For Nick Lappos, Rich Lee and/or Shaun Coyle re B412 speed limitations
 

Guys,

On the Bell 412 the min IFR speed is 60 KIAS. I was wondering where this speed comes from. Is it to do with longitudinal static stability?

Also on the Bell 412 with one auto pilot disengaged max speed is 115 KIAS however, with both failed there is no specific limitation other than VNE. Why? Is it AFCS performance?

Thanks,

WBS

Minimum IFR speed is going to be derived from a whole host of factors. I suspect for the 412, it has something to do with any of the following-
a) heading hold reverts to the yaw channel somewhere below this speeed
b) starting to go to the back side of the power curve, and very difficult to fly accurately at this speed repeatedly (I know it can be done, but probably not well or for long)
c) trying to maintain ball centered flight is difficult below this speed (takes a lot of sideslip to generate enough sideforce to move the ball, or put another way, you can be in ball centered flight and have any one of large variety of sideslips)
d) gains for the autopilot may not be optimized below this speed
e) a go-around using the autopilot go-around feature, which uses (if memory serves me right) 70 or 80 knots, would be a complicated affair from a speed lower than 60 knots, particularly if single engine
f) some other reason I can't recall as the caffeine hasn't kicked in yet....

Personally, I'm suspicious of helicopters with a minimum IFR airspeed lower than 60 knots unless they have some sort of super-airspeed system. If you're at 40 KIAS for example, there isn't enough dynamic pressure below that airspeed to tell you anything useful. So if a gust knocks you back below 40 KIAS, what have you got?
If we use a parallel to FW, their approach speed is 1.3 times stall speed. We don't stall, but if we use the airspeed indicator as a loose parallel then we should have 1.25 times the minimum airspeed that indicates reliably. The 'reliably' part would include at maximum lateral CG, and with the slip ball not necessarily centered - high sideslip angles will really screw up most airspeed systems. Just a thought.

The current FAR/JAR requires a positive lateral gradient with sideslip - the aircraft must roll and the stick must go laterally to trim a side slip. Also, the pedal must trim to a new pedal position that supports the sideslip. Since helos are hover machines and have no natural gradients in a hover, there is a speed where the rules are no longer met, the aircraft is flat in its response to yaw, and for most helos it is 55 to 60 knots.

This is based on airplane flight characteristics. No helicopter has any natural roll with sideslips below about 55 to 60 knots, so no helicopter can be IFR certified below those speeds, in spite of autopilots that do all the work.
Also, the longitudinal static stability must be positive at Vmini, but this is seldom the reason for the current 55 to 60 knot Vmini.

I disagree with Shawn, many helos allow IFR flight below Vmini with good results - every SAR approach is an example. I have flown perhaps 200 approaches with a DGPS coupler to a hover, all very nice and all without a care about obsolete FAR requirements.

For the speed limitation with one helipilot engaged, I've always assumed (although I have no positive documentation for it) that it was based on the possibility of having the one good one fail. You can fly the 412 at any speed with both disabled, but if you lose them suddenly at high speed, you're going to be very, very busy for awhile, getting it under control. The 412 is flown, theoretically, totally hands off in ATT mode, and if you lose all helipilots without warning at high cruise speed, hands off the cyclic, it's going to be very ugly.

For the min IFR speed, have a read through the Advisory Circular for part 29 certification. There are many test points that have to be demonstrated. The fact that the min IFR speed is 60KIAS indicates all those points were demonstrated at 60KIAS and above. It would be dangerous to automatically assume that any of them failed below 60KIAS, but as Nick and Shawn have pointed out, there are some things that will fail at some point.

For reasons not associated with certification, I've done some of the certification tests on the 412 but not enough to offer a final answer. From that experience, I'd say that the weak longitudinal static stability at min speed in a climb is the test point with the highest min speed.


