar
You don't mention if this happens if throttled up on the ground. Also, if the engine is exposed or cowled.

From personal experience have to agree too lean, most likely on the High Speed needle. If too lean on LS it will usually just slow down and stop at lower throttle settings. Overheating should cause the engine to drag and quit. Have to recommend if this happens to open it up and have a look. Overheating can also cause low lube conditions where internals can be damaged. Had one where the rod physically welded to the crank pin.

My first 4-stroke was a Saito 91 and I found this when firing it up to break in and set mixture. At high RPM it came to an IMMEDIATE stop and the prop ejected with high energy. Never did find the double lock nut setup, nor the Higgley hub next used to hold on the prop. Once the HS needle was richened up and set correctly put close to 350 flights on it before selling the engine/airframe.

That risk of lean stops is the reason I cringe watching vids where anyone is within the half arc danger zone ahead of the prop and why I usually speak out if seeing this in person.
May 24, 2019, 08:53 AM
Registered User
Sounds like you got your answer, but I'd be inclined to try a 12x5 also, which will probably give you more thrust on that engine as well as make it a little less prone to preignition.

Jim
May 24, 2019, 09:02 AM
Ik heb het niet gedaan!
Gary Cee's Avatar
Quote:
Originally Posted by the pope
Had a magnum 25 throw a plug in flight once . Some idiot forgot to tighten it up , Guess who the idiot was ?

Nice try but,



I've never been in Australia!
May 24, 2019, 10:10 AM
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DPlumpe's Avatar
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Cougar-

In my nearly 70 years of running model engines and probably around 10,000 flights, I've never lost a prop on the ground and, before this, never lost one in flight, except once with a FS-26 which I'd just switched from a plastic MA prop to a wood one and likely didn't tighten enough to account for the softness of wood.

My FS-48 in the PT-19 is cowled, with pretty decent cooling, and has never shown signs of overheating, even in the past six S. Carolina summers. Air temperature at the thrown-prop incident was in the low-70s, humidity unexceptional.

The first time it threw was right after takeoff, just clearing the end of the runway, so it hadn't yet reached full operating temperature. I retrieved the prop and hardware (lucky!) and when my flight turn came again flew for several minutes before it decided to burp again. This time was over a cornfield and prop, hardware and cowl top were lost.

Since the unanimous advice is it was too lean, and that agrees with rational thought, I'll change my uninformed "too rich" prejudice and be more careful with needle settings on my 4-stroke engines.

Thanks to all
-Dave
May 25, 2019, 06:14 AM
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earlwb's Avatar
Well I had this happen once when the glow plug was loose. On take off right after the plane left the ground, the engine stuttered and quit. The prop and spinner spun right on off too.

In the hot summers, some days have high temperatures and low barometric pressure. That makes it appear to the engine that is at something like 10,000 feet altitude or even higher. The engines just do not like to run like that. You just can't seem to find a needle setting to get them to run OK. You windup going extra rich as though the engine is just being broke in. The high temperatures sort of act to increase compression as it acts to advance ignition timing causing detonation. The low pressure acts to starve the engine of some oxygen for combustion too. Thus the two things act against each other inside of the engines. You try to go more rich to retard the ignition/combustion due to the higher temperatures but the low pressure doesn't have enough oxygen for combustion so you want to go more lean to reduce the amount of fuel to balance the air to fuel ratios. Then of course a 100 feet or so of altitude could change the temperatures and pressure making it even more complicated.
May 25, 2019, 06:43 AM
Registered User
Quote:
Originally Posted by earlwb
Well I had this happen once when the glow plug was loose. On take off right after the plane left the ground, the engine stuttered and quit. The prop and spinner spun right on off too.

In the hot summers, some days have high temperatures and low barometric pressure. That makes it appear to the engine that is at something like 10,000 feet altitude or even higher. The engines just do not like to run like that. You just can't seem to find a needle setting to get them to run OK. You windup going extra rich as though the engine is just being broke in. The high temperatures sort of act to increase compression as it acts to advance ignition timing causing detonation. The low pressure acts to starve the engine of some oxygen for combustion too. Thus the two things act against each other inside of the engines. You try to go more rich to retard the ignition/combustion due to the higher temperatures but the low pressure doesn't have enough oxygen for combustion so you want to go more lean to reduce the amount of fuel to balance the air to fuel ratios. Then of course a 100 feet or so of altitude could change the temperatures and pressure making it even more complicated.
this goes against all tuning advice ive ever seen posted.

hot weather low pressure means less oxygen, which means less fuel to get the CORRECT fuel air mix.
May 25, 2019, 07:13 AM
Registered User
I have to agree.

High temps = less oxygen in the air = less fuel required = less power.

Cool temps = more oxygen = richer mixtures = more power.
May 25, 2019, 07:18 AM
Registered User
Yes, hot weather, lean the needle, cold weather, richen it.

Strange problem, I've never had it happen either, but I would try a richer mixture and a little less prop load. I think a 12x6 is on the high end for that engine. A 12x5 will give you better thrust anyway.

Are you sure the low needle isn't too lean? If it is, then it affects the high range too, and it can be hard to richen it with the high speed needle.

Is the tank in the right position relative to the carb?


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