Article: 2894 of rec.aviation.homebuilt Path: newshost.ncd.com!ncd.com!olivea!charnel!yeshua.marcam.com!hookup!batcomputer!munnari.oz.au!ariel.ucs.unimelb.EDU.AU!werple.apana.org.au!zikzak.apana.org.au!not-for-mail From: rando@zikzak.apana.org.au (Brett Paulin) Newsgroups: rec.aviation.homebuilt Subject: More Gyro info Date: 14 Mar 1994 22:22:18 +1100 Organization: Zikzak Public Access UNIX, Melbourne Australia Lines: 327 Message-ID: <2m1hdo$pju@zikzak.apana.org.au> NNTP-Posting-Host: zikzak.apana.org.au G'day people.. Well, I had a tremendous response to my first post on Gyro's and no flames, so this has encouraged me to continue with some more info.. First, I'll quote part of an e-mail message to another bloke about pre-rotators and stuff.. then see what else I can follow that up with.. ======================================================================== > read about gyros with pre-rotators, which in principle allow you to > lift off with zero take-off roll. Combined with what you describe > concerning proper landing procedures, one has in effect a helicopter, no? Well.. not really.. close but not quite.. B-).. A Gyro can't hover in still wind... although it won't stall like a plane, it will sink with zero airspeed.. you have to maintain about 10-20mph to hold altitude.. of course if there'e enough wind, you can _appear_ to hover by flying into the wind (and even appear to be going backwards if the wind is stronger than your minimum flight speed).. but you still have forward airspeed.. A vertical sink landing is only an emergency maneuver to use when you have nowhere safe to land (say an engine out over a forest or something..) and consequently want to impact with the minimum forward speed.. Without forward airspeed, the vertical sink rate of a Gyro is about that of a Parachute.. which will put _you_ down reasonably safely, but will probably bend up the machine... the normal landing _touchdown_ speed is 0, but the _approach_ speed is usually around 40-50mph. This is so you maintain enough air through your rotors to keep the revs up and thus be able to flare at the bottom. All the speed is dissapated in the flare before touchdown so a properly executed landing will have 0 ground roll, and if you're skillful enough, you can probably touchdown with just enough space to clear your blades, but you can't hover in like a chopper.. As for the prerotators.. A pre-rotator will greatly reduce your take-off roll (and in some cases will cut it to 0 if it's powerful enough).. but so will a good stiff breeze bring the rotors up to flight speed if you wait for them to spool up.. But the gyro cannot ascend vertically, even if it manages to break ground with no roll.. If you have enough engine power, then you could climb vertically by virtue of sheer grunt, but this wouldn't be a good idea, as an engine failure in this situation would leave you nose-high, no airspeed, and not enough altitude to pick up airspeed to flare out at the bottom.. which means you are about to do one of the aforementioned vertical sink landings and possibly bend your gyro.. A max performance _safe_ takeoff would involve breaking ground with little or no ground roll (depending on wind and prerotator strength), then putting the nose level and accelerating to best climb speed (usually about 40-50mph) before climbing out too steeply.. that way you have airspeed to autorotate with if the engine snuffs.. BTW, while a helicopter _is_ capable of ascending vertically, where a gyro (usually) is not, you will very rarely see one do it, unless the pilot _has_ to, to get out of a spot, or is showing off.. All Helo pilots know about the "deadmans curve" which is the height/airpseed graph.. it will show you that you shouldn't be above about 10 feet unless you have a reasonable amount of speed on the clock.. same reason.. a helo without power is a heavy Gyro and must autorotate in.. for this you need airspeed... So most helo pilots in a normal take-off will hover off the ground, then accelerate forward and climb out, just like a gyro.. The helo height/airspeed graph will show that you should have a minimum altitude (usually 300-500 feet depending on machine weight) before it is safe to reduce airspeed to 0. This applies to gyro's also.. that way you can lower the nose, and dive to pickup airspeed if the engine stops.. I've seen one Gyro totalled this way (no injury to the pilot). He was hovering into a strong wind at about 100 feet of altitude. His