Retreating blade stall on a 130X! ???

[tomatwalden]

Just tried to see how fast I could make my 130x go in fast forward flight ... and a strange thing happened ...

I went high, pushed the nose down and gave it alot of +ve collective. As it passed me, I pulled the nose up into level flight. Just after that, it violently pitched the nose up by about 90deg and shot skywards for 10/15 feet before I pushed the nose down again and retained a stable hover. It's the classic symptoms I would say (never having experienced it before) of a retreating blade stall, following by gyroscopic precession pitching the nose up in consequence.

The thing is, I didn't think it was realistically possible to push something as small as a 130X into this condition. Motor is stock, was in 100% idle up at the time, so the headspeed would have been way in excess of forward airspeed.

Could it simply have been as AS3X "brain fart" !? Any thoughts / ideas ?

T

[Guest]

BeastX and vbar have both done the same to me when maxing it out, seems far too slow to be retreating blade stall though, I put it down to fbl heebeegeebies. Maybe somebody will do the maths

[ghost2212]

I would think its much more likely to be a glitch with the AS3X stabilisation system

[KevDavies]

put a flybar on it problem solved
LOL

[Guest]

put a flybar on it problem solved

What goes wrong on those first when you do faster and faster barnstormers?

While unnerving, I'm deeply relieved that the failure mode on my helis is to raise the nose when they crap out anyway

[KevDavies]

What goes wrong on those first when you do faster and faster barnstormers?

While unnerving, I'm deeply relieved that the failure mode on my helis is to raise the nose when they crap out anyway

dunno?? Never lost control of a heli through speed. Have through many other scenarios tho just not speed. Yeah its a good thing they do climb out but in that same breath why is the electronics crapping out?? Yet another nuance thats putting me off fbl

[Baldie]

Wonder if its this...and why it happens to FBL ?

When you are hovvering, All you are using is lift.
When you move into FF, the lift is offset to allow the heli to move forward.
As you go forward, the heli will "slide" towards the floor as you have lost lift to "thrust"
So you give it more pitch to keep it from sliding to floor.
At some point, you will be at max pitch and therefore at your maximum forward speed, without loosing hight.

Now in all this the FBL controller is trying to stop the heli being moved about by the air hitting it.....

My guess is that the heli runs out of collective pitch and somhow cyclic pitch, and the FBL unit gets confused or cannot do any more.
As soon as the disk tilts upwards, you are still in max collective, but now you have lost LOADS of forward speed, so it all goes into lift and the heli rockets skywards....


?????

[helimadness]

I have the exact same problem with my mini cp from walkera so I think its more a generic FBL problem than a brand specific problem. Would love to know what the answer is.

[Guest]

I'm thinking the speed run people probably know a thing or two, from what I remember they are pretty careful over which FBL controller they choose

[Mike00top]

The 130x even with a pokey 450 cell i guess out of a dive would have to be flying at over 50kmh to get the middle retreating section stalled ( which isnt too bad ) , i think you would need to get over 80 kmh to get the tip near to stall as well , thats roughly based on 3500rpm which is likely to be the lowest rev on max pitch and cyclic added plus drag, even if you based it on 2/3rds or 3/4 of the blade stalled to give enough effect,

add a bit of blade flex and slop, ands its certainly not impossible !


what sort of collective were you at when it happened ? cos near 11 or 12 degs and a bit of cyclic juggling would make it possible

and once its fully developed NO flybarless system in the world could stop it, only its natural effect, thats provided the pushrods/ blades and swash havent fell to bits before hand with the horrific vibration LOL

[tomatwalden]

collective was probably pretty neutral-ish at the time. I'd given it bucket loads from the top of the dive, but as it passed me, I'd levelled the heli out more so that it was probably only a few degrees of positive pitch. Bear in mind, this all happened quite fast, so I couldn't say for certain.

