August '04 Update

August 2004 Update - G rating

The following letter was sent to all planholders on August 4, 2004.

It's time to update the UltraCruiser builders. A month or so ago, someone asked if we had done a static test on the wing to verify the calculated "G" rating. We had not, and it appears we were with the 50% of ultralight manufacturers that were relying on the calculations. We decided we would do a static test, the question was how and when. The idea of bending a perfectly good bird was enough to turn my stomach. It wasn't long and opportunity came knocking. The UltraCruiser we demonstrated at Sun 'n Fun was built for a gentleman in Indianapolis. He fell in love with the design at Oshkosh 2003 and hired us to build his UltraCruiser. His goal was to fly it on his 90th birthday which was in the middle of June. The day before his birthday, during a "taxi" session, he got airborne. He did a great job during his 25 minutes in the air. Slow flight, stalls, steep turns, all went well. The landing was a different story. He simply landed at 10-15’ up. The airplane mushed in, with most of the force on the left main. The left wheel went straight forward, twisting the spar at the strut attachment. Lots of popped rivets in the skins and the bird rolled 90 feet and came to a stop on the left side of the grass strip with the engine running and no prop damage. We are thankful that the only thing Edwin hurt was his feelings. Even the following day, he had no soreness or stiffness.

The up-side is that the damage was localized to the spar area around the left strut and wing attach points. We decided we would need to replace the center section and left wing panel. But first, we would do the static tests using the UltraCruiser in its present condition. We removed the left panel and cut the spar inboard of the left strut. We bolted a steel plate to the left spar stub as an anchor point for the test. The test fixture was assembled and the weights calculated.

The first test showed that the carry thru began to bow forward at less than 3.5 g's. We assumed there could have been some unseen stress from the hard landing, but everyone at Hummel Aviation including Morry, agreed this was not acceptable. Morry explained to us that the calculations are for tension and compression of the spar structure and that the bending or bowing had not been anticipated.

We decided to support the carry thru to eliminate the bowing and retest. One requirement in designing the improvement was that it had to be easy, as light as possible, and not require the skins to be altered. With that accomplished we retested the structure. With thirty 50# bags of salt and five 7# bars of steel stacked on our pride and joy, we heard a pop and decided that was enough. We had loaded the wing to 6 g's and one of the .063 support angles we used to brace the carry thru was beginning to buckle.

By the time you read this, I will have spoken to all the guys who are flying UltraCruisers to make them aware of the situation. THIS BRACING IS REQUIRED TO OBTAIN A 4G RATING FOR THE ULTRACRUISER.

Attached is a sketch of bracing and materials needed. Hummel Aviation will make available a materials package to facilitate this change. This package will cost $30 including shipping (additional shipping for overseas) and consists of the following:

Note: The items included have been revised since original update was mailed:

(1) 1 x 1 x 1/8 x 20” alum angle

(2) 1 x 1 x 1/16 x 16” 65º angle

(26) CCPQ-43 stainless steel rivets

(20) AN470 AD -11 rivets

(2) AN4-5A bolts

(2) AN4-7A bolts

(4) AN635-428A nuts

(4) AN960-416 washers

Photos of the stress test and the improved carry thru can be viewed on our website (www.flyhummel.com).

We will do everything we can to ensure that all planholders receive this update. We ask that you complete and return the enclosed postage paid postcard so that we can mark you off the list of those who need contacted. Please feel free to contact us if you have any questions.

Click here for sketch

Click here to view photos of the stress testing

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