Author Topic: About stunt engine runs...........? (Read 1823 times)

Terry Caron · « **on:** June 13, 2014, 07:07:32 PM »

Once again, I may start out asking the wrong question 'cause I'm not sure exactly what it is I want to know.
But it's about a proper stunt engine run, so I'll try this for starters:

For years the Fox .35 in a 4-2 break was a favorite, and still is today for many.
Apparently many also run other engines in 4-2 with success.
Does the Fox do it best?

These days OS 20 FP and .46 LA schneurles are often advised, with a high rpm/low pitch "constant speed" run.
(Some, perhaps notably the OS .40 LA, not so successfully.)

Will cross-scavenged engines not successfully run similarly?
Such as an Enya .35/10x3.5 prop, maybe.

Terry

Brett Buck · « **Reply #1 on:** June 13, 2014, 07:46:50 PM »

Quote from: Terry Caron on June 13, 2014, 07:07:32 PM

These days OS 20 FP and .46 LA schneurles are often advised, with a high rpm/low pitch "constant speed" run.
(Some, perhaps notably the OS .40 LA, not so successfully.)

The are not remotely "constant speed". In some cases the change is far too much. It just doesn't change "phase".

You know that when a 4-2 break motor goes into a 2-stroke, it's actually slower than it was when it was 4-stroking, right? That's *why* it went into a 2-stroke in the first place.

Quote

Will cross-scavenged engines not successfully run similarly?
Such as an Enya .35/10x3.5 prop, maybe.

It will run fast enough to get a decent airspeed, but it will very likely not have enough overhead to properly speed up/slow down in the maneuvers. It feels "dead".

Brett

Terry Caron · « **Reply #2 on:** June 13, 2014, 08:04:32 PM »

Quote from: Brett Buck on June 13, 2014, 07:46:50 PM

You know that when a 4-2 break motor goes into a 2-stroke, it's actually slower than it was when it was 4-stroking, right? That's *why* it went into a 2-stroke in the first place.

Brett

Brett, I'm a prime example of those who don't know a proper stunt run -
I did not know that - I thought the engine sped up in 2-stroke going up in a manuver and slowed down going "downhill" to approximate a constant flight speed.
And I thought a ".46 LA run" was trying for the same end.

Terry

RknRusty · « **Reply #3 on:** June 13, 2014, 08:19:28 PM »

Terry beat me to it while I was figuring out how to say about the same thing. I didn't know that either. So I just listened to a video of my fp35 four stroking(or wet stroking) around the flats, turning to a solid two during the climb into loops and overheads, and turning back into a wet stroke(four?) on the dive back out of the loops. Not as pronounced as a Fox, but clear and timely, nevertheless. I assumed the same thing, more thrust in the climbs, less in the dives.
Still got a lot to learn.
Rusty

RandySmith · « **Reply #4 on:** June 14, 2014, 01:08:32 PM »

MANY ENGINES do speed up and gain RPMs when they break into a 2 cycle, some engines are setup to where they actually slow down when they switch to a 2 cycle.
My setups will gain RPMs when switching to a 2 stroke, but the gain is slight.
It is also possible to get a huge gain in RPMs when switching to a 2 stroke.. which is why you will see some poorly setup engines go into warp speed and wind up very badly

When it comes to stunt run setups, you cam run a 4/4 , a 2/2, or a 2/4 .... in extreme winds I have ran a 4/8 setup...by force :-)

Randy

rich gorrill · « **Reply #5 on:** June 14, 2014, 02:05:10 PM »

I have been fooling around with an OSFP35 in a Fran Twister. With a 10/6 prop I got terrible run away. Switched to a 10/4 and things got better, more consistant run with wet 2 stroke. Tried adding one extra head gasket, set a little wet and got a pretty hard 4-2 break on 10/22 powermaster fuel. Maybe with 5/22 fuel the break will be a little softer. Biggest problem was the fuel consumption went way down. I was getting 5 1/2 minute with 3 oz. It went down to just over 4 min. That's why I call it foolingaround. The tank is a 4 oz. uni-flow.

Terry Caron · « **Reply #6 on:** June 14, 2014, 02:36:24 PM »

Quote from: RandySmith on June 14, 2014, 01:08:32 PM

It is also possible to get a huge gain in RPMs when switching to a 2 stroke.. which is why you will see some poorly setup engines go into warp speed and wind up very badly

Randy

Is this the downfall of running most engines (such as an older baffle) in a lg diam/lo pitch/hi rpm 2-stroke run like an .20FP?
Is it a matter of luck (or modding) whether one will run well in a 2-4, 4-4, 2-2 or whatever?

