I've always been fascinated by autogiros, even when I didn't know what they were. When I saw the Monotwirl in a recent magazine ('Silent and Electric Flight International', November 2007), I had to build one. It is all depron in construction and very similar to the current 'Shock Flyers'. I have to say that it is almost the worst plan I have ever seen with some inexplicable notations and some curious construction features that seem to deny the existence of friction. So, I had to build it more or less exactly as the plan to prove that it couldn't fly.
Mine came out at 165 g (5.9 ounces) with a CD-ROM motor and 1200 mAh 2 cell Li-Poly battery. A power/weight ratio that will prop hang and doesn't need the rotor. The 'design weight' is given as 7 - 9 ounces and a 3 cell Li-Poly battery is suggested.
One thing I HAD to change. The rotor hub has a brass tube which is designed to rotate on a carbon fibre shaft and retained with a wood block. I wasn't prepared to see if the wood block would last a whole flight or how long it would take for the carbon to delaminate. My shaft is piano wire and the rotor is retained with a wheel collet, which means that it is removable for transport. The wire pivoted laterally in another brass tube. The rear of the wire was bent sideways to accomodate an aileron clevis fitting.
The first flight took off into a near vertical left bank (the rotor is anticlockwise) but was controllable via the rotor tilt and I managed a left circuit holding on lots of right aileron and got it down in one piece, merely knocking the motor off. The second flight resulted in a low level circuit with the model refusing to climb, which I put downat the time to low temperature and an old Li-Poly battery. Unfortunately, the landing broke the fuselage at the cockpit opening. It stayed in one piece because the wood dowel reinforcing one side of the fuselage didn't break. The third flight - the next day - was very puzzling. I hand-launched, as on the second flight, and the model climbed away, rolled inverted and went straight in. Damage: it knocked the pylon off and pushed the motor sideways.
After more repairs it is clear that the fuselage needs another dowel along the other side to make it stronger/stiffer. The other point concerns the mounting of the lateral tilt servo. The loads in any impact detatch it from the depron fuselage. My misgivings about the construction methods seem justified.
Much more of a mystery is the roll inverted. It rolled right - AGAINST the rotor rotation. And just what sort of control force is needed to lift the fuselage and battery (92% of the model weight) above the rotor?
With some decent weather, I managed 5 flights in one day, although it needed minor repairs to the rotor after the first. The model was very reluctant to climb, with lots of up elevator needed, and flew quite fast. With no coning angle on the rotor there is no self-righting action and it needs to be flown all the time. It will only climb with the rotor parallel to the ground. Any degree of bank and it loses height. It's either underpowered or nose heavy. Its balanced on the rotor axis, as per instructions, which seems right, and it has enough power to pull the model vertically out of my hands.
The problem here is that all landings are made in fast forward flight which results in a nose-over (long grass at this time of year) and some damage to the rotor. The fuselage will only stand a certain amount of this before cracking again. A near vertical descent would be the answer if only enough height could be gained.
Further flying revealed that the model was very reluctant to raise its nose under any circumstances. Chopping the power completely produced a steepening dive rather than the expected descent on an even keel. Another days flying gave some positive input.. Having gained some height and done a steep descent, opening the throttle to flair revealed that the model flies quite well on reduced power. Closing the throttle further produced a floating glide. Then the penny dropped. There is too much downthrust. All that stuff in the magazine article about carefully working out the angles, etc. means nothing. I already had my doubts about whether the model had actually flown! I also repeated the right roll, the cause of which remains a mystery.
This is not meant to be any kind of review or criticism of a published design but when I saw the original article, the thoughts that went through my head were:
1 The rotor is too small.
2 The elevator is too small, and
3 It needs some coning angle on the rotor.
So, far, I can't see any reason to change my views, and there are clearly other problems.
But it does fly and is more or less controllable. I've now added a dowel on the other side of the fuselage to stiffen it and reduced the downthrust. The reduction in downthrust did nothing, but more flying revealed that it was still impossible to raise the nose and a power off descent lead to a steep dive, rather than an autorotation on an even keel. Having demolished two rotors, I made two more. Making one seemed like a waste of time!
Modifications included making the lower hub disc from 1/16" ply, so that the spruce leading edge could be attached to something solid. The spruce was continued across the full width of the rotor that it overlapped and the lower edge was chamfered to give some coning angle. The second rotor of the pair was made rather larger in diameter. This meant that the tail would have to be modified when it was used. The coning angle was nothing excessive, being about 1/2" on the end of each blade. The original magazine article stated that the blades adopted a natural coning angle and that this was assymetric. Both of my first rotors stayed resolutely flat in the air and looked rather odd.
While waiting for the weather, I've managed just one flight on the new rotor - straight into the ground to the left!
Guidance system now installed. The cranial indentation doesn't seem to affect performance
The third rotor was broken after only a couple of flights, all of which rolled into the ground to the left. After cutting away the tail so that I could use rotor number four, I got the same result. So, after a total of 29 flights (none of which exceeded 40 seconds) on four different rotors, the model was so tatty that I scrapped it. I'm convinced that the early rotors only worked because of flexing in the depron hub - destined to be short-lived.
Apart from the things already listed, I'm convinced that the direction of rotation was wrong and that the rotor head needs ballraces. The original design of a brass tube pivotting on a carbon rod I can only describe as a joke.