RRPM increase in the flare - HELP!

[RVV85]

Hey guys.

I have a fixed wing pilot trying to understand why the RRPM increases in the flare. I have tried explaining to him why and how but the guys just say that it does not make sense because if you increase the AoA the drag increases and that should decrease the RRPM.

How can I explain it to a fixed wing mental person???

Anything plz?

[oldbeefer]

Two reasons -1. If you apply G the blades cone up. This will cause the RPM to rise (the ice skater pulling her hands in). 2. In a flare, there will be an airlow opposing the Induced Flow, therefore reducing the induced flow. This will bring the relative airflow closer to the Plane of rotation which will move the Total Reaction closer to the Axis of Rotation which will, in turn, reduce the Rotor Drag therefore RPM will increase (all assuming no governing system, of course). Sit down with him with the vector diagram to make the explanation easier.

[Frenchrotorhead]

This is no as clear as it should be but might help :
The Effect of Disc Loading (Flare and Turn)

Disc Loading is an apparent increase in the weight of the aircraft supported by the rotor disc . This can be felt in the 'seat of the pants' as you are pushed harder into your seat. The Rotor RPM will be seen to increase with an increase in disc loading, due to an increase in coning angle producing what is known as 'The Coriolis Effect'. In a Flare the airflow from below the disc reduces the induced flow, tilting the total reaction (http://www.griffin-helicopters.co.uk/note/vectordiagramtest.asp) forwards and increasing RPM.

For more about helicopter aerodynamics : Effects Of Controls (http://www.griffin-helicopters.co.uk/lessons/effectsofcontrols.htm)

Hope this helps

[TiPwEiGhT]

Get him to sit on an office chair with his arms out, spin him around and then tell him to move his arms up above his head... can't really explain it any easier.

TiP:ok:

[Arm out the window]

Tell him nothing!
Those fixed wing plonkers have got no business poking their noses into secret rotary issues...

[Lord Mount]

Tell him its magic and you are a wizzard!
Tell him you speed the rotors at your will just to confuse the cr@p out of people.

[paco]

Collective is lowered as you try not to climb

Coriolis effect - increased TRT causes coning angles to increase and the C of G of all blades to move inwards

Increased load factor

The lift vector being more forward (because the RAF is shifted downwards) produces an autorotative force and reduces drag

Phil

[Torquetalk]

In addition to the explanations above you could also put to your student that he or she might be confusing angle of incidence with AoA: Assuming a fixed collective position, the gain in AoA during the flare leads a change in the position of the RAF: there is no increase in drag. The trade-off comes in the form of loss of airspeed.

You might also ask why planes flare to land; why does it work?

[helimutt]

Ok then, if the C of G moves inwards as the blades cone upwards, can someone tell me how far in the CofG moves in and how much reduction there is in disc diameter, if you liken this to an ice skater pulling their arms inwards during a spin. It's just i've never believed the amount of movement of CofG would be enough to cause such a massive rise in RRPM. Anyone got any other believable theories?

[chopjock]

Anyone got any other believable theories?

Yes, after the flare the rotor disc now behaves like a massive parachute, with a sudden increase of updraft through the blades.

[outofwhack]

I'd say one thing is for sure...

RVV85 is being shown up by a fixed wing pilot for not really understanding the effect. RVV85 if you don't really understand it yourself how can you expect yourself to be able to teach it.

Unfortunately, denigrating an 'aviator of another discipline' is all too common a trait.

Just my 2d worth. I'll get my coat...:rolleyes:

OOW

[[email protected]]

Increasing the AoA does indeed increase lift and drag - the FW puke is right about that but, as oldbeefer said, the reduction in induced flow tilts the total reaction forward (or less back) leading to an overall reduction in rotor drag. You haven't reduced Tq or moved the lever so you have the same power driving the rotor with less drag to overcome - RRPM increases!!

[Tallsar]

...and no use of colour either ..well done Crab!:)

[delta3]

We can also leave out Coriolis and Einstein...

Compare the flow in level flight of the heli with the level flight of a plane (forget the rotating part) : at sufficient high blade speed (outer part of the rotor) the wind blows in horizontally to a foil with a certain "positive" AoA, just as it does with an airplane wing : this creates lift and drag just as it does with the plane (the only difference is that drag becomes torque)

Now the flair or autorotative state : put the heli blade flat or even slightly negative and let the wind come well from underneath. With respect to the wind vector lift now becomes a "pull" the effect of which can become larger then the drag and this can ultimately drive the rotor. This again is the same for a plane: putting the plane in a descend will create the same difference in geometry (wind blowing from below) that drives that plane as a glider.

(sorry no time to make the drawings that go with the text above, but hope it helps)

d3