Fail Safe

[jimbob]

Was just skimming though the PPM/PCM debates and I was prompted to ask the following:

Apart from throttle at idle (obviously) what other settings should be used in fail-safe for a heli?

I was thinking of pitch, gyro mode etc... and then realised that the governor needs turning off too...

Whats the recognised norm for all this then?

[Disc]

Hi,
At our Club the following rules apply PPM.

You must fit a throttle Cut device of some kind. If you have a Govoner
and that was throttle cut then thats okay. But some sort of Throttle
cut is required.

PCM just switch it to kill the throttle and on my predator sends all other
servos to netural. So if my Radio goes dead or some upsets it basically
my baby won't beat itself to death but will still make a big mess.....

[ChrisB]

With PCM you have to set all sticks to centre position and throttle to either hold or idle,this will automatically disengage any governor,gyro is irrelevant whether its in HH or Normal.If the Rx allows control back to the pilot you may be able to recover it,but if it does not give control back meaning its a switch on by someone else or a serious hit then its going in.I have seen many helis hit the ground under power,mine included and the damage is almost always far worse than a non powered crash,you may get a funkey chicken where the heli will beat itself to death etc,if the heli hits the ground with engine at idle then you only have the initial impact to worry about,unless that initial impact damages the radio tray allowing the throttle to open and the funkey chicken to take place.

[Ade_Law]

i set idle for motor, everything else to last position that way it carries on doing what you wanted but if it doesnt come back the energy is going.

Ade

[jimbob]

Would it then be a good idea to set pitch to 0 for the same reason ade?

[Ade_Law]

No, i want the model to continue on the path i was on. so that if it does come back to life its not lept accross the sky in the process.

Ade

[G-DAVE]

Used PCM failsafe on my models for donkeys -- agree with Ade totally -- Motor to tickover - others hold last position.
Dave

[jimbob]

Been thinking about the pitch issue in failsafe and thought i'd jot down my ideas to provoke some discussion...

1) If pitch stays the same, then the direction will stay the same, but the head speed will reduce. Energy is used up quicker - desirable but less chance of a save.

2) If pitch goes to 0, then the model will continue in the same direction but lose directional speed. This is of course desirable unless the direction is straight down, but the odds are against this. Head speed is maintained increasing the chance of a save but conserving energy so increasing damage potential.

3) max +ve pitch. Head speed is reduced very quickly, at the expense of a quick +ve acceleration - I wonder how quickly the speed would die with no throttle... If the purpose is to lose energy (as in the throttle), then this could be considered however odd is may seem.... Perhaps someone could tell me what happens if you switch to 0 throttle and max +ve pitch from a hover?
This is surely only feasible (however unfeasible) is the heli is upright for the majority of time. 3Ders need not apply, but for circuit fliers?

4) max -ve pitch. same as 3, but with a much increased chance of the ground becoming a factor.
Same caveat as 3, but for helis that spend their time inverted? Adam Turner?

This is purely to promote a discusion. I wrote it down to make myself think about it, and its not necessarily cut and dried although on balance I think Ade and Dave are probably bang on with option 1...

Interesting though.

[Janek]

Ahh, the good old failsafe debate, haven't had one of those in a while

My 2 pennies :

1) Failsafes are for the SAFETY of you and any bystanders, therefore the setting should buy you as much time as possible, for shouting warnings,and getting out of the way. Saving the model, even if you lucky enough to get signal back, is a secondary consideration.
2) Throttle to idle, and govenor off (throttle cut is TERMINAL, and IMHO stupid !)
3) Cyclics to centre
4) Pitch to zero (you have remembered to "unlink" the pitch and throttle, haven't you , you don't buy yourself much time with an idling engine and -10 degress of pitch ! )
5) Gyro to rate mode (so the heli weathercocks)

There are arguments that "hold" on the cyclic and pitch are better, and in SOME circumstances this might be true, but the setting above will give more time in the vast majority of circumstances.

Let the arguments commence

[Ade_Law]

at zero the model will auto quite happily not really pulling much speed out of the blades... = bad

remember that there are 2 sorts of energy.. stored energy in the blades and the models speed through the air at max/min pitch you are going to put a lot of speed into the model whilst the headspeed is dropping off which wont take long but the model will continue to travel in the direction it started going in.

with it set to hold last value the chances are its going to load the rotor system to some extent reducing headspeed slowly without putting momentum into the model.

Ade

[jimbob]

Course theres no right answer cos it all depends on the helis attitude at moment of failure.
I agree with Janek that it should be all about safety rather than saveing the model.
What do you think about my maximum pitch scenario? Its not meant to be a serious suggestion - just a point of discussion - if the idea is to remove the maximum energy from the heli ASAP...

[Ade_Law]

although its removing headspeed its converting that into modelspeed. which if it happened whilst pointing at a person might be interesting

your right, there is no one setting that is ideal in all conditions.

what we really need is a rotor brake.

Ade

[Janek]

Don't forget safety and TIME.

I agree that a heli with zero pitch will happily auto, and maybe keep the blades spun-up, but it also gives you more time for shouted warnings and getting people out of the way. Dead blades equals a brick, and bricks don't stay aloft very long. . . . .

If you have a lockout, you should accept that the heli is probably already lost, and that safety is now the number one concern. . . . . . . if you happen to get it back and save it, that's a bonus, but not one that should be relied on.

[Janek]

Maybe one day, if someone is rich enough, proper testing can be done for a selection of PCM failsafe settings, and flight scenarios.

[G-DAVE]

On the pitch thing.
Many years ago I witnessed a fly away - it was a Kalt Gasser (the model before the alpha's ) the pilot was running it up on the ground and turned to the pits and said he had no control at all.
It was at the lift off point just sitting there very light on the skids- after 5 mins or so while we puzzled about what to do - it took off very slowly and climbed to a dot in the sky .
It drifted over another field out of site to us. We followed it for 15 mins then the engine stopped. It landed several fields over and when we got to it the only damage was a cracked blade (wood in those days) and we think this happened when it fell over on landing.
What happened ? How did it land so softly? Did the blades stop then run backwards ? this always puzzled me .
By the way it is a genuine story - it happened.
Dave