Helicopters: Collective Pitch vs First Limit Indications

[peeush]

This is regarding using 'Power' in Helicopters.

In particular, looking for advantages of using the 'First Limit' vis-a-vis the 'collective pitch' (Collective pitch indications are related linearly to actual blade pitch) for determination of power available and power being used in an helicopter.

The 'first limit' indication would be necessarily preceding limit used when climbing through an altitude, where in, Torque limits would be the one to be hit at lower density altitudes first, and Engine Limits (Turbine temperature or Optimum gas generator turbine related) at higher density altitudes.

My assumption that use of collective pitch would be sub optimal utilisation of either the engine or transmission limits, when analysed for Lama helicopter, was found to be incorrect.

So, if both the indicating systems allow optimal use of both engine and transmission system, is it only the difference in design philosophy or something more that I didn't consider.

Need education and comments.

Thanks

[GipsyMagpie]

One massive problem with pure collective pitch (CP) indication is you have no idea of the torsional stress you are putting through the transmission. If you do a sharp roll (left for american helos) you get a torque spike. Totally unseen in CP. Also the tail rotor power is not considered.

[peeush]

May agree to the first part about a 'Sharp' roll. But torque indications also do not include tail rotor torque either.

[GipsyMagpie]

They do if they are engine torque. Some helicopters with mast torque (412?) wouldn't but most will.

[ShyTorque]

This has already been done. The Puma HC1 had no torque meter, only a collective pitch gauge.

Problem is, you need to know the CP limits in your head, because they are different depending on the flight regime you operate in.

E.g. the limit was lower in the climb than in the hover (and in the cruise) and again different for underslung load work. They also changed depending on DA.

There was no account taken of transient torque spikes so the limits were probably set lower than the aircraft could actually take.

[GipsyMagpie]

Having flown cp-only, cp + torque, torque only and FLI, I can unreservedly say torque only is the way to go. CP only is rubbish as discussed. CP + torque is just more limits and FLI is just pointless when operating in one climate (sure it takes the effort of what is limiting when changing between environments where torque, engine temperature or engine speed might be the limit - I can do that pretty easily with a glass cockpit thanks). It also lags too much for the dynamic flying I do. Torque (as long as it is displayed as engine torque not the ridiculous gauge on AS332) is simple and gives a fair idea of what you're asking of your machine. But give me a decent glass cockpit for it, whatever you do...no moving strips for airspeed, rad alt and bar alt thanks.

[Pilot DAR]

Interestingly, in addition to the manifold pressure (sort of equates to torque) and tachometer, the Schweizer 300CBi's I trained in also had a rivet in the slide plate, which formed a part of the collective friction mechanism. I asked what it was for. I was told that if you continued to pull collective, the friction discs would bump up to the rivet, that was a tactile indication that you were probably in the MP limiting range of power. But, you could continue to pull, and the friction discs would pass the rivet, and you could pull more if you needed to. A crude system, but it seemed to work. I only felt the contact with the rivet a couple of times, and never felt the need to pull past it.