Cinderella's Magic Slipper
"The whole airplane is a point design."
"Many aircraft get into an upward weight spiral during development but the Eclipse 500 is different."
"...the E22 engine launched the Eclipse on a downward weight spiral, where less weight led to less wing area and less weight again."
So sayeth Oliver Masefield, V.P. Engineering during a 2002 interview with Aviation Week.
Oliver, I am proud of you for understanding the holy grail, the fundamental truth in aircraft design!
Hit these goals and you can wear Cinderella's magic slipper, get to kiss the golden lips of the princess and all good things will come to you.
But you got in that awful upward weight spiral and are now destined to suffer the same fate as Cinderella's ugly step-sisters!
Had any of us sat in on that 2002 interview and suggested to Oliver that his "point design" was perhaps a "blob design" and that the Eclipse would only achieve 72% of the expected range and would need 27% more fuel to even achieve the reduced number, he would have probably jumped up and beat us to death with his slide rule.
His "point design" certainly did not assume a heavier weight. In fact, he said, "We eliminated the temptation to build in weight for a growth version. We can't afford that." He got the heavier weight anyway and fortunately for him, the structure was so over designed it could absorb the weight increase with no further testing.
But tip tanks, for God's sakes! What a terrible compromise. During most of a flight, they are empty...the airplane is carrying dead weight and dead drag. Nobody puts tip tanks on airplanes today!
At Oshkosh, Vern said that 97% of the airplane was redesigned for the Pratt engine. If you have a two year hiatus for an engine change and are in a 97% do-over mode, why not scale the wing up 8% and gain an additional 26% of fuel volume, eliminate the need for tip tanks, the stall speed would be lower and the increased wing area would provide better high altitude performance.
(Scaling up the wing would increase span 8%, chord 8% and thickness 8%. Doing the math:
1.08 x 1.08 x 1.08 = 1.26 or 26% more volume)
During this same 2002 interview Oliver also said:
"Some items - from seats to oxygen bottles - do not scale down as the airplane gets smaller: as a result, smaller airplanes tend to have higher empty weight fractions than larger craft."
Oliver, you understood the equations, why were they ignored?
Empty weight fraction = empty weight divided by gross weight
(the lower the number, the better)
Empty weight fractions for three similar Cessna airplanes:
6,765 lbs/10,800 lbs = .63
7,725 lbs/12,500 lbs = .62
8,300 lbs/13,870 lbs - .60
The Cessna numbers validate Oliver's point, smaller airplanes tend to have bigger empty weight fractions, now let's see what he did:
Eclipse empty weight fractions:
Projected with Williams engine
2,700 lbs/4,700 lbs = .57
Projected with Pratt engine
3,390 lbs/5,640 lbs = .60
3,550 lbs/5,920 lbs = .60
Oliver, unless your engineers have developed skills way beyond those at Cessna, your empty weight fraction by your own dictum is probably closer to .64 which would mean an empty weight of 3,789 lbs and if so, would decrease your range another 200 miles or so.
I hate to pile it on, but Eclipse is typically showing the aircraft in a five seat configuration and I suspect the empty weight reflects that number as well. Want to carry six total, add another 40 lbs for the sixth seat. Let's look at a weight build up for a high density flight:
Empty weight 3,789 lbs
Sixth seat 40 lbs
Six at 190 lbs 1,140 lbs
Fuel 951 lbs
Total 5,920 lbs
Eclipse has not released sufficient performance data to accurately estimate the range with six on board, but I would guess between 400 - 500 miles. That does not stop Vern from saying in one breath, we have a 6-place, 1,125 nm range jet. This guy would wear out a two man truth squad on his tail from morning to night.
And if Vern were to read this blog, he would probably say it is all a bunch of bullshit. Vern, my pile of crap is not as big as yours nor does it smell as bad.