A160 Hummingbird rigid rotor design [Archive]

Ian Corrigible

23rd Dec 2006, 23:45

IDR gave the best description a couple of years back:

The unique and Frontier-patented feature central to the A160 is that the revolutions per minute of its main rotor can be reduced to as little as 40% of its maximum value, reducing drag and fuel consumption by half. This cannot be done with a conventional articulated rotor because it would lead to catastrophic vibration, so the A160 has a fully rigid rotor. The blades change in pitch, but the blades and hub are hingeless and stiff in the flapping plane - the hub is a solid forging from AerMet 100 high-strength steel - so that the aircraft is controlled as much like a fixed-wing aircraft as a helicopter. Roll and pitch moments are transferred directly from the rotor to the vehicle.

Reportedly, tip speeds dip as low as M0.25. The A160 can operate at a range of rotor RPMs: the original three-blade carbon-graphite optimum speed rotor spanned 150-350 rpm, while the new four-blade OSR system runs at 200-400rpm.

The other interesting part of the program is the ultra-efficient 650shp coupled diesel being developed: see OPOC thread (http://www.pprune.org/forums/showthread.php?t=235125).

I/C

Dave_Jackson

24th Dec 2006, 00:18

I think that both answers are correct. I.e. the ability to widely vary the RRPM without damaging oscillation, plus the ability to operate at very slow RRPMs without excessive coning.

The related patent is US 6,007,298. As I recall, Karem also received a later patent that is almost identical to this one, except that it is for a tiltrotor.

slowrotor,
The idea could well be a good one, however it may not be meet your objectives. I believe the intent is for this rotor to operate near the optimum lift/drag ratio. This maybe ideal for an unmanned UAV but the increased possibility of stalling the rotor is probably not too attractive for a maned craft.

Dave

unstable load

24th Dec 2006, 13:28

The blades change in pitch, but the blades and hub are hingeless and stiff in the flapping plane - the hub is a solid forging from AerMet 100 high-strength steel

It looks to me like the blades do not have accomodatin for pitch changes either. The rectangular fitting at the root end cannot rotate about the centre of the hub in any way that I can see. :confused:

Are we seeing the first of the "intelligent" blades that will accomodate pitch changes along their length by variable twisting? Interesting times.:D

Graviman

24th Dec 2006, 16:12

I notice that the Program Manager Steve Glusman worked on Comanche. This head looks roughly how i envisioned that head to look, from various descriptions. From the TV program "Ultimate Helicopter" i remember freeze framing the image, but never seeing a pitch link system. At the root is a "damper system", which looks to me as if it also acts as an electric pitch actuator. That ring at the base is either an azimuth sensor, power slip ring (either DC brushed AC brushless) or both.

I gather it can also absorbs blade flexural modes, so maybe blade torsional wavespeed has been tuned to to equal blade centrifugal tension wavespeed at midrange rpm. This means that through active pitch control the blade dynamics can be damped down at the aerodynamic source - this requires intense real-time computational modelling of blade dynamics. Alternately there may just be a damping system built into the blade root. :\

Since Comanche achieved either 12% or 15% effective offset hinge (i forget which), then Nr change induced coning could be accomodated by blade flexure alone. The Nr range would allow efficient silent hover, while beeping up for forward flight or manouvre.

----

Ian, somehow i missed that OPOC thread. Don't quite see/remember how crank mech works, but it is this designs best feature. The debate about 2-stroke vs 4-stroke is an old one, and for my money 4-stroke wins. The power advantage for a given displacement is at best x1.5, which becomes less as turbo charging does more of the compression (since turbine captures more energy in 4-stroke). Also at high RPM 2-stroke scavenging struggles, so the power advantage is further reduced.

This design will suffer from the 2-stroke need to lubricate the piston rings, so that you cannot help but let oil into combustion chamber or exhaust. The emission standard is not quoted, so i can only assume they are relying on helicopters having loose standards. This is the main reason 2-stroke are being dropped for land/sea machines (although there are some interesting designs carried over for historical reasons). Partially burnt oil is absolutely fowl, believe me. :}

Mart

slowrotor

24th Dec 2006, 16:14

Thanks Ian,
I wonder if the A160 rotor has large tip weights to help add centrifugal force at the low rpm.
With no flapping, the dissymetry of lift in forward flight must be equalized solely with cyclic pitch.
Also, why not make the forged steel rotor head larger to save weight?
Dave,
I think low rpm is good for efficiency and needed for entry level rotorcraft. The cruise speed would be slow. The helo would not be for transportation, but only sport and training or other local uses.
Thanks,
slowrotor

I looked at the patent and found the blades do not have tip weights. The patent is for a rotor blade design that is thick and stiff and light but apparently requires a hingeless hub flexbeam (at least thats how I interpret the patent legal jargon) Thank you Dave,, for the patent number

IFMU

24th Dec 2006, 18:07

To me, it looks like a tension torsion strap sticks out of the hub to restrain the blades from flinging off, while allowing the blade to pitch, The root end of the blade just pivots on bearings which are housed in the part of the head you can see. I'm guessing the pitch horn is on the root end of the blade. You can see the swashplate in a fairly conventional position.

