Achieving reliable short takeoff

[mad max]

I saw that guy on U tube a couple of yrs. ago. He got airborne and headed straight for the camera man. A great pre-rotator. No torque reaction. Hydrogen peroxide I assume?

[maulonir]

I built a pre rotator using a pocket bike engine I bought on ebay. 50cc with a built in centrifugal clutch. Set up through a bendix similar to the one in the picture Muz put on here. With it running flat out I couldn"t get any more rotor rpm out of it than the electric starter motor setup that it replaced which was about 150rpm. Rob

[gyroo]

Thanks Birdy, Tim, Mad Muz and Rob,I"m eagerly taking all this information in, and will certainly have it all in the forefront of my mind as I start to experiment with spinning up the rotor.Of course, the advantages of a 2-stroke pre-rotator are that (1) the centrifugal clutch at the motor end guarantees a "soft start" at the ring gear (or in my case, the final hub sprocket); (2) the 2-stroke can be run indefinitely, such as during lengthy taxying, or when holding to get onto the runway at a regional airport, or for when doing careful and painstaking spinups for dynamic balancing of the rotor; and (3) the 2-stroke is throttle-able over a wide rpm range after the centrifugal clutch fully takes up (with the lower rpm range being used for taxying, the higher obviously immediately pre-takeoff).A further obvious advantage is that the 12 volt battery will not be tortured nearly as much as with an electric pre-rotator, plus, with this setup there is no heavy electric starter motor hanging off to the side of the torque tube or torque bar (which has been the topic of some debate on this forum in the past).A disadvantage is that the whole setup is easily 10kg heavier than even an electric setup, taking into account the mounting arrangements for the heavy Yamaha KT100, the chains, shafting, and the quite heavy Ringspann Corp 3/4 inch freewheel sprague bearing ($840 alone).Still, it"s gotta be more straightforward and reliable than the extremely strange serpentine belt arrangement pre-rotator on some of the Butterfly gyros!Cheers,Mark R

[MadMuz]

I built a pre rotator using a pocket bike engine I bought on ebay. 50cc with a built in centrifugal clutch. Set up through a bendix similar to the one in the picture Muz put on here. With it running flat out I couldn"t get any more rotor rpm out of it than the electric starter motor setup that it replaced which was about 150rpm. RobThe point I was trying to make before, is that any power unit trying to prerotate our rotors, faces a very steep Hp required curve.... and there will be like a brick wall at some stage, I believe about the 60-80% rpm, due the the pitch in the blades. A small motor, be it electric, petrol, or friction wheel/bendix..... all will easily achieve 50 rpm, quite easily get to 80 rpm, some should make it to 125 rpm, might make it to 150 rpm..... but all would struggle to get any more rpm, once they hit the "brick wall" that the individual set of blades you are spinning up (each will be different) Hydraulic and the long belt and pulleys that run pretty well directly off the engine crank would have the best chance of gaining max possible rpm, but then the weight and complication (and what can go wrong) increases dramatically. If you could achieve any percentage of overspeed with any type of prerotator, you would be able to hover on the prerotator whilst doing rapid, dizzying rotations

[MadMuz]

It sounds great Mark and what I like about the idea, is that the little motor runs entirely separately to the actual propulsion engine..... it can be mounted where you want it,there is nothing to jam in flight that could effect the propeller engine, no cables near the prop, no wheel running on the prop hub, like you said no huge battery, heavy wiring, soft start resistors etc..... simple, just a tiny little petrol engine.... totally self sufficient and independent..... I hope you will put some pics on here (so we can all copy it) ;D

[birdy]

I"v got abit of time tnite, so I"ll elaborate.Mark, the 2 biggest hurdles in your way are the blades pitch, and the square root thing. ( if it takes xx torque to get the blades to 150 rpm, it"ll take xx times 4 to get them to 300 rpm.)I ran numbers ona gadget I"m too old to continue with, and found I needed, from memory, 140foot pounds to get a set of 26" blades to 500 rpm, set at 0 lift pitch.That sorta torque will screw the head off the mast in pretty short time ona regular gyro, so a lot more beef would be needed up top.I don"t want to sound like a knowall, but you list a few advantages of your 2bamger setup.The similarity to the modified RAF spinner are,1;the clutch on the RAF setup is as soft start as you could get.2; the clutch can be engaged as long as you like, even after take off.3; solong as you have the correct drive ratios, the rotor rpm will be well up before the prop starts pushing you forward, and the " down wash" of the rotor on the backstop means you don"t even need ground breaks.But, wen you are ready to blast off, just open the tap fully, the engine overruns the clutch and your off, with torque still transferred to the rotor as your accelerating.And I"v found the trick to short, steep take offs is not so much the initial prespin rrpm, but keepn the torque go,n to the rotor till your clear of wotever your tryn to fly over.A rudder sunk in the blast of a angry 912 has plenty of authority over any torque the prespiner applies.There"s no other engine, no other fuel, not much heavier n an electric spinner, the applied torque is easily regulated by a bike clutch leaver on the throttle and, with over 1500 hours on one ona 912, and 1400 ona 914, without even need for a belt replacement, it"s bullet proof.

[birdy]

Ho, and one more point, the amount of torque you will need to apply to the OFFSET gyro head means there"s a serious leaverage on the stick.Don"t know how much torque this RAF thing applies, but even it"s pissy power is enuff to mean you need to hold heavy side stick input to keep the spindle virtical during prespining.The more pressure you put on the clutch, the more side stick pressure you need to fight.

[GyrOZprey]

;D Sorry about that Sounds like a great idea, there is a guy in the states with that, might be one of the butterflies? He starts the little motor and takes off with it running, then just reaches behind his head and switches it off. It is on youtube, but I am scared to look, I od"d on youtube last week and used $80 in prepaid credit in 8 days

[MadMuz]

The gyro in the pics here is Matt Pearson"s awesome

[GyrOZprey]

Bugga, I had heard about steves accident, I didn"t know it was that machine..... Was the bearing upside down (and directionally loaded) or was that the one where the big washer was left out when reassembling the head? Either way, what a terrible thing..... hope his family are doing ok Here are the accident details & rotorhead pictures/discussion from RWF ...http://www.rotaryforum.com/forum/sho...php?t=39244and the tribute to Steve.http://www.rotaryforum.com/forum/sho...php?t=39243His wife Jan .....last I heard, - was still struggling coming to terms with it!

[gyroo]

Ho, and one more point, the amount of torque you will need to apply to the OFFSET gyro head means there"s a serious leaverage on the stick.Don"t know how much torque this RAF thing applies, but even it"s pissy power is enuff to mean you need to hold heavy side stick input to keep the spindle virtical during prespining.The more pressure you put on the clutch, the more side stick pressure you need to fight.Thanks again, Birdy, for all your thoughtful information (especially valuable because it comes from practical experience).Reducing the anticipated effect of torque on the control stick is precisely why I have the drive shaft coming up to the rotor head at 9 degrees rear tilt with a small tripot plunging CV bearing (capable of up to a total of 30 degrees deflection) at the top of the shaft, which mates with a small rotating stub shaft under the torque bar positioned at the virtual intersection of the pitch and roll pivots. The CV joint enables the head to tilt right through the normal range of head tilt for any gyro, while the "plunging" capability will absorb any relative vertical movement between the mast and the shaft caused by mast deflection and/or any relative movement between the top of the mast and the airframe from where the pre-rotator drive shaft comes up from.The drive into the stub shaft under the torque bar then goes into a small chain sprocket, then via a #35 chain to a larger sprocket at the front of the torque bar. The drive then goes into a 3/4 inch vertical shaft going upwards which enters the (expensive) Ringspann Corporation sprague freewheel bearing mounted on the front of the torque bar, and a further chain sprocket is mounted to the top of that. A 2nd chain takes the torque directly to the final sprocket which is in the place of a conventional ring gear under the hub bearing block. This arrangement is necessary because it (a) ensures that the 1 inch gimbal offset can be maintained while also allowing that any Hooke"s joint effect caused by the deflecting drive joint is almost completely eliminated; and (b) allows for a simple 2-stage stepdown with an arrangement that is fully and completely visible (ie, visually inspect-able) and where the chains can be replaced at whatever intervals the wear rate requires. The Ringspann sprague bearing enables the rotor to freewheel and override the pre-rotator drive arrangement. Even though there is obviously a centrifugal clutch attached to the Yamaha engine down in the gyro frame, (theoretically allowing the drive train to spin continually with the rotor, detached from the stopped Yamaha), for safety"s sake I obviously wanted a proper helicopter-like freewheel capability to be as close as practicable to the rotor hub. With the overall arrangement I have settled on - yes - it will be totally feasible for Yamaha power to still be feeding into the head during the take-off roll, and once the take-off is completed and the Yamaha shut down, the Ringspann freewheel will virtually eliminate any residual pre-rotator drive-train drag on the rotor head.When the Yamaha is running I expect to be able to clearly "feel" in the control stick that drive is going up to the rotor, but my expectation is that any potential for the heavy resistance in the stick that you write so vividly about will be minimized by this arrangement.Dr Bensen, in the experimental arrangements Tim McLure mentioned, bypassed the torque-induce-stick force issue by putting the engine right up on the torque bar. In my case the Yamaha is just too big to go up onto the torque bar.Cheers,Mark R.

[birdy]

Sounds like you got that issue covered Mark, I"ll shuddup then. I"d love to see sum pix, sounds interesting.

[gyroo]

Soon, Birdy, soon - All the bits are disassembled from the frame at the moment as I"ve been doing a lot of painting and re-fitting the modified main-wheels and keel. I"ve also re-ordered another small CV from "the CV Man" in Jessup, Maryland, USA, where all sorts of smaller CVs from quads are lurking on his shelves. I wasn"t completely happy with how I had seated the first into the drive shaft.I anticipate the final re-assembly will take place when current pressures of work ease off and the Melbourne weather gets more tolerable out in the shed!!Cheers (and thanks for your very very highly valued input).And thanks to everyone else for their input as well.I"ve tried to anticipate every dynamic, reliability and safety aspect of the setup and this is the final arrangement I came up with. For years I"ve been tinkering with small engines, v-belts, chains, epicyclic gearboxes, and all sorts of other stuff that might make a good pre-rotator, and this is finally the setup I decided upon (after a long process of trial and error, and elimination of alternatives).Mark R

[Des Garvin]

Hi Mark, One small point of interest is how do you plan on keeping your chains in tension so you do not get whipping slap as the load is applied and released. All chain drives that I have had dealings with have this action, some more so than others in a short space of time operational wise.Will be watching your creation with much interest as so far you have nudged my curiosity of this build. looking forward to the pics when done.

[gyroo]

Hi Des,Nicely spotted, old bean. You will have read that there is a lower chain (under the torque bar) and an upper chain (above the torque bar). At the moment I have made provision on the upper chain only for a Lovejoy square-tube (with rubber inserts) chain tensioner. The primary chain tensioning occurs by means of the vertically mounted Ringspann freewheel bearing being moved forward out from the front of the torque bar. However, I figured that the chances of having both chains tension up by the same amount would be low because the upper and lower chains have sprockets of different diameters and teeth numbers, and the chains are of slightly different overall lengths. Hence the precaution of making provision for the Lovejoy tensioner.Why I use the phrase