I have a basic question on the general benefits of higher nitro in a fuel. I do understand that higher nitro offers more power.

Just as an example (say in an OS 25LA engine), if I can achieve a desired ground rpm with 5% nitro and achieve the same ground rpm with 10% nitro (with associated needle adjustment), is there really any additional benefit to the higher 10% nitro from a power perspective?

  Nitro is an oxidizer. It adds oxygen to the mix. In hotter temps and higher altitudes the air is thinner, with less oxygen, so you have to turn the needle in to get a certain power setting so the fuel/air mixture is correct. Adding nitro replaces the oxygen that isn't there, so to speak. If you are flying on 5% at a certain needle setting, and then switch to 10%, you notice that you have to open the needle to get that specific RPM setting and your run time goes down. The nitro is adding extra oxygen and you have to open the needle more to let in more fuel. The extra power you mention comes from the extra fuel that you are letting in with a more open needle setting. The opposite happens if you go down on nitro, and the result is a more closed needle, maybe a bit less power and a longer engine run. It's a common thing to do that if you are a little short on fuel for the pattern, drop down on nitro to get the extra time you need, or add some other combustible liquid such as lantern fuel to the fuel that leans out the mixture (puts less alcohol in your mixture)  so you have to turn the needle in to get the power back, and thus increases you run time on the same amount of fuel.
  Type at you later,
   Dan McEntee

Was also wondering if 10% (vs 5%) at the same ground rpm, during the combustion process, if it is a 'hotter' burn which will increase piston pressure thereby offering more torque on the shaft.

I have a basic question on the general benefits of higher nitro in a fuel. I do understand that higher nitro offers more power.

Just as an example (say in an OS 25LA engine), if I can achieve a desired ground rpm with 5% nitro and achieve the same ground rpm with 10% nitro (with associated needle adjustment), is there really any additional benefit to the higher 10% nitro from a power perspective?

  Potentially - because you are running it richer in the second case, putting in a different place in the response curve. Generally this will make it more responsive in flight. Whether it is better or not depends on where you were before you changed it. The in-flight response is critical and any successful system requires a load response in the maneuvers - 4-2 break, medium 2- hard 2, fat 4 to medium 4, something like that.

   An example, 1999 NATs (I think). Ted and I were running 10% SIG, just like at home. We get to Muncie, and while we were getting the right in-flight speed, with our setup, the engine was running about 50-50 4 and 2, and I melted the front end of my pipe. I had to turn the needle in about 1/2 turn. In-flight performance was rather wimpy compared to what we were used to. We were getting by, until Thursday afternoon, Ted said, "hey, lets try some 15%". Tried it, immediately, the needle went right back to the original position, fuel consumption was back to normal, the engine ran in a 4-stroke almost the entire flight, and it damn near pulled my arm off. At a later NATs, 2001, I think, someone complimented me on my engine run, saying my 4-stroke ran better than everyone else. I had to break it do him it was a PA61 - ran the entire flight in a thundering 4-stroke.

     For the 25LA, I would start with 10%, and make sure that at a satisfactory flying speed it would never fall into a 4-stroke, because its too responsive running that way. If it went over the top lean in the maneuvers, I might try 15% to richen it up.

     Brett

Thx to all for the good info. One often reads simple statements like more nitro = more power. Guess it is not that simple.

Thoght it was releasing hydrogen & oxygen molecules when it ' burnt ' .  Some Dragrace ' fuel ' class info would get all tecnical about it . Hemis going 2000 Hp.  50 years ago on it . MOPAR Rules . !   



Dosnt slow you down , anyway . IF you adjust the chamber volume to suit . Ignition Point is actually crucial on OUTPUT . The modern Wizzers having somewhat more elaborate IGNITION
theyre less prone to meltdown .

This sucker lived up the hill from us , around 1970 . 2000 Hp. rebuilt mid 90s for methanol 1000Hp. Got the season championship . Relinered & iron heads to hold the cracked block together .


Accidently used nitro insted of the second methanol bottle on the three twins in the wideo clip . So was around 60 % . Good ' Bump ' / kick back . Needed a stronger flick & kicked one prop loose .

Speed Turkeys know the how to get the head right trick . Seems running rpm's they go just off full lean ( hopefully ) whereas aerobics ( with a T ) you go rich - more fuel & oxygen . Sounds like a Y block
running well .,

"Speed Turkeys know the how to get the head right trick . Seems running rpm's they go just off full lean ( hopefully ) whereas aerobics ( with a T ) you go rich - more fuel & oxygen . Sounds like a Y block
running well .,"

Speed model aeroplanes typically are VERY rich when released, but it depends on the fuel tank setup. Even more radically rich with a piped engine, and those usually run very rich for laps while building heat in the system. Getting the thing to come 'on pipe' takes a lot of piloting skill. Hold it back (reverse whipping) to get it hot, then whip to get it on pipe, if it doesn't jump to Warp 9 on its own, then scurry like a rabbit to try to keep up.

That was decades after I stopped flying speed. What I found was that high compession gave buckets of TORQUE, and the limit was the piston & glowplug durability. We'd take .040" off the squishband and .040" off the gasket seat of our ST .29rv's and they'd hold up well on 65% nitro. Still in the days with no pipe and wood props. Then, stuff changed $$$$ and I went off to fly F1A for 15 years.    Steve

 I just ran 65% nitro this past Sunday in my Fox 36X BB turning a 8x8 prop and using a baby pacifier as a fuel tank. I was setting the needle running the engine from the rich side, I pinch the fuel tubing to here the RPM peak up. I noticed that with the high nitro the engine was slow to spool up and would even cut out when I gave a little pinch to the fuel line. It’s a little tricky to set, I have to turn the needle in real slow to peak and back it up richer. When I use 10% nitro the engine spools up quick with a little pinch of the fuel line and is easy to set the needle. It’s like hitting something with a sledgehammer compared to using regular hammer, slower to pick up but it hits with more power at impact.
Al

, and suffered a lot of blown seals. No ice cream involved, if you've heard that old joke.  Steve

Oh!
Nice!!

Tell the joke, I don’t know it. 👎

It is not a PG joke.