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True Speed & permissable RPM ?

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  • True Speed & permissable RPM ?

    Having spent a few days in France, Blois & beyond in my Rans S6-116 the economical cruise of approx 95 mph is slow for long steps. Three hours was a long time against a mild headwind from Chauvigny tp Abbeville last Wednesday.

    At Blois on Saturday I briefly looked at the hotter ships in the ULM & VLAC categories, all w satisfyingly with the flat 4 Rotax up front. However due to enormous cost and even greater difficulties in using a RTF European a/c in LAA land, alternatively start with the paper drudgery of starting up with a new kit to the U.K. & you'll understand why I am attracted to the extant Eurostar !

    I met two separate crews at Abbeville who praised it's high cruise of 100 kt, say 115 mph, and economy. BUT reading the LAA TADs shows a low limit is imposed on cruise RPM of 4,800 which ought to negate some of the Eurostar's advantage ?

    If the Rans is cranked up to 5,000 rpm I'll get a GPS true 100 mph, but not much more from even higher rev's, even though no restrictions up to 5,500 rpm continuous. I have to say that is with a pair of draggy fat 'Tundra' tyres which do grant strip use in winter and without the 'need' for a chute, the max a.u.w is a healthy 499 kg [1100 lb.].

    So the trusty beast has its good points.

    Please come clean with the pro's con's of the Eurostar please.

    mike hallam.

  • #2
    True Speed & permissable RPM ?

    Hello Mike
    "economical cruise" is always variable as quoted by different people and sometimes on the same types. There should be an industry standard as in the car industry (MPG at 56 mph for example), because cruise speed is related to fuel burn. as you say at 5000 rpm your rans will cruise at 100 mph, but what the fuel burn at that setting?

    I have flown my 80 hp eurofox alongside my friend with the original 80 hp Eurostar design for over 5 years, on many long overseas and local trips. we both show 100 mph on the ASI at about 4200 rpm which burns about 11 litres, can be 4300 sometimes depending on loading. We check actual speed with the GPS as well. Different aircraft of the same design will differ depending on the prop setting and prop type as well as the engine size, my friend has his woodcomp classic set at 4600 static to give these figures, other props on the Eurostar will be different again. of course everything is a compromise, have a coarser setting on the prop and you lose a bit of oomph during ground roll

    so when comparing aircraft, I always ask what is the fuel burn and rpm setting at 100 mph. as I know it the foxbat for example will "cruise" at 100 mph, with a setting of 5000 rpm +/- on 100HP and fuel burn of about 16 litres, a skyranger swift on 100HP will be at about 4700 for 100 mph, but again depends on prop type and settings and engine size

    all aircraft should have their props set to meet the noise certificate settings, but I'm sure there are variations to the standard...

    hope this helps


    • #3
      True Speed & permissable RPM ?

      Thanks Roger,

      I see that the EuroFox ~= the Eurostar performance at a decent cruise, ditto fuel consumption.
      And you've kindly confirmed they're GPS cross checked and not a poor static location, or perhaps wearing pink spectacles when reading the 'cruise' asi.

      Had to help Vince in DEvon dismember his Rans S4 mid week and load a lorry with it, destination a buyer in Finland, so been too busy (400 mile round m/c trip Wed & today) to try some qualitative 5,200 rpm S6-116 trial. Though I think she'll swallow 16 l/h and at best show 105 mph true. [Plus lots more noise etc.]

      For long grass, as is alongside my strip when the hay is tall, generally means a high wing a/c is preferable as it allows more clearance should one wander from the straight & narrow !

      mike hallam.


      • #4
        True Speed & permissable RPM ?

        On your original question about max count RPM - on a microlight the manufacturers 'falsely' derate from the engine manufacturers figure so as to allow a higher max zero fuel weight. - this is because the certification code says you must have the capability to lift 2x 86 kg persons and enough fuel for one hour at max cont RPM. So the more you lower the placard value of Max cont the heavier you can build the plane. So just a paperwork dodge really. You won't harm anything by operating at a higher RPM - up to Rotax's max cont value.
        But you will have some very short flights if carrying two 86kg folk and keeping legal on that 450kg.

        If you start with 13 litres ( pretty much what a 268kg microlight can carry with those two 86kg folk and no baggage ) and want to land with 5 as min reserve , and fly at 5500 RPM - you will be burning 20 litres or so an hour so will have around 25 minute flights. Might just get you across the channel if there are some good fields close to the coast on each side....!



        • #5
          True Speed & permissable RPM ?

          Excellent logic Paul, thanks,

          Went to the LAA AGM Saturday last, murky down south with some dodgy flying to abort for an hour or so till sufficient gaps allowed Sussex to Northampton to be done after all. Returning I did crank the S6-116 to 5,200 with a fairly fine Warp Drive adjustable pitch setting (this year's trial at 13 degrees at the tips).

          Ground speed aided by some 12 mph tail wind - may be more at 3,000 ft when Controlled airspace allowed for the first half of the way - was up to 130 mph and about 105 airspeed. Quite thirsty at some 17 l/hour. Which at 0.45 lb per hp hour means approx. 75% continuous hp.

          Very satisfying to find my t/off to t/down average was 105 mph.
          And mpg as opposed to l/hr or even l/100 mph isn't actually a lot worse than grundging along at 4,600 rpm. Particularly useful option when in a hurry and a darned sight cheaper for a few extra litres MoGas than buying a 55k whiz bang a/c !

          Thanks again,

          mike hallam.