Empty C of G SS7

[Esser]

Wow, this is really eye opening. I realize the SS7 was designed for heavy engines but using the Kitfox W&B example in the build manual. An 814# empty SS7 would have to have an empty C of G of 7.48" to get a 9.5" C of G with a 170# pilot and 3 gallons of fuel.

Is this even possible without adding ballast?

I am interest to see John Kuzmic's as he has a 914 and an airmaster prop. All heavier than a ULS and a ground adjustable. Plus he has a nose wheel. Not sure how much of a difference that makes since the tail wheel has such a long arm.

[Esser]

Doing a quick and dirty calculation, is your plane was 801# empty, you would need 45 lbs on your tailwheel and 378 on each main to get an empty C of G of 7.49

[Esser]

Sorry to beat this thread to death but here is a sample W&B I did with my theoretical plane. Now to see if it's doable.

I went a little over on the baggage area to push it.

[Esser]

Bringing this thread back to life as currently my empty weight C of G is 5.49...

Considering everyone said this was impossible and I didn't try to load weight into the front I'm slightly amazed and annoyed lol. As it stands right now, to get me to 9.00 inches with me in the seat I need 40lbs of fuel and 38 lbs in the cargo bay.

I'm thoroughly confused. I know the Airmaster is a heavier prop and the prop extension that came with it is heavy too. But looking what everyone else has said, I'm scratching my head. I will double check my measurements tomorrow but as she sits now, I'm 418 on each main and 42 on the tail....

[jrevens]

If your weights are accurate, it sounds like you are very nose heavy, Josh.


I have the Whirlwind 72" 3 blade GA, 912 ULS, Desser 22' 8.50 tires, T3 tail spring, lightweight Matco dual fork 8" tail wheel, Heavy "standard" battery, a relatively heavy panel with steam gauges, extended baggage with aluminum floor panels, single axis auto pilot, and a few other "extras" (like a ground-handling handle on both sides), etc. And Oratex of course.


Empty weight is 819# with header tank fuel, all engine and brake fluids, & fire extinguisher.


Empty CG is at 9.843". So, like so many others, it's impossible for me to even get close to the forward limit with someone in the airplane. My empty tail weight is 59#.


As an example, with 2 180# people, full fuel, and 130# in the cargo bay my CG would be at 15.9". I haven't flown it loaded to give that CG condition yet, so I don't know how it will handle there, but the probable weight carrying ability is pretty impressive to me. In that regard, for those of you flying, how do these airplanes (SS 7) fly/feel when close to the aft limit?

[Esser]

I think I must have something messed up because I think even if you hung the extra weight I have in my plane off the tip of your nose, it still wouldn't be that nose heavy. I'll keep everyone updated

[David47]

Josh, I just did a quick comparison between you and Ross (look at the thread re SS7 weight). You both have Airmaster CSP's, but blades may differ, and a quick look at the Rotax 912 iS and 914 iS shows there's not much weight difference (roughly 160 lbw each installed).

His empty weight and CG are 874 lbw and 9.66" aft of the datum from my rough calc's.

As a rough calc for yours: (and assuming your weights and main wheel measurements are ok) I'm getting 878 lbw and CG around 5.89" aft of the datum, assuming the same t/w moment arm as Ross' i.e. well forward of the forward CG limit.

From what I've seen over a few posts from weighings, the 60lbw tail weight or thereabouts is a reasonable guide to go on based on average weights that other builders are getting.

In short, looks like you need some ballast and the best bang for buck is right down the back. Any chance of strapping a steel plate inside the fuselage above the tailwheel spring ?.

So you don't feel too glum, I have a Lyc 0-233, which is heavy and I'm expecting to have some ballast down the back ....

FWIW.

[redbowen]

Josh,

I looked at your CG calculation sheet and compared it to mine and I notice you have -2" for the L/R main arm which gives you a negative moment. On my sheet, which is the same form but for a model 6/7, I have +2" arm for L/R Main which gives me a positive moment. This would change your empty CG from 7.49" to 11.27". I also have the 1 degree wing sweep which moves my workable range to 9.50" - 14.75" instead of 11.37" - 16.00", I assume that you do not have a wing sweep on a ROTAX install.

[Esser]

I actually have a -1.4 moment on the mains. The mains are ahead of the leading edge so they should be negative and the tail should be positive. Maybe it's different with sweet wings but I would double check your measurements again. The mains are only + if they are tri gear mains.

I'm -1.4 on the mains and +167 on the tail. The SS7 workable range is 9.00 to 16.00