Schluter launched a model that was to change my life and open up the way for me to make a full-time living from model helicopters. The Schluter Heliboys was advertised as being aerobatic – and it certainly was. The main reason for this was its ‘pod and boom’ design that made it light and mechanically simple. In fact this was one of the first ever pod and boom model as kits up to this point were based on full size helicopters with fuselages that covered the engine and mechanics. It was also easy to modify – with a few tweaks you could increase the throw on the paddles and rotor blades making it very manoeuvrable which suited my style of flying. Fortunately the rest of the mechanics could handle the stress that this placed on them.
One day a friend of mine said to me “Why don’t you fly inverted?” This was the one manoeuvre that most people thought was impossible, due to the many technical challenges that are involved.
For example, when upright the spinning rotor generates lift by ensuring the blades have a positive pitch. But to fly inverted, the direction of lift must be reversed or the helicopter will ‘suck’ itself into the ground. This means changing the pitch of the blades to be negative. However, engine speed is typically connected to the pitch of the blades. The more positive the pitch the higher the engine speed, which is fine for flying normally. But when, and only when, flying inverted we need to maintain engine speed while decreasing pitch. With today’s computer controlled transmitters this isn’t a problem, but at that time transmitters were very basic and there was no simple way to make this happen.
Another problem is to do with the flow of fuel to the engine. In those days, fuel was picked up from the bottom of the tank via a pipe that had a weighted end. Air vents were placed at the top of the tank to ensure a vacuum doesn’t occur as the tank empties otherwise fuel would stop flowing to the engine. The only trouble is that when inverted, fuel would block the air vents causing the fuel supply to the engine to be disrupted. The last thing you need when inverted is for the engine to splutter or stop!
As well as the technical challenges, there are challenges facing the pilot in how he sees and reacts to the attitude of the model. As already mentioned, transmitters were simple affairs and they didn’t have reversing switches. This meant that if you did get the model inverted, then for control purposes you had to reverse your brain for some of the functions. For example, to ascend normally means that the pitch stick has to move up, but when inverted, to go ‘up’ involves moving the Pitch stick ‘down’. This of course assumes that engine speed is being controlled independent of pitch position.
To add to the confusion, when inverted, turns to the left via the tail rotor now go to the right while forward and back cyclic direction is also reversed. But bizarrely, left and right cyclic direction stay the same. Many would-be helicopter pilots struggle to ‘reverse’ controls when the helicopter turns in front of them when flying the right way up, flying inverted greatly increases complexity. But these were challenges to relish and so I came home and thought about what I had to do.
As mentioned above, most helicopter setups have the engine speed on the same control as the rotor blade pitch. The control is usually arranged so that at low engine speed there would be negative pitch on the blades that would gradually increase to positive pitch at full engine speed. (It’s actually not quite that simple as the engine speed is not directly proportional to pitch but we can ignore this for our purposes). To achieve both normal and inverted flight, I knew that I had to have full throttle at both the low and full pitch positions, and at the mid throttle point I needed zero pitch. As far as pitch was concerned, I knew I needed to have +8 degrees to fly normally and -8 degrees to fly inverted.
After puzzling over how I could do this, I put the throttle onto a switch so that it was either ‘full on’ or ‘tick-over’, leaving the ‘throttle’ stick on the transmitter to operate just the pitch of the blades. This made the model quite hairy to fly, so you had to know where the zero degree pitch was.
I solved this by placing a groove on the transmitter ‘throttle stick’ ratchet so that as the stick moved to zero degrees there was a ‘click’ that could be felt through the stick. This meant I could now invert the model and use the remaining ‘throttle stick’ to put in negative pitch. This worked quite well. Of course while inverted I had to ‘remember’ to reverse the other controls for left and right manually. This was fairly easy to do provided you concentrated and no one spoke to you.
The first time I went inverted, the model did a couple of pirouettes and a lot of backward flying. This was because I had forgotten to reverse the cyclic control to make it go forward, but I didn’t crash. In fact I can’t ever remember crashing through pilot error but the engine spluttered a few times due to fuel starvation
To fix the engine problem I first of all put in top and bottom vents to the tank and fitted them with one way valves to stop the fuel leaking out. It didn’t work. What I needed was a constant fuel supply so I decided to pressurise the tank – something that wasn’t done in those days. I didn’t know how to do this but after experimenting with connections to the manifold and exhaust pipe, I finally got there.
I soon perfected inverted flight. I wasn’t aware of anyone else who had done this although I learned later that Mike Mass and Ernie Huber in the US had also managed to achieve inverted flight. I’m not sure which of us was ‘first’, but I was certainly the first in Europe."
To view Len flying inverted, check out the following YouTube video of the first F3C world championships held in 1985. Len's flight is at 7:35.