A real life flying Sycamore at the Helicopter Museum

[Fareastdriver]

I have posted this before but for those who have not seen it here it is again.

Powered by the Alvis Leonides nine cylinder radial mounted horizontally across the fuselage. The control system was a gimballed rod that went up through the rotor shaft to a spider on top that was connected to the pitch horns on the blades from above. A system later used on the Scout. This gave you two benefits; appalling power margins and limited control authority. It was used as the Royal Air Force’s basic helicopter trainer up to 1965 and as so was a brilliant choice because if you could fly a Sycamore you could fly ANYTHING. The drive system was a centrifugal clutch so when starting the engine one had to be careful not to crash engage the rotor if the engine rpm went over 1,500.

The starting was typical Leonides. Fuel pump on, magnetos on, starter motor switch with one hand, primer switch with the other and throttle with the third. The fourth would be holding the cyclic in case the rotor brake slipped. A failure to start was normally accompanied by a conflagration from the exhaust in which case the drill was to gun the starter motor until either the fire went out or grew in intensity so that everybody, including you, left the immediate area. When it burst into life it was stabilised at about 1,200 rpm to warm up. Whilst this was happening the generator was on line so you could switch on electrical services. This gave you a necessary opportunity to check the fore/aft CofG. As I mentioned before, control authority was somewhat limited. It didn't matter so much laterally but there was insufficient for all circumstances fore and aft. To cater for this there was a hydraulic trimming system; basically a tank of water under the pilot’s seat and a smaller tank in the boom by the pylon. The front tank had a gauge and the contents were adjusted to suit the single /two pilot and/or passengers. Get this wrong and when you lift into the hover you go backwards or forwards quite rapidly.

After this you engage the rotor. On modern helicopters you check the needles and hydraulic pressures as the rotor accelerates. On the Sycamore, no such problem, no hydraulics. It had manual controls with Q feel, i.e. big adjustable springs that had trim wheels, fore & aft and lateral, to enable you to retain control. At flat pitch and IIRC 245 Rrpm you checked the cyclic movement and the trim effect. You also checked the freewheel because if that didn't and the engine stopped you were dead. Once you had set the trim to two notches right and on notch back you lifted into the hover. They had installed a pilot's aid in the Sycamore. It took the form of a cam on the collective linkage that opened the throttle progressively as you raised the collective.

Unfortunately it used to drag and lead quite ferociously. To overcome this and maintain a constant Rrpm one had to close the throttle for the first third of travel and open it for the last bit. The throttle was mounted laterally on the end of the collective pointing inwards to the pilot so it worked in the opposite sense to a motorcycle which caused a multitude of amusing situations.

Once in the hover you adjusted the trim and checked available power. The Leonides as fitted to the Provost T1, my previous experience with it, had a normal take off rating of 2,800 rpm, 4.5 lbs boost/38 in MAP for five minutes and an emergency rating of 3,000 rpm 8 lbs boost/46 ins MAP for two minutes. A Sycamore with two up and full fuel would hover at 2,850 rpm and 42-44 lbs MAP so you didn't do it for long. Should you for various reasons, temperature, altitude, then one could use the previously mentioned 'jump takeoff'. This involved speeding the rotor up tp 275 Rrpm, and then pulling the collective up as high as you could at the same time applying full throttle. The aircraft would 'jump' into the air and when you were airborne you would trickle the aircraft forward so that you acquired transitional lift before the Rrpm died off too much. To give you an idea of how much collective you had available there was a notched plate on the floor with an indicator pin.

Once airborne the incredible efficiency of the Hafner rotor blades in forward flight would show itself. One could sit back, trim out the cyclic so that it flew by itself at 125 knots in perfect comfort. Any other manoeuvre required brute force and an excellent memory because if you wanted to do what you were doing again you noted the control positions because that was where you were going to have to put them. Being a training aircraft in its later days one had to teach and practise engine off landings. Because of the afore mentioned throttle cam the engine had to be shut down because otherwise the cam would rev the engine when cushioning the landing. This gave rise to the following procedure on finals to a suitable landing area, ideally an airfield from an altitude in excess of 1,000 ft. Throttle closed, collective down to the bottom, flare, wait for the rotor to start to overspeed then pull the collective up two notches on the gauge to stabilise Rrpm. Autorotate at 60 kts and accelerate the engine to 1,200 rpm to stabilise and cool it. This is to make sure it starts after landing. After, preferably 30 seconds, shut the engine down. At about 200 ft start flaring off the speed and as soon as it drops to approx. Zero add two more notches of collective. The aircraft will now descend quite rapidly so at about ten feet before entombment pull half the available collective. This will arrest the rate of descent sufficiently to enable the remaining collective to cushion the landing. However:---------- one cannot land vertically because of the design of the undercarriage. This is mounted in such a way that when the weight goes on the wheels they move out sideways. At best this would cause the tyres to roll off the rims and at worst one would dig in and leave the aircraft lurching over with a decaying rotor and no authority. To overcome this, the aircraft is moved forward at the last moment so that it rolls on to the ground.

Then the panic No 1 starts! The droop stops were notoriously unreliable so the engine had to be started before the Rrpm dies away and the blades hit the boom. That being achieved heart conditions advise a return to dispersal and coffee. On shutdown the engine is throttled to idle and above the windscreen is a mirror so that you can see if static droops stops have engaged. When the three rods are vertical then the engine can be shut down. Should one or more not engage then Panic No 2 comes into play. The blades will not clear the boom if the droop stops are not engaged. The mass balance on the end of a blade sticks out forward of the tip and resembles an inert 20 mm. cannon shell. In extreme condition to stop these chopping the pylon off a large leather patch is glued to the boom to encourage them to bounce of. At base we were slightly more sophisticated. To prevent the blades hitting the boom the fire section would come out with a high pressure hose. They would aim a jet of water over the boom/pylon joint from the starboard side so that as the blades slowed and lost lift they would bounce off the water and over the boom.


The RAF stopped using them for training in 1965 after a series of rollovers so they were only used by 32 Sqn at Northolt, day only. I flew one there after a few years on Whirlwinds and I realised that I was ducking at 3 Rrpm all the time. It was because I had forgotten how close the rotor blades missed the cockpit roof by. As I said before, the best training air the Air Force ever had; an experience never to be repeated.