The Blind Leading The Stupid!

[jetbox]

I think I probably owe you fellas an apology, my past experiences obviously differ greatly than yours

No offense taken I hope.

[VERTICAL REF]

In a straight and level descent off the eastern slopes - yes full blown servo transparency! And this boy don't cowboy!!!

 

 

Skully,

 

SERVO TRANSPARENCY has everything to do with DISC loading! You even said it yourself in the next line down!

[CTD]

I've only seen it once. It was in very heavy turbulence in a D model towing a snowmobile, and lasted the usual couple seconds.

 

It's surprising that I didn't get into it more, given the types of flying I've done with the D, B, BA, B2 and TwinStar. Towing bird in the rocks made for some pretty serious disc-loading events, very agressive applications of collective, and several hundred hammerhead turns all day to get back on 50 or 100 metre spacing. Never saw it.

 

I agree the system could be better, but I also have a feeling that the awsome governor and rock-solid Np/Nr performance of the 350 series makes us do things with them that we wouldn't try with other machines. I can't imaging doing some of the bird runs I did with an L.

 

Any thoughts?

[recirqe]

Me too,

80 % of gross weight levelling out from a descent at 135 KTS at 5000'.Lower collective reduce severity of maneuvre. Grabs your attention though.

[sharkbait]

Being a glutton for punishment I will take a shot at it.

Seeing as the Astar has a low pressure hydraulic system you can get yourself in a situation where the disc loading overpowers the hydraulics.

Basicaly the forces down from the rotor system equal the force the servos can send up to the rotor system so you start to feel feedback forces in the cyclic - not unlike hydraulics off in a jetranger for example. ( or flying a 47 G if you want to really wallow in the past.)

The RFM ( as I recall ) tells you you have reached the G limit of the a/c.

We used to demonstrate this by initiating a steep turn an then slowly tighting up the turn ( at a prudent altitude ) when feedback was felt we would hold it long enough to show that even though feedback was felt the controls still worked albiet with muscle power needed to move the controls. A reduction in the turn returned the rotor disc to below its max load and as you did so the feedback disappeared.

I would assume that if you tightened the turn and further loaded up the disc the feedback forces would increase a "G" increased.

In Discussion before I briefed that not unlike a spiral in a fixed wing the easiest way to recover was to reduce power, roll the wings to level and recover from any dive.

The demonstration was enough to alert pilots of the scenario and we never got any feedback from the field about any problems. Then again we were eastern flatlanders not used to mountain turbulance and stuff.

I do not recommend anyone trying this without a training pilot and the permisssion of the Chief pilot. Test pilots we ain't.

When the company I worked for was purchased by another company they did not demonstrate this scenario ( in fact did not know what the heck I was babbling about but thought it was a bad thing ) so I never taught it again.

Worst case would be High gross weight, high power, high turn rate.

As stated before the twinstar having a dual hyd. system this would not happen so there was a LIMIT light on the caution panel to tell you you had arrived at the "G" or discloading limit.

All this was on AS350D models so the B, B2 ect may be more susceptable (SP?)to this given higher gross wieghts and, I believe the same pressure Hyd. System. Never flew a B2 or anything so can not say.

If you get into this situation while trying to avoid terrain then you have put yourself in a bad place by having to pull to the G limit of any a/c.

Clear as mud????

Perhaps someone with a 350 RFM can quote the actual applicable section or give a more scientific explanation.

Gentlemen you may fire at will.

[Blackmac]

SharkBait, I guess you are not as old as moi.

The 47G was the first Bell that I know of that had hydraulics.

The 47D1 had irreversables (no hydraulics), we had one at Viking in the early days. It was fun to fly. Identical to the "Mash 47D1's."

If you really want to develope a strong left arm try flying a Vertol H-21 when the collective is out of rig.

 

Cheers,Don

[sharkbait]

Geez blackmac - a H21 - that is the first Helicopter I ever saw. Was that You???

Actually we had a 47G at Canadore in 1976 and I recall that the Hyd. system was not as good as the G-2. and I recall you really felt some feedback in the controls - could be wrong - Long time ago. Underpowered little critter too ( Franklin (SP?) engine) Auto Al and I nearly dumped it in the trees trying to get out of a hole 1 fine day. Big leap in the learning curve that day ( still have a mild concussion from Al's clipboard - which is why helmets were mandatory ) Someone rolled it over in a hole NE of YYB later that winter so that was that. C-FSCJ I think.

Thanks for your reply. Always interesting. :up:

[skullcap]

VR:

 

I said it has less to do with disc loading and more to do with lock number which is derived using the dimensions of the blade itself and the load put on it. That is why the B blades are more susceptable to it. As far as disc loading the B and B2 have the same disc loading at 4300 lbs but you take your B to 10 grand on a windy day and try to cruise at max cont. hang on. Plus at higher altitudes the pitch on blades is higher for same torque setting thus imposing a greater load to the hydraulic system which is why it is more easily encountered at altitude.

 

There is a blurb in the book "the art and science of flying a helicopter" on lock number, makes it easier for us simple folk.

 

CTD:

 

I have towed a bird in the rocks with BA and never got close to ST, yes you are doing hard manuevers but at lower airspeeds(80 kts and less) and thus the blades/hyd aren't overloaded. Have done lots of high altitude slinging and production skiiing but the most have encountered ST is sightseeing in level flight at max continuous power in gusty wind conditions. Why sightseeing, well we would climb up to 12000 feet to go and sometimes enroute it got windy. Heliskiing doesn't normally have you trekking across the sky in level flight as often.

 

I also encountered it while pulling out of high speed descent as used to from Gazelle days and woke me up.

 

SB;

 

We used to train the same way but I feel that since you have to put the a/c through a relatively high load with only two people in it to encounter ST that was not getting the message to the pilots. What we did was to brief the pilot on the maneuver and while flying at 80 kts turn off the hydraulics on collectve, wait for the pilot to lower collective slightly then turn them back on. Would work up to 100 kt doing same manueaver, and was an good way to show the feedback associated to ST.

 

It is also referred to as jack stall, and there has been much written about it in the US.

 

It is a bit hard to get some pilots head around the fact that you don't have to be pulling any G's to get into ST it is just that most a/c will be flown at lower altitudes and probably encounter ST more by being flown overly aggressive. The reassuring thing about ST in turbulence is that once the blades are unloaded slightly it goes away, many times before I could even lower the collective. But a few months in the rockies or coastal range will be enough experience to enlighten most.

 

In my experience the bigger blades on BA,1,2 are much better and would assume the same with B3, although wonder about being over 6000 lbs with external load in windy day.

 

####, imagine your hand out the window doing 40 kmhr and feeling the aerodynamic forces then go to 120 kmhr and do the same. No G force difference only higher load on arm, g force would be felt by body being flung out on road when door opens. Ha, ha.

 

I need hockey fix aaaaaaaaaaaaaaah. GO FLAMES GO.

 

Thats better.

sc

 

:up:

[CTD]

Skullcap, you're right of course. The only thing that confuses me (about this ) is that the only time I've personally seen it, I was doing 60 kts towing a snowmobile. I had a good buddy who was putting off an impromtu airshow at a camp where we both worked, and got into it at the top of a layover turn at virtually zero airspeed.

[skullcap]

CTD;

 

Odd at that low a/s, was is turbulent, if doing 60 were you pullling max continuos?

 

The airshow problem, could have been inputting mucho t/r thus using up all the hyd capacity, as the fm says not to complete 360 pedul turn in less than 6 sec.

 

Have heard of pilot experiencing st in hover due to harsh t/r handling at high DA's.

 

sc