As a Chipmunk owner/operator, W&B has always been of interest. There seems to be a dearth of data regarding "balance", just a lot of myths.
I
thought I had both diagrams recently posted, although on closer comparison my diagram is actually for a Mk.20 (the export T.10) whereas Coff's drawing is for a Mk.21 (the quite rare Chipmunk certified "from the ground up" as a civil aircraft). Given that the airframes are identical, the data shown (in particular the Moment Arms) here won't change for a T.10/T.20/Mk.21/Mk.22.
As part of my Chipmunk restoration, I purchased from DHSL at Duxford (at not inconsiderable expense; I only have one book at home that cost more, and that's a collector's item) an Approved Flight Manual (AFM) for a Mk.22. Now I could have registered it as a T.10 (permissible in Australia) but at the time the paperwork path of least resistance was to register it as a Mk.22.
But a Mk.22 is identical to a T.10 apart from some placards. I went to great lengths to keep the aircraft as it came out of the RAF in 1994, i.e. a "late" Chipmunk with the "Canadian" heater/exhaust, strakes and the broad-chord rudder. The following changes occurred:
1. Slick magnetos replaced the original BTH items - not only infinitely better but allegedly slightly lighter (I don't have the actual weights).
2. Metal 24 gallon tanks - again allegedly slightly lighter than the bag tanks but again I have no precise figures.
3. Radios comprised a King KY96 VHF Comm, a KT-76 Xpdr and an altitude encoder - total weight with rack 11 lbs (17 lbs lighter than the STR9X).
4. Dual Concorde RG25-XC 12V batteries at a total weight of 47 lbs, which I'm told is slightly lighter than the Hawker "two batteries contained in a single box" kit that the RAF fitted.
Back to the AFM - Section III Limitations:
Much as Coff described (I'm always left wondering how, unless you're carrying lead ingots, you're ever going to squeeze 40 lbs into the locker). However while the MAUW is quoted at 2100 lbs, it then says that flight at 2200 lbs is permissible provided that aerobatics are not flown and the aircraft not engaged in either "...public transport or aerial work". My Chipmunk has an empty weight of 1478 lbs (perhaps a little lighter than the standard RAF T.10) but with full fuel (24 gals), 2 X 200 lbs occupants and the mythical 40 lbs in the locker I gross out at 2091 lbs. Overloading then is very unlikely, without having to use the higher weight limit. This section also confirms the Moment Arm for both the rear seat occupant (max 250 lbs) and the locker are the same as in the diagrams. Note however, the permissible CofG range is defined as 6.8 to 0.77 inches forward of the datum which is what Coff's Mk.21 diagram states but NOT what's on the T.20 sheet.
Section VI - Weight & Balance:
Now this really piqued my interest as there's only minimal information. I've got a certificate confirming the datum as 1067 mm aft of the firewall (42 inches) and giving an empty weight, Arm and Index Units - and that's it. No other loading/balance information. Oddly, my brother-in-law's Mk.22 AFM has an additional sheet stating something like "provided the aircraft remains in the normal configuration it is unlikely that the balance range will be exceeded", a very sweeping statement indeed. However, working with the T.20 diagram it appears that I am also unlikely to ever exceed the aft CofG limit, which is interesting since changes 1 to 3 as shown earlier would tend to shift my CofG position aft. My "worst-case" scenario is with a 200 lb rear occupant, 40 lbs in the locker and 6 gallons of fuel, the CofG lies at -0.9" (limit is -0.77"). And it's an unlikely situation too, as I was always taught not to have anything in the locker for aerobatics, while 6 gals is close to 45 minute's fixed reserve (you should be landing/have landed by then).
As an aside, since the front seat occupant has no effect on balance it would seem that there is no reason (from the W&B standpoint at least) why the Chipmunk can't be flown solo from the rear seat. Or am I missing something?
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