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  • Question for the pro's

    All GA aircraft that Im familliar with have a prop wash that cause the AC to yaw to the left. Some racers at Reno have contra-rotating props, would this then cancel the yaw effect? Could a plane like White Lightining run the left motor clockwise and right counter clockwise to cancel prop wash also? Just Wondering!

  • #2
    Re: Question for the pro's

    I'm not a pro with aircraft, but I'll have a go. Short answer yes, counter-rotating props would negate P-factor, as would a twin with counter-rotating props.

    Long answer, mostly yes. There are more things on an aircraft than just the propeller rotating. There is the crankshaft, fly-wheels, gears (if it is a geared engine), and so on to start with. All of these components rotating add to the total rotational inertia of the engine/propeller, which adds to P-factor. A good illustration of what I mean by rotational inertia is a gyroscope. You push on it and it reacts 90 deg. from where the force is applied. So counter-rotating propellers would negate the portion of P factor due to asymmetrical thrust, and most of the P-factor caused by rotational inertia, (opposite directions subtract form total inertia) but not all of it unless done right.


    Phew! Hope this answers your question.
    Fast Flying Female, both on the ground and in the air!

    Comment


    • #3
      Re: Question for the pro's

      The P-82 twin Mustang had conter rotating props. In fact one of the major hang ups in the restoration of that AC is finding a reverse rotation prop and governor set up. The CAF's P-82 crashed when the reverse prop had a seal failure. I'm sure Hamilton Standard could build a prop for the right price, but it would be a huge price I'm sure. The Merlin on that side has to run backward as well if I remember correctly, but it might have a reverse rotation gear box on that side????

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      • #4
        Re: Question for the pro's

        if i'm not mistaken the caf's f-82 is a later model, if it is, then it would have an allison engine...
        heh heh alriiiight

        Comment


        • #5
          Re: Question for the pro's

          Hi !

          Well as far as I have now experienced with my R/C model a fast revolving prop causes also a roll effect to the left ( if prop rotates right ).

          As long as the plane is on the main gear the effect can be only controlled with a rudder...if tried to pull up at this lower than normal airspeed the plane would turn to the left on its back. So the more speed the craft has gained is an assurance that nothing funny will happen...as far as I remember from a guy who tested Mustang at low speed with a sudden increase of power the plane will do something very nasty.

          Steady hand with the throttle and a low pitch in the prop will get the plane airborne. I remember reading several casualties with FAF flying Bf-109s when the plane weered of to the left in the start...a stall at 5 metres sometimes just wrecked the plane but from higher the planes usually hit a forest with deadly effect.

          In the WARBIRDS WORLDWIDE there was once a Hark Hanna interview how to fly an Me-109..I recall the rudder was full left and throttle use was more regulated until to certain speed had been gained..I am not 100 % sure..possibly he also had stick back too for a while to make sure the plane is controllable during the taxiing.

          Hope this helps...and correct me if I am wrong.

          Juke
          http://max3fan.blogspot.com/

          Comment


          • #6
            Re: Question for the pro's

            OOPPS ( just read the story again ) !

            The rudder to the right of course and at low rpm the stick is at the control panel to be able to turn the tailheavy 109 at taxiing.

            Rudder does rapid adjustments all the start time.

            The rudder full to the right ( and use the throttle ) was the Walter Eichhorn technic !!!

            I had about almost everything wrong there.

            JT
            http://max3fan.blogspot.com/

            Comment


            • #7
              Re: Question for the pro's

              Shane,

              Most American airplanes have an engine that turns the prop counterclockwise when viewed from the cockpit. This creates a yaw and roll to the left.

              A counter-rotating set -up on a twin or a contra-rotating set-up on a single engine airplane will negate the roll portion, which we call torque. It is the force that pushes the left wing down on first application of throttle. After the airplane accelerates to a certain airspeed the aileron control authority will correct the roll.

              Assymetric thrust from the descending blade at positive angles of attack will cause a yaw to the left. This we counter with rudder, the faster you go, the less rudder you need.