With one AP disengaged, the remaining channel has to double its gain. The series actuator moves twice as much distance for the same correction, in order to compensate for the lost system. Because of the extra distance it travels, it is easier to saturate the actuator. If this happens, it will feel like you have lost both APs in the one channel, until the actuator is driven back into range. If you keep your speed below 115KIAS (and fly normally) you shouldn't be able to saturate the actuator. With both failed, you don't run the risk of saturating anything as neither actuator is moving, so there is no limit with both APs disengaged.

Matthew.

How did we manage all those coupled approaches to the hover in the dear old Wessex and Sea King, without even touching the controls :confused:


;)



Nick, 200 is just getting qualified, you young pup ;)

"Bell 412 with one auto pilot disengaged max speed is 115 KIAS however, with both failed there is no specific limitation other than VNE."

I would say the restriction is for servo actuator hard-over proctection.

With 2 auto pilots operating, should one servo actuator go hard-over the other auto pilot will sense the anomaly and cause its corresponding servo actuator to motor in the opposite direction therefore mitigating the attitude excursion.

With 1 auto pilot operating a servo actuator hard-over my produce an attitude excursion that could be difficult to recover from if flying IFR.

It should say something about it in the IFR supplement for the aircraft.

Thanks very much for the info guys. Much appreciated.

Cheers,

WBS.

60 Kts min IFR
 

In addition to all the above, on the 412EP fitted with AP (as opposed to Helipilot) with Flight Director and Coupler, the Coupler heading hold stops working below 60 knots.

This is, in my experience, a common cause of total loss of control during the final stages of IFR approaches even with experienced (but rusty scan-wise!) IFR Captains (luckily in the Simulator on all occasions that I have witnessed)

In particular whilst flying a fully coupled IFR approach single engine with visibility at or below minima with a "must land regardless" emergency (e.g. Baggage bay fire or insufficient fuel for a go around) I would recommend holding IAS at or slightly above 70 kts (top side of the minimum power curve) since even a minor scan hiccup allowing speed to fall slightly below 70 kts will result in a rapid further decrease in IAS (as the power requirement increases) resulting in IAS falling below 60 kts, then heading hold ceasing to work then usually disaster (simulated) :{

Why Bell 412 with one autopilot disengage max speed is 115 KIAS
 

Why Bell 412 with one autopilot disengage max speed is 115 KIAS
IS it about servo actuator hard-over.
Please give me a reason and referent source to account for the reason.:confused:

Help me please
 

from yours reason i trust in yours post 100 %
and i want referent document to account for 1 auto pilot operating a servo actuator hard-over,because i get along with yours reason but i can't find source of referent document to describe to the other pilot in my sqdn.

Something else to consider are flying coupled GPS copter approaches that have a 70 knot speed at the MAP. A 10 knot margin between a coupled VminI of 60 knots and a required max airspeed of 70 knots needs 100% attention to the correlation of power and airspeed. Inadvertantly letting airspeed bleed under VminI may find you in a very strange configuration very, very quickly! Not much fun when you fail to break out. Using the IAS mode throughout the approach will keep the heat off.

Yes it is, as a matter of fact that limitation is not present when both Helipilots are Inop.

Likewise the CH-47. You can bimble along at 115kts on 1 AFCS system or disengage both and pull the lever [from memory].

If my memory serves me correctly, when the Bell 412 auto pilot is operating in the dual channel mode, each channel’s SAS actuator operates at 50% authority at half gain. This allows for a corrective response should one channel experience a hard over. With one channel failed the remaining channel has to work at 100% authority with double the gain. In this case, there is no corrective response should the remaining channel experience a hard over. At speeds greater than 115 KIAS the aircraft’s response to a hard over during single channel operations could be pretty exciting.

Jack's response is correct - the reason for a VNE with one channel inop is for hardovers.

And for those others who are worried about the small difference between min and max speeds, why not fly the machine with airspeed hold engaged at 65 knots?