airspeed was way back (probably 0) and he was nose high. He went to advance the throttle to power out of the hover, and the throttle cable slipped though the lever (he used a poor cable-crush type of clamp which loosened off..) and the engine went back to idle. He got the nose down and started to dive, but didn't have enough height to regain sufficient airspeed to flare at the bottom. The machine landed level, but hit fairly hard (due to the low rotor revs caused by the low airspeed) and one of the vertical suspension struts bent and snapped.. The gyro toppled over with the rotors still spinning.. rotor ground strike and bits flew everywhere.. (Even spinning slowly, a 23 foot diameter rotor packs a lot of momentum !) a $4000 lesson in safety (new rotors, prop, mast, control rods etc).. > One worry, of course, is how much complexity (and increase in > failure probability) the prerotator introduces. Do you have any > thoughts? Apart from the above ? B-) A prerotator is a nice option to have on a gyro.. it saves you from developing a few arm muscles from hand-starting the rotors.. (you usually have to spin them up a bit by hand before the wind will take over bring them up for you).. and it will allow you take off in much shorter distances on a calm day with little wind.. on a windy day, a non-prerotator equipped gyro will get off in just as short a distance.. he just has to wait a bit longer for the wind to speed the rotors up rather than powering them up.. They're handy to have if you are limited in strip length or are lazy.. B-) They add a bit of weight, and a fair bit of cost.. most prerotator setups will add around $1000 Aus Ato the cost of the machine.. (Hydraulic or cable drive).. you can do it a bit cheaper by using a starter motor and a ring gear on the rotor head, but thats heavy, and means you must also carry a fairly hefty battery to power it.. (unless you already have one from electric start on your engine.) Failure likelyhood.. not much.. even if it does fail, it's not likely to be dangerous.. just inconvenient when it doesn't work and you have to hand spin the blades.. I have heard of one fatality caused by a prerotator.. The cable-drive type usually have a little rubber wheel which friction drives off a drum mounted in front of the propellor. This wheel is usually pulled into engagement by a bicycle-type brake handle and cable to the wheel-pivot mechanism. The handle is usually mounted on the control stick a bit lower down than the normal holding position.. In this case, the pilot had fitted too long a cable to the engagement mechanism, and just left the excess floating unsecured about the engine.. at high forward airpseed, the slack cable blew back into the propellor and got tangled around the drive drum.. This wrenched the stick forward (seeing as how the engage cable was attached to the handle on the stick) beyond the pilots strength to counteract, and the gyro proceeded to dive into the ground at high speed.. One of those "mechanical failure caused by pilot/builder error" cases I mentioned.. The ASRA (Austrlian Sport Rotorcraft Association) now reccomends that no secondary controls (prerotator, wheel brakes etc) be mounted upon the control stick.. One last thing on prerotators.. another way to get a gyro off the ground in short order is the "Jump takeoff" technique. This usually involves de-pitching the rotors (Gyro Rotors are normally fixed in pitch), accelerating the blades beyond normal flight speed (which is around 300-400 rpm) to 500+ rpm with a sufficiently powerful prerotator, then suddenly throwing in the normal flight pitch.. This results in the Gyro leaping vertically into the air (up to 50 feet given sufficient weight (inertia) in the blades) from which point, you can accelerate to flight speed.. This feature was introduced to Autogyro's in the 1930's, and worked well (The sudden leap was probably a bit hairy !), but so far, no one has come up with a functioning jump takeoff system for the ultralight style gyro's we fly.. the Popular Rotorcraft Association currently has an $5000 (and increasing each year) prize to the first person to come up with a practical jump takeoff system (must meet certain criteria) for ultralight Gyro's So, given all of the above waffle, the summary is, a Gyro is not quite capable of helicopter-type takeoff's and landings, but can come very close for a lot less cost and complexity.. > When you say 'watch out for negative Gs', are