RRPM below 80% and still flying

Two-seat R22 and four-seat R44 helicopters.

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RRPM below 80% and still flying

Unread post by Scotsman » Thu Jan 01, 2015 7:04 pm

Hi Guys,

I am only getting towards half way into the heli PPL and all the instruction and reading I have done suggests that you would have catastrophic blade stall at around 80% RRPM at sea level. Take a look at this clip which seems to a R44 at below 80% (by assumption at sea level) you have to watch from 3:25 onwards...how is the aircraft still flying (looks like 50% at 3:30)? (ignore the complete failure to take the required low RPM corrective course of action)

https://www.youtube.com/watch?v=fe2CN0SIwTk
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Thu Jan 29, 2015 3:16 pm

Scotsman wrote:Hi Guys,

I am only getting towards half way into the heli PPL and all the instruction and reading I have done suggests that you would have catastrophic blade stall at around 80% RRPM at sea level. Take a look at this clip which seems to a R44 at below 80% (by assumption at sea level) you have to watch from 3:25 onwards...how is the aircraft still flying (looks like 50% at 3:30)? (ignore the complete failure to take the required low RPM corrective course of action)

https://www.youtube.com/watch?v=fe2CN0SIwTk
Hi Scotsman, only just seen this :D

The blade stall that you are talking about is for an unpowered rotor. The 'driving' section of the unpowered rotor becomes too small to overcome the drag and the rotor blade loses RPM catastrophically. When powered you can run the RPM down until the engine can no longer overcome the drag (much lower rpm) or the Angle of Attack of the blades exceed approximately 16deg and stall, whichever comes first.

There is an interesting technique that I was taught when dealing with "stuck left pedal" tailrotor emergency. Maybe you can test it on your instructor ;) Admittedly I haven't done it in a 22 and he might have some good reasons for not wanting to try it.

With stuck left pedal the tailrotor is providing too much thrust (Nose Left of track). Because of the gearing the tail rotor spins at a higher rpm than the main rotor so if you manually reduce the power (by rolling off the throttle) you will reduce the lift created by the main rotor (compensated for by increased collective) but the reduced thrust in the tail rotor will be relatively far greater (by a factor of the gearing). Hey presto, the view out of the front window improves markedly!
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Re: RRPM below 80% and still flying

Unread post by Scotsman » Thu Jan 29, 2015 4:10 pm

Makes sense on the stuck pedal due to the gearing ratios.

I wonder what rrpm the 22 stalls at with power? I would not have thought that it would be hugely reduced from the unpowered version as I would have thought that you would have hit the critical angle by the increased amount of pitch required to maintain height with a 24 percent plus reduction in rrpm.
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Thu Jan 29, 2015 4:21 pm

Scotsman wrote:Makes sense on the stuck pedal due to the gearing ratios.

I wonder what rrpm the 22 stalls at with power? I would not have thought that it would be hugely reduced from the unpowered version as I would have thought that you would have hit the critical angle by the increased amount of pitch required to maintain height with a 24 percent plus reduction in rrpm.
Haven't been much below 80% in the 44. I would think (ie. opinion, not fact) that the limiting factor is Angle of Attack, hence in forward flight you should get a lower RPM before stall than in the hover and the collective will be just under your armpit :D .

Anyway, from your video it looks like you get buffeting very similar to VRS so should be possible to recognise. Must say there was no indication of that at 80%.
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Re: RRPM below 80% and still flying

Unread post by Scotsman » Thu Jan 29, 2015 4:28 pm

Distinctly sweaty armpit! :lol: :lol:

I see what you are saying on forward flight due to increased inflow reducing the angle of attack...seems logical. Also would the angle of descent that the heli embarked on also reduce the angle of attack therefore lessening the pitch requirements giving a bit more space between where they were and the critical angle.
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Thu Jan 29, 2015 4:31 pm

Scotsman wrote:Distinctly sweaty armpit! :lol: :lol:

I see what you are saying on forward flight due to increased inflow reducing the angle of attack...seems logical. Also would the angle of descent that the heli embarked on also reduce the angle of attack therefore lessening the pitch requirements giving a bit more space between where they were and the critical angle.
If the heli is descending then the rising air column should increase the angle of attack, I think.

