Espritmodel.com Telemetry Radio
Reply
Thread Tools
This thread is privately moderated by mnowell129, who may elect to delete unwanted replies.
Old Jan 01, 2007, 09:52 AM
edi
Triplane addict
edi's Avatar
Elstertrebnitz, Germany
Joined Aug 2004
1,713 Posts
Mick,

two more hours of sleep and four re-readings made the lot clear to me. However, I still don't quite get why the *left* tip produces more lift. Doesn't that depend on CW or CCW rotation? And as a whole, will the behaviour of the model differ depending on whether the left rotor rolls CW and the right CCW or whether it's the other way round?

Dan,

I was thinking about tilting both rotors to the left and to the right for roll control.

Guys, please let me add that I haven't had so much fun with a thread for a long time!!!

A Happy New Year to you!
edi is offline Find More Posts by edi
Reply With Quote
Sign up now
to remove ads between posts
Old Jan 01, 2007, 10:09 AM
Registered User
Joined Nov 2004
3,171 Posts
Quote:
Originally Posted by edi
Mick,

two more hours of sleep and four re-readings made the lot clear to me. However, I still don't quite get why the *left* tip produces more lift. Doesn't that depend on CW or CCW rotation? And as a whole, will the behaviour of the model differ depending on whether the left rotor rolls CW and the right CCW or whether it's the other way round?
By left tip, I mean the left wingtip, not rotor blade tip.
The left wing/rotor is yawed forward because we've put in right rudder, this yaws the aircraft to the right, left wingtip forward, right wingtip aft. The left rotor shaft gets tilted back, right tip tilted forward. Left rotor speeds up, right slows down.....
Doesn't really matter about the rotation, except if coning sneaks in. This might explain why the rotation of twins seems to favor the advancing blade to the outside.
mickey
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Jan 01, 2007, 10:12 AM
Registered User
Joined Nov 2004
3,171 Posts
Quote:
Originally Posted by edi
Dan,

I was thinking about tilting both rotors to the left and to the right for roll control.
You gotta ask yourself. If the roll in a single rotor comes from misaligning the rotor lift with the CG how does that work with rotors at the tips? I don't think it's going to be that effective. Differential fore/aft tilt is likely to work much better.
Twin rotors, like the Osprey do roll control with differential collective (differential increased lift of the rotors), not by rolling the individual rotors.
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Jan 01, 2007, 11:38 AM
edi
Triplane addict
edi's Avatar
Elstertrebnitz, Germany
Joined Aug 2004
1,713 Posts
Fact is, I am about to build a Twirl, and I was thinking how to move on from there. Of course, I could add ailerons to the wings which would be easiest, but how effective would that be? I want to keep things as simple as possible, so I would like to keep away from rotors with collectives and friends.

As you can guess from my avatar, I like unusual scale subjects. I have thought that some experimental helicopters (like the Berliner helicopter or the Focke-Achgelis 61) could be turned into a successful autogyro with much less hassle than building them as helicopters. I am also interested in the early Ciervas (especially in those that didn't fly in full-size ).
edi is offline Find More Posts by edi
Reply With Quote
Old Jan 01, 2007, 12:15 PM
Keep it simple
alfoot's Avatar
Salisbury,England
Joined Jan 2005
970 Posts
Edi,

It just occurred to me that , if you are building a Twirl, one way of achieving differential fore-and-aft rotor tilt might be to replace the thread bracing that goes from the undercarriage blocks to the bottom of each rotor post with an "aileron" type linkage so that you could twist the wing and rotor posts.

I must add that I have not tried this, the reason for the original thread bracing is to reduce the tendency for the rotors to tilt aft under flying loads. I guess that the dihedral would need to be reduced a bit too.

You will also find that, due to the dihedral and the other effects that Mickey alluded to in his earlier post, it turns pretty well with just the rudder, and in fact it will roll on that control alone as designed.

Like you, I think that the Focke-Achgelis 61 would make an interesting autogyro project.

Al
alfoot is offline Find More Posts by alfoot
Reply With Quote
Old Jan 01, 2007, 12:24 PM
Registered User
Joined Nov 2004
3,171 Posts
Both the FA61 (Model Builder) and the Osprey (Model Airplane News) have already been done as gyrocopters.
I have both sets of plans.
The MAN plans are still available. I think someone is
selling the model builder plans.
I'll look for the article.
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Jan 04, 2007, 05:37 PM
Registered User
Joined Nov 2004
3,171 Posts
Follow me, follow you...

