I read somewhere on the forum that higher nitro is used when flying stunt in "hot" temperatures. Someone please explain why this is done.

Second question,is higher nitro used when flying at higher altitudes? I normally use 5% fuel at sea level,should I use more nitro at  1200ft above sea level? Thanks,PhillySkip

Skip my understanding, based on where I am Texas, where for each period of the day the temp changes dramatically

Early morning cool air flights vs late afternoon hot air flights--- a shift from the 10% N in the morning to 15% N in the hot air yields about the same engine run

I know for competition flying this rule of thumb has a lot of OTHER different variables, like humidity, pressure density altitude, temp

Each of these, as well as altitude, effect combustion, drag, lift, prop efficiency

All interesting to me; BUT

I sport fly--- so I only really concern myself with using 5 ~ 10% N on morning flights and use the 15~20% N for late hot afternoon flights...just because;

 Tim, Mark, Brett, and others told me to....grin

The combustion of nitromethane releases oxygen into the process, and although swopping out nitro for methanol will lower the overall calorfic value of the fuel it enables more power to be obtained from the resultant mix.

But there is a point of excess where the base calorific value is robbed too far and adding more will simply be worse.

Can you cite any specific case where a engine actually goes slower because of too much nitro, as opposed to an engine blowing up or getting shredded because of too much nitro?

'cuz AFAIK the normal limit is that the engine can't take any more, and it expresses this displeasure by either melting into a blob or fragmenting.

(and to the point of this discussion -- with any modern stunt engine, either of these points will happen well beyond the point where more nitro does not equal more good.  If you have to run 25% nitro to get a good-enough run, you need a better engine!)

Thanks all for jumping in. You've answered my question. PhillySkip

Hi guys,
           I am more than prepared to say that I have the wrong of it but what I base my opinions on is based on what Bil Wisniewski  of speed fame once quipped, that of adding more than 60% had no further benefit.

That and using  more nitro means a progressively  richer mix and unless you drastically reduce compression you will detonate. At 90% nitro are you sure that a glow engine will even start because of extremely low compression needed for survival  and much less run because of hydraulic lock considering that nitro has an AFR of 1:1.7?

There has to be a point of hystersis in an engine that is optimised to actually run through a schedule rather than just hand grenading after 5 seconds of dieseling down 400 meters.

So anyone cite cite a stunt run attempt using even 50% nitro? Was it better than half that percentage?

There  is a bit about nitro in the  setup tips  below, it may help some to read this:


I would like to talk about the basics for setting up your engine to get a better machine. Someone once said a good stunt ship is 40% design 60% motor.
In a nutshell:
  Remember the needle is for setting the fuel mixture, not for setting the plane speed.
The prop pitch is for setting the speed of the airplane.
The prop diameter is for loading the engine properly and getting maximum thrust from the motor.
  Nitro is for controlling how much or how little break you have, this is to say how strong the motor will come on in the maneuvers. The other 2 items that work with this are :
 Compression and  venturie  size. This is not all there is to getting perfect engine runs. Many things work in conjunction with each other to achieving this goal, and most everything I am going to mention affects the others to some degree or another

The 8 basics to helping yourself to a better engine setup are:

1. Engine: Pay careful attention to matching the engine to the airframe, Don’t overpower or under power your plane, make sure your power plant is an acceptable weight for the ship it in and matches well with it, there is room to go bigger or smaller but remember engine weight and power.

2. Correct Fuel: Fuel is one of the most important things in tuning an engine, You must make sure that you have the correct oil type and percentage for the engine your using, example Fox 35s, OS 35s, Older McCoy’s and such need high oil content fuels 24to29% is common percentages. Half castor is good in these type engines as long as percentage is right.
Modern ABC and AAC engines will use much lower oil, 18 to 22 % percentages are the norm here. Nitro percentage is also key to getting the best from your power plant, There are so many ways to run stunt engines it is impossible to print anything but guide lines.
 I have written many times about fuel, you can get a copy of my Care and feeding of a stunt engine from the PAMPA archives, or on Stunt Hangar.
Typical Stunt engines   will use 5% nitro in cold weather going to 10 or even 15% in hot months, You can get in trouble using, say 15% in January, this setup would need 25% or more to be the equivalent in August. So unless you really have this working it is best to setup your engine for lower nitro in cold months and higher nitro in hot months, this will help keeps the run constant thru out the year. Also it will help control power in your motor.

3 .Correct Props: Props are also critical for achieving good engine runs; a prop needs to “load” the motor correctly without over or under loading the motor. It also needs to be the correct size and pitch to pull the airplane. This is an over simplication, but generally the diameter will be what you use to “load” the engine and the pitch will be what you pick to set the plane speed
Some things you will run into when over propping an engine are, Hard to set the needle on the ground, The engine will unload a lot and go rich in the air, The engine will run hot, or not cycle very rapidly.
When under propping generally you will notice the engine will also not cycle correctly, They at times will just 4 stroke thru everything, sometimes going into a 2 cycle at weird times.
A properly loaded engine will use a prop that if running a 4-2 break, will come onto a 2 stroke a 10 O’clock and back to a 4 at 2 O’clock. There are variations on this, you can have a strong motor just beep 2 stroke at the tops of maneuvers, but you shouldn’t have one that 4 strokes uphill and switches to a 2 stroke on the downhill parts of maneuvers.

