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SSDR Clutton Fred

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  • SSDR Clutton Fred

    This is no more than an idea at this stage. Don't panic.
    I am looking into the possibility of finishing a part-built Clutton Fred to SSDR spec.

    The original design has a MTOW of 350kg, empty weight 250kg. This presents the first and probably most significant hurdle in getting the weight down to 200kg empty.

    The original has a claimed stall speed of 40mph (35kt) which at the reduced weight should be somewhat lower.

    The aircraft currently exists as a fuselage (uncovered) and tail surfaces (in need of some attention).

    I have begun a spreadsheet of component weights. A Rotax 582 represents a good few kg saving compared to the VW based engine. Modern aluminium tube construction wing and tail surfaces should offer further savings, especially with Oratex or similar covering. There is further room for weight reduction in the undercarriage arrangement and the control linkage (original has a steel tube pushrod for elevator control). I would propose to copy an existing microlight wing construction using the dimensions of the Clutton wing. The low AR of the Clutton wing will reduce stress in the spars, as the overhangs are all shorter and the fore and aft spars can be spaced further apart giving greater torsional stiffness.

    The first question which has arisen concerns the wing section. Eric Clutton specified a section with a significant undercamber. This presents problems with covering and will likely add weight to the wing (in fastening the covering to the underside).
    If I understand correctly, an undercambered wing is more efficient at low speed, but does not offer any more lift than a flat bottom section. The effeciency advantage becomes less as speed increases.
    Thus, using a flat bottom section should not change stall speed, but will be more draggy at or near stall speed.
    Can anyone comment on the comparison between undercambered and flat bottom sections?
    Can anyone spot a terminal flaw in the general concept of a lightweight Fred?
    Pete T.

    "A closed mouth gathers no feet".

  • #2
    SSDR Clutton Fred

    Depends on your construction methods, should be possible to make an undercambered section with the undersurface battens bent that way and secured to the corresponding upper surface battens with the same method as HG use to control undersurface blow down at high speed.

    You can always do a comparison with something like Javafoil.

    Comment


    • #3
      SSDR Clutton Fred

      Generally an undercambered section has a greater Clmax than a flat bottomed one - so will result in lower stalling speed for same wing area. But that can be recovered somewhat if you fit flaps - which then return it to similar to that undercambered section!

      At the end of the day you need to work out whether you want a flapped wing or not and run the numbers for what wing are you need to meet the stall speed. Around 25-30kg/M for an unflapped flat bottomed wing section ( Thruster T600 is 28kg/m) Machines like Skyranger Swift and C42 with simple flaps meet the stall speed at 36kg/M.

      Shouldn't be too hard to shed that 50kg off the weight using light weight fabric and ditching the VW for a two stroker, and sorting out the obviously heavy other bits. Fred's signature feature is the folding wings so would be good to retain that.

      Paul

      Comment


      • #4
        SSDR Clutton Fred

        Splendid - two responses from respected individuals and no show stoppers!
        The folding wings would indeed be nice to retain, but are in direct conflict with any requirement to increase wing area.
        At 10.32m the wing loading at 300kg is just shy of 30kg/m. The implication is that an unflapped, flat bottom wing will just make the stall speed requirement.
        Pete T.

        "A closed mouth gathers no feet".

        Comment


        • #5
          SSDR Clutton Fred

          You could build in an insurance policy on the stall speed by using full span ailerons and making provision for a flaperon mechanism.

          Paul

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          • #6
            SSDR Clutton Fred

            Good plan, Paul. Especially so as MTOW will likely be a little over the 300kg with a BRS fitted.
            I'd still like to aim for 300kg and keep the wing loading below 30kg/m with the original wing dimensions.
            Pete T.

            "A closed mouth gathers no feet".

            Comment


            • #7
              SSDR Clutton Fred

              My old escapade had an under-cambered wing which was covered in oratex; no additional underside fabric restraint other than the oratex glue was required.

              Just a though!

              Comment


              • #8
                SSDR Clutton Fred

                Thanks Rick. I have gained some confidence from other sources regarding Oratex and undercambered wings.

                I now have a tubular wing design worked up and some calculations. I have gone for what is essentially a tubular version of Eric's wooden wing, with mainspar and rear spar in the same locations. This allows the option of folding wings to be built in at a later stage if weight permits.
                Pete T.

                "A closed mouth gathers no feet".

                Comment


                • #9
                  SSDR Clutton Fred

                  After following the thread "SSDR - does it really exist" I have become concerned that I may have a problem in registering the aircraft as SSDR. The project was issued with a registration when it was started (many years before I became aware of it). Thus it technically exists as a light aircraft, but not as a microlight. Fred also appears on a list published by the LAA as "unlikely to be eligible for deregulation" http://www.lightaircraftassociation.co.uk/2014/News/ssdr_exemption.html
                  Is it worth trying to get the aircraft re-classified as SSDR under its existing registration, or would It be simpler all round to declare it as a new design?
                  Any advice appreciated.
                  Pete T.

                  "A closed mouth gathers no feet".

                  Comment


                  • #10
                    SSDR Clutton Fred

                    Yes, I'd heard that Francis Donaldson had negged the Fred as SSDR suitable. So I think you'd need to discuss with him before going any further.
                    Dave

                    Comment


                    • #11
                      SSDR Clutton Fred

                      I've exchanged Emails with Francis. He replied:

                      "Based on the calculations below we would not object to the proposal provided the max gross weight did not exceed 680 Lbs
                      Wing area 111 sq ft
                      Max gross weight 190 +80 + (25 x 0.71) = 288 Kg = 633 Lbs
                      Wing loading = 5.71 Lbs/sq ft
                      CL max required to achieve 35 knot (= 40.25 mph) stall speed = 5.71 /((40.25 x 40.25) /391) = 1.38
                      Basic CL max of Gottingen 535 aerofoil 1.75
                      Aspect ratio = 22.5/5 = 4.4
                      Estimated CLmax for AR = 4.4 corrected for aspect ratio 0.85 x 1.75 = 1.48
                      This exceeds 1.38 therefore it is feasible that a stall speed less than 35 kts could be achieved."

                      I have not yet broached the subject of the existing registration. Good to get the basic practicalities sorted out first.
                      Pete T.

                      "A closed mouth gathers no feet".

                      Comment


                      • #12
                        SSDR Clutton Fred

                        I believe that, armed with the above, you'll have no trouble having the existing category changed.
                        Dave

                        Comment


                        • #13
                          SSDR Clutton Fred

                          I should add that Francis responded to my enquiry the same day and kindly provided his calculations.

                          From some other forum posts, one could be led to believe that Francis is the LAA's chief design rejection Ogre. On the contrary, he has taken an interest in my project and we have exchanged a few emails on the subject.

                          Even without considering that I am not even a member of the LAA, this is a level of service beyond that which I often receive from paid suppliers.
                          Pete T.

                          "A closed mouth gathers no feet".

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