Finally i got the reason for speed 115 knots From Bell Engineer
 

The airspeed limitation mentioned in the flight manual with one AP inoperative, is associated to AFCS hard over situation that could happen during flight.
The autopilot design is very strict, and the autopilot flight control authority is limited by the regulation. The general rule for an autopilot in SAS mode is that the linear actuator must not have more than 10% authority over the total flight control authority. This 10% measurement is for safety reason in case of autopilot failure in hard over situation.

In dual autopilot operation each linear actuators are sharing the work (flight control displacement) and the each linear actuator will only displace 50 % of its travel (equivalent of 5 % total flight control authority). So in case of an AFCS failure in hard over, the failed axis will have a little effect on flight control (roughly 5 % of total flight control movement) and the pilot can easily take over the control without any risk for safety of the aircraft and its crew at any airspeed.

In single autopilot mode of operation, the linear actuator can be displace as much as 100 % of its travel (equivalent of 10 % total flight control authority), So in case of an AFCS failure in hard over at an higher airspeed (over 115 kts), the failed axis will have a greater effect on flight control (this time roughly 10 % of total flight control movement), so the pilot in command will have less time to re-take control of the aircraft.
So the airspeed limitation (115 Kts) is only a safety measure allowing the pilot to take command of the flight control before the aircraft attitude become uncontrollable in the event of an AFCS failure in a hard over situation.

How quaint! Strange then that the EC225 minimum IAS in IMC is 30kts when coupled to IAS mode. Some day you will catch up on t'other side of the pond. (just like old times eh Nick! Even though an old thread, I missed it first time around!)

Helicomparator:
The Super Puma Mk 2 with 4 axis autopilot had a Vmini off 40 KIAS. Did a familiarization flight test when I was with Transport Canada to verify - set 40 KIAS in fully coupled mode.
Watched the airspeed needle drop to somewhere between 40 and the next slower (unlabeled!) line and stay there for several minutes. Never did get back to 40 KIAS. French test pilot agreed it wasn't very good. The limit for Canadian registered versions was set at 50 KIAS (which it held consistently), and resulted in several years of phone calls about 'Why is ours different than the French version?'
I doubt that there is enough dynamic pressure at 30 KIAS to work reliably.

Hi Shawn.

Doh, really wanted and argument with Nick for old times sake but you will have to do instead! Anyway, the L2 was 1980s technology and had an analogue autopilot. I say that knowing full well that it was in fact digital, but it was a digital representation of an analogue control system. It also had a different pitot-static system from the 225.

The 225 is rock steady at 30kts coupled (the ASI reads down to about 20kts). This is in part due to those cunning French using "hybridised" parameters for the control algorithms in a 225 .

In an L2, the autopilot would be looking at IAS only, and responding to any difference between IAS and datum, and responding accordingly. Too much gain and it would overshoot and start to divergently hunt the datum, especially at low speed, so gains had to be kept pretty low hence it was not that keen to get back to its correct speed.

225 uses mostly inertial data from the AHRS, so it knows by integrating the accelerometer data, the speed. The IAS is of course hybridised with this inertial data to remove the long term integration errors, wind changes etc which would otherwise inevitably build up. Because of this hybridised data and precise knowledge of accelerations, it can have much higher gains on the IAS control loop, and they don't have to be a representation of an analogue control loop with all the inherent issues that can create. Discontinuous functions are easy in software!

So the 225 can and routinely does fly at 30kts coupled in IMC, night etc and is rock steady even in turbulence.

When I had a downer on ATC (for reasons I won't bore you with!) I was asked to "slow down to minimum approach speed" by a rather grumpy approach controller. It was fairly windy, and with the minimum IAS coupled to the ILS with IAS mode being 30kts in IMC, the headwind would have resulted in the heli either becoming stationary, or even reversing back up the ILS, something it is quite happy to do. Fortunately sense prevailed (could be something to do with the large fixed-wings behind me) and in the end I only slowed to Vy.

HC