we talking about > maneuvers which deliberately invoke negative Gs, or very bumpy air? > While hangliding, I have many times 'gone over the waterfall', > exitting a strong thermal to find myself in major sink, effectively > putting me in the negative (I would be thrown up against the keel) for > several seconds till the glider was flying again. These are pretty > unusual conditions, but in encountering them, would a gyro flail > itself and flip as you describe? Not that I know of.. Bumpy air doesn't bother Gyro's too much, and I've come out the other side of a strong thermal and lost a fair bit of altitude myself on occasion without any sign of significant negative G's.. The Gyro wont suddenly "fall" when it drops out of thermal, it will just start to sink slowly, necessitating either some power input, or lowering the nose to build airspeed and fly out of it.. The rotors have a considerable amount of inertia, and a momentary loss of load shouldn't cause any problems, provided it's not a prolonged, strong negative G situation such as a "pushover" after a rapid climb.. There are other factors that affect this as well.. such as thrust lines etc.. I'll see if I can find a copy of some mail I sent to another bloke and post it ****** OK, here's the bit on Negative G's/Pushovers I sent to ****** ****** another Bloke ****** > I've heard a bit about what happens when the airflow gets on top of the rotor > how serious is that? Depends on what you mean.. Theres a couple of things that can cause that.. The most common is P.I.O. (Pilot Induced Oscillation), also known as porpoising (after the actions of a dolphin or porpoise leaping out of the water).. This is where the pilot (usually an untrained Gyro pilot) gets "behind" the machine, in that he does not compensate for the "Lag" in a gyro's control system and begins overcorrecting.. (like fishtailing in a car). The gyro dives down, he pulls back (too hard and too late) and the gyro zooms up, so he pushed forward.. again too hard and too late, and goes zooming down again.. What eventually happens is that he pushes forward too hard and fast, which results in negative G's (I'll explain in a moment) and a (usually fatal) crash. Negative G's causes a couple of things... The rotors turn by virtue of tha air passing _up_ through them. This requires there to be a load on the rotors (the weight of the machine and pilot) and forward airspeed. This "positive G's" where you feel firmly anchored into the seat, just like sitting down normally... If you fling the gyro into a tight turn, you increase the load on the rotors (and they automatically speed up to carry the load). and the positive G force increases.. which makes you feel squashed down into your seat harder. This is ok, and a Gyro will tolerate huge positive G forces without any worries at all. However, if you are climbing rapidly, or even cruising level at a brisk pace, and suddenly shove the stick forward, the machine will try to head downwards rapidly. it feels like you are going to float out of your seat (if you weren't wearing a belt, you probably could). The rotors are no longer carrying the load due the simulated weight decrease of the machine, and they begin to slow down. In addition to this, the airflow is now coming _down_ through the rotors (due to the inertia of the machine carrying it upwards or forwards with the rotors now tilted level or downwards ), which slows the rotors down even more. This means the centrifugal force that is holding these flexy rotors out nice and straight despite the load on them is decreased, and they will begin to bend downwards. This is problem number one. Problem number two has to do with thrust lines and drag lines.. it's difficult to explain without a piccy, so I'll have a go in ASCII.. Rotors ----------------------------------------------- | <----- Rotor Drag Line | prop | | | | 00 tail | - ### | 00pilot & -----> Propellor Thrust Line /\ | engine| 00seat etc / \ | | 00000 <----- Parasitic Drag Line / \ | 00000 ------------------------------------- O o Not very good, but it'll do.. :) Ok, we have our Gyro cruising along at say 60mph. The engine is operating at 3/4 throttle pushing out a fair bit of thrust. counteracting this is the parasitic drag (which is the drag created by the pilot, airframe,, etc). This drag is down fairly low.. usually about seat level. Up top we have the drag created by the rotors whirling away as we haul them through the air.. (it's a fair