Saturday, October 21, 2006

Watching the Eclipse - Weights

This is the second in a series of topics that will be revisited now that the Eclipse is certified and deliveries are about to commence.

The Eclipse web site is still showing the numbers released July 27th:

Max T.O. Weight 5,920 lbs
Fuel 1,686 lbs

The Type Data Sheet lists:

Max T.O. Weight 5,760 lbs
Fuel 1,524 lbs

These reductions shift the range-payload curve to the left and almost certainly reduce the published range figure to under 1,000 nm.

Until the company starts delivering airplanes, the true empty weight will remain a closely guarded secret. Somehow we will need to get Duke a leave from Bush's Bean Factory and have him infiltrate the Albuquerque airplane company. I am confident Duke could get us the actual numbers.
(editors note: Imagine the off-shore readers trying to figure out the Duke business!)

Back on subject, the range is predicated on empty weight and current numbers reflect a 3,550 lb airframe, a number I suspect is still not based on reality.

Now I will go out on some thin ice. Using the information I have available, the Type Data Sheet and a 3-view diagram from Jane's, I did some quick and dirty center of gravity calculations.

If the empty aircraft CG is at station 208, a single 170 lb pilot with minimum (300 lbs) fuel on board will push the aft CG limit.

If the same airplane is loaded with full fuel and 4 - 170 lb occupants in the four front seats, the airplane will be at gross and outside the forward limit.

The Eclipse does not have a large CG envelope. It is a short coupled airplane - the distance between the horizontal tail and wing, which ultimately limits CG travel.

The four front seats are all forward of the approved envelope. Occupants in the pilot seats weighing more than 170 lbs will really shift the CG location due to the long moment arm. Since most adults weigh more than 170 lbs, in addition to providing the hand held GPS with each delivery, Vern may also want to provide a bathroom scale with each airplane to ensure the weights and CG limits are not exceeded.

So Duke, while you are at it, get us a CG location as well.


SpeakTest said...

I think the weight discrepency is because the original TCDS does not include the larger tip tanks. The new tanks will increse the fuel capacity to 1686, and presumably they will get a MTOW increase to 5920 as well when they amend the TCDS.

I don't think empty weight will change the range very much, but it will have an impact on full fuel useful load, and you might not be able to take the whole family on a max range trip non-stop.

Certainly the CG envelope looks pretty tight, and it will get even worse if they increase MTOW. I wonder if they can expand the envelope with further flight testing of if major changes would be required (i.e. a new elevator.)

Strange that the CG diagram is even part of the TCDS. The Mustang's TCDS doesn't contain this info.

Stan Blankenship said...


Eclipse sailed thru static test, so the gross weight limit is probably not a structural issue.

The reduced weight and narrow CG envelope suggest a control issue which is why I suspect you mentioned a new elevator.

Another option is a trimmable horizontal stabilizer which I always thought was de rigueur for a jet.

Stan Blankenship said...


Both the Eclipse and Sino-Swearingen TCDS's show CG diagrams. Both aircraft certified out of the Ft. Worth ACO.

The Adam 500 does not have a diagram, was certified out of the Denver ACO.

No diagram for the Mustang, certified out of the Wichita ACO.