Help Smithcub W&B needed
- Thread starter Phil Sc
- Start date
skywagon8a
MEMBER
SE Mass MA6
The forward and reward limits are whatever you determine them to be during your flight test phase. I used Piper's numbers but added 1/2" to the aft limit determined during flight testing. http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/ab203ab0e89895af862572090071f7cd/$FILE/1A2.pdf
Piper's envelope is a bit longer than Cub Crafters.
Piper's envelope is a bit longer than Cub Crafters.
I have weighed it
I’m seeking the forward and aft limits and datum point in order to confirm I am safely within limits.
skywagon8a
MEMBER
SE Mass MA6
This makes me wonder how much you know about weight and balance? What did you use for a datum when you weighed the plane? What did you use for a reference when you leveled it for weighing?
This is the FAA Type Certificate for a PA-18: http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/ab203ab0e89895af862572090071f7cd/$FILE/1A2.pdf
Notice on page 6 that the datum is the wing leading edge. Some people use an arbitrary number ahead of the leading edge in order to not get confused with + & - arm numbers. If you use a spread sheet on your computer the + & - numbers are a non event. I use the wing leading edge because it is a known hard point from which I can measure rather than a point in space. The point in space just means more math which can be done wrong.
The Leveling Means on page 6 says: "Plumb from PK screw in upper door frame channel to center punch mark on enclosure door rear hinge."
We know that you do not have this on your plane. Alternatively you can refer to page 10 on the T.C. Landing Gear item 202: Tires are located at (+2) This means that when the plane is level you can drop a plumb bob from the wing leading edge and measure to the center of the axle 2". Another method of determining that the plane is level is to place your level on the firewall between the upper and lower engine mount fittings. The firewall should be plumb. We know this from the fuselage drawings. You may or may not have access to this drawing.
This is the FAA's weight and balance handbook:
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-1.pdf
Some airplanes do have published empty CG limits. There are no published empty weight limits for a PA-18. The published limits are applied when the plane is loaded for any particular flight. You can refer to the T.C. for the seat, fuel, baggage and other moment arm locations.
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Smith Cub was sold to Backcountry Cub and they have not been able to provide Weight and Balance information.
Typically in a "Kit" aircraft weight and balance information is provided. This usually consists of a sample W&B indicating the datum point and arm measurements of pilot and passenger seats, fuel, baggage etc. This is to guide the builder with his own using measurements and weights obtained from the aircraft itself. The forward and aft limits should also be provided to ensure safe operation. I have built 5 previous kits (not my first rodeo) and each of the previous kit manufactures had these published figures. I used the leading edge for my measurements and properly leveled the aircraft but I need to confirm I am within safe limits and also satisfy Transport Canada paperwork prior to flight.
So are you doing the first flight?
Cub clone? I'd start at standard Cub limits. My airframe is longer. BCSC provided me a sample W&B with fore and aft limits. Slats installed moves the aft limit back an inch. My testing says I need to move the forward limit back, too. PM me contact info and I can forward an excel spreadsheet. The column values can be edited. Pretty handy. Once established I entered my airplane info into Garmin Pilot's W&B page. That's very useful for changing sample loads to determine how the plane handles them.
Cub clone? I'd start at standard Cub limits. My airframe is longer. BCSC provided me a sample W&B with fore and aft limits. Slats installed moves the aft limit back an inch. My testing says I need to move the forward limit back, too. PM me contact info and I can forward an excel spreadsheet. The column values can be edited. Pretty handy. Once established I entered my airplane info into Garmin Pilot's W&B page. That's very useful for changing sample loads to determine how the plane handles them.
skywagon8a
MEMBER
SE Mass MA6
The Smith/Turbine Cubs of Wyoming/Backcountry Super Cub are all Piper PA-18 clones. As such in order to satisfy Transport Canada you should use Piper's limitations as published in the T.C. Mine is a TCOW 180 hp and I used Piper's numbers except as mentioned above I moved the aft limit aft. You will be safe using Piper's numbers.