This is different from Commanche, which was a bearingless MR design. All the classic degrees of freedom are achieved through a flex beam.

A rigid rotor doesn't flap, except by aeroelastics. It is hardly a new idea. The first was probably Juan de la Cierva's early autogyro, which tended to roll over and crash as speed increased. This ulitimately led to the invention of the flap and lead-lag hinges, much to the benefit of helicopters to follow. Additionally, the Sikorsky XH59 (ABC) had fully rigid rotors.

It will be interesting to see how well the Hummingbird works in forward flight. In a coax, like the ABC, the dissymetry of lift is canceled between the rotors, so it doesn't roll into the ground with increasing speed. Recently, the Phantom Works balled up the second Canard Rotor Wing, another rigid rotor. I wonder if they didn't have enough cyclic to counter the rolling moment from the rigid rotors, and merely did a historic reenactment of Juan de la Cierva's early work. In theory, you can stick in enough cyclic to make up for the missing flapping hinge, though in practice rotor flapping does an awful lot to equalize the dissymetry of lift.

-- IFMU

Graviman

24th Dec 2006, 18:47

That'll learn me IFMU - something about keeping it simple. :ouch:

I'm still puzzled, since i can't see any pitch links in there, and the root seems to have more going on than just attachment to the hub via tension-torsion straps.

Mart

Ian Corrigible

24th Dec 2006, 18:56

Mart,

APT is developing the OPOC engine for FEV Engine Technologies, so FEV’s website (http://www.fev.com/content/public/default.aspx?id=447) might provide some additional insight into the type of supporting tech you'll see going into the powerplant. As if the OPOC design, the converted 4cyl & 6cyl Subaru boxer engines and Pratt’s PW207 turbine weren’t enough engines for one program, there is a fourth manufacturer developing a 6-cyl diesel for the Hummingbird: Nivek R&D (http://www.nivek-rd.com/index.html).

Re: Comanche, as IFMU states the rotor system was designed around a composite flexbeam which allowed all but the thrust bearings to be eliminated. Pitch control rods were retained. IIRC, the offset hinge was 15%.

Slowrotor,

Can’t answer whether or not the A160 uses heavy tip weights. I know the team has emphasized the fact that the tip of the rotor is more flexible than the root in terms of flap, lag and torsion.

With regards to the rotor head, everything about this vehicle has been specifically designed to meet very exacting long-endurance goals, so lower drag was traded-off wherever possible. Airflow interference with the mast-mounted mission pod probably also influenced the numbers. As Dave Jackson notes, it’s a very mission-specific design.

Another supporting technology publicly revealed by the DoD is the use of “high-speed electrical actuators [which, in combination with the hingeless rotor] allows for precision control and improved performance in turbulent flight conditions.”

I/C

Dave_Jackson

24th Dec 2006, 19:35

It appears that the rotor may have individual blade control, by electrical rotary actuator.

..
slowrotor,

A potential advantage of 'Absolutely' ;) rigid rotors for your objective is that the craft should perform as an airplane if it stalls. In other words, if there is sufficient elevation, the nose will drop and RRPM will then increase.

This may necessitate twin main-rotors. However, twin main-rotors offer many advantages;

Here is an serious concept;
An interleaving rotorcraft with very rigid rotors.

Slow RRPM rotors, with small aspect ratio blades to minimize the length of the braces between the rotors and the fuselage.
Conventional pitch-arm and swashplate flight-controls. No active blade twist.
Implement the simple ABC concept to minimize the downwash on the fuselage and braces.
Arrange the power-train so that a pusher-prop could be implemented in the future, if desired.
Etc.
Etc.
Etc.Simple, low cost, much easier to fly, and probably much safer.

..
Mart;

The intermeshing offers many advantages, however the attractiveness of the interleaving increases as one looks deeper and deeper into it.

Dave

Graviman

24th Dec 2006, 22:22

Dave,

It appears that the rotor may have individual blade control, by electrical rotary actuator.
This is what the picture looked like to me. Maybe this is the trial of Ian's "High speed electric actuators". Maybe the resolution is just too poor, but i wondered about that cable.

