Mast angle

[MadMuz]

Ok, so I am just zipping up my flame proof suit and getting ready for the onslaught

[gyroo]

Hi MadMuzIntriguing. I"m very much in favour of responsible experimentation in the workshop or shed, and smaller scale versions can teach us a lot much, much more cheaply than full scale. Lean-back masts result in the Rotor Thrust vector (RTV) running more or less down the mast axis (I stress the "more or less"). Vertical will therefore be less in line with the RTV, and forward-lean obviously is way out of whack with the RTV. My guess is that a lean-back mast would probably be more likely to behave itself in situations where various forces and moments are acting on the machine in flight, but it"s only my guess. Some of those whiz-bang snazzy looking European prototype gyros have radically raked-forward masts, and I must say I have wondered myself how conceptually valid that configuration is.I"m actually wondering how valid your testing method was, however, because you (as I understand it) had the bottom end of the small scale mast fixed firmly in place and with your block of wood representing the engine mount junction 1/3 the way up? Perhaps we should ask was it really a valid experimental methodology? How representative of a gyro in flight was it, given that the base of the miniature mast was clamped or fixed in a way the real thing wouldn"t be?Intriguing, anyway, and it"ll certainly have me trying to replicate your findings out of interest and curiosity, when I get an opportunity in the workshop to do so.Thanks for mentioning it.Mark RMelbourne

[birdy]

The masts on the pretty lookn euro bathtubs need the rediculas forward lean so,s the head is in the rite position to get the required dangle angle.A 2seat tandem with a lite rotax means it"d fly very nose down with a virticaly mast.And yes, the 9* Bach lean is the strongest by far! coz most of all stresses onit are down the mast, in tension like you say.Put a 1/2 tun weight on the bottom of a 6" mast and Nuthn happens.As you lean it over, it"ll start to bend.It"ll probably beak before it got to horizontal.It"s simple phisics, and it"s why forward leaning masts need to be so strong, and ridged, which creates all sorts of other vibration and resonants issues.

[MadMuz]

Mark, as I said, my testing was just playing about with little mock up masts, very unscientifically done.... any real scientist would have just laughed at the simplicity of it all. But, like I said, what I did just proved to me, how right Mr Benson got it when he put together his machines mast angle.... in my own opinion, he was a clever dude and got it very right.I only did my little experiments because I could see, but couldnt explain, that the forward leaning and 90 degree (upright masts) looked wrong. To start with, before my experiments, I used to say to people who thought the forward masts were the new best thing, if you are mowing long grass, the strands of grass (paspallum) that get missed and stand up again are the ones that were leaning in the direction of the mower, the ones leaning towards the aproaching mower, well and truely get cut.... or if you strip a branch off of a tree, you are better to pull it the opposite direction to the direction it is leaving the tree....

[R Beadle]

What have
..........

[birdy]

One thing you gota remember Muz, Ol benson didn"t have the luxury of light horsepower like we have now.The heavier the engine, the more rearward the head placement for the dangle angle.I dout it was intentional from a mast strength perspective. It"s where it ended up after a hang test, and it just happened to be close to the RTV.As for " stronger" masts, be,n ridged, as in triangular ones like the gyroz and the twin mast tandems is not as desirable as first appears.Sure, they are stronger, but they are also much less flexable than a single beam.And flexability is a must if your spin,n a teetering 2 blade rotor, coz of the inherent 2 per rev vibration.If it can"t flex and absorb the vibes, it will only transfer the stress sum where else that"s less flexable.I"m sure you"ll find on the twin mast machines a "slider head" that absorbed the 2per instead of it shaken the crap outa the rest of the machine.From an engineered strength perspective, flexability is far stronger and durable.Can"t say I"v ever heard of a rosco mast bend, break or crack through just flyn it, no matter wot it"s orientation.Wen I first sat I"m Max"s old ea81 side x side rosco and drove down the street to his "strip" it felt like in was sitn ona trampoline it was so soft and wobbly.But strong it was.

[mad max]

Yep, driving a Rosco gyro down the main street, is as smooth as ;D If I am ever in doubt as to the strength of a mast I will place a swaged cable down inside. Keep the mast flexible. If s*#t happens, you may be able to have a safe scary landing. Better than leaving the planet early. Your butt is very valuable.

