exploreten

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   exploreten added a topic in Avidfoxflyers General Hangar    2 Sep 2010

   Avid Magnum going on Murphy Amphibs

   I found the post below relative to a Google search for "Avid Magmum floats" and would like to know if anyone has any feedback for me on putting Murphy 1800 Amphibs on the Magnum. The Plane has been flown for a few years, 150 lyc. powereed, I've made the floats 8'3" wide (outside dim.) in order to be able to trailer the plane and floats down the road leagally. I had a couple of Osprey Amphibians earlier in my career.
   
   The plane is almost ready to test fly. Obviously the stress of on water operations and the added weight of the floats is raising structural questions in my mind.
   
   Any feedback would be appreciated.
   
   Frank
   
   
   
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   Posted 08 March 2010 - 02:06 AM
   dholly, on 07 March 2010 - 03:50 PM, said:
   Randy,
   
   It is my understanding the Airdale uses the same 144"L x .083" wall T6061 spars as our Mk-IV HH and Avid+. After a lot of time researching this, I came to the conclusion that building new Airdale or Magnum wings for my Avid+ was alot of wasted time and expense. According to Steve W., the Airdale wings use steel vs. alum drag tubes w/gussets for the higher compressive loads, along with 1" x .049 wall 4130 lift struts and an aluminum spar stiffener identical to the original Magnum, which was similar in shape to the plywood MkIV HH stiffener but longer. Of course, there are other airframe and gear modifications inherent to the Airdale that together add up to the 1400#, which we cannot retrofit. What I did do in my own quest for increased mtow was modify my stiffeners. Here's an excerpt from my log explaining my rationale:
   
   The kit manufacturer's suggested gross weight rating for my old Avid Mk-IV with Heavy Hauler wing (144"L x .083" wall T6061 spars, 7/8" x .035" wall 4130 lift struts, aluminum drag tubes, ribs 12" o.c. and 1/4" x 16" ply spar stiffeners) was 1200lbs. By comparison, the suggested gross of the Avid+ with the exact same wing, excepting 1/4" x 18" ply wing spar stiffeners, is 1250lb., due primarily to a stronger fuselage. I decided to replace the 1/4" x 18" ply wing spar stiffeners supplied with the Avid+ kit (part W-24, inserted into the spars and secured under the lift strut brackets) in favor of a longer, stronger spar stiffener design similar to that used in the Avid Magnum. My goal in doing so is to safely increase useful load by an additional 70lbs, to the Light Sport Aircraft max of 1320lbs.
   
   The rationale... early Avid Magnum design at 1500lbs. gross, used identical 0.83" spars and ribs, but added 1" x .049 wall 4130 lift struts, steel drag tubes (vs. aluminum) and 1/4" x 24" long ply stiffeners. Shortly afterward, Magnum gwt was increased to 1650lbs. with the addition of 1/8" x 24" aluminum stiffeners similar in shape/size as the original plywood type, however, two strips of 1/8" x 1/2" aluminum were riveted along the top edge of the aluminum stiffener to form a kind of 'T-Beam'. All other wing components being equal, the new stiffener allowed for a 150lb. (10%) gwt. increase according to the kit manufacturer. Even later, gwt was raised to 1750lbs. with the addition of a 36" extruded aluminum 'I-Beam' stiffener. So obviously, adding beefed-up 'Magnum style' spar stiffeners will strengthen the Avid+ wing. Since 70lbs. would represent a 5.6% increase in gwt for the Avid+... about one-half the 10% factory increase on the Magnum for essentially the same stiffener modification... I do not think the possibility of raising my gwt by 70lbs. after the stiffener mod is either unfounded or dangerously unrealistic.
   
   However, I am mindful there are several other areas in the wing that run out of 'stress related' safety factor margin over 1250lbs that must be considered as well, ie., Heim adjustable rod end, rod end attach bolts, rod end welded screw thread, the root attach bolt and bushings, the wing attach bolts, the lift strut attach brackets and the carry through tubes. Further, the wing and related components are only one small part of the gross operating load capacity equation and there are many other inter-related items to take into consideration. Landing gear, bungees, tailwheel, tailspring, attach bushings, attach bolts, wheels, brakes, axles, fuselage tube i.d. and o.d., bigger rudder, larger elevator, trim tab requirement, heavier duty fabric etc., all come into play.
   
   To be conservative and safe, I will do calculations on the wing loading and consult the G forces per bank angle charts. For a slightly higher useful load, I would limit operation in turbulence and keep bank angles below 75*-80* degrees (probably not a huge problem with amphib floats hanging under!), if necessary. At 1320lbs. gwt., useful load with pilot, passenger and gear would increase from 450lbs. to 520lbs. (or from 300lbs. to 370lbs. with full fuel, and from 375lbs. to 445lbs. with half fuel) on the amphib floats. In any event, this will be a good 'weight for safety' trade off. At 4.2ozs. for the original W-24 ply vs. 17.5ozs. each for my new wooden 'I-Beam' stiffeners, the weight penalty is 13.3oz. each x (4) stiffeners = 53.2ozs., or about 3-1/3lbs. total. One other alternative: adding a 12" length of hardwood shoe mold to both sides top and bottom of the supplied W-24 stiffener. This weighs out at 10.3ozs. each, or a total weight penalty of ~1-1/2lbs. total.
   
   Of course YMMV but, along with the foregoing armchair engineering, I think the Avid wing load test results and long history of all those Avids and Kitfoxes flying 70lbs heavy regularly lead me to feel comfortable with a mtow between 1250 and 1320 after simply modifying the stiffener. Since I made mine, I heard Just Aircraft came out with an extruded aluminum 'I'-Beam style stiffener that fits the I.D. of the .083 spar, might be worth a call.
   
   Cheers,
   Doug
   

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