Announcement

Collapse
No announcement yet.

Speed at the tips

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • Speed at the tips

    "A gyro’'s primary attribute, its ability to safely fly low and slow, makes it an ideal vehicle for chasing feral hogs around Florida bayheads but its inherent inefficiency – wings going 500 mph while the rest of it goes 50 mph, makes it as useful as an item of transportation as a rowboat." – C. Beaty

    OK, after reading that, I decided to do a quick calculation of how many mph the tips of my 28' rotor are going at any certain rotor rpm. The interesting coincidence is that, for a 28-ft-diameter rotor, mph is almost exactly the same number as rrpm!
    28π (feet per rev) x 60 (min/hr) = 5278 ft (per rpm per hour). There are 5280 ft in a mile, for those non-US folks reading this. So when I have 325 rrpm (typical), my blade tips are going just about 325 mph.

  • #2
    Fore and aft they are, and in a vertical descent. When going 75 MPH the right side is seeing 400 mph and the left 250.
    "Nothing screams poor workmanship like wrinkles in the duct tape!"
    All opinions are my own, I've been wrong before and I'll be wrong again. Feel free to correct me if I am.
    PRA# 40294

    Comment


    • #3
      You are talking about airspeed, but I was not (obviously). I was just remarking on the arithmetical coincidence for 28-foot-diameter spinning circle.

      Comment


      • #4
        This gets you into the fascinating world of mu ratio (aircraft airspeed/average tip speed). OK, fascinating in a nerdy way.

        NACA wind-tunnel testing in the 1930's discovered that a mu of about 0.35 is the best compromise, purely from an aerodynamic-efficiency viewpoint, between very low rotor RPM (low profile dag, but large losses due to large cyclic pitch changes between advancing and retreating) and high RRPM (higher profile drag, less cyclic flapping and the possibility of compressibility losses). A mu of 0.35 with a 325 mph average tip speed rotor yields a least-rotor-drag airspeed of about 114 mph.

        Most homebuilt gyros fly somewhat slower than this mu analysis would prescribe. That's because most gyro airframes are themselves pretty draggy -- so TOTAL drag (airframe plus rotor) is more tolerable at sub-100 airspeeds. Of course, this is especially true of totally bare-naked Bensens and the like. They tend to run out of available prop thrust at airspeeds where their rotors are still at mu = 0.25 or so. You can't go faster (though your rotor would like to) because your airframe has the drag of a barn door.

        Comment


        • #5
          The autogyro, as a result of its inefficiency, would have gone the way of the dirigible had Igor Bensen not resurrected it as a hobycopter.

          Helicopters are almost as inefficient but their ability to hover makes them nearly indispensable for certain applications.

          Comment


          • #6
            Originally posted by Doug Riley View Post
            This gets you into the fascinating world of mu ratio (aircraft airspeed/average tip speed). OK, fascinating in a nerdy way.

            NACA wind-tunnel testing in the 1930's discovered that a mu of about 0.35 is the best compromise, purely from an aerodynamic-efficiency viewpoint, between very low rotor RPM (low profile dag, but large losses due to large cyclic pitch changes between advancing and retreating) and high RRPM (higher profile drag, less cyclic flapping and the possibility of compressibility losses). A mu of 0.35 with a 325 mph average tip speed rotor yields a least-rotor-drag airspeed of about 114 mph.
            I AM fascinated, though... 325 does seem to be a fairly average rrpm (and thus tip mph) for my gyro, and I've set VNE at 115 mph, which looks a lot like your 114 number. I routinely cruise at 90 and frequently get over 100 for brief spurts, but I always keep well below 115. The Magni 16C (UK certified) POH has cruising speed at 90mph but VNE at just 100, which is definitely on the low side, it seems to me. I suspect that's mainly to keep the UK gov't happy, though.

            Comment


            • #7
              In my opinion the VNE for a particular model gyroplane is set by a test pilot who feels the aircraft is stable up to that speed and controllable by the average pilot.

              In my experience the rotor does not become unstable when higher tip speeds are encountered.

              Speaking generally The Predator reaches .35 at 100kts indicated air speed.

              310 rotor rpm, average tip speed 333 miles per hour, .35 equals 101kts.

              I have often exceeded 130kts in a descent with no loss of rotor stability or aircraft controllability.
              Regards, Vance Breese Gyroplane CFI http://www.breeseaircraft.com/

              Comment


              • #8
                Rotor tip speed is a function of blade loading (not disc loading). It is approximately equal to 66 x (square root of blade loading), ft/sec.

                Cierva standardized on a blade loading of 35 lb/ft² which results in a tip speed of 390 fps = 265 mph.

                Some Bensen type clones have operated with a blade loading as high as 70 lb/ft² which results in a tip speed of 552 ft/sec = 376 mph. At 50 mph air speed, the advancing blade tip speed = 426 mph, not a very efficient way of flying. When your trike goes 50 mph, its wings also go 50 mph (if all goes well).

                (fps/1.47 = mph)

                Comment


                • #9
                  Correctly designed rotor blades don’t become unstable at some high speed. Correctly designed rotor blades using airfoils with zero pitching moment and with CG located at the airfoil’s aerodynamic center result in a gradual forward stick movement with increasing airspeed until the forward stop is reached, occurring at an airspeed well below the speed of catastrophic retreating blade stall.

