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Blades stalling during autos


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Someone asked me a question the other day but I don't know enough to give them a good explanation.

Can someone here help them (and me) to understand this?

 

In reference to out-of-ground effect hovering autos;

 

If there is no forward airspeed and you enter an auto the airflow changes from above the blades downwards to below the blades upwards. This would make the airflow upwards and hit the blades at almost a 90degree angle, so how come the blades do not stall?

???  ???  ???

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Well, for starters, if the engine quits in the hover, you will be on the ground before the aircraft gets into autorotation, using just the stored energy in the rotor system. OGE, depending how far OGE, could be rather hairy.

 

If you are OGE at 500' agl or better, drop the nose and get some airspeed. But also note that the top of the Danger Avoid Curve is often higher than 500'.

 

The whole blade doesn't stall, because the pilot drops the collective to the floor. In auto, some of the blade is stalled, the rest is producing lift, but some of the lift is pointing backwards to also provide drag. Done properly, there is enough pointing forwards to keep your revs up.

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Firstly I am referring to autos above 500ft.

I know how to do them, it sjust the aerodynamics of them I was hoping to clear up.

 

My question is, if you push the collective down (enter auto) you get the air flowing upwards at the 90 degrees to the blades. Therefore hitting the flat surface of the blade and not flowing around, which is required to produce lift.

 

How come this doesn't just bend the blades straight up as they do when stalled.

 

With the air hitting the blades at such an acute angle how do they produce any lift at all.

 

I know someone out there can tell me.

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Think about what you have said.

 

When airflow comes from above, as it does in powered flight, your way of thinking would mean that the air flowed from above and hit the flat bit on top of the blade and bent it downwards??

 

The point you are missing is the rotation of the blades, and the vector sum of rotational airflow and induced airflow.

 

Think of it as a triangle with the horizontal bit at the top, not the bottom. The horizontal bit at the top is the rotational airflow, and its length varies with the part of the blade we are looking at. The vertical bit, running up to meet the flat bit at a right angle, is the Rate Of Descent airflow, and is constant over the length of the blade. The third side is the vector sum of the two, the Relative Air Flow (RAF) and its pointy angle varies with the length of the flat bit. This angle is part of the angle of attack, when added to the pitch angle of the blade.

 

At the hub, rotational airflow is almost zero, and the ROD airflow (from the bottom in an auto) acts from almost directly below. The angle of attack is huge. Yes, it is stalled. Lots of drag.

 

Move out a bit and the rotational airflow becomes more effective. Add it to the ROD and the angle of attack is reduced and the blade is coming out of the stall. Move a bit further out, and you reach the angle where the blade is producing lift. This lift (at 90 degrees to the relative airflow) is pointing forward of the blade, and is dragging the blade forward. But we need to look at the Total Reaction (TR) which splits into lift (at 90 degrees to RAF) and drag (in same direction as RAF). The TR is angled a bit back from the RAF.

 

A bit further out, and we are reaching the optimum lift/drag ratio. with the most lift pulling forward, and the least drag.

 

Further out, the rotational airflow is getting very big, and when added to the fixed size of the ROD airflow, the angle of attack is getting smaller. The TR is tilted further back, so the sad bit is the lift is also acting as drag, helping to slow the blade.

 

The overall forces acting on the blade are the inboard stalled section, causing drag; the middle lifting section, producing a driving force, and outboard dragging section, which still produces lift. With flat pitch, the forces will balance out to give a certain RRPM. Pull some pitch in, and the stalled section moves further outboard, the driving section moves further out, and drag starts to win - the revs drop.

 

Buy a book to learn some more, I have to go flying now... ::rotorhead::

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Hi Soon2B,

 

The simple answer is that as long as you keep RRPM in the green (and airspeed below VNe), you can't stall a rotor blade under any flight conditions (except settling with power, which is a mixed-up mess of aerodynamics). To repeat - as long as you keep RRPM in the green.

 

In an OGE hover, you have the collective raised quite high, meaning you have the blades pitched high also. When you enter autorotation, you lower the collective, which brings the blade pitch to zero. This more than makes up for the change in inflow angle, so the AOA will change only slightly.

 

Remember (as Eric H said) you have the immense "forward" velocity of the rotor blade (call it 350 - 400 kts). An helicopter autorotating at zero airspeed is descending at 2,000'/min, which is only around 20 kts. So the rotor is seeing the air arrive basically from the front, even in a vertical descent.

 

Think of an airplane going 350 knots and descending at 2000'/min - it's not stalled, is it - and a helicopter rotor sees exactly the same thing as the wing of that airplane.

 

Hope this helps!

::armsup::

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Soon2BStudent said:

"Firstly I am referring to autos above 500ft.

I know how to do them, it sjust the aerodynamics of them I was hoping to clear up."

 

When you say you know how to do them, does that mean you have done autos with an instructor? If so, their instructional technique is sadly lacking if you were not taught the theory of it all before doing it.

 

Always ensure you get:

1. A full "mass briefing" or lesson on the theory

2. A day or two later, after you have had time to revise and digest it, the flight can take place. Pre-flight briefing should revise the lesson and cover the sortie to be flown;

3. A post-flight debrief, and info on what is coming next.

 

Any school that doesn't give you all this is letting you down.

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Not really the instructors fault, he has been over the autos and I thought I understood OK.

When I look back over everything to try to keep it fresh I start to understand things more, but I also read things that  confuse me. This was one of those that confused me.

 

Thanks for your help, I just needed someone to give a brief explanation to point out what it was that I was missing.

You were right it was the rotational airflow.

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How come this doesn't just bend the blades straight up as they do when stalled

 

Being stalled is not what causes the blades to bend straight up. In a stalled condition, the blades stop being effective producers of lift and the rotor rpm starts to drop (how fast this occurs depends on the pitch angle of the blades) at such a low rpm the blades no longer are able to support the weight of the helicopter. This strain plus the force of the upcoming air will bend the blades past their critical cone angle and at some point along the blade it will crease and fold up.

 

Imagine trying to pick up an R22 or 300C by the blades only, the same result will happen.

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