Originally Posted by
high spirits
HC.
I'm kind of in agreement with crab, but open to discussion. How do you fly the other way round? Set a power setting on collective and then set a speed either side of the curve to get a rate of climb or descent? Not something I'm familiar with. Could you explain?
The simplest example is when you are cruising at max continuous power. That means you can't raise the collective any more. So if the heli starts to descend, you have to raise the nose slightly and accept a reduction in airspeed. You are in fact flying altitude on the cyclic and leaving the power alone.
In the context of autopilots, when you engage altitude hold on a 3 axis machine, or a 4 axis machine being flown in 3 axis, the altitude hold goes onto the cyclic (assuming you are not going too slowly). It all seems totally normal to me!
In a 3 axis machine flying a non-precision instrument approach you could chose either IAS or VS / ALT.A on the cyclic. The former has the advantage of ensuring an L2-scenario doesn't develop. The latter has the advantage of allowing precise control of the vertical profile including level-off at MDA. Which is best I think depends on a couple of factors: How fast you intend to go, and which is the most important parameter.
So for an approach flown fast, well above Vy, you might choose to couple to VS/ALT.A . For an ILS, where glideslope is the most important parameter to be precise with, I would want to couple it to the autopilot (presuming the autopilot was good at flying it!). However the pitfall in the event of a go-around is that you now have a climb being demanded on the cyclic, you must remember to raise the collective or risk running out of speed, and if you go OEI the most important parameter to have the AP controlling is the airspeed, at Vy.
For an approach flown near to Vy, I would definitely want to couple to IAS and control the vertical mode with the collective.
So there is no one-size-fits all answer. But the pilot of a modern autopiloted helicopter who can't make a sensible decision about which mode to couple and which to fly, and follow it through to a safe conclusion, is incompetent. Just like a pilot who can only turn right, but not left!
Whilst I am rambling on, also consider the interchangability of kinetic and potential energy 1/2 MV^2 = MGH. Cancelling out the constants M and G gives us 1/2 v2 proportional to H. Note the speed squared term. So at low speed, a modest change of speed has a small effect on altitude. At high speed, a very small change in speed (very small change in pitch attitude if you like) has a huge effect on altitude. So if you are intent on maintaining a certain speed with the cyclic whilst doing 150kts, tiny changes in cyclic will induce high vertical speeds that the collective is probably unable to contain, or at least it will be very uncomfortable if you try. It is just not the right way to fly at high speed because it doesn't work!