Daniel Reese
Senior Heliman
Location: Urbana OH
My Posts This: Topic Forum | Sure you can! EVERYTHING depends on the unique differences between your particular setup and every other setup you've read about.
I had searched THOUSANDS of posts about this project and found MANY MANY cases where builders had been successful at powering a 5-blade rotor with the stock motor & esc. When I tried to apply those lessons to my build I had mixed results.
Initially I used Align 325PRO wood blades. They worked but I mistook wind and pilot error for blade problems and switched to Align 325 Fiber blades (the blue plastic ones) and those were MUCH worse and so I had to shelve the project. In hind sight, it deserves another try with the wood blades and that would probably solve the problems- that and not trying to fly it in the wind.
There are some KEY variables at play in developing a setup that works- your decisions on those variables will determine if YOU can use the stock motor and esc. Each variable is related to each other variable- a change in one effects them all- but I'll try to disect them a bit:
First, I should mention a "variable" that isnt variable at all but is so intertwined with the rest that it cannot be ignored: Rotor sensitivity.
A two blade rotor spinning at a constant rpm, at a certain blade angle of attack, will generate a certain ammount of lift. If you apply that same rpm and angle of attack with the same sized blades but increase the NUMBER of blades you will increase the ammount of lift. What that means is that the 5 blade rotor, since it has nominally 2.5 times as much potential to generate lift as the 2 blade rotor did, then it will be 2.5x more sensitive to changes in wind (translational lift) and changes in blade angle of attack (pilot inputs). To "tame" this sensitivity you can change one or more of the constants: blade size/rpm/angles of attack.
BLADES: lighter = more sensitive = less "work" for the motor. Heavier = less sensitive (preferred) but also more "work" for the motor. The stock motor will have to work harder moving 5 blades, regardless of weight, than it did with just 2 so there's already increase in "work"; by increasing the weight of those blades you exponentially increase the "work" the motor has to do to move them.
HEAD SPEED: Faster = low blade angle of attack required to hover and smaller changes in blade angle of attack required for maneuvering. All that = relatively less drag = relatively less "work" or torque on the motor/esc.
Slower = higher blade angle of attack required to hover and greater changes in blade angle of attack required for maneuvering. All that = relatively higher drag = relatively more "work" or torque on the motor/esc. If you increase rpm, you will generally notice an increase in stability. However, changes in rpm are directly related to vibrations- changing the rpm- even slightly- is often the simplest solution to a vibration problem.
"WORK" is actually two different variables:
For the ESC, "work" relates to how much of the battery capacity the ESC is allowing to flow to the motor. For the MOTOR, "work" is more properly referred to as torque, or how much resistance to movement that the motor has to overcome. In both cases WORK = heat = damage to the component (Motor or ESC)
The ESC and the motor each have a sort of "Power Band" a range where they are running at peak efficiency. For the ESC that power band is near the upper end of the throttle curve when it's pretty much letting all the battery flow right trough it. It gets hot when you try to hold back some of that capacity and fly at 80% throttle or less- that excess capacity of the battery gets converted to heat. Heat = reduction in lifespan and if severe, can lead to a sudden and complete failure of the ESC.
The motor's powerband is at high RPM with relatively little torque. The more resistance you put on the motor- the more drag in the rotor system (the more torque required to overcome that drag)- the more amps/watts it will need to pull to keep going. That increase in amps/watts = an increase in temperature which = a reduction in it's lifespan and if severe, can lead to a sudden and complete failure of the motor.
"The Decision Tree"
The fuselage weight is both a variable and a non-variable. You're making the decision up-front about the weight and if you've decided to use the Align fuse, you've "committed" yourself to a heavy fuselage...that's one hit against the stock motor and esc but thats NOT a death sentence for it! You'll just need to compensate in some other way and it's those compensations that will decide weather or not you get to use the stock motor and esc.
A light rotor system WILL make the motor/esc happier but it will seriously challenge YOU as the pilot. A light rotor system is very sensitive and even with electronic stabilization, will be challenging to fly (not impossible, just challenging).
A "heavier" rotor system WILL make the motor/esc unhappy but it will dramatically reduce it's sensitivity and so reduce the challenges of flying it. Electronic stabilization systems will DRAMATICALLY improve the simplicity of flying a heavier rotor system.
The weight of the rotor system will determine the "WORK" or torque variable. Since you're starting with a heavy fuse, if you also use a heavy rotor system you will likely exceed the abilities of the stock motor & esc.
If you use a light rotor system you will likely still be within the capacity of the stock motor & esc.
The last variable is RPM. Slower rpm increases some components of drag and so increases that "work" variable.
If you combine a heavy fuse with a heavy rotor and low rpm, you will almost certainly exceed the abilities of the stock motor and esc.
If you combine that same heavy fuse with a heavy rotor but a HIGH rpm you may yet be within the capacity of the stock motor/esc.
The same inverse applies here too: If you combine that heavy fuse with a light rotor and low rpm, you may yet be within the capacity of the stock motor/esc.
If you combine that heavy fuse with a light rotor and high rpm you will almost certainly be within the capacity of the motor/esc but that bird would be challenging to fly.
I havent mentioned batteries yet...Regardless of the all these variables, a 5 blade rotor of any type will require greater battery capacity than a 2 blade rotor. It is perfectly feasable to fly a 2 bladed, pod-n-boom TRex450 on an 18c 2000mAh battery. It is impractical and well near impossible to fly a 5 bladed pod-n-boom TRex450 on that same battery. The battery threshold seems to be at about the 20c 2000mAh range. Anything less and you'll kill the battery (puff it); anything more and you're usually just fine. The higher the C rating the more capable the battery is of handling the heavy demands of the motor. The higher the mAh rating the more flight time you'll have.
Wow, that took a long time to say! I guess it boils down to decisions on variables. A decision on any one variable will impact all the others. The sum of all the variables- your unique setup- will determine if you can use your stock motor & esc & battery. |
