Myself and 3 of my co CH750 builders have been planning to install the Rotax 912iS in our CH750s since the day that Rotax announced the release of the iS. Up to this point, there has been little information available so all of us have searched for scraps of info that can feed our hungary appetite for information.

We talked to our (Canadian) Rotax distributor at Oshkosh and as luck would have it, they had bought a CH750 kit to install a 912iS and use as a demonstrator. That was great news for us, so since then we have been waiting for the 750 to be built and flown. Rotech Research have contracted the newest Zenair Build Centre in Vernon BC which is right across the field from them to the build the airplane. The project is progressing, but no airplane goes together by itself so we somewhat impatiently wait to see a 912iS installed in a 750.

Today there was another tidbit of info released. Their build is a feature on the forum/information site. The site has many videos on Rotax tips, product information and reviews, etc and today they released a new series called "Rotax Builder Series". The video released today has an introduction to the series with a first look at the CH750 airframe and some of the issues with installing a 912iS in that airframe. The video is at If you wish to look at the video, this site is user pay site and has annual fee of $29.95.

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Hi Bill,

I had forgotten about your fuel system thread. I must go back and review the whole thread. The last time I looked at it, I thought that I was installing a carbureted 912ULS.

Unfortunately, I do not have any recent photos of my fuel system components but my system has not changed much since these photos. I have not completed anything because I have been waiting on Rotech to design and install their 912iS on their 750. Now that I can see they have more or less followed the same process as me, I am going to get working on it again very soon. Here are a couple of photos. If you have any questions, fire away.


It looks like our tanks ideas are very similar.  It looks like your vent line comes out the top.  You must be raising the passenger seat to clear the elbow.

No, I am going to cut the fitting down some and then shorten the elbow threads (& tap the fitting threads deeper) so it will sit down below the seat pan. I think it will work but will have to wait until I get the tank done. Hope to do that within a couple of weeks.

UL power does not specifically recommend a header tank. They acknowledge that either a header tank or return line(s) to main fuel tank(s) could be appropriate ... depending. For me, the Zenith setup with rear tank outlets seemed to cry out for a header tank. And, for me, behind the panel was the logical place to put it. But there is not one "right" way to do it, to the exclusion of all others.

Yes Ken, you have that part right. There is no "right" way to do it. Hopefully with all of us here, we will all be able to share knowledge and experience so we can design and build a fuel system that will be safe and operate correctly for our own applications.

BTW, a pet peeve of mine of this forum is people splitting up thread topics and now I realize that I have added to the problem. Peter had a good thread going (thread that Bill referred to) of fuel system design with header tanks from a year ago and now I get this thread going. Sorry about that, all.

Don't worry Joe, we still have the electrical and other systems to think about :) I'm going to our local air show in the next few days and I will be looking for interesting stuff to share with all of you.

I am also one day away from finishing the deburring and priming of my rear fuselage - the job from hell.

I emailed Skytek about their FWF kit schedule and the use of 3/8 lines a few days ago and Keith agreed to let me share his response with you. I've edited it sightly:

Sorry for delay in getting back to you.  We are in the process of getting a firewall fwd kit put together but we still have not done all the testing to be sure that all parameters are met.  We do not want to make something that may be unsafe in certain flight conditions.  We all know that it is best to eliminate as many problems as possible before they become a problem.  This makes it slower as far as you are concerned as you are waiting on the kit.

There will be videos on how it is all done to make it easier.  We are in the process of building a small run of header tanks.  I can put you on a list for one if you want or at last let you know when they are available.  I do not even know the price yet as up until now we have only made prototypes for testing purposes.

It is very important to change your lines to 1/2".  Make sure that you still have the finger screen in place with restrictions.  The next video released will show this as the fill time of the header is important in ALL attitudes & conditions of flight even uncoordinated turns when a/c may be used as a trainer or simply a poor pilot like myself.  You may be able to get away with 3/8 vent lines but....I would not recommend it as there are cases that could get you in trouble on that one occasion where thing are not going quite as planned.  

We are trying to make a retrofit package for the vent lines but it will only work if your vent line is on the outboard side of the wing tank.  Early versions of the tank have the vent on the inboard end which may increase the fuel crossover rate when in a bank. This could lead to problems in fuel management when low on fuel.  It means removing the tank to get a second vent on the outboard end.   In the end it is all up to you as the builder how safe do you want your aircraft that you and your kids might be flying in.  Sorry no comment on the prop as I do not know anything about them.  This is still a learning curve for me.

May you holes be straight and rivets pull tight.  Cheers Keith

That is very useful information.  However, there are many ways to skin a cat (and to cook them).  We actually ran tests today in conditions we would NOT recommend for others - and the 912iS ran fine in all conditions.  We ran with 1/4" lines to our 8litre (2gal) header, shut off all the supply lines, emptied 4 litres of fuel from the header to put a massive air lock in place, and ran engine tests -

NB WHAT FOLLOWS IS NOT A RECOMMENDED CONDITION - IT REPRESENTS THE CONDITION OF POOR FUEL MANAGEMENT AND WORSE CASE SCENARIO THAT WE COULD THINK OF.  We ran a full 3 minutes with ALL TANKS CLOSED and NO fuel flow from the wing tanks to the header, creating a slight back-pressure in the header tank, until the fuel temperature started to rise, increasing a high level of vapour lock risk deliberately.  Then we opened 2 tanks, allowing flow from the higher header and 'bubble back through the supply line' on the other tank.  We ran for about 20minutes, until the fuel filled the header tank and the header tank supply lines had vented all of the air in the system (About 4 litres of air!) we then opened the other two tanks and watched the fuel lines back fill from the highest head tank too!.  The 912iS is an amazing engine and it recovered in the WORST possible scenario in our field tests.  WE DO NOT RECOMMEND THIS CONFIGURATION - IT WAS A TEST TO SEE WHAT HAPPENS.

