Aerodynamics?

[StevieTerrier]

The Bell 222 FM cautions the pilot not to fly in excess of 90% mast torque, as a sudden climbing turn to the right can cause a 10% increase in mast torque and a possible tq exceedance.

Is this due entirely to aerodynamics (loading the disc?) or is there some combination of factors? I also seem to remember some discussion about the engine torque on the 206 rising in a similar situation (fortunatel not by 10%) Did we ever sort that, and is it a similar phenomenon??

[Shawn Coyle]

Whenever you see something like this it's worthwhile to remember what it is that the torquemeter is reading - namely drag on the rotor blades.
If you are at 90% already and do a climbing turn to the right, then with a rapid enough roll the angle of attack can change significantly enough to increase the torque and cause you to overtorque.
Strange thing, this torque stuff.
If you fix the collective at a moderate airspeed and increase airspeed (by descending, obviously) then you'll see the torque decrease, due to the change in inflow, changing the drag on the blades. Reverse happens when you decrease airspeed from a fixed collective setting.
Something similar happens just as you go through translational lift- without moving the collective, the torque decreases!! Drag on the rotor blades. thats what is causing it.

[What-ho Squiffy!]

A dim, dark memory tells me that it is called roll-induced torque spike. The effect can be quite startling when hacking, flicking and zooming.

[tgrendl]

On the 222 it's actually a pitch up or a rollleft that induces the transient torque.

Once the disc has established it's new position the transient portion of the torque goes away.

Shawn could give a very detailed explanation of the why's and wherefores if you can convince him to come out of his nice, air conditioned hanger:D :D

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The increase in torque during a roll was very noticeable on the Gazelle and was explained as follows. During a roll to the right (towards the advancing side) the blades on the right flapped down and experienced an increase in AoA – the blades on the left flapped up and their AoA reduced. Th advancing blades have high AoA and high speed the retreating blades have lower AoA and speed and the overall result is an increase in rotor drag. Rolling to the left produced an opposite effect ie torque reduction. The torque spikes were only evident as the roll rate was increasing, once it reached steady state there was no appreciable difference from the S&L flt torque.
The Gazelle has a fixed spool engine with reset governor – it maintains a constant Nr with no static and minimal transient droop which may account for it’s sensitivity to changes in rotor drag.
Of course the Bell will have rotors turning the other way which does not explain the torque increase in a climbing right turn.
Over to you Shaun and Nick….

[StevieTerrier]

tgrendl - thanks for sorting out my left and right!! I did mean left but it actually came out in the posting as right. How weird is that? Anybody know where I can get a pair of those special pilot "L" and "R" gloves?

Shawn - thanks very much for that. Is the amount of torque change influenced at all by the number of blades? By that I mean that if the 222 had say 6 small blades instead of two Cessna 152 wings, would you see the same torque change?

Air conditioned hangar? Mmmmmmm.

[Shawn Coyle]

Air conditioned hangar - not quite. The hangar is at the current ambient, which is 102F (but it is a dry heat).
I'm not sure if the number of blades would make much difference, but I do remember my Canadian Air Force friends who were flying OH-58A models in Germany telling me that they could always win a dog fight with the French Gazelles across the river by rolling rapidly to the direction that would torque spike the Gazelle (and give a torque decrease on the OH-58). Not that dog fights between helicopters ever happened of course...

[Flare Dammit!]

Actually...and paradoxically, unloading the rotor causes a transient torque increase, and loading the rotor causes a transient torque decrease. Try it!

In turns, Bell 407's can suffer the same overtorquing fate as the 222.

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That is because loading the disc causes an increase in Nr (flare effect) due to the change in direction of the relative airflow increasing the AoA - blade flaps up, c of g moves inwards (towards the hub) and conservation of angular momentum increases the Nr. As torque required reduces as rotational speed increases, the indicated Tq will reduce. Conversely unloading the head will have the opposite effect. Try it in autorotation with the throttle wound off - we all know if you flare in auto the Nr goes up but try following it with a push forward and watch the Nr decay.