RRPM below 80% and still flying

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Re: RRPM below 80% and still flying

Unread post by Scotsman » Thu Jan 29, 2015 9:23 pm

Will do...just enjoying the flying and looking forward to a week off doing what I enjoy.
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Re: RRPM below 80% and still flying

Unread post by cage » Thu Jan 29, 2015 9:32 pm

Jettys are so reliable we don't have to worry about stuff like this.
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Re: RRPM below 80% and still flying

Unread post by cage » Thu Jan 29, 2015 10:17 pm

If rotor rpm drops rotor velocity drops. Remember coefficient of lift? Velocity squared? There would be an exponential drop in lift. Density and angle of attack will impact the amount of lift but not nearly as much as velocity.
At low rpm blades will also cone reducing surface area and lift further.
I doubt anything could "fly" at 50% rpm and i wouldn't want to test sub 80%.
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Re: RRPM below 80% and still flying

Unread post by Scotsman » Fri Jan 30, 2015 6:21 am

ah ha the cage arrives.....enter cage stage left with obligatory robbie comment! :lol: :lol: :lol:
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Re: RRPM below 80% and still flying

Unread post by cage » Fri Jan 30, 2015 6:57 am

Couldn't resist. I think @TF was in bed already with some hot chocolate. Robbie drivers :)
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Re: RRPM below 80% and still flying

Unread post by Scotsman » Fri Jan 30, 2015 7:00 am

cage wrote:If rotor rpm drops rotor velocity drops. Remember coefficient of lift? Velocity squared? There would be an exponential drop in lift. Density and angle of attack will impact the amount of lift but not nearly as much as velocity.
At low rpm blades will also cone reducing surface area and lift further.
I doubt anything could "fly" at 50% rpm and i wouldn't want to test sub 80%.
What's your view on the difference in the RRPM at which blade stall would occur in a powered and a non power (autorotative) condition?
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Re: RRPM below 80% and still flying

Unread post by cage » Fri Jan 30, 2015 8:58 am

The concept of "stalling" in a chopper vs a fixed wing are somewhat different.
Generally stalling is velocity related, which in a chopper should be constant (under power). As rpm decays so does velocity so does lift (squared).
Unlike a fixed wing, different parts of the blade are moving at different speeds producing differing lift across the disc (which is why blades are twisted to vary AoA and to even out lift).
Stall speeds are aerodynamic properties and irrelevant of whether or not you are under power.
Generally stalling in a chopper occurs at very high speed where the retreating blade will stall. Even in this conditions there are parts of the blade producing lift, just not enough of it (will leave it to @TF to do the lecture on blade regions, driving etc etc).
An aerofoil can also stall when you pass the max AoA - I don't believe any chopper allows you to pull sufficient collective to do this. At high settings the drag becomes an issue and you usually run out of power anyway where lift is still being produced by the aerofoil until rpm decays.

Attitude will have an impact as during a decent the relative wind will change, affecting the AoA which will give you more lift so you you could get better performance meaning you could operate at a lower rpm, this would be minor though as velocity has the biggest effect on lift and you'd still be going down.
Power isn't a consideration in any lift formula.
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Re: RRPM below 80% and still flying

Unread post by cage » Fri Jan 30, 2015 8:59 am

(I can sense @TF enthusiastically reading the principles of helicopter flight) :wink:
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Fri Jan 30, 2015 9:16 am

cage wrote:The concept of "stalling" in a chopper vs a fixed wing are somewhat different.
Generally stalling is velocity related, which in a chopper should be constant (under power). As rpm decays so does velocity so does lift (squared).
Unlike a fixed wing, different parts of the blade are moving at different speeds producing differing lift across the disc (which is why blades are twisted to vary AoA and to even out lift).
Stall speeds are aerodynamic properties and irrelevant of whether or not you are under power.
Generally stalling in a chopper occurs at very high speed where the retreating blade will stall. Even in this conditions there are parts of the blade producing lift, just not enough of it (will leave it to @TF to do the lecture on blade regions, driving etc etc).
An aerofoil can also stall when you pass the max AoA - I don't believe any chopper allows you to pull sufficient collective to do this. At high settings the drag becomes an issue and you usually run out of power anyway where lift is still being produced by the aerofoil until rpm decays.

Attitude will have an impact as during a decent the relative wind will change, affecting the AoA which will give you more lift so you you could get better performance meaning you could operate at a lower rpm, this would be minor though as velocity has the biggest effect on lift and you'd still be going down.
Power isn't a consideration in any lift formula.
Hey Scotsman, you ask a simple question (Can you fly at a lower rpm under power than what you can in autorotation) and look what you get??

