gyrojake;n1136530 said:
Come on guys, this is elementary.
It would be A, at or near the ENGINES COM.
It is better to have your G/G equal or above the thrust line.
Putting your engine C/G under the thrust line makes for a High thrust line, compared to your overall CG.
Now I would have said Center of Gravity instead of center of mass because they change with effect of gravity and we ain't space ships !!
If this is an Air Command installation with a Yamaha engine, it wouldn't matter, It's a Piece-O-Sh!t to start with !!!!
Greg, if you smoke the good weed and stay off the booze you would have actually known the answer
Brian Jackson
He did give specifics that the THRUST line won't change and all else stays stationary in the new engine installation.
HOT DANG! We have a winner. Jake hits the nail on the head - yet again - and with a thorough explanation which answers all of the questions asked, completely. (Sans the brutality of the "Wicked" attack, which I take as a good-natured Jakester jokester jestering and not as a personal mean-spirited hater hit.)
The question came up the other day in a discussion we were having regarding a new gearbox that is coming out that is designed for Yammie tractor fixed wings. The gearbox has three stacked gears in order to place the prop shaft as high up on the Yamaha conversion as possible, so that it winds up being where a Rotax 912 would be in the same airplane. You've seen the one done on a YG3 by the Frenchman in a thread on here a few years back, a one-off. Soon there will be one in production for an Apex YG4i.
This one guy started asking about pre-rotator attachments for the gearbox. I said, whoa, hoss, there's something here you need to take into consideration before putting this on a pusher gyro.
When I told them all that: (A) the prop on a gyro is a fixed position; (B) you can't raise/lower it because of clearance considerations to the keel and the rotor; (C) that the engine would therefore be lowered three full inches to accommodate the tall gearbox; (D) and that doing such a thing is bad juju in a gyro already at its design limits for any minor HTL and this install change will make the gyro unstable if/when unloading the rotor in a low G situation such as rounding out at the top of a climb; well, the screaming at me commenced. All of my attackers, to a man, were FW guys who know nothing at all about gyros. Probably never even been in one.
Me against the world. They said, "BULL crap! RAISE the BS flags! Ban the man! Shoot him! Kill. Kill. Kill!" I'm not exaggerating. One guy pulled a sawed-off and stuck it in my face while I calmly played out my full house hand. You know how it goes with me. I knew that lowering ~150lb of installed engine mass 3 frikkin inches would certainly will lower the gyroplanes overall COG significantly, resulting in the prop thrustline becoming that much higher above COG. As it is, with a typical gearbox, and virtually every single gyroplane Yammie conversion done since our Todd Reick built the first one in 2007, and the MANY dozens that have followed since, the prop thrust goes pretty much right through the engines own COG/COM.
So lowering it 3 inches makes no sense at all for an otherwise properly balanced gyro. By doing so you will unnecessarily unbalance it by simple slapping this gearbox-engine combination on a gyro without making EXTENSIVE additional alterations to everything else to compensate.
Thanks Jake. And as in the sage lyrics of Country Joe and the Fish, "Don't bogart that joint, my friend. Pass it over to me..."
And thank you all who commented and joined in. Nice chatting. Out.
