Autogyro study aids
Join Date: Sep 2006
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Age: 84
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For the benefit of "this is my unsername"
I did spend about a year helping a friend of mine figure out why his Merlin wouldn't fly properly. Rotor pitch angle seemed to be the problem, after much faffing around machining new head plates, pitch control bars, etc thinking it had been set up incorrectly, we put an inclinometer on the mast.
At the bottom end it was correct, at the top it was angled forward of correct by 5deg and bent sideways 3deg.
Some investigation from previous owner revealed it had fallen off a trailer into a ditch!
An "engineer" (gyro) fixed it, signed it off. The owner operated out of a long Tarmac strip I believe so didn't notice the lack of pitch up control and sold it in good faith.
So having spent a while involved in the principles I'm not quite devoid of knowledge of them. Fixed wing pilot or not.
Question.
The Ken Wallis designs all had relatively short masts compared to later ones.
He could push the rotor round while strapped into the seat.
Looking at the side view of photographs of them they seem more "longitudinal" rather than "vertical" in profile, for want of a better phrase.
How does the high mast affect pitch control?
Would an unloaded rotor have a tendency to accelerate/rotate around the CofG when it has a longer moment arm?
Going fixed wing mode for a minute, the thrust line/lift line are not very far apart.
Gyro mode, there is a considerable vertical distance between thrust line and draggy rotor lift plane.
I may be out of order, all wrong, but if so can someone please explain why?
Being a fixed wing pilot doesn't make me ignorant of the gyro and I object a bit at the "expert" remark. I don't need to fly one for hundreds of hours to understand the principle, so can we drop the "fixed wing pilots know bugger all about them" thing please?
I did spend about a year helping a friend of mine figure out why his Merlin wouldn't fly properly. Rotor pitch angle seemed to be the problem, after much faffing around machining new head plates, pitch control bars, etc thinking it had been set up incorrectly, we put an inclinometer on the mast.
At the bottom end it was correct, at the top it was angled forward of correct by 5deg and bent sideways 3deg.
Some investigation from previous owner revealed it had fallen off a trailer into a ditch!
An "engineer" (gyro) fixed it, signed it off. The owner operated out of a long Tarmac strip I believe so didn't notice the lack of pitch up control and sold it in good faith.
So having spent a while involved in the principles I'm not quite devoid of knowledge of them. Fixed wing pilot or not.
Question.
The Ken Wallis designs all had relatively short masts compared to later ones.
He could push the rotor round while strapped into the seat.
Looking at the side view of photographs of them they seem more "longitudinal" rather than "vertical" in profile, for want of a better phrase.
How does the high mast affect pitch control?
Would an unloaded rotor have a tendency to accelerate/rotate around the CofG when it has a longer moment arm?
Going fixed wing mode for a minute, the thrust line/lift line are not very far apart.
Gyro mode, there is a considerable vertical distance between thrust line and draggy rotor lift plane.
I may be out of order, all wrong, but if so can someone please explain why?
Being a fixed wing pilot doesn't make me ignorant of the gyro and I object a bit at the "expert" remark. I don't need to fly one for hundreds of hours to understand the principle, so can we drop the "fixed wing pilots know bugger all about them" thing please?
Join Date: May 2009
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The original Bensen was powered by a high-revving 2-stroke drone engine with a small direct-drive prop, and as a result the required rotor to prop clearance could be achieved with a relatively short mast. I'm told that this put the thrust line of the engine on the original Bensen was pretty close to the vertical CG - as per the later Glasgow recommendations. Some of the later Bensen derivatives were fitted with geared Rotax 2-strokes fitted with gearboxes which allowed longer, more efficient props to be fitted, requiring a taller mast. Whilst the taller mast did have the effect of raising the CG, that was somewhat counteracted by the engine being lower relative to the thrustline due to the "cranked" nature of the gearbox and the engine thrustline moving up - and so a significant offset between the vertical CG and the thrustline was introduced. The original Merlin with a 582 and relatively light rotor blades had some interesting handling qualities - for example it would pitch significantly nose down when power was applied and pitch nose up when power was reduced.
