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Old 07-07-2009, 11:21 AM   #21
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Quote:
Originally Posted by Danal Estes View Post
The "requirement" for negative pitch is a common mis-perception among model helicopter pilots. Full scale does not require negative collective pitch to sustain an auto. If so, a gyrocopter could never sustain flight.

See here
I read through the entire thing. The gyrocopter has a component of forward power that is not being supplied by the main rotor. Thus yes this is very different and look at the angle of attack of the rotor. But read through this and chapter 3 and no where does it say for a helicopter negative pitch is not needed. It simply says reduce collective and maintain a proper head speed. What pitch is used to obtain that no where in here does it say. I understand diagram 16-4 and forward momentum and thus angle of attack BUT what if you have no forward? So while negative is not needed for a gyrocopter due to the forward movement not provided by the rotor, I still argue a heli is not the same and still may use negative pitch to obtain rotor speed. Hey I could be wrong still but it doesn't make sense. Model are not that different.

I agree you don't need much negative to auto and more forward airspeed the less you need to maybe even 0 or +1'ish. In fact I see guys use a lot to drive the heli down thinking this is better. In fact my best auto's are MAYBE -1 where just like this link you provided says. Best auto is maximum glide rate and a slowest decent rate. Makes sense to me from experience BUT does require some negative. I cant believe a real heli would be much different. It's all Bernoulli's principles of lift and drag. But hey... A aerodynamics engineer I am not. So if I am wrong on this so be it.

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Old 07-07-2009, 11:30 AM   #22
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Real helis don't do vertical autos, that's the reason they have a height/velocity chart. While they can do vertical autos from a low hover when they do that they are using the energy stored in the rotor to get the machine down.
OK so that makes sense now. Is it that they CANT do vertical and if not why? If they had negative available then I don't see why not?

Then I have to ask what happens in a real heli if you are in a 900 foot hover and the engine quits? To get forward momentum you would have to use energy from the rotor and change the angle of attach of the rotor to forward. In doing so wont you dump head speed without having negative to compensate?

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Old 07-07-2009, 12:30 PM   #23
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Good discussion, I did not know that carrier helicopters drove some negative to help keep it placed on the deck. Deffinitely makes sense though.

In light of the original question though, is do any helicopters have any appreciable negative pitch to use in an aerobatic maneuver, not when the baldes are dissengaged in an auto.

So, lets say, at the top part of a loop, are there any helis that could drive any appreciable negative pitch? If you bottom out the collective and into any neg the blades would dissengauge in a lot of helicopters right? Or am I wrong? The Red Bull helicopter uses 0 deg at the top of the loop, unless something has changed that statement was directly from the Red Bull pilot. I would think driving any real neg at the top of the loop could be bad in full scale world.

Just a thought
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Old 07-07-2009, 06:11 PM   #24
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Originally Posted by Finless View Post
OK so that makes sense now. Is it that they CANT do vertical and if not why? If they had negative available then I don't see why not?

Then I have to ask what happens in a real heli if you are in a 900 foot hover and the engine quits? To get forward momentum you would have to use energy from the rotor and change the angle of attach of the rotor to forward. In doing so wont you dump head speed without having negative to compensate?

Bob
A real heli can auto vertically but the rate of descent would be extremely high. Most height/velocity graphs allow for a stationary hover above 400 AGL. From that height you have enough altitude to build up enough forward speed for a normal auto. From a low hover there is enough stored energy in the rotor to cushion the landing. A medium altitude hover is bad, not enough altitude to build forward speed and not enough energy in the rotor to get the machine down.
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Old 07-07-2009, 08:37 PM   #25
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Quote:
Originally Posted by joediamond View Post
A real heli can auto vertically but the rate of descent would be extremely high. Most height/velocity graphs allow for a stationary hover above 400 AGL. From that height you have enough altitude to build up enough forward speed for a normal auto. From a low hover there is enough stored energy in the rotor to cushion the landing. A medium altitude hover is bad, not enough altitude to build forward speed and not enough energy in the rotor to get the machine down.
+1. For the model world these height/speed charts don't make sense due to the power, weight, and energy components. Reading full scale procedures don't make sense with our power to weight ratios. But it is really interesting.
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Old 07-08-2009, 01:05 PM   #26
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Quote:
Originally Posted by Finless View Post
OK so that makes sense now. Is it that they CANT do vertical and if not why? If they had negative available then I don't see why not?

Then I have to ask what happens in a real heli if you are in a 900 foot hover and the engine quits? To get forward momentum you would have to use energy from the rotor and change the angle of attach of the rotor to forward. In doing so wont you dump head speed without having negative to compensate?

Bob
You dump collective to maintain rotor RPM, then push forward to get airspeed and then you enter a normal auto with forward motion.

From 900 feet, not a big deal.

Great fun from 200 feet or so. In an R-22 you want about 60 knots forward speed, IIRC. Basically the top of the H-V chart at 0 airspeed gives you just enough altitude to get the needed airspeed and to flare for the landing. But the H-V chart is based on a "normal" pilot with a 2 second reaction time.

But a real helo cannot do a straight vertical auto, from above about 15 feet (for a helicopter with good rotor energy). An R-22 will hit pretty hard with a 15 foot hovering auto.
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Old 07-08-2009, 01:41 PM   #27
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A 22 from 15' will spread the skids..

As for negative pitch, the pitch of the blades in relation to the mast has very little to do with the angle of attack. Remember, AoA is the angle between the relative wind (rotational *AND* flight path) and the chordline of the blade. The AoA will in all likelyhood be positive regardless of the blade's pitch due to the flightpath being forward and down.