              Slipstream effect is caused by the corkscrewing path of the propellers thrust around the airplane, over the left wing under and behind the right wing, around the bottom of the fuselage and then up to strike the left side of the fin. This is countered with left rudder as well.

              As you can see, we have a lot of de-stabilizing factors working on the airframe as the prop does it's work.

              Also positive "G" load will cause the nose to swing right because of gyroscopic precession of the prop, when unloading it swings left. The Dreadnought got it's high topped fin because of this.

              A few notes:

              Prop wash is an old term for wake turbulence, it is felt by the airplane behind the one making a wake.

              The F-82 has Aeroproducts propellers, not Ham. Standards.

              The P-38 swings it's props from inboard to outboard, the F-82 from outboard to inboard.

              The rotational forces of the engine on the contra-rotating prop Precious Metal are minimal compared to the forces of the propeller. The moment of inertia for internal engine forces would be very low compared to the huge moment for the prop blades. I wonder if any can be detected at the stick and rudder.

              A great article can be found in the March 1969 (I think) Air Progress about the effects of propeller forces on prop fighters. It was written by Mike Dillon, and entitled, "Fly your own Fighter Plane; or Death in a Beautiful Package" or something to that effect. A good read.

              See ya,
              Chris...

              .

              Comment


              • #8
                Re: Question for the pro's

                On Griffon engined airplanes their counter rotating propellers eliminate torque. But if you look the Red Barron, Precious Metal, and Miss Ashley II with the torque eliminated why do they have such tall vertical tails? I heard they have a tendency to fly sideways. Is this true?
                Jarrod

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                • #9
                  Re: Question for the pro's

                  The CAF F-82 is an earlier model airplane, It as Chris points uses the Aeroproducts propeller and uses a special dash number Merlin which through an idler gear rotates the engine in the left hand direction. If they are having trouble finding parts they are either not calling the right people or they have alienated them. In the last five years I have personally seen two engines and one propeller that fit those descriptions.
                  John Slack

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                  • #10
                    Re: Question for the pro's

                    It is true the rotation of the engine parts would contribute less to adverse yaw (or P-factor) than the propeller. However, many race engines rotate at very high RPM compared to the propeller. Even though the engine parts may have less mass and smaller diameter then the propeller, I argue the higher RPM of the engine parts would make their contribution to adverse yaw worth considering, especially because the RPM is squared in the equations.

                    But I'm not a race pilot or mechanic, just a lowly engineer, so this is all theory. Still interesting though.

                    I don't mean to hi-jack the thread, but wasn't there a radial engine built during WWI where the ENGINE rotated? I seem to remember it had trouble turning because of the huge gyroscopic effect of all that rotating mass. Anyone know more about this?
                    Fast Flying Female, both on the ground and in the air!

                    Comment


                    • #11
                      Re: Question for the pro's

                      It's not just torque. Props tend to have a drag effect, even in rotation.....

                      The Northrop XB-35 flying wings were said to be a bit more stable than the jet-powered versions (YB-49), as the drag of the props on the trailing edge of the wing acted somewhat like tailfins (I'm sure gyroscopic effect had something to do with it as well). That's why the YB-49s had several small tailfins added to the back of the wing after the props were eliminated, as an attempt to replace the lost stability...

                      Also, the first Me-262 jet pilots use to complain about how hard it was to slow the planes down to lose altitude for landing, since they had no propeller to act as a speed brake when rpms were reduced, as the conventional piston-engine fighters had...


                      -->SO, with the extra blades of a mix-master up front, a larger tail would seem necessary to counter drag on the nose (especially in dead-stick flight!)

                      There must be greater gyro effect from more mass spinning up front too, so I would think it would take more rudder to swing the nose about...Just speculating there...



                      (The first post of this thread made so long ago asked "Could a plane like White Lightining run the left motor clockwise and right counter clockwise to cancel prop wash also?")

                      ....The P-38 already has counter-rotating props (Chris McMillin mentioned the rotation direction). Lefty use to fly single-engine loops and rolls in front of the crowd between races to show how stable the P-38 was (if flown properly of course)...



                      I wonder if the counter-rotating prop layout creates greater loads on the tail surfaces...The airflow patterns behind one must be pretty turbulent I imagine....

                      Comment


                      • #12
                        Re: Question for the pro's

                        Originally posted by jarrodeu
                        On Griffon engined airplanes their counter rotating propellers eliminate torque. But if you look the Red Barron, Precious Metal, and Miss Ashley II with the torque eliminated why do they have such tall vertical tails? I heard they have a tendency to fly sideways. Is this true?
                        Jarrod

                        The taller tails are a result of the Mustang's inherent stability as it's speed increases...regardless of the p-factor cancellation. It's not just about rudder control. That's the reason that the H-model had a taller tail, the same reason that the original Precious Metal had an H-model tail, and the same reason that all three Griffon racers have one too. Same goes for the Cavalier conversion of the Mustang.

                        Do you sacrifice the drag that comes with the added stability? Most people think it's not worth it...but I do know that the pilots who flew the original Precious Metal felt that it was a much easier plane to fly around the pylons than a stock-tailed Mustang.

                        Comment


                        • #13
                          Re: Question for the pro's

                          I don't mean to hi-jack the thread, but wasn't there a radial engine built during WWI where the ENGINE rotated? I seem to remember it had trouble turning because of the huge gyroscopic effect of all that rotating mass. Anyone know more about this?
                          I think nearly all of the radial engines in WW1 warbirds spun on their crankshafts to improve cooling of the cylinders....

                          -------------------------------------------


                          You can see a few at the USAF museum piston engine section of their website. There's a lot of unusual and unique engines there:


                          samples found there
                          Gnome 9-N Rotary Engine
                          Oberursel UR-2 Rotary Engine


                          -------------------------------------------

                          Finally, a pretty good description of early rotary engines (not wankels) can be read at:

                          Comment


                          • #14
                            Re: Question for the pro's

                            Originally posted by Chris McMillin
                            Most American airplanes have an engine that turns the prop counterclockwise when viewed from the cockpit.
                            Um, Chris: you meant 'clockwise when viewed from the cockpit', yes? Otherwise your post gives a good summary.


                            Originally posted by Chris McMillin
                            A counter-rotating set -up on a twin or a contra-rotating set-up on a single engine airplane will negate the roll portion, which we call torque. It is the force that pushes the left wing down on first application of throttle. After the airplane accelerates to a certain airspeed the aileron control authority will correct the roll.
                            Just to be clear: if the engine's generating power, it's applying torque to the propeller shaft full-time, not only when you first apply the throttle; that's just when you first notice it, as the torque suddenly increases from some negligible value. I think that's what you wrote, too, but it's not quite clear. At any steady state flight condition all the torque the engine supplies to the prop shaft is being absorbed by the propeller, which pushes back with equal force; otherwise, the prop speed would continue to increase. You're right, of course, in stating that aileron control authority increases with airspeed, and will counteract the tendency of the aircraft to roll to the left; you could also rig the airframe with various asymmetries to relieve the ailerons from having to make the effort, at least at one particular power setting and airspeed.

                            Every kid with flying in his blood has hands-on experience with the effects of engine torque: remember when you would wind up one of those rubber band-powered balsa models, hang onto the prop and let go of the fuselage?
                            SteveZ

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                            • #15
                              Re: Question for the pro's

                              Originally posted by Unregistered
                              The CAF F-82 is an earlier model airplane, It as Chris points uses the Aeroproducts propeller and uses a special dash number Merlin which through an idler gear rotates the engine in the left hand direction. If they are having trouble finding parts they are either not calling the right people or they have alienated them. In the last five years I have personally seen two engines and one propeller that fit those descriptions.
                              John Slack

                              Knowing the CAF they burned all bridges that lead to the parts they needed. If I remember correctly, their Merlin was servicable, but they had no prop. I guess it doesn't matter now that they traded away the plane for a P-38.

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