Hey, I can post again! Apparently I'm limited to 5 posts per day, so I'll try to group responses until I'm out of probation.
Originally Posted by
Ascend Charlie
Ahh, now you are getting into Translational Lift, which as you correctly say, is moving into air that has had little chance to start moving downwards through the disc. And push it down you must, in order to hold the aircraft up. A lot of theory books concentrate on the Bernouilli equation without mentioning that the airflow at the back of the airfoil is headed downwards - the flow over the top doesn't "have to meet the flow underneath", and in fact it gets there well before the lower flow, hence the movement downwards, the downwash.
The term Gyroscopic Precession has used up many threads on this site. It is only a means of understanding the dynamics of the rotor system, but the rotor is NOT a gyroscope. Phase Lag is the correct term, and it is fixed by adding an Advance Angle to the inputs from the swash plate to the disc. The lag is approximately 90 degrees, but gets as low as 72 degrees on the Robinson. But if precession is the way to grasp this concept, go for it, but don't tell anybody that you believe in it. A bit like being a Flat Earther.
My understanding was that it was gyroscopic precession when an outside force is acting on the disc, and phase lag when we are changing the disc itself through control inputs. You're saying it's always phase lag?
Originally Posted by
Ascend Charlie
Regarding Dissymmetry of Lift, it only happens until the pilot pokes the cyclic forward to stop the flapback. After that the lift on both sides is exactly the same, in steady flight. The retreating blade is suffering, with the relative airflow reduced by the forward velocity, so the advancing blade has to "throw away" all that beautiful lift it gains from forward movement, to match the poor cousin on the other side.
If I'm understanding this correctly, flapback is moment of dissymmetry of lift, and the rotor system needs a nudge from us in order to move back into symmetry. Once in forward flight, it's actually a control input that creates a moment of lift dissymmetry, after which the rotor system will re-balance?
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Yes, as AC says above - phase lag
You can perform a very good demonstration of inflow roll and flapback - start in about a 10' hover into wind if there is any (best done with zero or very light wind) and with the AP disengaged if you have one.
Maintaining the collective position, initiate the forward movement with a small amount of cyclic and hold it. The aircraft will start to move forward and descend slightly - then in reasonably quick succession the nose will pitch up and the aircraft will roll towards the advancing side of the disc.
Once your students understand what the aircraft wants to do, they will better counteract it by maintaining the disc attitude during the transition.
I do this with students currently, and while I could explain the roll toward the advancing side, I couldn't explain the nose pitching up. You guys are definitely helping me understand that part of it.
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Originally Posted by
paco
If it helps...
The combination of inflow roll and flapback makes the disk move up or down at a point just left of the nose, but they have their greatest individual effects at different speeds. Those of inflow roll are greatest at low speeds, on leaving the hover, and decrease significantly with speed. At higher speeds, dissymmetry of lift will be the most dominant force.
As the effects of flapback are greatest at high speeds, the Tip Path Plane rises near the retreating blade when first moving from the hover, then moves round towards the front as speed increases.
As for having to push forward, one simple explanation is that, as you push the cyclic forward, the body lags behind, but, when it does catch up, it centralises the cyclic, assuming you haven't moved it. So you have to push forward to keep things going.
Phil
So the large forward cyclic input during takeoff is, at least in part, due to the weight shift of the fuselage as it swings under the moving rotor disc, plus the changing angle between the fuselage and rotor disc altering our cyclic inputs?