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Old Dec 01, 2012, 04:06 PM
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how to get more speed from gas engines?

i am new to gassers and i have a few questions regarding speed. i have a 50cc great planes revolver. it calls for a dle 55 motor. i have seen this plane fly on this motor and it does about 120mph.. i was wondering how people get more speed out of gas airframes. do i go up to a larger motor, make a 85cc. an info will help
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Old Dec 01, 2012, 08:26 PM
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Maybe a larger engine, but it depends on if the engine will fit OK, and whether the CG is still Ok. Plus the plane needs to stay streamlined as well. You can lookup what they do to modify two stroke motorcycle engines and those techniques apply to the RC gas engines as well.

Basically they do things like shave the base of the cylinder a little bit to increase compression and advance the port timing a little bit too. Then there is porting and polishing the intake and exhaust passages. Check the reed valve block and make sure it is clean and smooth as well. A larger bore carb, advance ignition timing maybe, a better exhaust system such as preferably a tuned exhaust system. But if the engine isn't all that good to start with, you may be looking at a better connecting rod and a better crankshaft too.
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Old Dec 01, 2012, 10:52 PM
TigreJohn
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The Tim Taylor mentality is apparently still around. In my humble opinion, the power recommendations for most airframes, if not all, is too much power. In your case, a 120 mph or more Revolver is unnecessary. Its an aerobatic airframe, not an oversized pylon racer. 50cc is more than enough for this airframe. You want thrust, not speed for this plane

As an example. I fly a GP Super Sportster which calls for a 40-46 engine. I am flying it with an old OS .32SF. With a low pitch prop, this sucker with climb out of a hover.
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Old Dec 02, 2012, 12:00 AM
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I think there is a phase most people go through where they tend to overpower planes, and also see how fast they can get something to go as well. Later it changes and having enough power to fly the plane like you want to is enough.

I forgot to mention that if you are going to over power the plane, you will very likely need to reinforce it a lot too. The control surfaces need to be carefully setup to minimize the air gap and also to help prevent flutter. The tail needs to be reinforced so you don't tear it off in a power dive too.
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Old Dec 02, 2012, 12:13 AM
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i have two revolvers already, the 50 and 70 version. they are both setup with more powerfull electric setups than what is recommended..the 70 does 140mph level and the 50 does 134mph level...both with no reinforcement..the revolver is a good beefy airframe that doesnt require any reinforcements.

i only ask these questions because i dont feel like setting up and testing different electric setups for a 50cc size revolver..i have always like the simplicity of gassers and just want to know if i can match my electric setup speeds with a gasser.

a revolver its not really an aerobatic ship if you've ever flown one. and seeing one cruising around doing only 95 or 100mph doesnt look good..my revolver is not even the fastest one at my field, we have a nitro one that is faster....
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Old Dec 02, 2012, 06:53 AM
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For more forward speed (at the expense or some reduction in thrust), you can always opt for a smaller diameter prop, with a higher pitch.

All things being equal, these engines turn about the same RPM and this one will do 6,400-7,200 with most suitable props.

A 20x14 prop will give you a lot of forward speed.
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Old Dec 02, 2012, 08:12 AM
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I had forgotten about it....but the unlimited giant scale pylon racing folks tend to convert their bigger racing engines over to run methanol and nitromethane fuel instead of gasoline. They also tend to go with fuel injection too. But their planes are larger and they are using 170cc modified DA engines or something else now.

But anyway converting the engine to burn glow fuel is one way to boost the power output.
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Old Dec 02, 2012, 08:39 AM
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Originally Posted by earlwb View Post
I had forgotten about it....but the unlimited giant scale pylon racing folks tend to convert their bigger racing engines over to run methanol and nitromethane fuel instead of gasoline. They also tend to go with fuel injection too. But their planes are larger and they are using 170cc modified DA engines or something else now.

But anyway converting the engine to burn glow fuel is one way to boost the power output.
I would not subscribe to that, Earl.

Walbro carburettors contain gaskets and membranes that are incompatible with methanol.
Within a few months of use, these items will deteriorate and become mush...

Besides, to increase speed two-fold; you need to provide eight times the horsepower. Yes, eight times more.
Methanol will in theory increase power by ~24% over gasoline. The speed increase that will result will only be about 7.7%.

With all other things remaining the same (prop, design, ambient conditions, Etc.), if your model was doing 80 mph, it'd now do 86 and use fuel costing nearly 10 times as much to do it. You'd also need 2.5 times the fuel capacity, for the same flight duration...
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Old Dec 02, 2012, 09:00 AM
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Very good points Dar.
Your post slaps you back to the reality of how difficult and expensive extra speed is.

