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Stan, If your out there. I found this forum. It might be of some interest to you.http://www.rcgroups.com/forums/showthread.php?t=324822Joe Carola
Edited 3/16/2007 11:21 pm ET by Framer
Joe: I am on dial up and the link wont connect. Thanks anyway....<G>
Stan
Stan,Here's the first post of this guy on a thread he started. I have know idea what he's talking about. Maybe a hip roof or something.......;-)Jan 20, 2005, 08:45 PM #1
mnowell129
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Join Date: Nov 2004
Posts: 1,051
The challenge of gyrocopter design.
________________________________________
Having studied for some time and designed and test flown my gyrocopters for the past few months I came to a tiny amount of understanding about gyrocopter design. This is the beginning of a thread intended for future designers that I intend to discuss the challenges of gyrocopter design that I have encountered. It may generate a discussion among experienced designers as well and that would be welcome. According to David Ramsey this is part 1 of "the book of mickey". I thought I would start with some basics. A rotor in autorotative forward flight with coning is diagrammed. The rotor in side view and top view is shown. Clock wise (from the top) rotation. The "V"s are local air velocities. Vf is the forward speed but it is drawn backwards since this is what the blade "sees". Note that the rotational air velocity vector is drawn backwards to the rotation because that is what the blade "sees". In side view is the rotor with coning with Vf resolved into the parts down the blade and perpindicular to the blade. The part down the blade does nothing to change the lift of that blade. The part perpindicular to the blade has the effect of increasing the local angle of attack and local velocity of that blade. note that the forward blade has a bigger increase than the backward blade due to coning. If the blades were straight the increase would be the same on both.On the top view the left blade sees the Vr+Vf, the right blades sees Vr-Vf, creating the asymmetric lift problem.
There are two ways to resolve the asymmetric lift problem (ALP), feathering or flapping. Feathering and flapping are BOTH forms of cyclic pitch (more later).
Either one or both can be used to resolve ALP due to forward flight differences. Here lies the first great gyrocopter challenge. Flapping cannot be used to resolve the ALP due to coning. If flapping were used to resolve the coning induced lift asymmetry, the forward blade would flap up on the right and the back blade would flap down on the left. The rotor disk would then be tilted to the left by some amount. The rotor always produces a force perpindicular to the plane formed by the rotor tips, called the tip path plane (TPP). So if the TPP is tilted left a left roll is created that must be trimmed out to fly level. This has to be done with feathering. This is accomplished by either shaft tilt feathering or swashplate type feathering. The effect being the same. The blade in front is pitched down as it crosses the front, the one in back is pitched up, this keeps the rotor level. (In my models I make the head rigid with very little coning, thus very little roll trim is needed and it changes very little with airspeed.)So here is the first great gyrocopter challenge. If you build in flapping hinges to alleviate the forward motion ALP you allow more coning angle to develop. The higher coning angle requires roll trim to stop the coning induced ALP. Further, the faster you go, the more roll trim is required. In fact the forward flight speed becomes limited by running out of roll trim! This is also true of helicopters! On the other hand going faster speeds the head up, causing it to flatten out, reducing coning. The question becomes does the rotor speedup that reduces coning and therefore reduces the roll trim needed end up being more than the increased effect a higher forward speed on coning creates that increases the roll trim needed!!!! Obviously some models will favor one vs the other and is related to the rotor speed, flight speed and flapping stiffness....First frustrating outcome : Fixed rotor head gyros with coning and shaft tilt to correct for coning are only usually trimmed at one airspeed. If this is takeoff speed then cruise is wrong, if this is cruise speed then takeoff speed is wrong. This is frustrating indeed. What does this lead to? Hand launches right into cruise speed.The great compromise is just enough flapping to relieve forward motion ALP, but no so much as to allow so much coning that you run out of roll trim....
First conclusion : coning is bad, flapping hinges aggrevate coning.
......
First design choice : Roll cyclic either by shaft tilting or swashplate is probably a good idea, even if you don't use pitch cyclic....so you can maintain roll trim at multiple airspeeds.
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Stan,Maybe you recognize this. I copied the first part of the forum.Welcome to RCGroups, the most active R/C community on the Internet.Registering with our site is easy and completely free. Registration provides you with the ability to join in on the public discussions, communicate with other members via private messages, respond to polls, upload your photos and videos, post to your own blog, buy and sell RC gear in the classifieds area, and much more. If you have any problems with the registration process, please contact us for additional help.If you've previously registered, please login using the login form below.Go Back RC Groups > Aircraft - Exotic and Special Interest > Auto Gyros
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The challenge of gyrocopter design.
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Auto Gyros · This forum includes information on Building, Designing and flight of RC Auto Gyro's.http://www.gws.com.tw/Joe Carola
Stan,Here's some more. Jan 24, 2005, 07:21 AM #9
mnowell129
Registered User
Join Date: Nov 2004
Posts: 1,051
Questions that remain that I intend to address...
What are the characteristics of the two bladed teetering head (bensen/wallis) ?
Why does a scale benson head not work right?
Why is the flybar used on your model?
What does collective do and does it help the gyrocopter?
What is following rate and why is it important?
What's the big deal about CG correcting and/or tip weights on
the blades?If you have questions yourself, please add to the list.
mickey
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Old Jan 24, 2005, 08:44 AM #10
mnowell129
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Join Date: Nov 2004
Posts: 1,051
Text that goes with the spider cyclic diagram.