bit of drag too.. most gyro's only have a 2:1 lift/drag ratio.. so for every 2 pounds of lift they make.. they create 1 pound of drag.. Compare this with a glider where lift/drag ratio's around 30 or 40:1 aren't uncommon) In the middle we have the Thrust line.. which is the line through the center of the propellor. Normal flight, all is cool. But if you suddenly shove the stick forward at high speed without reducing power first, all that rotor drag vanishes instantly. However, the parasitic drag and the thrust do not. Because the thrust line is _above_ the parasitic drag line, the thrust attempts to push the gyro nose-down. Which is exactly what the rotors are trying to do as well.. The usual result is that the Gyro flips upside down rapidly, and due to our rotors having slowed down (from being unloaded and reverse airflow), they bend downwards, usually strike the tail of the machine and quite often cut it off.. (The tip speed is around 350Mph !). This of course does nothing to help, and the Gyro usually tumbles to ground and crashes, killing the pilot.. This sort of stuff is fairly scary talk to most people.. but then I usually point out to them.. "If you are travelling down a two lane, bidirectional highway at 100km/h (or 55mph for americans) and merely yank the wheel to the right (left for U.S.), chances are you'll be just as dead from crashing into the oncoming traffic.. but that doesn't bother you.. you know better than to do that and are aware of the consequences of doing so.. NOT shoving the stick forward suddenly and rapidly without first reducing power or airspeed is just part of your training.... > I guess I should subscribe to this 'Popular Rotorcraft' you > mentioned, but in the meantime, do your back issues say anything about > dual-gyro instruction area ? I would love to go for a spin > and judge for myself before I commit time and money to building... Unfortunately, I know little about US geography, so what I'll do is list a couple of the bigger Gyro companies that provide instruction, and if they're not local to you, I'm sure they will be able to reccomend who is.. These addresses are out of the latest PRA I can find about which is April 92.. I hope they're still current.. let me know if there's a problem and I'll see what else I can dig up.. Try.. Bill Parson's Gyro's Flagler Co. Airport Bunnell, Florida 32010 (904) 437-7671 or Farrington Aircraft 4460 Shemwell Ln. Paducah, KY 42003 (502) 898-2403 Heres a number I found in the PRA for their office.. who should be able to reccomend a local CFI to take you for a ride.. and also put you in touch with the local chapter of the PRA so you can chat to some other pilots.. contact PRA on (504) 683-3545 If none of that works, let me know and I'll see what else I can find.. > It just occurs to me, wouldn't gyros be just as effective and > much cheaper than the helicopters currently used for cattle mustering? You bet.. There are lots of cattle musterers flying Gyro's in Australia. either on their own property, or as contracters.. Australia's (and possibly the world's) highest time Gyro pilot is Rusty Ferguson.. a full time musterer with over 12,000 (yep!) hours in Gyro's. He's crashed a couple (due to engine failure over inhospitable terrain he often has to muster over), plain worn out quite a few more.. and is the bloke the Gyro designers go to if they want some new part or piece of machinery endurance tested.. it's not uncommon for Rusty to clock up in excess of 30 or 40 hours a _week_ (as much time as the average bloke spends behind his desk in an office job), which is more than some recreational pilots manage per year.. ! ======================================================================== Ok.. that'll do for this post.. If you have any specific questions you'd like to ask.. just e-mail me and I'll see what I can do.. If the answer is of general interest, I'll probably also post it here.. Thanks to all those who sent messages saying how much they enjoyed my first gyro post.. I hope this one lives up to your expectations, and I will endeavour to keep posting as long as I've got something to say and people seem interested.. If there are any other Gyro pilots out there.. why not join in.. ? I don't know everything about Gyro's (far from it), and I'd welcome any help from others with info to contribute.. Till next time.. Catcha... -- ------------ Brett Paulin Rando@zikzak.apana.org.au Zikzak public access UNIX, Melbourne, Australia.