Roll axis (stupid) question .....

[rattle&hum]

Hello All!

Me confused!

I always though that Helicopters would, in the roll axis, behave differently to fixed wing aircraft. In an airplane you start the turn by putting the apropriate control input in and then when you reach the desired bank set the control back to neutral to keep the turn. To roll back you have to move the yoke to the other side until you fly straight and level again and then centralize (or at least thats how I remember it).

Now, I was under the impression that in a helicopter you would do the following: To start the turn move the stick lateral until you reach the required bank - then maybe a bit back but NOT to the central position and keep it there until you want to roll out. To level you just put the stick back to the middle and the helicopter will eventually roll level (not considering any change in pitch or power).

Am I right or wrong? And what is the aerodynamic reason for the difference? (I though it might be because the airplanes turning force acts around the centre of gravity e.g. the roll axis lies in the CoG - and no opposing moment of gravity results in a turn whereas in a helicopter the gravitational force will do so - but I might be wrong :confused: )

Thanks for your replies!

rattle&hum

[Eurochopper]

In fixed wing, to hold a steadily banked turn you probably have to "hold off" the bank with, eg, a touch of left aileron for a right turn. This becomes more pronounced at low airspeed and is because of the speed differential between the two wings (outer wing goes faster so produces more lift)

In a helicopter, this is very much the case due to its natural instability in roll - ie you initiate the turn with a small amount of cyclic but then need more cyclic in the other direction to stop the bank increasing. What may be confusing you is that in helicopters with autopilots (mainly the larger ones), you "fly through" the autopilot - ie the autopilot is always engaged in flight.

The autopilot is trying to keep the wings level or at least resist rates of roll, and in some cases ( AS332 etc ) you need to hold into-turn cyclic - letting go results in the autopilot returning you to wings level. Try doing that with the autopilot switched off and you will be upside down before you know it!!:eek:

[Shawn Coyle]

Sorry, Eurochopper, but that's not the case in an unaugmented helicopter. Having done lots of flight testing and flight test training to show the spiral stability of helicopters, it's exactly the same as a fixed wing.
In constant airspeed above minimum power speed, roll rate is commanded by the lateral cyclic. Roll into a turn requires lateral cyclic, but when the desired bank angle is reached, the roll rate is stopped by re-centering the cyclic.
If you can measure the cyclic out of turn position or force in a small helicopter like the Jet Ranger or Hughes 500, then you've got better eyes and hands than most - there is effectively none.
Try it - in forward flight, roll to a bank angle, and when steady, release the force on the stick - the helicopter will stay at the bank angle.
In a helicopter with an AFCS, all bets are off- the reaction will depend on the architecture of the AFCs.

[Eurochopper]

Shawn - I bow to your greater knowledge of small helicopters - its such a long time since I flew my only 2 small types (B47 and B206) that I may well be wrong. However, an AS332L with autopilot switched off is as I describe - of course its not really designed to be flown AP off so the roll stability is probably allowed to be worse than it would be for a small AP-less type. As you say, this effect is certainly more noticeable around min power speed that it is in the cruise. Why is that?

[GLSNightPilot]

Eurochopper, I've never flown the AS332, so I can't comment on how it flies. However, on every other model I've flown, it works just as Shawn says. And he said any speed above minimum power speed - I don't notice much if any difference with airspeed, as long as it's kept constant. Roll into the turn, then neutralize the cyclic, & you'll have a constant bank angle, more or less, depending on turbulence, etc. If you use the Chinese hat to roll into the turn, then generally you just beep to the angle you want & stop. The AFCS will take care of centering the cyclic, however it works on that particular model. Most AFCS systems aren't designed to be flown against; move the cyclic and you'll likely confuse the system. But there are exceptions, & the AS332 may well be one.

[Eurochopper]

Shawn

It turns out that I share an office with a former ETPS tutor of yours! We discussed the subject of spiral stability and he suggests that I give it a try (when there are no passengers!).

I am not a test pilot but do have the advantage of running our Flight Data Monitoring program so will have access to the flight data recorder data, which of course includes control position information to a resolution of around 0.1% of full travel. It seems that the primary problem with spiral stability is getting the aircraft steady enough so as not to have to move the controls. It occurs to me that I can average the lateral cyclic position in level flight and again in banked flight to see what the difference, if any, is.

However one issue springs to mind immediately - do you keep the pedals in the same place, or apply pedal to keep balanced flight in the turn? If the former, there will be a roll effect from the yawed flight when banked that would tend to reduce any tendancy for the bank to increase, and this would be more powerful with increasing speed. If the latter I suspect that out-of-turn cyclic would be required to maintain bank angle. Perhaps this is where our differing opinions stem from - in my case I assumed balanced flight but in your case perhaps not?

I'll let you know what I find!

[Shawn Coyle]

There is a relatively easy way to do the test in a light helicopter without an AFCS. Don't move the controls (both pedal and lateral cyclic) and lean to one side (if you have passengers, get them to lean as well. Lean for a few seconds, until you get a bank angle going, and then sit upright again. What you have done is introduced a change in the balance of forces and then taken it out. The helicopter, if it has neutral spiral stability, will stay at the bank angle it had when you all sat upright.
In most light helicopters, there is no need to adjust the pedals when you roll to an angle of bank of up to 20 degrees of bank. In some, it is positively a help to take your feet lightly off the pedals as you might actually introduce some force/displacement without knowing it - pedal forces are way too light in general.