A potential advantage of 'Absolutely' ;) rigid rotors for your objective is that the craft should perform as an airplane if it stalls. In other words, if there is sufficient elevation, the nose will drop and RRPM will then increase.

If a heli rotor stalls you fall from the sky like a stone - there is NO mode of recovery. The only way to avoid this would be designing a "pushable" collective for -ve pitch to force rotation (like windmilling a prop).

The intermeshing offers many advantages, however the attractiveness of the interleaving increases as one looks deeper and deeper into it.

At the serious risk of threadjacking an already interesting subject:
I am a believer in identifying the problem before attempting to design the solution. Coaxial is the most flexible system but intermeshing has hub packaging and possible shaft weight advantages. I champion neither, but recognise both as solutions to the problem of high speed heli flight. Remember my interest here has always been to identify the technological lmlitations of future helicopter development.

----

All fellow Rotorheads,

Have a Merry Xmas, and thank you for developing my understanding of rotorcraft!!! :ok:

Mart

Dave_Jackson

25th Dec 2006, 02:02

Mart,If a heli rotor stalls you fall from the sky like a stone - there is NO mode of recovery.
Not so fast. :)
You recover from a stall in an airplane by lowering the pitch of the wings.
You recover from a stall in a helicopter by lowering the pitch of the blades.
Remember, the blades are 'Absolutely' rigid. They will not 'fold up'.

The only way to avoid this would be designing a "pushable" collective for -ve pitch to force rotation (like windmilling a prop). A possibility. Another possibility would be to install a rotor-governor and not let the rotors stall.

identifying the problem before attempting to design the solution.
I looked at Nick's book 'No Free Lunch'. It says that all four configurations (Side-by-side, Interleaving, Intermeshing and Coaxial) have their pros and cons vis vie each other. It suggested getting a piece of paper and drawing four columns, then ........ :O

Dave

IFMU

25th Dec 2006, 02:10

It appears that the rotor may have individual blade control, by electrical rotary actuator.
Dave,
This is what the picture looked like to me. Maybe this is the trial of Ian's "High speed electric actuators". Maybe the resolution is just too poor, but i wondered about that cable.
Guys,
No way. They are flying the hummingbird, at least in hover. Individual blade control is still a couple of decades behind phasers and transporters. I'll believe in Santa Claus before I believe they are using IBC on that thing. Those wires are more likely instrumentation. With a prototype aircraft you are going to want to know the forces on this new rotor system.
The pictures are poor, but look at them and I'll try and clarify my guess as to how it bolts together. The tension torsion strap is secured to the head with one bolt, and to the blade with one bolt. When it is bolted into the head, it sticks out of the hole that you see in the end of the head, and goes out a few inches. You slide the blade over the end of the head, the tt strap slides into the blade. You sock the blade down to that rectangular fitting, the tt strap hole lines up with a hole in the blade root. Sock a bolt through said hole and the blade is restrained against CF. Now, I'm guessing that somewhere near the rectangular fitting is the pitch horn, or an attachment (read: threaded holes) for the pitch horn to bolt to. This pitch horn would be attached to the swashplate visible in the crappy picture with a very conventional pitch link.
Merry Christmas to all.
-- IFMU

Ian Corrigible

25th Dec 2006, 02:40

They are flying the Hummingbird, at least in hover.

Although it didn't get much publicity, the design has already logged a 12-hour, ~1,050 nm flight in a circuit at FL40 (which culminated in the fleet's third crash...).

Merry Christmas, Happy Hanukkah, Jolly Kwanzaa or whatever the locals are celebrating where you're flying today.

I/C

Graviman

25th Dec 2006, 10:33

Mart, Not so fast. :)
You recover from a stall in an airplane by lowering the pitch of the wings.
You recover from a stall in a helicopter by lowering the pitch of the blades.
Remember, the blades are 'Absolutely' rigid. They will not 'fold up'.

Dave, you recover from blade stall in a helicopter by crashing into the ground, and becoming another statistic in not letting Nr drop too low during a throttle chop or inoperative donk. Why do you think autogyros spin up their blades before flying? The collective pitch varies between roughly 0' to 20', since autorotation requires some AOA to maintain lift. This way the Lift vector is vertical so the induced drag is not slowing Nr.

Another possibility would be to install a rotor-governor and not let the rotors stall.

Yes, this is the crux of my discussion with BugDevHeli. I have not yet heard from him via PM (if you're out there Bug), but won't discuss concepts further.

...all four configurations (Side-by-side, Interleaving, Intermeshing and Coaxial) have their pros and cons vis vie each other.

Yes, but you have not come up with a single technical justification for SBS or interleaving. A single teetering rotor has dihedral as a result of the aero-gyroscopic forces, and "rigid" rotors are not so far removed in behaviour (but feel less stable). The only problem is rotor dynamic response, and the fact that the cyclic does not auto centre. The Honeywell SPZ7600 system Nick sited takes care of that.

----

No way. They are flying the hummingbird, at least in hover. Individual blade control is still a couple of decades behind phasers and transporters. I'll believe in Santa Claus before I believe they are using IBC on that thing. Those wires are more likely instrumentation. With a prototype aircraft you are going to want to know the forces on this new rotor system.

HeHeHe - point received and understood! Thanks for the explanation - it does make much more sense now. :ok:

----

From earlier post - too much Xmas spirit. ;)

...As if the OPOC design, the converted 4cyl & 6cyl Subaru boxer engines and Pratt’s PW207 turbine weren’t enough engines for one program, there is a fourth manufacturer developing a 6-cyl diesel for the Hummingbird: Nivek R&D.

I'll watch that space Ian. I suspect 2-stroke will survive in aero for a while after 4-stroke is standard everywhere else, just for that possible x1.5 weight advantage. Ultimately Extreme boost turbo charging is the way for smaller aero engines to go, and poppet valves are the only way to reduce oil loss. The auto industry flirted with 2-strokes in the 90's, but tier 4 emissions clamped that pipedream to reality....

Merry Xmas all!

Mart

slowrotor

25th Dec 2006, 15:34

I am still trying to figure out if the Hummingbird has an "absolutely rigid" rotor as Dave would say. Or does it have a hub flexbeam.

There is mention of a flexbeam in the patent. But why have a stiff blade and flexable hub? The patent also says the rotor works from 0 to full rpm. That would suggest an absolutely rigid rotor.

This is of interest to me as I am looking at the idea of an absolutely rigid rotor, a slow rotor that does not have much centrifugal force.

Graviman

25th Dec 2006, 17:08

Dave_Jackson

25th Dec 2006, 17:38

IFMU,

You make good points. The blade root just seem to have a larger diameter than what should be required for the radial bearings or the blade strength. All very interesting.

Mart, :confused: :confused: :confused: :ugh:
Dave, you recover from blade stall in a helicopter by crashing into the groundForget the generalistic 'statement of fact'. Please give a reason why a helicopter with 'absolutely' rigid rotors will crash into the ground.

Why do you think autogyros spin up their blades before flying?HOW do you think autogyros spin up their blades before flying? (excluding prerotors)

A single teetering rotor has dihedral as a result of the aero-gyroscopic forces, and "rigid" rotors are not so far removed in behavior (but feel less stable). The only problem is rotor dynamic response, and the fact that the cyclic does not auto centre. The Honeywell SPZ7600 system Nick sited takes care of that.

What are you trying to say?

An absolutely rigid rotor is just not achievable Who says that an 'Absolutly' rigid rotor (http://www.unicopter.com/B329.html#ARR) is not achievable.

slowrotor,

Just an observation. The vertical distance from rotor to fuselage (center of gravity and drag) is decreased as the rotor's rigidity is increased. The picture of the Hummingbird shows a rotor that is very close to the fuselage.

I will list out a few reasons why the interleaving MAY be attractive.

Dave

Dave_Jackson

25th Dec 2006, 20:21

slowrotor,

Here is some information that may be of interest; (http://www.unicopter.com/1447.html)

Dave

slowrotor

25th Dec 2006, 22:17

Dave,
Most of the development in very stiff rotor research , such as Sikorsky's ABC, was for higher cruise speed. But a stiff blade would also be needed for any rotor that has a low tip speed, such as the human powered helos. Most of the early designs did not use centrifugal force, they were wire braced.

I wonder if anyone has built a flying replica of the first helicopter?

IFMU

25th Dec 2006, 23:07

Slowrotor,
An absolutely rigid rotor is just not achievable, since we are dealing with large inertias at relatively high RPM. Comanche achieved 15% effective hinge offset, and has about the most rigid rotor practical. This reduces the azimuth lead angle from 90' to about 75', but we are still dealing with an aerogyroscopic system.
The flexbeams mentioned in the patent with be the tension torsion packs that IFMU discusses. These are basically a pack of steel laminate "feeler guages", which transfer bending loads and tension but allow torsional freedom. The result is a high effective hinge offset. The only potential downside is that they can suffer reduced fatigue life from lead-lag forces, since guages can buckle with in plane bending.
Aerokopter Sanka example mentioned under title "Rotor System" (http://www.aerokopter.co.za/Technical-Details-and-specifications.htm)
Mart

I think that the XH59 would have been stiffer than the commanche. A tension-torsion strap is made up of steel feeler gauges, but a flexbeam is typically composite. The tt strap is very stiff in centrifugal, more pliable in twist, and the bearings take up all the bending loads. A flexbeam allows the rotor to move in lead lag, flapping, and pitch. It is a different animal.