An new topic.
Suppose your gyro is just happily flying along and you want to roll to the right. You input your cyclic, either by swashplate, direct spindle tilt, or indirect spindle tilt (moving the fuselage in some way).
The rotor responds. What the figure shows is the next 3 revolutions of the rotor after your cyclic is input. The rotor advances from A0 to A1, then A2 etc.
The real question is how big is the step from A0 to A1 or stated another way how quickly does the rotor follow your control input. That is, what is the following rate?
Lets consider some absurd situations. Suppose that A3 represents a 30 degree tilt and that you have a rotor that responds by going from A0 to A3 in .001 seconds, essentially really fast. You as the pilot would find this impossible to fly, much like a fixed wing aircraft with way too much aileron throw. You'd probably crash on takeoff by rolling it upside down (which would only take .006 seconds or put another way 83 rolls per second). The problem is clearly that the aircraft is just too responsive.
Lets take the other extreme. Lets say that the time from A0 to A3 is 5 minutes. Clearly this would be difficult to fly. You'd never get a correction in time before crashing, not to mention that staying in the field would be a problem.
The question arises, what are the practical ranges of following rate? The question was answered by engineers at Hiller Helicopter in a study that showed for a human to control the rotor it had to move 30 in between and 2 seconds. Faster than the pilot couldn't keep up, slower than 2 seconds the pilot overcontrolled.
Now you ask, what controls the following rate? The answer is the the following rate increases with RPM, decreases with blade mass and increases with blade area and lifting capability. This should make sense. A heavy blade is going to respond slower. A faster rotor is going to respond faster. A large area blade with big lift will overcome the mass and respond faster.
So what this means is that a 30', 400 rpm rotor on a full sized gyrocopter is going to have a drastically different (read this lower) following rate than a 3', 1000 rpm model rotor.
In the next installment we'll talk about how to moderate the following rate and get to flybars.
Notice that most full sized helicopters don't have flybars but virtually all models do. Ever wonder why? The answer has to do with following rate.
Every wonder why an exact scale bensen hasn't been built yet in a small size. The answer is also following rate.

more later
mick
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Jan 05, 2007, 12:42 AM
FPV really is fun.
Scotth72's Avatar
Sedona, Az
Joined Sep 2001
892 Posts
Good times. As you know, I like to fly your model designs with a carbon flybar, for more immediate response. I have come to think that this modification, while making cyclic response quicker due to less mass, allows for more headspeed variation. Strong cyclic moves decrease headspeed. I have learned to really get on the throttle to keep the headspeed up on strong cyclic moves. It is also interesting how sluggish the control can get with low headspeeds, as the control is solely thru the flybar paddles. I still feel this is the best way to control these small autogyro models..... A Bell/Hiller mix would be quite interesting. I tried, but I think that my lead/lag bolts were not tight enough, and the blades got out of shape. Too broke to fix, and my current models fly so well....
Scott
Scotth72 is offline Find More Posts by Scotth72
Reply With Quote
Old Jan 05, 2007, 07:41 AM
13brv3's Avatar
Navarre, FL
Joined Mar 2002
3,829 Posts
Mickey,

I've really enjoyed the series so far, and look forward to any further topics you have in mind. I'm curious if one of those topics will be tractor vs. pusher configurations.

It would seem that the first, and most successful full scale gyros were all tractor configuration. In recent decades, it seems like pushers are just about all that exist, despite the terrible manners of some of them. In the model world, virtually all gyros are tractors, with only a couple pushers, including your BEGi. If you have any insight into why this is, I'd love to hear about it.

Thanks,
Rusty (almost finished with a PT-Profile, and planning a full scale gyro this year)
13brv3 is offline Find More Posts by 13brv3
Site Sponsor
Last edited by 13brv3; Jan 06, 2007 at 08:28 AM.
Reply With Quote
Old Jan 05, 2007, 11:38 AM
Registered User
Joined Nov 2004
3,171 Posts
Follow some more

Now that we've set the stage for following rate the more mathematically minded can go back and read the excerpt from John Drake's book in post #37
http://www.rcgroups.com/forums/showp...6&postcount=37
and look at the math. Note that the radius of gyration of the blades, symbol k, is the blade radius divided by 1.73.
Although the math is helpful in pointing out the relationships, actually calculating the values doesn't really help me that much. The math does tell you what direction you need to go.
Let's do some history now. Go look at www.autogyro.com and look at some older models. Consider the gyrocopter models with a tilting spindles, soft flapping hinges and lightweight balsa blades. What can we infer from the basic design. First with lightweight blades and soft flapping hinges we get a lot of coning. With light blades we get a high following rate (read that sensitive control). To tame the control you reduced the control throws, but this left you with too little throw to compensate for coning induced roll, etc. etc. etc. It should be easy to see how hard it is to get this configuration to fly right.
More recent designs, the micromum and minimum come to mind, you get stiffer hinges, heavier blades and lower following rates.
Mostly for a given blade size, rpm, etc. the following rate is reduced by the moment of inertia, I, of the blade. The easiest way to increase the blade's inertia is to add tip weight.
So why not just weight the heck out of the rotor tips on your gyrocopter. The answer is that this has two negative effects. One is that the rotor takes longer to spin up. This isn't a killer problem generally. Second, go back to the discussion of tilting spindle cyclic and review the centrifugal force loads being fed back to the servos. Increasing the blade mass just increases the load on the servos. This is the bigger problem. So you're kinda stuck on a tilting spindle with lightweight blades and a high following rate. What can you do to tame the aircraft down? You add damping so the combination of a twitchy rotor and a damped fuse results in a flyable aircraft. What makes the aircraft damped? A couple things: a wing (whistler), a tall mast (many designs) and mostly a horizontal tail. A tail is required on an airplane (reflexed airfoil IS the tail on a flying wing BTW). But a tail isn't required on a gyrocopter strictly speaking for stability like an airplane. It is required to tame the following rate, even on full sized gyros.

