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HomeAircraftHelicopterHelicopter Main Discussion › Max tail blade pitch reopened
10-13-2010 03:15 AM  8 years agoPost 1
Heli_KV

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Just reopened the topic, because I am not sure that everybody who had something to say were given opportunity to do so. So please, consider that as continuation. All oppinions are please welcome.

I personally not sure of what I say is exactly right, though I am sure theory is right, so I welcome to hear opinions. If no continuation, then so be it, this topic will dye by itself. Theory of flight is quite interesting and complicated to a degree subject and we as humans took a long road investigating this (just a century back). We all use pretty much high tech electronics stuff in helis, but aerodynamics is the basis I guess.

PS. I am a bit frustrated, trying to post a long answer to opponents and finding on button click, that topic was closed. So that's why it is reopened. My answer was vanished . And I agreed with some of posts, but not with all of them!

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10-13-2010 03:26 AM  8 years agoPost 2
Aaron29

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10-13-2010 03:58 AM  8 years agoPost 3
Heli_KV

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Thanks Aaron29 for reply. I can assure you, I do fly. And guess what my max pitch is? Right. Max. I do fly. But I do not fly 24h a day. I do not fly that good and that's why I am learning and that's why I question or support others questions, which I think right. Anyway, any opinion is good, so thanks.

As for nuts. Well, not really of nuts, but, there is really interesting story in our human life, which I already posted, but not many people investigated why it was so way back then. Let me put more stress on it. The fact that Sun is around Earth or vice versa was purely practical. We (our ancestors) needed to navigate ships. How? By stars only. In open ocean - only stars. No GPS that days. So, some theory with practical applications was created. And Earth was in the center! (Sure religious bells and whistles around). And it worked, and worked better, then any other theory. Ships, navigation meant money, new lands, new slaves, you name it. So anything others, were not good, so kill it! Time past, precision requirements increased, theory improved and vualya - world changed. Now it is opposite. But the lesson learned - it is all from practical point.

And it is just proved now - just go fly. It works! So I do, but I still have this looks like simple question (stress on 'looks like' (not me, but still, looks like I kind of inherited it ). Looks like among us, there is no engeneer, who can answer this question directly.

Or it is possible to say: 'Somebody has too much time on hands. Just go fly'. I do fly with max deflection possible...

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10-13-2010 04:01 AM  8 years agoPost 4
GetToDaChopper

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I'm very sorry Heli_KV i thought you was done posting till after Thanksgiving day.

i didn't mean to cut you off, sincerest apologys.

please continue.

As for proper computations, it is enough to take Navier-Stocks equations for gas dynamics and to solve then numerically. It is enough to have two-dimensional model, which will significantly reduce computation time. The only problem is that equations cannot be solved analytically in general case, so numerical approximation is required. For example using finite differences (or finite elements) methods. If to get rid of viscosity, it is possible to take equations of ideal gas dynamics, which does not count viscosity. It will be close enough or for that matter, if taking into account, that Mach number having our speeds is less then 1, it is possible to take linear approximation of such equations, which will significantly reduce computational and algorithmic efforts and maybe will allow analytical solution. I was not specialized in linear approachs, so maybe it is way simplier then it seems to be and does allow analytical solutions. Btw, for a number of cases even nonlinear equations can be solved analytically and that's used in hyperspeed flow engine creations (kind of jet engine, where it is nesessary to heat gas (air) to most possible temperature to get desired effectiveness). But that's completely different subject for hypersonic flows.

All we need to look at what speed vortexes after the blade start to appear and at what speed and what angle of attack. I think, that approximation of blade as an infinitely thin plate will be good enough. I may be mistaken in my latest assumption. Let specialists in gas dynamics linear approximation say their word and give us a light.
If you have to ask then you need a better tail gyro. Set it to max limits and let the hard-earned coin you spent on your gyro cover the rest
Sorry, but no. It is not good enough. Let's take a look at this scenario: You have a gyro, fast enough, but not infinitely fast and fast but not infinitely fast servo. Let's say we fly tail forward with high speed. Ok. So, tail starts to deviate from desired direction. What will gyro + servo do? Right, it will deflect tail blades to compensate for rotation, keeping heli heading. But, look, speed was not enough to compensate at the very beginning of started deflection (Gyro/Servo has finite speed), so task now is more complicated, we need more force to apply. So, what will gyro do? Right. It will continue to increase tail pitch and ahh, unfortunately up to the level of loosing effectiveness of blades. If having max pitch as recommended in 'manual' and having long enough tail shaft, gyro will keep going and going, increasing pitch (btw, at the same time increasing load on motor), but it results in further and further non effectiveness of gyro efforts. So what is the limit for gyro then? That's exactly what OP asked. But gyro does not know, but it can be limited (and as we saw speeds we are talking about are way less then tail rotation speed). So what do we need to tell to gyro? Well, depends on blades, their shape, size, flexibility, etc. Let's forget of fexibility of material (important, but hard to take into account). Maybe some rough numbers will help? That's exactly what OP was asking and I agree with him.

Sorry for spelling mistakes and once again, please no offence. The topic seems to me quite interesting.

In a nutshell, I think, gyro has to know of max angle of tail blade attack and we can set it by limiting gyro deflection. Simple enough, but the question is still unanswered: What is max angle of attack, let's say having blade like we have. At least an idea: 5 degrees, 10, 40?

Max angle is not good enough, at least for theoretical subject. Isn't max angle is too big, that we shoot ourselfs with this max angle and then horrible stories heard, that 'tail blows'?

Again i apologize for cutting you off, not intended and if you like i'll take the thread back.

P.S. again i say i'm not having any troubles or thinking about limiting tail slider travel, i do fly as much as time allows and heli flys great i just wanted an answer to something i was THINKING about, not something i was going to do on my heli. and "just do what the manual says" is not an answer to my question.

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10-13-2010 04:15 AM  8 years agoPost 5
Aaron29

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10-13-2010 04:23 AM  8 years agoPost 6
GetToDaChopper

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Thank you very much thats all i wanted to know +1,
just one small question if max pitch was allowed to go to 55 degrees would you consider that to be too much pitch under any situation ?

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10-13-2010 04:25 AM  8 years agoPost 7
drdot

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fwiw...

All airfoils have a defined stall point in terms of AOA...Airfoils play a part, Reynolds numbers a great deal....Big issue to the question to me is varying airflow during inverted and backwards flight....Most of my helis have mechanical t/r pitches in excess of 40 degrees each way,,,Double the angle needed to stall the blades in a conventional airflow..

The blowout you mention usually has little to do with the changing pitch of the t/r blades as a direct function of blade stall, but more the inability of the power train to deliver enough torque to the entire rotor system to maintain rotor speed....Lower t/r speed....higher aoa....stall or loss of sufficient thrust to prevent tail rotation...

Interesting topic.....

John.

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10-13-2010 04:29 AM  8 years agoPost 8
Aaron29

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10-13-2010 04:31 AM  8 years agoPost 9
GetToDaChopper

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So at the higher angles you mention more power can make the difference between the blades stalling or continuing to provide good thrust and tail authority ?
P.S. remember you set the max pitch up as what the gyro will command. When you move your stick all the way over, it does not necessarily reach that pitch. Only the gyro can decide how much pitch is needed.
Great point +1

But if the gyro dicides that more pitch is needed and it can go all the way to 55 degrees and it is too far the gyro will keep going and going, increasing pitch but it results in further and further non effectiveness of the tail to do what the gyro is comannding could it not ?

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10-13-2010 04:33 AM  8 years agoPost 10
Aaron29

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10-13-2010 04:40 AM  8 years agoPost 11
drdot

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fwiw...

Most setup errors I see at our field have something to do with excessive main rotor pitch, leading to exactly what you describe....Excess pitch loads the system too heavily, the rotor speed drops...Angle of attack increases, t/r and mains begin to stall....Imagine an airplane at 5000 feet in level suddenly having NO airspeed...the angle of attack of the main wings becomes 90 degrees...
Stalled?.... You bet!
First thing I recommend is dropping the max pitch range on the main rotor.....Not everyone can fly with collective pitch ranges of 15 degrees...Even fewer helis can sustain that pitch range for long...

John.

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10-13-2010 04:42 AM  8 years agoPost 12
GetToDaChopper

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A stalled blade produces less lift and lots of drag. You can power through that drag as much as you want and it still has less lift
well thats what i thought and if the tail get in to that situation and the tail is not doing what the gyro is commanding couldn't this situation keep getting worse by commanding more and more pitch ?

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10-13-2010 04:43 AM  8 years agoPost 13
Aaron29

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10-13-2010 04:44 AM  8 years agoPost 14
drdot

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fwiw...

Yes...And that's what leads to the dreaded "blow-out"....

John.

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10-13-2010 04:45 AM  8 years agoPost 15
Aaron29

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10-13-2010 04:46 AM  8 years agoPost 16
GetToDaChopper

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i know, how do they know when to back off ?

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10-13-2010 04:50 AM  8 years agoPost 17
GetToDaChopper

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maybe it has something to do with the gyro knowing the heli movement is getting worse and worse from the continuing command given from gyro it's self ? hummmmm

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10-13-2010 04:52 AM  8 years agoPost 18
Aaron29

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10-13-2010 04:54 AM  8 years agoPost 19
GetToDaChopper

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very true

i just have always like to know what go's on behind the scenes if you will. lol

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10-13-2010 05:18 AM  8 years agoPost 20
drdot

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fwiw..

A friend who is an EE has been talking to Art at Skookum about possible feed forward from the governor function...Might back off collective/t/r pitch as headspeed decays as a function of load...We've been noticing increased vibration levels on flights with the 720 as a direct result of high collective cyclic maneuvers...Directly attributable to buffeting of blade stall...Amazing the information the flight log provides!!

Good Times!!

John.

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HomeAircraftHelicopterHelicopter Main Discussion › Max tail blade pitch reopened
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