-- IFMU

Dave_Jackson

26th Dec 2006, 02:22

Originally posted by slowrotor;
Most of the development in very stiff rotor research , such as Sikorsky's ABC, was for higher cruise speed. But a stiff blade would also be needed for any rotor that has a low tip speed ...:ok: Now that's 'thinking outside the box'.
____________________
...
A few interesting quotes from the Sikorsky ABC reports:

"The result is that the aircraft is easy to fly, giving the pilot purer responses to his inputs" From ~ An ABC Status Report - Linden & Ruddell - After Jan 1981

"The high ABC blade bending and torsional stiffness permit such small elastic deflections that potentially unstable pitch-flap-lag couplings are precluded." From ~ Advancing Blade Concept (ABC) Dynamics - Abbe & Blackwell - May 1977

"Another indication of blade stiffness is the static deflection of the blade under dead weight load. For a typical ABC blade the static deflection is only about 0.2 percent of the blades radius. .[I]" As well, if the spar material was changed from titanium to boron fiber then the weight would be 0.74/1 and the deflection would be 0.46/1 as much." From ~ The ABC Helicopter-
M.C. Cheney Jr. - Feb 1969
__________________
...
You may not want the rotor to go too slow. As you know, even the airplane can loose a lot of altitude recovering from a stall.
A theoretical discussion of stall recovery with 'absolutely' rigid rotors might make for an interesting thread.
__________________

PS. Looking for a place to build the blade? :)

http://www.unicopter.com/Temporary/CNC_Table.gif

Graviman

26th Dec 2006, 10:49

Forget the generalistic 'statement of fact'. Please give a reason why a helicopter with 'absolutely' rigid rotors will crash into the ground.

Dave, rigid or compliant blade it makes no difference. If rotor RPM drops to the point where the blades are stalled there is no way of producing the torque required to continue rotation. Since collective only goes to ~0' pitch for autorotation then you can not easilly windmill start a heli rotor. This was discussed a while back, and is the result of the autorotation updraught requiring +ve AOA to produce lift (induced drag for finite rotor diam).

HOW do you think autogyros spin up their blades before flying? (excluding prerotors)

Whenever i have seen one, the pilot always spins up the rotor by hand first. Prerotors must require a shorter ground run to achieve full rotor spin up from the horizontal "wind" component. This may not be a good comparison anyway, since autogyro blades are optimised for flying in permanent autorotation so will be easier to windmill start.

What are you trying to say?

Your main arguement for lateral symetry is improved flyability. I am showing that the two key elements of static and dynamic stability may be addressed in a conventional rotorcraft. I don't see SAS as a band-aid, any more than your car's ABS or engine management system (and besides i was originally proposing a reliable mechanical solution).

The only real justification for a counterrotator is the retreating blade stall limiting high speed flight.

Mart

Dave_Jackson

26th Dec 2006, 17:24

Mart,

OK let's have a theoretical discussion about autorotation, as it relates to a theoretical 'absolutely' rigid rotor (ARR). This may be fun because I certainly don't know the answers. Perhaps no one knows.

You say;
rigid or compliant blade it makes no difference. I beg to differ. At the entry into autorotation the norm is to pull back on the cyclic, while lowering the collective. The backward movement of the cyclic is to counter the blades desire to flap down at the front, due to the reduced rotor speed. The ARR cannot flap down, therefore there MAY be no need to do anything with the cyclic. Remember that this elimination of the downward pitching moment on the craft will not be replaced by a rolling moment, since the two counter-rotating rotors will cancel out each others desire to roll.

If rotor RPM drops to the point where the blades are stalled there is no way of producing the torque required to continue rotation. We are talking about an ARR therefore the blades are not going to come off. Now, assuming we have enough elevation, point the nose of the craft at the ground.

If the engine is operational, it can recover the RRPM and then the craft can slowly pull out of the dive.

If the engine is not operational then shallow angle of the airflow coming upward through the disk should be similar to your gyrocopter traveling down the runway building RRPM before takeoff. In this case the craft would be VERY SLOWLY pulled out of the dive. Remember that both the helicopter and the gyrocopter can autorotate. The only difference is that the gyrocopter's blades have a positive twist off 1º. Give the ARR some Active Blade Twist and the rotor may perform better that the gyrocopter's rotor.

