So I was seeing some settling with power with my X3 as I transitioned to hover when I was using two 600 mm blades. So I looked at my thrust coefficient divided by solidity compares to other RC heli’s and found that mine was a bit higher. So I did a trade off study looking at thrust coefficient divided by solidity and calculated power consumption (Chris, this was the excel sheet I gave you). I chose based on my design goals to go with a 4 bladed rotor system with 550 blades and lower rotor speed. Chris brings up a good point regarding stability in high speed flight. With this aircraft I was looking to help the rotor with a wing and forward thrusting propellers to keep blade AOA down and hopefully stay away from blade stall. Crashed that aircraft a few years ago due to a gearbox failure and never got to back to it. No doubt more blades equates to more power to achieve similar high speed performance. But it all depends on design goals and constraints. If you want to lift more but need to constrain the dimensional size of the heli then you’ll need more blades to keep the hover AOA down. If you know that your mission will involve low speed forward flight, then maybe lower rotor speed would be fine as long as the controllability doesn’t suffer. But as Chris points out, there are limits and he has more experience in what they are.
If you look at two of the most popular trainers, the Robinson R-22 and the Guimbal Cabri G2, one uses a larger two-blade teetering rotor with coning hinges. The other uses a smaller diameter three-blade fully-articulated rotor.
The G2 is superior to the R-22 in every respect. It has more power, higher gross takeoff weight, higher service ceiling, better autorotation with it’s excellent rotor inertia. Is a more stable helicopter. All while having a smaller rotor diameter than its counterpart.
The results achieved with the G2 was thru careful engineering by Bruno Guimbal with the assistance of Airbus Helicopters to build the highest performance and safest light helicopter that has ever been built. And it is now the best-selling aircraft in general aviation.
I’m sure RC enthusiasts, thru experimentation and careful engineering, can also build a successful three-blade helicopter. But one of the major problems is that RC rotor heads are incredibly crude compared to full-size machines. Most three-blade RC heads I’ve seen are rigid with only lead-lag hinges. And this introduces inherent instability problems and hampers proper operation of the blade coning and flapping action in flight. Until somebody builds a decent RC head three or more blade RC helicopters are a non-starter.
When I say RC heads are incredibly crude compared to their full-size counterparts, this is a good example of what I’m talking about. Watch the action of the rotor on this Mi-26 making a takeoff run.
I’m not sure if ArduPilot can fly a fully-articulated head like that because ArduPilot depends on sensing frame angles for its attitude controller. But I’m working on building one to find out if it can be done and if it can’t, figure out how to make ArduPilot fly one. I know ArduPilot can fly one with the new acro mode. Not sure about any of the other modes.
So is your concern just the ground resonance issues or is there some flight stability issue you have experienced.
The X3 was a 4 bladed system and I didn’t experience any ground resonance issues. From a flight control tuning perspective, I’ve shown that I can push the flap regressive mode to a higher frequency with the stiffer head in flapping. Then I was able to attenuate the flap instability with a notch filter. This allowed more P and D gain to be used in tuning. I have to more work to see if that translates to better flying qualities throughout the flight envelope.
Both. The three-blade head on the Demeter is a rigid style with lead/lag hinges only and it is too quick responding to be suitable for UAV/scale use. Can slow it down and it’s better. But at normal tip speeds the helicopter is too “flitty” and unstable for my tastes. At least not as stable and easy to handle as a comparable two-blade rotor. Get some serious weight on the helicopter so it has 40-45 lb takeoff weight and it’s better.
I haven’t had time to go back to working on it recently. But I’m building a new fully-articulated head to try out, likely won’t get back to that until fall sometime.
I have another three-blade heavy lift helicopter project about 30% built too, that one has twin Zenoah piston engines on it**. What ever I learn with the Demeter project I’ll apply to the twin-engine one. With the summer schedule here, this will all be next fall/winter projects.
** For the twin engine one I have to make some mods to the governor to be able to tune two engines individually to get them both to pull the same amount under load. I got the code wrote for that, but then got busy with spring work and never got back to it.
Or I should be more clear, because I don’t have the governor working 100% for twin-engine helicopters yet either. I wrote code so the governor calculates two individual outputs for engine #1 and engine #2, using the same throttle curve, but settings for engine #2 that are the same as the current single-engine code.
