HobbyKing.com New Products Flash Sale
Reply
Thread Tools
This thread is privately moderated by mnowell129, who may elect to delete unwanted replies.
Old Nov 07, 2010, 08:14 AM
RC Group therapist
Boneswamped's Avatar
DeRidder LA.
Joined Nov 2008
1,961 Posts
Quote:
Originally Posted by Sky-walker View Post
Hi mnowell129

I have read most of what you wrote beside my previous knowledge in rotorcraft aerodynamics. But I have a kind of an interesting buggling question that I want to answer;

Is the cyclic of an "autogyro" REALLY the same of a "swashplate helicopter"

Lets pretend that the swashplate helicopter has no collective, . Let us imagine the heli is fully rigid(yes it is stupid) with no hinges but the feathering.

To keep it simple(I hope): the pilot pushes the stick forward, the "net" result of the autogyro rotor in every cycle(the fore and aft blades are at nose and tail), is as if we took a whole rigid(nonteteering) rotor-hub assembly and tilted it forward a few degrees? The "feathering" incidence of the fore/aft blades there, are zero offcourse (there are none but I write for clearence) correct? (I know I am, if you say no, then you misunderstood me already).

Now, if it was a rigid heli rotor instead, wouldnt the helicopter rotor head vary its feathering hinges whole the time so the front blade is minimum pitch( after traveling 90 degrees from the left position ) and the aft is maximum in pitch where on a gyrocopter the "front and aft blades" are at "zero feathering incidence". Or wait, are the helicopter blades at zero degrees feathering incidence at the front and aft as well???
Well, I will be guarnteed be misunderstood here, it is very hard to explain, these are very complicated matters and even worse when explaining a question.
Hello Sky
Your question sounds as though you're looking to argue, but I'll try to answer it anyway. You have not been misunderstood, of course you can always use that if you don't like my answer. I'm unsure of what your "previous knowledge in rotorcraft aerodynamics" is, but reading through the material Mickey had posted is always helpful. Everything he posts can be backed up, and isn't just pulled from his like a lot of folks on the forum, in short; he's a credible source of info.

To short answer your question; yes Cyclic feathering occurs in swashplate systems as well as flex plates (like I use in most of my models). There isn't a simple way to illustrate this for you, it would take many drawings and formulas to show what is really occuring. The main difference, as you mentioned; is mechanical feathering Vs. aerodynamic feathering. Many people confuse angle of incidence, and angle of attack. When a blade flaps up or down, the induced flow changes, thereby changing the angle of attack without a mechanical incidence change. Tilt a flexplate gyro hub, and you've in effect just changed the angle of attack with the above mentioned effects. In your example; Obviously the mechanical angle of the blades did not change.....they are rigid, but by changing the induced flow of the "Fixed blades" you change the aerodynamic angle of attack. This is the same end state as mechanical cyclic feathering. The two systems obviously have mechanical differences, but they are performing the same aerodynamic functions. Look into blade element theory; which will show you scalors and vectors for a given rotation cycle and snapshot of a given blade element. Might be a bit more than you are interested in, but a partial understanding of aerodynamics leads to incorrect assumptions most of the time.

Why is a fully rigid rotor system stupid?!? The full scale LUH-72/EC-145 helicopter that I fly on a daily basis has a rigid rotor system without lead/lag/flap bearings or hinges. It only has feathering as you described. I better let Eurocopter know the system won't work right away. In seriousness, all the lead/lag/flapping is done in the blade itself by being extremely flexible at the root.

Are you building a gyro currently, or do you already have one? Autogyros are fun to me because so few people in the RC community have flown them with any real success. If it weren't challanging there would be no point in it for me.

Regards,
-Mike
Boneswamped is offline Find More Posts by Boneswamped
Reply With Quote
Sign up now
to remove ads between posts
Old Nov 07, 2010, 02:16 PM
Registered User
Sky-walker's Avatar
Joined Jun 2005
1,025 Posts
Quote:
Originally Posted by Boneswamped View Post
Hello Sky
Your question sounds as though you're looking to argue, but I'll try to answer it anyway. You have not been misunderstood, of course you can always use that if you don't like my answer. I'm unsure of what your "previous knowledge in rotorcraft aerodynamics" is, but reading through the material Mickey had posted is always helpful. Everything he posts can be backed up, and isn't just pulled from his like a lot of folks on the forum, in short; he's a credible source of info.

