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Up-dates on the cause of "Precession".


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I read a book by Noel Whittle Paramotoring From the Ground Up ("2002", a good read) in it he talks about, Precession and it's effects and was wondering if any new reasons for why it may happen have come to the fore?. Also could along with turbulence could shortage of fuel (not lack but starvation)be a cause and what would be the result and how many times would/should you rotate and the speed etc?.

Thank you and regards,

Warwick Barnes. :?:

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Hi Warwick, welcome.

I will have a go at answering your question but hope that others will attend to it too if they have anything to add (jump in guys).

To recap, there are two main forces acting on the paramotor in flight, torque (engine) and precession (propeller). Engine torque produces a down force on the right hand riser and therefore a turning tendency to the right. Precession.... well, as you say in your post, Noel tells us in his book that there is no identifiable effect manifested by precession from the prop that he could detect.

To investigate this a little further, precession acts at 90 degrees in the direction of rotation from where it is applied. So if the propeller rotates to the left (viewed from the rear) and we apply a force to the left of the disk (yaw) that would be transfered 90 degrees and actually be felt as a pitch up on the rig.

Trouble is, we do not have a control that applies yaw in a paramotor. If we get a riser twist that is probably about the closest we come to it and that would certainly be felt though by how much is unknown to me. Theory has it that it should be noticeable.

Gyroscopic_precession_256x256.png

:?: How fuel would relate to precession I could not say, nor could I get a handle on what you were looking at when you mentioned rotation, but if liftoff was the focus of your question, yes - as we lift off, the motor/prop does tilt backward so we might expect a yawing moment to the left.

This graphic might suggest that this happened to me. My motor is very powerful and I did lean back at the point of rotation. I am not suggesting that is what actually happened, but it might explain the different directions being taken by the wing and the motor here and illustrate the point? Or was it just me.... I don't seem to have any right brake applied at the moment the picture was taken.

forces.jpg

I hope that was helpful and again, if anyone has anything to add, please step in.

Norman - too much time on his hands in Atlanta. :lol:

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Sorry Simon,

It is no joke and was caught on video you maybe able to get it from the people who took it,(ozbudd at bigpond.com,but do not say I told you or you will never see it.) it happened on the 7/4/03 at Bonny Hills (Laurieton)on the NSW, Mid North Coast and witnessed by 8 other pilots, two were instructors and it was my first PP/G flight and I had only 7.6 hrs P/G bit amazingly only one accident report was lodged and it disappeared from the HGFA Accident Data Base.So as far as I can say is yes it did happen,but ask any official and it never happened.

Warwick

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  • 1 year later...

Here's an example of precession in a similar model to a paramotor pegged for takeoff.

Note the wobble when he turns the rig on.

I think this is a significant force in daily flying that is misconstrued as torque steer since any change in prop disc plane results in a forward or back force application on risers.

Because a glider depends on a self centered load, in flight there is no reference. Blame torque steer since we feel that on the ground.

My friend has a paraski w 582 rotax and six blade prop (offset triples). Paraskis have rudders which offset torque steer, yet anytime he bangs the throttle or comes off it he gets riser twist to some degree. His rig is 500+lbs with him in it, but I can see this flying next to him. It is directly related to the rotational mass of the spinning disc and the speed of the plane orientation change. So, carbon props less so than wooden props.

So, when your rig is wound up at takeoff rpm and your feet swing out in front of you...

Note those folks who do the smooth lean back (eventual back problems) takeoffs don't get oscillations near so often.

Try this on steady cruise power with some altitude:

pull feet into harness, settle rig, then stick them out straight in front of you (change prop disc orientation via COG).

If you get some side to side wobble, this makes sense. Right?

Try it with different prop speeds - it should be most pronounced at takeoff power.

Please don't go all trapeze artist and wind up 18 times!

:lol:

This is my theory and I'm sticking to it until convinced otherwise, which may occur at anytime :wink:

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  • 2 months later...

Thank you t_andrews,

I have not been here for quite some time and did not see your reply,sorry.

Your reply sounds like a viable explanation and is the best that I have heard.

In fact the only one that I have had from any source and I have found it strange.

I had to find out about the Precession,thing on my own and was the only explanation to what,happened that I could come up with. Most people prefer to ignore that it happened or ridicule me over it and tell me it could not happen.

But it was witnessed and videoed and that is just the start.

Thank you again and best regards

Warwick Barnes.

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I still fail to understand what you mean by 'rotations'. Do you mean your risers twisted on themselves, or that you flew round in a circle eighteen times? If neither of these, what do you mean?

