Actual Flying Rotax Installs

[gcm52]

I am in the process of flight testing my Rotax 912 ULS (100hp) powered Onex. I am quite pleased with this airframe and engine combination. Today I did some speed checks at 6000 ft MSL. At 5300 RPM I was showing 140 kts true airspeed. This could be considered a high cruise power setting (max continuous for the Rotax is 5500 RPM. At 5000 RPM (75% power) I was showing 126 kts TAS.

[13brv3]

Contrats George. Great to see you back on the forum. I'd love to get a measurement from the firewall to the ring mount tube for the Rotax when you get a chance. I'm just about to get started cutting tubes for my mount (912UL Onex) and that measurement would be a very helpful reference.

Thanks,
Rusty

[WesRagle]

Hey George,

Congratulations! Very impressive!!

Wes

[CaseyCooper]

Yesterday I got my airworthiness certificate, today I flew my Sonex for the first time. Aside from a slight right roll, and a slight left yaw, it flies wonderfully. With the Rotax with my 3 blade tapered tip Warp Drive, my take off rpm is 5,300 rpm. Wot of 5,500. Cruise of 135 mph at 5,100 rpm. Climb of 2,000+ feet per minute with full fuel, I weigh 180 lbs, at a climb of 80 mph. Outside air temp was 72 degrees, airport elevation 2,120 feet. Density altitude unknown today. My Sonex empty weight is 666 lbs. Tomorrow I’ll be adding trim straws to dial the trim in, then begin flying off phase one.

[CaseyCooper]

Don’t mind the ugly mug

[CaseyCooper]

N7777X

[Matt541]

Wow, that's sweet lookin'! Well done.

[13brv3]

Congrats! What length prop are you running? 80 or 100 HP? Any pics of the engine installation would be excellent.

Cheers,
Rusty

[SonexN76ET]

Congratulations on your first flight! I look forward to hearing more as you continue your test flights. Good job on the Rotax installation!

Jake

[CaseyCooper]

This is a big one! But very insightful.

So I have some updates on my install and points of concern.
I have a 64” 3 blade warp drive, I develop 5200 rpm on climb out and 5550 in straight and level flight at full throttle. Right where is recommended by Rotax. Carbs are synced and everything is happy. BUT, my problem is not my engine. It’s related to it.
The Rotax mount from Sonex requires the use of their attach bars and the bushings used in mounting the Aerovee. When installed per Sonex instruction, I noticed my first problem. When installing the bolts that go through the mount, bushing, attach bar, and second bushing, I tightened the bolt until the bushings were slightly compressed/rounded, without the nut bottomed out on the shoulder of the bolt. The problem is, without an ability to tighten the bolt against something solid, you can keep tightening the bolts and compress the bushings until the cows come home. But I tried it anyway. Flew for 3 hours, began to smell burning fiberglass in the cockpit. Not a good thing. I thought my engine could be on fire, but I found it hard to believe, or some other problem arose. I quickly got on the ground and noticed my prop spacer had rubbed severely on my cowl. Which my spacer was centered before and after the flight. So I grabbed the engine and was able to rock it quite a bit up and down at the prop flange. Checked the bolts, they were not loosened and the mount bushings were still as I left them. The bolts were wiggling from the mount to the attach bars, because they had nothing solid to tighten against. So I turned on the lathe and machined some 5/16” id 0.030 wall pins/sleeves that would allow me to tighten the bolt good and tight, which removed the play a bit. So I put some more time on it.
I got to 10 hours and did another inspection because I began to feel a vibration through the fuselage that got progressively worse and then really bad all of a sudden. So, I began to check and double check a few things. My prop and spinner are balanced. Nothing changed, nothing out of place. My carbs were perfectly synced, no debris in the carbs, great fuel pressure. Pulled the magnetic plug to look for debris that might’ve come from a gear box problem, clean and clear. In flight I had perfect temperatures, no issues. Mag checks perfect, a 10 rpm difference. Flew it again having mentally felt better, then the terrible vibration in the pattern and back down I came.
So I had an epiphany, my mounts. So i took the bolts/pins out and inspected my mounting surfaces. My front attach points on the attach bars, were resting on my mount bolts. The bushings had shifted and deformed allowing this to happen. The rear however, was perfect. The Rotax produces far more torque than a jabiru 3300 and an aerovee because it’s gear driven. We are able to spin bigger props, more blades. Which in turn means more torque, and more load from p factor, which means a lot of pull back and forth at the prop flange upon acceleration, climb, and deceleration. So what I noticed was the attach bars, being mounted vertically, eventually spread the bushings apart from back and forth, and up and down movement, worked down to the bolts. It also doesn’t help that the bushings used, are 3 tiered. The large section rests on the outside, the second just in the attach bars, and the inner most around the bolt and between it and the attach bar. The finally tier is just about 3/32 of an inch of rubber separating the two from each other, in a vertical configuration, which is almost nothing. The second shelf is designed to help assist in shifting, but allowed them to shift anyway. The Jabiru bushings are wonderful, but too big for the application. The aerovee bushings for the aerovee ring mount, work great! The aerovee mount has pins, and the load is distributed differently on the ring mount, and they don’t put of the forces a Rotax does. A bushing that might work great is a lord mount, or Barry mount, but they are a different dimension than what the attach bars allow.
So upon recommendation by Mark at Sonex, I shimmed the outside bushings with more washers, and in this case, I used washers that were dished to encapsulate the bushings, and push them in tighter against the attach bars. I also noticed the front attach points (the engine mount) flex outward and inward slightly, about 1/8 of an inch, which can equate to a lot of movement at the engine. I flew it for 3 more hours and no luck in solving the problem. It was also suggested to try polyurethane bushings, that are much harder, which I wouldn’t believe would help the problem. It would cause less isolation from engine to airframe, which could translate the energy elsewhere and cause stress somewhere else.
With all this being said, I wiped out my mounts in 13 hours. I’m not going to fly it until I find a solution. I had told Mark at Sonex I’ll be happy to work with him any way I can to find a solution and make it work. But as of right now were at a stalemate. I’m waiting to hear back from Sonex about any improvements that can be made or changed. I feel like this is a great opportunity for us builders and Sonex to work together to fine tune these installs. The Sonex really is a wonderful aircraft, and the Rotax makes it that much better.
In no way am I attempting to poo poo Sonex. The company is extraordinary and produces some of the best bang for your buck aircraft money can buy. They are extremely thorough and wonderful people with a bulletproof reputation.

So if you made it this far, Jake I know you’re close, and anyone else that has a Sonex Rotax mount, this might be good information for you and things to look for. Let me know if anyone has any questions.