Friday, August 04, 2006

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

Latest iteration:
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.

1 comment:

flight guy said...

New update from Flight International:
(Info not found on Eclipse transparent Website)

Eclipse to replace its 500 VLJ's composite wingtip tanks with aluminium after certification tests fail, delaying approval
By Graham Warwick
Aluminium to replace original composite wingtip units
Eclipse Aviation planned to begin testing late last week of aluminium wingtip fuel tanks for the Eclipse 500 very light jet after it failed to secure certification of the original composite tanks. The need to replace the tanks is one reason the Eclipse obtained only provisional US type certification in July.

The replacement tanks have the same fuel capacity and essentially the same weight, so flutter clearance will be performed by analysis to enable Eclipse to obtain full certification around the end of August, says chief executive Vern Raburn, adding that preliminary panel tests indicate the aluminium tanks will pass lightning certification.

"It's Eclipse's fault. We screwed that one up," says Raburn, but he blames suppliers for other delays. "Meggitt has had technical problems with the autopilot, but I think we are through them," he says. "We are doing flight tests now." Albuquerque, New Mexico-based Eclipse has slowed the production ramp-up and will not deliver as many aircraft as planned this year.

Raburn reserves his strongest criticism for supplier Avidyne, which has failed to certificate much of the planned avionics functionality "and continues to slip". The hardware has been qualified, "and we have been flying the functionality for initial certification for a month and a half, but they have not been able to get it certified".

Initial certification will allow single-pilot, day/night visual and instrument flight rules operation, but a software upgrade is planned for October to add avionics functionality that DayJet needs to meet Part 135 operating requirements so that it can begin its air-taxi service. Initial certification will give DayJet what it needs to begin training, route-proving and work out its maintenance processes, he says.

Additional avionics functionality that Eclipse views as its "market differentiator" will not be available for another six to 12 months, says Raburn, adding: "We will certificate with what most aircraft have, but we will not have many of things needed to realise the potential of this aircraft." As the hardware is already installed and approved, he says, Eclipse will add the functionality as software upgrades.

The company is working on enhancements, including larger aluminium tip tanks that will extend range. The heavier tanks will need flight testing, planned for later this month. Plans call for the "first dozen aircraft" delivered with smaller aluminium tanks to be retrofitted with the larger units once they are certificated, says Raburn.

Makes you wonder what they are actually certifying?