ENGINE PLACEMENT and THRUSTLINE

NoWingsAttached

Unobtainium Member
Start with a typical pusher gyroplane with a fixed prop position, fore/aft placement and height of thrust line. No changes to the airframe nor head placement allowed. You have two options: the engine can be placed with its COM (A) at or near the prop thrustline, or (B) 3 inches below the prop thrustline.
Q: 1. Does it make a difference? 2. If yes, which installation is best? 3. Why?

Thanks, looking forward to your responses! I already know the answer, just wondering what the responses will be. Hopefully some experts will take the time to chime, too!
 
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WaspAir

Supreme Allied Gyro CFI
Which are you asking about, the "engine's COM" as it says above, or the aircraft's COM ?
 

gyrojake

Gyro Rehab Candidate
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.
 

Brian Jackson

Platinum Member
Jake, the thrust line is whichever direction the propeller is pointed, and I'm told that isn't moving. Only the CG of the engine. But I would have no way of discerning which is the best placement of that weight because he didn't tell me if the ship was CLT or not to begin with. Did I miss something?
 

gyrojake

Gyro Rehab Candidate
Brian Jackson;n1136532 said:
Jake, the thrust line is whichever direction the propeller is pointed, and I'm told that isn't moving. Only the CG of the engine. But I would have no way of discerning which is the best placement of that weight because he didn't tell me if the ship was CLT or not to begin with. Did I miss something?
The existing C/G doesn't matter, he's installing without modifications or concerns. That's the problem with the ; That's close enough engineering.
He was only interested of the engines C/G compared to the stationary prop thrust line.
The machine may or may not be CLT, but that was not the concern.
It's better LTL than HTL, and CLT is optimum, so I will alway go with weight equal or higher than thrust line.
Plus, ya just gotta know Greg to appreciate his madness LOL !!
 

Brian Jackson

Platinum Member
That's kind of where my thinking was leading as well, but trying to avoid that gotcha moment Tyler mentioned earlier.
 

HighAltitude

in transition
Isn't this just a redrive vertical offset from engine COG question? It's not a variable that 99.9% of us can control.
 

gyrojake

Gyro Rehab Candidate
HighAltitude;n1136535 said:
Isn't this just a redrive vertical offset from engine COG question? It's not a variable that 99.9% of us can control.
All it takes is a 7/16 wrench and a tube of caulk to flip the gear box. Pretty easy task.
The feel of his statement leads me to new motor mounts for either application.
Now modifying the machine takes some skills, but still can be done by the average gyro owner with minimal mechanical knowledge.
Crap, I build em from scratch and I'm retarded compared to most of you guys.
 

j bird

Gold Supporter
Come on Jake, retarded? this is 2018!!!, your mentally challenged, I think you need some of my hybrid tomatoes, I got 4 or 5 varieties, I'm sure one of them would give you more insight into bent and broken rotor blades.
 

HighAltitude

in transition
Can you easily change the redrive offset on a yamaha engine? All along I have assumed that the OP was referring to the Yamaha since that is his expertise.
 

gyrojake

Gyro Rehab Candidate
j bird;n1136553 said:
Come on Jake, retarded? this is 2018!!!, your mentally challenged, I think you need some of my hybrid tomatoes, I got 4 or 5 varieties, I'm sure one of them would give you more insight into bent and broken rotor blades.
Yes Please !!
Yeah, I'm pretty ignorant but that's who I am, and so appreciate the fact that you accept me for who I am. Thank you Brother !!
So mail the mayters so I could be,............... what were we talkin about
 

gyrojake

Gyro Rehab Candidate
HighAltitude;n1136555 said:
Can you easily change the redrive offset on a yamaha engine? All along I have assumed that the OP was referring to the Yamaha since that is his expertise.
Nope. the gear box is on an offset and has to be set up with adapter to change positions.
What does OP mean, Original Post?
 

Vance

Gyroplane CFI
In my opinion there is no correct answer to a question with insufficient information to make a decision.
 

NoWingsAttached

Unobtainium Member
WaspAir;n1136507 said:
Which are you asking about, the "engine's COM" as it says above, or the aircraft's COM ?
EXACTLY what it says. This is one of those detective mystery shorts where you have all the information you need to answer the question.
 

NoWingsAttached

Unobtainium Member
Brian Jackson;n1136526 said:
You didn't mention the aircraft's current center of gravity. Engine placement affects that.
Well, there ya go. You DON'T need to know the all-in aircraft COG to determine that engine placement height will affect it.
 

NoWingsAttached

Unobtainium Member
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.
 
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