Guys,
Was wondering if there is a rule of thumb for PA Stunt about the thrust needed for the weight of the ship? The trend has been to bigger props and larger engines. What is the latest thinking on this. Seems it you take most ships and point the nose straight up once the power comes on it is just about equal to the weight maybe a little more but not enough for a VTO.
Best, DennisT
My planes have slightly greater thrust, and will all go up if pointed pointed straight up and let go.
However this tells you little about how the plane/engine will work when flying stunt, You will still need to get the power balance correct and the run quality correct, whether it be cycling ,or constant speed run.
There are many ways to setup an engine for stunt, below are some hints on ways to do this.:
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.
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.
Randy
Topic: Thrust vs weight for Stunt (Read 1924 times)