CG Limits

[jiott]

This is probably a question for the Kitfox factory (maybe John will chime in). The W&B worksheets that come with the SS7 kit as well as the spreadsheet that is available on the Kitfox website show the max allowable aft CG to be 16.00". This number is apparently independent of gross weight. However, when I was taking my training with Stick & Rudder in their SLSA, the POH showed an allowable CG envelope that was not so simple; it showed a max aft CG of 16.00" at 1320 lbs, but it then slanted back to 14.00" at 1050 lbs. Which is correct?

The main reason I ask is that I recently loaded up my SS7 for a camping trip with me alone (no passenger) but lots of gear in the baggage compartment. My weight was1250 lbs with an aft CG of 16.0". It did not feel unsafe in flight, but definitely felt tail heavy in the landing flare. According to the SLSA POH I should have limited the aft CG to 15.5". In my case here it is probably no big deal, but I could see that with a very lightweight pilot flying solo with lots of gear in the baggage area the discrepancies between these two aft CG envelopes could become quite large. Some clarification would be nice.

[t j]

My kitfox is a 4, but anyhow here's my story on weight and balance.

Everything Put in the plane moves the CG back. The baggage has a weight limit of 40 pounds. I load 40 pounds in the baggage then people and fuel to get a maximum gross weight and maximum aft CG. (15.8 inches...within limits for both the max aft limit and the Center of gravity Moment Envelope graph.)

As fuel is burned off in the wing tanks the CG moves forward but stays above the slanting line (bottom side) on the Center of Gravity Moment Envelope Graph so stays inside the box. I don't know if I was just lucky of if that is true for all kitfox models.

Does your baggage compartment have a maximum weight limit?

[jtpitkin06]

It is important not to confuse requirements for Special Light-Sport Aircraft certification with experimental aircraft requirements.
Manufacturers of S-LSA must meet very strict flight testing requirements. There is a specific flight testing schedule that must be followed. Part of the certification is flight at different weight and CG locations. If the LSA manufacturer is only able to load the airplane to a CG of 14.0 inches at 1050 pounds, then that is the maximum that will be shown on the weight and balance documents. It is easier to achieve an aft CG at heavier weights so the aft CG may have been demonstrated at a different location. The CG envelop may look quite different on a S-LSA when compared to the envelop on an Experimental aircraft.


As a builder of an Experimental LSA or Experimental – Amateur Built, you may put the aft CG wherever you desire but you make entry in the log book that you flight tested it at XXXX pounds and a CG location of XX inches with no adverse characteristics.



The answer is; yes, it is safe to fly your Model 7 at lighter weights with a CG of 16.0 It will obviously feel a little lighter on the controls with an aft CG.

[jiott]

Thanks for the reply John.
TJ, my baggage compartment was full but the total baggage weight was about 50 lbs-far below the max of 150 lbs. I don't see how you could ever get even close to the 150 lb limit without exceeding the aft CG limit of 16.0".

[Paul Z]

I have found the same issue, you need to pull the heavy stuff forward in the baggage area, and all the light stuff to the back. When I flew to oshkosh this definately helped.

[SWeidemann]

I'm asking advice about how to handle the small additional weight of adding a cabin heater and a couple of thermostats. I installed the coolant heater kit and at the same time I am changing engines (from a runout 912-ULS to a new one), with the addition of an oil and coolant thermostat. Is there any reason to do any more computations, or is a log book entry satisfactory?

Thanks much, Skot

[Danzer1]

Skot,

You have a couple of issues to deal with.

1st, if I recall, you bought the aircraft used. Did you ever obtain a repairmans certificate for it? If so, you can sign off on the changes. If not you will need an AP to sign off. Once you have that sorted out -

2nd, you need to read the operating limitations in your Airworthiness Certificate. Likely it is worded ambiguously enough to prompt a call to your local FSDO for clarification.

If they consider the changes substantial enough (likely) they will also (likely) require a new condition inspection and a new phase one flight testing for a certain number of hours. With a new condition inspection a new weight and balance could be required.

Changing an engine (even of same type) and adding accessories that weren't there before - will (likely) prompt any FSDO to require the above, but as we know, they all don't all interpret what is written the same, so it is best to check with them first.

Greg

[N981MS]

Skot,
As I understand it, anyone can work on an experimental. Change the engine to a turboprop. Wing airfoil. Make it a biplane. Put in a heater. Anything. No need for an A&P sign off.

The repairman's certificate allows the builder to do the annual condition inspection.

Crazy eh?

I do not think your weight and balance issues require FSDO involvement unless you are changing the current registered cg range. If you are just updating your empty weight and empty cg, I think you are OK.

At least one DAR I know says things that change flight characteristics are what requires going back in to phase one for a duration determined by the FSDO. ie change the prop. Change Engine type. Change gross weight.

I am not a DAR and did not stay at a Holiday Inn last night so YMMV.

[Danzer1]

Max,

As I understand it, anyone can work on an experimental. Change the engine to a turboprop. Wing airfoil. Make it a biplane. Put in a heater. Anything. No need for an A&P sign off.

The repairman's certificate allows the builder to do the annual condition inspection.

For clarity - I did not say he couldn't do the work himself - I was intending to refer to signing off the annual condition inspection as you have mentioned.

If he does not have the repairmans certificate - he then needs an A&P to sign off on the annual and that would include all changes made since the last annual. The conditions of the airworthiness certificate usually mention logging modifications and I would expect any good A&P to notice the changes made in review of the logs and therefore include them in the annual inspection.

At least one DAR I know says things that change flight characteristics are what requires going back in to phase one for a duration determined by the FSDO. ie change the prop. Change Engine type. Change gross weight.

I agree, so the question would be for the DAR/FSDO - does adding an oil cooler, thermostats, cabin heater, piping, fan, etc - change the CG and the gross weight and distribution and hence flight characteristics? Would be up to his local DAR/FSDO to clarify - as mentioned.

Skott also mentioned in previous threads/posts, adding strut fairings, fuel pump, new prop blades, replacing engine and mount. So again, up to the DAR/FSDO as to whether any of this would change not only the weight, but also the flight characteristics of the aircraft.

Still think it's best to check (FWIW).

Greg

[jrevens]

I know that practically everyone reading this knows that if it's licensed as experimental amateur-built (EAB), you can't get a Repairman's Certificate unless you're the builder.

If you bought the airplane, it would be a good idea for your own peace of mind & safety to do a weight & balance computation regardless of any changes, and especially if doing something like an engine change, IMHO. The small amount of time & effort is worthwhile.