Now what about that scaled down bensen. Consider again the flapping tilting spindle head with 3 or more blades. The flapping hinge is somewhere out along the radius. This hinge offset provides some leverage for the blade. Once the blade flies up in response to cyclic it can now pull on the shaft by virtue of the hinge offset. In fact the more the hinge offset the more the blade can pull on the shaft. This in fact gives you more powerful control and adds some control power that isn't simply the rotor thrust being misaligned with the center of gravity. What's this got to do with following rate?
Well if the shaft is providing resistance to the rotor due to hinge offset, the rotor can't move as fast, hence the following rate slows. What about three blades? In this case because of centrifugal force the servo can't put in too much cyclic and the following rate naturally follows the amount of cyclic that can be put in.
And finally what happens when you make the flapping offset 0 and go to a two blades system. You get the basic teetering rotor on all the "bensen" type full sized gyros. You know enough now to see that with only two blades there is no centrifugal force to stop you from putting in as much cyclic as you want and with 0 hinge offset (teetering remember) there is no fuselage damping to slow down the following rate. Now add to this a very small, low mass rotor at high rpm and what do you get. An extremely high following rate, certainly far exceeding the second human controllable requirement. What does this say for the prospect of a small scale bensen? Not much.
I know some of you are aware of a big scale bensen in France. But it HUGE, 18 pounds or something. You know enough now to realize that size makes a big difference in following rate. The blades are heavier and turn slower and you can see from the following rate equation that this lowers the rate.
So here is the basic aerodynamics that get you to the typical present day gyrocopter, 3 blades with flapping offset, puller or pusher doesn't matter. Relatively tall mast for roll damping, elevator for pitch damping. Essentially a very touchy rotor with aerodynamic damping from the fuselage that results in a nice flying aircraft.
Now how is it that G3PO can have a short mast and no horizontal tail at all?
Tune in next.
mick
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Jan 08, 2007, 11:39 AM
iPhly R/C with iPhone
iter's Avatar
Silicon Valley, Calif
Joined Jan 2005
1,723 Posts
Feature request

Mickey, thanks for writing this thread. I think we should eventally clean up the chat and make it a sticky.

I have a topic request for this thread. Could you elaborate on lead-lag hinges in a future installment?

Ari.
iter is offline Find More Posts by iter
Reply With Quote
Old Jan 08, 2007, 12:19 PM
Winging it >
leadfeather's Avatar
Joined May 2006
9,414 Posts
I second the idea for making this a sticky. There is great value in this thread to any one interested in autogyros.

Mickey, there may not be a lot of feedback to your posts but believe me, people are reading them and learning...keep up the good work!!

Dan
leadfeather is online now Find More Posts by leadfeather
Reply With Quote
Old Jan 08, 2007, 12:20 PM
Registered User
Joined Nov 2004
3,171 Posts
Quote:
Originally Posted by iter
Mickey, thanks for writing this thread. I think we should eventally clean up the chat and make it a sticky.

I have a topic request for this thread. Could you elaborate on lead-lag hinges in a future installment?

Ari.
You are welcome.
I'm okay with it being a sticky.
Will add lead lag hinges as a topic.
Lets see thats now, tractor vs pusher, flybars, lead lag hinges, anything else?
Please tell me that I'm either a) explaining something
that you didn't understand in what you already built or
b) are finding this helpful in something you are building.
It's a shame to put all these words down if they are not finding useful application....
mick
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Jan 08, 2007, 12:23 PM
Registered User
Joined Nov 2004
3,171 Posts
Quote:
Originally Posted by leadfeather
I second the idea for making this a sticky. There is great value in this thread to any one interested in autogyros.

Mickey, there may not be a lot of feedback to your posts but believe me, people are reading them and learning...keep up the good work!!

Dan
Thanks Dan,
It is kind of a weird classroom. At least when I taught I could see who was sleeping and who was putting the move on the good looking freshmen. Here I'm tempted to ask the paraphrased question "If a tree falls in the cyberwoods did it make a sound?".
Feedback is welcomed.
mnowell129 is offline Find More Posts by mnowell129
Reply With Quote
Old Jan 08, 2007, 12:53 PM
Registered User
imsofaman's Avatar
Nowhere
Joined Aug 2003
6,178 Posts
Noce job Mickey....lots of info to digest. I agree with the sticky....we cant loose this and it would be great to reference it from time to time.

Dave
imsofaman is offline Find More Posts by imsofaman
Reply With Quote
Reply


Thread Tools