I doubt that precession could be blamed for either of these, as it is very much a transitory effect, i.e. the force of precession will only be felt for the duration that a force is applied to the prop disc, trying to change the plane in which the disc is rotating. I.e., at take off, when you suddenly lean back, you can expect the force to precess during that lean. However, as soon as you become stabilised in a new position, then the resultant force will stop.

Torque effect on the other hand will continue as long as the prop /engine are rotating. The effect should be worse on a belt driven unit as opposed to a gear driven one, as in a belt drive, both the engine and prop are rotating in the same direction, so the torque effect is the sum of their separate effects. However, with a gear driven prop, engine rotation is in the opposite direction to prop rotation, and so the separate torque effects will tend to cancel out.

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Thank you Phil_p for your input.

I have no idea if it was precession it was just a theory as there,were no others available.

And yes it was flying around in circles and I stopped,facing the opposite direction to my direction of travel.

The distance between the canopy and pilot,was about a metre shorter than it should be.

Anything else that you want to know just ask.

Best Regards,

Warwick.

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Got to say this is one video I'd love to see to compare to my imagination.

I know some risers - once crossed - have very little resistance to further twist, but to climb up toward the wing for a meter takes some momentum!

Figure skaters have to practice to get to 18!

I cannot envision this without Wile Coyote in the harness, sorry...

Not saying it didn't happen, just that my brain won't let it happen without dumping some reason or using cartoons.

I have seen a video of a fellow launching who pitched way up from too much power and did the absolute wrong thing by pegging it further.

The wing was about to stall and he spun (motor only) 180 in an instant, slowing his forward speed and letting gravity take over...

18 though has got to be a record.

Get a copy of that video and call Guinness mate.

Really.

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Got to say this is one video I'd love to see to compare to my imagination.

I know some risers - once crossed - have very little resistance to further twist, but to climb up toward the wing for a meter takes some momentum!

Figure skaters have to practice to get to 18!

I cannot envision this without Wile Coyote in the harness, sorry...

Not saying it didn't happen, just that my brain won't let it happen without dumping some reason or using cartoons.

I have seen a video of a fellow launching who pitched way up from too much power and did the absolute wrong thing by pegging it further.

The wing was about to stall and he spun (motor only) 180 in an instant, slowing his forward speed and letting gravity take over...

18 though has got to be a record.

Get a copy of that video and call Guinness mate.

Really.

G'day again'

I think the vid you are thinking of is - Pilots name is Olvik from Sth America somewhere about 2004 vintage on you tube.

Mine was on you tube about late 2003/04 but was removed, (see ozbudd / P/G forum) I think about 2005 later that year I was told,by ozbudd (aka Johnno Thorpe) that "I could not have the vid because we thought,that you would cause trouble with it".

In 2007 I found some people listed as his friends (gizzo70 & pgpete),so I left a comment to contact him about my vid with a brief description of it.

One year later (Nov 08) I got a reply telling me they had seen the vid,to get off his case about it and to rack off (Aussie speak for F**K off),so I wish I could get it. If you want to try let me know what the result is,I am very interested to here what happens.May be if enough people ask for it it will show up.(All comments were later removed).

The motor was a NIRVANA the Canopy was a SKY Atis (DHV 1-2),Nirvana ignored my requests for any explanation on what or why it happened,with basically me no speaker de English and Sky,once they realised I was not blaming them for what happened ( but for making a really safe glider) thanked me for what I said.

Best Regards,

Warwick.

PS: why am I not getting a notification to these replies the box is ticked.

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  • 5 weeks later...

1) I used to work at a quarry right behind the beach at Bonny Hills at the time of this accident.

2) I heard about it over the VHF band, (and the panic in the voices of the people speaking)we used to record all transmissions on this band, for our records and the pilots were always interfering with them because they used the same frequencies.

3) I used to spend my lunchtime on the beach and watched the pg show when they were flying.

4) I always intended on learning the sport but was so put off by this accident that I had nightmares about it,then had an accident of my own which will prevent me from ever doing it.

5) I did not see all of the incident,so I do not have any idea of how many times he actually rotated but he was still going around when I finally got him in my sight through the binoculars and I saw the entire untwisting of himself from his predicament and I have no idea how he did it,how he kept his head at such a time.

6) All I remember of that was him kicking his legs like he was on a bicycle and it seemed to take forever.I cannot fathom what it must have been like for him.But it was a damn fine effort and he obviously survived to talk about it,a pity people choose not to believe him.There maybe a lesson or three to learn from.

IT DID HAPPEN and as he said.

Regards,

Mark-O.

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