To short answer your question; yes Cyclic feathering occurs in swashplate systems as well as flex plates (like I use in most of my models). There isn't a simple way to illustrate this for you, it would take many drawings and formulas to show what is really occuring. The main difference, as you mentioned; is mechanical feathering Vs. aerodynamic feathering. Many people confuse angle of incidence, and angle of attack. When a blade flaps up or down, the induced flow changes, thereby changing the angle of attack without a mechanical incidence change. Tilt a flexplate gyro hub, and you've in effect just changed the angle of attack with the above mentioned effects. In your example; Obviously the mechanical angle of the blades did not change.....they are rigid, but by changing the induced flow of the "Fixed blades" you change the aerodynamic angle of attack. This is the same end state as mechanical cyclic feathering. The two systems obviously have mechanical differences, but they are performing the same aerodynamic functions. Look into blade element theory; which will show you scalors and vectors for a given rotation cycle and snapshot of a given blade element. Might be a bit more than you are interested in, but a partial understanding of aerodynamics leads to incorrect assumptions most of the time.

Why is a fully rigid rotor system stupid?!? The full scale LUH-72/EC-145 helicopter that I fly on a daily basis has a rigid rotor system without lead/lag/flap bearings or hinges. It only has feathering as you described. I better let Eurocopter know the system won't work right away. In seriousness, all the lead/lag/flapping is done in the blade itself by being extremely flexible at the root.

Are you building a gyro currently, or do you already have one? Autogyros are fun to me because so few people in the RC community have flown them with any real success. If it weren't challanging there would be no point in it for me.

Regards,
-Mike
Thanx Mike for your reply,

I am not arguing with Mickey. what he wrote is correct. It is just that there is one unclear point in my head that I "can not visualize or really understand" and that is iratating me. No more no less.


Yes, I know that helicopters like LUH-72/EC-145 are "hingeless"
but they are really hinged in that they are "flexible" on all axes instead of using bearinghinges. When I said rigid I meant ofcourse absolutely rigid and that is plain stupid in a functional helicopter but I wanted to use that as an analogy!, like the earlier Hiller coaxial(yes its a different case, because coaxials dont rollover because of dissemetry of lift) and yes a fully rigid single rotor wouldnt fly becuze it will tip over as fast as it gets forward speed.



Any way, I discovered that the best way to understand the part I am wondering is to watch a slow motion camera footage of rotating helicopter rotorblades. Nothing would be beter.. hmm, but where to find such footage

"Are you building a gyro currently, or do you already have one? Autogyros are fun to me because so few people in the RC community have flown them with any real success. If it weren't challanging there would be no point in it for me."

BTW Are you wondering if I own or flew a real gyrocopter?


Kalle
Sky-walker is offline Find More Posts by Sky-walker
Reply With Quote
Old Nov 07, 2010, 03:58 PM
Registered User
Stevie-G's Avatar
Joined Mar 2010
4 Posts
Hi!

Quote:
Originally Posted by Sky-walker View Post
... Or wait, are the helicopter blades at zero degrees feathering incidence at the front and aft as well??? ...
Yes, they are.
Look at this picture:


I hope this helps.

-=Stevie=-
Stevie-G is offline Find More Posts by Stevie-G
Reply With Quote
Old Nov 07, 2010, 04:49 PM
Registered User
Sky-walker's Avatar
Joined Jun 2005
1,025 Posts
Hi Stevie, so if the rotorblades are ultrastiff and if there are no flap hinges at all and no "o-rings in a RC model case," the helicopter wont move anywere to aplied cyclics?
Sky-walker is offline Find More Posts by Sky-walker
Reply With Quote
Old Nov 07, 2010, 08:16 PM
Mickey from Orlando. Really.
Joined Nov 2004
3,524 Posts
The tilting hub gyrocopter puts direct cyclic pitch into the blades. For example when you tilt the hub forward, the advancing blade, when at the right or left side (depending on CW vs CCW rotation) gets some down pitch. This down pitch is zero at the tail, maximum at the side, zero at the nose and positive on the retreating side, this is cyclic pitch. It seems this next point is misunderstood sometimes. This pitch is varying around the circle with respect to the plane of rotation of the blades, not the spindle. The plane of rotation of the blades doesn't change easily, so when the spindle moves it doesn't directly affect the tilt of the rotor, but forces the pitch to vary around the circle in such a way that the rotor moves to the new position perpendicular to to the spindle. Note that the rotor moves to the new position at a speed that is a function of the amount of cyclic and the relative inertia of the blades and other factors such as how far out the flapping hinge is. So the tilting spindle creates the exact same feathering cyclic pitch as the swashplate. The only difference is the swashplate doesn't have to overcome the centrifugal forces that the tilting spindle must. In the previous example when the spindle is tilted forward, the blades, when they are fore and aft try to oppose the spindle motion (in a flapping hinge model). This force must be overcome by the servos. This opposing force on a swashplate model is applied to the shaft, not the servos.
This force feedback fighting the tilt of the spindle is always present with a 3 or 4 bladed model (earlier question) because there is always a blade somewhere over the nose or tail, opposing the tilt. A two bladed rotor however goes through parts of the rotation when the blades are at the sides where nothing is opposing the forward tilt of the spindle, thus there are long periods of time with no force feedback, letting the spindle tilt easily. This allows maximum cyclic input to take place providing much more sensitive control response. In a true two bladed teetering rotor the flapping hinge offset is 0 providing no force feedback in the rotor, thus it will respond very quickly. The two bladed teetering rotor is the most challenging to design and fly simply because it responds much too quickly to control inputs, the usual solution on models is the flybar, which slows the control response down. Now electronics and servos have become fast enough to slow down the control response without the flybar, so now you have model helicopter heads like the most recent photo in this thread. The previous solution to this problem with helicopters was large amounts of tip weight.

