Hi guys, I'm in need of some advice of getting longer flight time with a gopro gimbal on a trex 450 pro stretched.
Currently I'm using a DFC head with 350mm Spinblades asymmetrical blades, with a 5000mah 3s lipo I have a flight time of around 13min. I would like to try a 3 or 4 blades rotor head, looks like it can increase my flight time with lower head speed. It would be great if the flight time can be increased to 20min with larger battery.
These SB blades are huge and heavy, and I'm wondering whether it's possible to use such blade on the 4 blades head. I'm afraid of low efficiency of the airflow due to smaller space between the blades, wanted to calculate it out but found it too difficult for me.
At the mean time I found the SB 350mm scale 4 blade set are also asymmetrical, but they are scale blades which is narrow. I'm not sure whether these scale asymmetrical blades have enough lift to increase flight time.
I don't know which combi is better or worse, or is that a good idea to use 3/4 blades head. Any information or advice is much appreciated.
Replies
Rob, eigenfrequency is the frequency of free vibration of a system, we use it to design a controller and avoid disaster.
We have the first eigenfrequency, second eigenfrequency and third... forth...etc. Normally we talk about resonance, which means the disturbance hits an eigenfrequency. A good example is the tuning fork, if you hit it, it will vibrate at a certain frequency, it's the first eigenfrequency we hear. It also has higher frequencies but the magnitude of the vibration in higher eigenfrequencies is very small so we can neglect them. So basically it's the first eigenfrequency we care about, sometimes the second one as well.
It's the same for many things, also for our heli. When we have the disturbance (the rotor rotation in this case) and its rotating exactly at the eigenfrequency, the resonance occurs. The heli will shake very badly, and most of the time you'll see the controller will or about to lose its control.
When the rotor accelerates from still, the heli will start to shake, at a certain rpm the frame will shake violently, thats the resonance at the first eigenfrequency. As the rpm increases and pass that eigenfrequency, you'll see the shaking is gone, that means the frame is not responsive to the disturbance any more, we call it decoupling. The higher the frequency is, the less responsive the heli is to the disturbance.
So it's good to have a high rpm in this viewpoint (of course only the rotor is well balanced and the rotor doesn't hit the higher eigenfrequency), although the magnitude of the disturbance is higher, the frame just feels like there is less vibration.
I think I'm gonna do some experiment by adding weight, adjusting the HS, measuring current and the mAh, then using the data to calculate the expected battery capacity and the expected flight time etc, seems doable. I don't expect to nail it but I guess I can find some improvement.
The vibration problem seems not very big as I found the eigenfrequency of my heli is somewhere around 800-1000 rpm of the rotor, but I'll pay attention to it because I'm going to mount the APM and PX4 with foam later, which may cause a higher eigenfrequency of the sensor itself.
The bandwidth of the control system makes me worry though. I'll start with 1900 RPM, I guess with a well tuned PID and enough power it should be fine. My wild guess is a max thrust to weight ratio of around 1.7:1. Under this ratio I still have extra force about 20% of the mass in the vertical direction with 45 degree rolling, which seems about enough to keep the heli stable if the disturbance is not so big. It's a pity I can't find a way to calculate the thrust of the asymmetrical blades but I can test it out.
I'm in the Netherlands, a crazy windy country, the 80+ km/h wind nearly blew my quad away last year. All of a sudden the quad rolled to the right about 45 degrees but still hovering with slow drift to left, I was lucky enough to land it quickly before it left the landing area and went into the forest, the power was just not enough to fly against the wind.
A very stable "flying battery" will be awesome for casual hover and video making, but still I'm not gonna fly it in 80+ km/h wind :)
Thanks very much for the info Rob, that helps a lot. I'll stay with my current setup.
Am I already close to the limit of flight time? I'm always thinking of head speed / blades vs. battery capacity, higher HS / more blades can carry larger battery but at one point the energy consumption will start to dominate, I have no idea when the combi would be optimal though.
Xin, going to a multiblade head is the wrong thing to do if you want improved flight times. Less blades are more efficient than more blades. To the point that people have played with the idea of single-blade heads. That's a bridge-too-far for me though.
Multi-blade heads are used when you need to get more lift out of a minimum rotor diameter. They also help with better responsiveness out of a given headspeed.
I use spin blades on a 600 sized Heli. I found you need to get the RPMs well down to have the increased efficiency but it is not as great as advertised.