All hypothetical.

The only real justification for a counterrotator is the retreating blade stall limiting high speed flight. You forgot the tail-rotor.
Hopefully, everyone will eventually forget the tail-rotor. :)

Dave

unstable load

26th Dec 2006, 21:57

We are talking about an ARR therefore the blades are not going to come off.

Dave,

I am an not versed in pilot matters, so pardon me in advance if this is a particularly inane question to you guys who fly them.....
surely the concern if blades stall is more a lack of lift and the lessened ability to regenerate sufficient RRPM in auto to regain the lost RPM?
I realise that at seriously degraded RPM there will be the problem of loss in rigidity of the blades and subsequent departure of the blades, but how low in the rev range will this occur (notwithstanding said ARR)?

Graviman

26th Dec 2006, 23:25

...surely the concern if blades stall is more a lack of lift and the lessened ability to regenerate sufficient RRPM in auto to regain the lost RPM?

Yup, spot on UL!

I realise that at seriously degraded RPM there will be the problem of loss in rigidity of the blades and subsequent departure of the blades, but how low in the rev range will this occur (notwithstanding said ARR)?

It will be gradual, but as long as the blade can handle the strain (ie deflection) i don't see a problem here. ARR blades would be stiffer which is good. The stiffer the blade the higher the natural frequency of the blade flexing above&below the hub plane - so that it it higher than natural pendullum oscillation. Engineers (and TPs) relate this to a number called the effective hinge offset, which is the the equivalent hinge offset as a percentage of blade radius. Comanche achieved 15%, but reducing Nr will increase offset. There are many texts on this subject, with my preference being RW Prouty.

----

Dave, i'd rather this was bounced off guys with more heli flight experience than myself (M'aidez!). Theoretical exercises often fail to capture the important dynamic characteristics.

OK let's have a theoretical discussion about autorotation, as it relates to a theoretical 'absolutely' rigid rotor (ARR). This may be fun because I certainly don't know the answers. Perhaps no one knows.

Well, having done auto and VRS exercises with a heli pilot who has a lot more experience than myself i felt reasonably inclined to listen to his advise during the post flight briefing. The rest is just 2nd year aeronautics and common sense.

I beg to differ. At the entry into autorotation the norm is to pull back on the cyclic, while lowering the collective. The backward movement of the cyclic is to counter the blades desire to flap down at the front, due to the reduced rotor speed. The ARR cannot flap down, therefore there MAY be no need to do anything with the cyclic. Remember that this elimination of the downward pitching moment on the craft will not be replaced by a rolling moment, since the two counter-rotating rotors will cancel out each others desire to roll.

Drop collective first, then flare to compensate for loss of flapback and to convert any forward momentum into lift. Nr will also go up slightly as the rotor cones. Don't forget to unload pedal anti-torque. Agreed rigid rotor will alter dynamics, but i have only flown teetering. :sad:

The problem is that the collective only goes down to 0' pitch, so if Nr drops to rotor stall you may as well whistle Dixie for all the good fiddling with the cyclic will do.

We are talking about an ARR therefore the blades are not going to come off. Now, assuming we have enough elevation, point the nose of the craft at the ground.

And if Nr has dropped to the point of rotor stall, and the collective only goes to 0' pitch? I used to enjoy doing stall/spin reversals in wooden gliders, but heli dynamics are quite different. As i say for a stalled rotor, rigid or teetering, the outcome in inevitable.

If the engine is not operational then shallow angle of the airflow coming upward through the disk should be similar to your gyrocopter traveling down the runway building RRPM before takeoff. In this case the craft would be VERY SLOWLY pulled out of the dive. Remember that both the helicopter and the gyrocopter can autorotate. The only difference is that the gyrocopter's blades have a positive twist off 1º. Give the ARR some Active Blade Twist and the rotor may perform better that the gyrocopter's rotor.

Gyros do not have collectives, so can never get into a rotor stall situation. If the rotor has stopped, and collective only goes to 0' pitch, and donk has quit, where is this magical rotational force going to come from?

You forgot the tail-rotor.
Hopefully, everyone will eventually forget the tail-rotor.

Indeed and positive yaw authority will just be a distant memory in the programmers software.

Mart

IFMU

27th Dec 2006, 01:53

I beg to differ. At the entry into autorotation the norm is to pull back on the cyclic, while lowering the collective. The backward movement of the cyclic is to counter the blades desire to flap down at the front, due to the reduced rotor speed.
Dave,

Are you a helicopter pilot? If not, I think the single biggest thing you could do to help your quest of designing a helicopter is to go out and get your private helicopter rating. Even if it means flying with a tail rotor, nobody will hold it against you.