4. Needle setting: Don’t try to use the needle to set the speed of the airplane; the needle is for setting the fuel-air ratio that goes into the engine. Typical settings are so the motor will be in a very fast 4 stroke when in level flight. If you set the needle to rich it will delay the engine switching and make the engine come on late in the maneuvers. If you set it too lean, you can sometimes run the risk of too much 2 stroke and going sagging lean in the tops of maneuvers, killing your drive and over heating the engine. Once you get this set , you will have  a little lead way in tweaking the needle in or out for conditions.

5. Airspeed: Most all planes like to fly at the airspeed they work best in, even exact or what is supposed to be the exact same design , will a lot of times want to fly at different lap times.  Example I have flown many SV-11s, at the same weight that fly at different speeds. Try to find the optimum speed for the plane your flying. This will depend a lot on the weight of the plane and also the power of the engines

6. Compression: Setting the correct compression for you motor isn’t a simple thing and will vary with nitro and prop size. Try to set the compression so when your engine hits into a 2 stroke it doesn’t come on too hard or too soft, This will work in conjunction with nitro and venturie size. Generally you will use higher compression for low nitro and  lower the compression when using higher nitro fuels.

7. Venturie : Venturie size is one of the keys to get a proper switching 4-2, while not coming on too hard or too soft or too late or too early. If you are constantly accelerating too much when your engine cycles you may need to go down on venturi size. If you are not cycling much or have too soft of a break or a late break , you may need to open the venturi up a size or 2. This will work in direct relationship to nitro and compression.

8. Fuel Tanks: A large portion of engine problem I see are really not engine problems, They are fuel delivery problems that many times relate to fuel tanks, Tanks are maybe the most critical component of your power train. Make sure you have a solid mount, and a tank with no leaks or cracks in the tubing, inside or out. I see many engines either speeding or slowing lap times at parts of the flight, this can many times be a cracked tubing inside the tank, particularly on uniflow tanks.When in doubt I suggest trying a new tank, if there is a difference, you may have bad tank. Don’t forget to use a good filter and make sure there are no holes in the fuel tubing. It is also advisable to try to keep the tank as close to the engine as possible.

9.  Plugs  can also be  a  major cause of trouble, and poor runs.  When you first crank the plane, notice if it goes rich and sags slightly when the battery is removed; if so, the plug is normally too cold.  This is  critical to getting a proper Stunt run. You could say the heat range of the glow plugs sets the "timing" on the engine, A hotter plug also need a hot plug to keep the engine run clean , a cold plug will allow the engine to stumble and burp , and even shut off if your running a deep 4 cycle.

Most plugs are designed to provide a  colder range than we want in C\L aerobatic engines , and you should try to get the right range for the motor. This range is almost always a "HOT" range, Many days of testing and much time and expense buying almost every plug on the market has yielded these results ;Thunder Bolt R\C long, T Bolt #3 , T Bolt 4 stroke, Glo Devil RC #300 long, Merlin, and the McCoy 59  , Enya 3 & 4, Fireball RC long, the Hobby Shack RC long, SIC RC long  a few of the OS hotter plugs and some of the FOX long and  Miracle plugs are best plugs for our use.  In almost all instances, use a long plus, as they will be substantially hotter than the shorts, plus they are deeper in the combustion chamber and this tends to keep things hotter and keeps  the plug elements cleaner, thus keeping the run cleaner.

A lot of times the plug problems show up as rich inside maneuvers and leaner outsides; this happens because gravity and centrifugal force ,forces the oil-fuel charge down on the element on insides, thus cooling the coil and pulls it away on the outside maneuvers, letting it naturally go leaner.  I have seen this problem instantly cured by simply changing plugs. Please  do not be  afraid  to put in a  new  glo plug , or try different types of plugs

These are just basic suggestion to help you tune your engine, and by no means covers it all. There are literally thousands of engine setups that would take volumes to cover, But I hope you can use this as a basic guide to helping get a better engine run.

Regards
Randy Smith

Tim and Brett, there was a 'why' in the subject line which in my own bumbling way tried to address,  and I said that too much vitamin N will make the run worse not slower.

Tim, can I cite where adding more nitro will make an IC engine slower?
Yes, just throttle down an engine that is optimised running huge amounts of nitro to say  10% of its ability, like say a drag car around the pits.
Is it slower to drive, more difficult to keep a constant speed than an engine that is optimised to run 0% nitro at a more open throttle setting?
Stunt engines are throttled devices and running too volitile a fuel  at partial settings will lead to a very narrow tuning range, overheating, thermal runaway and extremely hard starting.

To date I have never heard of a stunt engine employing more than 25% nitro at sea level as a normal diet and that was using a decompressed  Rojett.

And Brett, 'yes' using a hot fuel that is unsuitable will slow you down as every attempt to control it will rob usability.