Classic IV Thrust Line - 912 and Jabiru

[Geowitz]

Trying to figure a few measurements out before I go and spend a lot of money. Here's what I need if you can help...

Classic IV measurements only please...

1 - Is the 912 engine mount the EXACT same for the classic IV whether you use the round cowl or smooth cowl?

2 - What is the thrust line of the 912 in the round and smooth cowl? ie. - How high is the middle of the crank shaft above the bottom of the lower fuselage tube at the lower firewall on a 912 install with the round and smooth cowl?

3 - Jabiru flyers - What is the thrust line of the Jab 2200 using the JabiruPS engine mount? ie. - How high is the middle of the crank shaft above the bottom of the lower fuselage tube at the lower firewall on your Jab 2200 install.

Easiest way to do this - Level the airplane(flying attitude). Run a level string line from the tip of the spinner along the side of the fuse. Measure from the string line to the bottom of the lower fuse tube at the firewall.

Thanks,

George

[Geowitz]

I am not concerned with how one thrust line performs compared to another nor am I confusing thurst angle. I need measurements to compare the usefulness of cowlings with the Jabiru. The classic IV smooth cowling as far as I can tell has not been tried with the Jabiru since its availability is somewhat recent.

Typically, with a Jabiru 2200 engine, there are several options for cowlings. You can either use the round bump style with modifications or the skyfox(Ausie) cowl with mdifications. I am looking into the possibility of using the Kitfox Smooth cowling, but I need the designed thrust line of that cowling and others to compare with the Jabiru thrust line to see how close it is. If it designed with a higher or lower thrust line it will make the Kitfox cowling difficult to use without serious modification.

[Geowitz]

I need the trust line of a 912 in the round cowl and smooth cowl Classic IV. I also need the thrust line of the Jabiru in the skyfox cowlon a Classic IV. Level the airplane. Level stringline from spinner tip along fuse. Measure from line to bottom of bottom tube at firewall. Pretty simple...

[SkyPirate]

George ,..it's pretty easy to find out the answer to your question yourself,.. using the motor mount locations on your firewall as your "zero" or starting place,..you have the firewall forward kit for the 912,..go to the rotax site and bring up the diagram of the engine config,..it will give you the dimensions of the motor ,,including center of PSRU ,..might have to do a little math,..which will give you a very close to perfect answer,..but due to 4 points that are mounted in rubber on the mount ..this can vary in small increments,..once you have this dimension,..make up a carboard blank and place it on the nose of your plane so your thrust line will be possible to measure,.. keeping in mind ..the 912 and the 582 both produce different thrust lines with the 582 being closer to center of the round cowl opening ..the HKS's actually sit lower in the opening then the rotax engines,.. so just between the 3 motors I mentioned there is close to a 5 inch difference in comparison for the thrust line,..as for the jabiru,..if you do the same and go to the website for the jabiru ,.. http://www.jabiru.net.au/4%20cylinderbodyFrame-1.htm and find info or ask them what the thrust line is in relation to the motor mount for the kitfox firewall package for the motor your going to use,..I'm sure they can tell you,..then make another cardboard blank that reflect's your finding's,..you'll get as close to an exact measurement for the differences as you can ,..taking into concideration the rubber mounts in the system,..hope this helps

Chase

[SkyPirate]

don't forget to do the math for both the horizontal and vertical axis of the thrust line concerning the motor mounts,..some try to null "P" factor in the construction of the mount ..some don't

Chase

[HighWing]

I think I can help a bit. I just finished making a new engine mount for a Model IV. First, I can't imagine a new engine mount to accommodate the smooth cowl. It is much easier, if you are already designing a new cowl, to build it to fit an existing engine location.

What I did to find the thrust line, and I suspect what you are asking for the 912 is the prop thurst line from the gear box rather than the crank level. While at the Desert Fox fly-in, I took several pictures of Model IV airplanes zoomed from a distance to reduce parallax with the camera at the approximate level of the prop hub. All these were in the three point position. Then on Photo Shop, I leveled the thrust line by drawing a vertical lint through the prop tips and correcting the angle incrimentally until I got the prop exactly vertical. Then it is easy to draw a line through the spinner tip aft. The only math I used was to develop ratios from the actual dimensions and the picture dimensions to get a numerical measurement above the under cockpit longeron.

I am visiting in Tennessee until Monday and if you don't get a better answer by then, drop a personal note and I would be happy to send the photo I used and the dimensions I used after I get home. Can't help with the Jab, but if you can get similar photos from the side with that installation, a good approximation can be obtained.

I plan on using the original 582 cowl with the necessary mods - tons easier than making an engine mount.

Lowell

[SkyPirate]

I'm pretty dawg gone sure the PSRU axis is parallel with the crank axis as far as the thrust line goes on the 912 ,.but I could be wrong
Personally I'd use the dimension the manufacturer lists for the offset to do the calculation ,..but ,..pictures are nice once you get it done