Can Helicopters fly inverted? (Merged threads)
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Nick,
For the fun of a hypothetical discussion;
>In trimmed upside down flight, the rotor would behave exactly as it now does.<
I believe you are saying that if there was any coning it will be upward away from the earth, but from the inverted pilot's perspective, it is 'downward' toward the fuselage. In other words, from a remote observer's perspective, the only thing unusual about the rotor disk is that the mast is above it.
>At trimmed speed, an acceleration would require forward stick, and the rotor would back-flap in a stable manner. <
IMHO acceleration will require aft stick, since the thrust from the rotor disk is now in the opposite direction (negative pitch).
I will now bend over and assume the 'Lu' position.
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Twin 1,
>As to the question Why bother? <
You may have come up with an industrial size weed-eater.
For the fun of a hypothetical discussion;
>In trimmed upside down flight, the rotor would behave exactly as it now does.<
I believe you are saying that if there was any coning it will be upward away from the earth, but from the inverted pilot's perspective, it is 'downward' toward the fuselage. In other words, from a remote observer's perspective, the only thing unusual about the rotor disk is that the mast is above it.
>At trimmed speed, an acceleration would require forward stick, and the rotor would back-flap in a stable manner. <
IMHO acceleration will require aft stick, since the thrust from the rotor disk is now in the opposite direction (negative pitch).
I will now bend over and assume the 'Lu' position.
______________________________
Twin 1,
>As to the question Why bother? <
You may have come up with an industrial size weed-eater.
Last edited by Dave Jackson; 16th May 2002 at 18:57.
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Oh it hurts is an understatement.
You're upside down level forward flight. Pull stick back, rotor disc tilts so the total thrust is now further forward. You now have a moment about the c of g that will pitch the nose down....errr up...you see more sky (or is it less ground). Anyway, to stop that pitching motion you add forward cyclic. So do you slow down now??
HeLP!! Where did I go wrong?
You're upside down level forward flight. Pull stick back, rotor disc tilts so the total thrust is now further forward. You now have a moment about the c of g that will pitch the nose down....errr up...you see more sky (or is it less ground). Anyway, to stop that pitching motion you add forward cyclic. So do you slow down now??
HeLP!! Where did I go wrong?
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Yes, well .... now you have a taste of the mental gymnastics us model heli pilots make when we invert the machine
By the way, setup of the machine are just as neat, I run what we call a 3D curve on my radio.
One has throttle and coll pitch curves on the radio, typically we set them to maintain RPM throughout the range of collective.
But for 3D, I set my collective pitch from max inverted pitch at low end to max at high end, zero in the middle, and the throttle now is in a "V" curve, meaning full throttle at both ends.
Can make for some of the more interesting maneuvers some of you have seen models do.
Another interesting thing is that we can also do inverted auto's, flipping to upright near the end and flaring/landing.
(not that I'm doing them yet, but it can be done)
By the way, setup of the machine are just as neat, I run what we call a 3D curve on my radio.
One has throttle and coll pitch curves on the radio, typically we set them to maintain RPM throughout the range of collective.
But for 3D, I set my collective pitch from max inverted pitch at low end to max at high end, zero in the middle, and the throttle now is in a "V" curve, meaning full throttle at both ends.
Can make for some of the more interesting maneuvers some of you have seen models do.
Another interesting thing is that we can also do inverted auto's, flipping to upright near the end and flaring/landing.
(not that I'm doing them yet, but it can be done)
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I understand (as taught by RAF CFS) that a conventional heli in the hover has static stability (pendulum effect) and dynamic instability (flapback causes overshoot of original attitude and divergence)).
Invert the aircraft and you have the opposite.
A 1960s design took advantage of this (I have seen pictures of it but I can't recall its name), it was a simple manned platform with the rotor below the C of G. It was the closest I've seen to a hover board! The pilot stood on the platform and fired it up, holding onto a handrail arrangement on top of the engine. It was controlled by weight shift and it would hover just about hands off. Not a good device to abandon ship from though
I think it never went into production because it was too stable to be a sensible mode of transport.
Invert the aircraft and you have the opposite.
A 1960s design took advantage of this (I have seen pictures of it but I can't recall its name), it was a simple manned platform with the rotor below the C of G. It was the closest I've seen to a hover board! The pilot stood on the platform and fired it up, holding onto a handrail arrangement on top of the engine. It was controlled by weight shift and it would hover just about hands off. Not a good device to abandon ship from though
I think it never went into production because it was too stable to be a sensible mode of transport.