The teetering rotor does provide interesting proof that a tilting head steers by cyclic and not by brute force, for with the teetering head the only force that the tilting spindle can apply to the rotor is to change it's pitch, since the teeter prevents the spindle from applying any "direct" force to the rotor.
mnowell129 is offline Find More Posts by mnowell129
Last edited by mnowell129; Nov 07, 2010 at 08:24 PM.
Reply With Quote
Old Nov 08, 2010, 03:05 PM
Registered User
Sky-walker's Avatar
Joined Jun 2005
1,025 Posts
It is a real shame, that real helicopters use ultra complicated swashplates when they could use the very simple teetering hinge. Ofcourse the only reason for the swashplates is the collective. If we would fly at very low altitudes, we really dont need collectives as much and a teetering hinge would work perfectly. introducing a collective mechanism to a teetering hingle is as complicated as installing a full swashplate and therefore they introduced the swashplate as the key of the future. IMHO, a swashplate is delicate and complicated. I would rather ride in a teetering hinge heli-craft(not many helis got that) with a new solution to autorotations.

I am having a hard time finding any helicopters using a tilthub. The only one I found was the BJ "schramm" I think it is called
Sky-walker is offline Find More Posts by Sky-walker
Reply With Quote
Old Nov 08, 2010, 04:20 PM
1.21 gigawatts
EDDIEGIGOWATTS's Avatar
Fort.lauderdale Florida
Joined Jul 2006
724 Posts
Mickey
Thank you so much for this thread
it has help Me understand rotor aircraft to a great extend
never knew how important all this info is till now
Looking forward for more


Eddie
EDDIEGIGOWATTS is offline Find More Posts by EDDIEGIGOWATTS
Reply With Quote
Old Nov 09, 2010, 06:48 PM
Registered User
Stevie-G's Avatar
Joined Mar 2010
4 Posts
Quote:
Originally Posted by Sky-walker View Post
I am having a hard time finding any helicopters using a tilthub. The only one I found was the BJ "schramm" I think it is called
Do you talk about the Helicycle?
Aero-TV: Helicycles Aloft -- BJ Schramm's Legacy (9 min 25 sec)


Although it has tilthub, it also has swashplate.
Like most of UL helicopters.
For example: R-22, Mini 500, Mosquito Air, etc.
The Bell JetRanger has tilthub too.

ps: sorry for my english, I don't speak it too well.
Stevie-G is offline Find More Posts by Stevie-G
Reply With Quote
Old Nov 09, 2010, 07:19 PM
Registered User
John235's Avatar
Sydney, Australia
Joined Mar 2006
1,315 Posts
Thanks for the video. There is a nice view of the teeter-head at 2:02.
John235 is offline Find More Posts by John235
Reply With Quote
Old Nov 09, 2010, 09:55 PM
Mickey from Orlando. Really.
Joined Nov 2004
3,524 Posts
This thread seems to be confusing the teetering head with the tilting spindle/hub.
The helicycle does not have a tilting hub. It does have a teetering head.

Collective is not the only reason for using a swashplate. The tilting spindle mechanism has it's own set of problems.
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Nov 10, 2010, 06:37 AM
Registered User
Stevie-G's Avatar
Joined Mar 2010
4 Posts
Quote:
Originally Posted by mnowell129 View Post
This thread seems to be confusing the teetering head with the tilting spindle/hub.
The helicycle does not have a tilting hub. It does have a teetering head.

