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Main Forum - Helicopter Talk R/C Helicopters and the people who fly them. VENDOR TOPICS DO NOT GO HERE. Full Scale Heli threads go in OT please |
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#21 (permalink) | |
Team Taco VP
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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. Bob
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#22 (permalink) | |
Team Taco VP
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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
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#23 (permalink) |
NERD
Join Date: Apr 2008
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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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Norman Fun times and good friends. |
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#24 (permalink) | |
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#25 (permalink) | |
Registered Users
Join Date: Oct 2008
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Ed Always Something New! ------------------------------------------------------------------------------------ T-Rex 500ESP, GY401/9257, 6s 3000mAh 100% Flat TC X9303; Nano CPS |
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#26 (permalink) | |
Registered Users
Join Date: Sep 2006
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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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Terry AMA#47402, IRCHA # 3395 Blade CP "Pro", Trex 450SE, PiccoZ, Quick of Japan EP8v2 EX, Hurricane 550, Hurricane 200, JR Vibe 50, Blade mCX, Bergen Intrepid Gasser, Pantera 50, Blade mSR, Novus CP |
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#27 (permalink) |
HF Support
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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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-Kevin No helicopters were harmed in the creation of this post. AMA CD # 7309, Turbine Waiver# RW 890 Click Here For Bergen Intrepid Turbine Build Videos |
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#28 (permalink) |
Registered Users
Join Date: Dec 2006
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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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#29 (permalink) |
Registered Users
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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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#30 (permalink) |
Registered Users
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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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#31 (permalink) |
Registered Users
Join Date: Dec 2007
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![]() Great discussion
![]() ![]() 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. ![]() ![]() ![]() ![]() Bruce |
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#32 (permalink) |
Registered Users
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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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#33 (permalink) |
Registered Users
Thread Starter
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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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#34 (permalink) |
HF Support
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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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-Kevin No helicopters were harmed in the creation of this post. AMA CD # 7309, Turbine Waiver# RW 890 Click Here For Bergen Intrepid Turbine Build Videos |
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#35 (permalink) |
Registered Users
Join Date: Sep 2006
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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. ![]() ![]()
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Terry AMA#47402, IRCHA # 3395 Blade CP "Pro", Trex 450SE, PiccoZ, Quick of Japan EP8v2 EX, Hurricane 550, Hurricane 200, JR Vibe 50, Blade mCX, Bergen Intrepid Gasser, Pantera 50, Blade mSR, Novus CP |
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#36 (permalink) | ||
Registered Users
Join Date: Dec 2006
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#37 (permalink) |
Registered Users
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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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#38 (permalink) | |
Registered Users
Join Date: Sep 2006
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Seems a bit much. But you would know. ![]()
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Terry AMA#47402, IRCHA # 3395 Blade CP "Pro", Trex 450SE, PiccoZ, Quick of Japan EP8v2 EX, Hurricane 550, Hurricane 200, JR Vibe 50, Blade mCX, Bergen Intrepid Gasser, Pantera 50, Blade mSR, Novus CP |
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#39 (permalink) |
Registered Users
Join Date: Dec 2006
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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.
![]() By the way, the BO-105 has -8.5 at the tip.
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#40 (permalink) |
HF Support
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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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-Kevin No helicopters were harmed in the creation of this post. AMA CD # 7309, Turbine Waiver# RW 890 Click Here For Bergen Intrepid Turbine Build Videos |
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