Empty C of G SS7

[Esser]

Hello everyone. I am starting to contemplate W&B as I try to position things in the airplane. From what I understand with the Kitfox, everything you add to the plane basically puts the C of G aft. It would be very helpful if a few people could post their empty weight, prop and engine combo, tri or tailwheel, and the empty weight C of G.

Has anyone tried to get the C of G as far forward as possible with min fuel and one person on board? Does anyone have their aircraft at the forward limit?

The reason I am asking is Jiott had a thread where he was loaded to 1320# and already at 16". I am planning on using the full 1550# and I want to be limited as little as possible by being too far aft.

Let me know, I would be interested to see what the average ends up being.

[jiott]

My empty wt. (with oil, coolant, and unusable fuel+full header tank) = 834 lbs
912uls engine with Whirlwind 3-blade GA prop
Taildragger with Alaskan Bushwheel tailwheel (those babies are heavy)
Empty CG is at 13.40"
I have the lightweight Aerovoltz battery on the firewall.

All this puts my CG fairly far back. I have never got close to the forward CG limit with any combo of loading. The amount of baggage I can carry with full fuel seems to always be limited by the 1320 lbs with 16.0" CG. A lighter tailwheel and a heavier standard battery would help quite a bit I think. The W&B spreadsheet on the Kitfox website allows you to play around with lots of combinations.

[Dave S]

Esser,

With our trigear (which is heavier than most @884#) the closest I can get to the forward GC is with zero fuel, zero baggage, no passengers no pilot, empty header tank, empty toolboxes etc.....CG ends up at 10.37".....in otherwords it's impossible to load to exceed the most forward limit without hanging lead bars on the prop for our aircraft. Noteworthy that one of my odyssey batteries is behind the pilot's seat.

On the other hand......it is entirely possible to exceed the aft limit with careless loading. I have an extended baggage bay with the baggage area divided into 3 loading zones/stations.....the only thing that goes in the far back is the sleeping bags.

Ditto on the KF spreadsheet...although a person can easily make up their own W/B spreadsheet with excel if you have any W/B stations that vary from stock.

Sincerely,

Dave S

[phintz]

perfect timing of this question, my FAA checkride dude made me figure both empty and max cg of my series 6. Empty weight of 750 lbs with oil and unusable fuel was 11.19 , Max loaded with full fuel , 2- 200 lb people I was 15.16 in. . If you would like I have a printed spread sheet of this also!

Patick -series 6
tri gear , rotax 100 hp

[JohnB]

My SS7 taildragger, Rotax 912ULS, Whirlwind 3 blade, came in at 808 lbs. and an empty CG of 11.2". A little porkier than I has hoping, but I also have the heavier Desser 21x8 tires and installed the extended baggage compartment (at least that's my excuse and I'm sticking to it.) As others have said, I have found it is impossible to get to the forward CG limit but fairly easy to exceed the aft limit when you start loading baggage.

[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.

http://i1042.photobucket.com/albums/b423/joshesser/Screen%20Shot%202015-05-25%20at%2011.35.39%20PM_zpsromjcrzn.png (http://s1042.photobucket.com/user/joshesser/media/Screen%20Shot%202015-05-25%20at%2011.35.39%20PM_zpsromjcrzn.png.html)

[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.

[Esser]

Hey David. If I needed weight back there, that’s where I’ll put my first aid and tool kit. It still doesn’t make sense to me looking at Ross’ numbers. It should be very similar

[David47]

Unless you have a lead cowl, I agree, but what about prop comparisons or are they the same ?. You're effectively looking at a 3000 in.lb difference in your moment, which is a lot for this size airplane, and given the weight of yours, that's about a 3.3" difference in CG position. Maybe get someone else to do it again as an independent check ?. Or, get another set of scales and do it again .... I've weighed many aircraft and you occasionally get one that just doesn't add up. Maybe time to step away and change the approach ... wish I could offer more than just check everything again, change scales, change to someone else ....

[PapuaPilot]

Hey Josh, did you measure the actual arm of your wheels or just use the numbers in the W&B chart? Your

EWCG seems off, but 42 lbs for the tail seems too light.

[jrevens]

Hey Josh, did you measure the actual arm of your wheels or just use the numbers in the W&B chart? Your

EWCG seems off, but 42 lbs for the tail seems too light.


Good point Phil. I found that the center line of my mains is actually 2" forward of the wing leading edge datum in a level attitude, as opposed to aft of the Datum on the Kitfox drawing. Gotta' measure everything accurately... I'm sure Josh did.

[AirFox]

Hi Josh, Your EFI W&B vs Ross's 912IS may explain the difference in your calcs.

[Esser]

My mains are -1.4” my EFI set up is lighter than the carbs. I weighed everything. Extra weight came from the extra alt and intercooler. I have the same prop as Ross. I have a lithium battery up front. My prop spacer is solid steel and could probably be lighter. My exhaust is ceramic coated which could add some weight. I’m really at a loss to explain it. I’m 42lbs on the tail with an 8” tundra lite and T3. Double checked everything today.

[jiott]

Josh, if you have the W&B spreadsheet it should be a simple matter to subtract out the estimated weight and CG of each extra item you have added (ext. alternator, intercooler, ceramic exhaust, EFI, prop spacer, etc.) and explore with the spreadsheet until you figure it out. Its all just mathematics and the spreadsheet makes it quick and easy.

[rosslr]

Hey Josh,

Not sure I can add much - Dave did some nice research re the similarities/differences between yours and my plane. I am sure you will check everything but if your figures are right, then having to fly with 40lb of fuel and 38lb in the luggage compartment is not a bit deal - in fact I would seldom fly with less fuel than that and always have a toolbox and other stuff in the luggage compartment. If I am by myself and light on fuel, I have a bag of diver weights that I put in the baggage compartment as it is much more balanced (especially using flaps and trim for final) with weight back there.

Or, as suggested by others, a bit of lead back near the tail wheel makes a heap of difference. I tried it but didn't like it back there where I couldn't secure it well and it was not convenient to take it in and out as needed.

If it is what it is, then apart from your Kitfox being a bit heavy, I don't see it as being a huge issue.

best

ross

[Esser]

Hey everyone. Thanks for all your input. As it stands right now, moving some items such as the fire extinguisher more aft, fired kit, tool kit etc everything will work out. Taking 6lbs off the mains and getting it to the tail fixed everything. The good news is I can REALLY use my extend baggage area!!!

[redbowen]

It still seems odd. I have a Lycoming O290 and my empty weight is 920lbs and my empty Arm is 9.94". This is with one of the heavier firewall forward set ups. The battery is in the tail which makes a huge difference. Is there possible something with the extended grove gear you have that could be moving the landing gear forward effecting the forward CG?

[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

[jrevens]

... The SS7 workable range is 9.00 to 16.00




I believe the info that I worked from showed the limits as 9.5 forward and 16.0 aft. Doesn't make any real difference to me though, since I can't come anywhere close to the forward limit. My empty CG is just aft of that forward limit, so I guess I could send it up by itself to fly without me... if it wanted to. :eek: