jliltd

jliltd's Activity

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   jliltd added a post in a topic Today's Dumbass Stunt    23 Nov 2009

   Yet another great feature of a light tractor airplane like an Avid. Just a little embarrassment.
   
   If you were in a Rutan canard you would have probably taken out your prop. I watched a pilot and his wife (both very nice people) auger in after they forgot to replace the fuel cap on their Vari Eze and it went through the prop on takeoff. It was a near vertical impact with a big puff of flame and smoke. Unsurvivable.
   
   I took off with my fuel valve closed in my Avid one day and it got my attention on climb out (landed in the grass at Arlington, WA). And then to top it off I took off with the radiator cap off on an initial test flight. I had landed and checked the coolant level and then left the cap off so it could cool better. The rest was history. Didn't hurt the engine but it was a long flight back to the airport smelling steam and seeing coolant drizzling down the firewall near the rudder pedals.
   

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   jliltd added a post in a topic Setting wings to fuselage (a.k.a. calculating wing sweep)    27 Sep 2009

   Doug,
   
   I have really been enjoying your builder's web site and just joined this site after a tip from your resident C5 flier.  It brought back lots of memories of when I built an Avid years ago.
   
   I just read your posts about your wing sweep.  The 27.5" distance between front and rear spar doesn't enter into the equation since the whole wing swings as one fixed unit.  With your plumb bob points projected on the level shop floor we are left with a long skinny right triangle.  The far (opposite) side from the sweep angle would be the 1/4" forward your measured at the tip of the spar.  Per your description the hypotenuse of the triangle is 144", if in fact 144" is the distance along the spar from the fuselage pin point out to the area you measured at the tip.  Otherwise if the overall spar length is 144" from cut end to cut end then the hypotenuse of the triangle shall be shorter (by the distance of material between the new pin hole and inboard end of spar - "edge margin" in engineering parlance).  I am assuming you had the aircraft completely level on pitch and roll datums with the tail in the air over a level slab when you did the plumb-bob projections.
   
   Mathematically the sine of the sweep angle equals the opposite leg's distance (1/4") over the hypotenuse (144").  So the sweep angle itself is the arcsine of the opposite over the hypotenuse, or asine(.25/144) which equals a smidgen under 1 tenth of a degree (.0994712 degrees) which is probably negligible.  If you want to change the pin joint to tip distance from 144" to another number you can recalculate the angle with the above formula.  If you want to use Microsoft Excel the formula is "=ASIN(.25/144)*180/PI()" where you can see the 1/4" and 144" numbers in between the first set of parenthesis.  Leave the 180/PI() alone since Excel gives the answer in radians by default and what we want is degrees.  If the 144" dimension is shortened then the sweep angle becomes even more negligible.
   
   I went one step further to check this out by laying down the geometry in a CAD program showing the sweep of a 144" spar resulting in a 1/4" tip movement.  It came out the same as the math.  I make mistakes sometimes so I like to cross-check things when I can and didn't want to post wrong info (been there and done that and won't let that happen again [).
   
   I'm not sure how important the value of the sweep angle is compared to the tip movement distance (again one tenth of a degree taint much).  The Kitfox recommendation to use a 1 degree forward sweep angle for heavier engines would yield a 2.5" forward measurement at the tip (assuming the same 144" spar distance)!  2.5" sounds like a lot to me but it's all based on a right triangle with the same formula (144*sine(angle)) where you just plug in any sweep angle you want.
   
   If wings were lengthened this tip deflection distance will increase.  Shorter wings would have a lower deflection distance for the same sweep angle (i.e. a wing clipped 3/4 length would have a forward tip movement of 3/4 of 1/4" inch or 3/16").
   
   What is important that your sweep change is negligible and now you have good edge margin in your spar root fitting joints.  That's not a bad thing since the design of the Avid/Kitfox plane spar to fuselage joints are 90 degrees out of common.  Usually strut-based wing spars are pinned horizontally and don't transfer bending loads to the fuselage; but our planes are pinned vertically to facilitate wing fold pivot and do transfer some bending lift loads across the joint.  Not a big deal with Dean Wilson's design due to good safety margins and the fact that the wing struts take the majority of the lifting loads from the wing spars.
   
   I'm a windy bastard at times and a lot of the above is probably overkilling an answer to a simple question  .
   
   I most want you to know I enjoy your builder's installments and documentation.  You're doing a great job.
   

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