Collective is not the only reason for using a swashplate. The tilting spindle mechanism has it's own set of problems.
You are right, I confused the two different things. Sorry for my mistake.
As I said, I don't speak english well.
Online dictionary is not enough help to understand such technical language clearly.

Regards.
Stevie-G is offline Find More Posts by Stevie-G
Reply With Quote
Old Nov 10, 2010, 08:23 PM
Registered User
Sky-walker's Avatar
Joined Jun 2005
1,025 Posts
Stevie, no, it is not the helicycle which is a standard typical swashplate non-tilt helicopter

I made some search but it is hard to find which "heli version" is the tilt-hub. I think it is
Here and is
called Scorpion 2 by rotorway and sometimes called by B.J schramm, so confusing

could be either this
http://www.aviastar.org/helicopters_...orpion_too.php
or
http://www.aviastar.org/helicopters_eng/scorpion.php


You can clearly see it doesnt have the typical swashplate, but has only a seperate "push-link" to the collective
Sky-walker is offline Find More Posts by Sky-walker
Reply With Quote
Old Dec 08, 2010, 07:39 AM
Winging it >
leadfeather's Avatar
Joined May 2006
9,729 Posts
stability vs controllability in rotors

Here is something that's been on my mind; stability vs controllability.

1) If I define stability as the propensity for a rotor to return to level flight, and I define control-ability as the speed that the rotor responds to controls; faster response equaling more controllable. (I know that a "too fast" responding rotor seems uncontrollable;maybe a better term is unflyable; but that is because the pilot is too slow, not because the rotor is not controllable).

Based on these definitions, what forces and what design techniques can improve stability?

It seems that some of the things one can do to make a rotor slower responding so that the pilot can keep up, will also reduce stability. For instance by making the flapping hinges stiffer, the rotor will become slower responding for the pilot, but it will also be slower responding to the pendulum forces of the autogyro trying to level the rotor.
leadfeather is online now Find More Posts by leadfeather
Reply With Quote
Old Dec 08, 2010, 03:06 PM
Registered User
Stevie-G's Avatar
Joined Mar 2010
4 Posts
Quote:
Originally Posted by Sky-walker View Post
Stevie, no, it is not the helicycle which is a standard typical swashplate non-tilt helicopter

I made some search but it is hard to find which "heli version" is the tilt-hub. I think it is
Here and is
called Scorpion 2 by rotorway and sometimes called by B.J schramm, so confusing

could be either this
http://www.aviastar.org/helicopters_...orpion_too.php
or
http://www.aviastar.org/helicopters_eng/scorpion.php


You can clearly see it doesnt have the typical swashplate, but has only a seperate "push-link" to the collective
In fact, the pictures (on the links you gave) show it's NOT a tilt-spindle helicopter.
It has a "normal" swashplate for cyclic control.

Look these closer pics:

(Pictures from here: http://www.used-helicopter.net/for-s...ead-new-2/614/)

I think these are what you were looking for:
Nick's Ultra-Lite Heli Rotor Head 0001 (1 min 56 sec)
Nick's Micro Heli Rotor Test 2 (5 min 35 sec)

AirScooter (0 min 53 sec)

These do have tilting spindle.

Regards.
Stevie-G is offline Find More Posts by Stevie-G
Reply With Quote
Old Dec 09, 2010, 08:20 AM
Winging it >
leadfeather's Avatar
Joined May 2006
9,729 Posts
more on stability/control-ablity

Below are three examples of rotor craft with different stability/control-ability characteristics. I should note that I'm directing this topic at pitch and roll axis only.

1) the 450 helicopter has a 90 degree flybar, very rigid blades (no flapping). It is very controllable but it is unstable. Adding weights to the flybar will make it less unstable but also slows the control response. This helicopter never returns to level on it's own.

2) the Blade SR 120 (best small heli I've ever flown) has a 45 degree flybar, no flap, is very stable and has pretty good control-abilty. At high forward speed I have to put in some left aileron to keep it level; assymetric lift?

3) Mr. Twister has flapping hinge blades. It is very controllable and very stable. No assymetric lift.

Here is where I am going with all of this. I want to make the 450 more stable to make it easier to fly. So far I think I have two choices. 1) Add a flapping function to the blades and 2) change the flybar angle.

I may experiment with both. I was thinking of modifying the head so that I could ground adjust the flybar to any angle to see what effect it has.

I would appreciate thoughts and suggestions from the rotor scholars/experts.
leadfeather is online now Find More Posts by leadfeather
Reply With Quote
Reply


Thread Tools