I'm building a Mini Max 1600R and I have chosen a MZ201 engine instead of the Rotax 377 that the plans indicate.  I have build my fuselage and I now need to install the motor mount base, but I am not sure how to position it vertically.  I can do it as per my plans with the belt reduction installed so that the propeller is lower than the engine crankshaft, or I can lower the position of the mount base as per the Eros plans (designed for a Rotax 503) and install the belt reduction the opposite way. The two options are shown in the picture attached. Either way because the MZ201 is not the same size as the engines specified in the plans I need to know what the vertical distance between the thrust line (the propeller axis) and the top of the top longerons should be.  Can anyone please help me with this information?  How critical is the exact position of the thrust line?

Quoted from gangeli

I'm building a Mini Max 1600R and I have chosen a MZ201 engine instead of the Rotax 377 .. . . . . . because the MZ201 is not the same size as the engines specified in the plans I need to know what the vertical distance between the thrust line (the propeller axis) and the top of the top longerons should be.  Can anyone please help me with this information?  How critical is the exact position of the thrust line?

I have seen many Maxes ( including mine ) where the center of the prop hub is level with the motor mount base of the airframe, ( as in Figure 1 above ). This thrust line is perfect with no pitch up or pitch down bias in my Max. This configuration also allows for better free-air-cooling, as redrive does not obstruct ram airflow / propwash to the cylinders.  (Cooling aspect evidently not consequential if you are using forced air cooling with fan and/ or cowling the engine.)

Not sure if there is a recommended placement for thrust line, I too would like to know. Hope someone answers this.

The thrust line should go through the airplane cg in order to avoid pitch changes with power changes.  To get a reasonable guess where the Minimax cg is vertically, consider the cg of each of the major parts, wings, engine, full gas tank(s), undercarriage and pilot.  The pilot cg is about at the belly button.  

Quoted from Bob Daly

The thrust line should go through the airplane cg in order to avoid pitch changes with power changes.  

No, the thrust line should go through the centre of drag. This avoids the need for a constant trim force in straight & level flight.

Passing it through the c.g. will only avoid the need for trim during temporary throttle inputs.

There is a whole thread on the HBA forum on this very topic of where the thrust-line should pass - through CG or thro center of drag ?

https://www.homebuiltairplanes.com/forums/threads/center-of-drag-versus-center-of-gravity.37107/

Quoted from beragoobruce

No, the thrust line should go through the centre of drag. This avoids the need for a constant trim force in straight & level flight.

Passing it through the c.g. will only avoid the need for trim during temporary throttle inputs.

For the Minimax, this is a distinction without a difference because drag (and thrust) acts more or less through the cg.

Tail loads at 63 mph and gross weight can range from -45 lbs to -75 lbs depending on forward or aft cg location.  If the wing lift at aerodynamic center is also acting through the airplane cg, the tail load is -60 lbs. This load x tail arm (~8.6 feet) balances the aerodynamic moment of the cambered wing, -516 ft-lbs (-0.1 C_M x 4.5' chord x 112.5 ft² wing area x 10.2 psf dynamic pressure, (63mph)²/391). The thrust and drag will be about 123 lbs.

What about the Himax where wing drag, and therefore the drag center, is above the cg? The tail load ranges from -39 lbs to -66 lbs because the wing drag does relieve some tail load.  But the thrust line is essentially unchanged.

BTW, welcome to the ETLB Gangeli.  Please share some photos of your project. Where U located?

Thanks flydog, I'm in Victoria, Australia. I did introduce myself some time ago in the 'Who's who and where?' thread.  I believe it's still the last post there.  Very happy to see that this forum is still active.  Thanks to everyone for the contributions.

Quoted from Keith103

There is a whole thread on the HBA forum on this very topic of where the thrust-line should pass - through CG or thro center of drag ?
https://www.homebuiltairplanes.com/forums/threads/center-of-drag-versus-center-of-gravity.37107/

With this: "In accelerated flight, where the prop thrust is not balanced against aero drag, so the airplane is speeding up or slowing down, all the other forces are still there, but we add in a pitching moment nose down for the moment equal to (Fthrust-Fdrag) * (yprop - yCG)."  Billski, a genuine aero engineer, and I are in complete agreement.

So, to answer gangeli's question we need a weight and balance exercise. The pilot's cg (170(?) lbs) will be about 6" below the top longeron.  The wing cg (70 lbs) will be about 3" below the top longeron. The fuselage itself (50 lbs) will be about 10" below the longeron. The full gas tank cg (30 lbs) will be about 5" down. The tail cg (25 lbs) will be slightly above the longeron, maybe 2" and the undercarriage cg (25 lbs) will be maybe 30" below the longeron. Use actual weights and distances if available. Then the cg of the whole is 7" below the longeron. If the engine and prop weigh 90 lbs and the prop+engine's cg is about where the cylinders meet the case,  then mounting the engine high like the first picture moves the airplane cg to about 5.5" below the longeron and the prop thrust might produce a very slight nose up moment.  Mounting the engine low like in the second picture might put the airplane cg at 7.2" below the longeron and produce a very slight nose down moment. How slight?  Well at 70 mph, the thrust will be about 140 lbs. Then even a large 6" offset thrust produces a moment of just 70 ft-lbs, small compared to the wing's 628 ft-lbs moment.

From Morry Hummel's 1/2 VW plans: