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Adjust Low Thrust Line closer to Centerline thrust on RAF Sparrowhawk?

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  • Adjust Low Thrust Line closer to Centerline thrust on RAF Sparrowhawk?

    I have an RAF Sparrowhawk with the standard low thrust line (LTL) and I'm curious if anyone has made adjustments to bring it closer to centerline thrust (CLT).

    My understanding is it was designed to be CLT at full gross. The problem is I fly 95% of the time by myself. When I add or reduce throttle, I end up chasing it with trim since otherwise the stick forces are very heavy. The thrust line is supposedly around 2" low when flying solo. I was thinking of raising it 1" to be 1" LTL solo and 1" HTL at gross.

    Has anyone raised their engine or maybe tilted the engine up to bring it closer to CLT?


  • #2
    I think the "chasing it with trim" is an effect of the cruciform tail, which causes the gyro to "hunt" as the pressures build and release unevenly from each quadrant.
    I.e., like gimbal thrust.

    Thus, I doubt that changing its vertical cg will make any difference.

    PP - ASEL complex (Piper 180, C206, RV-7A), SP - Gyro (Calidus, RAF, SC2), soloed in gliders; checkride soon

    Wasn't happy with my RAF's pitch instability, so I installed a Boyer H-Stab to my great satisfaction!

    When an honest but mistaken man learns of his error, he either ceases to be mistaken -- or he ceases to be honest."


    • #3
      I believe this gyro have very High Thrust Line with two passengers. Less high with only one passenger.

      Click image for larger version

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      • #4
        How does that compare to a High thrust line, Magni M24, or a Cavalon type gyro?
        The government cannot give anything to anybody that the Government does not first take from somebody else.

        I prefer dangerous freedom over peaceful slavery.
        - Thomas Jefferson

        Scott Essex....Flying H Ranch


        • #5
          A cruciform tail unit does not lead to "hunting" or a need to "chase" trim, in my experience. I taught in a Dominator tandem for several years. This aircraft, of course, has a cruciform tail and LTL. The low thrustline does make it necessary to adjust trim with significant throttle changes -- but once you adjust it, the machine flies on rails. In practice, I used essentially full forward trim at takeoff, and full aft trim during an engine-idle approach. Cruise naturally was somewhere in between, the precise position governed by one's selected cruise speed.

          A wide, blunt body pod with insufficient tapered streamlining back to the tail could result in the tail operating in the body's wake turbulence. But I don't think that was the question.

          I'd start by examining the incidence of the H-stab. If, in the aircraft's normal flight stance, the H-stab is down-loaded (leading edge lower than trailing, with respect to the slipstream), then adjust the H-stab's incidence to remove the down-load. That ought to be easier than moving the whole cabin, or the engine, up or down. If the H-stab is already at zero, though, and you still have excessive LTL behavior, you're likely stuck with moving the weights or the prop thrustline itself.

          The aircraft's CG moves a fraction of the distance that you move a weight -- in the ratio as the moved weight is to the all-up weight. So, in a 1500-pound aircraft, if you move 750 pounds two inches, the aircraft's CG will move one inch, in the same direction.


          • #6
            Jean Claude, your photo is of an RAF, but Eric has the Sparrowhawk variant with much lower thrust offset to cg.

            Doug, the "hunting" characteristic I mentioned was, as I understand it, more unique to the Sparrowhawk's tail vs. all cruciform tails.
            However, since Eric's issue seems related to throttle settings, you may be on the right track.


            PP - ASEL complex (Piper 180, C206, RV-7A), SP - Gyro (Calidus, RAF, SC2), soloed in gliders; checkride soon

            Wasn't happy with my RAF's pitch instability, so I installed a Boyer H-Stab to my great satisfaction!

            When an honest but mistaken man learns of his error, he either ceases to be mistaken -- or he ceases to be honest."


            • #7

              I am a gyro newbie, but have flown many fixed wing aircraft (no jets), from a Boeing B-17 to a Piper Cub in 50 years of flying. All of them exhibit the same phenomena - if you change power setting, you need to adjust the trim to maintain altitude. I believe this is a simple aerodynamic principle.

              You might also consider the fact that the prop thrust provides most of the airflow over the h-stab, and not the relative airflow related to airspeed. I am sure there is that some component is affected by the blunt, un-aerodynamic cabin shape, but what is that percentage, and how does it change with airspeed? Prop thrust will ALWAYS be the greater component.

              I also think my RAF Sparrowhawk has heavy stick forces compared to most FW aircraft, and the few gyros, that I have flown. Adjusting the h-stab angle of incidence appears problematic, whereas adjusting the prop thrust angle may be done by adjusting the rod ends connected to the engine PSRU plate, and by adding or removing washers on the rear engine mount. Have you performed the double hang test to determine the vertical CG location as a starting point? Of course this location changes with passenger operation. Let us know what you find.
              CFI-ASMEL-IA, A&P
              @ 6450' MSL in W. Colo.


              • #8
                This same issue has cropped up in the investigation of the recent Lion Air Flight 610 Boeing 737 MAX crash. Here's some text from an Aviation Week electronic newsletter about the investigation:

                Like all turbofan-powered airliners in which the thrust lines of the engines pass below the center of gravity (CG), any change in thrust on the 737 will result in a change in flightpath angle caused by the vertical component of thrust. This is a moment resulting from the horizontal thrust component caused by a CG offset and a trim stability change.

                Although the Leap 1B is designed to have thrust levels similar to the 737NGs CFM56-7B, the newer engine is heavier and has a larger fan. Because of its greater size, Boeing had to maintain adequate ground clearance by cantilevering the engine farther forward on a heavier strut, adding to the offset. The Leap 1Bs 18-blade composite fan is 69.4-in. in diameter, compared to 61 in. for the CFM56-7s 24-blade titanium fan.

                Each Leap 1B weighs 6,129 lb., 849 lb. more than a CFM56-7B. For the 737-8, the added structure of heavier struts and nacelles, beefed-up main landing gear and supporting structure add 6,500 lb. to the green aircraft weight, but operating weights are boosted by 7,000 lb. to preserve full-fuel and payload capability.

                MAX pilots are therefore trained to know that although the aircraft has natural speed stability through much of its flight envelope, there is also inevitable thrust-versus-pitch coupling at low speeds. The 737-8 has a speed-stability augmentation function that helps compensate for the coupling by automatically trimming the horizontal stabilizer according to indicated speed, thrust-lever position and CG. Boeing advises that pilots still must be aware of the effect of thrust changes on pitching moment and make purposeful control-wheel and pitch-trim inputs to counter it.


                • #9
                  I'll risk being tagged as a "nanny" liberal. I think that aircraft that are marketed to beginner aviators, as gyros generally are, should be as user-friendly and low-workload as possible. The 737 is flown by professionals. In that context, some pitch reaction traceable to LTL isn't unreasonable. In a recreational craft, OTOH, the less of this sort of thing, the better.

                  Gyro designers, of course, have been violating this notion since Bensen himself.

                  Dunc, it's true that the SPEED of the airflow over the H-stab is augmented by the propwash (and a good thing it is), but the DIRECTION of the flow is heavily affected by the direction of the flow INTO the prop disk. In effect, it's garbage in, garbage out -- if the flow into the prop disk is turbulent (because of a blunt pod upstream), the flow OUT will be turbulent, too. This effect accounts for the "growling" noise that pusher props tend to make.

                  It's fortunate for us that the flow into the prop disk does affect the direction of the propwash, since that way the H-stab still "sees" changes in the aircraft's angle of attack, and produces a restoring moment.