Fortunately I am often proved wrong :wink:
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Re: RRPM below 80% and still flying

Unread post by Scotsman » Thu Jan 29, 2015 4:56 pm

maybe I should shut up and go and read up on principles of flight for a bit! :lol:
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Thu Jan 29, 2015 6:29 pm

Scotsman wrote:maybe I should shut up and go and read up on principles of flight for a bit! :lol:
No, your studying the stuff at the moment so you could always be right.

Thinking a bit more about it the immediate effect of lowering the collective is to reduce the angle of attack, as you said.

But, if your VSI is steady and your ASI is steady then your angle of attack should be similar to (return to) horizontal flight AoA. You are using less pitch to support the weight of the helicopter but the AoA is the same due to the 'descent rate' of the helicopter.

Anyway, feel free to correct me with the facts :D

Not sure if you have "the book". You should have. Here it is online:
http://www.faa.gov/regulations_policies ... 083-21.pdf

PS: We should get cage involved :roll: Give him something to do rather than annoying the plank drivers :lol:
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Thu Jan 29, 2015 6:38 pm

Back to the 80% rule.

Up on the highveld I believe 87% is the live/die number.

If you want to extend your autorotation range you can pull a bit of collective and reduce your RRPM to around 90% without dying. During your training you will find the low RRPM horn frightening but later, as you become more aware of your RRPM you will find it more annoying than frightening.

Admittedly the gains of reducing the rrpm are a couple of 100 metres. I have yet to land short on an autorotation so I know when I am in trouble I have a bit spare.

Have fun. I'm looking forward to you giving me a flip!
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Re: RRPM below 80% and still flying

Unread post by Scotsman » Thu Jan 29, 2015 8:31 pm

Yep 80% plus 1% per thousand feet. You'll need to update the life policies before flying with me! Doing ok with the autos but need work on the 180s.
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Re: RRPM below 80% and still flying

Unread post by Scotsman » Thu Jan 29, 2015 8:33 pm

Think cage enjoys baiting the fixed wing fairies too much to join the conversation.....not to mention that we will have to put up with all the Robinson jibes!
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Thu Jan 29, 2015 8:37 pm

Scotsman wrote:Yep 80% plus 1% per thousand feet. You'll need to update the life policies before flying with me! Doing ok with the autos but need work on the 180s.
My wife will be happy if you have a couple of bar passenger insurance on the H&F policy 8-[
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Thu Jan 29, 2015 8:48 pm

Scotsman wrote:Think cage enjoys baiting the fixed wing fairies too much to join the conversation.....not to mention that we will have to put up with all the Robinson jibes!
Thats' just because he is jealous.

I heard he tried to pull you to the dark side.
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Re: RRPM below 80% and still flying

Unread post by Scotsman » Thu Jan 29, 2015 9:12 pm

He did but I am quite happy with the Robbies and looking forward to converting onto the 44 before the end of the syllabus. Can't afford or justify the increased jetty rates for a weekend warrior and the 44 is pretty fast as helis go anyway. Only got 30 hours time at the minute on helis so some way to go still but taking a week off soon to push through some of the hours.
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Re: RRPM below 80% and still flying

Unread post by tanglefoot » Thu Jan 29, 2015 9:18 pm

Scotsman wrote:He did but I am quite happy with the Robbies and looking forward to converting onto the 44 before the end of the syllabus. Can't afford or justify the increased jetty rates for a weekend warrior and the 44 is pretty fast as helis go anyway. Only got 30 hours time at the minute on helis so some way to go still but taking a week off soon to push through some of the hours.
Take your time.

A lot of the experience is in flying through the seasons. Some winter flying will do you good too.
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