So now I have the choice of applying the output for engine #2 to the tail DDVP output and use that for engine #2. Or was hoping to be able to assign two SERVO outputs to HeliRSC so I can individually control two engines. Never got to finishing the actual output part yet.
This is where you have to adjust the ghost using the rate and accel limits. I think the one that would be most effective in adjusting the quickness of the aircraft pitch and roll response in stabilize mode would be ATC_INPUT_TC. But with all of these settings there is probably some fine line where if you make it suit your flying style and it is too slow that it could affect the autopilot. The autopilot basically flies the aircraft in stabilize mode. Now in acro, the settings you make like ACRO_RP_P doesn’t affect the autopilot. And that sets your max pitch and roll rates commanded by your stick. So you could slow the aircraft response in acro to give more full scale feel.
Thank You guys so much fo help 
So far i’m still waiting for new blades becouse last set is totally damaged.
We’ll raise RPM do about 1300 and change collective pitch and cyclic pitch angle.
What about thouse sevo lags 
is it normal for them to work like that? This behaviour is the same even with different power suply… thouse lags apears before we used Pixhawk controler. With MSH Bain V2 there were no lag what so ever 
What do you thing about using blades from 600mm class with this setup that we have and changes that you sugested? Will the performance be better or should we say on 700mm blades?
I doubt that would work. The gear ratio in your helicopter’s transmission likely can’t spin a 600-size rotor fast enough. Keep in mind that 1300 rpm is still dangerously slow rotor speed for a 700 class helicopter.
Edit:
For folks that like to run excessively low headspeed on RC helicopters, please compare from the full-size world.
Heavies:
MiL Mi-26 Halo - 105ft rotor diameter, 8 blades, 725 ft/s blade tip speed
Medium light:
Bell 206L Long Ranger, 37ft rotor, 2 blades, 763 ft/s
Bell 407 (206L with 4-blade rotor), 35ft rotor, 759 ft/s
Light:
Bell 206B Jet Ranger, 33.25ft rotor, 686 ft/s
Light Utility:
Guimbal Cabri G2, 23.55ft rotor, 3 blades, 663 ft/s
Ultralight:
Mosquito, 18ft rotor, 2 blades, 471 ft/s.
The heavies run disc loading up to 14lb/ft^2 and can go 160kts
The medium light disc loading is 4lb/ft^2 and can go 120kts
The light class disc loading is 3.8 lb/ft^2 and can go 110kts
Light utility disc loading is 3.5lb/ft^2 and can go 110kts
Ultralight is 1 lb/ft^2 disc loading and can go 55kts.
Your RC heli with 1500mm rotor @ 1300 rpm is 335 ft/s blade tip speed.
Your RC machine is similar to an ultralight. About the same disc loading in most UAV configurations. The only reason it can fly at all at blade tip speed that low is because you fly it at disc loading well below 1lb/ft^2. But the speed it can go is totally dependent on power and blade tip speed. At 335 ft/s it won’t be able go faster than 30kts before it becomes highly unstable and rolls into the retreating blade.
You need that blade tip speed up to at least 400 ft/s, and even that is pretty low for the helicopter to be stable. If you only intend to hover it around, and fly it at very low speeds you can get by with 1300 rpm to try to get more flight time out of it. But it won’t be stable in the wind and you’re going to see another crash eventually because the helicopter refuses to respond to control inputs.
You can run a little 450 at 325 ft/s and it’s fine, but it only flies at maybe 0.5-0.6lb/ft^2 loading and it’s not a fast helicopter. And 450’s are known for being twitchy, unstable machines because of their small rotor and the low blade tip speed they run. Everybody knows in RC, the bigger they get the more stable they are in the wind, and that’s because the bigger rotor can achieve higher tip speed.
I’ve seen other people try to run low headspeeds with 3D heli’s converted to UAV use with the idea they are going to get more flight time, and it never works out because they got constant attitude controller tuning problems. The helicopter is unstable and refuses to meet control rate demands, it has the classic “tail bob” due to running the rotor too slow, in any amount of wind it’s almost impossible to hover it accurately.