Terry

RandySmith · « **Reply #7 on:** June 14, 2014, 03:11:32 PM »

Quote from: rich gorrill on June 14, 2014, 02:05:10 PM

I have been fooling around with an OSFP35 in a Fran Twister. With a 10/6 prop I got terrible run away. Switched to a 10/4 and things got better, more consistant run with wet 2 stroke. Tried adding one extra head gasket, set a little wet and got a pretty hard 4-2 break on 10/22 powermaster fuel. Maybe with 5/22 fuel the break will be a little softer. Biggest problem was the fuel consumption went way down. I was getting 5 1/2 minute with 3 oz. It went down to just over 4 min. That's why I call it foolingaround. The tank is a 4 oz. uni-flow.

Are you running the stock non free flowing muffler with the extremely tiny outlet?
Randy

RandySmith · « **Reply #8 on:** June 14, 2014, 03:31:59 PM »

Quote from: Terry Caron on June 14, 2014, 02:36:24 PM

Is this the downfall of running most engines (such as an older baffle) in a lg diam/lo pitch/hi rpm 2-stroke run like an .20FP?
Is it a matter of luck (or modding) whether one will run well in a 2-4, 4-4, 2-2 or whatever?

Terry

Terry , that depends entirely on "which" motor, some , but very few older baffle engines will turn the RPMs to use the low pitch of a more modern loop charged engine, so as a result you use more pitch and less RPM, many find trouble when using modern loop charged motors with high pitched props, as they make much more power and have extra om-mph to convert into a much larger difference in break RPM s and you get the run-a-way. So the lower pitch prop help, as you need to run the harder and use up much more of the power the engine has, and as a benefit from the higher RPMs, you get increase tension and better drive thru the manouvers

Randy

Brett Buck · « **Reply #9 on:** June 14, 2014, 03:58:14 PM »

Quote from: RandySmith on June 14, 2014, 01:08:32 PM

MANY ENGINES do speed up and gain RPMs when they break into a 2 cycle, some engines are setup to where they actually slow down when they switch to a 2 cycle.

Not immediately. The mechanism for breaking is that the load on the engine is increased, and this only happens when the airplane slows down causing the engine to bog down a bit (because the prop take a bigger bite). Later, it usually overshoots and does wind up faster, due to thermal effects.

BTW, it's not because of fuel pressure. The first thing that happens in a corner is the fuel pressure at the engine end of the system rises dramatically, due to the deceleration.

The mistake most people make is to interpret the 4 stroke or 2 stroke in the air as having the same effects as screwing the needle in and out on the ground. This is easy to disprove. Start a Fox 35, set it to a solid 2-stroke on the ground, measure the RPM. Launch it, and if everything is normal, it will go into a 4 stroke when the speed stabilizes. If you then measure the in-flight RPM, it's 800-1000 faster. And, the fuel pressure (with a conventional tank setup) is lower in the air than on the ground.

Brett

RandySmith · « **Reply #10 on:** June 14, 2014, 09:37:53 PM »

Quote from: Brett Buck on June 14, 2014, 03:58:14 PM

Not immediately. The mechanism for breaking is that the load on the engine is increased, and this only happens when the airplane slows down causing the engine to bog down a bit (because the prop take a bigger bite). Later, it usually overshoots and does wind up faster, due to thermal effects.

BTW, it's not because of fuel pressure. The first thing that happens in a corner is the fuel pressure at the engine end of the system rises dramatically, due to the deceleration.

The mistake most people make is to interpret the 4 stroke or 2 stroke in the air as having the same effects as screwing the needle in and out on the ground. This is easy to disprove. Start a Fox 35, set it to a solid 2-stroke on the ground, measure the RPM. Launch it, and if everything is normal, it will go into a 4 stroke when the speed stabilizes. If you then measure the in-flight RPM, it's 800-1000 faster. And, the fuel pressure (with a conventional tank setup) is lower in the air than on the ground.

Brett

Another mistake people make is thinking the "in air" RPM in a 2 cycle does not increase over the same 2 cycle on the ground. The 2 cycle RPM speed does increase just as the 4 cycle does.

Randy

Author Topic: About stunt engine runs...........? (Read 1823 times)

Terry Caron

About stunt engine runs...........?

Brett Buck

Re: About stunt engine runs...........?

Terry Caron

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RknRusty

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RandySmith

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rich gorrill

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Terry Caron

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RandySmith

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RandySmith

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Brett Buck

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RandySmith

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