Dump the collective to maintain RPM (and who cares what the pitch is) pitch (the aircraft, not the blades) for proper airspeed and tweak the collective to maintain proper RPM.
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Old 07-08-2009, 02:27 PM   #28
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Haven't seen this mentioned but thought it worth mentioning, on full size helicopters the main rotor blades are twisted so the tips are -10 or more degrees to minimize tip vortex, even the lift load across the span of the blade and improve handling during hover. If you could get the theorectical -10 pitch you would be at -20+ degrees at the tip (or +20 inverted) which would do very, bad things.
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Old 07-08-2009, 05:13 PM   #29
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The story in RC Heli mag about the Red Bull helicopter said that when inverted he was at 0 pitch.
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Old 07-08-2009, 07:22 PM   #30
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I don't recall seeing this on BlackHawks. I worked at Sikorsky for a year as an intern, and all of the blades were zero twist. They did have a sweep at the tip, but no twist.
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Old 07-08-2009, 08:08 PM   #31
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Great discussion. As was already said, it seems some have a few deg negative for particular reasons, but as we could all appreciate having a full size heli actually maintain a hover would be a tremendous task, even if the fuel delivery systems & lubricant system etc etc were up to it, there is a stack of weight for such long blades to keep inverted.

Back to your original idea for the heli to be inverted to avoid combat, assuming we have sorted all the mechanical, weight, blade issues, what about the poor pilot rocketing up with such force.. I would assume you would need a gravitation field around the heli to negate the effects on the human body, then that introduces another whole host of issues for electronics. Technology is not up to scratch just yet, but gravitation fields are starting to pop their heads up now, in our belief system, but in actualality have been around for some time. (Germans in WW2)

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Old 07-09-2009, 12:43 AM   #32
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Guys/girls, remember a truely round loop has +4g at the bottom, entry, +2g in the vertical and +1g over the top, with the same on the downward side, ie +2g, then +4g on exit. So there is no need to have negative over the top, look at your collective position the next time you do a perfectly round loop, you might end up with zero stick/pitch position, but the relative airflow due to forward speed still gives the heli positive angle of attack on the blades to hold the +1g over the top.

So for full scale helis to do a loop is theoretically possible on all helis, but head design, ie rigid or not, power to weight ratio etc plays a more important part in the aerobatic capability of full scale helis.
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Old 07-09-2009, 12:52 AM   #33
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After all this, academic material and all, I'm still not totally sure if a heli that is not traveling forward, but is instead dropping out of the sky with boom at 90 degrees, at perfectly zero pitch along the blades, generates any ongoing lift..

my guess from what has been said so far: with no motor, lift diminishes quickly to zero as rpm diminishes to zero
with motor maintaining constant RPM, lift is generated in proportion to downward velocity (which is why you don't get light on the skids at zero pitch) so some equilibrium speed is reached..

if so, one can reach the strange conclusion that a heli will fall slower powered at 2 degrees negative pitch than with stopped blades.
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Old 07-09-2009, 01:08 AM   #34
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Yep, because again, the physical pitch of the blades doesn't necessarily mean the AoA is in the negative....
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Old 07-09-2009, 07:59 AM   #35
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Wile full scale do have some twist to the blades, not 10 degrees worth. A few degrees yes.

And Skiddz, I was going to say a 15 foot hovering auto makes the smile bar frown. Place I did most of my R22 lfying had one like that. Done by an instructor. Who no longer worked there.
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Old 07-09-2009, 09:16 AM   #36
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Quote:
I don't recall seeing this on BlackHawks. I worked at Sikorsky for a year as an intern, and all of the blades were zero twist. They did have a sweep at the tip, but no twist.
Those must have been factory rejects. As a CW3 UH-60 pilot that squeaked through Maintenance Test Pilot school I can assure you those blades are twisted sisters. I'm not Chuck Yeager and I haven't slept in a Holiday Inn Express lately however, I do know a thing or two about the Blackhawk.

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Wile full scale do have some twist to the blades, not 10 degrees worth. A few degrees yes.
The Army's TH-67 (basically a Bell 206, yes?) has -10 degrees twist from root to tip. Blackhawk has over -13 degrees twist from root to tip.
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Old 07-09-2009, 09:31 AM   #37
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I don't believe this has been brought up yet but the forces involved are just too great.

In order to avoid an incoming missile strike, a so called helicopter would have to apply a whole lot of negative pitch in a minute but finite length of time. I believe such forces will tear the helicopter apart. When size increases the forces do as well - only they increase exponentially.

Also consider the phalax. No moving object in existance can avoid that.
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Old 07-09-2009, 04:29 PM   #38
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Quote:
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Those must have been factory rejects. As a CW3 UH-60 pilot that squeaked through Maintenance Test Pilot school I can assure you those blades are twisted sisters. I'm not Chuck Yeager and I haven't slept in a Holiday Inn Express lately however, I do know a thing or two about the Blackhawk.



The Army's TH-67 (basically a Bell 206, yes?) has -10 degrees twist from root to tip. Blackhawk has over -13 degrees twist from root to tip.
That would mean the root is at 18 degrees of pitch at 5 degrees at the tip.

Seems a bit much.

But you would know.
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Old 07-09-2009, 04:59 PM   #39
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I know it sounds like alot and frankly guys don't pay attention to it because it's really just "nice to know" info. The swept tips add to the effect of the negative blade tip as well, sound reduction is a happy byproduct. It's worth mentioning the twist is non-linear, to take advantage of various aerodynamic and physical phenomena that frankly, I'll never understand. But yeah, the tips are usually flying at minus something in relation to the root.

By the way, the BO-105 has -8.5 at the tip.
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Old 07-09-2009, 09:01 PM   #40
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R22 and R44 are both like -8 degrees of twist so -10s not out of the question.
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