Q-talk 57 - May/Jun 1996 - index
- Category: Q-Talk Index
- Published: Sunday, 30 June 1996 07:11
- Written by Tom Moore
- Hits: 1171
ISSUE NUMBER 57
QUICKIE BUILDERS ASSOCIATION
AN ACCUMULATION OF TOLERANCES
by Jim Masal
I once remember hearing the above phrase out of the month of Gene Sheehan ... "an accumulation of tolerances". He was using this to explain a misfit of assemblies during Q construction. That was a neat phrase and, in fact, I used it again myself just recently. For those of you who have never heard it, let me explain.
Suppose you have an assembly of, say, 17 parts that have to come together. Something like the connection from the control stick to the right and left elevators would be a good example. And suppose during the cutting or drilling of each component you were 1/32 of an inch off, say, the width of a pencil line too much. At the end of the operation you would be over a half inch (17/32) out of alignment. If you only erred half that much you would only be a quarter inch out of alignment and so on. If you made half your errors on the plus side (too much) and half your errors on the minus side (too little), your assembly would be dead on, just as it would be if each cut or hole were perfectly made. In reality though, each assembly is a random combination of a little too much, a hair too little and some dead on the money. And so, during construction I found myself continuously making small adjustments to account for minor misfits due to an accumulation of tolerances.
In the specific case mentioned above, i.e. elevator control, I have seen more than a couple Q-1's in which the torque tube from the control stick to the elevator bellcrank had to be bent or otherwise modified to clear the top of the tank. That's an accumulation of tolerances because the plans don't call for a bend, nor does every Quickie need one. Mine doesn't.
When you turn off the highway down the rocky, pot-holed road called "changing the plans", be prepared to fight a constant and bloody battle with this tolerance situation in addition to the other dimensional snags and snares you create for yourself by making changes in the first place.
All of this came to mind last weekend when I got a close look at a Dolphin ... not the fish. Old timers will remember way back in issue #2 of Quicktalk the story of two Texas builders who were building an "enlarged" Q-1 to be powered by a full HAPI VW engine and to be named Dolphin. Because of a ... how can I put this ... personality abrasion, I broke off contact with those guys. But I heard snippets of scuttlebutt over time. Both Dolphins were completed. One flew, but with great difficulty centered on the engine. At one point there was a roundabout probe made about any interest I might have in test flying it for comparison (I had just finished test flying mine), but I let that suggestion slide.
10 years later now, one of the Dolphins reappeared, hanging from a truss in a hangar near my work. It is now owned by Rick Markle who is close to finishing a beautiful metallic green Dragonfly. Rick, a one time avid ultralighter, bought the engineless Dolphin to be refinished as a "local, fool around the area" airplane. A Kawasaki 440 engine is planned.
Before Rick got it, it must have been nosed over because both wheelpants are roughed up and a new unfinished cowling is attached. The rest of the airframe is nicely contoured and in finish paint. There must have been some problem with the fuel tank, as there is a big section cut out of the top and there are ripples in the fuselage skin under the tank. There appears to be a dry area in the floor of the tank with evidence of some attempt to inject epoxy. Otherwise the workmanship appears to be fine.
But what about how the airframe was enlarged? We brought out the tape measure. It appears that only cockpit width was changed. We used my plans built Quickie for comparison in the numbers that follow. Rail to rail, ID cockpit width is 21" (mine is 19"). Inside ID, mid cockpit is 23 3/4", (mine: 21 3/4"). So we have a wider cockpit here, but then the builder put in right and left consoles starting about mid-thigh tapering out and forward such that it seriously cramps my fit as opposed to the legroom in my own cockpit. Furthermore, I don't fit as deep into the Dolphin which restricts my shoulders at the rails, whereas in my Quickie I only feel the pinch at my elbows.
Measuring the distance at the root from leading edge main wing to L.E. canard yields 70.5" Dolphin vs. 69" Quickie. Root to tip, wing: 7'6" (vs. my 7'7"), root to tip canard: 7'1" (mine: 6'10"). Note, these measurements are not precision and that in the airfoils you must account for the larger Dolphin fuselage, wider wheelpants and the fact that I added 3" of curled up wingtip to each side of my main wing. It appears that airfoil dimensions of the Dolphin were not substantially changed. That keeps the CG range under control along with the fact that the firewall is moved back 6-8 inches over the canard.
We could make many more measurements, but that would only be a minor point. It wouldn't be unlike me to come up with a moral to this story, would it? In fact, I'll bet you were expecting it. OK? This is what you can sometimes get when you depart from the plans. Two guys had a great idea. They even had the high skill and long perseverance to produce nicely finished examples of their dream machines ... not just one, but two. And here we are 10 years later with zero performance data for our information pool because neither plane, so far as I know, was successfully and thoroughly test flown. Now these guys were not QBAers and they have no obligation to share their knowledge with anybody else in the world, but wouldn't it be nice? And these builders never got to enjoy the many hours of enjoyable flight that was also part of their dreams. Why they gave up is unknown, but building a kit airplane is not just a physical process, but a mental and emotional one as well. Not only can divorce, old age, family illness, career or location change, etc. put a stop to a project, but sometimes you can just plain get worn out from lack of success. Facing the rebuild of a damaged aircraft can be the last straw. So sorry I had to be the one to tell you this.
So if you were wondering why I don't jump up and down, bouncing off the walls and ceiling while speaking in tongues after you enthusiastically tell me how you plan to make YOUR Quickie a far superior plane than it is now by modifying the plans ... now you know.
Other Articles In This Issue
You can order a PDF or printed copy of Q-talk #57 by using the Q-talk Back Issue Order Page.