nitro model engine factors

On 4/12/2004 1:01 PM Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:

What are you trying to maximize the performance for ? Racing, pattern flying, etc ?

For instance, For racing, you are not concerned about engine longevity, just maximum rpm/speed. For pattern, you are looking for consistent, reliable performance.

So far the general idea is correct, however, the humidity on any given day will also effect the performance of the engine (see comment under fuel). I have had excellent results with OS#8 plugs for general sport and fun fly flying at sea level.

You said you have a Leo. This is primarily a European engine. If memory serves me correctly, European engines are designed to run on FAI fuel (0% nitro) to a MAX of 5% nitro. To use a higher percentage nitro, you will most likely have to add head shims (extra head gaskets) to lower the compression (or risk burning a hole in the piston).

You also need to be aware of your altitude. An engine that performs a certain way on 5% fuel at sea level will require 15% - 20% nitro at

3,000' - 4,000' (and usually a different plug) to perform the same way due to lower air density. Humidity also plays a big part. In humid conditions, not only is the air denser, but the moisture in the air helps to cool the fuel mixture.

AND/OR a smaller diameter and pitch prop.

Still on track (in general terms)

I am not familiar with MCP or "Kraut" mufflers, but this is pretty much correct. You do need to be careful that you maintain adequate back pressure for pressurization of the fuel tank, or run a fuel pump.

There may SEEM like excessive oil in the exhaust, but it is a safety measure. In case of a "lean run" it will help to minimize or eliminate damage to the engine. The manufacturer usually states the minimal percentage of oil the engine should have. I have found you can SAFELY go 2% or 3% less. I would stick with the manufacturer's suggested percentage of oil, at least until the engine is completely broken in.

If noise is a minimal concern, remove the internal baffle in the muffler. That will usually provide extra power while still maintaining a good back pressure for the fuel tank.

There is an excellent article in Fly RC (Premiere Edition) on tuning your engine. It covers several of my comments in detail.

You've found one of the clues to getting the best out of an engine, and that's the plug heat range. Going to a hotter plug advances the ignition point and seeing you gained revs then you're closer to optimising for the particular fuel/compression ratio of your engine. Basically it means that with your original medium plug you could have either increased the nitro content or raised the compression with the 10% fuel. If you now tried a hot plug and saw a rev DEcrease then you'd know that the combination of fuel/compression/medium hot plug was nearing the ideal.

Remember with plugs though that some manufacturers may only provide 3 plugs from hot to cold while others may have 12 or more in their range. To confuse it even more, a medium plug from one manufacturer won't necessarily be the same as a medium plug from another.

As far as oils go, so long as you don't go to ridiculous extremes then oil has no affect on the power produced.

Brian Hampton Adelaide, South Oz

I would like to optimize for thrust without spending too much money (and keeping it light).

So during hot and humid midwest days I should see higher rpm?

Of course in those conditions airfoils generate less lift so actually overall performance actually decreases.

It seems excessive compared to my past engines (all lasted 5+ years). The manufacturer recommends 17% oil, so having 80/20 synthetic/castor

18% blend seems much above recommendations. I'm going to use it for the first gallon and then switch down to 16%. If I pick 200rpm for each little optimization it will add up to something noticeable. That is my strategy.

This one doesn't have a baffle.

thanks, I'll try to look it up.

Michal

Nitro is used in engines to reduce the amount of oxygen needed to combust a given volume of fuel. High reving engines need lots of nitro because they can't get enough oxygen to burn all the fuel.

If you want lots of power you need to burn more fuel, that requires delivering a lot of oxygen to the engine. If the carb is small, then you can't get enough oxygen, but if you substitute methanol with nitromethane, you can burn more of that mixture.

In this case you have plenty of oxygen to burn so you can use close to straight methanol for more power.

FAI speed and FF run happily only when throttling isn't required. They are optimized for high rpm only and suck at everything else.

Seemed like more than my other engines have in their exhaust. Maybe because it was a cool day the exhaust oils felt thicker. I wouldn't go radically lower, just 1-2%.

Thanks for oil info.

Michal

I agree 100% on plugs. On my old OS40fp with 11x4 APC, I got 600rpm increase by going to a hot 4-cycle plug. I know it didn't suffer from preignition sice that engine lasted 6 years :)

Michal

Sorry Ted, but higher humidity means the air is *less* dense. Water vapor is invisible and is *lighter* than air. When the water vapor in the air condenses it becomes fog/clouds. Most think that since water is heavier than air, moist air must be more dense than dry air.but that is not correct. Increased humidity increases the density altitude and that means longer TO distances and poorer engine performance... Took me awhile wrap my brain around that fact - almost ran out of runway on a hot, humid day and that convinced me. Ed F.

The problem is heat, not water vapor.

Takes more heat to increase the water content of the atmosphere, and higher total heat (latent and sensible) in turn raises the Brownian motion, pushing the individual molecules farther apart.

Don't get wrapped around the humidity thing too tightly, since dew point is only part of the density altitude equation.

A google search with turn up bunches of density and pressure altitude calculators, equations, and dissertations. Cheers, Fred McClellan The House Of Balsa Dust

Hi Fred, I'm not sure I understand you point. I was responding to Ted's statement "...In humid conditions, not only is the air denser..." that statement isn't correct.- Temp 90°, humidity 80%, performance will be poorer than 90°, 40% humidity. That's the way Uncle Sugar teaches it. :-).

Here is the layman's reason for why humid air is less dense than dry air.

Humid air is less dense because water is H2O which has 2 hydrogen atoms with an atomic weight of 2 each plus one oxygen atom with an atomic weight of 16 for a total of 20. An oxygen molecule on the other hand is O2 or 2 atoms with an atomic weight of 16 for a total of 32 vice 20 for water. The other molecules that make up air are also heavier for the most part and the water vapor displaces them thus making the air less dense. Less dense air makes for poorer performance for both engines and wings.

Density altitude is a function of pressure altitude with temperature and humidity (dew point) figured in. Being a meteorologist, I get to play with this stuff all the time. Just ask a full scale helo pilot about PA/DA, I'm sure he/she can give you an earful. Higher temperature and humidity with a given pressure altitude equals higher density altitude and thus poorer performance.

Jim W

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