See, as a helicopter pilot, I would see the entry into auto a little differently. Collective is used to hold RPM. Cyclic is used to adjust pitch attitude, and therefore capture an airspeed. Why does the nose pitch down? In S&L flight, just before the auto, there is a balance of forces and moments. One moment is the rotor thrust pulling up, near the CG. When you drop the collective, this big up force (equal to the weight of the helo) goes away and the nose drops.

In all honesty, I am no super experienced helo pilot. I have about 60 hours of helo time, not counting RPVs. These days, I get about 2 hours of helo time a year, thanks to the benevolence of others. But I tell you this: getting the rating was the single biggest thing I did to improve my fundamental understanding of helicopters. There are plenty of engineers who wouldn't know a helicopter if they bumped into one while walking along looking at their feet.

-- IFMU

Dave_Jackson

27th Dec 2006, 04:44

unstable load;

It appears that 400 fps is the minimum tip speed to have sufficient stored kinetic energy for autorotation. ~ Prouty

IFMU;

You've got about 60 more hours on helicopters than I have. :) When I first got into flying I bought an airplane before starting to take flying lesson. Not wanting to be 'normal', I initially derived a perverse pleasure from the possibility of building an easy to fly helicopter before learning to fly the thing.

Perhaps the most sensible reason for not getting a license is there are hundreds of pilots on this forum who collectively have far, far more knowledge about piloting then I could ever hope to acquire. Their contributions are much appreciated.

Mean while back in the land of theory; :rolleyes:

Mart and IFMU;

It appears that the three of use are talking about the same thing, but using different words, when it comes to the cyclic and autorotation (My source is Prouty ~ Vertiflite - Fall 2003 :), which I should have taken the time to acknowledge.)
Mart said;
As i say for a stalled rotor, rigid or teetering, the outcome in inevitable.
&
Gyros do not have collectives, so can never get into a rotor stall situation. If the rotor has stopped, and collective only goes to 0' pitch, and donk has quit, where is this magical rotational force going to come from?

Don't give up yet.

I asked the guys on the Rotary Wing Forum if prerotation (by hand or pre-rotor) was necessary before a gyrocopter started its take of roll. They said 'Yes' and also mentioned that a rotor would turn in reverse if the craft sat facing the wind.

One post mentioned a minimum speed of 50 RRPM before starting the ground roll. In addition, a video showed a rotor turning at approximately 30 RRPM before starting the ground roll. This works out to a tip speed of only approximately 60 fps.

IMHO, it appears that conventional helicopter blades must have 400 fps to not collapse, however, if they are rigid enough the rpm could go down to 60 fps before there was a problem. This being the case; a slight negative pitch [as Mart mentioned] might overcome the problem.

I still believe (suspect) that if the engine was still operational the RRPM could be regained, as previously mentioned.

In addition a rotor governor (http://www.unicopter.com/Governor.html)should negate any chance of a problem, unless it is the pilot's time for the Darwinian award.

Dave

Graviman

27th Dec 2006, 11:58

IFMU, thanks for answering my M'aidez - In way over my head there! :ok:

Dave, i'm glad we've got that one sorted. Agree that -ve pitch (maybe against a spring) and a rotor govenor will help. I am hoping Bug will take up the concept i proposed, since it would greatly reduce the likelyhood of pilots being caught out. There was a good thread on this, which i took a copy of before removing my posts. I won't discuss the concept further until Bug indicates his intentions.

I am with IFMU though about you getting some heli time. I really can't afford to get as much experience as i would like, but have made a point of getting some heli experience just so i could have a better understanding of any points explained by experienced pilots - the gliding experience meant i was quickly able to progress. This forum (and Nick Lappos in particular) has been a goldmine in learning about rotorcraft. :ok:

I once offered to work for Westlands for free just to get some heli industry experience, but timed my original graduation wrong so began in the auto industry. I like to think that at some point i will be able to seemlessly move over to the heli industry, with a CV that demonstrates the required engineering knowledge and experience. Considering my primary source of heli knowledge, this is most likely to be Sikorsky. Time will tell, and i have a great deal of work to do before i am ready.

Mart

IFMU

27th Dec 2006, 17:02

IFMU;
... When I first got into flying I bought an airplane before starting to take flying lesson. Not wanting to be 'normal', I initially derived a perverse pleasure from the possibility of building an easy to fly helicopter before learning to fly the thing. Dave
There's a difference in philosphy. Before I began building my airplane, I got my license and some experience so I would know what the heck I wanted.
-- IFMU

Dave_Jackson

27th Dec 2006, 18:12

The literature on the subject of autorotation appears to suggest that trying to explain the vectors, along the span of the blade at the different azimuths, during autorotation is as much an art as it is a science;

Here is a later and different response to the gyrocopter question.It depends on the type of blades.
Iv seen glass blades usually start to spin backwards in a gentle wind whether the stick is full back or centered forward.
The alu ozy blades i use always spin forwards.