Greg
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Old Dec 02, 2012, 10:01 AM
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glow is perfect for speed, but its so messy and expensive and i would rather stay electric than go glow. i am familiar with modding 2 stroke engines but would like to stay away from modding the engine..

so would going up in engine size provide more speed and rpm's on a similar prop?
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Old Dec 02, 2012, 10:06 AM
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kremecheze, back to your original post. Yeah the 85cc engine would work as a power boost. The issues would be the weight and whether there is enough ground clearance for the larger propeller or not. Weight may or may not be a issue. Especially with the CG on the plane.

A DA 50R weighs about 2.94 pounds or 1,492 grams. A 22x12 prop for speed looks promising.
A DA 85 weighs about 4.3 pounds or 1,950 grams. A 26x12 prop for speed looks promising. So it boils down to whether it would fit OK inside of the cowl or not, whether the prop would clear or not, and whether the CG can come out OK or not.

Walbro makes a methanol carb too. The WT-499 alcohol carb for racing. So there are parts one can get to take care of the diaphragms deteriorating in a methanol environment. But if you use a gas carb and convert it, you'll need to do something about the jets and making them larger. Plus in some locales they use E85 fuel and it plays havoc on all engines thsat aren't setup for it.
The regulator side of the carb doesn't appear to be as bothered by alcohol as much as the pump side and Walbro makes a blue plastic diaphragm and a grey or white color teflon diaphragm for their carbs to handle the alcohol better.

Yeah years ago, I had been there and done that with pylon racing.



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Old Dec 02, 2012, 10:08 AM
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Originally Posted by DarZeelon View Post
I would not subscribe to that, Earl.

Walbro carburettors contain gaskets and membranes that are incompatible with methanol.
Within a few months of use, these items will deteriorate and become mush...

<<snip>>

Dar,

We started getting ethanol in pump gas in Calif over 10 years ago to reduce smog in large urban areas. Walbro re-engineered the gasket materiel to an alcohol safe formulation, so all Walbro carbs (in the U.S. at least) are now alcohol safe, and you can get an alcohol safe carb kit for just about any older carb.
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Old Dec 02, 2012, 10:30 AM
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S3NFO,


OK, so the methanol sensitivity issue is solved, I understand, by using a compatible Walbro carburettor (or just retrofitting the 'methanol soluble' items within it).

But, as I wrote in the part you snipped, that is the least of the problem...
Using methanol on a daily basis is far from being cost-effective.
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Old Dec 02, 2012, 10:58 AM
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Quote:
Originally Posted by DarZeelon View Post
And, as I wrote in the part you snipped, that is the least of the problem...
Using methanol on a daily basis is far from being cost-effective.
It is not that bad.... Methanol, at least in large parts of Europe, is cheaper than gas, and when you convert a big gasser, that means you also only need oil contents as for a big gasser....

I pay roughly 2 Euro 55 for one liter of 2-stroke mix gas (5% oil), and I pay roughly 1 Euro 80 for one liter Methanol wit 5% oil.

So yes, it is still more expensive to run Methanol, but the golden rule in engineering is:
Twice the power is four times the money.

And there's simply no way around that, even if you would just use gas, it would be more expensive in higher fuel cosumption, cost of tuning an engine/tuned parts/bigger engine....

Personally, I think changing over to Methanol is infact the cheapest way to get more power out of the same engine, and most definitely the cheapest way to get more power into your plane.

Brgds, Bert
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Old Dec 02, 2012, 11:08 AM
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If you want more speed out of a gas engine and maintian the low cost of gasoline? Run 92 octane, A high quality synthetic 2 stroke oil at 45-50:1 and as someone already stated a smaller prop with more pitch. That would be the easiest and quickest way to see the improvment.

DLE 55 loves a 20-14 or 20-12 for more speed.

If still not satisfied with it's performance then your going to spend alot of money only to go just a little faster.

A larger engine was suggested and I do not recomend this as it will increase the wing loading and make landing a bear and decrease the over all speed due to increased weight!. Power to wieght ratio is key. Keep it light, Keep it simple, Enjoy the hobby!

I have a DLE 20 in a H9 P-47 running a 4 blade 15.5 X 12. I snipped about an inch off of each blade and reballanced. The 4 blades makes it sound awsome! and the 12 inch pitch pulls it through the air around 80MPH.
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