It went past me at about 10feet off the ground and flew for about 15 feet level(ish) before it violently pitched. I'd say it was going quite fast mind. A friend of mine had just bumped into me flying, and we got to discussing about how fast such a small heli could go. It was my 3rd or 4th attempt, getting a bit braver each time before it did this.

It's possible I guess that as I levelled out, the pitch was nearing the point of a some form of stall. Maybe any +ve increase I might have done instinctively in level flight might have just pushed it over the edge so to speak?

Does the retreating blade have to completely stall - could a big enough dissymmetry of lift cause the pitch up? Maybe cold dampers not allowing enough teeter ?

[Mike00top]

The retreating blade seldom fully stalls on the full semi circle as the effective chord increases near 12 oclock rotor position (12 being straight ahead) as soon as there is enough retreated area stalled it will pitch, but it depends on so many things, the blade AOA the heli AOA its airspeed, rotor rpm, the blade airfoil shape, frontal and airframe drag, CofG, centre of mass, centre of drag, etc, etc, etc

its a slim possibility that for a fraction of time you flew through it, and it occurred,

if it wasnt linear servos i would have almost said the head over come the servos too,


only one way to find out ! have another go ! ! ! just remember though it does put a massive amount of stress on the heli and its not good to keep doing it LOL
as for the possibility of the AS3X doing it would be unlikely, unless the severe vibration at point of stall throws the gyros ? ? ? you have checked your 3 in 1 board is firm between the main frames, if its loose in the frames (mine was) 4 dollops of silicon in the corners sorted mine.

[tomatwalden]

Unfortunately, my lunch break is now well and truly over and I'm back in front of my laptop. I'm intriuged now though, so I will try it again. Next time I'll stick an 808 camera on my bonce so if lightning does strike twice, I'll have evidence!

[Tean]

A while ago I built a couple of larger flybarless helis using CSM gyros on the head. I could provoke this sort of behaviour by using too much heading lock gain and not enough damping gain. Usually it would be on the exit from a loop on a gusty day. I have a theory as to why this is.

All single rotor helicopters tend to pitch the nose up in forward flight. This is due, as you say, to the asymmetric lift which produces a rolling moment. This is converted to a nose up moment by the gyroscopic precession of the rotor.
Now I'm old enough to have flown a few flybarless models before we had electronic didgerydoos and it was very noticeable that you had to hold in forward cyclic in fast flight to keep the nose down. If you let the cyclic off, the heli would balloon upwards. I remember the GMP Legend would almost loop without any back cyclic if you got it going fast enough and returned the elevator to neutral. Gusty winds would make the problem much more noticeable, especially going into wind.
The flybar removes much of the nose up effect, especially in the wind, but eventually it will run out of inertia and allow the nose to rise. This is not too much of a problem because it happens gently and is not difficult for the pilot to control. With a model, fast forward flight is often a run up to a loop or other rising maneuver anyway and it may even help beginners to control the model's speed.
With computerised gyros we have the option to add intergrated (heading lock) stability. I think this is what gives modern flybarless helis that "running on rails" feeling. They go exactly where you point them regardless of wind and turbulence or nosing up. But just as a heading lock gyro can easily push the tail right to the end of its travel, a "heading lock" swashplate gyro can keep pouring in the cyclic long after a mechanical flybar would have gently let go. The result can be that the heli holds perfectly straight until the retreating blade, that has say eight degrees of collective, gets hit with another ten degrees of cyclic and stalls in a gust, causing the nose to suddenly pitch up with no warning.
Now hopefully the gyro designer has seen all this coming and programmed all sorts of decay and gradual centering stuff in there so it all happens gently, but there's always that complicated little gust to catch his carefully planned compromise out.

[Mike00top]

The GMP Legend was awesome i flew mine with no flybarless controllers at all, the main quirk was if in level forward flight, if you dropped the collective quickly it would nose down slightly and if it was raised quick, it would nose up, otherwise with those heavy blades it flew great and was my flagship for many years. but mine didnt need to hold in forward cyclic ? with the CofG on the front of the mainshaft it would follow any route given.