Movement of driving area in fwd speed auto
 

Could someone pl tell as why the driving region moves to retreating side in fwd speed auto

Because there is now more airflow from the front, causing the area of reverse flow to increase (move outboard), taking with it the stalled area, the driving area, and the driven area.

Go fast enough, and a few things can happen :
(a) the area producing lift gets too small to support the weight of the aircraft;
(b) retreating blade stall comes along to stuff everything up, and
(c) the drag on the blade slows the RRPM down and it all turns to worms.

Also, the AoA on the advancing side is reduced, especially towards the outboard, pushing the driven area inward.

Remember that the advancing blade has to throw away most of its lift, because it cannot have any more than what the poor old retreating blade can come up with - otherwise you roll over. The faster you go, the more potential energy (altitude) you are trading in for drag and ROD, as the blades aren't doing too well.

True, but only above speeds above min pwr speed. Below that, the faster you go in auto, the lower your RoD.

Marly, the only way to go faster is to lower the nose and trade off the altitude for airspeed. Bigger loss of altitude = bigger rate of descent.

And anyway, how can you go faster if you are staying below min power speed?

On the power / airspeed curve, you have to replace the "power" curve with "altitude". At a certain speed, the loss of altitude is minimised. Go slower, you sink faster. Go faster, you sink faster, but get a bit more range.

Dissymmetry of Lift.

The faster you go the more your ROD.
https://cimg4.ibsrv.net/gimg/pprune....3d4e9fe709.jpg

Ascend Charlie, please look at paco’s graph, which quite clearly shows RoD REDUCING with increasing speed, up until the point marked ‘endurance’.

Paco, you seem unable to read your own graph.

“And anyway, how can you go faster if you are staying below min power speed?” WTF?? You lower the nose to accelerate, just like in powered flight.

I’m guessing you two have never actually autorotated anything. a vertical autorotation has a very high RoD. This reduces as IAS is increased up until around min pwr speed (the bit marked ‘endurance’. RoD then builds again as IAS increases.

I have demonstrated this in live aircraft hundreds of times.

I think you'll find Paco and AC have autorotated plenty of aircraft and both have a great deal of experience.

Paco's graph shows that from zero airspeed to min drag (bucket) speed, increasing the IAS gives a decrease in RoD but once you go past that speed, further increase in IAS gives increase in RoD.

Thankyou Crab@. Precisely as i said.

Nothing I wrote is in conflict with Paco's chart.

Think, also, about when you have been "in live aircraft hundreds of times" - you are in a steady auto at, perhaps, 30 kt.
When you want to speed up to 60kt for the flare, or if you have regained the profile for the aiming point, you lower the nose, and for each 10 kt you want to regain, you will burn off 100', rather rapidly. Once back in a steady state, the ROD will settle back a bit, but the speeding up greatly INCREASES the ROD until you reach the steady speed.

You are absolutely right.

You are guessing. But just a bit off.

Sorry, wasn't clear with the comments, in a hurry for McDonald's breakfast :)

I've done plenty of verticals, thank you very much :) It's the only way to get into some clearings and not much other choice with a longline on. Also an essential tool when flying 11 kv lines, which is why I used to teach them. To this day, I would rather do a vertical from the top of the curve on a jettie* than mess about trying to gain speed etc. You'll be too late with the surprise factor anyway.

*high inertia blades and built like a brick outhouse. Don't try it at home on anything else.

In the early 70s we used to do them in Hueys too - hold the aim point in the chin bubble, go past the "normal" flare height, and when you get to the bit where your bumcheeks take a bite out of the seat cushion, pull sharply on the lever. Then cushion on, wait for the heart rate to stabilise, and do it again.

​​​​​​​One suggestion was to fit a broomstick through a hole through the floor under the lever, which flared for you as it touched the ground. Never tried that one, though.

LOL! There is a secret weapon as pointed out by Dennis Venturi (no relation) in the old Canadian Helicopters magazine. Dynamic Stall (see NASA AMES) generates a vortex over the blades if you do a very quick pullup on the collective, so much so that he calculated you could double the lift on a 206 for a few seconds beyond the static stall so that gives you a little more leeway with the pucker factor at the end.