Actual Flying Rotax Installs

[Kai]

Anybody with factual CG findings?

I did the excercise on the scales last week: plane on the scales with the upper cockpit longeron level.

To refresh your memory, I have a R912 clone sitting in Sonex standard gear #0525 now. The face of its prop flange is 35mm further forward compared to a Jab33A with the shortest one. To the prop flange is bolted a Sensenich fp prop originally intended for the Sonex with a R914 plus the usual scull cap spinner. Different from most other taildraggers is that there is a 10 gal seat back tank in the fuselage, made up of 1/8’’ Al 6060T6 sheet. I speculate the empty weight to be some 15 lbs.

It came out heavier than expected. Total empty weight is 682 lbs, with 39 lbs on the tail wheel and 643 lbs on the mains. As far as I am able to find out, this puts the empty weight CG at 22% MAC. I am surprised the empty weight CG is not further forward.

Comments, anyone?

Thanks
Kai

[sonex1374]

Kai,

This probably doesn't apply to your situation, but it's a good time to remind everyone that the weight and balance measurements need to be collected while the plane is in a level attitude (upper fuselage longerons horizontal). This will make a surprising difference in the weight on the tail (up to about 5-8lbs more tailwheel weight when tail-down vs level position). Calculating the resulting CG for both sets of measurements, the tail-down CG is approx 1.5 inches further aft than the actual CG when level (or about 2% of MAC difference). This is the same effect as loading the baggage compartment with 40 lbs to get a rearward CG shift of 1.5", so it does make a significant difference.

The change in tail weight when you're tail-up vs tail-down is easy to see next time you pick up the tail of your plane to move it. You can sense the difference even without using a scale, and even a simple bathroom scale will measure the change. If anyone is not sure how they performed their W&B measurements and think they may have collected the weights in a tail-down position, I'd encourage you to re-do the W&B. It really does matter.

Jeff

[CaseyCooper]

I agree with Jeff. A good weight and balance is key. Mark and I have been working together on compiling data/information for Rotax powered Sonex’s, every build will be different of course meaning more/different data. But, more people flying them, means more results. Which will benefit everyone in the long run.
I ran a weight and balance with Mark just to be certain of figures and this is what we came up with. I’m at the forward limit for my weight and full fuel, but have no way of going further forward. As fuel burns off, with passengers baggage etc., it still remains in range quite well.
In my opinion, weight and balances don’t always match up on paper vs real world. Rigging comes into play (flaps up or down slightly for example will move your center of lift forward or aft), and of course slight irregularities in measurements, etc. It happens!
One of my intentions of running a 64” Warp Drive, which is solid carbon fiber, extremely durable, but consequently heavy, was to get my cg forward. Being a 3 blade it puts more weight in the front, which was a benefit of offsetting the lighter power plant weight. Also I’m running a 2” spacer the same diameter as the prop flange.

I’ve been attempting to upload a picture of the weight and balance we compiled but the website isn’t letting me. So I’ll try again later!

But for now here are some basic figures I have;

Empty weight: 666 lbs.
Empty cg: 16.85%
Max Forward with 180 lbs pilot and 96 lbs fuel: 20.31%
Max Aft with 180 lbs pilot 170 lbs passenger 38 lbs baggage 6 lbs fuel: 31.21%
Full Gross with 180 lbs pilot 170 lbs passenger 38 lbs baggage 96 lbs fuel: 27.61%

Obviously these percentages can vary given different weights and their location, and also different aircraft with different configurations.

[Kai]

All,

Good points and well taken! Of course I can’t disagree with any of them.

However, my intentions were to get your comments om my actual empty weight CG.

Wih the 912- should one aim for an e-w CG on front of the 20% MAC mark?

I am scratching my head here. Originally my Sonex had a Jab 22A up front. Its e-w CG was at 24% MAC. Takeoff from a grass field with a full tank, pilot and passenger, was an exercise only for the very interested ones. With the 912 I am at 22%.

With the 912 conversion one of the goals was to fully exploit the MTOW of 1150 lbs.
What do you say: go or nogo?

Thanks
Kai

[sonex1374]

Kai,

My Jab 3300 powered plane weighs 675 lbs empty, with an empty weight CG at 20% MAC. This works out very well over the entire range loading configurations. Solo with full fuel I'm at 21%, with a passenger and baggage and low fuel (flying to camping) I'm at 30%. If the E-W CG was further aft then I *could* run into a situation where I was very close to the aft CG limit when flying with baggage and low fuel.

My tailwheel weight is 34 lbs, and playing with my CG numbers if I increase it to 39 lbs (like yours) I also would have a E-W MAC of 22%, and this would cause my MAC to change to 31% in my passenger+baggage+low fuel scenario (but still within limits).

I think you're probably in the sweet-spot with your current W&B (E-W CG MAC of 20%-22%), and I wouldn't change anything.

Jeff

[CaseyCooper]

Kai,
I think you’re right on track. The only thing I would possibly worry about is if you’re at full gross, with your aux tank full and baggage, you might fly out your aft end if you don’t burn your aux fuel early in the flight.
With my empty cg of nearly 17% it’s allowed me to operate in all conditions safely without falling out the forward or aft end with fuel minimum and max gross of 1150 with a max aft of nearly 32%.
But with your calculations I would say you still have room to operate well in range. Like Jeff says, I wouldn’t worry. It looks good!

[Kai]

Guys,

Thanks- both of you.

Much appreciated!

Rgds
Kai

[garyb]

Hi all
My empty weight CG is 21% MAC at 645lbs I often fly it at 1150 lbs gross with no problems. The CG does track on the fuel cut off line maybe a bit over. I don't know how others with the Rotax engines get such Fwd empty weight CG's. I tried to keep the tail light.
Cheers

[peterp]

Hi all,
Mine has prop flange 1" forward and 3/4" up with a ring mount. 650 lb empty and 21% mac. Fly's just fine. Cheers Peter

[CaseyCooper]

I’ve posted this on Facebook but I realize not all of us are on there. I made a short video of an evening flight. Details of the days flight are in the description including performance figures. I’m going to make another video of my install with ins and outs that might help other pilots/builders. Once I get used to hearing my own voice on camera. Ha!