Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

What fuel to use?

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Old 03-16-2018, 08:30 AM
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Planedriver59
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Default What fuel to use?

I built a diamond dust delta wing with an os max V (40) ducted fan engine. What is the best fuel to use for max rpm and easy tuning? All I had was 30% heli fuel and it was nearly impossible to tune and kept burning out glo plugs. I ran it on 15% sport, it ran good but not the screamer I was hoping for. The local hobby shop has some 20/20 should I try this? Any help would be much appreciated.
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Old 03-17-2018, 02:10 AM
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You may only get marginally better results with more Nitro. 20% Nitro won’t do much better than 15% will. Go for some
Heli fuel that’s 30% Nitro and try that. If you don’t get the results you’re looking for, order a Dub Jett engine. OS engines aren’t much to sneeze at these days.
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Old 03-22-2018, 06:32 AM
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JohnBuckner
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You are running an appropriately sized tunned pipe with that engine ??
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Old 03-23-2018, 10:37 AM
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I flew a Diamond Dust with an OS 46 VRDF and used the APC CF Q40 pylon props on it - 7.4X8.25 I think was the size. It turned @22K on 10% and flew the DD very well.

I bet the same prop would be fine on the 40 VRDF.

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Old 03-24-2018, 03:59 PM
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The prop is it...what are you running?.
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Old 04-07-2018, 10:30 AM
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Without knowing the prop and the exhaust set-up, hard to be much specific help.

Impossible to tune and burning out glow plugs are two primary symptoms of a piped engine with too much prop load, i.e. pipe too short for the prop. With DF engines propped for sport speed RC use, this usually means the user has over-propped the engine. The engine essentially goes lean, cranky and hot as it tries to stage onto the pipe but can't, and they don't like that - overheating, burned plugs. You are also describing symptoms of over compression.. not that different in behavior and destructive ability. Reducing nitro may improve manners compared to 30% but I'd bet it isn't the root cause of your issues, just the equivalent of a couple of Tums on a sore stomach.

One thing that won't cause that kind of grief is under-propping then working up. That's effectively what you're doing when you bench test a tuned system then incrementally shorten the pipe to max rpm. You are raising the tuning point on a fixed load until the engine says "whoa, we're there" when the rpm plateaus. Starting under-propped on a fixed exhaust then adding prop load is not the typical approach, unless you are a competitor or a sport speedy guy with a collection of piped engines and a grab bag of appropriate props. However, functionally, both achieve the same result by starting from the safe side of the pipe/prop relationship and working towards max rpm. Whether that is max static or max unloaded in the air is an application-specific choice, think speed/pylon versus pattern for example.

C/L stunt fliers learned how to use the governing effect of exceeding the tuning point of a tuned pipe system by running the pipe on the long side.. the idea being to have the pipe at peak resonance when the aircraft needs maximum pull in vertical lines, then progressively over-running the pipe as the load decreases in level flight or more so in downlines. That pushes the mixture rich and clamps down the rpm, providing a kinda more modern approach to traditional 4/2 stroke operation.
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Old 04-09-2018, 01:08 AM
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Add a couple of gaskets to the head and lower the compression. This was standard procedure for flying Formula One back in the day.
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Old 05-25-2018, 06:58 AM
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Don't forget on a ducted fan engine you need to make sure you're running about 23% oil or you're going to have issues as well.
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Old 09-08-2018, 04:28 PM
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Originally Posted by GhostofAuger View Post
I flew a Diamond Dust with an OS 46 VRDF and used the APC CF Q40 pylon props on it - 7.4X8.25 I think was the size. It turned @22K on 10% and flew the DD very well.

I bet the same prop would be fine on the 40 VRDF.
Just for reference, a good, current big flange Nelson Q40 engine will turn a current APC 7.4x7.5 prop at about 26.000 to 27.000 rpm static. On 15% nitro/18% synthetic oil.
They are $425 new...it may be beneficial in the long run to just get a new engine and stop burning plugs and messing with pipes. Set exhaust timing to about 197-198 degrees. These things will blow your mind with user friendlyness and plug life...if you don't run 'em lean...one lean run will take hours of life out of it.
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Old 09-08-2018, 07:48 PM
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Agreed... Kind Of. The only problem is the current nelson Q40 engine, the monster that it is, has no throttle. I enjoy a ducted fan engine on a speed plane for its ability to throttle it. I also like my many Dub Jett engines for the same reason. Also agreed on the prop load. Put less prop on the OS fan engine, keep the 30% in it, use a cold plug, and fly the crap out of it.


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