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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.
You should talk to Andy Elliot in Arizona. His Corvair-powered 601 is registered experimental – not light sport. He IS an aeronautical engineer. He certified his 601 to a higher weight – I think 1,450. Also he chose NOT to do the full “B” upgrade because he feels that not all the "recommended" upgrades are needed.
Also, I know a pilot who has logged hundreds of hours in a 601 well over-gross. Many of the hours were PRIOR to the "B" upgrade. As far as I know the plane has not fallen from the sky yet. When the plane is heavy it will burn more fuel and the engine will run hotter.
You pretty much have the balance worked out. Heavier plane stalls at a higher speed and puts more stress on the aircraft structure (especially the wings, but the entire structure works harder). A wing that breaks at a 5 G loading at LSA weight may only be able to stand 3 G's at a 1500 pound aircraft weight. Unless you want to test a wing to destruction you will never know for sure where that breaking point is for higher weights. Zenith did test the 601XL wing to some pretty high loads during the initial design and it did not break, so there is some robust structure there to experiment with. As already mentioned, Dr. Andy Elliot (who IS an aeronautical engineer) certified his 601XL to a higher weight and the plane has been flying for quite some time now. Here are some pics of some of the XL wing testing. Since the plane is right side up with the test weight on the top of the wing, this must be a negative G test - to test positive G's the wing is upside down with the weight placed on the bottom (upper) side of the up side down wing. http://www.zenithair.com/zodiac/6-photo-testing.html
The CH750 can be certified for 1440# if registered as EAB. It looks like it applies to both STOL and the CRUZER.
Gross Weight Increase for STOL CH 750 Light Sport Utility Kit Plane
July 26, 2010 - AirVenture Oshkosh, Wisconsin:
Zenith Aircraft Company has announced a design gross weight increase for the STOL CH 750 aircraft. The aircraft design gross weight has been increased from 1,320 lbs. (600 kg) to 1,440 lbs. (650 kg), an increase of 120 lbs. (50 kg). The increase adds significantly to the load carrying capability of the “light sport utility” aircraft first introduced at AirVenture (Oshkosh) in July 2008. The gross weight increase applies without airframe modifications to all aircraft built to drawings Edition 2 (drawings, dated July 20, 2010), and operated as “Amateur-built – Experimental” aircraft. STOL CH 750 aircraft registered as SLSA and/or operated by Sport Pilots are not affected as the weight limit per FAA rules for Light Sport Aircraft (LSA) continues to be 1,320 lbs.
Several rigorous static load tests were performed on the structure in preparation of this gross weight increase. Aircraft built to Edition 2 drawings incorporate structural changes to justify the extra load, especially to accommodate larger (up to 160-hp) and heavier (up to 280 lbs. installed weight) engines. “We have not changed the basic design, since the CH 750 offers such great performance, capabilities and features����,” stated Zenith Aircraft Co. president Sebastien Heintz. “With the Edition 2 drawings we have increased the capabilities of the aircraft, especially for pilots using the aircraft as a utility plane and to allow for additional larger engines, such as Lycoming’s new 233 powerplant. Sport Pilots [in the United States] will continue to be limited to the 1,320 lbs. gross weight, but with the added peace of mind that the additional margins provide, and private pilots can take full advantage of the added useful load now available for the STOL CH 750 registered as a Amateur-Built – Experimental (EAB) aircraft.”