Regarding Attitude having an impact. Mr cage will have to admit that we have all been taught to fly 'attitude' for a specific speed. This attitude is the same (for a specific speed) in horizontal flight or descending. The relative wind changes but so does the pitch so to all intents and purposes the AoA stays much the same for any chosen speed and a constant rate of descent compared with horizontal flight.

Now Mr cage. Please answer the Scotsman's question!
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Re: RRPM below 80% and still flying

Unread post by cage » Fri Jan 30, 2015 9:26 am

Thought I had!
There should be no difference between powered and unpowered flight as far as determining when a blade will cease to produce lift.
The RPM determines the point at which lift will cease irrespective of the engine running or being in autorotation.
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Re: RRPM below 80% and still flying

Unread post by cage » Fri Jan 30, 2015 9:37 am

tanglefoot wrote: The relative wind changes but so does the pitch
Isn't pitch different in an auto vs controlled descent?
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Fri Jan 30, 2015 9:40 am

cage wrote:Thought I had!
There should be no difference between powered and unpowered flight as far as determining when a blade will cease to produce lift.
The RPM determines the point at which lift will cease irrespective of the engine running or being in autorotation.
But you can agree that with an engine the blades can turn at a lower rpm than autorotation and still provide sufficient lift to support the weight of the helicopter?

The catastrophic failure in autorotation is as a result of RPM decay and not lack of lift at that point in time.
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Fri Jan 30, 2015 9:40 am

cage wrote:
tanglefoot wrote: The relative wind changes but so does the pitch
Isn't pitch different in an auto vs controlled descent?
Yes, so is the relative wind. Either way the blades have to provide enough lift to support the weight of the helicopter otherwise you would have vertical acceleration.
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Re: RRPM below 80% and still flying

Unread post by cage » Fri Jan 30, 2015 10:16 am

tanglefoot wrote:
cage wrote:Thought I had!
There should be no difference between powered and unpowered flight as far as determining when a blade will cease to produce lift.
The RPM determines the point at which lift will cease irrespective of the engine running or being in autorotation.
But you can agree that with an engine the blades can turn at a lower rpm than autorotation and still provide sufficient lift to support the weight of the helicopter?

The catastrophic failure in autorotation is as a result of RPM decay and not lack of lift at that point in time.
the aerodynamic principles and limitations are the same. However.
You'd also need to know the point at which the aircraft weight etc would exceed the lift produced buy the rotors.
The difference is how the rpm is produced - in an auto there would be a significant ROD which would negate any lift produced so the "safe" rpm threshold would be higher.
Under power you would only have to deal with the differential of lift produced at full to low rpm until the point where the rpm cannot support the aircraft weight at which point it would start to descend.
In my mind the same amount of lift is being produced in both conditions the effectiveness of the lift is however quite different between the two scenarios.
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Fri Jan 30, 2015 11:18 am

cage wrote:
tanglefoot wrote:
cage wrote:Thought I had!
There should be no difference between powered and unpowered flight as far as determining when a blade will cease to produce lift.
The RPM determines the point at which lift will cease irrespective of the engine running or being in autorotation.
But you can agree that with an engine the blades can turn at a lower rpm than autorotation and still provide sufficient lift to support the weight of the helicopter?

The catastrophic failure in autorotation is as a result of RPM decay and not lack of lift at that point in time.
the aerodynamic principles and limitations are the same. However.
You'd also need to know the point at which the aircraft weight etc would exceed the lift produced buy the rotors.
The difference is how the rpm is produced - in an auto there would be a significant ROD which would negate any lift produced so the "safe" rpm threshold would be higher.
Under power you would only have to deal with the differential of lift produced at full to low rpm until the point where the rpm cannot support the aircraft weight at which point it would start to descend.
In my mind the same amount of lift is being produced in both conditions the effectiveness of the lift is however quite different between the two scenarios.
So now we have lift and 'effective' lift :roll:

I've highlighted the important point in your post.

I have experience that 80% on the highveld in an R44 under power is possible and we know that catastrophic RRPM on the highveld under autorotation occurs around 87% so it is true to say that it is possible to fly at a lower RRPM under power than it is under autorotation??

I disagree vehemently about your argument regarding 'how the lift is produced. The rate of descent also has nothing to do with it.

Nevertheless, we are probably just confusing Scotsman and not getting anywhere. It's ok, you can go back to the main board now :twisted:
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