I have a trex 470 iv been building as an FPV platform. Just started flying it on ardupilot and still have all the default tunes. It flies pretty well if feels like to me. I had the 74mm tail on it and am running the governor around 1750 so sorta low headspeed. I tried flying and a little tuning for some smaller old 450 tail blades I have. It flies fine and seem to turn plenty fast enough for me. Certainly not going to be good for an experienced 3d pilot though. However if I collectives up and down too fast the tail does blow out. I have tried tuning it out a little but it looks like the tail is going to max so it probably just does not have enough tail blade (speed or size) to handle no matter what. I know I can find out more in the logs and stuff.
My question though, Is this even a problem? I want to keep the max main rotor AOA sorta high to get as much speed in forward flight as I can. So are tail blow outs something I can just use piloting to cope with or is it a problem I really need to address?
I have not had a chance to fly it in much space to see if the tail weathervanes enough if forward flight but assuming at higher collective when its moving forward it can keep the tail straight it seems to me like it should be fine.
I forgot to mention, the only reason I was wanting to try the smaller tail blades is for more ground clearance. (plus I have carbon ones that look cool for 450 size) The 74s still end up with grass stains on the tips when I test in my yard.
It is on straight not raked landing gear off a 500 so it sits a little higher over all but the same ish height of the tail as a stock 470.
the 1750 head speed is because I am currently running this 4s and that’s just what I have landed on for feels goodish. Lots more testing here needed to see what gearing and head speeds feel the best.
Being able to run 2 different head speeds would be fantastic but that’s a different topic.
I am curious as to what you mean by this. What limitation are you concerned with in high speed forward flight where you want high AOA (low rotor speed) or do you mean that you are trying to get as much flight time as possible in forward flight?
The limitations to high speed forward flight are advancing blade exceeding speed of sound and the retreating blade getting into stall. I think the advancing blade achieving Mach 1 at the tip is pretty unlikely. I think the bigger concern would be retreating blade stall. Which would require a higher rotor speed rather than a lower rotor speed.
It will almost certainly turn into an unpleasant FPV experience. Remember that you are also running low tail rotor speed and putting smaller T/R blades will likely make things even worst (tail wagging, loss of authority an so on) - something that tuning will not fully mitigate.
I’d advise not count too much on weathercock effect, contribution of that vertical fin on this scale and stream velocity is pretty much negligible.
@bnsgeyer Was just trying to set it up for as much forward speed as I could but with in the limits of the what its built. I don’t think I’m anywhere close to having retreating blade stall issues, I doubt its going to be very fast no matter how I setup my max pitch. I also plugged it it to a tip speed calculator and show that it should be over 350mph for my current setup. I just wanted more max pitch available for when its moving horizontally. My reasoning is that I think it will have more torque available even on 4s once it gets moving similar to constant speed propellers on planes combined with translational lift means that more pitch can be applied and gain some more forward speed.
I don’t really have a specific idea of what this heli should do right now other than be fun and help me learn. As such its the worst type of aircraft, one that I want to do everything. Having to do everything usually means doing everything badly lol.
Ultimately Ill need to decide what its for then set it up for that. For now Id like it to be as fast as possible, fly as long as possible, be very agile, be able to carry large batteries, be an FPV platform…you get the idea.
I put in speed of 100mph and 50mph to the tip speed calc to find our how much of the rotor would be at or near 0 if the heli where to fly 100mph (probably dreaming) or 50mph (more realistic?). At 100mph only about 1.5-2in of the actual rotor are at or under 0mph and at 50mph only the head not the rotor will be at or under 0. Of course that means the rest of the rotor is also still slower relative air speed and I have no idea how to relate or apply this information to a guess at how it will be affected. I just thought it was interesting.
Iv read over and over in many places that something like 200m/s (like 450mph) is the golden number for tip speed. Iv also read that as low of a head speed as possible will be most efficient but I think that usually just means longest flight times in a hover and not most ground covered since efficiency means different things to different people in RC.
More experimenting needed.
@bnsgeyer@Ferrosan Do you have a target head speed or tip speed you just aim for with all builds? Or do you play around with each one to get it how you want?
I didn’t get anywhere close to that when I calculated your tip speed. The main rotor tip speed based on (500mm) 1.6 ft rotor radius and (1750RPM) 183 rad/s came to 204 mph.
At 50 mph for a rotor system with 200mph tip speed, the inner 25% of the rotor blade in the retreating side would be in reverse flow (<0mph). It isn’t an exact correlation but I think a forward speed much above 0.6 times the tip speed would be close to retreating blade stall. I suppose 100mph could be possible without going into retreating blade stall but I doubt it since these are probably untwisted blades.
So I use the rule of thumb that Chris Olson taught me which I think is pretty good. Set your rotor speed so that your collective blade pitch in a hover is around 5 deg.
@bnsgeyer considering the headspeed he is aiming at (1750 rpm) he will roughly achieve half of that value, with collective close to max out in level flight condition (not a smooth fpv ride).
To get to those speed ranges I think headspeed would need to be increased to around 2300 rpm , with severe limitations to endurance (rotor head permitting). @spova as a general rule i aim at around 1900-2000 rpm for that scale FPV sport flight, speed 95km/h tops in a light dive condition.
This heli is 870mm rotor diameter.Trex470l with 390mm blades. No twist. I have been plugging those values in here Tip Speed Calculator - Calculator Academy. Maybe their calculator is wrong.
It does hover at just over 5 degrees positive blade pitch. Ill double check this though.
Note: Yep looks like that sites calculator is inaccurate comparted to doing the math my self. I get closer to 175mph when I do it my self.
Thank you both that helps a lot knowing what to sorta aim for. Finding out the calc site I was using is way if is lame lol. Looks like the 200m/s is over 4000RPM. I didn’t have any particular reason for 1750 other than it seemed like about what it wanted to do with the stock gearing and 4s.