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Short run with strong prelaunch ?

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  • Short run with strong prelaunch ?

    Here are my calculated comparison of classic takeoff with very powerful launchers. No hope by this means

    Rotor inertia: 190 kg.m2
    Static Propeller Boost: 2400 N decreasing to 2100 N at 45 mph
    Coefficient of rolling in the grass: 4%
    Area of parasitic drag: 7 sq.ft
    Induced drag corrected by ground effect

    Click image for larger version

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  • #2
    Jean Claude,
    Interesting, but may I ask for some clarification?
    What are you trying to say when you write "No hope by this means"?
    Where did the "385 rpm in flight" note come from? The graph seems to show it taking off in both cases at about 320-330 rpm (and 9 seconds?)
    T

    PS "tp 265" seems to be a typo

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    • #3
      Tyler,
      What are you trying to say when you write "No hope by this means"? I want to say : No hope to significantly shorten the run by boosting the power of the launch beyond 10 Hp.
      The graph seems to show it taking off in both cases at about 320-330 rpm (and 9 seconds? Yes, full throttle, the duration of the run appears identical. Takeoff is dependent on the forward speed and the acquired rpm. (here, 325 and 335 rpm still full growing)
      Where did the "385 rpm in flight" note come from? This rotor carrying 470 kg reaches 385 rpm steady during the cruise
      Yes, "tp" is typo. "to" is better.

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      • #4
        Thanks for explaining further.
        Are you defining the takeoff as all wheels off the ground? I normally spin my rotor to 220 rpm before starting the run; a little higher if it's turf. I'll have to start measuring my TO distance.
        My cruise rrpm is usually around 325, but that's with only about 425 kg of load.

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        • #5
          Are you defining the takeoff as all wheels off the ground? Yes
          If the pitch of the blades remains unchanged, then your 220 rpm requires about 6hp,Tyler.

          What model of gyrocopter do you own?

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          • #6
            Magni M16 :)

            PS Incidentally, here's the problem I have with the word "gyrocopter". Clearly it's a back formation from "helicopter", but "helicopter" has really two parts: "helico" (spiral) and "pter" (wing). If we substitute "gyro" (circle) for "helico", we ought to end up with just "gyropter", no?
            Incidentally "helicopter", the word, first seen in french in 1861, pre-dates the actual machine by the better part of a century.

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            • #7
              You need to achieve both Rotor RPM and a flyable velocity to takeoff. Just increasing Rotor RPM with pre-rotator doesn't do much.
              That is why when some companies claim they can reach 320 Rotor RPM with pre-rotator, I have to ask "And Why?". After about 200 to 220 Rotor RPM from the pre-rotator, the best bang fr the buck would come from increased thrust from the prop (more HP from the engine) which can allow faster acceleration to Vx

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              • #8
                The french word used for the FW is "avion", coming from the latin word "avis" which means bird. This name was invented by Clément Ader for his steam airplanes imitating bats. Yet bats are not birds!

                Click image for larger version

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                • #9
                  Cool photo. Where is that "rara avis" exactly?
                  Most folks seem to like birds more than bats. It was probably a marketing decision. :)
                  Of course the mammalian order for bats is chiroptera ("hand wings").

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                  • #10
                    Originally posted by fara View Post
                    You need to achieve both Rotor RPM and a flyable velocity to takeoff. Just increasing Rotor RPM with pre-rotator doesn't do much.
                    That is why when some companies claim they can reach 320 Rotor RPM with pre-rotator, I have to ask "And Why?". After about 200 to 220 Rotor RPM from the pre-rotator, the best bang fr the buck would come from increased thrust from the prop (more HP from the engine) which can allow faster acceleration to Vx
                    Yes, that's exactly what I was taught, actually.
                    What about the rationale for slightly higher spin-up on a soft-field takeoff?

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                    • #11
                      Originally posted by Tyger View Post
                      Cool photo. Where is that "rara avis" exactly?
                      Most folks seem to like birds more than bats. It was probably a marketing decision. :)
                      Of course the mammalian order for bats is chiroptera ("hand wings").
                      It's the 'Avion III' built by Clément Ader in 1891. And it really flew for a short distance. That airplane can be seen (along with many other fascinating, historical machines) in the Musée des arts et métiers (Paris)

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                      • #12
                        It is presented in France as the first "heavier than the air" having taken off thanks to its own power.
                        But very unlikely in my opinion, because its highly arched wing could not give the required pitch stability.
                        The witnesses only certified the disappearance of traces of the wheels in the grass.
                        His steam engine and his boiler is magnificent of lightness

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                        • #13
                          It looks like it's hanging higher in the museum than it ever got under its own steam.

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                          • #14
                            Yes, yet his alcohol burner, tubular boiler, double expansion motor, steam condenser, had a total weight of
                            145 kg + 26 kg of water and gived 25 hp to 480 rpm
                            That gives 15 lbs / hp, almost as good as the Wright brothers' internal combustion engine.
                            But he was still very far from reaching their competence in aerodynamic.

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                            • #15
                              The bottom line is that if you need to induce a specific amount of kinetic energy in the system before flight, nothing is going to change that.
                              For the sake of argument and simplicity, let's assume the average power produced during the takeoff run is 100hp.
                              This means the gyro needs 900hp•sec of energy to attain flight.

                              If you pre-rotate higher, the rotor presents greater drag and slows the acceleration down the runway.
                              If you limit the pre-rotation, the lower drag allows for faster acceleration but requires more time for the rotor to attain flight speed.
                              it is a no-win situation. You can't short-change the Energy Demon!

                              In reality, most gyros utilize runways (paved or otherwise) that are more than 500m long, with many exceeding 1000m, so whether you get off in 100m or 200m is mostly just a matter of bragging rights.

                              What might be of more practical importance is how long you spend tying up the runway.
                              Taking 20 or 30 seconds pre-rotating with someone on final and closing at over a mile a minute is wasted dead time.
                              Reduce your pre-rotation speed and get your Butt moving!

                              For that matter, pre-rotating with the brakes ON is wasting energy.
                              One the rotor has stabilized and spun to ≈50rpm, release the brakes and let a little bit of ground speed build.
                              The engine will be at less than 3000 rpm but it will get you moving enough that the rotor might not slow down at all once you reach 200rpm and pull the stick back and apply takeoff power.

                              Everyone will be much happier if you can get airborne within 30 seconds of crossing the hold line.



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