On a day with strongish wind, when i had plenty of time to waste, i managed to get them to a speed where they would have gone on to flyn speed, without touching them. Just a lot of patient trimming with the stick.
________________

More dry theoretical stuff:

If a rotor consisted of symmetrical blades (say NACA 0012), the blades had 0-deg pitch and no twist, I suspects that an airflow parallel to the disk will rotate the rotor in the correct direction. This is because the profile drag of reverse flow on one blade will be greater than the profile drag of conventional flow on the blade on the other side. :8

Dave

Graviman

27th Dec 2006, 20:57

Corrected some hastily typed goofs - please reread before responding...

The literature on the subject of autorotation appears to suggest that trying to explain the vectors, along the span of the blade at the different azimuths, during autorotation is as much an art as it is a science;

Dave, It is a very well understood science, but not always well explained. Often what is forgotten is that the rotor is of finite length, and so will be operating in an effective downwash from the rotor edge vortices. This is the same thing as tip vortices on a fixed wing, requiring an increase in pitch for the overall downwash - which is known as induced drag. This effective downwash offsets the autorotation upwash, which is why the local airstream reduces velocity through the rotor. The increased pitch required to counter this effective downwash means that the blade will generate +ve lift in an updraught at ~0' pitch, so the rotation just continues with no torque input.

Besides, an easy check is that the startup/shutdown procedures are done with collective in same position as auto. ;)

If a rotor consisted of symmetrical blades (say NACA 0012), the blades had 0-deg pitch and no twist, I suspects that an airflow parallel to the disk will rotate the rotor in the correct direction. This is because the profile drag of reverse flow on one blade will be greater than the profile drag of conventional flow on the blade on the other side.

In theory yes, but most of the drag is induced so it would take a lazy afternoon to get them up to flying speed. A triffle slow if the ground is rushing up to meet you.

This needs confirmation, but does make sense:
It has more to do with the rotor TPP plane being at an angle to the flow, and autogyros being designed with a small amount of -ve pitch for autorotation. The advancing blade offers a low AOA while the retreating blade offers high AOA. Thus retreating blade just acts as a sail "spinica" and causes rotation.
If the gyro has a +ve pitch then the rotor goes backwards...

---

BTW, i wasn't sure if my point about static stability in a conventional heli was picked up earlier. Basically flapback operates in any direction the heli is flying, so can be considered longitudinal and lateral dihedral. It just takes a pilot to hold the cyclic in position, correct for rotor dynamics, and deal with the forces of asymetry. However, SAS as part of AFCS nicely handles these last points.

This is why i believe the complexity of counter rotation is only justified for improving figure of merit for a heli capable of high speed flight.

Mart

Dave_Jackson

28th Dec 2006, 19:50

Mart,

Questioning that which is presented as an 'Educated Assumption' can be constructive and fun.

Questioning that which is presented as a 'Statement of Fact' takes the argument up a notch or two.

I defer to you superior factual grasp of the subject.

Dave

Graviman

28th Dec 2006, 23:21

Well maybe, Dave. I figure we're all here to learn about helicopters. :ok:

I did consider starting another thread for your earlier point:

The intermeshing offers many advantages, however the attractiveness of the interleaving increases as one looks deeper and deeper into it.

What i am pointing out is that you have come to the conclusion about interleavers because of your conviction about absolutely rigid rotors. By allowing the rotor to have only a finite effective offset hinge then the rotors do not need to be seperated to produce lateral dihedral. This means that Flettner rotation intermeshers with high effective offset hinge rotors offer ideal flight characteristics, including long & lat dihedral, and sideslip/yaw coupling. A coaxial requires a tail vertical stabiliser.

In case you wondered, i am backing the symetry arguement. A single rotor with high offset hinge will suffer cross coupling, and more neutral static stability. A single teetering rotor is statically stable, but at the cost of control at reduced g. Counterrotating rotors with high offset hinges will allow cancellation of cross coupling while maintaining static stability. Dynamics may still require SAS though.

The tail rotor arguement can still only really be won by using twin pushers to replace the yaw authority. You stated a while back you were looking for the ideal rotorcraft without stability band-aids. For GA usage i would go with Flettner rotation intermeshing, with twin pusher props - more-or-less your original concept...

Mart