droopystop asked (good question!):
How then does the S61N needs more power to hover IGE at 5kt (factored) headwind than in still air? Or have I misinterpreted the graphs?
Nick sez:
The power required to hover is determined in flight test and we accept all data below 5 knots, so we really don't plot or measure the data below that. Thus the IGE power required charts are flat (I don't believe they show more power needed at 5 knots, BTW) up until 5 knots. The effect of the ground vortex roll-up strikes between 5 to 8 knots (depends on disk loading) and increases power a bit, which is why the helo dips a bit while accelerating during an iGE takeoff.
Here is a way to prove that the aircraft uses no more power in rearward flight at say 10 knots:
In still air, accelerate forward very gently and increase collective to keep power constant (the torque or MP will drop if you don't).
Now do the same thing but gently accelerate rearward, and note the same behavior. If you do not reduce the collective, you will take off rearward, because the power is less than the steady hover.
Many folks posted about the "barn door" and "high drag" of the fuselage as the reason why the power goes up in rearward flight. This really is not a big factor, it is virtually zero drag at 5 knots or even 10 knots, especially rearward. The drag of a square foot at 10 mph is about 3 lbs (hold a card out your car window to verify!) If the Robbie has a total flat plate area of 20 square feet (like a big triangle of 4 foot by 10 foot) it has a sideward speed drag at 10 mph of about 60 lbs. This is achieved by tilting the thrust about 2 degrees. The lift lost in this 2 degrees of lateral tilt is .06% of the thrust (cosine of 2 degrees is .9994). Peanuts.
For those who comment on the control issue with some helos, that is correct for some helos at some speeds, but all helos are approved for 17 knots of flight in any direction at any CG before they run within 10% of the control stops. Those who say they run out of cyclic at 10 knots rearward are not corrrect, but it feels that way because the stick is pretty far back in their stomachs.