In aircraft with a high engine thrustline relative to the vertical CG then unloading the rotor (eg by rapidly pitching forward and reducing "G") whilst at high power settings can result in the aircraft pitching nose down as a result of the pitching moment of the thrustline around the CG when the drag from the rotor reduces. This in turn can result in a further reduction in "G" resulting in an increasing pitching force and if uncorrected can have a very unhappy outcome. Most modern designs seek to address this by bringing the engine thrustline closer to the position of the vertical CofG and/or use of a horizontal stabiliser.
Homebuilt gyroplanes can be put together with a range of different engines and props and are operated by pilots of varying weights. When they are built they are subject to a "hang check" where the aircraft is lifted by the teeter bolt with the pilot and a typical fuel load on board to check the angle at which it hangs. The reference for the angle (eg mast or keel) and the acceptable range vary depending on the design. A gyro with, for example, a heavy pilot and a light engine/prop will hang nose down - meaning that in normal flight the rotorhead will be operating closer to it rearward limit of movement than it would be with a light pilot / heavy engine. That can becomes an issue when flaring to land as the pilot may run out of rear stick travel before the rotors are at the required pitch angle for the flare. Conversely with a nose-up angle the pilot may run out of forward stick movement limiting the speed of the machine. The normal adjustment for this is to move the position of the rotorhead forwards or backwards until the required hang angle is reached.
In aircraft with a high engine thrustline relative to the vertical CG then unloading the rotor (eg by rapidly pitching forward and reducing "G") whilst at high power settings can result in the aircraft pitching nose down as a result of the pitching moment of the thrustline around the CG when the drag from the rotor reduces. This in turn can result in a further reduction in "G" resulting in an increasing pitching force and if uncorrected can have a very unhappy outcome. Most modern designs seek to address this by bringing the engine thrustline closer to the position of the vertical CofG and/or use of a horizontal stabiliser.
Homebuilt gyroplanes can be put together with a range of different engines and props and are operated by pilots of varying weights. When they are built they are subject to a "hang check" where the aircraft is lifted by the teeter bolt with the pilot and a typical fuel load on board to check the angle at which it hangs. The reference for the angle (eg mast or keel) and the acceptable range vary depending on the design. A gyro with, for example, a heavy pilot and a light engine/prop will hang nose down - meaning that in normal flight the rotorhead will be operating closer to it rearward limit of movement than it would be with a light pilot / heavy engine. That can becomes an issue when flaring to land as the pilot may run out of rear stick travel before the rotors are at the required pitch angle for the flare. Conversely with a nose-up angle the pilot may run out of forward stick movement limiting the speed of the machine. The normal adjustment for this is to move the position of the rotorhead forwards or backwards until the required hang angle is reached.
Join Date: Sep 2006
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I agree with all of that.
It seems that any increase in power, larger dia prop, is going to cause problems as the mast gets longer.
Somewhere about 30 odd years ago I had the quaint notion that a smaller dia ducted fan system might work.
I built an enclosed turbine motor device about 4 inches dia, ran it on compressed air at 10bar and got about 12000 revs, no load, still got it somewhere. The idea was to use a series of fans in a Venturi tube, prob about 3 feet dia 2 feet long driven by a hydraulic pump/engine. Two of those side by side driven off one decent engine placed wherever suited the CofG.
The whole idea was to get the rotor/mast down as close to CofG as possible.
I never got the chance to build anything so it remains in the daft ideas tray.
I still think the rotor drag is not being considered enough. CofG and thrust line, yes but that long mast bothers me.
It seems that any increase in power, larger dia prop, is going to cause problems as the mast gets longer.
Somewhere about 30 odd years ago I had the quaint notion that a smaller dia ducted fan system might work.
I built an enclosed turbine motor device about 4 inches dia, ran it on compressed air at 10bar and got about 12000 revs, no load, still got it somewhere. The idea was to use a series of fans in a Venturi tube, prob about 3 feet dia 2 feet long driven by a hydraulic pump/engine. Two of those side by side driven off one decent engine placed wherever suited the CofG.
The whole idea was to get the rotor/mast down as close to CofG as possible.
I never got the chance to build anything so it remains in the daft ideas tray.
I still think the rotor drag is not being considered enough. CofG and thrust line, yes but that long mast bothers me.
