Q-talk 129 - So What Is Your Gross Weight?
- Category: Q-Talk Articles
- Published: Wednesday, 23 December 2009 16:24
- Written by Doug Humble
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So What Is Your Gross Weight?
There has been a lot of discussion on the Q-List about what the gross weight of our airplanes actually is. Mike Perry is not an engineer, but has done a nice job of researching this subject and submits the following article.
Loaded up your plane lately? Thought about gross weight? This article is a review of Q2/Q200 gross weight and load factor issues. I'm trying to tease out what we know, what we guess, what we should worry about.
Part 1: A Personal Odyssey
This article really began June 13th 2007. That day Jim Patillo was preparing for "The Great Race" and he needed a Pilot Operating Handbook (POH) for the Q200. Unfortunately, there isn't one. Quickie Aircraft Corporation(QAC) never produced a Q200 POH. They sent out a Q2 POH with a four page supplement for weight and balance, and Jim needed more than that.
I first got involved by sending Jim an editable POH file. The file is for the Q2, but it is easy to edit in Microsoft Word or in WordPerfect. I also pointed to the graphs in QAC newsletter 20. However, Jim also needed weight and balance graphs for a gross weight of approximately1300 lbs. After some discussion on and off the list, I realized I had something almost no one else had: a QAC weight and balance graph showing a gross weight limit of 1300 pounds.
When Quickie Aircraft Corporation first announced the Q200, they set a gross weight limit of 1100 pounds, but said "We will possibly increase the gross weight above 1100 lbs. after further testing." They never made an official announcement, but a few high serial number kits shipped with a weight and balance chart that went to 1300 pounds. After talking with Jim and a few others, I set out to make that chart available.
The chart I had was in poor condition: a worn, patchy photocopy with hand-written text and smudges. The scan we made was worse. The chart in QAC Newsletter 20 was much better quality, and so were the performance graphs. Scan, cut, copy, paste; reduce the resolution to make a file easy to E-mail; then do it all again. Eventually we had a set of diagrams that you could use to make your own POH. (I would like to thank my daughter Meg for all her help with a high resolution scanner.)
Those charts are now posted on Jon Finley's website along with the editable POH at:
http://www.finleyweb.net/JonsStuff/Quicki eQ2Docs/tabid/58/Default.aspx. You can download and edit to your heart's content. (But remember, the idea is to produce a flying airplane, not a perfect POH.) The Q200 POH Supplement file has a longer explanation of the files and some cautions.
Part 2: But is it safe?
Shortly after the files were posted on Jon's website, I started to get questions about Marc Waddelow's analysis of the wing and the allegedly marginal strength of the main wing. If the strength is marginal at 1100 lbs. you certainly don't want to fly at a higher weight. There was also some discussion on Q-List.
Meanwhile, Larry Severson kept posting claims that the Q2 canard was designed to 30 Gs. Eventually I re-read everything I could find on Waddelow's analysis and all the old materials I have on the Q2xx from QAC and others. Here is what I found:
This is an early description of the canard and wing design: "At the drawing board stage, the Q2 rear wing was designed for a positive 12 G limit load and the canard, since it doubles as the main landing gear is required to withstand over 30 G's of positive in flight loads and a 500 ft./min. landing impact." (Sport Aviation, May 1981, as reprinted by QAC for distribution) Please note this would be 12 Gs at a gross weight of 1000 lbs.; the Q200 was not released until 1983.
The only documentation I have of Marc Waddelow's analysis of the wing is in QuickTalk # 28, pages 9 & 10. This is a summary of an exchange of letters between Marc and Gene Sheehan. Initially Marc noted a stress concentration at BL 40 and concern about the strength of the wing. As his analysis became more and more refined, he got much closer to QAC's numbers. Eventually he wrote: "I don't think the wing will break if built per plans. However . . . I am still convinced the lay up plans could be improved." He goes on to suggest tapered spar caps, similar to what Sam Hoskins recently posted on his blog.