We will be fitting 3/8" lines, to and from the header and to and from the tanks in future installations, and are aware that the 'bubble back' is better on the larger lines and that venting is better still, but we have established that other configurations can work well, provided installed appropriately.  We install so that we ALWAYS flow from 2 tanks at once since that appears in all of our tests to ensure sufficient flow and permit 'bubble back' on a supply line in our conditions and installation.  We have already operated lots of hours on 2, 3 and 4 tanks simultaneous flow with 1/4" supply and no return line in our in flight tests.   THESE ARE TESTS FOR OUR LEARNING THAT WE SHARE FOR INFORMATION OF OTHERS.

There are many solutions, and we must respect each others and not cause unnecessary concerns in those who have chosen a different road.  Our needs are specific to our environment, which is hot, fuel poor, dusty and vapour lock risky.  If we look at all of the different installations for the 912iS and similar fuel demand engine solutions we can make our own minds up.  Clearly the safe bet option is always going to be the largest fuel line, but there remains the question of weight, complexity and necessity.  From our tests, on the ground and in flight, we have not had any problems with smaller lines sizes.  I hope to post a video of the tests soon.

Finally, I got around to the document for this... it is on its own blog post  -  Feel free to ask questions, I may not have answers, but will tell you what we think and what we have found - all decisions on fuel systems are yours though!

Well Jonathan, I have put my money (and it IS substantial money....) where my mouth is. I have a bank draft in my wallet and I am driving 8 hours tomorrow to Rotech Research (Vernon, BC, Canada) to pick up an iS. I am very excited! After I look at their 750 installation, I might have more questions for you. Thanks again for all your documentation and telling us of your experiences.

Okay folks, Dave and I am home from Vernon, British Columbia with my and Travis's 912iS's. We spent most of the day at Rotech and Skytek where we saw and learned lots. First; Both places are top notch businesses which made us feel like we had made the correct choice with our airframe and engine choices. Rotech and Skytek have done a huge amount of design work with installing the 912iS on the CH750 airframe. As with any never done before installation, there is a lot of design, trial fitting, redesigning, making new parts, etc and to us it looked like no shortcuts have been taken. I do believe that they have found every rock and turned them all over.

We were hoping that when we arrived we could look at the 912iS installation in the 750 but it was across the airfield at Skytek. I was a little concerned that I might not be able to look at it closely, being in the fab shop. I could not have been more wrong. Keith and Darian Swartz are SUPER NICE guys. They each spent more than a couple of hours with us showing us what they have done and were very interested in what we had done with our projects. Their shop is uber equipped and they are genuine craftsmen. If you want a show-quality airplane then they are the guys to get. We talked at length about the 750 and about the 912iS install and not once did they make us feel like we intruding on their time even though I know we were because in any business, time is money. Did I say before that they are truly nice guys?...!

Rotech owns the airplane and are the Rotax experts so they are doing most of the installation design and are also doing a lot of the installation work of the engine themselves. Even though the airframe has had the 912ULS mounted successfully there are quite a few differences. Most of which are in the fuel system but I will get to that issue in a bit. The engine mount uses the ring mount unlike the Zenith ULS engine mount which is a bed mount style. The engine mount that bolts up to the ring mount has been finalized based on this aircraft and we have ours ordered which should be here in a few days. The cowl is the same cowl that is used on the ULS installation and I believe it is the same one used on 912 CH601/650 installation. The exhaust system has been a problem because of delivery problems from Rotax (Austria) and the fact that almost every aircraft (brand) needs a little different exhaust configuration. Rotech has used a fabrication shop to build their CH750 exhaust. I hope that we are able to get that same exhaust because it fits very well and the quality of welds and mandrel bends is excellent. As it currently stands, though, we do not have an exhaust system yet available for this airframe. Rotech say that it should be sorted out very soon. A few other issues include: the correct length prop spacer needs to be chosen and the coolant radiator fits much better if the outlet spigot has a mandrel bend outwards to miss the exhaust header.

Now for the fuel system. I have been confident in my fuel system design but now that I have been there and seen their system and heard the efforts that they have taken to design, build, and test their system, I have to admit I had not been as thorough as I thought that I was... A key issue that I missed was that the fuel is being pushed under pressure through the fuel rail and as it exits under lower pressure it is producing vapour (I will call it "air") and is returning it entrained in the fuel return line to the header tank. Therefore the header tank will always have "air" in it that will have to be vented away. In my case my header tank vent line going to the main tanks it only 1/4 inch and I am now concerned that "air" may not be able to get out of the header tank through the-always-flooded vent line quick enough to keep the header tank mostly full. Rotech also found that returning fuel entrained with "air" would entrain the fuel with air that was being pumped to the engine which of course causes problems. Basically when the header tank volume was low and the inlet and outlet were too close the fuel was "frothing" and aerated fuel was being delivered to the injectors. Not good.

At this point I am not so sure how I am going to address my fuel delivery design, but I am seriously considering removing some of the components that I have installed and replacing them with Rotech/Skytek's design. I will let you know.

If you haven't looked at the 3rd video in the "Builder Series" video I recommend that do so. 

It can be seen here.

Anyone heard anything about the availability of ch750 FWF kits for the 912 iS?


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