All,

My question is a bit unusual given that our airplanes were well-designed to fit inside the light sport category (1320# Gross Weight, 138 mph maximum continuous power...).

Can a CH650B or CH750 be certified above the light sport weight limit?

Of course, why would anyone want to do this as it requires a PPL instead of a sport license to operate, but (to an extent) builders do have freedom to certify their airplanes as they see fit?  Specifically, I'm looking to optimize my CH650 for ultra long range hauls (trans-Atlantic).  The Great Circle Route via Labrador / Greenland / Iceland / Scotland requires a maximum flight length of 676 nm (see here).  Since weather at these northern airports in inherently unpredictable, carrying extra fuel is critical to safety.  We'll also be on floats to make putting down in the North Atlantic a bit safer.  Then if we can't get into the airport, landing in any Greenland fjord for a few hours while the weather clears becomes an option.

I'm crazy and I know it.  Our eventual goal is to circumnavigate the globe, but this North Atlantic leg is the most bothersome.  Since other nations (except the Bahamas) don't recognize LSA, this operation will require a PPL anyway and certifying the aircraft above 1320# isn't limiting factor.

According to my flight manuals, operating above gross weight endangers life by over stressing the airframe and making the aircraft too heavy to climb.  This said, we can't forget the scandals a few years ago when 650's appeared to be plagued with "flutter-induced" wing failures.  It's now the most researched LSA on the market today.  With the revisions integrated into the airframe from the 601XL to 650B, I feel a comfortable safety margin exists to add the extra weight if operated responsibly in smooth air.  However, I'm not an aeronautical engineer nor is this assumption supported by any comment or recommendation of the Zenith Aircraft Company - I'm an amateur aviation hobbyist practicing our liberties of experimental aviation.

With all of the engine options available to Zenith aircraft from the 80 hp Rotax 912 to 130 hp UL Power 350iS, I find it hard to fathom an aircraft with these engine ranges would have the same gross weight limitations.  I learned to fly in a Cessna 150 with a 100hp O-200.  That aircraft had a gross weight limit of 1600 lbs and climbed like a slug with me, my obese instructor and 15 gallons of fuel on a 90 degree day.  The Cessna 150 has a NACA 2412 airfoil and 160 sqft of wing area.  The CH650 has a fat 65-018 airfoil and 134 sqft of wing area.  The two aircraft have approximately the same wing loading at gross.  However, a CH650 will out climb any C150 when each is loaded at gross weight.

If I can recall my limited knowledge of aeronautical engineering principles, there's something about lift increasing by the square of the area.  Shortened 'speed wings' trade lift/drag for speed.  By the same equation, an increase in lift is sustained by increasing airflow over the wing (going faster).  In other words, heavier than 1320#, my aircraft would no longer be eligible for LSA rules because its stall speed would be higher than 45 mph.  We can test the principle ourselves by practicing stalls when heavy and noting stall speed and again after burning most of the gas and kicking out the copilot.  Following this to its logical conclusion, a wing's maximum theoretical lifting capacity is limited by the maximum airspeed of the aircraft; however, performance becomes increasingly unpredictable as that threshold is approached.  Vso < Vne

In short, the CH-650 is designed to balance the requirements of the LSA rules.  That is, 1320# at 138 mph with a 45 mph stall speed.  If a large 130 hp engine is mounted and 200 extra pounds of fuel added for endurance, the aircraft is no longer a LSA because its weight is greater than 1320# and demonstrated stall speed is going to be greater than 45 mph.  Is the aircraft still operable?  I'm sure many here have occasionally operated beyond 1320# without incident.  I know I used to fly a Flight Design CTLS at 1450 pounds without serious issues - however, I would never recommend doing such on regular occasions or into turbulent conditions.

Okay, time to turn this over to other armchair pilots for deliberation ;).  I'd be especially interesting in hearing from any of you with aircraft flying who have taken an excessively overweight passenger with lotsa luggage for a flight.