One quote from Sheehan: "As to your [Waddelow's] suggested modifications, I can't see anything wrong with them other than an increase in weight. This may seem to be a small matter to you, but my experience has shown that the typical homebuilder who doesn't trust the designer and adds a little beef here and there usually ends up with a very heavy airplane. He also insists on flying over gross weight. So instead of having a stronger airplane, he may actually have less margin. . ." Furthermore, Sheehan reported non-destructive testing to 8Gs, and recommended any modification of the main wing or canard be tested to "at least 50% above what you wish to use as your limit G loading."
I did not find any documentation of testing to actual limits; that is, no one built a wing and loaded it until it broke. I think QAC should have, but they didn't. We have only Sheehan's report of non-destructive testing.
Part 3: Number, Number, Who's got a Number?
Let's talk about load factors and load limits. Load factor is the ratio of the load supported by the aircraft wings to the weight of the airplane, usually represented as "G-forces" or Gs. Load limits represent structural limits of the airframe, and are chosen to ensure aircraft safety. The three categories of load limits for small aircraft are Normal (+3.8 Gs, -1.52 Gs), Utility (+4.4 Gs, -1.76 Gs) and Acrobatic (+6 Gs, -3 Gs). Composite aircraft are usually designed to twice these limits for a safety margin. Static testing is to 1.5 times the design limit. With that information, where does the Q2xx fit?
The QAC Q2 POH says: "The Q2 is intended to be operated in the utility category. The utility category is intended for limited acrobatic operation. . . . Flight Load Factors +4.4 g, -1.76 g" (page 4-1). However, the Sport Aviation article quoted above says the wing was designed to 12 Gs. Also, Gene Sheehan writes to Marc Waddelow, "The Q2 rear wing has been load tested numerous times all over the world. Our own testing was to about 8 G" (QuickTalk # 28). Confused yet?
My take on all this: QAC designed the Q2 to acrobatic limits, but recommenced only utility limits to give a margin for builder variability. I think the Q2 main wing was tested to 9Gs when the gross weight was 1000 lbs., or about 8.2 Gs at 1100 lbs. (hence Sheehan's vague "testing to about 8g").
What about higher gross weights? 12 Gs at 1000 lbs. gross is the same load as 9.23 Gs at 1300 lbs. or 8.89 Gs at 1350 lbs. We should still be able to fly in utility category with a safety factor of 2 if the wing is built properly. You will have to decide for yourself about the stress concentration Marc Waddelow reports at BL 40.
Part 4: Other Information on Load Limits
We do have other data: there are some amazingly heavy Quickies flying. Some Q2xxs were built with O235s, full panels, design mods and the kitchen sink. In Larry Koutz's listing of flying Qs in the USA, there are several planes with empty weights over 825 lbs. Charlie Harris of Littleton Colorado has a Q200 weighing 832 lbs with 1000 hours. Many Q200 pilots are using a gross weight limit around 1300 lbs.
Then we have the weight and balance info sent with my kit, showing a gross of 1300. I am unaware of any additional testing or engineering documentation to support this.
I am aware of three wing failures. One was an improper repair (expanding foam), one plane had a secondary gas tank above the main wing and a fuel leak eroded the foam, and a Canadian plane was improperly constructed with foam cores built up of smaller blocks and not properly aligned or glued. Improperly constructed or improperly repaired wings do break.
I am not an engineer, but I think the Q2xx main wing is safe up to a gross of 1300 lbs. I think anyone flying at gross weights over 1100 lbs should read the discussion in QuickTalk 28 and make your own decision about the safety of the Q2 main wing. I would also plan to fly the Q2xx in normal category if over 1100 lbs (limit 3.8 Gs, no aerobatics or steep turns) and utility category under 1100 lbs (limit 4.4 Gs). "Normal" may sound like an inferior standard, but we usually fly Pipers and Cessnas in the normal category.
Part 5: But then . . .
Since I started this article, I learned that N39425 was involved in a landing accident in which the main wing broke without striking the ground. This was a very unusual accident involving a jammed trim device, and I hope to learn more about this accident. (Pictures were posted on the internet briefly, but are no longer available.) Also, I've started a conversation with John tenHave about the strength of the Q2xx main wing, and he is working on a comparison of the Quickie and Q2xx main wing. So I expect to have more to report in an upcoming issue.
You can order a printed copy of Q-talk #129 by using the Q-talk Back Issue Order Page.