oldfart
Elite Veteran
Location: Vancouver, Canada
| Personally, unless I have a graph that can show me the HP curve of the engine/fuel/muffler (EFM) combination along with the torque curve of the same EFM, it is all a bit speculative.
Also I have yet to find any EFM combination, on any engine that occurs at the same RPM. With the max torque usually occuring at a lower rpm then the MAX HP.
For max acceleration in and out of maneuvers, I will always try to keep my engine's rpm somewhere between the peak torque and peak hp rpm of my EFM, but closer to that of its' peak torque rpm....keeping in mind that the kinetic energy in the rotor can, and should be, used as part of the equation. So the choice of gearing becomes relatively important here.
Just as the type of laoding (driving/flying), weight of the vehicle and the size of its's tires are a big consideration in gearing of any vehicle - so should they be important considerations here.
Also consider our ability to match our throttle (relative power) to the pitch (relative loads) with our radios. This can consider a type of transmission - one that is constantly changing gears for us. Just drive one of the new cars that have these type of trannies and see what I mean. They sound weird because they stay at the same rpm as they accelerate to max.
Of course, as in anything that is power (rpm) to load related, once one loads the system more then the max power available (motor and kinetic) to maintain the rotor speed, the speed will bleed off.
What presents the load to the system is the blades, the rotor speed and the pitches (both collective and cyclic) that we select at any point in time.
The one thing in the load equation that we cannot change in the air is the length, or type of blades. But we can change the pitchs and the rotor speeds. So if we select a rotor speed that matches well with the available power at the selected EFM compromise, then we will aways be "good to go".
So now that we have a set blade size, and a set rotor speed, the only thing that we can change relative to load, is the collective and cyclic pitches. So as long as we do not present the system with a combination of these that will overload we should never have a "bog".
Sounds simple doesn't it. But it can get a little complicated through a maneuver. So let us look at that.
Let us say that you do a full climb at 1900 and wide open throttle and your rotor speed declines. Well, we can't get more power, as we already have the engine tuned properly and the throttle barrel is wide open. So if we want to maintain that rotor speed (RS) then the only factor we can change is the amount of laod (pitch) we have dialed in at the top. Once you dial that down so that you can climb at full stick without a decrase in RS, then you will know that you are "maxed". So let us say that is at 10 degrees. You know konw that your EFM can pull 10 degrees of pitch at 1900 and come back smiling.
But what if I add some cyclic to turn at full stick - oops the RS starts to bleed off - Why? Because you have added more load then the "max". So basically, you know as long as you keep your total "pitch" loading to 10 degrees, it will all be fine.
Now in a stationary hover (say you have 4 degrees of collective at 1900), you will find that the throttle cannot be wide open, or it will overspeed because you have way less load then at 10 degrees. So your throttle curve will have a setting that allows the EFM system to open the carb just enough to give the power needed to hover, say 50%. So that means I have a lot of power waiting to be tapped, so I can add 6 degrees of cyclic (for a flip for example)as 4 + 6 = 10.
But OOPS it starts to bog if you do not have a governor. Why? because the throttle is still only half open, but when you pulled full aft cyclic and fed in that 6 degrees, you presented the system with a full load!!! So unless you have a 50% elevator to throttle mixed in there, to open your throttle all the way, and give you back the throttle opening you need for a full load, you will lose RS.
So if you have governor or a properly set throttle curves and mixes to handle all the load changes, you can bleed off head speed very quickly.
This also shows you the importance of learning collective and pitch management. As everytime you overload the system past what power it has available, you will have to give it time to recover. And remember any internal cumbustion engine cannot recover as fast as an electric can (almost instantly as long as the battery has the capacity).
And as you overload and bleed of RS, the tail rotor will be loaded further to take up the changing torque, this will present a further load to the available power...making it worst, so the sooner you back off on something (ease off some collective, some cyclic or a bit of both), the better.
Back to the beginning - for my 3D flying, using a G231 with the old V1 Century muffler and RT710 blades I find the stock 6.28 works well at sea level or close to it. At 4000 feet, I would probably change the blades and tweak it differently. Ditto for drag racing or for AP work. |
