Motor Set-up Tips v. Randy Smith

[Shoestring]

I have been asked many times lately to explain how to setup an engine for a good “stunt run”. All to often I see motors running backwards from the ideal. Four stroking up hill and breaking into a 2 stroke downhill, going blubbery rich on insides, screaming lean on outsides, etc, etc.
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

 

[Shoestring]

OS LA 46 Tips von Brett

Question: The 4-2 break is sweet. Any hard and fast rule for keeping it under varying temperature conditions? E.g., cooler temperatures would require a bit of needle adjustment in what direction? Or am I over thinking this?


Generally, cooler air is thicker, with tends to make it go lean, so you would need to open the needle to get the same "setting". Hot air is thinner, generally requiring the needle to go in.

However, two other things are also operating at the same time - thicker air gives you both more power, and better propellor "bite", both of which make it go faster, and hot, the converse. If you hold the same "setting", that is, the same place on the 4-2 break curve, it can go substantially faster when it is cool, and slower when it is hot. To hold the same speed, you have to exaggerate the needle motion to run further toward the 2-stroke "setting" when it is hot, and deeper into a 4 when it is cool.

If you are using a tach, it gets even more complex, because if you set it to the same RPM, and ignore where it is relative to the 4-2 break point, you *usually* have to set it to a *lower* RPM when it is hot, and "faster" when it is cold, because depending on the prop load and how it responds, it will unload (speed up from ground to air) more in hot weather than in cold, meaning that you need to set it slower on the ground to get the target in-flight RPM.

Depending on how the engine is set up, these effects can be pretty strong, or weak. There are a variety of ways to deal with these issues, this is a big part of learning to run them. Depending on what a particular combination does, you might change:

prop pitch - generally more when hot and less when cold, to get the same "bite" (but maybe change the break characteristics since this also varys the load feedback during maneuvering)

compression - more when hot, less when cold, to mitigate the power variation from varying air density OR, to make the break stronger or weaker (stronger when hot, since you need more when it it flying slower and less than you are flying faster)

venturi - bigger when hot, smaller when cold, to mitigate the power variation to retain the same "setting"

nitro - less when cold, more when hot, to maintain the same run time and same power level, and "setting"

muffler restriction - more restriction reduces power, less increases power, also changes pressure feedback into tank.

There are a wide variety of things like this to do, they all interact, and which one works or doesn't for a particular set of conditions can only be determined by experiment. Your guesses get better as you have more experience. There are a wild array of obscure things you can do to props to make the system work differently.

All these things are also present for tuned-pipe systems, the generally the variables are far less important because the pipe regulation RPM doesn't really change much with conditions, and the performance is generally so much higher, that even "off" a little bit you have abundant power. With 4-2 break systems you have to try to optimize them all the time, because the performance is low enough that you have to make it work nearly perfectly to just get in a good flight.

Of the adjustments above, by far the easiest to execute, with minimal chance of screwing up, is the nitro. The last one you need to be dealing with is compression (which, unfortunately, seems to be the FIRST thing people do). If you are nominally running 10% in the cold, and it's hotter than normal, then try using 1 ounce of 15%, the rest 10%, so you get 12% net or something like that. My general rule is to change the nitro until the run time comes out the same at the same needle position. The fuel needs to be otherwise identical, because changing the fuel viscosity (by mixing fuel with 18% oil and fuel with 22% oil also changes the fuel viscosity, thicker, the further out the needle needs to go).

I would suggest that to make any real sense of the problem, you are going to need to get a reliable stopwatch and learn proper techniques to time laps. You might make a change, and it feels more "powerful" or pulls harder, but you need to know whether that is because of the way you changed the characteristics, or it just sped up 2/10 of a second a lap - which will have a dramatic effect on what you feel.

I am not trying to intimdate or discourage you, this is the reason it takes decades to become a genuine expert at all this stuff, and we are all still learning on just about every flight. You have to be very careful to control your conditions, and have a good grasp of the underlying principles. It's also why people went to tuned pipe systems as soon as it was practical, and now electric, because they are far less prone to these variations.

Brett

 

[Shoestring]

From a PAMPA article

Randy Smith
1047 Sawgrass CT SW,
Lilburn, Georgia, 30047
(678)407-9376
randyaero@msn.com

The Golden Age of Stunt

I’ve known we are living it; however, while at the VSC 18, this thought was solidified. The Golden Age of Stunt is Today. At no time in history did we have more planes, motors, props and all the stuff we use. It is amazing the amount of choice that is available to every stunt flyer.
From Laser cut Old Time and Classic kits to modern AAC motors to the light , super strong Carbon Fiber items that seem to grow out of the woodwork everyday.
The planes were absolutely stunning and comparing motor runs to 30 years ago, well suffice to say “no comparison”.
I have been asked many times lately to explain how to setup an engine for a good “stunt run”. All too often I see motors running backwards from the ideal. Four stroking up hill and breaking into a 2 stroke downhill, going blubbery rich on insides, screaming lean on outsides, etc, etc.
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. The performance of some of the airplanes at VSC proved this is true.
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
conjuction with each other to achieving this goal, and most everything I am going to mention affects the others to some degree or another. Please do not think of the following as an oversimplification, it is not. It would take a volume of text to try to explain all the relationships that one item has to another, this is just to try to add a little clarity, and to give you a place to start.
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 is in and matches well with it
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. For example Fox 35s, OS 35s, older McCoy’s and such need high oil content fuels 24 to 29% are common percentages.
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
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 keep 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 of the 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. You can setup the run with a very strong 4 cycle that doesn't break anywhere, but will still cycle and increase/decrease power in maneuvers somewhat, don't try to get so deep into a 4 cycle that the engine goes even richer when starting a maneuver, or slows ans speed in weird places, generally this setup should still be in a high power setting that is close to the 2 cycle beep without going into a 2cycle.
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 too 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 leeway in tweeking 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. Venturi : Venturi 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 problems 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. 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
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.

Randy Smith