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PostPosted: Sat Mar 22, 2008 7:06 pm
by mgodwin12
the prop will be like the picture but the mount will use the face of the engine and will mate ( hehe) with the mount ring from the original water pump.

PostPosted: Sun Mar 23, 2008 5:22 am
by xgary
Good work and commitment on the re drive project.
What will sell them is once you post some numbers, ppics and videos of the better performance.

I gave Rick at GSC all the lazair prop sizing last year for different engines but not on the re-drives. I forget the pricing now but can find out. Not alot of margin in props to offer discounts in my opinon. I sell GSC props and Rotax parts and can get you a price if needed.

Call direct toll free ,

PostPosted: Sun Mar 23, 2008 8:09 am
by uscgairdale
It's cool to see the redrive thing happening. How much of a concern is it to have the thrust line move that much below the chordline of the wing? Some other redrive Lazairs, Carel's Hirth redrives in particular, have a significant difference from the original thrustline.

Perhaps this is a good topic for Chappy, as I recall his redrives on his Series I kept the thrustline in the original plance. I'm not trying to rain on anyone's parade here, just curious about the affects of the change.


PostPosted: Sun Mar 23, 2008 8:25 am
by xgary
That is the same concern I voiced a few years ago about Carel's plane. Guy would never respond and I not sure if he ever flew that highly modifed Lazair.

Having the thrustline pulling evenly through the nacelle would be my first choice of course. And with Ser 1 and early 2 wings I would think it might be abit much for the D cell.

My new D cells are built the same as 2 seater and/or elite specs so the first 4 feet are vastly stronger than the early ones and I would only gues that that would take a 185 with an offest re-drive more so than a early model wing

How bout a pair of 277s with or without redrives ?

PostPosted: Sun Mar 23, 2008 12:16 pm
by mgodwin12
on the lazairs here that have that redrive there was never any problems with the change of thrust line or with the added thrust on the d-cell. my airplane is an '82 III so i'm not worried. with the design of the redrive frame it may be possible to put it out the side rather than underneath. this would make the props slightly further apart. more asymmetric thrust issues? we can't have everything. i thought about making a planetary drive gear box like on a radial engine but the cost, weight and everything else is prohibitive. that would have been cool though.
i will post numbers etc as soon as i get them. the trouble with this project is that i can't test them until they're built and i can't build them untested???? does that make sense? i got a quote from a different machinist to do a one off copy of the bc drive and it was 3k!!!! so here i go with the 5k investment hopefully to bring it down to something reasonable.
i have a plan to build a test rig on my truck and i'll get some numbers static as well as at different airspeeds. and hopefully won't attract the attention of the law with this thing bolted to my poor truck. i'll add any cost of tickets and fines to the r&d costs. haha

PostPosted: Sun Mar 23, 2008 2:25 pm
by xgary
my airplane is an '82 III so i'm not worried

ser III are a 020 copy of the orginal 016 d cells .

2 seater has a 4 foot doubler plus several extra stiffners that were used to beef up the ser III wing to accomdate the larger engines with more thrust.

food for thought anyhow

PostPosted: Sun Mar 23, 2008 4:28 pm
by JPXman
yep i'm not worried about the thrust centreline moving too much, as its still just 9HP behind the prop. With carel's design there was a 60% increase in HP (9HP to 15HP) with no associated increase in structure design.

The 2-seater wings were designed for direct drive 25HP KFMs. So taking a series III wing and beefing it up to 2-seater standard will be overkill for a 185 redrive, but i do recommend the long bolt mod for ANY lazair :)

The big advantage to the big prop will be higher thrust levels at take off speed. I love that you're going to do a test in your truck. Mike - if you don't have the rig already made, we do here in edmonton. ship us the redrive and we can have the test done in a day or so. we have a test stand with a rolling nacelle attachment with a spring scale to measure thrust.

i think that at 20mph, in a test of redrive propped 185 vs bi-propped, we'll see double the thrust.


PostPosted: Sun Mar 23, 2008 7:22 pm
by xgary
i think that at 20mph, in a test of redrive propped 185 vs bi-propped, we'll see double the thrust.


yah , now you got me interested.

like you said double the thrust but not on centreline of nacelle.
Is it a problem ? I am not sure. and you gotta remmber that the prop will be pulling not from 28 inches but from a 44" circle which will put 50% more P factor pull on the nacelle and the rubber mounts.

PostPosted: Mon Mar 24, 2008 10:44 am
by JPXman
before we get shorty all fired up, i want to ask 2 questions:

the redrive flange - is it too late to make changes to its shape? if not, could we incorporate shapes if necessary so that if i wanted to drill different holes in it i could mount it to the face of a JPX engine?

Could we add a section on part of the flange to support the mounting of an electric start motor?


PostPosted: Thu Mar 27, 2008 10:55 am
by mgodwin12
this mount is being designed to fit the crankcase of the rotax so re-drilling it to fit a JPX won't work, sorry. the machinist said he could make one to fit but it would be a whole new mount and he'd have to make an engine pulley as well. with the electric start mounting flange its more complicated than just sticking a mounting flange on the side of the mount. the gear teeth have to mesh perfectly and there are quite a few calculations required with this and he'd have to redesign an engine pulley to fit the ring gear to. plus the cost of the starter motor ( 190$ each) etc. basically its all a matter of money and the point of this exercise was to make a rotax redrive as cheaply as possible. we're using his "off the shelf" prop pulley with a mount and engine pulley designed to fit the rotax 185.
right now he's working on the pulley spacing to fit the available belt length.
i took some pictures and i'll see if i can figure out how to put them on this page. hopefully today. i'm etching my shop floor with acid, man. should be fun.......
thanks for the testing offer, Tyler, but by the time he's got the drives ready i'll have the rig ready and i'm looking forward to tearing around the field rather than drywalling and etching.
with regards to the increased thrust off center etc. i was out in my buddies re-drive equipped lazair and had an engine failure ( the only one he's ever had, go figure!). the spark plug blew out of the cylinder. even with the prop windmilling for a bit, i had no control problems at all and flew the 2 miles back to the airport. it was a no brainer. i had the other engine at full power and flew normally. i couldn't believe it. i even taxied to his hanger. mind you his hanger was kind of in line with the runway so it wasn't a big deal, but i had no yaw issues or anything. not like a regular twin engine airplane........
so hopefully these things will be ready sooner than later, at least in time for xmas in may/june.