The Cabri heliciopter - comments on the belt drive and fenestron etc.

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TheGreenAnger
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The Cabri heliciopter - comments on the belt drive and fenestron etc.

#1 Post by TheGreenAnger » Tue Jul 26, 2022 3:06 pm

Prompted by the sad news of a passenger killed by contact with a helicopter tail rotor I was apt to think of the Cabri light helicopter which has a fenestron which is less likely to injure or kill the unwary and its rather unique clutch system compared to say the R44,

The R44 system comprises of a vee-belt sheave (belt pulley) which is bolted directly to the engine output shaft. The vee-belts are tensioned by an electrical motor which jacks the upper sheave up or down to tighten the pulleys which then transmit power to the upper sheave which has a freewheel unit (sprag clutch) much like the one used in a bicycle, contained in its hub. It works, is simple and best of all it is light and relatively simple to maintain.

http://www.copters.com/R44_SkinsOff/Upp ... heave.html

The Cabri helicopter clutch belt system and fenestron are more esoteric.
The clutch system is neat, a hydraulic ram pivots the entire engine to tension the drive belt. It only moves a couple of mil, but that’s enough. It has dual ignition but only one magneto, mounted on the back of the engine, the second spark comes from a plasma ignition system, providing smooth delivery at low revs. The oil filler is easily accessible for topping up, unlike some helicopters I could mention. The tail cone, made of a single composite moulding, is bolted to a tubular lattice with multiple load paths to obviate the consequences of a failure, and tail rotor pitch actuation is by an enclosed semi-rigid steel rod that looks like a Bowden cable but isn’t: Andy insists it’s a ball bearing-supported flexible linear shaft and it’s rather more expensive.

Except that we shouldn’t call it a tail rotor because it’s a fenestron and the pilot needs to be fully aware that a fenestron is not a tail rotor. More of this later, for now, note that the fenestron has seven blades enclosed in a duct that markedly reduces the chances of one of the most common fatal accidents helicopters suffer, with people walking into tail rotors. The blades feel flimsy and are loose on their bearings, but they’re remarkably efficient and damage tolerant, they’ve sawn one through at 30 per cent span and run the tail at max revs for 100 hours without any further damage and even rotating nine times faster than the main rotor, it’s much quieter than a conventional tail rotor. The vertical fin is offset to aerodynamically unload the fenestron in cruise flight where hovering requires 80 per cent tail rotor thrust, cruise requires only 40 per cent... Deceleration and spot turns are where you need to understand your fenestron. Unlike a conventional tail rotor, where the thrust to power relationship is linear, a fenestron is more effective at low pitch, but less effective between about 40 and 80 per cent pitch. From 80 to 100 per cent it is significantly stronger. This means that the last inch of pedal travel packs a mighty punch, but the pilot who thinks it’s a tail rotor is unlikely to stamp it all the way to the floor. So coming to the hover was the next thing I made a mess of. The tail starts wagging below 40 knots when you lose the aerodynamic effectiveness of the vertical fin, and as you begin to bring the lever in you need lots of right pedal or you’ll arrive sideways, as I did.

Turning left, you need to understand that non-linear fenestron response. If you overdo the pedal, the helicopter will start spinning on the spot. If you counter with the amount of pedal input you’ve been used to in other helicopters, it’s not going to stop. And if, God forbid, you pull up the collective, it’s going to spin even faster. The Cabri is the only piston-engined helicopter with a fenestron, and the instant availability of torque without turbine lag speeds the yaw. The cure is to get your right foot all the way to the floor and sit there until it stops, and don’t touch the lever. A pilot trained on the Cabri will start a spot turn left with a twitch of left pedal, and then advance the right foot to control the rate.

Recovering from autorotation, there’s an added factor if you let the RRPM get a little low. As the governor comes back in at 2,000ERPM it will want to boost the rotor revs, and there will be a noticeable yaw kick as it does so. If at the same time you’re decelerating through the 40-knot loss of aerodynamic anti-torque zone, it might be a bigger kick than you’re prepared for.

I don’t want to overstate this, if you were coming to it new, you’d hardly think about it. But a lot of pilots have habits that are hard to break, and they can’t afford to lose concentration. The accidents that have befallen the Cabri have all been because the helicopter was better than the pilot. G-UIMB had to be bashed straight after an instructor got crossed up and went into an uncontrolled yaw on arrival. Turned out he’d not flown the Cabri for two weeks and had spent that whole time flying other-way-round tail rotor helicopters.



https://pilotweb.aero/airecraft/flight- ... 2-6241440/
My necessaries are embark'd: farewell. Adieu! I have too grieved a heart to take a tedious leave.

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