                  Rotors using airfoils having a negative pitching moment can enter a runaway condition if the negative pitching moment sufficient to cause a rearward stick movement with increasing forward airspeed.

                  The first gyro fatality occurred in the UK when the pilot of a Cierva C-30 got into a high speed dive from which he was unable to recover. The early C-30 rotor blades used a Gottigen 606 airfoil that had too much negative pitching moment.

                  Many gyro pilots learned this back in the days of rotors with adjustable pitch settings. The common metal rotor blades of that era, Bensen, Rotordyne, Stanzee, etc could still be started with the pitch adjustment cranked all the way up to the limit with interesting results. Typically, top speed was 20 mph or so with the stick on its forward stop; an increase of engine power resulting in a climb, still at 20 mph, a reduction of engine power resulting in a descent, still at 20 mph, all with stick hard up against the forward stop.

                  Comment


                  • #10
                    Originally posted by C. Beaty View Post
                    Correctly designed rotor blades don’t become unstable at some high speed. Correctly designed rotor blades using airfoils with zero pitching moment and with CG located at the airfoil’s aerodynamic center result in a gradual forward stick movement with increasing airspeed until the forward stop is reached, occurring at an airspeed well below the speed of catastrophic retreating blade stall.
                    It sounds like you are saying that, with correctly designed blades, I shouldn't have to worry about retreating blade stall, at least in level flight, because the stick will hit the forward stop before that happens. Do I understand you correctly?

                    Comment


                    • #11
                      Rotor stall in a gyro is totally different than rotor stall in a helicopter.

                      Helicopter retreating blade stall begins at blade tip and total, catastrophic blade stall is just a tic away.

                      In a gyro, the inboard region of the retreating blade is always stalled in forward flight and stall moves outward as airspeed increases, increasing cyclic “flapping” angle and necessitating forward stick movement to avoid a climb. That is, if the blade is correctly designed.

                      Comment


                      • #12
                        Originally posted by Vance View Post
                        In my experience the rotor does not become unstable when higher tip speeds are encountered.
                        Speaking generally The Predator reaches .35 at 100kts indicated air speed.
                        310 rotor rpm, average tip speed 333 miles per hour, .35 equals 101kts.
                        I have often exceeded 130kts in a descent with no loss of rotor stability or aircraft controllability.
                        μopt. of .35 indeed gives the best L / D of the rotor, but it is not the limit of stability. The unstability is reached for higher or lower μ, depending on the pitch setting and the stall angle of the airfoil.
                        Only when this μmax is reached, the flapping angle diverges, and it becomes impossible to exceed the speed.(Example in fig 9 of NACA report 515)

                        μopt. of alone rotor seems to me weakly interesting, because the airframe drag is preponderant.
                        The best L/D of entire gyroplane is always < μopt.
                        Click image for larger version  Name:	Sans titre.png Views:	1 Size:	35.5 KB ID:	1140542

                        Comment


                        • #13
                          Originally posted by Jean Claude View Post

                          μopt. of .35 indeed gives the best L / D of the rotor, but it is not the limit of stability. The unstability is reached for higher or lower μ, depending on the pitch setting and the stall angle of the airfoil.
                          Only when this μmax is reached, the flapping angle diverges, and it becomes impossible to exceed the speed.(Example in fig 9 of NACA report 515)

                          μopt. of alone rotor seems to me weakly interesting, because the airframe drag is preponderant.
                          The best L/D of entire gyroplane is always < μopt.
                          J-C, what value(s) of μ would you say is optimal/typical for a modern, tandem, open gyro in its entirety?

                          For anyone else that's interested, there is a link to that entire report here: https://ntrs.nasa.gov/archive/nasa/c...9930091587.pdf

                          Comment


                          • #14
                            Tyler, I have never drawn this curve with my spreadsheet, because the μopt. does not seem relevant to me.
                            I can only say for two blades with "modern" aspect ratio to aerodynamic pitch of 3.5° it gives μ maxi = .35 and L/D = 10.5

                            Comment


                            • #15
                              Poorly-designed gyros with unstable airframes become very pitch-unstable at rather modest airspeeds. For example, a high-thrustline gyro without adequate H-stab compensation flies more and more nosedown as throttle and airspeed increase. Control reversal as Chuck Beaty describes can occur, even if the rotor is properly designed. The flight sensation is one of riding in a wheelbarrow, about to be dumped forward, and being restrained only by steady back pressure on the stick

                              My original-issue Air Command flew like this at any speed much over 70. It was nerve-wracking in calm air and intolerable in the rough stuff. My Dominator, on the other hand, was LTL and had a potent H-stab. At 100 mph, it was still happy to go faster. The nose didn't tuck at all, and it felt as if it were flying on rails. I V-limited it only because Ernie warned me that, at some speed, the windshield would fold back and/or depart.

                              Neither aircraft's behavior had anything to do with the rotor. On both craft, the rotor at 80-100 ASI was just settling into happy mu territory, except...

                              The McCutchen blades on the Air Command were smooth at low airspeeds, but developed a 2-rev vibe at higher airspeeds that added to the pucker factor. I figured at the time (wrongly) that the rotor was about to go berserk as i approached 80 mph. Since the airframe was also trying to go berserk (in a PPO kind of way), there were powerful clues to slow down. As a result I got left behind a lot when group flying.

                              Comment

                              Working...
                              X