Empty C of G SS7

[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