T-bar shaped Stick question?

[tonydenton]

what"s the little engine behind the seat for , surely not prerotator back in days ??

[gyroo]

Hi Tony -Yeah, it actually is. Here"s another picture of the Bensen "Spirit of Kitty Hawk" in the Smithsonian Annexe at Dulles Airport in Washington - on this one the 2-stroke pre-rotator is up on the torque tube. It seems to be the same type or similar type of engine in each photo. Looks to be in the 30-50cc range.Cheers,Mark

[Tim]

Hmm, seems I posted the same time as Mark and mine disappeared.Bensen claimed he could run up to 22 hp and still control it with the rudder. Recons that he could take off from a tennis court (and clear the fence?). He also experimented with a wheel driven road able gyro which is also a possibility - although there could be a shaft drive up to the bottom of the spindle head? A bit hard to determine from this photo.Bensen did a plethora of experiments in the early days so I wouldn"t put it past him to combine the ability to drivie a prerotator and the wheels with the same auxiliary engine.

[mad max]

Overhead stick, Scary. If you stop thinking of your bum you come unstuck :"(

[gyroo]

Sorry Tim, for crowding you out!I don"t have the capacity to interfere with the process of posting, so I dunno what"s happened to yours.My Yamaha KT-100 go-kart engine installation creeps ahead, and that puts out 15hp at 10,000 rpm. I reckon I"ll be test running it up at Ian Morcombe"s toward the end of the year. I"ll finally be able to publish something about 2-stroke pre-rotators that comes from first hand experience and experimenting. I"m engineering it so that there"s no Hooke"s Joint Effect within the drive train, but of course any time the rotor teeters transiently away from being perpendicular to the spindle axis conservation of momentum will introduce a bit of Hooke"s Joint Effect coming from the rotor itself. I didn"t want to have 2 sources of Hooke"s Joint Effect within the system, fearing that they"d fight each other harmonically and set up bad vibration.All I can say is stay tuned. I remain open minded about whether potentially spinning the rotor up beyond 300rpm will truly result in a dramatically shorter takeoff roll, because we shouldn"t forget that the blades will still need a bit of time and takeoff distance (and obviously airspeed) to transition from a driven state to the autorotative state.Cheers,Mark Regan

[tonydenton]

ok thanks, so what is hooks joint effect, mark ?

[MadMuz]

Mark, I am looking forward to your progress with your prerotator.... I personally think that the best thing about maximum revs with any prerotator is only good so that by the time the forward movement gets the air moving up thru the rotors, rather than being pulled down by the blades being driven..... that the rotor speed will not have dropped too much.....

[gyroo]

Yeah Muz,It should be interesting. Because our blades are almost always set up at a geometric pitch of 1.5 degrees (and have a zero lift at negative 4 degrees), then I suppose it"s possible to calculate a rotor speed at which such a low fixed-pitch could get a stationary gyro airborne. My guess is something like 600 to 700rpm.What many people overlook is that gyro flight is only possible because of the phenomenon of translational lift, and translational lift always requires airspeed over the disc to achieve a positive rate of climb. In choppers you can really feel it start to kick in between 15 and 20 knots, and in between 40 and 50 knots you can generally set up a 1200 fpm climb on the same throttle setting as was needed to barely hover. So, in both choppers and gyros, they only start to really perform when the disc is able to "work" on a comparatively large mass or volume of air per unit of time, and this comes from healthy forward airspeed.So, while speeding up a rotor to something normal flight rpm will theoretically help a gyro - because you don"t need to use up runway coaxing the rotor up to a practical rpm - in nil wind conditions you will still need a roll to 30 knots and beyond to get the translational lift really going. I suppose if you"ve got enough installed power you"d possibly be able to "horse" the gyro off in the 20 to 25 knot range and climb slowly and steeply, but as I wrote in the last gyro news when talking about Greg Gremminger"s "show take-off" diagram, if you lose your engine "horsing" through 50 feet in a steep slow climb you"re very likely to be very seriously injured or killed when you crash back down to earth.Of course, the really enjoyable fun starts when there"s a steady 15 knot wind blowing. In those circumstances I think it"s likely that my Yamaha KT-100 pre-rotator single seater will get off in no time flat.Time will tell.Mark R

[Tim]

Hooke"s joint and Cardan joints are "joints that are now more commonly known as Universal joints. Way back many moons ago a cove called Hooke analysed the rotating motion of the joint and found that its speed of rotation was nonuniform, ie if the input shaft was driven at a constant velocity then the output shaft rotating velocity varied by 2/rev unless the input and output shafts were perfectly in alignment. This variation in output shaft rotating velocity would cause a vibration, which got worse the further the input and output were out of alignment. Such is the case when the rotor spin axis is out of alignment with the rotor head spin axis. The rotor has too much mass to vary rotational velocity so the rotor head cops it all. If the head was to be driven by a pre-rotator shaft with misaligned universal joints then the resultant harmonic could cause nasty vibrations.There are ways to minimise this which no doubt Mark has taken care of.

[tonydenton]

thanks tim , we use cardan joints but I hadn"t heard the "hooks joint terminoligy " I had watched a really joint you tube video.[ maybe muz can find it and link it to here] it was a display bench set up. it has 3 large sprockets on the drive line . 1 on the input end . another in the middle and the 3rd on the out put end of the drive line . the narrator has a bank card type of plastic which he holds against the teeth of the sprocket as it is being powered .

[Tim]

Never thought of that Tony. That would certainly be a good way to demonstrate it.

[tonydenton]

ok I went looking but I don"t know how to post links. the best one is " drive shaft velocity " and another is "driveshaft angle trainer". they both give good displays [ although the 1st one is the best.]

[Tim]

I"ll try it.Drive shaft velocity - YouTubewww.youtube.com/watch?v=gmV4qwLfOMY

[MadMuz]

Theres the other one, many more to see.....

[RossM]

different couplingThompson Constant Velocity Joint - TCVJ https://www.youtube.com/watch?v=UEvaOg7glKk