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Aerodynamics killed the design
To: Shy T
If you are referring to the “Flying manhole cover” developed by Hiller it never went into production because of instability rather than being too stable. Air was drawn down into a Venturi duct and passed through two counter rotating props, which accelerated the flow. When the pilot would lean in any given direction the craft would fly in that direction. With the Venturi duct tipped from the vertical axis the air would flow into the duct faster on the leading edge (direction being flown) and there would be an increase in lift due to the aerodynamic shape of the duct. This differential of lift would cause the craft to return to the vertical and the pilot would have to start all over. It is similar in some ways to flap / blowback in which case the pilot must add more forward cyclic.
If you are referring to the “Flying manhole cover” developed by Hiller it never went into production because of instability rather than being too stable. Air was drawn down into a Venturi duct and passed through two counter rotating props, which accelerated the flow. When the pilot would lean in any given direction the craft would fly in that direction. With the Venturi duct tipped from the vertical axis the air would flow into the duct faster on the leading edge (direction being flown) and there would be an increase in lift due to the aerodynamic shape of the duct. This differential of lift would cause the craft to return to the vertical and the pilot would have to start all over. It is similar in some ways to flap / blowback in which case the pilot must add more forward cyclic.
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Lu,
Thanks for that; I'm not sure what I am thinking about was a ducted fan...
Aha!
I just noticed Dave J's links at the top of this page. Having followed them, what I remember seeing was more like the "Flying Platform" than the Hiller, although the former machine is obviously a much more recent thing.
The Hiller design does look unstable; seems to have quite a small diameter rotor and probably a high C of G. Presumably, having a duct around the fan would reduce the flapback effect quite considerably.
Thanks for that; I'm not sure what I am thinking about was a ducted fan...
Aha!
I just noticed Dave J's links at the top of this page. Having followed them, what I remember seeing was more like the "Flying Platform" than the Hiller, although the former machine is obviously a much more recent thing.
The Hiller design does look unstable; seems to have quite a small diameter rotor and probably a high C of G. Presumably, having a duct around the fan would reduce the flapback effect quite considerably.
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Shy Torque,
You are quite correct, the duct of a ducted fan isolates the fan from the free stream flow, so that there is almost no back flap. This is the reason why Comanche can do the snap turn at 100 knots - its fan does not produce any strong flapping forces or moments while at large sideslips. A typical tail rotor would produce very high moments and flapping forces if subjected to the same maneuver.
For the same reason, a fenestron or ducted fan is a poor device for yaw stability at high speed, The fan simply does not produce any change in thrust for sideslip variations. That is why fenestrons reqyure such great vertical fin areas (and endplates).
Nick
You are quite correct, the duct of a ducted fan isolates the fan from the free stream flow, so that there is almost no back flap. This is the reason why Comanche can do the snap turn at 100 knots - its fan does not produce any strong flapping forces or moments while at large sideslips. A typical tail rotor would produce very high moments and flapping forces if subjected to the same maneuver.
For the same reason, a fenestron or ducted fan is a poor device for yaw stability at high speed, The fan simply does not produce any change in thrust for sideslip variations. That is why fenestrons reqyure such great vertical fin areas (and endplates).
Nick
Gentleman Aviator
To elaborate on ShyTorque's reference to CFS(H) training; the C of G arguments about stability above are mistaken. (I only speak of real helicopters, not RC models, of which I know nothing).
In an unstabilised helicopter (no autopilot, autostab, ASE), the fact that the C of G is below the disc does make it dynamically UNstable in the hover.
Consider: an unstabilised conventional helo in a steady hover diverges to one side, thus creating a relative airflow over the disc. Disc flaps away from the movement, creating a correcting movement of the rotor disc. However, (isn't there always a "however" in helicopter P of F), the pendosity of the body then tilts the disc even further, thereby OVER correcting, and leading to movement in the other direction, beyond the original hover point. All this WITHOUT cyclic movement, rapidly leading to the undamped phugoid characteristic of dynamic instability. Which is why hovering an unstabilised helo is like balancing a stick on your finger. Put the C of G ABOVE the disc, and you get dynamic stability - not sure how you get on or off though!! And certainly we were taught that gave stability to the "flying platform" of the 50s/60s
That's the CFS (H) version anyway, as taught to generations of military helo pilots, by such luminaries as Chris Tinkler, Ray Lawrence, Ron Jones and Mike Ramshaw - where are they all now?
In an unstabilised helicopter (no autopilot, autostab, ASE), the fact that the C of G is below the disc does make it dynamically UNstable in the hover.
Consider: an unstabilised conventional helo in a steady hover diverges to one side, thus creating a relative airflow over the disc. Disc flaps away from the movement, creating a correcting movement of the rotor disc. However, (isn't there always a "however" in helicopter P of F), the pendosity of the body then tilts the disc even further, thereby OVER correcting, and leading to movement in the other direction, beyond the original hover point. All this WITHOUT cyclic movement, rapidly leading to the undamped phugoid characteristic of dynamic instability. Which is why hovering an unstabilised helo is like balancing a stick on your finger. Put the C of G ABOVE the disc, and you get dynamic stability - not sure how you get on or off though!! And certainly we were taught that gave stability to the "flying platform" of the 50s/60s
That's the CFS (H) version anyway, as taught to generations of military helo pilots, by such luminaries as Chris Tinkler, Ray Lawrence, Ron Jones and Mike Ramshaw - where are they all now?
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Nick,
Thanks, your answer brings back happy memories of the Gazelle too (which I haven't flown for 10 years or so now); although not being a test pilot I always managed to keep mine in PERFECTLY balanced flight (LOL), apart from the odd roll off the top where the ball sometimes got half a width out
).
Teetering,
Not forgetting Lofty Marshall of course! .
I understand Ray Lawrence is sadly no longer wth us. What genius that modest man had. I got on very well with him as a student and was privileged that in a quiet moment he once showed me his photo album of his inventions / projects, mostly from the 1960s. They were incredibly good and they all worked as intended. He apparently seldom showed them off as he did not like to blow his own trumpet.
For example:
His man-carrying helicopter powered by a 500cc Triumph motorcycle engine.
A Triplane designed, built and flown by himself in Borneo.
His (large!) radio controlled model helicopter with all the mechanicals and radio gear made by himself. The one-piece flywheel / centrifugal clutch was seen in the 1960s and copied by a German enthusiast who now manufactures them (sadly Ray never patented it, which he should have done).
Model 3-cylinder and a 5-cylinder radial aero engines, all of his own design and manufactured on a lathe.
A 5-inch reflecting telescope; the mirror and lenses made by himself.
A hovercraft. A caravan. A speedboat.
All his own design and made by his own hands. Sadly missed by me, he was a great inspiration at the beginning of my career, not forgetting his clever explanations of things aerodynamic and rotating at CFS(H).
Wherever you are, Ray, thanks and good on yer!
Sorry for getting well off track there, folks
Thanks, your answer brings back happy memories of the Gazelle too (which I haven't flown for 10 years or so now); although not being a test pilot I always managed to keep mine in PERFECTLY balanced flight (LOL), apart from the odd roll off the top where the ball sometimes got half a width out
).Teetering,
Not forgetting Lofty Marshall of course!
.I understand Ray Lawrence is sadly no longer wth us. What genius that modest man had. I got on very well with him as a student and was privileged that in a quiet moment he once showed me his photo album of his inventions / projects, mostly from the 1960s. They were incredibly good and they all worked as intended. He apparently seldom showed them off as he did not like to blow his own trumpet.
For example:
His man-carrying helicopter powered by a 500cc Triumph motorcycle engine.
A Triplane designed, built and flown by himself in Borneo.
His (large!) radio controlled model helicopter with all the mechanicals and radio gear made by himself. The one-piece flywheel / centrifugal clutch was seen in the 1960s and copied by a German enthusiast who now manufactures them (sadly Ray never patented it, which he should have done).
Model 3-cylinder and a 5-cylinder radial aero engines, all of his own design and manufactured on a lathe.
A 5-inch reflecting telescope; the mirror and lenses made by himself.
A hovercraft. A caravan. A speedboat.
All his own design and made by his own hands. Sadly missed by me, he was a great inspiration at the beginning of my career, not forgetting his clever explanations of things aerodynamic and rotating at CFS(H).
Wherever you are, Ray, thanks and good on yer!
Sorry for getting well off track there, folks
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Here's a web page with lots of neat information. 
http://www.howtoadvice.com/Sky-High
Twin 1,
Perhaps your inverted rotor may be an excellent project. The above article is interesting and it does suggest that there are practical applications for this configuration.
ShyTorque,
The following picture, from the above site, may be the craft that you are thinking of. http://www.howtoadvice.com/Image/Pho...li-Vector.jpg. [The link does not appear to work. It's an enlargement of the fifth picture in the above web page.] Prouty says that this craft "had hingeless blades with high inherent hub stiffness, and consequently its stability did not benefit significantly from its unique rotor location." This may be aerodynamically similar to the ducted fan and the comments by Nick.
teeteringhead,
A teetering head may be what you are referring to. If the head is rigid, as mentioned above, then its rotational inertia should provide some dynamic stability, in dampening the oscillations. Just a thought.
Lu,
Were any of your distant relatives in aeronautics? This web site describes the following patent by a Mr. Zimmerman. http://www.howtoadvice.com/Image/Dia...mmerman1Lg.jpg
http://www.howtoadvice.com/Sky-High
Twin 1,
Perhaps your inverted rotor may be an excellent project. The above article is interesting and it does suggest that there are practical applications for this configuration.
ShyTorque,
The following picture, from the above site, may be the craft that you are thinking of. http://www.howtoadvice.com/Image/Pho...li-Vector.jpg. [The link does not appear to work. It's an enlargement of the fifth picture in the above web page.] Prouty says that this craft "had hingeless blades with high inherent hub stiffness, and consequently its stability did not benefit significantly from its unique rotor location." This may be aerodynamically similar to the ducted fan and the comments by Nick.
teeteringhead,
A teetering head may be what you are referring to. If the head is rigid, as mentioned above, then its rotational inertia should provide some dynamic stability, in dampening the oscillations. Just a thought.
Lu,
Were any of your distant relatives in aeronautics? This web site describes the following patent by a Mr. Zimmerman. http://www.howtoadvice.com/Image/Dia...mmerman1Lg.jpg
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Can't claim a relationship
To: Dave Jackson
Sorry Dave the only thing I have in common with Mr. Zimmerman is that my last name begins with Z and ends in man. However I did work with a couple of people while in service that invented Pop-Out- floats, Friction locks for flight controls, The hydraulic hoist and Hands off stable flight for helicopters among several other things on helicopters to include the rescue basket used in SAR.
My only contribution to helicopters was in the maintenance arena which allowed the rigging of helicopters without the use of a universal propeller protractor and having to level the helicopter. And, a device that allowed the lubrication of rod ends without removing the control linkage from the helicopter.
Sorry Dave the only thing I have in common with Mr. Zimmerman is that my last name begins with Z and ends in man. However I did work with a couple of people while in service that invented Pop-Out- floats, Friction locks for flight controls, The hydraulic hoist and Hands off stable flight for helicopters among several other things on helicopters to include the rescue basket used in SAR.
My only contribution to helicopters was in the maintenance arena which allowed the rigging of helicopters without the use of a universal propeller protractor and having to level the helicopter. And, a device that allowed the lubrication of rod ends without removing the control linkage from the helicopter.
Last edited by Lu Zuckerman; 19th May 2002 at 20:43.
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Dave,
Ah yes, the Amphibious Heli Vector. The first aircraft to be fitted with airbags? Invented by that famous inventor chap McCarty, latterly known as "Shorty".
His demise came whilst trying to carry out a one-man rotors running refuel whilst cattle mustering to avoid cycles on the engine....I guess he just forgot.....
Ah yes, the Amphibious Heli Vector. The first aircraft to be fitted with airbags? Invented by that famous inventor chap McCarty, latterly known as "Shorty".
His demise came whilst trying to carry out a one-man rotors running refuel whilst cattle mustering to avoid cycles on the engine....I guess he just forgot.....
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Dave,
on a more serious note..
My understanding is that an inverted rotor system will only benefit from enhanced stability if it can flap back. A "rigid" rotor would roll laterally instead (as Cierva found out until he used flapping hinges on his autogiros). Two contra-rotating rigid rotors presumably would not exhibit either effect, as per a ducted fan.
on a more serious note..
My understanding is that an inverted rotor system will only benefit from enhanced stability if it can flap back. A "rigid" rotor would roll laterally instead (as Cierva found out until he used flapping hinges on his autogiros). Two contra-rotating rigid rotors presumably would not exhibit either effect, as per a ducted fan.
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ShyTorque,
You expect a serious response after your previous post ??????
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I agree with all you say.
The drag of the pilot will tend to pitch the 'nose' of the craft up. The reason that Lu's long lost relative has wings and headgear is probably to enhance this parasitic drag.
As you mentioned, ducting should eliminate any potential nose up pitching resulting from the rotors.
Your points about un-ducted rotors make sense, if the craft's rotors have 'absolute' rigidity, plus no coning. As you said, rolling is then prevented because of the two counter rotating rotors.
If we assume that the rotors do not have 'absolute' rigidity, then there will be a small amount of flap back taking place.
Nick and Roberto Celi have already corrected me on something related to the following and Nick may have cause to do so again. If the rotors have coning as well, then there will be some additional lifting taking place at the front. This is because the leading blade will have a slightly higher angle of attack then the retreating blade.
All three things probably contribute to the nose up pitching and thus the static stability of the craft. The greater the rigidity, the more that the coning will cause nose up. The less the rigidity, the more that flap back will cause nose up.
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If you buy this, then there's a bridge in Brooklyn we can talk about.
You expect a serious response after your previous post ???
???_________________________
I agree with all you say.
The drag of the pilot will tend to pitch the 'nose' of the craft up. The reason that Lu's long lost relative has wings and headgear is probably to enhance this parasitic drag.
As you mentioned, ducting should eliminate any potential nose up pitching resulting from the rotors.
Your points about un-ducted rotors make sense, if the craft's rotors have 'absolute' rigidity, plus no coning. As you said, rolling is then prevented because of the two counter rotating rotors.
If we assume that the rotors do not have 'absolute' rigidity, then there will be a small amount of flap back taking place.
Nick and Roberto Celi have already corrected me on something related to the following and Nick may have cause to do so again.
If the rotors have coning as well, then there will be some additional lifting taking place at the front. This is because the leading blade will have a slightly higher angle of attack then the retreating blade.All three things probably contribute to the nose up pitching and thus the static stability of the craft. The greater the rigidity, the more that the coning will cause nose up. The less the rigidity, the more that flap back will cause nose up.
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Dave,
Cheers for that, some interesting reading there.
All,
Thanks for the replies!
If anyone could recommend a good web-site to understand the basic principles of the helicopter I would be most grateful.
Thanks Again,
Twin
Cheers for that, some interesting reading there.
All,
Thanks for the replies!
If anyone could recommend a good web-site to understand the basic principles of the helicopter I would be most grateful.
Thanks Again,
Twin
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Twin 1,
This may be of some help: http://www.unicopter.com/B285.html
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>"2 heads are better than 1"<
Two questions;
This may be of some help: http://www.unicopter.com/B285.html
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>"2 heads are better than 1"<
Two questions;
- Are you referring to rotorheads?
- May I plagiarize this, occasionally?
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Inverted Helicopter Flight
At the farnborough show today during the Apache display where it was doing loops, the thought came to me.
When a helicopter is inverted during a loop, is it in controlled flight? In a loop a helicopter will fly from normal to inverted to normal flying position so is the inverted stage an controlled section of flight or is physics just moving the a/c back to an position where the pilot is in control?
By controlled i mean that the pilot can make positive inputs to the a/c as they can do in the normal flying position
I ask the question because i cannot see how the rotors would work in the inverted position...
(my excuse for the weired question is the sun... i'm a nice shade of pink now after today )
Thanks in advance!!
z...
When a helicopter is inverted during a loop, is it in controlled flight? In a loop a helicopter will fly from normal to inverted to normal flying position so is the inverted stage an controlled section of flight or is physics just moving the a/c back to an position where the pilot is in control?
By controlled i mean that the pilot can make positive inputs to the a/c as they can do in the normal flying position
I ask the question because i cannot see how the rotors would work in the inverted position...
(my excuse for the weired question is the sun... i'm a nice shade of pink now after today
)Thanks in advance!!
z...
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Getting inverted in a helicopter is simply a matter of inertia - you make the appropriate input and wait! It is most definately not contollable whilst the thing is inverted, in fact some very curious things can happen if you try! Most helicopters can do the manoevre but the limiting factor tends to be the oil in the various sumps - It starts off in the right place but ends up somewhere else! so as you roll out the CWP lights up like a xmas tree.