[Brian]

I"m not going to comment on mast angles but I will say that a rosco with a suby 2.2 on the back needs a new mast at around the 1500 hour mark. Wouldnt say it was going to break or anything but after rosco put a new mast on, the same rotors were noticeably smoother!! So my observation was that masts do have a life, would a 912 powered rosco be the same ??...[ 1500 hours ]

[Tim]

Mast angle is a compromise, as is the C of G of a fixed wing. There are too many variables to mandate a set angle. Suffice to say that the Gyro must hang at the correct angle to ensure that the cyclic stick is as close to the centre position as possible in the most common phase of flight. The hang test then determines the position of the rotor head, irrespective of the mast angle. That rotor head must be must be connected to the frame in the most convenient way possible, taking into account sound engineering practices and strength, with a definitive margin of safety built in.Rotor thrust is always perpendicular to the tip plane axis of the blades, so if the rotors on a Bensen Gyroplane had a tip plane axis 9* back from horizontal, then the rotor thrust vector would be at 9* back from vertical. With a 9*mast, the thrust would have been directly in line with the mast, and the mast would have been purely in tension with no bending moment. As Gyroplanes evolved over the years, many of the variables changed. Some of the things that affect the rotor thrust vector are aerodynamic loads (up or down forces), variable loads (pilot, fuel and baggage), rotor efficiency, air speed and cyclic stick position.To take two extreme examples, a heavy gyro travelling slow with a portly pilot, some down-force from his legs (or anything angled to the incoming airflow), and an inefficient set of rotor blades, might have a rotor thrust vector of ~ 12*, whereas a light gyro at high speed with more modern and efficient blades might have a rotor thrust vector of ~ 6*.And then there"s the cyclic stick position. At full forward stick the thrust vector would be 0* and at full back stick it would be 18*. In practice however this doesn"t normally happen because the airframe quickly follows the command from the changed thrust vector, and the gyro climbs or descends.What all this means is that the mast angle at 9* is not so relevant these days, although it is a good starting point for the uninitiated. With rotors that have a good lift drag ratio, and a faster cruise than the old days, probably a 5* to 7* mast would be more appropriate.Most of the vertical mast machines have very light Rotax 2 strokes, and to effect a correct hang test, purely from an engineering point of view, it is far better to have a vertical mast than to have a 9* mast with cheek plates hung 8" forward from the top of the mast (which has a built in negative bending moment even before you compound the bend by pushing it in the middle with 300lbs + of thrust). With the light overall weight of a Rotax 2 stroker the mast is way over strength anyway.The case for the tandems with an inclined forward steel mast is not favourable for aerodynamic or rotor balance vibrations. Personally, I don"t like them but one would have to assume that the manufacturers have done their scientific tests and calculations and come up with a safe product.While Madmuz"s small scale experiment was probably irrelevant, it does demonstrate the vulnerability of masts in marginal conditions, especially if a gyro is built from the incorrect grade or size material.

[MadMuz]

Hi Tim, all so true, and

[birdy]

The redundant cable down the mast, well, least you"ll die quick.Unless the top is anchored zactly under the rotor bearing bolt, any weight after mast separation will very violently apply cyclic inputs to the rotor, choppn everything in its path.Better to never fly higher n you know you can jump.

[MadMuz]

Its called wishful thinking Birdy Was before I knew just how strong that mast material was ;D

[gyroo]

Hi guys,I"m with Birdy on this one. From the very large numbers of accident reports I"ve seen and also from the mounting number of accident investigations I"ve now been involved in, it strikes me that one of the most interesting aspects of gyro accidents is that the mast usually fractures and snaps just above the engine mount junction and about half the time the mast and rotors end up some distance away from the frame, and the other half of the time the fractured detached mast and rotor are not too far away because one, or both, of the vertical control rods is, or are, still connecting the detached rotor to the frame.

[birdy]

I"v heard said by people"s smarter n me ( that"s pretty much everyone) that the standard oz mast is up to ten x engineered on singles.Even if it"s only 500% engineered! that"s as good as 5 x redundancies.

[Brian]

During a phone call from a slightly unhappy fellow member, they commented on how little information there was to be had on the forum & that the forum was basically worthless drivel....well, not in so many words !I hope that member is looking at the forum & checking the topics cause there has been a lot of very helpful info & technical information to be had here .Its disappointing to see members critising people just because they dont have 10,000 hours in a gyro etc etc but you dont have to be einstein to see that people like Mark Regan do have a lot of technical information stored away & that whilst he might not know it all, he will take information on board, research topics etc and as such, ASRA & the forum are a much better place to gather good information. He isnt afraid to put what he knows up on the board for the world to see.Whilst I mentioned Mark, its not taking away anything from our very learned Tim McClure, Allan W, Birdy, & a host of others. Those who frequent the USA forum will see that they have their Chuck Beatys etc who are a wealth of information. Anyway, great stuff fellas !!!