This is the next step in cyclic control beyond tilting the shaft. You still tilt the shaft but you add a hollow hub mounted on bearings around the shaft. You now fix the blades to the hub which is connected rigidly through a big bearing to the fuselage. Now the flapping forces of each blade are carried straight to the fuselage and not carried by the control servos or human. The tilting shaft now tilts in a ball link in the center of the hollow hub. A "spider" turns with the rotor and has linkages attached to blade holders. The blade holders have feathering bearings so the spider can move the pitch of the blades positive and negative. If you look at the left side of the diagram you will see that the shaft tilts and all the blades follow in pitch all around the circle just as if you had tilted the shaft in direct control. Because the linkages are attached at 90 degrees with ball links, only shaft motion perpindicular to a blade causes any change in pitch. Thus if you tilt the shaft and follow a blade around you will see that it receives cyclic pitch just as it would with a direct control head.
This form of control puts the same cyclic pitch inputs in that are present with shaft tilting with two distinct advantages. 1) The flapping forces are not fed into the servos 2) The control power is much higher because the flapping forces are fed directly into the fuselage, so you end up with roll or pitch forces applied directly to the body.I read someone state on autogyro.com ( I believe Jim Baxter) that with direct control the rotor moves first and then the body. This is in fact true and is indicative of the low control power. The misalignment of the rotor thrust and CG must take place to create motion. With the spider controlled cyclic you get the same rotor thrust tilt but is rigidly attached to the body so the whole unit rolls together, indicative of the much higher control power present.THe right side of the diagram is an idea of how to build a spider control head without lots of bearings and linkages. The spider is now a thick slotted piece. THe blades now tilt on plain hinges (radially oriented). The little block and CF rod fit into the spider. Now tilting the spider has the same on/off axis effect (or lack of effect) on each blade. This might work if the hinges were good enough to not bind under load and the slot in the spider were smooth enough to also not bind under load.The complication with this type of control system is the big hollow hub and bearing required. This is a practical, not aerodynamic problem.The practical solution is to move the spider to the bottom and put IT on the big bearing and let the rotor spin on its normal sized shaft.
What you can probably guess is that this turns into the swashplate and the nominal arrangement for helicopters.Maybe the next installment will show how a bottom spider works and how this easily turns into a swashplate.The general result however is this:
1) Non controlled rotors need flapping hinges to correct for forward flight asymmetry.
2) Non controlled rotors need roll trim to correct for coning induced roll.
3) Non controlled rotors can be trimmed in roll for one flight speed.
4) Non controlled rotors can be trimmed with a wing or winglets (the original Cierva's did this)
5) Cyclic pitch MUST be used to control both asymmetric lift and coning induced roll trim.
6) Cyclic pitch can be peformed by shaft tilting, however the control forces are high and the control power is low.
7) Cyclic pitch can be performed with feathering bearings and a cyclic spider. THe control forces are light, the control power is high. The hollow hub is hard to employ.
8) A swashplate provides the same control power and light controls as the cyclic spider with less mechanical difficulty in construction, and results in the same effective cyclic pitch as shaft tilt.This chain of results is why helicopters have swashplates.
This is why my gyrocopter model uses a cheap commercially available swashplate, etc. (no manufacturing required on my part).WHat a designer should know is that for a fixed head you are going to have to solve the asymmetric lift problem and the resulting coning induced roll trim problem. This can be tedious and without ailerons, likely only to work at a narrow range of flight speeds.If you put roll control in with flapping hinges you can expect high servo loads, and low control power but you can still correct the coning induced roll trim problem.If you do spider cyclic you can get pitch and/or roll control, get by with no flapping and very little coning and need very little roll correction due to coning and will work at all flight speeds.
If you use a swashplate you get all the benefits of the spider cyclic, it is easier to construct and components are commercially available at low cost.(http://www.micro-flight.com is guy who makes very tiny helicopters (< 12"), he has very small swashplates, linkages, etc. if you are into the tiny indoor problem, For anything bigger GWS dragonfly, Century hummingbird or bigger helicopter parts are available. http://www.like90.com has the trex450 parts, including blade holders, swashplates, etc for dirt cheap, these would be appropriate for 1.5 - 2 pound sized models.).The bottom line is that gyrocopters are controlled by cyclic pitch, whether by flap or by feather, its just which one you chose and what effects you get from your choice.The next question, "what about that flybar or those tip weights?" is very important. THe answer will explain why your swashplate or spider controlled (or two bladed direct controlled ,etc. ) gyro rolls over on takeoff faster that you can say spit...(and why you can't make a scale bensen! without electronics) And what to do about it.TTFN
hope somebody is reading this chatter with some benefit...
mickeyJoe Carola
Joe: Hey....thanks. That is our gyro pshyco babble we rotorheads spew out.
I am getting near having hi-speed internet. All I have to do is run a wire to the roof. I have been saying that since last fall. The internet is not that big of a deal for me. I mainly go to my rotorcraft site.....and seldom post here.
Thanks again Joe.
Stan
Stan,I got all exited when I saw that site, even though I don't know a thing about them but have enjoyed your posts and pictures of them. They have tons of pictures of people flying in them like you have. Here's a couple. One is picture is of three models someone made and the other is a cool picture. I felt like posting on that site saying that I know Stan Foster and you should see his pictures.....;-)Hope the pictures are small enough for you to open. If they're not let me know.Joe Carola
Joe: Thanks...those are neat pictures. I have been kind of wanting a little electric helicopter model...something small and cheap for when I wreck it.
Stan
So you probably weren't able to open that site in the first post.
It was a video link to a guy that built a gyro copter on a motorcycle with a foldable prop so you could ride away on the ground with it.
http://www.thebutterflyllc.com/
every court needs a jester
Rez: I didnt get to see your link..but I a very aware of that gyro/motorcyle combo. I know the guy.
Stan